A composite tectonic–eustatic origin for shelf sandstones at the Cambrian–Ordovician boundary in North Greenland

Abstract

Shelf sandstones deposited at the Cambrian–Ordovician boundary in central and western North Greenland have a sheet-like large-scale geometry, and extend for over 300 km along strike within a shallow marine, carbonate-dominated, platform sequence. Conodont faunas from units above and below the most extensive sand sheet, the Kap Coppinger Member of the Cass Fjord Formation and the Permin Land Formation, indicate that deposition of the sands spanned the proavus, intermedius and lindstromi conodont biozones. Sand was derived by reworking of older, quartzite-dominated formations following the uplift and unroofing of a platform bulge to the east. During the Cambrian, this sediment was transported northwards from the uplifted area across an unstable platform into a deep-water trough. However, during sea-level lowstands within the Cambrian–Ordovician boundary interval this clastic material was deflected westwards and transported across the stable platform by progradation of tide- and storm-dominated shorelines. Facies sequences in the thickest part of the Permin Land Formation suggest at least three such regressive cycles combined to form a composite sand body. Each regressive cycle comprises a shallowing upward transition from tide- and storm-deposited marine sediments to clean sandstones of marine or terrestrial origin. The cycles are separated by thin but laterally extensive dolomites interpreted to represent drowning events. Thinner sequences further to the east are attributed to reduced accommodation space as a consequence of lower subsidence rates close to the uplifted source area, whilst thin sequences in the west are ascribed to the effect of increasing distance from the sediment source so that only the youngest of the three regressive cycles is present.