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The Origin of a Late Pleistocene and Holocene Marl Deposit
P. Michael Terlecky, Jr.
A late Pleistocene to Holocene, non-marine carbonate deposit was studied in the Rochester, New York, area to ascertain its origin and the environmental changes that affected the basin during the last 11,000 years. The deposit approaches 95% CaCO3 and occupies 1.5 mi2; it is not indurated for the most part and consists of fine-grained 1 to 3 µ calcite. Cortication tubules, oogonia, and encrustations around the alga Chara vulgaris are also present.
The origin of this marl deposit is attributed to the physicochemical precipitation of calcite in shallow pond waters caused by the loss of CO2 to the atmosphere and to photosynthetic activity following the removal of ice cover and the onset of physical and thermal agitation. A minor role is assigned to direct biochemical precipitation. Springs supplying water to the basin are supersaturated with respect to calcite; a pond in the area had an increase of from 1.4 to 5.4-fold supersaturation with respect to calcite from January to April. Simultaneously, pH values rose from 7.5 to 8.0. Springs in the area supply Ca+2 and SO4-2 rich waters to the basin from limestone and gypsiferous-shale bedrock.
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