Morphology and origin of ripple-drift cross-lamination, with examples from the Pleistocene of Massachusetts

Different types of ripple-drift cross-lamination occur on forsets of a kame delta near Concord, Mass. A morphological classification is proposed, based on end members of a sequence of forms: type A, characterized by non-preservation of stoss-side laminae, and consisting of climbing sets of lee side laminae; and sinusoidal ripple lamination (new term), a series of ripples with symmetrical, sine-wave profiles and continuous laminae across the system. One intermediate type (B) is defined between end members. Extension into a genetic classification is based on ratio of sediment fallout from suspension to sediment moved as bed load. Cross-lamination originated from density underflows of sediment-laden meltwater flowing into a glacial lake; the different types are due to small fluctuations in current velocity and variations in composition and concentration of suspended sediment.