A Theoretical and Experimental Study of Climbing-Ripple Cross-Lamination, with a Field Application to the Uppsala Esker

Abstract

The angle of climb of ripples in this widespread structure depends theoretically on the net rate of deposition on the bed, the bed-load transport rate in the absence of net deposition, and a deposition rate difference between upstream and downstream ripple faces. Theoretically, too, the local size-sorting in cross- laminated deposits becomes less marked as the angle of climb increases. Again theoretically, with increasing steepness of climb, the internal structure of the deposit changes from cross-laminae filling erosional troughs to laminae that are very continuous laterally. These various predictions are broadly confirmed in a series of laboratory flume experiments covering a 300-fold range of the angle of climb. Additional confirmation of the prediction concerning local size-sorting is obtained from measurements made on cross-laminated sands in the Uppsala esker at Löfstalöt. Making use of a further theoretical relationship enabling mean flow velocity to be calculated from sediment mean size, rates of sediment deposition were estimated for beds with climbing-ripple cross-lamination in the Uppsala esker. The calculated rates are mainly of the order 10^-3 to 10^-2 g cm^-2 s^-1. The flows depositing the beds appear to have varied in discharge on a time-scale varying between several hours and a few tens of hours. Such a periodicity is consistent with what is known of discharge fluctuations in modern glacial streams. The analysis of climbing-ripple cross-lamination on these lines may in future be helpful in improving our understanding of processes important in geomorphology and geology.