Abstract

Deep-marine strata consisting of a sandy basal part overlain sharply by a muddier and mud-clast-rich upper part are increasingly being recognized in the deep-marine sedimentary record, and have been termed linked debrites, cogenetic debrite–turbidite beds, hybrid event beds, transitional-flow deposits, and bipartite beds. These composite strata are generally reported to develop downflow of clean (less muddy) sand, but the variable distance (hundreds of meters to tens of kilometers) over which this lithological change is reported to take place has resulted in a variety of physical depositional models. As such, the details about the origin and spatial evolution of these admixed sand–mud strata still remain poorly understood. Part of this uncertainty is that many earlier studies, with a few exceptions, have been based on macroscopic observations in drill core or discontinuous outcrops, and therefore interpreted bed continuity is based on similarity of textural or other sedimentological attributes. However, in slope to proximal basin-floor deposits of the Neoproterozoic Windermere Supergroup, and distal basin-floor deposits of the Ordovician Cloridorme Formation, exceptional outcrop exposure permitted individual two-part (bipartite) facies and their associated strata to be continuously traced and lithological changes to be documented along the strike of the outcrop.

In both the Windermere and the Cloridorme, the bipartite facies consists of a lower sandy (25–60% matrix) part overlain sharply by a muddier (40–80% matrix), commonly mud-clast-rich upper part, and occurs downflow of muddy sandstone and upflow of sandy mudstone in a hundreds-of-meters long proximal to distal depositional continuum. These strata are then overlain everywhere by a thin- to very thin-bedded, traction-structured sandstone and/or silty mudstone cap. Notably, the interface separating the lower sand-rich part from the upper mud-rich part in the bipartite part of the depositional transect is always planar in the Windermere, but often alternates between planar and irregular in the Cloridorme. The continuum is interpreted to represent deposition downflow of an avulsion node that resulted in erosion of the local mud-rich seafloor and charged the flow with fine-grained sediment, resulting in increased effective fluid viscosity and significant changes in the characteristics, intensity, and transfer of fluid turbulence needed to maintain the particle suspension. Collapse of the suspension and development of overcapacity conditions resulted in rapid particle settling in the now rapidly depleting and negligibly sheared mud-enriched suspension that formed along the margins of the avulsion-wall jet in proximal-basin-floor and slope deposits (Windermere), or at the downflow terminus of the flow in more distal basin-floor deposits (Cloridorme). Additionally, when normalized to the total length of the continuum, the transition from muddy sandstone to bipartite facies and bipartite facies to sandy mudstone are equally proportioned in all examples, suggesting that, once formed, the depleting suspension evolved in a spatially systematic and dimensionally proportionate manner—a consistent depositional evolution difficult to reconcile with two mechanistically different parts to the flow. Moreover, differences in the morphology of the bipartite interface (i.e., planar vs. irregular) is not a primary depositional feature, but instead is interpreted to be a consequence of local postdepositional deformation of a previously continuous planar surface.