Temporal and spatial changes in style of accretion at the bend of a sinuous turbidite slope channel (channel–levee Complex 5, Tachrift System of NE Morocco)

This outcrop study describes variously accreted deposits constituting the fill of a sinuous channel from the Tachrift System (upper Miocene of the Taza–Guercif Basin, NE Morocco). Detailed correlation of fifty-nine logs from three outcrops spanning a left-turning bend allow recognition of four stratigraphic units recording channel establishment and levee development, aggradation and bend translation and expansion, and switching of the parent channel. Results show that lateral-accretion packages (LAPs) formed at the inner bank represent the main intra-channel depositional element, co-occurring with equally sandstone-rich and coarse-grained deposits accreted at the outer bank. The best-exposed and larger example of LAPs occurs in two correlated exposures c. 500 meters apart along-stream, which most likely straddle the bend apex. In the upstream outcrop, cut-and-fill features and sandstone amalgamation make it difficult to recognize lateral accretion. Here, LAPs consist of bedsets of amalgamated sandstones bounded by thalweg-dipping erosional surfaces that are lined up with mud-clast breccias in the deeper part of the channel fill. In the downstream outcrop, the correlated LAPs become lithologically more layered along most of their profile. Moving up the accretion dip, to the LAP top, they comprise finer-grained, and more structured sandstones alternating with mudstones. In the downstream outcrop, LAPs transition into a mud-rich heterolithic deposit characterized by trough-cross inclined stratification, which dips at a high angle to mean downstream paleocurrent. This deposit extends over a distance of a few hundred meters, straddling the inner- to outer-bank transition, and is interpreted as the expression of bank-attached bars (here termed turbidite counter-bars) forming in a flow separation zone. This study provides insights into longitudinal heterogeneity of LAPs and lithological composition of outer-bank bars and turbidite counter bars never before observed at outcrop.