Paleoenvironment of a late Quaternary mammoth-bearing sinkhole deposit, Hot Springs, South Dakota

摘要

Mammoth-bearing laminated sediments fill a steep-walled sinkhole which formed 26,000 yr ago in the southern Black Hills of South Dakota. The depression formed as a collapse feature over a solution breccia pipe. Artesian water issuing principally from one marginal spring quickly established a lake in the sink and thereafter maintained an energy gradient which affected sediment dispersal. Sediments were supplied to the pond by storm runoff from proximal uplands and by spring erosion of sinkhole walls. Three successive and gradational phases of sedimentation are recognized in the sinkhole: (1) relatively rapid deposition of poorly sorted gravels and sands as predominantly subaqueous talus accumulations adjacent to near-vertical sinkhole walls, and concomitant sedimentation of micrograded sands and silts in the central pond area; (2) slowing of sedimentation rates and widespread deposition of finer grained, rhythmically laminated, but not varved, sands and clayey silts; and (3) progressive and fairly rapid reduction in spring discharge and water depth, ending pond sedimentation. Renewed downcutting of major streams in the area and synchronous decline of regional ground-water tables terminated spring discharge to the sinkhole. Paleontological and other indirect evidence strongly suggest that the sinkhole pond was fed by heated springs which maintained a year-round temperature of at least 35 °C (95 °F). Although warm water may have attracted a variety of megafauna to the sinkhole, only mammoths were trapped there. When in the water, mammoths deformed pond strata and generated several types of sedimentation events. Pond depths probably did not exceed 4 to 5 m except in the spring conduit area.