Lateral accretion in a deep‐marine channel complex: implications for channellized flow processes in turbidity currents

Some deep‐marine channels show striking similarities to fluvial channels, despite major differences in the properties of the flows that they conduct. Some field observations from deep‐marine channel deposits within a Late Cretaceous palaeo‐canyon in the Rosario Formation of Baja California, Mexico, that bear on these comparisons have been reported. These channel deposits contain erosively based lateral accretion sets. These sets are generally a few metres thick and resemble fluvial point bar deposits. Sediment movement and deposition within these accretion deposits was induced by turbidity currents several to many times thicker than the depth of the channel, moving at several metres per second. The inclined sets define laterally migrating and sinuous channels locally at a high angle to the confining canyon. The instantaneous channel widths varied from 6 to 39 m, the depths from 2·5 to 4 m and the sinuosities from 1·3 to 3·1. Palaeocurrent data, taken mostly from clast imbrication in conglomerates, indicates current modes along the channel thalweg, but with other directions representing either secondary flow (oriented primarily up the point bar) or over‐passing canyon‐confined flow. It is suggested that, at times, the lower part of the turbidity currents flowing down the channels behaved similarly to within‐bank fluvial currents, with a cross‐channel component of flow towards the cut bank, and return flow at the bed sweeping up the point bar. At other times, this secondary circulation may be absent or reversed, which may be related to changes in flow thickness, coupling with the overriding flow and possible flow separation.