The Copan Lagerstätte: Exceptional Crinoid Diversity
and Preservation in the Pennsylvanian of Midcontinental North America
James
Thomka (Department of Geology, University of Cincinnati, Cincinnati, Ohio,
USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
28 January 2011
The
Pennsylvanian is a unique time for crinoids. The Pennsylvanian doesn’t
get much attention in terms of crinoids. There are many crinoids in the
Pennsylvanian, diversity-wise.
Looking
at the history of crinoids, the Pennsylvanian falls between two significant
crinoid events:
1)
Mississippian - when crinoids hit their peak, abundance and diversity-wise; get
entire rock types from crinoid remains at this time.
2)
end-Permian mass extinction - the worst ever; crinoids got decimated.
Four genera of crinoids cross the Permian-Triassic boundary. All modern
crinoids evolved from them.
The
Pennsylvanian seems bland in comparison.
Pennsylvanian
crinoids generally don’t occur as articulated specimens - don’t see many
crowns.
The
only significant exception is the LaSalle Limestone of Illinois.
Now,
a second Pennsylvanian crinoid deposit with lots of articulated crowns has been
found - the Upper Pennsylvanian Copan Lagerstätte of Oklahoma. It’s named
after the small town of Copan in northern Washington County, northeastern
Oklahoma. This unique deposit is not regionally extensive - <12 m2
- very restricted geographic distribution. It’s also very thin - 0.5
meters thick - very restricted, stratigraphically.
The
Copan Lagerstätte is in a concretionary mudstone interval in the lower
Barnsdall Formation.
This
crinoid deposit is late Missourian (Upper Pennsylvanian) in age.
The
site was discovered in the 1940s by Strimple, who is one of the few
Pennsylvanian crinoid specialists. In the 1940s, the site was called the
“Tankdike locality”, in reference to it being a petroleum storage tank
cut. There was an abundance of crinoids in float at the cut - rich in new
crinoids.
Eight
new crinoid taxa were found and described from here in the 1950s to 1970s.
The
fossils consist of fairly articulated individuals - cups and crowns.
The
crinoids were never found in-situ, only in float.
The
site was rediscovered in 1984 by Dan Mosher, an amateur. He found lots of
Pennsylvanian articulated crinoids. He wanted to find the source
horizon. He used pennies on the outcrop for every float crown he
found. He used the distribution of the pennies to locate the source horizon.
He sieved and collected lots of crinoid crowns. Mosher took the crinoids
to Roger Pabian.
Heavy
equipment was brought in later - the overburden was removed and they benched
off the source horizon. Collecting resulted in an impressive yield of crinoids.
Some were preserved in siderite concretions. Some long lengths of stems
were found, plus some intact crinoid holdfasts.
1250
articulated or partially articulated specimens were recovered from an
only-inches-thick horizon.
The
Copan Lagerstätte produced 44 genera and 50 species of crinoids. This is
a higher crinoid diversity than the Crawfordsville, Indiana crinoid fauna
(Mississippian).
Copan
is the most diverse Pennsylvanian crinoid lagerstätte in the world. The
next most diverse Pennsylvanian crinoid lagerstätte has 32 genera.
What’s
leading to this diversity and preservation?
In
the Pennsylvanian, midcontinental North America was under shallow seas.
The Copan, Oklahoma site was close to the Ouachita Mountains.
Iowa
had extensive shallow-water carbonates. Nebraska was a more open-ocean,
shallow marine shelf with algal mound environments in places.
Cyclothems
are important in the Pennsylvanian Midcontinent. Each cyclothem
represents one relative rise and fall in sea level. Oklahoma has Kansas-type
cyclothems - the most important unit in KS-type cyclothems is the core shale -
black to gray shale representing dysoxic to anoxic conditions and having
low-diversity fossils, if any.
Core
shales actually consist of an upper core shale, which is light-colored (often
greenish) with diverse fossils, and a lower core shale, which is dark gray to
black and horizontally laminated.
During
deposition of the upper core shale, the seafloor was hospitable to life.
The upper core shale represents the first shallowing from the deepest part of
the cycle. Benthic invertebrates fluorished during upper core shale
conditions.
The
Copan Lagerstätte site has concretionary shale in the upper core shale and the
crinoid lagerstätte in the lower core shale - black shale.
Most
of the Barnsdall Formation is blocky, blah-gray shale.
The
0.5 meter interval of interest is not just monotonous mudstone - it has 3
discrete crinoid horizons.
The
lowest one has 44 genera and 50 species of crinoids - this is what Mosher found
- >1200 specimens.
The
paleoenvironment is interpreted as an oxygenated, low-energy, low
sedimentation, distal shelf setting.
The
shale intervals between crinoid horizons have productid brachiopods, clams, and
scaphopods.
The
crinoids occur in concretionary shales. Concretions are indicators of
low-sedimentation rates - they form in the shallow subsurface.
Very
large siderite concretions indicate long periods of sediment starvation on the
distal shelf. Thousands of years of sediment starvation,
apparently. Very sediment-starved conditions.
Sedimentation
was very slow - these conditions are associated with the well-preserved
crinoids.
Kope
Formation concretions are smaller than Barnsdall Formation siderite
concretions.
Encrustation
rates on hard biogenic substrates are high in crinoid-rich, concretion-rich
intervals.
There’s
also evidence for rapid deposition - how else does one get
beautifully-preserved crinoid crowns?
Mobile
organisms were there too - their skeletal remains are found in the horizons of
interest, but they are not well preserved. Occasionally a productid
brachiopod is found in living position. Occasionally a bryozoan sheet is
found. Complete regular echinoids are never found here.
Burial
events were thin enough to only affect immobile organisms, like crinoids.
One
specimen of a crown of a flexible crinoid has one side well preserved/intact
and the other side a plate hash. This indicates partial burial (thin
sediment cover) - the crown was partially exposed on the seafloor.
Other
evidence for thin burial - infaunal scavenging is inferred from partially
disturbed crowns.
Burrowing
bivalves and scaphopods in life position are seen in the intervening thick
shale/mudstone intervals.
There
are multiple horizons (3) of thin, crinoid-bearing, siderite concretionary
horizons. These horizons have no winnowing, no graded bedding, and no
piles of skeletal material.
There’s
no evidence that the rapid burial was high-energy - no big blast. In-situ
holdfasts are present - crinoids were toppled over in-place and buried.
No violent burial.
Thicker
layers have abundant, sharp-based lags - rapid deposition of considerable
material under much higher energy conditions. These thicker mudstones
also have large, siderite-filled burrows. The burrows have sharp-walled
boundaries - indicates firmground conditions. These firmground burrows
were temporary substrates - they indicate minor erosive episodes - the soupy
upper layer got removed.
So,
seeing a proximality difference.
Crinoid
horizons experienced sediment starvation and episodic, very thin, rapid burial
events.
Multidirectional
currents associated with rapid burial is diagnostic of storm sedimentation in
distal shelf settings. These are tempestites - not single event surges.
Some
crinoids were buried alive or prior to any major degradation.
Shaving
brush posture in crinoid crowns - the animal was trying to protect critical
parts/anatomy from stressful environmental conditions.
Can
also see brief periods of incipient decay prior to final burial.
All
of this is expected/associated with distal storm events.
Taphonomy
- can see the loss of arm plate tips before the first losses of
pinnules!
So,
there was a brief period of decay.
Gonads
are the most nutrient-rich parts of the crinoids for scavenging organisms.
Some
partially (inferred) scavenge-disrupted crinoid crowns were growth sites for
siderite concretions. The scavenged portion of a crown will have more
water movement, resulting in more concretion mineralization potential.
The
Copan Lagerstätte consists of thin, distal tempestites. Sediment
starvation allowed for stacking of obrution layers with very little dilution by
background sedimentation.
This
motif is probably seen/present in many other areas. We’ll see.
It’s
possible that original seafloor topographic control explains the limited
geographic distribution of the Copan Lagerstätte. The Copan was deposited
in a seafloor depression.
Many
different arm types are present in the recovered crinoids, indicating many
different feeding styles.
Sediment
channeling (funneling) may have been playing a role.
Inferred
water depth - there are no algae present to indicate photic zone depths -
probably ~100 m deep?
Not
basinal.
Crinoids got asphyxiated
by a little fine-grained sediments - they died and fell over. The
crinoids were not buried to the height of the upright animal. Death
preceded burial. Burial was by a ~few cm of uncompacted mud, but it
varied, apparently.