Tilloids in the West Congo geosyncline

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Abstract
In the West Congo geosyncline, which stretches from Gaboon far into Angola, the Lower and Upper Tilloid Formations extend continuously over more than 100 000 square kilometres. Of Late Pre-Cambrian age, they are most probably equivalent to the Grand and Petit Conglomérats of Katanga. They have generally been thought to consist of tillites and periglacial beds, but detailed study in north-west Angola has led to their reinterpretation as submarine mudflow deposits. The tilloid formations are generally well-bedded sequences consisting of tilloids (till-like pebbly mudstones and greywackes), conglomerates, greywackes, mudstones, quartzites, and limestones, with transitional rock-types. The tilloid clasts comprise angular to rounded intrabasinal fragments eroded from preceding strata, well-rounded extrabasinal crystalline stones, and (in the Lower Tilloid Formation only) debris from intraformational spilite lavas. Graded bedding occurs, sometimes in the tilloids and frequently in the quartzites and greywackes. Each formation marks the beginning of a sedimentary cycle within the West Congo geosyncline major cycle, and was laid down in a sharply subsiding basin after emergence and slight erosion of the preceding beds. Each is followed by rhythmically bedded turbidite formations. Tilloids at other levels in the West Congo System are also described. In each formation marine sediments are intercalated among tilloids, ruling out the possibility of a continental origin. Lithological, stratigraphical, and other evidence is over-whelmingly against a glacio-marine origin. It is suggested that the tilloids were deposited from mudflows formed by the slumping of unconsolidated sediment on the shelf, incorporating mud, sand, gravel, and stones up to large boulder size. This material, of both intrabasinal and extrabasinal provenance, was thoroughly mixed as it flowed far into the geosyncline to be redeposited as tilloid. It is proposed to distinguish ‘flow tilloids’ of this kind from ‘slide tilloids’, less well mixed and smaller in size, formed by less mobile submarine slides. It is thought that they reflect a special tectonic environment of deposition (abrupt subsidence of the geosyncline with only slight elevation of the adjacent land areas) which gave rise to unusually dense turbid flows; tilloid deposition is thus seen as a special type of geosynclinal turbidite sedimentation. Flow-tilloid formations appear to be characteristic of Late Pre-Cambrian geosynclines. The acceptance of a non-glacial origin for the West Congo tilloids would necessitate the re-examination of the evidence for a Late Pre-Cambrian glaciation.