Abstract

In contrast to many depositional environments, lakes are characterized by their variability. They are among the most diverse geologic features. Lake water ranges from pure to water with more than 31% dissolved solids. Size varies from tiny playa ponds to the Caspian (area, 436,400 km²; mean depth, 946 m; volume, 79,319 km³) or Lake Tanganyika (area, 34,000 km²; mean depth, 572 m; volume, 18,940 km³). Lake shapes may be circular, subcircular, elliptical, subrectangular elongate, dendritic, lunate, triangular or irregular. The water chemistry of lakes is so varied that it defies generalization. Similarly, biological activity shows considerable diversity, from lakes that are sterile to those choking in their (and our) wastes. Lacustrine sediment runs the gamut of clastic, carbonate, evaporite, and organic sediments.

There are two major trends in paleolimnology: (1) study of Pleistocene and Recent deposits; (2) application of standard stratigraphic techniques to lacustrine rocks, rocks that generally are older than Pleistocene strata. Such ancient lakes fall into two distinct size classes: (1) small lakes and ponds whose geological record is generally ambiguous and incomplete, and (2) large persistent lakes with well-documented lacustrine strata.

For ancient lakes in the western United States two observations on age are made: (1) as the age becomes older, there is a marked and progressive decrease in the number of lakes; and (2) there are concentrations of lacustrine successions in Triassic, Cretaceous, Eocene, and Late Tertiary/Pleistocene beds. The Tertiary was a time of continental deposition and the record is well-preserved. Younger strata are more likely to be preserved and diagenesis has been less than in older beds. Extensive lacustrine deposits record times of optimum paleogeographic and tectonic conditions for the development of large lakes.