A stratigraphic example of the architecture and evolution of shallow water mouth bars

Abstract

Improved understanding of mouth bar morphodynamics, and the resulting stratigraphic architectures, is important for predicting the loci of deposition of different sediment fractions, coastal geomorphic change and heterogeneity in mouth bar reservoirs. Facies and architectural analysis of exceptionally well-exposed shallow water (ca 5 m depth) mouth bars and associated distributaries, from the Xert Formation (Lower Cretaceous), of the Maestrat Basin (east-central Spain), reveal that they grew via a succession of repeated autogenic cycles. An initial mouth bar accretion element forms after avulsion of a distributary into shallow standing water. Turbulent expansion of the fluvial jet and high bed friction results in rapid flow deceleration, and deposition of sediment in an aggradational to expansional bar-form. Vertical bar growth causes flattening and acceleration of the jet. The accelerated flow scours channels on the bar top, which focuses further expansion of the mouth bar at individual loci where the channels break through the front of the mouth bar. Here, new mouth bar accretion elements form, downlapping and onlapping against a readily recognizable surface of mouth bar reorganization. Vertical growth of the new mouth bar accretion elements causes flattening and re-acceleration of the jet, leading to channelization, and initiation of the next generation of mouth bar accretion elements. Thus the mouth bar grows, until bed-friction effects cause backwater deceleration and superelevation of flow in the feeding distributary. Within-channel sedimentation, choking and upstream avulsion of the feeding channel, results in mouth bar abandonment. In this study, mouth bars are formed of at least two to three accretion elements, before abandonment happened. The results of this study contrast with the notion that mouth bars form by simple vertical aggradation and radial expansion. However the architecture and facies distributions of shallow water mouth bars are a predictable product of intrinsic processes that operate to deposit them.

Evolution from shallow‐water deltas to fluvial fans in lacustrine basins: A case study from the Middle Jurassic Shaximiao Formation in the central Sichuan Basin, China

“…Backset cross‐bedded sandstones, as the result of supercritical flow deposits, are common in subaqueous channels (Wang & Plink‐Björklund, 2019, 2020). Moreover, the erosion of sandstone beds into subaqueous anoxic thick‐bedded greenish‐grey mudstones provides further evidence for subaqueous channel depositional processes (Cole et al ., 2021).…”

“…Interpretation : LA 7 is interpreted as mouth‐bar deposits. The inverse grading and upward coarsening patterns are typical characteristics of mouth‐bar deposits (Sáez et al ., 2007; Cole et al ., 2021; Xu et al ., 2021). Climbing ripples indicate the influence of the distal flow from the rivers (Sáez et al ., 2007).…”

“…The lithofacies and lithofacies association analyses were conducted on more than 1000 m of core. The terminology used in this study to describe lithofacies follows the principles of Miall (1985Miall ( ), S aez et al (2007 and Cole et al (2021).…”

Variability in fluvially‐dominated, fine‐grained, shallow‐water deltas

“…However, by the standards set out above, the study cannot claim to be either detailed or complete.L 1 Continued analysis may increase or refine the range of river‐mouth deposits and the known mechanisms for channel elongation. 2 The analysis has focussed on the sandy portion of river‐mouth deposits, the larger, mud‐prone part, more likely to be moved by basin currents and deposited from plumes (Roberts, 1998), has not been addressed. 3 Almost all of the case studies would benefit from fieldwork, in particular to confirm interpretations based largely on satellite imagery and to describe the sedimentary record of each river‐mouth deposit style, and so bridge the gap between plan forms described here and the rock record. For example, possible links between the diverse plan‐form types described here and the steeper clinoforms seen in many ancient examples (Panther Tongue 1°–8°; Olariu & Bhattacharya, 2006; Ferron 1°–7°, Howell et al ., 2008, Ahmed et al ., 2014; Xert Formation 7°–24° Cole et al ., 2020) are unexplored. Similarly, Fidolini & Ghinassi (2016) provide the only known descriptions comparable to the tongue and splay‐shaped mouth bars documented here.…”

“…Many ancient systems have been interpreted similarly: lobate deltas characterized by friction‐dominated mouth bars are widely recognized (Flint et al ., 1989; Harris, 1989; Tye & Hickey, 2001; Olariu & Bhattacharya, 2006; Turner & Tester, 2006), and several systems are considered analogous to the bar‐finger sands of the modern‐day Mississippi Delta (Elliott, 1976; Okazaki & Masuda, 1989; Pulham, 1989). By contrast, more recent outcrop studies suggest that diverse channel‐termination processes and depositional elements can co‐exist, such as: (i) coeval, proximal, friction‐dominated mouth‐bar deposits, and distal, inertia‐dominated hyperpycnal deposits (Ahmed et al ., 2014; Jerrett et al ., 2016); (ii) lunate, high‐inertia mouth bars lain down during high stage, which divert low‐stage flows to form marginal, smaller, friction‐dominated mouth bars (Fidolini & Ghinassi, 2016); and (iii) an evolution that commences with a jet which fills the water column, deposits on the centreline and drives dunes basinward down low‐angle foresets, but which detaches from the bar front (as aggradation occurs and water depth increases) to form an avalanche foreset (Cole et al ., 2020).…”