English: Fossiliferous asphaltic limestone from the Pennsylvanian of Oklahoma, USA.

In paleontology, the term "lagerstätten" refers to sedimentary deposits having exceptionally well-preserved fossils, especially soft-bodied fossils. Such fossils can result from various geologic processes, including freezing (e.g., 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; nacreous aragonite). The Buckhorn dates to the late Middle Pennsylvanian, which makes it the oldest deposit in the world with aragonitic nacreous shelly fossils.

What makes the Buckhorn special? It's an asphalt deposit - a paleo-seafloor oil seep deposit. A patch of seafloor was saturated by heavy oil (asphalt) that seeped out during the Pennsylvanian, well before the sediments were buried or lithified. Modern seafloor petroleum seeps are known in many places. Normally, the aragonite (CaCO3) and nacre (mother-of-pearl) of shells gets destroyed by recrystallization during diagenesis and post-diagenetic alteration events. At the Buckhorn deposit, the asphalt prevented the nacre from recrystallizing.

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. The shells have the original aragonite and often have the original iridescent nacre. The mollusc fossils are principally cephalopods, bivalves, and gastropods. Some bellerophontid molluscs have also been reported. 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 sample seen here is 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 saturated with petroleum.

Stratigraphy: Buckhorn Asphalt, Deese Group, middle Desmoinesian, upper Middle Pennsylvanian.

Locality: Buckhorn Asphalt Quarry, west of Route 177, ~10 miles south of Sulphur, southeastern Murray County, Arbuckle Mountains, southern Oklahoma, USA.

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Mostly synthesized from info. provided by Alex Nutzel and Thomas Yancey.