Paleocurrent Directions from Two-Dimensional Exposures of Cross Laminae in the Devonian Flysch of Maine

Abstract

A common problem in paleocurrent analysis in flysch sequences arises when the strike and dip of cross laminae cannot be measured, even though a general paleoflow direction is evident from two-dimensional, cross-sectional exposures. A new method of paleocurrent analysis is described which makes use of these imperfectly exposed sedimentary structures. It is assumed that younging direction and the attitudes of bedding and fold axis are known; and that the cross laminae within a coset all appear to dip in the same direction. The line of intersection between the outcrop surface and the cross lamina of unknown attitude is measured in the field. A simple stereographic manipulation yields two numbers: an estimated paleocurrent direction corresponding to the tilt-corrected apparent dip direction of the cross lamina, and an uncertainty, normally in the range of $$\pm 60^{\circ}$$ and never greater than $$\pm 90^{\circ}$$. All paleoflow directions within this range correspond to potential cross laminae with acceptable initial dips. Data are plotted on a modified rose diagram. Although individual readings may give misleading results, a large sampling adequately replicates the vector mean paleoflow direction determined from true dip directions. This conclusion is based on (1) a field study of cross laminae in Devonian turbidites in Maine, which yielded comparable paleocurrent directions using the true dip and apparent dip methods; and (2) application of the apparent dip method to cross beds of known attitude intersected by random surfaces of inspection. The new method should prove valuable in areas where traditional paleocurrent studies have been difficult or impossible owing to a lack of three-dimensional exposures of cross laminae.