Morphology and Origin of Ripple-Drift Cross-Lamination, with Examples from the Pleistocene of Massachusetts

Abstract

ABSTRACT Many different types of ripple-drift cross-lamination occur in juxtaposition on the gently sloping foresets of a late Wisconsin kame delta near Concord, Massachusetts. A morphological classification is proposed, based on the two end members of a complete sequence of forms. Type A ripple-drift cross-lamination is characterized by non-preservation of stoss-side laminae, and hence consists only of climbing sets of lee side laminae. Sinusoidal ripple lamination (new term) consists of a series of ripples with symmetrical, sine-wave profiles and continuous laminae across the ripple system. One intermediate type (B) is defined between these two end members; the type is characterized by climbing sets of lee side laminae and by preservation of sandy stoss side laminae. The morphological classi ication is extended into a genetic classification, based upon the ratio of sediment fallout from suspension to sediment moved as bed load. Type A represents dominant bed-load movement, and sinusoidal ripple lamination represents dominant fallout from suspension. The cross-lamination described in the paper originated from density underflows of sediment-laden meltwater flowing into a glacial lake. The different types of ripple-drift cross-lamination are attributed here to small fluctuations in the current velocity, and to variations in the composition and concentration of suspended sediment. It is believed that at times of high fallout from suspension, the presence of fines stabilized the lake floor, and sinusoidal ripple lamination was formed. Flume experiments on the formation of sinusoidal ripple lamination tend to confirm this conclusion.