Large fluvial systems: the Atane Formation, a fluvio-deltaic example from the Upper Cretaceous of central West Greenland

Abstract

Recognition of major river deposits are crucial to the understanding of palaeogeography and basin evolution. This paper describes the geometry of 2–30 m thick fluvial sand-bodies which occur in Upper Cretaceous strata deposited in a major river delta in central West Greenland. The deposits are cyclic and each cycle can be divided into four genetic facies associations which reflect deposition: (1) at the delta-front, (2) on the delta-plain, (3) in fluvial channels, and (4) during transgression of a delta lobe. The cyclicity is attributed mainly to autocyclic shifting of delta lobes. The study area is very well exposed and an advanced, new photogrammetric system was used both for making accurate vertical profiles and for mapping the horizontal extent of individual sand-bodies.Based on horizontal extent, internal architecture and relationship to other facies associations, the c. 100 investigated sand-bodies from the fluvial channel association can be divided into two important types. Distributary channel sand-bodies are the dominant type (80%). They vary in thickness from 3 m up to as much as 30 m and reflect the migration and infilling of rather narrow distributary channels that existed on a flat, vegetated delta-plain. The relationship with the surrounding sediments as well as faint epsilon cross-beds which extend right through the sand-bodies indicate that the thicknesses of the sand-bodies approximate the bankfull depth of the palaeochannels. Many small and medium-sized sand-bodies have very narrow cross-sections and show no signs of lateral migration. The preserved geometry of these sand-bodies consequently represents the original channel morphology. The sand-bodies show an increasing degree of migration with increasing thickness and the width/thickness ratio increases from c. 6 for the smaller sand-bodies up to c. 20 for the largest. The distributary channels were only moderately sinuous with an average sinuosity of 1.2.Channel mouth complexes are the other type of fluvial sand-bodies (20%). They are 3–8 m thick and have a large lateral extent with a width/thickness ratio of 50 or more. They have an erosive base and occur in the top of the coarsening-upward sequences made by the delta-front association. Internal bounding surfaces suggest that these sand-bodies were generated by lateral amalgamation of many small channel fills in fan-shaped sub-deltas at the sea-ward end of distributary channels.The discharge through the distributary channels has been calculated using the Manning equation. These calculations indicate that the river system which supplied the Atane Delta was very large, and may well have drained most of the presently ice-covered interior of Greenland.