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Paleocurrent Analysis: Important Tool in Basin Analysis

John W. Buza

A paleocurrent study of the Chiwaukum graben, a major structural depression in central Washington, illustrates the usefulness of paleocurrent indicators in determining a more complete picture of a basin's geological history--beyond the more traditional uses regarding depositional environment and provenance. For the Chiwaukum graben, paleocurrent mapping has given considerable insight into the structural development of the area.

The Chiwaukum graben is bounded on the east and west by the Entiat and Leavenworth faults, respectively, and preserves a lower and a middle Tertiary sequence of fluviatile sandstones, shales, and conglomerates. Paleocurrent studies of the lower Tertiary sandstones demonstrate that the dominant paleotransport direction in the central portion of the graben was southwesterly (233°).

An integrated interpretation of paleocurrent and other data suggests that the area of the Entiat Mountains was the primary source area and a topographic high during deposition. Paleocurrent data also support the existence of exposed bedrock hills within the graben during sedimentation. Due to a lack of northeast-oriented paleocurrent vectors near the Leavenworth fault, the area southwest of the fault probably had little topographic expression and possibly was a site of deposition during at least part of lower Tertiary time. Paleocurrent analysis suggests that major relief along central and northern portions of the Leavenworth fault may be postdepositional.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.