Lake Thompson, Mojave Desert, California: A Desiccating Late Quaternary Lake System

摘要

In late Pleistocene time, Lake Thompson rose to 710 meters above sea level and covered 950 square kilometers of the western Mojave Desert, California. During the Holocene time, the lake desiccated and is today represented mainly by Rogers, Rosamond, and Buckhorn Dry Lakes, which cover 200 square kilometers of Edwards AFB. Elsewhere the former lake basin is characterized by exposed lake beds and beach ridges or mantled by aeolian and fluvial deposits. This study reports on the spatial and temporal components of former Lake Thompson. The spatial dimension identifies seven major geomorphic and lithostratigraphic units within the former lake basin, of which the most important are the modern playa, former lake system, aeolian mantle, interfingering fluvial deposits, and various bedrock outcrops. These units and their subdivisions are presented on a map entitled 'Geomorphology and Quaternary Geology of Lake Thompson within Edwards AFB, California.' The temporal component is represented by a chronology of Lake Thompson based on accelerator mass spectrometry dating of the stratigraphic sequence. Although a former deep lake beneath the modern dry lakes had long been inferred from borehole data, its age and development remained unknown. The present study recovered four cores for stratigraphic and sediment analysis and dating. Ages for the deep lake range from 30,000 to 17,000 BP, a humid interval typified by frequent inputs of fluvial sediment. After 17,000 BP, the lake began to desiccate, and its exposed floor was lowered by deflation. However, shallow perennial lakes returned during the late Pleistocene and early Holocene time, prior to the present phase of desiccation. Clay minerals from the cores support this scenario. High smectite values reflect deposition in a large lake under humid conditions around 30,000 BP, followed by diminishing smectite as conditions became drier. A more saline, alkaline lake existed under drier climatic conditions before 30,000 BP. (33 refs7).