ABSTRACT

An extensive carbonate platform fringed the North Greenland cratonic margin in the Middle Ordovician-early Silurian and was bordered by a steep, erosional bypass escarpment dipping about 45°. In Silurian times a huge, elongate, sand-rich submarine fan developed in the deep-water basin north of the platform, sourced from the rising Caledonian mountain belt to the east. Longitudinal siliciclastic turbidite deposition in the basin was punctuated by coarse-grained carbonate debris flows and turbidity currents derived from the carbonate platform. Four main episodes of carbonate sedimentation are recognized, represented by composite sheets tens of meters thick. Transport mechanisms included rock-fall, grain flow, debris flow, sandy turbidity current, and dilute low-density muddy turbidi y current.

The four conglomerate units span a period during which the height of the scarp decreased from 1300 to 0 m due to progressive infilling of the basin by turbidites. The carbonate conglomerates deposited when scarp relief was at a maximum primarily record rock-fall and viscous debris flow processes. They form persistent sheets that extended up to 50 km out onto the basin floor, and they constructed significant topographic highs that may have influenced siliciclastic turbidite dispersal patterns in the basin. The youngest conglomerate unit was deposited following foundering of the outer platform and onlap of basinal sediments to the lip of the platform. Debris from carbonate build-ups to the south flowed northwards to be deposited as thick conglomerate beds immediately north of the relict platform scarp. The conglomerates show giant load structures, pronounced pinch-and-swell, indistinct low angle "foreset" bedding, and abrupt snout-like distal wedging out of beds. Such features may have resulted from a change in flow properties on passing over the buried scarp from the lithified outer platform onto rapidly deposited water-rich sandy turbidites. The decrease in scarp height from 1300 to 0 m thus influenced transport mechanisms of carbonate gravity flows, but the overall geometry and organization of the individual base-of-slope units remained unchanged. They all have the character of debris sheets; organized systems, such as fans, are absent.