BUCKHORN LAGERSTÄTTE
In paleontology, the term lagerstätten refers
to sedimentary deposits having exceptionally well-preserved fossils, especially
soft-bodied fossils. Exceptionally preserved fossils can be the result of
various geologic processes, including freezing (Siberian permafrost),
desiccation (mummified carcasses in desert sands), and resin encasement
(amber), just to name a few.
The Buckhorn Lagerstätte of Oklahoma doesn't have soft-bodied fossils, but is
remarkable in having aragonitic shelly fossils preserved with the original
iridescent, colorful mother-of-pearl (nacre). The Buckhorn dates
to the late Middle Pennsylvanian, which makes it - by far - the oldest deposit
in the world with aragonitic nacreous shelly fossils.
What makes the Buckhorn special? It's an asphalt
deposit, that is a paleo-seafloor oil seep deposit. A patch of
seafloor in present-day southern Oklahoma was impregnated by heavy oil
(asphalt) that seeped out during the Pennsylvanian, well before the sediments
were buried or lithified. Modern seafloor seeps of petroleum are known in
many places. Normally, the aragonite (CaCO3) and nacre
(mother-of-pearl) of shells gets destroyed by recrystallization during
diagenesis & post-diagenetic alteration events. At Buckhorn, the
asphalt prevented the nacre from recrystallizing.
Age -
middle Desmoinesian, upper Middle Pennsylvanian.
Locality
- Buckhorn Asphalt Quarry, W of Rt. 177, ~10 miles south of Sulphur,
southeastern Murray County, Arbuckle Mountains, southern Oklahoma, USA.
Buckhorn Asphalt (12 cm across at its widest) - fossiliferous
asphaltic limestone with nacreous nautiloid cephalopod shells.
The portion of the Pennsylvanian-aged Deese Group that
contains the Buckhorn Asphalt is a fine-grained to coarse-grained siliciclastic
and carbonate succession of marine rocks having well-preserved molluscs and
plant fossils. In the past, the Buckhorn was considered to be
shallow-water marine, but it’s been reinterpreted as an offshore, deeper-water
deposit.
Stratigraphy:
limestone-clast conglomerate
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unconformity
sandstone with carbonized logs
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mudstone
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thin gilsonite interval
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asphaltic limestone - has the best-preserved fossil
material
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asphaltic sandstone
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chert-clast conglomerate - has woody debris, including
large silicified Callixylon logs
The molluscs in the asphaltic layers are extremely
well-preserved (down to the micron level) with the original aragonite and, in
many cases, the original iridescent nacre (mother-of-pearl). The mollusc
fossils are principally cephalopods, bivalves, and gastropods. Some
bellerophontid molluscs are also known from here. The asphaltic interval
includes a cephalopod-rich association and a bivalve-gastropod-rich
association. Some researchers have been particularly excited by the
presence of preserved protoconchs in Buckhorn fossil snails. The
protoconch is the most juvenile portion of a snail’s shell, and its morphology
has phylogenetic significance.
The Buckhorn Asphalt rock samples shown above &
below are asphaltic limestones from the cephalopod-rich association. The
cephalopod-rich rocks are dominated by fossils of the body chamber portions of
the shell (= where the squid hung out). These cephalopod-dominated rocks
have been interpreted as submarine mass flow deposits. The sediments
making up these flows were already impregnated with petroleum.
Buckhorn Asphalt (8.9 cm across at its widest) - fossiliferous asphaltic
limestone with nacreous nautiloid cephalopod shells.
Buckhorn Asphalt (2.4 cm from top to bottom) - nautiloid shell
preserved with original iridescent nacre (mother-of-pearl). This deposit
has the oldest fossil nacre known anywhere in the world.
Mostly synthesized from info. provided by Alex Nutzel
and Thomas Yancey.