LAKE SHORELINE EVIDENCE OF HYDROLOGIC CONDITIONS IN THE SOUTHERN BASIN AND RANGE PROVINCE DURING THE LATE PLEISTOCENE AND EARLY HOLOCENE: PALEOCLIMATIC AND ARCHAEOLOGICAL IMPLICATIONS

Paleolake shoreline deposits throughout the southern Basin and Range (SBAR) signify past intervals of steady-state climatic conditions and possibly regional landscape stability during the Pleistocene-Holocene (P-H) transition, while associated surface artifact scatters provide evidence for Paleoindian and Archaic occupations of lake margin environments in now-dry basins. Inexact knowledge about the age of fossil shoreline deposits—due to ¹⁴C related uncertainties and incomplete dating of shorelines—has left great uncertainty about regional climatic and surface hydrologic conditions during the period of interest. Several studies collectively reveal that lake levels fluctuated several times during the P-H transition. Lake Estancia highstands occurred from ~12.9-11.8 Ka and by ~10.0 Ka to >9.3; lowstands from ~11.5-11.0 Ka or later; and complete desiccation by 9.3 Ka. Shoreline evidence from the Willcox Basin indicates two highstands of Lake Cochise prior to 13 Ka and, correlating to Estancia, one from ~12.9-12.5 Ka, and by 8.0 Ka, Lake Cochise had risen to nearly the same level for an unknown duration. The shorelines of Lake Cloverdale have been dated to the LGM and late Holocene, with none dating to the P-H transition interval. In the Playas Valley (this study), shorelines constrain a highstand of unknown duration beginning by 12.2 Ka, dropping to lowstand levels by 11.8. The relative dearth of fossil shorelines deposits in the SBAR dating to the P-H transition period suggests that conditions then were drier than during the LGM and subsequent period of deglaciation (~20-13 Ka), when several of these lakes experienced highstands. Better control on the timing of late Pleistocene highstands could reveal whether early Clovis immigrants arrived when lake levels were full, completely desiccated, or somewhere in between. A possible Younger Dryas (YD) lowstand at Estancia, if confirmed, would mean that early Clovis people in the SBAR had access to lake marginal resources during the latter half of the YD—prior to complete desiccation by the terminal Pleistocene and the reemergence of permanent lakes in the early Holocene.