Upper Ordovician Mud Slide: A Crinoid Biocenose Becomes a Thanatocenose
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Transcript of Upper Ordovician Mud Slide: A Crinoid Biocenose Becomes a Thanatocenose
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.