Marine debris flows: Late precambrian “tillites” of the Avalonian-Cadomian orogenic belt

Abstract

Correlative Late Precambrian strata of the Avalonian-Cadomian orogenic cycle, deposited after 670 and before 550 Ma are now widely dispersed around the margins of the North Atlantic. These contain several prominent diamictite horizons, long interpreted as “tillites”, within thick turbidite successions associated with volcanic horizons. The Granville Formation of the Armorican Massif in Normandy, France contains many pebby mud (diamicite) beds up to 20 m thick that have been interpreted as glaciogenic. Regionally, the Granville Formation occurs within the lowest part of the Upper Brioverian Supergroup which consists of several kilometres of turbidites and volcanogenic rocks. At Granville, diamictites can be traced along striked into deformed and intermixed graded conglomerates and mudstones. Diamictites are identified as debris flows generated on a submarine slope by the emplacement of gravel turbidites on a mud substrate and their mixing during downslope slumping. A glacial influence cannot be identified. In North America, the Conception Group of the Avalon Peninsula, Newfoundland, Canada, contains nearly 4 km of turbidites with a conformable sequence of diamictites (the Gaskiers Formation) near its base Massive and crudely-stratified diamicite beds (< 25 m thick) have a tabular geometry and are interbedded with thin-bedded, fine-grained turbidites. Glacially-striated and faceted clasts identify a previous phase of glacial transport but clasts and matrix are dominantly of volcanic origin; air-fall bombs are present. Diamictites were deposited as marine debris flows; associated “disrupted” diamicite sequences are identified as slump deposits. The diamictites record downslope remobilization of large volumes of unstable volcanic and glacial debris; restricted glaciation of volcanic cones along a complex island-arc is suggested. The diamictite successions of the Granville and Gaskiers formations provide significant data regarding facies types, associations and the geometry of resedimented diamictite sequences in deep marine basins along active plate margins. Similar deposits at the same tectonic-stratigraphic horizon occur in New England and Africa. This paper emphasizes the difficulty of identifying a direct glacial influence on diamictite sedimentation in such settings and the need for a sedimentological reappraisal of other supposed late Precambrian “tillites” preserved in orogenic belts. This is a necessary first step prior to modelling late Precambrian paleoclimates.