Abstract

Ripple marked surfaces with the same surface expression as rib-and-furrow marks are found in the Chugwater Formation of Wyoming, but the paleocurrent directions obtained from these two current direction indicators are mutually perpendicular. These pseudo rib-and-furrow marks are apparently the result of truncation of ripple marks in which the crests of the laminae forming the ripple mark and the crest of the ripple mark are not parallel. The mechanism (or mechanisms) which produced this angular difference is unknown. Observations of the Chugwater structures suggest that rib-and-furrow marks and pseudo rib-and-furrow marks originate in several ways, and that their usefulness as a paleocurrent direction indicator depends on their mode of formation and their relationship to other paleocurrent direction indicators.

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