Abstract: Freshwater Oncolites, Onondaga Lake, New York

Walter E. Dean, Jane R. Eggleston

Onondaga Lake is a moderately saline, eutrophic lake characterized by waters rich in calcium, sodium, chloride, and bicarbonate. Large quantities of calcium carbonate precipitated in the lake result from input of excess calcium from calcium chloride wastes produced by soda-ash manufacturing. Beaches along the leeward (southeastern) side of the lake are composed almost entirely of oncolites ranging from a few millimeters to several centimeters in maximum dimension. Offshore, in 1 to 2 m of water, the oncolites are biscuit-shaped concretions as much as 15 cm in diameter. The oncolites are composed mainly of low-magnesium calcite, but dissolution of the carbonate with dilute acid results in a mass of blue-green algal filaments of the same approximate size and shape as the or ginal oncolite. Scanning electron micrographs (SEM) reveal that growth of the oncolites is through trapping and binding of sediment particles (mainly calcium carbonate) by mucilaginous sheaths of blue-green algae and later cementation by calcite. SEM observations reveal further that the moot common nucleus is the hollow stem and cortication tubules of the calcareous green alga, Chara (stonewart). Chara is not found in Onondaga Lake today although it is very common in other hard-water lakes in central New York State. Chara most likely was eliminated by the marked increases in salinity of the lake that resulted from the introduction of soda-ash manufacturing on the lake shores in the 1880s. This suggests that growth of the oncolites began at least 100 years ago.

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