Upper Ordovician Mud Slide: A Crinoid Biocenose Becomes a Thanatocenose

Gene Ulmer, George Myer & Steve Peterson, Dept of Earth & Environmental Sciences,

Temple, University, Philadelphia PA 19122 USA

A story of the dislocation and burial of a living colony of crinoids

Oval is Ordovician Outcrop

100km

Ohio

Cincinnati, Ohio

OhioOval is Ordovician outcrop in Ohio area

Pink is Ordovician

100km

HG

FE

D

Stratigraphy of an Ordovician/Richmond outcrop at Dent, Ohio

Vegatative cover

Brow

n Lim

estone

Bryozoan fauna

Gray lim

estone with

Calym

ene parts

Very thin (~1 cm)brown limestone with Reserella & Rafinesquina Shaly brow

n

limestone w

ith

Platystrophia

ponderosa

Gray lim

estone

with Isotelus parts

North

DE

FG

H

N

N’M

M’

Schematic of cross-cutting, sediment-filled trough that ended as Shown in a three-toe geometry. Cross-sections along M-M’

and N-N’ are presented in later figures. Scale given by MM’ which is ~17 meters.

Northeast

Schematic section along line N-N’ which is about 2.0m long

N N’

G

FE

D~30cm

Cross-cutting, sediment-filled trough

Unit G at surface at N-N’

2m

Schematic section along line M-M’

M M’

G

H

FED

~0.6 m

Topographic slope from construction lot-leveling

?

?Sediment filled trough

20m

Toe

DE

FG

H

N

N’M

M’

Schematic of cross-cutting, sediment-filled trough that ended as shown in a three-toe geometry. Cross-sections along M-M’

and N-N’ are presented in later figures. Scale given by MM’ which is ~17 meters.

Northeast

Glyptocrinus:Ord/Sil

FromGeo/Univ Texas.edu

Columnals, stem

Hold fast, anchor, root

Calyx, head, flower

Integumen

Ambulacrals, arms

Pinnae

Arrows showing layout direction of fronds. Note near vertical orientations marked O

2.54 cm

Two basic orientations of buried crinoids calyxes:Vertical with bent-under fronds(left) and

horizontal with sub-horizontal fronds(right)

Channel infill:Subtle bedding

Evidence for mild motion with little shear: • Few columnals still attached

•Dislocation of longer columnal stems that are missing•Two orientations with fragile fronds bent under

but not torn off

Apex of calyx (A)

Basis of size measurements reported was from point A to point B

Branching at edge of calyx (B)

Note that branching within calyx was not used

9 10 11 12 13 14 15 16 17 18 19 20 21 22

Histogram of sizes & orientations of crinoids involved in this study

X represents horizontally-oriented samples

O represents near vertically-oriented samples

Note: several vertical sample are still buried too deep to measure

Size in mm: distance measured from point A to point B

Pop

ulat

ion

dens

ity

of c

rino

ids

XO

O

OOO

X

XX

X

XX

X

X

Photo of Dent, Ohio Crinoid fromA SEA WITHOUT FISH

Figure 12.9 B, p 176Indiana University Press (2009)

Since the snails are found inthe inter-ambulacral area and near the

anus of the calyx, one must break such unique samples to expose

the presence of the snail. The slab featured in this manuscript has not

been cleaned in such a way as to show snails, but smaller slabs from

the same channel toes did allownon-destructive observations that

suggest at least 10% of the calyxes are showing snails in this

anal-feeding position

A few initial observations from the paleobiology:

•Note the wide range of sizes may substantiate a living colony assemblage

•The final orientation of any sample seems to be independent of it’s size

•The missing columnals & missing evidence of hold fasts lends support

to relocation theories•The symbiotic feeding of Cyclonema

Gastropods in anal calyx positions also has been literature-accredited

as being a living-assemblage feature

Stokes LawVelocity settling = 2/9 sin θ(δparticle – δfluid) g r2/μ

θ is the slope angle for the flowing fluid; (δparticle – δfluid) is the density contrast

between the sediment particle and the fluid;G is gravitational constant; r2 is the square

of the effective particle diameter and μ is the viscosity (fluidity) of the fluid

If the mud slide is too slow, the crinoids would not be pushed along,

but settle out before the toe end of the channel. If the mud slide is too fast, turbulence would shear not just the

columnals, but tear up the fragility of the arms and make the resting orientations much more chaotic than observed.

Type of Stokes Law parameters utilized to try to define the range of possible velocities involved in this modelled crinoid transport:

•Density of the columnals being Calcitic•The effective diameter of the columnal’s individual sections•The shuttle-cock like shape of the calyx with fronds attached

•The effective density of non-muddy saline sea water•The effective density of sediment-slurry mixtures

•Viscosity slurry ranges from 1 poise to 5 poise (oil to thick milk shake)•Slope of ocean floor, far from continental- or basin-margin

Examining these variables in many various sensible combinations with Stoke’s Law

shows a probable range :

From 50 cm/sec to 254 cm/ sec Which in more familiar daily terms is

~1 mph to ~5.5 mph

mild turbulence

Channel infilling materialPhysical Characteristics:Light grey, almost white

(cross-cut beds are dark grey to light brown)Sub micron to micron size, unconsolidated particles

XRD analyses:No dolomite, nor bentonite, nor clay

Very well crystallized qtz + Cc= MajorHydrogrossularite garnet = Minor

Zeolite (Gusmiondine) = TraceIllite = trace

Conclusion:This in not a mineralogy of fluvial sediments,but rather more typical for aeolian sources,

such as ash or loess. Since there is no bentonite, nor meta-bentonite, loess seems most likely !

Ordovician Glaciation and Loess ?

Wikipedia: “ As the Ordovician progressed we see evidence of glaciers on

the lands now known as Africa and South America. …These land masses at that time were at the South Pole

and covered with ice caps.This ice age probably was of a short duration only from 0.5 to

1.5 million years long Channel-fill Mineralogy:Submicron to micron size

Major: qtz, cc Minor:

hydrogrossularite garnetTraces:

illitezeolite(gismondine)

No: Dolomite

Mineralogy of Channel-Filling Material

The sediment in the cross-cutting channel is light gray,almost white, in sharp contrast to the dark grey to light brown colors of the units that are cross-cut.

The sediment particle size is sub-micron to a few microns.

XRD examination shows:Major: calcite and quartz

Minor: hydro-grossualite garnet and zeolite (gismondine)Trace: Illite

No clays, nor dolomite

This suggests an aeolian, non-fluvial source.Volcanic ash or loess are likely sediment sources,

and the mineralogy argues most strongly for loess which is known in the Ordovician literature.