Bottomset Architecture Formed In the Troughs of Dunes and Unit Bars

Abstract Bottomsets are formed in the troughs of migrating bedforms. The flow and sediment dynamics downstream of bedforms differ, and consequently, the thickness, grain size, and internal structure of bottomsets vary greatly. A classification of the controls on bottomset formation is proposed related to two- and three-dimensional processes, flow and sediment unsteadiness, and pre-existing deposits. Structures and deposits created by flow and sediment unsteadiness as well as pre-existing deposits are also considered in relation to changes they can induce in subsequent trough processes. The classification is illustrated by observations from laboratory flumes, the modern Burdekin River (Australia), and the rock record (The Roaches Grit, England, and Hawkesbury Sandstone, Australia). In bottomset examples described in this paper, internal variability, both vertically and laterally, was much greater than that within the dominant foreset component. Complex compound structures were created by the interaction and predominance of different bottomset controls. This, combined with their high preservation potential, makes them a highly useful paleoenvironmental indicator. Bottomset variation (e.g., changes in the abundance of mud laterally) will influence the permeability heterogeneity of fluvial reservoir rocks. Bottomsets formed under relatively steady conditions are likely to be laterally extensive and have similar characteristics over their entire length. They may act as significant barriers to vertical flow. Conversely, bottomsets generated in unsteady regimes tend to be highly variable, potentially creating conduits between cross-bed sets aiding inter-set permeability.