ABSTRACT

Lower Cambrian sandstone interbedded with pebble and cobble conglomerates exposed at L'Islet make up the fill of a submarine channel, 750 m in width. The channel cuts into a minimum of 88 m of thin limestone turbidite and claystone beds. Beds of the fill are lenticular, with thicknesses ranging from a few mm up to 3 m.

Four structures commonly present in the beds of the fill are graded bedding, parallel and oblique stratifications, and imbrication. Most of the beds show grading of both the maximum and mean diameters. Three sequences of structures are present in these beds; the first two occur in conglomerate, and the third in sandstone. In the conglomerate, grading may either be overlain by a division of crude parallel stratification, or by a division of cross stratification which is in turn overlain by parallel stratification. The former sequence is observed only in beds which have maximum grain diameters smaller than 20 mm, and the latter is seen only in beds with maximum grain diameter ranging from 20 to 100 mm. A third sequence, found in the sandstone beds, is a "normal" coarse-grained turbidite sequence with a graded division at the base, overlain by parallel laminations, and ending with dunes at the top.

Both conglomerate and sandstone of the fill have characteristics of sediments transported in suspension and by traction. Relatively high density (1.18 gm/cm³) turbidity currents with flow velocities of about 800 cm/sec could have transported the coarsest clasts found in the channel.