PALEOHYDROLOGIC IMPLICATIONS OF LATE PLEISTOCENE AGE SHORELINE AND SPRING-COMPLEX DEPOSITS IN WILLCOX BASIN, SOUTHEASTERN ARIZONA

Fossil shoreline features in closed basins of arid regions record the surface area of ancient lakes, reflecting changes in hydrologic balance from one lake cycle to the next. Records from the northern Great Basin (GB) indicate relatively high lake levels during the last glacial maximum (LGM: 24-19 Ka), and that unprecedented levels were reached from 17.5-15.5 Ka during the Heinrich 1 stadial. Competing hypotheses about the causes of these cycles hold that southern Basin and Range (SBAR) lakes experienced their highstands during the LGM, rendering shoreline chronologies from that region of critical importance to improving current understanding of regional atmospheric dynamics during the last glacial-interglacial transition.

In southeastern Arizona, a highstand of Lake Cochise formed a large beach ridge in Willcox Basin (WB) from 17-16 Ka. Recent chronostratigraphic investigations of shell-bearing deposits buried beneath and inset into the ridge constrain their emplacement to ≥19 Ka and from 16-13 Ka or later, respectively. It is possible that the lake reached the same elevation repeatedly, either due to equivalent levels of effective moisture or a lack of geohydrologic closure, although the presence of higher shorelines dismisses the latter scenario. Alternatively, these are not lake deposits; some of these units resemble deposits in southern Nevada, at latitudes well below the most southerly pluvial lakes in the GB. Previous research on the sedimentology, fauna, and geochemistry of those deposits has revealed their origins in groundwater discharge environments, suggesting the same for deposits in WB. However, the periodic presence of a lake in WB complicates the task of differentiating lacustrine from spring-related deposits—given the depositional complexity that characterizes the interface between these two distinct environments in modern analogues. Given the similarity in appearance between these and other “shoreline” deposits in closed basins throughout the SBAR and in other arid regions worldwide, resolution of this problem has profound implications for understanding the global hydrologic cycle during the last deglaciation. We examine evidence from recent investigations aimed at making this critical distinction as well as the regional paleoclimatic significance of these findings.