Reflected turbidity currents on an Ordovician basin floor, Canadian Appalachians

Abstract

Sedimentary structures associated with turbidity current deposits show either partial or complete Bouma T_a–e sequences¹ that suggest deposition of sediment from a waning (decelerating) unidirectional flow². Bathymetric highs and basin margins can cause turbidity currents to reflect back along their original flow paths, a rare situation in both ancient^3,4 and modern⁵ marine basins. The internal structures of such contained, or reflected, turbidites have only been described in one case, the Contessa bed (ref. 4 pp. 268,300). Our data from 1–6-m thick graded, mud-dominated turbidites from the Quebec Appalachians, using mainly sole marks, megaripple, ripple and grain fabric data, suggest that during the deposition of these beds the turbidity currents were reflected several times from either internal basin highs or basin slopes. The recognition of such beds not only provides an insight into the depositional environment (deep-water basin floor), but also allows detailed bed-by-bed correlations over tens of kilometres of coastal sections since the beds are reliable chronostratigraphical datum lines.