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.

Preservation of autogenic processes and allogenic forcings within set-scale aeolian architecture I: numerical experiments

“…Secondly, self-organization into groups of dunes, and subsequent group-based deposition of cosets is an important process in the creation and preservation of aeolian stratigraphy. This hypothesis is comparable to preservation mechanisms proposed by previous studies which have focused on set-scale preservation occurring within nested scales of fluvial morphological elements including bars, channels, and channel belts (Miall, 2015;Reesink et al, 2015;Paola et al, 2018;Herbert and Alexander, 2018). Additionally, the forward motion of dunes within dune groups drives modest dune climb, and the scouring action of dune group troughs may create outcrop scale truncation surfaces, which within this experiment, arise from autogenic processes, but would typically be recognized as parasequences and super-surfaces, and therefore possibly thought to originate from allogenic boundary conditions.…”

Controls on channel deposits of highly variable rivers: Comparing hydrology and event deposits in the Burdekin River, Australia

“…Mud accounts for a very small percentage of the channel bed deposits seen in trenches. In some years, mud drapes have been observed on the surface of the bed, most often restricted to local lower areas (Herbert & Alexander, 2018). They have been recorded also in some trenches (Herbert et al, 2020).…”

“…1C) was undertaken initially with the rationale to consider whether large individual events, such as that caused by TC Debbie, would produce features in the deposit that could be diagnostic of such events in ancient deposits formed within comparable, highly-variable discharge systems. The observations in 2017, combined with previous observations at the same site (Amos et al, 2004;Fielding et al, 2005b;Herbert & Alexander, 2018;Herbert et al, 2020) led to analysis of the variation of rainfall-discharge responses in this catchment and the associated sedimentology. All of this research aims to refine the understanding of the sedimentology of variabledischarge rivers (cf.…”

“…1B). The channel bed is easily accessible and has been studied at intervals since 1998 (Amos et al, 2004;Fielding et al, 2005a,b;Alexander & Fielding, 2006;Herbert & Alexander, 2018;Herbert et al, 2020). Fieldwork has included topographic surveying, observation of river and floodplain sediments, vegetation surveys, trenching, coring, sediment sampling (subsequent granulometry and compositional analysis), ground-penetrating radar surveys, observations and sampling of discharge events, and acquisition of rainfall and discharge data from other sources.…”

“…The prolonged exposure of large areas of riverbed allows riparian vegetation growth (ranging from grasses to trees), which subsequently influences sedimentary processes (Fielding et al, 1997;Nakayama et al, 2002;Herbert & Alexander, 2018). During wet periods, seeds germinate and plants continue to grow rapidly if their roots can reach the water table in the channel deposits.…”