OPTICALLY STIMULATED LUMINESCENCE (OSL) DATING OF SEDIMENTS ASSOCIATED WITH MAMMOTH REMAINS TO INFER AGE SPAN FOR THE WACO MAMMOTH NATIONAL MONUMENT, WACO, TX

The Waco Mammoth National Monument (WMNM) is a Late Pleistocene paleontological site located in central Texas, where the remains of approximately twenty Columbian mammoths (Mammuthus columbi) and several other vertebrates have been discovered at the confluence of Brazos and Bosque rivers. These remains, in varying states of articulation, are resting on channel and bar deposits within the second terrace of the Bosque River; and within flood plain sediments with noticeable pedogenic alteration. Ichnologic studies indicate the mammoth skeletons were exposed for months to years and heavily scavenged resulting in some disarticulation and displacement prior to burial. The cause of death for the suspected matrilineal herd is unresolved, though recent evidence is consistent with a drought association. Prior OSL dating of quartz grains from upper and lower fossiliferous levels indicate a minimum age span for the site of ca. 68 to 52 ka. This study determines OSL ages on recently secured monoliths from large, jacketed sediment blocks removed and curated from the original excavations. Also, eight previously dated samples were re-prepared and dated to evaluate the first generation of OSL ages. Sand-sized quartz extracts from these sediments were dated by single aliquot protocols, thermal-transfer methods, and UV approaches. These grains yield OSL emissions with a highly dominant fast component and light levels >10⁵ photon counts/s: ideal for a variety of dating approaches. OSL ages for the lowest sediment from the site, below the primary bone strata, have yielded ages of 75 to 80 ka, which indicates the site spans from late Marine Isotope Stage (MIS) 5a into Stage 3, with variable and episodic sedimentation rates as the climate cooled across North America.