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Ice Age Research Program, Museum of the Rockies, Montana State University, Bozeman, MT, 59717

Abstract: Lithostratigraphy and Stable Isotopes in a Pleistocene Basin: Sedimentary and Geochemical Responses to Climate Change

Changes in the sedimentology and stable isotope record of a Pleistocene-age basin situated in Saharan North Africa appears to reflect paleoclimatic fluctuations. The sequence may reflect last interglacial pluvial (wet) episodes alternating with intervals of aridity.

Stratigraphic exposures show facies variation between margin and central basin deposits. Basal sediments composed of sands and gravels are overlain by a paleo-inceptisol. Sands appear to have washed into the basin at the beginning of a wet interval. The sands are overlain by gyttia-like sediments. An interval of aridity is reflected by desiccation cracks. A series of marls and silts above these deposits indicate the presence of a small lake during a pluvial interval. The marls contain gastropods (Melanoides). Overlying, sands are interpreted as indicating drier conditions. The top of the sequence contains silts which appear to reflect another wet interval.

Stable isotope records for oxygen and carbon from authigenic carbonates (marls and gastropods) exhibit distinct trends which can be correlated with the lithostratigraphy. In the lower part of the sequence oxygen and carbon isotope values increase as marls begin to dominate. The values show a gradual change throughout the marl sequence consistent with evaporative conditions connected with increasingly and paleoclimatic conditions. The silts in the upper part of the sequence have higher isotopic values than the interbedded sands. The increase in isotopic values in the upper silts is compatible with the presence of less evaporative and wetter paleoclimatic conditions. Thus, sedimentologic and geochemical records can be used to interpret changing paleoenvironmental conditions within depositional basins and may help to infer paleoclimate change.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana