Abstract

About 10 meters of probable Oligocene or Miocene lacustrine sediments which crop out on top of Sentinel Butte in western North Dakota consist mostly of greenish-gray zeolitic mudstone interbedded with relatively thin layers of dolomite, limestone, and chalcedony. The dolomite layers are several centimeters thick and are mostly dolomicrite, with laminae or lenses of arkosic sand and pellets. They probably formed by deposition of protodolomite in shallow, closed, saline lakes. The limestone layers are up to 30 centimeters thick. Their basal portions contain varve couplets of micrite and organic matter. Graded bedding in micrite laminae and concentration of aeolian silt in organic laminae suggest that deposition of carbonate took place during a relatively brief portion of each year. Preservation of varves, fossil fish, and coprolites in the limestone imply deposition in a meromictic lake. In contrast, poor preservation of fish and bioturbation in the upper parts of two limestone beds record transition to holomictic conditions. Some variations in varve thickness appear rhythmic and possibly related to the sunspot cycle. Several asymmetric cycles of contrasting carbonate facies, ideally consisting of massive dolomite overlying clay and underlying laminated limestone, imply that the ancient lake in which the sequence was deposited underwent several episodes of shrinkage and closure during which its salinity increased, followed by expansion accompanied by abrupt development of ectogenic meromixis. Changes in authigenic mineralogy and upward increase in thickness of limestone beds relative to dolomite beds record a long-term trend toward freshening of basinal hydrology. Models involving ectogenic meromixis can explain intimate associations of evaporitic facies with varved sediments in other sedimentary sequences and may be applicable to some Precambrian iron formations.

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