ABSTRACT

Detailed study of a flysch sequence provides information on the variability and spatial relations of characteristic sedimentary structures, on the geometry of beds, and on the dynamics of the depositing currents. Excellent exposures of the Middle Ordovician Cloridorme Formation in northern Gaspe Peninsula permit correlation of individual beds up to 7.5 km and well defined units up to 10.5 km in the direction of current flow.

Flutes and grooves are the most abundant directional structures. The orientation of grooves is more variable than that of flutes in vertical section (S.D. of ± 30-37° vs ± 19°) and laterally within individual beds (± 28-31° vs ± 11-18°). The variability of direction indicators is greater in graywackes than in either finer-grained or more calcareous rocks and is greater in vertical section than laterally within individual beds.

Few longitudinal trends in the development of sedimentary structures were detected, but locally eroded fragments of the finer-grained sediments are important as textural elements, as tools, and as erosional indicators.

Individual graywacke and calcisiltite beds are very discontinuous in longitudinal section. Beds disappear downcurrent at a rate of about 50 percent in 4 km; the number of beds within a unit decreases by 50 percent in 10 km. The thickness of the small units decreases about 7 percent per km. Local current bypassing, imbrication of boils, and differential current deposition of argillite produce an intricate mosaic in longitudinal section. A volume of 10 to 30 10⁶ m³ is estimated for an average graywacke bed.