Criteria For Recognizing Lacustrine Rocks1

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Abstract
Lacustrine deposits can be recognized through the use of a variety of criteria that include physical, chemical and biologic aspects of sedimentary rocks. Because large lakes closely resemble shallow, epicontinental seas in physical properties, large differences in lithologic characteristics, sequences, facies relations, sedimentary structures, paleocurrent patterns and other physical parameters are not to be expected. Comparisons of these features in lacustrine and epicontinental rocks indicate the scarcity of significant diagnostic differences. In contrast, fluvial and lacustrine environments differ considerably in physical aspects, and the two environments can be differentiated on the basis of the physical characteristics of the rocks. Large lakes and epicontinental seas differ mainly in size, chemistry and biota. Size differences are evident: few lakes exceed 10,000 sq. mi. in area. Regional stratigraphic and paleogeographic relations are useful therefore in distinguishing former lakes and seas. Geochemical differences are more definitive, however. Normal sea water has been relatively constant in composition for most of geologic history and changes related to evaporation, precipitation or dilution are predictable. In contrast, the chemistry of lacustrine water is not uniform, but is determined almost wholly by the lithology and climate of the hydrographic basin. The geochemical character of lakes therefore differs widely from one area to another and rarely approximates that of sea water. Accordingly, lakes and seas can differ in authigenic and diagenetic minerals. The products of evaporite cycles are especially useful for environmental interpretation. Marine and nonmarine environments commonly are distinguished by their faunas. Paleontologic differentiation of nonmarine environments is uncertain, however, and requires further study. Despite the qualifications mentioned here, reconstructions of lacustrine environments can be made with confidence. Lacustrine or marine environments can be differentiated from fluvial environments on the basis of physical properties of the deposits. Lakes and seas can then be differentiated by geochemical and biologic criteria.

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