ABSTRACT

Climbing ripples in Quaternary lake silts in the glaciated part of Nebraska are defined by continuous laminae over some of the ripple systems, but more commonly, by laminae that are discontinuous and, in part, cross-laminated. Laminae on stoss sides of the climbing ripples are formed of trough sets of micro-ripple-drift, which except for their smaller size, are similar to erosional Type A and gradational Type B and S sets of ripple-drift described by Jopling and Walker (1968). Most of the sets are erosional Type A micro-ripple-drift cross-lamination with heights between two and 10 millimeters. Gradational sets of micro-ripple-drift usually occur on the upper three-quarters to two-thirds of the stoss side where parallel laminations pass laterally downcurrent into Type S, then Type B, a d finally, into Type A micro-ripple-drift. Patterns of micro-ripple-drift on the backside of the ripples are indicative of ripple fans, which migrated up the stoss side of the larger straight-crested ripples and cascaded clown the slipface that is made up of alternating coarse-fine laminae. The size of the larger ripples ranges up into the dimensions of dunes and down into the dimensions of ripples with no apparent hiatus in scale. These dimensions for ripples have not been reproduced in flume experiments using silt size sediment (Southard and Harms, 1972). The scale of the larger bedforms, therefore, is believed to be the result of a difference in scale for "ripples" and "dunes" because of the smaller grain size and/or the result of rapid disequilibrium deposition. Patterns of micro-rip le-drift indicate a movement of fluid over the larger ripples like that predicted by Allen (1968) for flow patterns over straight-crested ripples with grains transported and deposited froth bedload and suspension.

The angle of climbing displayed by the larger ripples is defined by the thickness of sets of micro-ripple-drift preserved on the stoss side and the thickness of sediment added to the slipface from avalanching of grains, cascading of micro-ripples over the brinkpoint, and deposition from suspension. Thickness of sediment added to the slipface contemporaneous with migration of a single set of micro-ripple-drift cross-lamination over the stoss side to the brinkpoint, is the major factor that determines the angle of climbing of the larger ripples. This relationship implies that the downcurrent climbing of these ripples is significantly influenced by the rate of migration of micro-ripples over the stoss side onto the slipface and the amount of sediment supplied to the stoss side and slipfa e by bedload sediment transport and from suspension.

The ripple-drift cross-laminated silts were deposited by density currents (under flow) in a small shallow lake, perhaps no more than 7 m deep, that existed for only a few years. Topography on the lake floor was irregular and mounds of gravel may have reached the lake surface in a few areas. Silt layers of varves deposited over these mounds show changes in thickness, type of ripple-drift cross-lamination, and changes in ripple shape which probably were caused by increased flow or the shallow water depths associated with the larger mounds. Cyclical repetitions in the steepness of climbing of ripples in the silt "summer" layer of the basal varve couplet indicate a simple-harmonic variation of local flow velocity with time. Such simple fluctuations characterize the diurnal runoff from gla iers today. In the early Pleistocene, similar diurnal variations in meltwater added to the lake from nearby glaciers could have significantly influenced underflow on the lake floor.