Abstract: Sedimentology of Late Quaternary Mammoth-Bearing Sinkhole Deposits, Hot Springs, South Dakota

Robert L. Laury, Larry D. Agenbroad, Barbara L. Dutrow

Thinly laminated very fine-grained sands, silty sands, and clayey silts bearing an exceptionally rich mammoth fauna are interpreted as the fill of a steep-walled sinkhole. Sedimentation which occurred in a spring-fed pond shows progressive upward shallowing. An indigenous aquatic fauna of ostracods, mollusks, and (rare) fish is preserved in the sediments. Bottom waters were oxygenated and supported an infauna of worms. Except for a gravel facies which veneers the walls of the sinkhole, most of the fill sediments probably entered the pond through artesian-spring feeders. Spatial distribution of sedimentary facies shows that one of the two identified spring conduits supplied most of the sediment. The gravels, which interfinger with the finer pond sediments, represent a marg nal and probably earlier facies derived by colluvial reworking of proximal upland fluvial-terrace deposits.

Most of the mammoth and associated megafauna were trapped during the early to medial history of pond sedimentation when the marginal pond floors had dips of 30 to 40°. Sediments deposited on those steep slopes commonly display deformation structures (convolute laminae) ranging in magnitude from individual mammoth tracks to more massive flows. Sands deposited near the most active spring feeder show intense micro-faulting, slumping, and disruption related to, respectively, dewatering adjustments, spring undercutting, and animal-generated spontaneous liquefaction of the "quick" sands. Spring activity and pond sedimentation probably ceased when fluvial downcutting in the Fall River watershed substantially lowered the regional water table.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma