coarsening-up alluvial

shallowbraided alluvial plain

Reworkedbasalt clasts

Palaeosols braided alluvial plain

phyllite

bivalvesin green-greysilt

Lake and ephemeral streams

distal alluvial plain

reworked basalt clasts

Palaeosols interbedded with

gravel sheets, rare channels

red-brown alluvial plain sediments

basalt

coarsening-upalluvial

massive conglomeratedebris fan

Channellisedalluvial plain

7.82 ± 0.32 Ma

6.93 ± 0.15 Ma

13.6 ± 0.5 Ma

12.2 ± 0.5 Ma

12.0 ± 0.5 Ma

QTt: Quaternary-Tertiary Tortoral Fm.

Tpz: Tertiary Playa del Zorro Fm.

Tl: Tertiary lacustrine

Tpa: Tertiary Penas Azules Fm.

Tb: Tertiary basalt

pC: Precambrian (?) basement

Tgm: Tertiary green mudstone marker

Penas Azules Formation

~

Sections ARG1 - 6 Section ARG8 Section ARG 9

Playa del ZorroFormation

TotoralFormation

300

m

Basement

Sequ

ence

1Se

quen

ce 2

Sequ

ence

3Se

quen

ce 4

3000

3000

3500

4000

3500

3000

3000

3500

2500

3000

3000

?

?

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Ql

Ql

Ql

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

Qt

QtQt

Qt

Qt

Qt

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

Qls

T

T

T

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tpa

Tf

Tpz

Tpz

Tpz

Tpz

Tpz

Tpz

Tpz

Tpz

Tpz

TpzTpz

Tpz

QTt

QTt

QTt

QTt

QTt

QTtQTt

QTt

QTt

QTt

QTt

QTt

QTt

pC

pC

pC

pC

pC

pC

pC

Tb

Tb

Tl

Tl

Tl

Tb

Tb

Tb

Tb

Tb

Tb

Tgm

Tgm

75

40 40

60

6080

25

10

2426

17

35 26

25

12

08

1020 20

354558

60

60 30 1825 16 14 15

7571

70 65

21162020

70

40

51

20

10

06

11

25

20

10

10

30

10

50

12

21

02

04

04

0710

10

08

08

15

0606

0615

07

1534

0915

3945

32

6520 4125

04

4632

65

20

30 4720 35

25

36

28

16

3230

32

28

40

25

32

75

20

7.5 m separation

12.0 ± 0.5 Ma

7.82 ± 0.3 Ma6.93 ± 0.2 Ma

12.2 ± 0.5 Ma

5.71 ± 0.4 Ma

11.8 ± 0.4 Ma

11.6 ± 0.5 Ma

Basin Compartmentalization in the Foreland: El Cajon Basin, Southwestern ArgentinaJonathan R. Pratt - The Ohio State University, Lindsay Schoenbohm - The Ohio State University, Estelle Mortimer - University of Leeds

Basin compartmentalization in a thick-skinned set-ting: In a thin-skinned setting there is a fairly accuratemodel for basin compartmentalization, where thrustspropagate in sequence utilizing bedding planes in sed-imentary rocks. However, in thick-skinned settingswhere reverse faults occur at higher angles, this modelis compromised by the importance of pre-existing weaknesses.

Approach: Analyze the spatial and temporal evolut-ion of one basin, the El Cajon Basin of Northwest Argentina as an example of a basin compartmentaliz-ation in a thick-skinned setting.

Data: As part of a collaborative effort, several data are combined to understand the process and controls that compartmentalized the El Cajon Basin.

1. Geologic mapping of the northern basin, (Fig 2a), produced by L. Schoenbohm.

2. Stratigraphic analysis (Fig. 1), produced by E. Mortimer.

3. Rigorous cross sections (Fig 3a,b), produced by J. Pratt.

4. Seismic data from southern basin (Fig 3c) produced by E. Mortimer.

5. U-Pb dating of intercalated tuff beds (Fig 1 & Fig 2a), produced by J. Pratt

E

E’

F

F’Si

erra

de

Chan

go R

eal

Sierrra de Q

uilm

es

Santa Maria Basin ->

<- Puna Plateau

45

45

Qal: Quaternary alluvium

Qt: Quaternary terrace deposits

QTt: Quaternary-Tertiary Tortoral Fm.

Tpz: Tertiary Playa del Zorro Fm.

Tl: Tertiary lacustrine

Tf: Tertiary coarse fan deposits

Tpa: Tertiary Penas Azules Fm.

Tb: Tertiary basalt

T: Undifferentiated Tertiary

pC: Precambrian (?) basement

Ql: Quaternary lacustrine

Qls: Quaternary landslide deposits

Thrust fault with dip

Thrust fault- concealed

Normal fault with dip

Thrust reactivated as normal fault

Syncline axis

Anticline axis

Fold axis- approximate

Contact

Contact- approximate

Contact- concealed

Thrust fault- approximate

Strike and dip of bedding

Strike and dip of overturned bedding

45

45

Richard YuretichRichard YuretichRichard YuretichRichard YuretichRichard YuretichRichard YuretichRichard Yuretich 11.8 ± 0.4 Ma

5.00 ± 0.14 Ma

0s

1s

2sARG01553

SQS

Seq. 1

Seq. 2

Seq. 3

Seq. 4

TWTT

Sier

ra A

conq

uija

Sier

ra d

e Q

uilm

es

Chango

Rea

l

Santa Maria Basin

Hualfin Basin

El Cajon Basin

N

Puna Plateau

26ºS

67ºW 66ºW

27ºS

2 0 2 4 6 KILOMETERS

~13 Ma

~10 Ma

~6 Ma

Chango Real uplifts, creating accomodation space

Sierra de Quilmes begins uplifting in the north, beginningcompartmentalization.

Sierra de Quilmes propagates further south and the Sierra Aconquijabeings to uplift, effectively compartmentalizing the foreland.

Geologic Background:

The El Cajon basin is one of several intermontane basins along the southeastern margin of the Puna Plateau (Fig. 2b). This part of the Andes is an active, foreland-propagating, thick-skinned fold and thrust belt, which is superimposed on the Cretaceous Salta Rift. Once part of a continuous foreland stretching from the Hualfin basin to the southwest to the Santa Maria basin to the east (Fig. 2b), the El Cajon was compartmentalized by the uplift of the Sierra de Quilmes.

The El Cajon basin contains three Miocene to Pliocene alloformations:

1. Penas Azules: a coarsening-upwards alluvial sequence between11.8-13.6 Ma. Deposited before significant deformation.2. Playa del Zorro: a coarsening-upwards alluvial sequence show-ing growth structures and containing a lacustrine unit. Syntectonic, between 6.93-11.8 Ma. in age.3. Tortoral: Massive fan conglomerate, regionally widespread and sits above an unconformity in the El Cajon. Upper age of 5.00 Ma.

Sierra

deFiam

bala

Sierra

de A

ftoHua

siSie

rra de

Hua

lfin

Sierra

deAco

nquij

a

Orogenic Front

Sierras PampeanasPuna

Farallon NegroVolcanics

Regional Cross-section from just south of the El Cajon. The area encompassing the El Cajon’s east-west extent is boxed in red. Published by V.A. Ramos et. al.

Fig. 1

Fig. 2a

Fig. 2b

F F'

distance (kilometers)1 32 4 5 6 7 8 9 10 11 12 13

elev

atio

n (m

eter

s)

3500

3000

2500

2000

1500

1000

500

0

-500

Tpz

Tpz

Tpa

pC

pC

Tb

Tf

High angle, basement faults

Alluvial fan along basin margin

Growth strata within Playa del Zorro fm.

Fig. 3a

Fig. 3c

elev

atio

n (m

eter

s)distance (kilometers)

E E'

1 32 4 5 6 7 8 9 10 11 12 13 14

3500

3000

2500

2000

1500

1000

500

0

-500

pC

pC

Tpa

Tpz

QTt

TlTb

Fig. 3b

Fig. 4

Fig. 5

High angleblind reverse fault

Fault propagation fold

Angular unconformity- Middle El Cajon

- Tpa thins eastward

- Less deformation along the Chango Real than in the north.

- Fault pair younger than in the north.

- Northern El Cajon

- Tpa thins eastward

- Eastern margin gives constraint on Tpa thickness. Potential pre-existing topo- graphy.

- Faults are most likely reactivated normal faults.

Thin Playa del Zorro fm.over Penas Azules fm.

Landslide deposit

Lacustrine unit

Discussion:

By using the basal age of the sedimentary sequence of 13.6 Mato approximate the onset of loading along the Chango Real, we can estimate that this is when significant uplift along the fault began.

The lacustrine bed serves as an indication of a switch from foreland drainage to internal drainage, which was most likely caused by uplift of the Sierra de Quilmes basement anticline asit propagated southward reactivating Salta Rift structures. The age of the lacustrine bed is bracketed between ~8 & ~12 Ma therefore the uplift of the Sierra de Quilmes is also bracketed between these ages.

The Sierra Aconquija is east of the El Cajon basin and is theeastern bound of the Santa Maria basin. Previous work by Strecker has dated the Aconquija uplift at 5.5 Ma.

The evolution of the El Cajon basin is therefore an in-sequencecompartmentalization following a propagating thrust-wedgemodel with deformation strongly controlled by pre-existing structures as is shown simplified in Fig. 5.

1

2

3

1 Mortimer et al, 2007, Fragmentation of a foreland basin in response to out-of-sequence basement uplifts and structural reactivation: El Cajon-Campo del Arenal basin, NW Argentina: GSA Bulletin: May/June 2007, p.637-653 2 Ramos, V.A., et al, 2002, The Pampean flat-slab of the Central Andes: Journal of South American Earth Science v. 15, p.59-783 Sobel, E.R. & M.R. Strecker, 2003, Uplift, Exhumation and Precipitation: Tectonic and Climatic Control of Late Cenozoic Landscape Evolution in the Northern Sierras Pampeanas, Argentina: Basin Research, p.1-21

-Seismic section from south of the El Cajon subbasin

-Shows both high angle reverse faults and growth strata as exist in the north.

-SQS stands for South Quilmes Structure, which is the buried extension of the Sierra de Quilmes.

-Tpa thins, but to a lesser extent than it does in the north.

Produced by E. Mortimer and modified by J. Pratt

Produced by L. Schoenbohm and modified by J. Pratt

Above:

Geologic map of the El Cajoncontaining locations of theU-Pb dated tuff samples andthe cross-section lines corresp-onding to Fig. 3a and 3b.

The ashes were dated usingan ion microprobe at the Univer-sity of Los Angeles with theaid of Dr. Axel Schmidt.

Left:

Stratigraphic column showingthe rock types and interpreteddepositional environments.

U-Pb ages are placed in thecolumn and are shown in red.

Of note is the bracketing of thelacustrine unit between 7.82 &12.0 Ma.

13.6 ± 0.5 Ma10.7 ± 1.7 Ma

5.00 ± 0.14 Ma