SEDIMENTOLOGY AND DEPOSITIONAL FACIES OF THE LATE PLEISTOCENE MORGAN MAMMOTH SITE IN NORTHERN CADDO COUNTY, OKLAHOMA

A large disarticulated Columbian mammoth (Mammuthus columbi) was discovered in northern Caddo County, Oklahoma, in an erosional tributary of the Canadian River. Recent erosion exposed a two-meter-long portion of the mammoth skull with a meter length of its tusk, a cervical vertebra, a fragment of a femur midshaft, and a portion of the scapula. Prior to excavation, it was envisioned that Clovis people may have killed the mammoth, similar to the Domebo mammoth kill site documented in southern Caddo County. Allometric measurements from the left tibia approximate the height and mass of the mammoth at 3.4 m and 11 tonnes.

A bioapatite date of the mammoth bone provided an age of 28,549 RCYBP. Excessive occlusal wear on its 3^rd molars suggests an age between 55‒60 years old. The mammoth skull has a trampling indentation with an area of 503 cm², a perimeter of 90 cm, and a depth of 11 cm. Tooth compressions were detected on a fragment of a mammoth long bone.

Additional fossil material includes several isolated bones of Equus and ?Stockoceros. A bioapatite date for the horse is 18,205 RCYBP. The proximal end of a partial horse humerus displays trampling. The mammoth, horse, and likely the antelope died during the Wisconsin LGM.

The disarticulated mammoth, horse, and antelope bones were preserved in the bottom of a small distributary fluvial channel. The agglomeration of the different-aged bones into the distributary channel occurred by ephemeral channel migration and incorporation of the bones from the floodplain. During excavation, both channel levees were exposed. The channel geomorphology exhibits a width of 5 m and a depth of 50 cm. The Pleistocene channel sediments are angular (0.96), moderately well sorted (0.60), and very fine-grained (3.52 Φ). The proximal floodplain sediments are angular (0.99), moderately well sorted (0.67), and very fine-grained (3.74 Φ).

Crevasse splay sediments from a subsequent channel infilled the primary channel from the south forming a single large trough cross bed. The crevasse-splay sediments are subangular (1.12), moderately well sorted (0.68), and very fine-grained (3.36 Φ). The overlying Holocene floodplain sediments are subangular (1.20), moderately well sorted (0.53), very fine-grained (3.28 Φ). Radiocarbon dates of the overlying Holocene sediments infer a ≈15,000 yr. period of nondeposition.