Formation of mega-scale glacial lineations on the Dubawnt Lake Ice Stream bed: 1. size, shape and spacing from a large remote sensing dataset

Mega-scale glacial lineations (1VISGLs) are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly-flowing ice streams. Knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets, but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses. This paper presents the results of a remote sensing survey of 46,000 bedforms on the Dubawnt Lake palaeo-ice stream bed, focussing on a central transect of 17,038 that includes highly elongate bedforms previously described as MSGLs. Within this transect, lineations exceed 10 km in length (max. >20 km) and 23% have elongation ratios >10:1 (max. 149:1). Highly elongate features are interspersed with much shorter drumlin-like features, but longer bedforms are typically narrower, suggesting that their length develops more quickly than, or at the expense of, their width. Bedforms are broadly symmetrical in plan-form and have a preferred lateral spacing of 50-250 m, which implies a regular, rather than random, pattern of corrugations. Comparison with drumlins reveals that the more attenuated MSGLs simply extend the tail' of the distribution of data, rather than plotting as a separate population. Taken together, this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity, but existing hypotheses of MSGL formation are either not supported, or are insufficiently developed to explain our observations. Rather, we conclude that, under conditions of rapid ice flow, MSGLs attain their great length relatively quickly (decades) through a probable combination of subglacial deformation, which attenuates ridges, and erosional processes that removes material from between them. (C) 2013 Elsevier Ltd. All rights reserved.