Depositional Mechanics of Atypical Turbidites, Cloridorme Formation, Gaspé, Quebec

Abstract

<p> Thick turbidite beds, belonging to the β1 member, Cloridorme Formation (Ordovician), are exposed on the wavecut platform between St. H‎élier and Fame Point, Gasp‎é, Quebec. The local paleocurrent direction (to the west) is approximately parallel to the east-west strike of the vertical beds.</p> <p> These thick turbidite beds display a sequence of sedimentary structures which differs from the sequence defined by Bouma. Three broad divisions are recognised: a basal division consists of either limestone or quartz granule to pebble conglomerate (0-4 cms thick) or coarse sand greywacke or calcareous wacke (0-15 cms thick). Basal divisions of calcareous wacke frequently display ripple-lamination, parallel lamination, or upstream inclined laminae. Where the upstream inclined laminae form a single set, they occur below a sinuous profile (wavelength 40-80 cms, and amplitude 2-5 cms).</p> <p> A second division (0-330 cms thick) consists in most places of spindle or globular shaped calcareous nodules scattered in an argillaceous matrix. In some beds streaking and lobing of light coloured carbonate bearing material is associated with these nodules. The upper division consists of fine grained siltstone and shale.</p> <p> Thick broken blocks which have fallen from the cliffs, display the internal characteristics of the second division. Hemi-ellipsoid structures, arranged en echelon and convex towards the base of the bed, are displayed by one block.</p> <p> The upstream inclined laminae in the basal divisions of calcareous wacke beds are interpreted as the result of the upstream migration of antidunes. The nodules within the second division developed as 'pseudonodules'. The hemi-ellipsoid structures resemble damped large scale (macroturbulent) eddies associated with the flow of dense grain dispersions. The directions of the internal motions are shown by the orientation of grains and graptolite stipes.</p> <p> Correlation of these beds has been achieved over a distance of 7-1/2 miles. Over this distance the beds show little change in their characteristics. Basal divisions of granule and pebble conglomerate persist over this distance and show that coarse particles may be transported by turbidity currents over long distances. The sedimentary structures of the basal divisions of several calcareous wacke beds give the appearance (probably misleading) of an increase in flow regime downcurrent.</p> <p> The beds were probably deposited from initially low concentration but high density turbidity currents accompanied by a period of coarse grain traction and suspension near to the bed. In the case of calcareous wacke beds this period of traction formed rippled, flat or antidune bed forms. Stratification has been preserved by the rapid deposition of sediment en masse from the subsequent high concentration body of the current. The formation of a succession of 'quick' beds led to the sedimentation of the second division.</p> <p> Calculations suggest that the currents moved over slopes of much less than 1°.</p>