A tempestite-stromatolite-evaporite association (late Vendian, East Greenland): a shoreface-lagoon model

Abstract

Sections of the Canyon Formation (310 m) and overlying Spiral Creek Formation (25 m) have been studied on Ella Ø (Ella Island), central East Greenland. These strata cap Vendian glacial deposits and are overlain disconformably by probable basal Cambrian sandstones. The Canyon Formation is informally divided from bottom to top into mudrock (250 m), siltstone (15 m), flakestone (25 m) and stromatolitic (27 m) members. Overlying a thin basal sandstone interpreted as littoral facies, the main bulk of the mudrock member consists of variably dolomitic silt-shales (offshore deposits below wave base). Bituminous films in the upper half of the member are thought to represent microbial mats. The overlying units represent a major shallowing-upwards sequence. The siltstone and flakestone members consist largely of sharp-based coarse dolomitic siltstones (0.05-0.6 m thick) containing combinations of: gutter casts, parallel and undulatory lamination, wave-ripple lamination and flexed dolostone intraclasts. A tempestite origin is indicated. Dolostone clasts in the flakestone member are analogous to shell fragments in Phanerozoic tempestites and are derived by reworking of thin dolomite crusts within the background sediments of the flakestone member. There is a transitional boundary to the overlying stromatolitic member which consists dominantly of finely crystalline dolostones with thin (<0.05 m) storm layers and stromatolite biostromes and bioherms. The lower half of the member lacks evidence of desiccation, but does contain subaqueous shrinkage cracks. The upper half contains a number of scoured and desiccated dolostone surfaces, abundant rigid intraclasts and several thin oolitic beds. Stromatolite morphology is characteristically broad (metre-scale), fairly smooth domes, whose synoptic relief tends to decrease upwards in individual beds and overall in the member. Scattered anhydrite crystal and nodule pseudomorphs are common, varying in abundance at different horizons. The Spiral Creek Formation contains a basal cross-stratified sandstone, with desiccated dolostone layers, interpreted as a tidal sandflat deposit. The central part of the Spiral Creek Formation consists of centimetre-scale alternations of dolomitic sandstones with short-wavelength wave ripples, and mudrocks with desiccation cracks and halite pseudomorphs interpreted as playa lake deposits. The overall facies assemblage above the mudrock member is interpreted as representing the progradation of a mudrock-dominated evaporative lagoon-sandflat-mudflat-playa assemblage over a shoreface dominated by storm action. The lagoon may have been partly barred by stromatolites, but is seen to be transitional with the shoreface. This requires a new facies model in which muddy coastal and lagoonal carbonate facies are not bounded seaward by a continuous reef or sandbody. This is indicative of low wave energy, except during infrequent storms, and a low tidal range, so that strong tidal currents are only locally developed.