THE ARCHITECTURE OF PRECAMBRIAN HIGH-ENERGY TIDAL CHANNEL DEPOSITS - AN EXAMPLE FROM THE LYELL LAND GROUP (ELEONORE BAY SUPERGROUP), NORTHEAST GREENLAND

Within the 2.5 km thick Late Proterozoic Lyell Land Group of the Eleon ore Bay Supergroup in East Greenland, numerous successions of stacked sheet sandstone bodies, 10-70 m thick occur. Each succession consists of several, 5-50 m thick multistorey sand sheets, separated from each other by 0.2-2 m thick heterolithic mudstone, or sandstone beds. indiv idual sand sheets are 0.2-3.0 m thick. They have highly erosive lower contacts, show a rough fining-upward trend and are capped by a thin mu dstone veneer containing desiccation cracks. Dominant sedimentary stru ctures within the sand sheets are sets of planar cross-bedding showing a range of characteristic tidal features, such as successive bundles of foresets, herringbone cross-stratification, ebb and flood caps and numerous reactivation surfaces. Horizontal lamination is abundant and may in some instances comprise more than half of the structures within a sand sheet. The sand sheets represent high energy tidal channels, w ith the multistorey sand sheets constituting larger tidal channel comp lexes. The heterolithic beds represent mixed tidal flat sediment, depo sited during periods of tidal channel complex abandonment, probably ca used by channel aggradation and changing drainage patterns. The tidal channels were wide, shallow and very sandy, with a very poorly develop ed channel morphology, caused by lack of fine-grained sediments and ve getation: Most likely they evolved within a meso- to microtidal, sandy back-barrier setting where the channels migrated swiftly across tidal flats and reworked most interchannel sediment. An architecture was de veloped consisting of stacked sand sheets interbedded with rare hetero lithic mudstone beds. The model presented incorporates sedimentary par ameters and processes characteristic of a pre-vegetative landscape and it has no direct modern analogue.