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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 53 (1969)

Issue: 3. (March)

First Page: 736

Last Page: 736

Title: Lacustrine Criteria: ABSTRACT

Author(s): M. Dane Picard, Lee R. High, Jr.

Article Type: Meeting abstract

Abstract:

Although among the least abundant of depositional environments represented in the geologic record, lakes exceed their limited occurrence in interpretive significance. Lacustrine rocks yield important information concerning paleogeography, paleoclimatology, and tectonics. Therefore geologists should seek to expand their ability to identify and interpret lacustrine environments.

Modern lakes are present in a variety of geologic settings, but most lakes are too small and ephemeral to be geologically significant. Only large, long-lived lakes, such as those in structural basins with interior drainage, can accumulate major lacustrine sedimentary sequences. The recognition of lacustrine rocks requires a variety of techniques. Inasmuch as unique criteria generally are lacking, the geologist must infer origin from scattered, commonly unrelated, and often contradictory data.

Lakes closely resemble shallow, epicontinental seas in physical properties. Large differences, therefore, in lithologic characteristics, sequences, facies relations, sedimentary structures, paleocurrent patterns, and other physical aspects should not be expected. Comparisons of these features from lacustrine and epicontinental rocks indicate the Previous HitscarcityNext Hit of significant diagnostic differences.

Large lakes and epicontinental seas differ mainly in size and chemistry. Size differences are evident; few lakes exceed 10,000 sq mi in area. Thus, regional stratigraphic and paleogeographic relations can be used to differentiate lakes and seas. Geochemical differences, however, are more definitive. Normal seawater has been relatively constant in composition for most of geologic history and changes related to evaporation, precipitation, or dilution are predictable. On the other hand, the chemistry of lacustrine Previous HitwaterTop is not uniform, but is determined partly by lithology and climate in upland source areas. The geochemical balance of lakes, therefore, varies widely in different areas and rarely approximates that of seawater. Accordingly, lakes and seas can differ in authigenic and early diagenetic minerals. Especially useful are evaporite cycles.

Marine and nonmarine environments commonly are distinguished readily by their fauna. Paleontologic differentiation of nonmarine environments is uncertain, however, and requires further study.

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Copyright 1997 American Association of Petroleum Geologists