Morphology and distribution of fossil soils in thePermo-Pennsylvanian Wichita and Bowie Groups, north-centralTexas, USA: implications for western equatorial Pangeanpalaeoclimate during icehouse–greenhouse transition
NEIL JOHN TABOR* and ISABEL P. MONTANEZ�*Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA(E-mail: [email protected])�Department of Geology, University of California, Davis, CA 95616, USA
ABSTRACT
Analysis of stacked Permo-Pennsylvanian palaeosols from north-central Texas
documents the influence of palaeolandscape position on pedogenesis in
aggradational depositional settings. Palaeosols of the Eastern shelf of the
Midland basin exhibit stratigraphic trends in the distribution of soil horizons,
structure, rooting density, clay mineralogy and colour that record long-term
changes in soil-forming conditions driven by both local processes and
regional climate. Palaeosols similar to modern histosols, ultisols, vertisols,
inceptisols and entisols, all bearing morphological, mineralogical and
chemical characteristics consistent with a tropical, humid climate, represent
the Late Pennsylvanian suite of palaeosol orders. Palaeosols similar to modern
alfisols, vertisols, inceptisols, aridisols and entisols preserve characteristics
indicative of a drier and seasonal tropical climate throughout the Lower
Permian strata. The changes in palaeosol morphology are interpreted as being
a result of an overall climatic trend from relatively humid and tropical, moist
conditions characterized by high rainfall in the Late Pennsylvanian to
progressively drier, semi-arid to arid tropical climate characterized by
seasonal rainfall in Early Permian time. Based on known Late Palaeozoic
palaeogeography and current hypotheses for atmospheric circulation over
western equatorial Pangea, the Pennsylvanian palaeosols in this study may be
recording a climate that is the result of an orographic control over regional-
scale atmospheric circulation. The trend towards a drier climate interpreted
from the Permian palaeosols may be recording the breakdown of this pre-
existing orographic effect and the onset of a monsoonal atmospheric
circulation system over this region.
Keywords Late Palaeozoic palaeoclimate, palaeosols, Permo-Carboniferous.
INTRODUCTION
Outcrop and numerically based modelling stud-ies of the Late Palaeozoic geological record indi-cate that Earth’s climate system underwentsignificant evolutionary change during this per-iod. It is generally believed that climate changeoccurred in response to the formation of thesupercontinent Pangea, an accompanying trans-
ition from icehouse to greenhouse states and aprobable global-scale reorganization of atmo-spheric circulation systems (Rowley et al., 1985;Cecil, 1990; Parrish, 1993; West et al., 1997; Gibbset al., 2002; and references therein). This plethoraof studies indicates that the mid-latitude andequatorial regions of Pangea became progres-sively more arid, with more seasonal preci-pitation, throughout the Late Palaeozoic,
Sedimentology (2004) 51, 851–884 doi: 10.1111/j.1365-3091.2004.00655.x
� 2004 International Association of Sedimentologists 851
strongly supporting the development of northernhemisphere monsoonal circulation during thisinterval. Furthermore, it has been hypothesizedthat monsoonal circulation began in the LateCarboniferous and reached its peak in the MiddleTriassic based primarily upon lithological andpalaeontological studies of North American andEuropean terrestrial strata (Parrish, 1993).
Geological proxy records (e.g. coals, evaporites,aeolianites, tillites) and numerical models canprovide an excellent framework for the long-termdevelopment of climate systems over Pangea. Fewdetailed records exist, however, that offer insightinto higher resolution spatial and temporal varia-tions in climate regimes that accompanied theevolution of Pangean monsoonal circulation.Significantly, recent studies of terrestrial depositsand their associated palaeosols in the south-westUSA suggest that monsoonal circulation was well-established over western equatorial Pangeaby earliest Permian time (Kessler et al., 2001;Soreghan G.S. et al., 2002; Soreghan M.J. et al.,2002; Tabor & Montanez, 2002). These studies,coupled with continuing discrepancies inmodel-data comparisons for low-latitude Pangea(Patzkowsky et al., 1991; Rees et al., 2001; Gibbset al., 2002), highlight the necessity of understand-ing the evolution of specific Pangean climatezones.
The palaeosol-rich, Permo-Pennsylvanian ter-restrial strata of north-central Texas hold thepotential to provide insight into climate changeover low-latitude Pangea, given that climatevariation would have been most pronounced inwestern equatorial Pangea because of the confi-guration of the supercontinent and its attendanteffects on atmospheric circulation and precipita-tion patterns (Dubiel et al., 1991; Parrish, 1993).Furthermore, rich faunal and floral collectionsand interstratified regionally correlatable fluvialsandstone bodies and fusulinid-bearing lime-stones provide a chronostratigraphic frameworkin which to define spatial and temporal trends inpalaeosol morphology, mineralogy and geochem-istry and the degree of palaeopedogenic devel-opment. This study evaluates the influence oflocal- to regional-scale autogenic and larger scaleallogenic processes on the stratigraphic distribu-tion of palaeosol morphologies and compositionsthrough detailed field, petrographic, mineralogi-cal and geochemical analyses of � 200 Permo-Pennsylvanian palaeosols. These palaeosolsprovide a detailed data set for comparison withpreviously published palaeoclimate trends basedon geological data and numerical models. The
results of this study help to refine the nature andtiming of climate change in western equatorialPangea during the early stages of supercontinentformation, major change in global atmosphericcirculation and the onset of deglaciation.
GEOLOGICAL SETTING
The study area in north-central Texas (Fig. 1) waslocated in the western coastal zone of equatorialPangea during the Late Palaeozoic (Ziegler et al.,1997)1 and remained between 0 and 5�N of theequator throughout this time period (Golonkaet al., 1994; Scotese, 1999; Loope et al., 2004).Major tectonic elements such as the MuensterArch and the Wichita, Arbuckle and OuachitaMountains (Fig. 1) had developed by latestPennsylvanian time (Oriel et al., 1967), and tec-tonic quiescence has characterized the regionsince (Donovan et al., 2001)2 . Late Pennsylvanian(Virgilian) and Early Permian (Wolfcampian andLeonardian) terrestrial strata of north-centralTexas, along with subordinate marine sediments,were deposited upon the broad, gently slopingEastern shelf of the Midland basin as it subsidedthroughout the Late Palaeozoic (Fig. 1; Brownet al., 1987; Hentz, 1988). A minor component ofdetrital sediments representing the Upper Penn-sylvanian and Lower Permian strata of the EasternMidland basin may have been derived from EarlyPalaeozoic sediments and metamorphic rocks ofthe distal Ouachita and Arbuckle Mountainchains (Hentz, 1988). However, the principalsource of detrital sediments during this timewas probably derived from second-cycle terrigen-ous-clastic sediments from Middle Pennsylva-nian fan-delta and fluvial facies that wereeroded from the proximal Ouachita foldbeltand Muenster Highlands (Fig. 1; Brown, 1973).Although there is no evidence for a major changein the lithology of source lands during Permo-Pennsylvanian deposition, a progressive decreaseupsection in grain size and thickness of fluvialsandstones, coupled with a decrease in strati-graphic frequency of thick, multistorey sand-stones, is interpreted as recording significantdenudation of the source areas by the close ofthe Early Permian (Hentz, 1988).
The primarily terrestrial deposits of the Bowieand Wichita Groups on the north-eastern portionof the Eastern shelf reach a thickness of up to530 m in the east and thin westward towardsmarine-dominated strata (Fig. 2). Three terrestrialfacies are recognized in the Late Pennsylvanian
852 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
and Early Permian terrestrial strata of theEastern Midland basin: (1) sand- and gravel-richchannel-bar facies; (2) point-bar facies; and (3)floodplain facies (Hentz, 1988). Sand and gravel-rich, channel-bar facies were deposited by brai-ded stream systems fringing the north-easternhighlands, whereas the point-bar facies recordpenecontemporaneous meandering stream sys-tems that developed on the Eastern shelf to thesouth and west of the braided stream system.
Alluvial floodplain deposits formed primarily inassociation with meandering streams given theirstratigraphic predominance in the western andsouthern portions of the study area. Based onregional facies distributions, Hentz (1988) definedthree physiographic provinces for the latestPennsylvanian to Early Permian landscape ofthe Eastern shelf: (1) the piedmont provincedominated by sand- and gravel-rich channel-barfacies and, to a lesser degree, point-bar facies; (2)
Fig. 1. Regional distribution of source terranes and physiographic provinces in the latest Pennsylvanian to EarlyPermian, Eastern Shelf of the Midland Basin. Province boundaries are defined for the period of maximum regressionin the earliest Permian. Structural and tectonic elements shown are for the pre-Virgilian palaeogeography of north-central Texas; the Muenster and Ouachita highlands remained significant topographic features in the Early Permian.Encircled numbers delineate locations of palaeosol-bearing stratigraphic sections used in this study. Diagrammodified after Brown et al. (1987) and Hentz (1988).
Permo-Carboniferous palaeoclimate from palaeosols 853
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
the upper coastal plain province characterized bypoint-bar facies; and (3) the lower coastal plainprovince dominated by floodplain facies and, to alesser degree, point-bar facies.
A high-resolution chronostratigraphic frame-work for Permo-Pennsylvanian terrestrial strataof north-central Texas has been developed
through correlation of multiple fusulinid-bear-ing limestones and intercalated multistorey flu-vial sandstone deposits, which were described(ss units in Fig. 2) and regionally correlated byHentz (1988). The Pennsylvanian–Permianboundary (301 ± 2 Ma; Rasbury et al., 1998)occurs within a few metres of the contact
Fig. 2. Regional correlation of Late Pennsylvanian and Early Permian strata in north-central Texas. Light grey bedsdelineate regionally extensive fluvial sandstone sets; black horizons are coals; white regions represent alluvialmudstones and intercalated siltstones and thin-bedded, fine-grained sandstones; dark grey beds are marine lime-stones. Encircled numbers show the stratigraphic distribution of studied palaeosols – numbers correspond to sites inFig. 1. Diagram modified after Hentz (1988).
854 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
between the Markley and Archer City Forma-tions of the Bowie Group (Dunbar & Committeeon Stratigraphy, 1960)3 . The Wolfcampian–Leo-nardian boundary (283 ± 2 Ma; Ross et al.,1994) is defined by the fusulinid-bearing ElkCity Limestone at the base of the PetroliaFormation of the Wichita Group (Fig. 2).
BURIAL HISTORY
The entire stratigraphic thickness of the LatePennsylvanian and Early Permian rocks in thestudy area is � 1250 m, with the lower 1000 mdominated by sedimentary rocks characteristicof terrestrial depositional facies (Hentz, 1988;Nelson et al., 2001; Tabor & Montanez, 2002)4 .Deposition in this area during the Triassic andJurassic was insignificant (Oriel et al., 1967;McGowen et al., 1979). However, Cretaceousstrata were probably deposited in the area, asshown by outliers of Cretaceous marine rocks inthe extreme western portion of the study area,but these probably did not exceed 330 m inthickness (Barnes et al., 1987). Tertiary and earlyQuaternary burial was insignificant in this area(Barnes et al., 1987). Therefore, the maximumburial depth of Virgilian strata in the EasternMidland basin probably did not exceed 1600 min thickness. This shallow burial history isreflected by relatively mild burial temperaturesthat never exceeded 40–45 �C in the EarlyPermian strata along the flanks of the Midlandbasin (Bein & Land, 1983).
METHODS
Thirty-one stratigraphic sections were dug orpicked back 30–60 cm to provide a fresh surfacefor description and sampling. The geographic andstratigraphic positions of these 31 sites are shownin Figs 1 and 2. Palaeosols (n ¼ 190) from the 31stratigraphic sections of the Bowie and WichitaGroups were logged and described in detailwithin the chronostratigraphic and sedimentolog-ical framework defined by Hentz (1988). Palaeo-sol profiles were recognized and described usingestablished criteria (Retallack, 1988; Kraus &Aslan, 1993; Kraus, 1999). Profile tops wereidentified on the basis of a marked change ingrain size and/or colour, as well as preservation ofprimary sedimentary structures. Profile baseswere delineated at the lowest occurrence ofunaltered parent material. Palaeosol and sedi-
ment colours were identified from dry samplesusing Munsell colour charts (Munsell Color,1975). Palaeosol classification primarily followsthe USDA Soil Taxonomy system (Soil SurveyStaff, 1975, 1998).
Palaeosol matrix was sampled at 0Æ1–0Æ2 mvertical spacings; rhizoliths and nodules weresampled where present in the profile. Palaeosolmatrix samples were disaggregated overnight byultrasonic agitation in a dilute Na2CO3 solutionand analysed for grain-size distribution. Percent-age sand, silt and clay was determined bycentrifugation and filtration. Thin sections(n ¼ 78) of representative samples were exam-ined for their micromorphology and mineralogyaccording to the approaches and terminology ofBrewer (1976) and Wright (1990).
X-ray diffraction of the < 2 lm size fraction wascarried out for identification of clay mineralogy.Samples were exchange saturated with K or Mgon filter membranes and transferred to glass slidesas oriented aggregates. A split of the Mg-saturatedclays was also treated with glycerol. Orientedaggregates of all Mg-treated samples were ana-lysed at 25 �C. K-treated samples were analysed at25 �C, 300 �C and 500 �C after 2 h of initialheating at their respective temperatures. Stepscan analyses were performed on a Diano 8500X-ray diffractometer with CuKa radiation be-tween 2 and 30�2h with a step size of 0Æ04�2hand a 1 s count time.
PALAEOSOL TYPES ANDPALAEOENVIRONMENTALIMPLICATIONS
Field-scale indicators of pedogenesis in the stud-ied Permo-Pennsylvanian terrestrial strata includecolour, mottling, structure, slickensides, accumu-lation of oxides, clays and carbonate and fossilroot traces. Palaeosol profiles were subdividedinto horizons on the basis of downprofile changesin macro- and micromorphological features, claymineralogy and abundance and geochemicalcharacteristics. Eight major pedotypes (sensuRetallack, 1994; Table 1; Figs 3 and 4) are recog-nized based on distinctive characteristics thatinclude the aforementioned macromorphologicalfeatures as well as profile thickness, degree ofdevelopment (maturity), relationship to strati-graphically adjacent palaeosols (i.e. composite,compound or cumulate palaeosols; sensu Marriott& Wright, 1993; Kraus, 1999) and clay mineral-ogy. Palaeosols assigned to each pedotype are
Permo-Carboniferous palaeoclimate from palaeosols 855
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
Morp
holo
gy,
min
era
logy
an
dch
em
ical
ind
ices
of
rep
rese
nta
tive
pro
file
sof
ped
oty
pes.
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lmC
lay
min
era
logy�
Pro
vin
ce
A:
His
toso
lM
ark
ley
Fm
.O
i(0
–0Æ2
0)
Dark
gre
yto
bla
ck,
mass
ive
an
dorg
an
ic-r
ich
cla
yst
on
e.
Wh
ole
Neu
rop
teri
sle
aves.
Non
-calc
are
ou
s.
Org
an
icla
yers
vari
able
thic
kn
ess
(3–70
cm
);th
incla
stic
part
ings
com
mon
.R
are
jaro
site
rep
lacem
en
tof
rhiz
o-
lith
s.
––
Low
er
coast
al
Aj
(0Æ2
0–0Æ2
7)
Lig
ht
gre
y(5
Y7/1
)cla
yst
on
ew
ith
fin
egra
nu
lar
stru
ctu
re.
Com
mon
med
ium
tocoars
eyell
ow
(5Y
8/8
)m
ott
les.
Den
sen
etw
ork
of
thin
(<1
cm
),ta
bu
lar
yell
ow
(5Y
8/8
)rh
izoli
ths.
Non
-calc
are
ou
s.
Jaro
site
(40%
)K
,I
(62%
)
C(0Æ2
7–0Æ3
5)
Gre
y(5
Y6/1
),m
ass
ive
silt
ycla
yst
on
e.
Non
-calc
are
ou
s.–
K,
I(5
9%
)
B:
Ult
isol
Mark
ley
Fm
.A
Bt1
(0–0Æ1
5)
Red
(10R
4/4
)cla
yst
on
ew
ith
gra
nu
lar
tosu
ban
gu
lar
blo
cky
stru
ctu
re.
Com
mon
dis
cre
te,
fin
e-s
cale
yell
ow
(5Y
8/6
)verm
icu
lar
mott
les.
Abu
nd
an
tF
e-
(less
soM
n-)
oxid
ecoati
ngs
an
dori
en
ted
cla
ysk
ins
on
matr
ixaggre
-gate
s.N
on
-calc
are
ou
s.
Cla
yaccu
mu
lati
on
inu
pp
er
pro
file
svari
es
from
mod
era
teto
sig-
nifi
can
t.
–K
,I
(67%
)L
ow
er
coast
al
Bt2
(0Æ1
5–0Æ6
0)
Red
(10R
4/3
)cla
yst
on
ew
ith
suban
gu
lar
blo
cky
stru
ctu
re.
Few
dis
cre
te,
fin
e-s
cale
yell
ow
(10Y
R8/8
)su
bsp
her-
ical
mott
les.
Abu
nd
an
tF
e-
(less
soM
n-)
oxid
ecoati
ngs
an
dori
en
ted
cla
ysk
ins.
Non
-calc
are
ou
s.
Haem
ati
te,
goeth
ite
(10%
)K
,I
(78%
)
BC
g(0Æ6
0–1Æ0
2)
Mott
led
gre
y(5
Y6/1
)an
dre
d(1
0R
4/4
)si
lty
cla
yst
on
ew
ith
suban
gu
lar
blo
cky
stru
ctu
re.
Dis
cre
te,
bro
wn
-yell
ow
(10Y
R6/8
)to
red
(5R
4/8
)vert
icall
yori
en
ted
tubu
lar
mott
les
an
dn
od
ule
s.N
on
-calc
are
ou
s.
Haem
ati
te,
goeth
ite
(15%
)K
,I
(42%
)
856 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
(Con
tin
ued
).
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lmC
lay
min
era
logy�
Pro
vin
ce
Cgv
(1Æ0
2–1Æ2
0)
Gre
y(5
/N)
mass
ive
silt
ycla
yst
on
e.
Larg
e(�
10
cm
);h
ori
zon
tal,
red
(5R
4/8
)F
e-
an
dM
n-o
xid
en
od
ule
sth
at
coale
sce
tofo
rma
part
i-all
yin
du
rate
dp
oly
gon
al
netw
ork
inp
lan
vie
w(F
ig.
5B
).N
on
-calc
are
ou
s.
Haem
ati
te(6
0%
)K
,I
(43%
)
C:
Incep
tiso
lM
ark
ley
Fm
.A
g(0
–0Æ2
8)
Oli
ve
gre
y(5
Y5/2
)m
ud
ston
ew
ith
fin
egra
nu
lar
stru
ctu
re.
Abu
nd
an
tfi
ne-
tom
ed
ium
-scale
,su
bsp
heri
cal
wh
ite
(5Y
2Æ5
/1)
mott
les;
fin
e-s
cale
Fe-o
xid
en
od
ule
s.N
on
-calc
are
ou
s.
Vari
able
hori
zon
develo
pm
en
tfr
om
poor
tom
od
era
te.
Ped
stru
ctu
revari
es
from
mod
era
tely
tow
ell
-d
evelo
ped
,an
dm
ed
-iu
mto
coars
ean
gu
lar
an
dw
ed
ge-s
hap
ed
.
Haem
ati
te(1
0%
)K
,I
(22%
)L
ow
er
an
du
pp
er
coast
al;
pie
dm
on
t
Bv
(0Æ2
8–0Æ4
9)
Oli
ve
(5Y
4/4
)m
ud
ston
ew
ith
med
ium
an
gu
lar
blo
cky
stru
ctu
re.
Fin
e-
tom
ed
ium
-sc
ale
,d
usk
yre
d(1
0R
4/4
)h
aem
ati
ten
od
ule
scoale
sced
tofo
rma
sem
i-in
du
rate
dla
yer.
Non
-calc
are
ou
s.
Haem
ati
te(2
5%
)K
,I
(25%
)
BC
ssg(0Æ4
9–1Æ3
1)
Dark
oli
ve
gre
y(5
Y3/2
)m
ud
ston
ew
ith
wed
ge-
tole
nti
cu
lar-
shap
ed
stru
ctu
ral
aggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Com
mon
,m
ed
ium
-to
coars
e-s
cale
yell
ow
(10Y
R7/6
)to
du
sky
red
(10R
4/4
)m
ott
les.
Non
-calc
are
ou
s.
–K
,I
(32%
)
Cg
(131-c
over)
Dark
oli
ve
gre
ym
ass
ive
top
oorl
yla
min
ate
dm
ud
ston
e.
Abu
nd
an
tm
ed
ium
-scale
,sp
heri
cal
tola
min
ar
ligh
tre
dd
ish
bro
wn
(5Y
R6/4
)m
ott
les.
Non
-calc
are
ou
s.
–K
,I
(29%
)
Permo-Carboniferous palaeoclimate from palaeosols 857
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
(Con
tin
ued
).
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lmC
lay
min
era
logy�
Pro
vin
ce
D:
Vert
isol
Mark
ley
Fm
.A
Bw
(0–0Æ1
5)
Pale
red
(5R
4/2
)m
ud
ston
ew
ith
med
ium
rhom
boh
ed
ral
stru
ctu
re;
sup
eri
mp
ose
dfi
ne
gra
nu
lar
stru
ctu
re.A
bu
nd
an
tfi
ne-
tom
ed
ium
-scale
,li
gh
tgre
en
-gre
y(8
/5B
G)
mott
les.
Cla
stic
-fill
ed
dykes
exte
nd
thro
ugh
hori
zon
.N
on
-cal-
care
ou
s.
Haem
ati
ten
od
ule
sm
ay
be
lackin
g.
–I,
I/S
,K
(28%
)U
pp
er
coast
al;
pie
dm
on
t
Bgc
(0Æ1
5–0Æ3
5)
Gre
y(6
/N)
mu
dst
on
ew
ith
med
ium
rhom
boh
ed
ral
stru
ctu
re.C
om
mon
med
ium
-sc
ale
du
sky
red
(10R
3/4
)m
ott
les
an
dh
aem
ati
ten
od
-u
les.
Cla
stic
dykes
exte
nd
tobott
om
of
hori
zon
.N
on
-cal-
care
ou
s.
Haem
ati
te(1
0%
)I/
S,
I,K
(33%
)
Bss
(0Æ3
5–0Æ7
5)
Red
dis
h-b
row
n(5
YR
5/4
)m
ud
ston
ew
ith
wed
ge-
shap
ed
stru
ctu
ral
aggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Com
mon
med
ium
-to
coars
e-
scale
,li
gh
tgre
en
-gre
ym
ot-
tles
(8/5
BG
).N
on
-calc
ar-
eou
s.
–I,
K(3
9%
)
2C
(0Æ7
5–1Æ0
7)
Lig
ht
gre
en
-gre
y(8
/10Y
)p
oorl
yla
min
ate
dm
ud
ston
e.
Non
-calc
are
ou
s.
–I,
K(3
6%
)
E:
En
tiso
lM
ark
ley
Fm
.A
rch
er
Cit
yF
m.
Nocon
aF
m.
Petr
oli
aF
m.
Waggon
er
Ran
ch
Fm
.
AC
(0–0Æ9
1)
Du
sky
red
(7Æ5
R3/2
)m
ass
ive
tow
eakly
bed
ded
fin
e-
gra
ined
mu
dd
ysa
nd
ston
e.
Bra
nch
ing
netw
ork
sof
fin
e-
scale
mott
lin
gan
dth
ick,
vert
ical,
tubu
lar
ligh
tgre
y(7
/N
)m
ott
les.
Non
-calc
are
ou
s.
Vari
able
develo
pm
en
tof
red
oxim
orp
hic
fea-
ture
sin
clu
din
ggle
yed
matr
ix,
mott
lin
gan
d/
or
Fe-o
xid
en
od
ule
s.W
eak
develo
pm
en
tof
slic
ken
sid
es
–K
,I
(31%
)L
ow
er
an
du
pp
er
coast
al;
pie
dm
on
t
858 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
(Con
tin
ued
).
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lmC
lay
min
era
logy�
Pro
vin
ce
Cc
(0Æ9
1–1Æ4
0)
Weak
red
(5R
3/3
)fi
ne-
gra
ined
mu
dd
ysa
nd
ston
e.
Com
mon
,fi
ne
tocoars
ed
us-
ky
red
(7Æ5
R4/3
)m
ott
les
an
dh
aem
ati
ten
od
ule
s.N
on
-cal-
care
ou
s.
Haem
ati
te(5
%)
K,
I(2
9%
)
F:
Alfi
sol
Arc
her
Cit
yF
m.
Nocon
aF
m.
Petr
oli
aF
m.
Waggon
er
Ran
ch
Fm
.
AB
(0–0Æ0
8)
Dark
yell
ow
-bro
wn
(10Y
R6/
4)
mu
dst
on
ew
ith
suban
gu
lar
blo
cky
stru
ctu
re.F
ew
fin
e-
tom
ed
ium
-scale
,p
ale
red
(10Y
R4/4
)verm
icu
lar
mot-
tles.
Non
-calc
are
ou
s.
Vari
able
thic
kn
ess
pro
file
s(0Æ7
mto
2m
).In
som
ety
pe
Fp
ala
eo-
sols
,carb
on
ate
isn
ot
pre
sen
tor
occu
rsover-
lyin
gor
sup
eri
mp
ose
don
cla
y-r
ich
hori
zon
s(i
.e.
poly
gen
eti
cso
ils)
.
–S
,K
(30%
)L
ow
er
an
du
pp
er
coast
al;
pie
d-
mon
t
Bt1
(0Æ0
8–0Æ2
3)
Pale
red
(10R
6/4
)si
lty
cla
y-
ston
ew
ith
med
ium
an
gu
lar
blo
cky
stru
ctu
re.
Com
mon
,m
ed
ium
-to
coars
e-s
cale
very
pale
red
(10R
4/3
)vert
ical
tubu
lar
mott
les.
Abu
nd
an
tcla
ysk
ins
on
ped
surf
aces.
Non
-calc
are
ou
s.
–S
,K
(56%
)
Bt2
(0Æ2
3–0Æ5
3)
Very
pale
red
(10Y
R4/3
)si
lty
cla
yst
on
ew
ith
med
ium
an
gu
lar
blo
cky
stru
ctu
re.
Com
mon
,m
ed
ium
-to
coars
e-s
cale
red
dis
h-y
ell
ow
(7Æ5
YR
6/6
)vert
ical
tubu
lar
mott
les.
Abu
nd
an
tcla
ysk
ins
on
ped
surf
aces.
Non
-calc
ar-
eou
s.
–S
,K
(61%
)
Btk
(0Æ5
3–0Æ7
9)
Bro
wn
-yell
ow
(10Y
R5/3
)si
lty
cla
yst
on
ew
ith
med
ium
an
gu
lar
blo
cky
stru
ctu
re.
Abu
nd
an
tcla
ysk
ins
on
ped
surf
aces.
Abu
nd
an
tst
age
IIcarb
on
ate
nod
ule
s.
Calc
ite
(10%
)S
,K
,I/
S(5
7%
)
BC
ss(0Æ7
9–1Æ0
6)
Red
(7Æ5
R4/6
)m
ud
ston
ew
ith
wed
ge-s
hap
ed
stru
c-
tura
laggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Very
weakly
calc
are
ou
s.
–S
,I/
S,
K(3
3%
)
Permo-Carboniferous palaeoclimate from palaeosols 859
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
(Con
tin
ued
).
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lm
Cla
ym
inera
logy�
Pro
vin
ce
C(1Æ0
6–1Æ7
6)
Dark
yell
ow
ish
-bro
wn
(10Y
R4/4
),m
ass
ive
tow
eakly
lam
inate
dm
ud
ston
e.
Non
-calc
are
ou
s.
–I/
S,
S,
K(3
3%
)
G:
Vert
isol
Arc
her
Cit
yF
m.
Nocon
aF
m.
Petr
oli
aF
m.
Waggon
er
Ran
ch
Fm
.
AB
k(0
–0Æ2
7)
Red
dis
h-b
row
n(2Æ5
YR
4/4
)m
ud
-st
on
ew
ith
med
ium
pri
smati
cst
ructu
re;
sup
eri
mp
ose
dm
ed
ium
an
gu
lar
blo
cky
stru
ctu
re.
Abu
n-
dan
tst
age
IIcarb
on
ate
nod
ule
s(F
ig.
8G
).
Carb
on
ate
devel-
op
men
tla
ckin
gin
som
ety
pe
Gp
ala
eoso
ls.
Calc
ite
(10%
)S
,K
(33%
)L
ow
er
an
du
pp
er
coast
al;
pie
dm
on
t
Bkss
1(0Æ2
7–0Æ5
7)
Red
dis
h-b
row
n(2Æ5
YR
4/4
)m
ud
ston
ew
ith
wed
ge-s
hap
ed
stru
ctu
ral
aggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Tra
ce
red
dis
h-b
row
n(5
YR
5/3
)sp
heri
cal
mott
les.
Cla
stic
dykes
exte
nd
thro
ugh
hori
zon
.A
bu
nd
an
tst
age
IIcarb
on
ate
nod
ule
s.
Calc
ite
(15%
)S
,K
(30%
)
Bkss
2(0Æ5
7–0Æ9
8)
Red
dis
h-b
row
n(2Æ5
YR
4/3
)m
ud
ston
ew
ith
wed
ge-s
hap
ed
stru
ctu
ral
aggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Cla
stic
dykes
exte
nd
thro
ugh
hori
zon
.A
bu
nd
an
tst
age
IIcarb
on
ate
nod
ule
s.
Calc
ite
(10%
)S
,K
(30%
)
BC
ss(0Æ9
8–1Æ4
0)
Red
dis
h-b
row
n(2Æ5
YR
4/3
)m
ud
ston
ew
ith
wed
ge-s
hap
ed
stru
ctu
ral
aggre
gate
sd
efi
ned
by
slic
ken
sid
es.
Few
toabu
nd
an
tm
ed
ium
-scale
,re
dd
ish
-bro
wn
(2Æ5
YR
5/3
)m
ott
les.
Cla
stic
dykes
exte
nd
tobase
.V
ery
weakly
calc
are
ou
s.
–S
,I/
S,
K(2
7%
)
C(1Æ4
0–2Æ1
1)
Du
sky
red
(2Æ5
YR
3/2
)p
oorl
yla
min
ate
dm
ud
ston
e.F
ew
med
ium
gre
y(6
/5B
G)
sph
eri
cal
mott
les.
Very
weakly
calc
are
ou
s.
–S
,I/
S(2
3%
)
860 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
1.
(Con
tin
ued
).
Ped
oty
pe
an
dst
rati
gra
ph
icoccu
rren
ce
Hori
zon
dep
th(m
)*M
acro
morp
holo
gy
Typ
ep
rofi
levari
ati
on
Nod
ule
an
drh
izoli
thm
inera
logy�
£2
lmC
lay
min
era
logy�
Pro
vin
ce
H:
Ari
dis
ol
Arc
her
Cit
yF
m.
Nocon
aF
m.
Petr
oli
aF
m.
Waggon
er
Ran
ch
Fm
.
Bkm
(0–0Æ2
2)
Red
dis
h-g
rey
(2Æ5
YR
6/1
)m
ud
ston
e;
stage
III
carb
on
-ate
s.
Degre
eof
carb
on
ate
develo
pm
en
tvari
es;
morp
holo
gie
scan
be
nod
ula
rto
tubif
orm
(e.g
.F
ig.
8E
).M
atr
ixst
ructu
revari
es
be-
tween
pri
smati
c,
an
gu
-la
rblo
cky
an
dsu
ban
gu
lar
blo
cky.
Som
ety
pe
Hp
ala
eo-
sols
inW
aggon
er
Ran
ch
Fm
.h
ave
slic
k-
en
sid
ep
lan
es
fill
ed
by
calc
ium
carb
on
ate
at
dep
th.
Calc
ite
(95%
)I,
KL
ow
er
an
du
pp
er
coast
al;
pie
dm
on
t
Bk1
(0Æ2
2–0Æ3
9)
Yell
ow
(10Y
R7/6
)m
ud
ston
ew
ith
med
ium
suban
gu
lar
blo
cky
stru
ctu
re.
Abu
nd
an
tst
age
IIcarb
on
ate
nod
ule
s.
Calc
ite
(35%
)I,
K(2
2%
)
Bk2
(0Æ3
9–0Æ5
8)
Pale
red
(5R
4/3
)m
ud
ston
ew
ith
med
ium
tocoars
ean
gu
lar
blo
cky
stru
ctu
re.
Abu
nd
an
tst
age
IIcarb
on
ate
nod
ule
s.
Calc
ite
(10%
)I,
I/S
,K
(25%
)
2B
ss(0Æ5
8–0Æ7
6)
Pale
red
(10R
5/3
)sa
nd
ym
ud
ston
ew
ith
wed
ge-
shap
ed
aggre
gate
stru
ctu
red
efi
ned
by
weakly
develo
ped
slic
ken
sid
es.
Calc
are
ou
s.
–I,
I/S
,K
(22%
)
3C
s(0Æ7
6–1Æ3
0)
Red
dis
h-g
rey
(10R
5/1
)sa
n-
dy
mu
dst
on
ew
ith
wed
ge-
shap
ed
aggre
gate
stru
ctu
red
efi
ned
by
weakly
develo
ped
slic
ken
sid
es.
Few
fin
e-
tom
ed
ium
-scale
gre
y-b
row
n(1
0Y
R5/2
)su
bsp
heri
cal
mott
les.
Very
weakly
calc
are
ou
s.
–I,
I/S
,K
(25%
)
4C
(1Æ3
0–1Æ6
0)
Gre
y-b
row
n(1
0Y
R5/2
)m
ass
ive
tow
eakly
bed
ded
fin
e-g
rain
ed
san
dst
on
e.
Mod
era
ted
evelo
pm
en
tof
fin
e-
tom
ed
ium
-scale
yell
ow
-bro
wn
(10Y
R5/6
)verm
icu
lar
mott
les.
Very
weakly
calc
are
ou
s.
––
*D
ow
n-p
rofi
led
ep
thfr
om
the
inte
rpre
ted
pala
eosu
rface.
�P
erc
en
tage
of
the
matr
ixre
pre
sen
ted
by
Fe-o
xid
e,
jaro
site
or
carb
on
ate
nod
ule
san
drh
izoli
ths.
�C
lay
min
era
logy
pre
sen
ted
inre
lati
ve
ord
er
of
abu
nd
an
ce
in<
2lm
size
fracti
on
;n
um
bers
inp
are
nth
ese
sare
perc
en
tage
of
matr
ixre
pre
sen
ted
by
this
size
fracti
on
:K
¼kaoli
nit
e,
I¼
illi
te,
I/S¼
inte
rlayere
dil
lite
–sm
ecti
te,
S¼
smecti
te.
Cla
ys
refe
rred
toin
this
stu
dy
as
illi
teare
form
all
ycla
ssifi
ed
as
mic
a-l
ike
min
era
lsor
seri
cit
e.
Permo-Carboniferous palaeoclimate from palaeosols 861
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
interpreted as being the product of similar soil-forming environments in which local and regionalprocesses combined to produce a distinctiveprofile. Pedotypes are thus analogous to a soilseries in a modern landscape (Retallack, 1990;Bestland et al., 1997).
In the following section, the pedotypes aredescribed based, in part, on individual measuredprofiles judged to be most representative of allpalaeosols assigned to a given group. Majorvariations in morphology or composition fromthe type profile are discussed and summarized inTable 1. The pedotypes are related to the USDAsoil taxonomy classification (Soil Survey Staff,1975, 1998), the spatial and stratigraphic distri-bution of the associated palaeosols delineatedand the palaeoenvironmental implicationsassessed.
Type A palaeosols
DescriptionThe type profile of pedotype A is composed ofthree non-calcareous layers that define a thinprofile (0Æ4 m; Figs 3 and 4A, Table 1). Theuppermost dark grey to black, massive layer isorganic rich and dominates the profile (Fig. 5A).The underlying light grey claystone layer exhibitsangular structure, is composed of quartz siltand low cation-exchangeable clays (kaoliniteand subordinate amounts of illite; Table 1) andlacks labile minerals (biotite, muscovite, feld-spars) present in the unaltered parent material.The basal grey claystone layer contains asmall amount of labile minerals and lacks clearpedogenic features. Redoximorphic features(e.g. Vepraskas, 1994) are abundant in type A
Fig. 3. Key of symbols and horizon nomenclature used in Figs 4, 7 and 9–13.
862 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
palaeosols and include overall low chroma matrixcolours (gleying), tabular networks of downward-bifurcating rhizoliths and medium to coarseyellow mottling. Rhizoliths in the type palaeosolare replaced by jarosite.
Type A palaeosols are developed within siltymudstones of the floodplain facies; all type Apalaeosols are laterally discontinuous over tens tohundreds of metres (e.g. see sections 1 and 2 inFig. 6). These palaeosols occur as thin (averaging< 0Æ5 m) composite profiles (sensu Marriott &Wright, 1993; Kraus, 1999) with common thinclastic partings. They are stratigraphically restric-ted to sites 1–3 in the lower half of the UpperPennsylvanian Markley Formation (below ss11 inFig. 2).
Interpretation and classificationLaterally discontinuous, organic matter-richlayers in type A palaeosols are interpreted to besurficial accumulations (O horizons) that formedby in situ accumulation of plant material (Figs 4Aand 5A). Angular blocky structure in the under-lying claystone layer is interpreted as being pedstructure that formed by differential shearing ofplastic materials (clays) in the profile. The gleyedmatrix, lack of weatherable labile minerals, loworganic content and abundance of kaolinite in the£2 lm size fraction suggest that the claystonesdirectly underlying O horizons formed as eluvialA horizons in response to intense hydrolysis oracidolysis in poorly drained profiles (Fig. 4A; cf.van Breeman & Harmsen, 1975)5 .
Fig. 4. Measured sections of palaeosol profiles representative of each of the eight pedotypes. (A) Type A palaeosolfrom the Late Pennsylvanian Markley Fm., site 1 (Fig. 1). (B) Type B palaeosol, Markley Fm., site 1. (C) Type Cpalaeosol, Markley Fm., site 4. (D) Type D palaeosol, Markley Fm., site 5. (E) Type E palaeosol, Markley Fm., site 6.(F) Type F palaeosol from the Archer City Fm., site 11. (G) Type G palaeosol from the Waggoner Ranch Fm., site 26.(H) Type H palaeosol, Waggoner Ranch Fm., site 28.
Permo-Carboniferous palaeoclimate from palaeosols 863
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Type A palaeosols are interpreted as beinghistosols (Soil Survey Staff, 1998) given therelative thickness and concentration of organicmatter in the O horizons of the palaeosol profiles(Fig. 4A). Modern histosols form in low-lying,waterlogged regions characterized by always wet
conditions and anoxic pore waters that promotein situ accumulation of organic matter. Thepredominance of gley matrix colours and lowchroma mottles and root traces in type A palae-osols, as well as the paucity of discrete Fe-oxidenodules, supports prolonged saturation of these
Fig. 5. Macromorphologies of Permo-Pennsylvanian palaeosols. (A) Couplets of dark–light banding are type Apalaeosols; dark layers are O horizons, light layers are A horizons. Approximately 1Æ7 m thick dark horizon at base ofoutcrop is a type B palaeosol. Palaeosols are from the Upper Pennsylvanian Markley Fm., site 2. (B) Haematite redoxconcentrations, oriented into a polygonal pattern (1Æ5 m across), in the Cgv horizon of a type B palaeosol, MarkleyFm., site 1. (C) Slickensides in a type G palaeosol, Lower Permian Nocona Fm., locality 15. Knife (0Æ25 m long) forscale; blade points in the direction of mineral orientation along the slickensurface. (D) Type G palaeosol showingpedogenic slickensides in cross-section, Lower Permian Waggoner Ranch Fm., site 26. White ‘blebs’ are pedogeniccarbonate nodules. Field of view is 1Æ5 m. (E) Calcareous rhizoliths from a type H palaeosol, Waggoner Ranch Fm.,site 31. 35 mm lens cap for scale. (F) Type G palaeosol, upper Waggoner Ranch Fm., site 31. Overlying light-colouredbeds are marine calcareous silts and limestones of the Maybelle Limestone.
864 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
profiles (Simonson & Boersma, 1972; Wildinget al., 1983; Schwertmann, 1985). The shallowand tabular distribution of jarosite-replaced rootsin fossil A horizons further reflects the poorlydrained palaeosoil conditions (cf. Retallack, 1988,1990). Jarosite, however, may represent post-pedogenic oxidation of Fe-sulphides despite itsneomorphic replacement of roots (cf. Kraus,1998).
Late Pennsylvanian type A palaeosols areinterpreted as having formed in poorly drained,shallow depressions on the regionally extensivecoastal plain given their morphological charac-teristics and laterally discontinuous nature. Thepredominance of stacked, thin, composite profiles(Fig. 6) reflects slow, episodic sediment accumu-lation in geomorphic lows on the latest Pennsyl-vanian coastal plain of western equatorial Pangea.
Fig. 6. Palaeosol-bearing sections ofUpper Pennsylvanian Markley For-mation. Palaeosol tops are delinea-ted by wavy ‘lines’ to the left of eachmeasured section. Capital lettersrefer to palaeosols of a given pedo-type; circles delineate pedotypeprofiles discussed in the text. Sec-tions 1 and 2 are from two outcropsseparated by 8Æ5 km; correlation be-tween outcrops based on sandstoneset 10 (ss10) in the middle of theMarkley Fm. Measured section 4underlies sandstone set 12 (ss12) inthe upper portion of the MarkleyFm. Measured section 5 underliesss14 from the uppermost MarkleyFm., proximal to the Pennsylva-nian–Permian boundary. See Fig. 2for stratigraphic position of sand-stone sets.
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Type B palaeosols
DescriptionThe type profile of pedotype B (Figs 4B and 6) iscomposed of four distinct units: (1 and 2) twoupper, red claystones that exhibit fine angular tosubangular blocky structure and oriented clayand Fe-oxide coatings along matrix aggregatesand lining channel voids; (3) an underlying,mottled red/grey silty claystone that exhibitssubangular blocky structure; and (4) a basal,
massive grey silty claystone (Fig. 4B; Table 1).Redoximorphic features are present throughoutthe profile as redox depletions (i.e. low chromamottles and matrix colours) and as redox concen-trations (i.e. Fe- and, less commonly, Mn-oxidecoatings on matrix aggregates and Fe-oxide nod-ules). Oblate Fe-oxide nodules, ranging from 0Æ05to 0Æ2 m in length, are laterally oriented and forma partially indurated horizon at the base of thetype profile (Fig. 5B). Type B palaeosols with lesswell-differentiated horizons commonly lack thickaccumulations of Fe-oxides.
Kaolinite and trace amounts of illite dominate the£2 lm size fraction of all claystones; labile miner-als are generally lacking (Fig. 7A). The abundanceof the clay-dominated, £2 lm size fraction decrea-ses downprofile (e.g. from 78% to 43%; Table 1).
Type B palaeosols occur primarily as thin tomoderately thick (< 1Æ5 m), strongly developedprofiles within mudstones of the floodplain facies(Figs 5A and 6). Type B palaeosols are strati-graphically associated with type A palaeosols.Type B palaeosols are primarily developed uponmudstones and are stratigraphically limited to thelower half of the Upper Pennsylvanian MarkleyFormation (below ss11 in Fig. 2).
Interpretation and classificationRedoximorphic colouration and the abundance ofmoderate to large Fe-oxide nodules in the lowerportion of profiles (BCg and Cgv horizons inFig. 4B) indicate that type B palaeosols werewaterlogged for prolonged periods (i.e. hydro-morphic soils), given that the degree of chemicalweathering of parent material and the size andabundance of Fe-oxide nodules commonly in-crease with increased duration of waterlogging(Simonson & Boersma, 1972; Sobecki & Wilding,1983; Retallack, 1990). However, the predomin-ance of kaolinite in the clay fraction indicates thatthese hydromorphic soils were characterized byepisodic or seasonal changes in soil moistureconditions (Wilding et al., 1983; Schwertmann,1985; Dixon & Skinner, 1992).
Clay-rich horizons in type B palaeosols areinterpreted as being zones of clay accumulationor argillic horizons (ABt1 and Bt2 in Fig. 4B)that formed by downward translocation of clayminerals (i.e. illuviation). Concentration of clayminerals, in particular kaolinite, in the inferredBt horizon and the presence of continuous, orien-ted clay coatings (argillans of Brewer, 1976) onpeds and lining channel voids suggest extensivepedogenic kaolinite formation. The presence ofFe-oxide (and less commonly Mn-oxide;
Fig. 7. XRD patterns of clay minerals from the £ 2 lmsize fraction in select type palaeosols. Clay mineralogyof: (A) a Bt horizon; type B palaeosol, Upper Pennsyl-vanian Markley Fm.; (B) Bt horizon, type G palaeosol,Lower Permian (lower Leonardian) Petrolia Fm.; (C) Bkhorizon, type H palaeosol, Lower Permian (mid-Leonardian) Waggoner Ranch Fm. K ¼ kaolinite;I ¼ illite; S ¼ smectite; C ¼ hydroxy-interlayeredminerals; Q ¼ quartz.
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ferri-mangans of Brewer, 1976) lining peds andvoids, in addition to the downprofile increase inFe-oxide nodules, records downward transloca-tion of Fe and Mn in addition to clay minerals.Translocation and mineral accumulation also indi-cate that type B hydromorphic palaeosols under-went periods of free drainage (Birkeland, 1999).
The haematite- and goethite-cemented layers atthe base of type B palaeosols are analogous tomodern plinthites that form in soil horizonscharacterized by a seasonally fluctuating watertable (Duchaufour, 1982; Soil Survey Staff, 1998).The thickness of the palaeoplinthite horizon(� 1 m) and its depth below the palaeosol surfacesuggest that the local palaeowater table probablyfluctuated within 1–2 m of the land surface(cf. Duchaufour, 1982).
Type B palaeosols are interpreted as ultisolsbased on morphological evidence for argillic hori-zons, prolonged saturation and extensive hydro-lysis interrupted by episodes of free drainage.Modern ultisols form in warm, humid environ-ments (e.g. modern tropical rainforests) character-ized by high rates of chemical weathering. Thesesoils minimally require periodic drainage inorder to form argillic horizons through illuviation(Retallack & German-Heins, 1994; Gill & Yemane,1996). Although Late Pennsylvanian ultisolsformed penecontemporaneously with type A pala-eosols, ultisols are interpreted as having formed onstable landscapes such as interchannel highs thatmay have developed during minor episodes offluvial downcutting. This interpretation is basedon evidence for their strong pedogenic develop-ment and for their having been at least episodicallywell-drained in the upper argillic horizons ofthe profile (cf. Markewich & Pavich, 1991;Bestland et al., 1997), and on the inferred lowsedimentation rates recorded by their thin,composite profiles (cf. Marriott & Wright, 1993).
Type C palaeosols
DescriptionThe type profile of pedotype C consists of fourpoorly to moderately developed horizons (Table 1)that exhibit fine to medium angular blocky struc-ture. Larger scale wedge- to lenticular-shapedstructural units are defined by randomly orientedslickensides that overprint the finer scale angularblocky structure in the lower mudstone (BCssg inFig. 4C). Although all type C palaeosols exhibitsome structure that is indicative of pedogenicdevel-opment, most profiles only have weak horizonation.Abundant redoximorphic concentrations (yellow
to reddish-brown mottles and Fe-oxide nodules)and reductions (gleyed matrix colours) occurthroughout type C palaeosols (Fig. 8A). A semi-indurated layer of coalesced submillimetre-sizedhaematite nodules (Fig. 8D) occurs near (28–49 cmdepth) the interpreted palaeosurface of the typeprofile (Fig. 4C); analogous Fe-oxide layers arelacking in some type C palaeosols. The < 2 lm sizefraction contains a moderate amount of clayminerals (22–32%) that are dominated by illiteand subordinate amounts of kaolinite and hydro-xy-interlayered mineral (HIM) clays.
Type C palaeosols occur as thick compoundand composite profiles (Fig. 6) developed inintercalated thinly bedded mudstones, siltstonesand fine-grained sandstones. They are strati-graphically restricted to overbank mudstonedeposits associated with channel and crevasse-splay sandstone deposits in the Upper Pennsyl-vanian Markley Formation above fluviogeniccycle ss11 (sites 4 and 5; Figs 2 and 6).
Interpretation and classificationThe laterally continuous, semi-indurated Fe-oxidelayer in the upper part of the type profile isinterpreted as being a palaeoplinthite (Bv horizonin Fig. 4C). The development of a plinthite proxi-mal to the palaeosol surface records a shallow,fluctuating palaeogroundwater table (Table 1; cf.Duchaufour, 1982). Further evidence for fluctu-ating soil moisture conditions is the occurrenceof large-scale wedge- to lenticular-shaped pedsdefined by slickensides in basal mudstones.Pedogenic slickensides form in clay-rich modernsoils as a result of soil mass movement and struc-tural modification driven by wetting and dryingcycles (Dudal & Eswaran, 1988; Wilding & Tessier,1988). Limited development of large-scale pedsand slickensides in type C palaeosols, however,suggests modest fluctuations in soil moisturecontent in the lower portions of the profile and/or profile immaturity (Brewer, 1976; Jim, 1990),given that these features form rapidly in soilscharacterized by repeated wetting and drying(Yaalon & Kalmar, 1978; Birkeland, 1999).
Type C palaeosols are classified as inceptisolsgiven their moderate development of ped structureand horizonation. These probably immature pro-files formed in floodplain deposits proximal tofluvial channels, as evidenced by their close lateraland stratigraphic proximity to channel-filling andcrevasse-splay sandstones; their compound andcumulate profiles record steady, and sometimesrapid, sedimentation in these environments (cf.Marriott & Wright, 1993; Kraus, 1999).
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Type D palaeosols
DescriptionThe type profile of pedotype D is composed offour red to grey mudstone units that exhibitabundant redoximorphic features including small(� 10 mm) haematite nodules, manganese oxidecoatings and low chroma soil matrix (or gley)colours (Fig. 4D; Table 1). The top two unitsexhibit medium-scale (50–200 mm long) rhombo-hedral structural aggregates; the uppermost clay-stone exhibits superimposed angular blockystructure defined by clay-lined microslicken-sides. Clay-rich mudstones in the lower half ofthe profile exhibit wedge-shaped structural aggre-gates defined by larger scale slickensides. Clayand Mn-oxides typically coat slickensides. Sand-filled V-shaped dykes (Figs 3D and 6) cross-cutthe upper half of the type profile. The < 2 lm size
fraction in the type profile is dominated byhydroxy-interlayered minerals and smectite andlesser amounts of kaolinite (Table 1).
Type D palaeosols are developed within mud-stones of the sand- and gravel-rich channel-barfacies (Fig. 6) of the uppermost Upper Pennsyl-vanian Markley Formation (above ss13 in Fig. 2).Their distribution is limited to the eastern portionof the study area (sites 5 and 6 in Fig. 1).
Interpretation and classificationWell-developed, medium-scale rhombohedral-and wedge-shaped structure and associated ped-ogenic slickensides in type D palaeosols areanalogous to ‘parallelepipeds’ in Holocene verti-sols (Dudal & Eswaran, 1988). In modern soils,vertic features develop during mass movementand shearing in smectite-rich soils that becomesaturated during seasonal rainfall (Yaalon &
Fig. 8. Photomicrographs of redoximorphic features in Upper Pennsylvanian palaeosols. (A) Haematite nodules (e.g.dark spots in area marked by white ovoids) and ferrans (dark area) in a claystone (Bv horizon) with angular blockystructure; type C palaeosol. Scale bar ¼ 1 mm. (B) Redoximorphic depletions (light grey) and concentrations (darkgrey) in an AC horizon; type E palaeosol. Scale bar ¼ 1 mm. (C) Redoximorphic depletions (light grey) and con-centrations (black) of both diffuse and nodular haematite in a Cc horizon; type E palaeosol. Scale bar ¼ 1 mm. (D) Aplinthite horizon altered to an ironstone; the ironstone is composed predominantly of haematite nodules (black) anda minor amount of clay minerals; type C palaeosol. Scale bar ¼ 1 mm.
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Kalmar, 1978; Dudal & Eswaran, 1988; Wilding &Tessier, 1988). The thin clay coatings on super-imposed angular blocky peds probably formed as
stress cutans (Jim, 1990) due to differentialwetting, and shrinking and swelling forces drivenby repeated wetting and drying of the soil matrix.Decreasing soil moisture and shrinking of soilclays resulted in cracking of the soil matrixduring drier intervals. The V-shaped dykes thatcross-cut the upper portions of the profilesresulted from back-filling of these surficial cracksfrom sediment-laden water during subsequentflood events.
Type D palaeosols are classified as vertisolsbased on abundant and well-developed verticfeatures, their clay content (28–39%) with asignificant percentage of smectite and the pres-ence of deep V-shaped cracks developed withinthe upper horizons (Soil Survey Staff, 1998).Modern vertisols form in seasonally moist andtypically tropical to warm-temperate climates,with four to eight dry months yearly (Dudal &Eswaran, 1988; Buol et al., 1997). The relativelylow kaolinite content and preservation of expand-able 2:1 layer-lattice clays (smectite) in type Dpalaeosols coupled with the occurrence of clasticdykes indicate periods of drying that were suffi-ciently long to preserve expandable clay mineralsand to allow for significant shrinkage of the clay-rich matrix (Duchaufour, 1982; Retallack, 1990).However, the presence of gley matrix colours andcommon to abundant redox depletions requiresthat periods of palaeosoil moisture deficiencyin type D palaeosols were temporally limited(Daniels et al., 1971; Duchaufour, 19826 ; Buolet al., 1997). Based on their stratigraphic associ-ation with overbank mudstones as well as themorphological and chemical characteristics ofthese palaeovertisols, they are interpreted ashaving formed in interfluve muds on the LatePennsylvanian upper coastal plain and piedmont.
Type E palaeosols
DescriptionThe representative profile of pedotype E con-sists of two units that are very weakly devel-oped in muddy fine-grained sandstone and are
Fig. 9. Palaeosol-bearing sections of Lower PermianArcher City Formation. Palaeosol tops are delineatedby the wavy lines to the left of each measured section.Capital letters refer to palaeosols of a given pedotype;circle delineates pedotype profile discussed in the text.Correlation of three sites (sites 9, 10 and 11 in Fig. 2)based on sandstone set 8 (ss8). Sites 9 and 10 are sep-arated by 0Æ3 km, and sites 10 and 11 are separated by0Æ2 km. See Fig. 3 for key to symbols.
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distinguished by their colour and nature of theredoximorphic features (Fig. 4E; Table 1). Theupper red unit exhibits a vertically oriented,branching network of light-grey, fine- (< 10 mmwide) to coarse-scale (10–25 mm wide) tubular
mottles (Figs 6 and 8B); superposition of differentgenerations of mottles is observed. The lower palered unit contains fine to coarse dark red mottlesand haematite nodules (Fig. 8C). The < 2 lm sizefraction in the profile is dominated by kaoliniteand illite (Table 1). In addition, some type Epalaeosols also exhibit weakly developed slic-kensides.
Type E palaeosols occur as thick (1–2 m)cumulate and compound and composite palaeo-sols throughout the Permo-Pennsylvanian terrest-rial strata in the study area (sites 1–3, 5, 6, 9–11,12–15, 19–22, 24, 25 and 29 in Fig. 1). They aredeveloped within or stratigraphically adjacent tofinely laminated to slightly bioturbated silty
Fig. 10. Palaeosol-bearing sections of Lower PermianNocona Formation. Palaeosol tops are delineated by thewavy lines to the left of each measured section. Capitalletters refer to palaeosols of a given pedotype. Corre-lation of the siltstone- and claystone-dominated sectionfrom site 16 with the sandstone-dominated section atsite 15 (80 km apart) based on contemporaneity of ElkCity Limestone and ss11 defined by Hentz (1988). Keyto symbols as in Fig. 3.
Fig. 11. Palaeosol-bearing sections of Lower PermianPetrolia Formation at sites 17 and 18. Palaeosol tops aredelineated by the wavy lines to the left of each meas-ured section. Capital letters refer to palaeosols of agiven pedotype. Key to symbols as in Fig. 3.
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mudstones and siltstones and fine- to medium-grained sandstones of the channel bar and pointbar facies (Figs 6, 9, 10, 12 and 13).
Interpretation and classificationType E palaeosols are interpreted as entisolsbased on the very weak development of pedogen-ic structure and horizonation (Soil Survey Staff,1998). Modern entisols exhibit little to no devel-opment of horizonation, and typically form uponunstable, wet landscapes (Buol et al., 1997).Entisols record stunted horizon developmentresulting from prolonged saturation and/or lim-ited periods of pedogenesis (tens to hundreds ofyears) caused by rapid sedimentation rates (Buolet al., 1997). The occurrence of redoximorphic
depletions and concentrations in many type Epalaeosols indicates a certain degree of hydro-morphy (waterlogging; cf. Simonson & Boersma,1972; Fanning & Fanning, 1989).
In contrast, type E palaeosols that formed insandy siltstones typically lack redoximorphicfeatures. Given that many type E palaeosolscommonly occur as weakly developed, com-pound or cumulate profiles in proximal positionsto fluvial siltstones and sandstones and estuarinesiltstones and mudstones (south-western portionof study area), they are interpreted as havingformed on unstable landscapes proximal tofluvial channels (e.g. levees and crevasse splays)on the coastal plain and piedmont and wetlandsof the lower coastal plain.
Fig. 12. Correlated transect (67 kmlong) of four palaeosol-bearing sec-tions of the Lower Permian PetroliaFormation beneath the regionallytraceable Beaverburk Limestone.The two sections (site 22) on the leftare siltstone and claystone domin-ated, whereas sections from sites 20and 21 contain abundant sandstonesof the point-bar facies. Palaeosoltops are delineated by the wavylines to the left of each measuredsection. Capital letters refer to pal-aeosols of a given pedotype. Key tosymbols as in Fig. 3.
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Type F palaeosols
DescriptionThe type profile of pedotype F is composed of sixwell-developed horizons (Fig. 4F; Table 1) thatare distinguished by their clay content, structure,mottling and mineral accumulations. The upper-most horizon is defined by a dark yellow-brown,non-calcareous mudstone with fine to medium
subangular blocky structure. Three significantlymore clay-rich layers (56–61% clay vs. 30–33% inother units) underlie the uppermost mudstone.These are red to brown, silty claystones thatexhibit fine to medium subangular blocky struc-ture. Thick, continuous and oriented clay coat-ings (argillans) occur around detrital grains andline aggregates in all three claystones. The uppertwo claystone layers are non-calcareous, whereas
Fig. 13. Correlated transect (31 km long) of three palaeosol-bearing sections (from sites 27, 29 and 30) of the LowerPermian Waggoner Ranch Formation beneath the Maybelle Limestone. Palaeosol tops are delineated by the wavylines to the left of each measured section. Capital letters refer to palaeosols of a given pedotype; circle delineatespedotype profile discussed in text. Key to symbols as in Fig. 3.
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the lowermost claystone contains discrete, cm-scale stage II carbonate nodules (sensu Machette,1985) and weakly developed calcareous rhizo-liths (Figs 9–12) that exhibit pedogenic fabrics(cf. Deutz et al., 2002). Faint tubiform red andreddish to yellow mottles in the mudstone andclaystone layers define a pattern similar tobranching root structures (cf. Retallack, 1988,1990) and probably record deeply penetratingroot systems. Smectite dominates the < 2 lm sizefraction of the claystone layers, with traceamounts of kaolinite and illite (Table 1). Thelower two horizons in the type profiles arered and yellowish-brown mudstones (Table 1);the upper horizon exhibits wedge-shaped struc-tural aggregates defined by randomly orientedslickensides, whereas the lower horizon pre-serves disturbed primary sedimentary lamina-tions (Fig. 4).
Some variants of the type profile lack thehorizon of stage II carbonate nodules. Moreover,some type F palaeosols contain carbonate-richhorizons that overlie or are superimposed on theclay-rich horizons with clay skins (cutans). Thesedominantly strongly developed palaeosols mayoccur as composite profiles and are found inLower Permian mudstones of the floodplainfacies (Figs 9–12) throughout the lower and uppercoastal plain and piedmont physiographic prov-inces (sites 9, 16 and 19 in Fig. 1). Given thatthese palaeosols are developed in fine-grainedmudstones and require good drainage, thesepalaeosols probably formed upon areas of thefloodplains that were removed from frequentflooding and depositional events associated withmajor stream systems (e.g. Buol et al., 1997).
Interpretation and classificationThe clay enrichment, abundant continuous argil-lans on detrital grains and peds and the non-calcareous matrix in the upper portions of type Fprofiles are evidence of clay translocation. Trans-location is facilitated through leaching of Ca2+
from clay exchange sites leading to the develop-ment of argillic (Bt) horizons (Franzmeier et al.,1985; Birkeland, 1999). Pedogenic carbonate canprecipitate in lower horizons of the profile ifsoluble minerals are incompletely leached fromthe soil horizon (Buol et al., 1997). The occur-rence of pedogenic carbonates in lower portionsof type F palaeosols, coupled with limiteddevelopment of redoximorphic features and illu-viated clays deep within the profile, indicate thatthese palaeosols formed well above the palaeo-water table and were well-drained (cf. Schwert-
mann, 1985, e.g. Kraus & Aslan, 1993). The darkyellowish-brown mudstones with fine-scalebranching root structures that cap most type Fprofiles are interpreted to be organic-rich ABhorizons.
The presence of well-differentiated profileswith argillic horizons suggests that type F palaeo-sols were alfisols (Soil Survey Staff, 1998).Modern alfisols form in humid to subhumid,mid-latitude to subtropical regions that have> 75–100 cm of annual precipitation (Strahler &Strahler, 1983; Buol et al., 1997). Given thatpedogenic carbonate typically accumulates insoils receiving < 70 cm mean annual precipitationin low-latitude regions (McFadden, 1988;Birkeland, 1999; Royer, 1999), carbonate-bearingtype F palaeosols may record formation at thesubhumid to semi-arid climatic boundary. Thepredominance of smectite and trace amounts ofkaolinite in type F palaeosols is consistent with asubhumid to semi-arid palaeoclimate. In addition,some type F palaeosols are characterized by thedevelopment of pedogenic carbonates above orwithin argillic horizons (polygenetic palaeosols).This superposition of calcic-over-argillic horizonsis climatically ‘out of phase’ and suggests thatthese type F palaeosols formed under two dis-tinctly different climatic regimes: an earlier, wet-ter climate that leached carbonate and formedargillic horizons in the profile and a subsequentdrier climate that accumulated carbonate andformed calcic horizons in the profile (cf. SoilSurvey Staff, 1975; Buol et al., 1997). Thesepolygenetic alfisols are stratigraphically limitedto the upper Archer City and Nocona Formationsas well as the uppermost Waggoner Ranch For-mation between the Maybelle and Lake KempLimestones (Fig. 2).
The development of multiple, well-definedhorizons, which commonly include superim-posed argillic horizons, and the composite natureof these palaeosols record prolonged periods (tensto hundreds of ky) of pedogenesis (cf. Bestlandet al., 1997). These characteristics coupled withevidence for well-drained profiles suggest thattype F palaeosols developed upon stable land-scapes distributed throughout the Early PermianEastern shelf for which the local groundwatertable was relatively deep. Evidence of large, deepfossil roots in type F palaeosols in conjunctionwith the presence of well-developed argillichorizons and blocky ped structure suggest thatwoodlands may have developed on such stableareas of the Early Permian alluvial landscape (cf.Mack, 1992).
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Type G palaeosols
DescriptionThe type G profile is composed of five well-developed mudstone horizons (Fig. 4G; Table 1)that exhibit wedge-shaped structure defined byclay-coated slickensides (middle three horizons;Fig. 5C and D) and contain stage II carbonatenodules and rhizoliths (upper three horizons;Fig. 5F), although a few other type G palaeosolsin the upper Waggoner Ranch Formation mayexhibit 50–100 mm thick stage III carbonateaccumulations near the bases of the profiles.Redoximorphic features are generally lackingwith red hues dominating matrix colours. Sand-filled clastic dykes extend throughout most ofthe profile (to a depth of 1Æ4 m). The < 2 lm sizefraction in the type profile is dominated bysmectite and lesser amounts of kaolinite andmica-like minerals (sericite; Table 1; Fig. 7B).
Type G palaeosols developed in mudstones ofthe floodplain facies throughout the studiedLower Permian strata (Figs 9, 10 and 12). Theirdevelopment at numerous sites (sites 7–9, 11–14,16, 18, 23, 24 and 26–31 in Fig. 1) reflects theirwidespread distribution in the study area.
Interpretation and classificationAnalogous to type D palaeosols, type G palaeosolsare interpreted as vertisols based on theirwell-developed vertic features, their smectite-dominated clay composition and the presence ofV-shaped cracks developed to significant profiledepths. Type G palaeosols, however, exhibit twomajor differences from type D palaeosols: (1)accumulation of pedogenic carbonate; and (2) apaucity of redoximorphic features. The lack ofredoximorphic features in type G palaeosolsindicates that these well-drained profiles formedwell above the palaeowater table. Furthermore,the significant carbonate accumulation, includingstage II and III carbonate horizons in the profiles,and the presence of deeply penetrating V-shapedclastic-filled dykes require prolonged periods ofdrying of the profiles and prolonged duration ofpedogenesis.
Type H palaeosols
DescriptionThe type profile of pedotype H consists of sixmoderately to well-developed horizons (Fig. 4H;Table 1) that exhibit medium subangular tocoarse angular blocky structure in the upperhorizons and wedge-shaped structural aggregates
defined by weakly developed slickensides in thelower half of the profile. Mottling is limited to thelowermost horizons and typically exhibits highchroma. A thick, indurated layer (‡ 150 mm) ofstage III carbonate accumulation (¼ petrocalcichorizon; Soil Survey Staff, 1975, 1998; Machette,1985) near the top of the profile and extensivedevelopment of stage II carbonate nodules andcalcareous rhizoliths in lower horizons of theprofile (Fig. 5H) help to distinguish the type Hpalaeosol and its pedotypes from all other car-bonate-bearing palaeosols. Hydroxy-interlayeredminerals and mica-like minerals with traceamounts of kaolinite and chlorite make up the< 2 lm size fraction in the type profile (Fig. 7C).The type profile has lower clay contents (< 2 lmsize fraction in matrix of 22–25%) than all otherpedotypes.
Type H palaeosols may occur as compound,cumulate and composite profiles developed inLower Permian mudstones of the floodplainfacies (Figs 9–13). Their occurrence at manysites (sites 7–9, 16–22 and 24–31 in Fig. 1)reflects their widespread distribution in thestudy area.
Interpretation and classificationType H palaeosols are interpreted as having beensimilar to modern aridisols that formed undersemi-arid to arid climate conditions based ontheir extensive carbonate accumulation, in par-ticular as petrocalcic horizons, and relatively lowclay contents in comparison with all other ped-otypes (Soil Survey Staff, 1998). In reality, thesepalaeosols may only be classified as inceptisolsin the US Soil Taxonomy, as climate conditionsare not clearly understood during Early Permiantime (Soil Survey Staff, 1998). Nonetheless, thepresence of chlorite in the fine clay fractionsuggests that type H palaeosols formed underconditions of low soil moisture given that chlor-ite is not stable in humid environments (cf.Yemane et al., 1996).
Type H palaeosols are interpreted as havingformed on infrequently flooded, stable regions ofEarly Permian floodplains given that they recordwell-drained and oxidizing soil conditions andthat the development of thick petrocalcic hori-zons requires extended periods of soil formationand soil moisture deficiency (Machette, 1985;Wright & Marriott, 1996). The occurrence ofwedge-shaped aggregates defined by weaklydeveloped slickensides in horizons underlyingthe calcic horizons, however, records fluctuatingsoil moisture conditions.
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DISCUSSION
Permo-Pennsylvanian palaeosols of north-centralTexas exhibit clear stratigraphic trends in pedo-type distribution and their mineralogical andgeochemical compositions. These superimposedtrends are interpreted here as recording theinterplay of autogenic factors inherent in thedepositional system (e.g. variability in sedimentaccumulation rate and parent material, localtopography, variations in floodplain hydrologyor soil drainage conditions) and basin- to regio-nal-scale allogenic processes such as climate,tectonic subsidence and eustasy. The overallstratigraphic distribution of the studied palaeo-sols delineates the time of appearance anddisappearance of pedotypes over the Permo-Pennsylvanian interval, as well as defininglong-term variations in the inferred degrees ofhydromorphy, free drainage and chemical weath-ering of the soil profiles and estimated soilmoisture conditions. These long-term variationsare interpreted here as recording the predominantrole that climate played in the development ofPermo-Pennsylvanian palaeosols associated withthe Eastern shelf of the Midland basin. Beforediscussing the application of these palaeosols toconstraining climate over western equatorial Pan-gea, the role of autogenic and basin-scale allo-genic processes other than climate in determiningthe development of the pedotypes and theirobserved spatial distribution is first assessed.
Superimposed upon the longer term strati-graphic trend are smaller scale patterns in pedo-type distribution defined by palaeosols withinthin stratigraphic intervals (metres to a few tensmetres; e.g. Fig. 14A and B). Lateral variations inpedotype distribution, degree of maturity of pal-aeosols and stratigraphic relationships betweenpalaeosols are also observed between age-equiv-alent deposits located hundreds of metres to tensof kilometres apart (Figs 6, 9, 10, 12 and 13).Reconstruction of palaeocatenas (e.g. fluvialchannels and their associated floodplains) in thisstudy was limited to a few laterally continuousoutcrops (hundreds of metres to 1Æ2 km long)exposed along Holocene stream valleys or inbadland exposures (e.g. Figs 9 and 13; Tabor,1999). The observed topography coupled withpalaeocatenary relationships inferred from metre-scale vertical stacking patterns of pedotypesindicates that geomorphic position on the aggrad-ing floodplain influenced soil developmentthrough differences in topographically controlleddrainage and sediment accumulation provided by
channel-margin deposits and shallow floodplainscours (cf. Bown & Kraus, 1987; Kraus, 1986)7 . Forthe latest Pennsylvanian period, the metre-scaleintercalation of type A and type B palaeosols infloodplain strata deposited on the lower palaeo-coastal plain (Figs 6 and 15A) necessitates thepresence of topography on the palaeolandscapeduring pedogenesis. Although both palaeosoltypes were hydromorphic, the presence of well-developed argillic horizons in type B palaeosolsindicates episodic free drainage of these profilesin comparison with the penecontemporaneouswater-logged type A palaeosols. However, theshallow depth to the palaeowater table inferredfrom palaeoplinthites in the ultisols, as well aslimited field relationships of channel-filling sand-stones and floodbasin strata, indicates that thepalaeotopography was probably not greater than afew metres.
Larger scale lateral variations (kilometres totens of km) defined by differences in the strati-graphic distribution of pedotypes observed with-in time-equivalent mudstone-dominated intervalsbracketed by laterally correlatable, major sandbodies (ss units in Figs 2, 6, 10, 12 and 13) furtherindicate that the geomorphic position in thePermo-Pennsylvanian alluvial basin influencedsoil development, probably through the influenceof river channel avulsion and broader patterns offloodplain drainage (cf. Kraus & Aslan, 1993;Kraus, 2002). Overall, these inferred meso- andmacroscale palaeosol–landscape associationsindicate that differences in sediment accumula-tion rates and topographically (or parent material)controlled drainage clearly influenced soil devel-opment on the Eastern shelf alluvial basin (e.g.Fig. 14B; Sobecki & Wilding, 1983; Wilding et al.,1991; Arndorff, 1993; Bestland et al., 1997; Kraus,1999, 2002; Kraus & Aslan, 1999). In turn, theimprint of landscape variability is recorded inthe vertical stacking patterns of pedotypes at themetre to tens of metre scale and accounts for thestratigraphic ‘noise’ observed in the longer termtrend.
Larger scale (tens to hundreds of metres) strati-graphic variations in the Permo-Pennsylvanianpalaeosols define a long-term trend upon whichthe aforementioned smaller scale variations aresuperimposed. The long-term trend records anoverall decrease in the degree of hydromorphy andintensity of chemical weathering of the soil profilesconcomitant with an increase in evidence for freedrainage of profiles and lower soil moisturethroughout latest Pennsylvanian (Virgilian) toEarly Permian time. This trend is defined by
Permo-Carboniferous palaeoclimate from palaeosols 875
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
floodplain palaeosols with morphologies thatreflect conditions of chemical weathering and soilmoisture regime (types A, B, D, F, G and H); no
equivalent trend is defined by the distribution oftype C and E profiles (Fig. 15). Type C and E pro-files are immature, poorly developed palaeosols
A B
Fig. 14. (A) Schematic stratigraphic column of fluvio-alluvial sedimentary rocks from the Early Permian successionof the Eastern Midland Basin, north-central Texas. The distribution of pedotypes between major fluvial channeldeposits suggests that depositional rate, position of the palaeogroundwater table and duration of pedogenesis acrossthe Permo-Pennsylvanian landscape played an important role in the stratigraphic stacking pattern of palaeosolmorphologies. (B) Interpreted distribution of soil types across the Early Pennsylvanian landscape of the EasternMidland Basin, north-central Texas. The stacking pattern of pedotypes between major fluvial channel sandstonedeposits is interpreted as representing migration of fluvial systems from proximal (positions 0 and 1) to more distal(positions 4 and 6) and back to proximal locations (positions 8 and 9). See text.
876 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
found throughout the three physiographic prov-inces. Their cosmopolitan nature probably reflectsthe abundance of young, episodically ‘disturbed’and poorly drained environments proximal tofluvial channels and ponds upon the floodplain(Buol et al., 1997).
In particular, the longer term variation inpedotype development records a relatively rapidshift over tens of metres from Late Pennsylvanianhydromorphic floodplain palaeosols (histosol-like type A; ultisol-like type B) to Early Permianwell-drained and oxidized profiles (alfisol-liketype F; calcic vertisol-like type G; and aridisol-like type H). Vertisols (type D) with abundantredoximorphic features but morphological evi-dence for periods of drying (e.g. clastic dykes inthe upper portions of profiles) make up animportant component of latest Pennsylvanianmature palaeosols. These palaeosols are inter-preted as recording intermediate soil moistureconditions and the initiation of a drying trendduring the transition at the Pennsylvanian–Per-mian boundary interval.
The Early Permian palaeosols (types F, G andH) record the onset of significant pedogeniccarbonate accumulation throughout the region,which could reflect a decrease in average soilmoisture levels and/or an increase in Ca2+ dustflux to the region. Moreover, a continuum of soilmoisture conditions is defined by these threeEarly Permian pedotypes (alfisols ¼ wettest toaridisols ¼ driest) by the previously discusseddifferences in their morphologies, abundanceand distribution of redoximorphic features andclay composition. Indicators of seasonal or epi-sodic fluctuations in soil moisture conditionsoccur in all mature palaeosols in the studyarea except histosols. However, the degree towhich the morphological indicators of soil mois-ture fluctuations and, increasingly, better soildrainage are developed in the studied palaeo-sols increases significantly from weakly devel-oped features in ultisols of the lower half ofthe Virgilian strata to strongly developed featuresin earliest Early Permian alfisols and calcicvertisols.
The stratigraphic distribution of Early Permianpedotypes (F, G and H) does not define a strongtemporal trend given that all three pedotypesco-exist in contemporaneous intervals. Type Fpalaeosols (alfisols), however, become somewhatless abundant upwards through the EarlyPermian strata (Waggoner Ranch Fm.) with aconcomitant increase in the abundance of type Hpalaeosols (aridisols; Fig. 15).
The longer term trend defined by the studiedpalaeosols could reflect a change in regional baselevel governed by tectonic activity or eustasy thatmay not have been directly linked to climatechange (cf. Marriott & Wright, 1993; Wright &Marriott, 1993). The palaeoshoreline migratednorth-north-eastward throughout the latest Penn-sylvanian to Early Permian in response to arelative sea-level rise, as recorded by the incur-sion of marine deposits on to the Eastern Shelf(Figs 2 and 14; Hentz, 1988). This shift in baselevel is expressed in the basinward shift in thegeomorphic position of palaeosol-bearing inter-vals in the studied Permo-Pennsylvanian stratathrough time (Table 2; Fig. 15). This shift shouldbe recorded by a progressive increase in hydro-morphy in response to shallower regional ground-water tables or overall increased maturity offloodplain palaeosols with increasing distancefrom sediment source areas (Wright & Marriott,1993; Birkeland, 1999; Kraus, 1999). The ob-served long-term trend of decreasing degree ofhydromorphy and intensity of chemical weather-ing of the palaeosol profiles concomitant withincreasing evidence for free drainage of profilesand overall falling soil moisture levels, however,is contrary to the trends anticipated in responseto a rise in base level. This indicates the predom-inant influence of an allogenic process(es) otherthan base level on the observed long-term trend inpedotype distribution. However, the greater abun-dance of type G (calcic vertisols) and type F(alfisols) on landscapes proximal to the palaeo-shoreline (0 to � 30 km; Table 2) and a domin-ance of aridisols (type H) in regions of the alluvialbasin proximal to sediment sources (> 60 kmfrom base level; Table 2) does indicate that adecline in topographic relief and sediment accu-mulation rates down palaeoslope on the EasternShelf may have had a subordinate influence onthe observed larger scale stratigraphic variationsin palaeosol morphology and maturity. The weakmanifestation of these basin-wide trends, how-ever, attests to the very low palaeoslopes on theLate Palaeozoic Eastern shelf. Temporal variationin Permo-Pennsylvanian parent material in thestudy area as an alternative allogenic control onthe large-scale stratigraphic variations in pedo-type is not considered likely given the lack of anyobserved systematic long-term variation in parentmaterial derived from the Ouachita or Arbuckleuplands (Hentz, 1988).
It is suggested that climate was the predomin-ant control on the origin of large-scale strati-graphic variations in pedotype distribution in
Permo-Carboniferous palaeoclimate from palaeosols 877
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
Table
2.
Rela
tion
ship
of
Perm
o-P
en
nsy
lvan
ian
pala
eoso
lsi
tes
rela
tive
toest
imate
dbase
-level
on
the
East
ern
shelf
of
the
Mid
lan
dB
asi
n.
Form
ati
on
Dis
tan
ce
from
rela
tive
sea
level
an
dp
ala
eoso
lm
orp
holo
gy*
0–10
km
10–20
km
20–30
km
30–40
km
40–50
km
50–60
km
>60
km
Sit
e#
Typ
eS
ite
#T
yp
eS
ite
#T
yp
eS
ite
#T
yp
eS
ite
#T
yp
eS
ite
#T
yp
eS
ite
#T
yp
e
Waggon
er
Ran
ch
31
H,F
,G26
H,G
,I29
H,G
30
H¢,E
¢–
––
––
–27
H,G
,I–
––
––
––
––
–28
H,G
,E–
––
––
––
––
––
––
–25
H,E
––
––
––
––
––
––
24
H,E
,G–
––
––
––
–23
G¢
––
––
––
––
––
–P
etr
oli
a–
–22
H,F
,G,E
––
––
––
––
19
H,F
,E–
––
––
––
––
–20
H,E
––
––
––
––
––
21
H,E
––
18
H–
––
––
––
––
––
17
H,F
––
––
––
––
––
Nocon
a–
–16
H,G
,F,E
––
––
––
––
14
G,E
––
––
––
––
––
––
15
E13
G,E
12
G,E
Arc
her
Cit
y–
––
––
––
–9
G,H
,F,E
––
––
––
––
––
––
10
H,G
––
––
––
––
––
––
11
G,H
––
––
––
––
––
––
––
8G
,H–
––
––
––
––
––
–7
G,E
––
Mark
ley
––
––
––
––
––
––
1A
,B,C
––
––
––
––
––
––
2A
,B,C
––
––
––
––
––
––
3A
––
––
––
––
––
––
4C
––
––
––
––
––
––
5C
,E–
––
––
––
––
––
–6
C,D
,E
*T
he
dis
tan
ce
from
base
level
was
dete
rmin
ed
by
measu
rin
gth
ed
ista
nce
dow
nd
ep
osi
tion
al
dip
from
the
stu
dy
site
sto
the
vert
icall
yd
ash
ed
lin
es
inF
ig.
2th
at
rep
rese
nt
the
stra
tigra
ph
iccu
t-off
betw
een
mari
ne-d
om
inate
dan
dte
rrest
rial-
dom
inate
dse
dim
en
tary
stra
ta(H
en
tz,
1988).
Th
ese
are
likely
tobe
min
imu
mest
imate
s,as
soil
form
ati
on
occu
rred
on
the
flood
pla
ins
du
rin
gti
mes
of
base
level
fall
or
low
stan
d(e
.g.
mid
-con
tin
en
tcyclo
them
s).
Th
ep
osi
tion
of
each
of
the
stu
dy
site
san
dth
eir
ass
ocia
ted
pala
eoso
lm
orp
holo
gie
sare
giv
en
inre
lati
on
to(1
)d
ista
nce
from
base
level
(in
ferr
ed
from
dash
ed
lin
es
inF
ig.
2);
an
d(2
)st
rati
gra
ph
icoccu
rren
ce.T
he
pu
rpose
of
this
table
isto
ass
ess
the
dis
trib
uti
on
of
pala
eoso
lsre
lati
ve
tobase
level.
Alt
hou
gh
more
data
shou
ldbe
gath
ere
dto
test
rigoro
usl
yth
eeff
ects
of
base
level
(an
dbase
level
ch
an
ge)
on
the
morp
holo
gy
of
pala
eoso
lsin
the
stu
dy
are
a,it
isap
pare
nt
that
there
isn
oro
bu
stch
an
ge
inp
ala
eoso
lm
orp
holo
gy
as
afu
ncti
on
of
dis
tan
ce
from
base
level
for
an
yon
eti
me
peri
od
.
878 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
the study area. The Late Palaeozoic is hypothes-ized to have been characterized by major changesin climate in response to the assembly of thesupercontinent Pangea, a transition from ice-house to green-house states and major changes inatmospheric circulation (Veevers & Powell, 1987;Parrish, 1993; Crowley, 1994). Climatically sen-sitive depositional and palaeontological recordsfor low-latitude Pangea (Rowley et al., 1985;Cecil, 1990; Dubiel et al., 1991; DiMichele &Aronson, 1992; Parrish, 1993; West et al., 1997;Kessler et al., 2001; Rees et al., 2001) coupledwith numerical model results (Parrish et al.,1986; Kutzbach & Gallimore, 1989; Patzkowskyet al., 19918 ; Gibbs et al., 2002) suggest that themid-latitude and equatorial regions of Pangeabecame progressively more arid and seasonalthroughout the Late Palaeozoic. The long-termaridification trend has been attributed to severalprocesses including: (1) northward drift of
Pangea out of the Intertropical Convergence Zoneinto the more arid low-latitude belt; (2) rainshadow effects created by uplift of the Alleghe-nian and Ouachita mountains; and (3) northwarddiversion of moisture-laden easterlies by thedevelopment of northern hemisphere monsoonalcirculation.
Significantly, recent studies have suggestedthat monsoonal circulation and attendant west-erly winds were well-established over westernequatorial Pangea by earliest Permian time(Kessler et al., 2001; Soreghan, G.S. et al., 2002;Soreghan, M.J. et al., 2002; Tabor & Montanez,2002; Loope et al., 2004), despite model resultsindicating that continental-scale monsoonal cir-culation would have strengthened progressivelyto its peak in the Middle Triassic (Parrish, 1993).The results of these lithological and geochemicalstudies, along with other discrepancies in model-data comparisons for low-latitude western Pangea
Fig. 15. Schematic diagram of the Permo-Pennsylvanian landscape through time based on palaeosol morphologicalfeatures. Roughly contemporaneous study sites from Figs 1 and 2 are plotted on each time period, as well as thepalaeosol types associated with each study site. See text for discussion.
Permo-Carboniferous palaeoclimate from palaeosols 879
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
(Patzkowsky et al., 1991; Rees et al., 2001; Gibbset al., 2002), highlight the need for a betterunderstanding of specific climate zones over LatePalaeozoic Pangea. Pennsylvanian to Permianpalaeosols from north-central Texas provide addi-tional constraints on the nature and origin ofclimate change over low-latitude Pangea giventhat climate change would have been most pro-nounced in western equatorial Pangea (Dubielet al., 1991; Parrish, 1993).
The long-term variation in soil moisture condi-tions and, in turn, precipitation inferred from thelarge-scale stratigraphic variations in Permo-Pennsylvanian pedotypes confirms the previ-ously defined long-term aridification trend forwestern equatorial Pangea. Notably, however, thepalaeosol data in this study indicate that thelong-term drop in inferred soil moisture levelsaccelerated at end-Pennsylvanian time. Incipientmorphological indicators of seasonality preservedin type D palaeosols of the study area suggest anabrupt onset of seasonality in conjunction with arapid decline in net soil moisture budget in thelatest Pennsylvanian.
The long-term aridification trend inferred fromthe palaeosol data cannot be interpreted asreflecting northward drift of Pangea given thatpalaeomagnetic studies indicate that the arearemained within a few degrees of the equatorthroughout the Early Permian (Scotese, 1999;Loope et al., 2004). The observed temporal trendmay record the development of a rain shadow inwestern Pangea in response to uplift of theequatorial Alleghenian and Ouachita mountains(Rowley et al., 1985; Parrish, 1993; Ziegler et al.,1997). However, the degree to which long-termaridification through the latest Pennsylvanianand Early Permian can be explained by a rainshadow effect is strongly dependent on theelevation of the equatorial highlands, for whicha significant degree of uncertainty exists (Otto-Bliesner, 19939 , 1998; Fluteau et al., 2001; Gibbset al., 2002). Moreover, the orographic effect ofthese prominent equatorial highlands on tropicalprecipitation (i.e. onset of aridity) should havebeen well-established by Late Carboniferous timerather than in the Early Permian as the palaeosolsof the Eastern shelf suggest (cf. Kessler et al.,2001; Soreghan G.S. et al., 2002).
The overall long-term palaeopedogenic trendand its inferred rapid decline in precipitationlevels and intensification of seasonality in west-ern equatorial Pangea in the latest Pennsylvanianand earliest Permian are strong evidence fornorthern hemisphere monsoonal circulation. For
western equatorial Pangea, this rapid change inregional climatic conditions may record a tec-tonic moderation of monsoonal conditions. Up-lift of the equatorial mountain chain in the EarlyPennsylvanian (Chesterian) would have acted tointensify the normal low pressure area of theequatorial belt, thus inhibiting the developmentof fully monsoonal conditions until their erosionin the latest Pennsylvanian and Early Permiandampened their climatic influence (Rowleyet al., 1985; Otto-Bliesner, 1993). Notably, thedelay in the development of fully monsoonalconditions may have promoted relatively rapidintensification of monsoonal conditions over thestudy area in the Early Permian as has beeninvoked to explain the origin of rapid variationin lithofacies of mid-continent cyclothems in thelatest Pennsylvanian to earliest Permian (Westet al., 1997).
Lastly, pedogenic evidence for intermediate-scale (103)105 years) climate fluctuations inwestern equatorial Pangea occurs in the poly-genetic alfisols characterized by well-developedcalcic horizons overlying or superimposed onargillic horizons. These polygenetic palaeosolsare restricted stratigraphically to the lower half ofLower Permian (Wolfcampian) deposits of north-central Texas, and reappear for a few tens ofmetres in the uppermost Leonardian-ageWaggoner Ranch and lowermost Clear Fork For-mations. These polygenetic profiles are analogousto those that occur within slightly younger,Lower Permian mid-continent cyclothems thathave been interpreted to record orbitally forced� 20 ky climate oscillations associated with wax-ing and waning of Early Permian continentalglaciers (Miller et al., 1996). Such precessionalforcing of climate would probably have beendampened following deglaciation and a trans-ition to greenhouse states. If polygenetic palaeo-sols from the eastern Midland basin recordprecessional-scale climate fluctuations, then theabrupt shift to more stable climatic conditionsmarked by the loss of polygenetic palaeosols inthe Early Permian (mid-Leonardian) successionmay be a sensitive record of the onset of degla-ciation. The brief reappearance of polygeneticalfisols in the later part of the Early Permianmight record a brief renewed episode of glaci-ation or a climatic cooling analogous to the lateQuaternary Younger Dryas. This interpretation, ifcorrect, places additional constraints on mostrecent estimates of a late Early Permian age forthe onset of deglaciation (Veevers, 1994; Gibbset al., 2002).
880 N. J. Tabor & I. P. Montanez
� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884
CONCLUSIONS
Late Pennsylvanian and Early Permian sedimen-tary strata of the Eastern Midland basin exhibit achange in palaeosol morphological, mineralogicaland chemical characters across the Pennsylva-nian–Permian boundary. This change is revealedby palaeosols with well-developed redoximor-phic depletions and concentrations in the LatePennsylvanian that are replaced by palaeosolsthat generally lack redoximorphic indicators inthe Early Permian. Furthermore, many of theEarly Permian palaeosols are characterized bypedogenic carbonate accumulation that is consis-tent with a net soil moisture deficiency, whereasLate Pennsylvanian palaeosols exhibit no pedo-genic carbonate. The clay mineralogical compo-sition of these palaeosols indicates a change frommore highly weathered and probably humidconditions in the Late Pennsylvanian to lessextreme weathering and potentially semi-arid toarid conditions in the Early Permian. Thesechanges may be a response to changing soilmoisture regimes from humid, poorly drained todry, well-drained conditions in the Late Pennsyl-vanian to Early Permian.
The inferred change in Late Pennsylvanian–Early Permian soil moisture regimes of westernequatorial Pangea is likely to be related to climatechange. It is suggested, based on the preservedlithological, mineralogical and chemical charac-teristics of these palaeosols, that a changeoccurred from a humid tropical climate charac-terized by high rainfall in the Pennsylvanian to aprogressively drier tropical climate characterizedby seasonal precipitation throughout the EarlyPermian. Furthermore, these data indicate theonset of monsoonal-type precipitation patternsnearly coincident with the Pennsylvanian–Per-mian boundary. Although these strata preserve ageneral trend of progressively drier and seasonalprecipitation through the Permian, polygeneticsoils preserved within the lower half of the EarlyPermian (Wolfcampian) and uppermost strata ofthe lower Leonardian Waggoner Ranch Formationmay indicate an oscillatory climate patternbetween more humid and drier climate duringdeposition of these stratigraphic packages
ACKNOWLEDGEMENTS
We are deeply indebted to Bill DiMichele and DanChaney, Department of Palaeobiology, Museumof Natural History, Smithsonian Institution, for
providing an introduction to the study area. JohnNelson and Dan Chaney provided excellent fieldguidance. This project was funded by NSF grantEAR-9814640 to I. P. Montanez and an NSF-IGERTgraduate fellowship to N. J. Tabor. Funding wasalso provided by grants to N.J.T. from the Geolo-gical Society of America, American Association ofPetroleum Geologists, Society for SedimentaryGeology and Sigma Xi. We thank Peter Haughton,Paul McCarthy and Susan Marriott for theirthoughts and helpful comments on this manu-script. All these reviewers (but particularly S.M.)have contributed significantly towards improvingthe quality and clarity of this contribution.
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Manuscript received 2 July 2003;revision accepted 22 March 2004.
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