https://doi.org/10.1130/G47601.1 Manuscript received 5 March 2020 Revised manuscript received 18 June 2020 Manuscript accepted 23 June 2020 © 2020 Geological Society of America. For permission to copy, contact [email protected]. Dune-scale cross-strata across the fluvial-deltaic backwater regime: Preservation potential of an autogenic stratigraphic signature Chenliang Wu1*, Jeffrey A. Nittrouer1, Travis Swanson2, Hongbo Ma1,3, Eric Barefoot1, Jim Best4 and Mead Allison5 1 Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, Texas 77005, USA 2 Department of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30458, USA 3 Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA 4 Departments of Geology, Geography and Geographic Information Science, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA 5 Department of River-Coastal Science and Engineering, Tulane University, New Orleans, Louisiana 70118, USA ABSTRACT phy over a large part (lowermost ∼410 km) of Dune-scale cross-beds are a fundamental building block of fluvial-deltaic stratigraphy the backwater-influenced reach (i.e., lowermost and have been recognized on Earth and other terrestrial planets. The architecture of these ∼500 km; Nittrouer et al., 2012) of the Missis- stratal elements reflects bed-form dynamics that are dependent on river hydrodynamic condi- sippi River (MR). tions, and previous work has documented a multitude of scaling relationships to describe the Nonuniform flow conditions arise where a morphodynamic interactions between dunes and fluid flow. However, these relationships are river channel approaches a standing body of wa- predicated on normal flow conditions for river systems and thus may be unsuitable for applica- ter, and the water-surface profile asymptotically tion in fluvial-deltaic settings that are impacted by nonuniform flow. The ways in which dune converges to base level. As the bed elevation dimensions vary systematically due to the influence of reach-averaged, nonuniform flow, and maintains a uniform slope, channel depth in- how such changes may be encoded in dune cross-strata, have not been investigated. Herein, creases, and, assuming a constant channel width, we explored the influence of backwater flow on dune geometry in a large modern fluvial cross-sectional flow area expands downstream channel and its implications for interpretation of systematic variability in dune cross-strata (Nittrouer et al., 2012). By principles of conser- in outcrop-scale stratigraphy. This was accomplished by analyzing high-resolution channel- vation, flow velocity must thus decrease, gen- bed topography data for the lowermost 410 km of the Mississippi River, which revealed that erating reduced sediment transport capacity, in- dune size increases to a maximum before decreasing toward the river outlet. This spatial channel accumulation (Ganti et al., 2014), and variability coincides with enhanced channel-bed aggradation and decreasing dune celerity, downstream fining (Nittrouer, 2013; Smith et al., which arise due to backwater hydrodynamics. An analytical model of bed-form stratifica- 2020). Recently, several efforts have sought to tion, identifying spatial variability of cross-set thickness, indicates a prominent downstream evaluate the impact of backwater hydrodynam- decrease over the backwater region. These findings can be used to inform studies of ancient ics on the rock record. A downstream increase fluvial-deltaic settings, by bolstering assessments of proximity to the marine terminus and in thickness of the fluvial bar forms (flow depth associated spatially varying paleohydraulics. indicator) and a downdip fining in median channel-bed grain size were recognized from INTRODUCTION terns of sediment accumulation (Nittrouer et al., the Castlegate Sandstone in Utah (Petter, 2010) Sedimentary rocks of fluvial origin are used 2012), and thus the development of stratigraphy and across the backwater reach of Holocene- to infer characteristic paleohydraulic param- (Wu et al., 2020; Wu and Nittrouer, 2020), but aged MR deposits (Fernandes et al., 2016). A eters of transport systems (e.g., channel slope it cannot be accounted for using empirically downstream narrowing of channel-belt deposits S and flow depthH ), which bolster assessments based reconstruction methods developed for was observed in the Ferron Sandstone of Utah of past surface environments on Earth (Foreman normal flow conditions (Bradley and Venditti, (Kimmerle and Bhattacharya, 2018) and in the et al., 2012), as well as other terrestrial planets 2017). To resolve the paleohydraulic signatures subsurface of the Mungaroo Formation in Aus- (Goudge et al., 2018). However, a major chal- of regions impacted by spatially varying flow, tralia (Martin et al., 2018). The spatial variability lenge in paleohydraulic reconstructions arises it is necessary to account for morphodynamic in the architecture of autogenic scours, assessed due to the probable existence of hydraulic vari- variations by coupling the adjusting fluid flow to arise due to nonuniform flow, has also been ability. For example, lowland fluvial channels field with sediment transport gradients and de- revealed to impact stratigraphy (Trower et al., nearing a receiving basin experience nonuni- termining bed elevation changes. Herein, we 2018; Ganti et al., 2019). form (backwater) flow. This influences the pat- used detailed observations of bed forms to To date, most research has focused on the inform an analytical model that explores the linkages between backwater hydrodynamics *E-mail: [email protected] evolution of outcrop-scale cross-set stratigra- and large-scale features (e.g., bars and channel CITATION: Wu, C., et al. 2020, Dune-scale cross-strata across the fluvial-deltaic backwater regime: Preservation potential of an autogenic stratigraphic signature: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G47601.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47601.1/5096082/g47601.pdf by Rice University user on 24 August 2020 Results from the MR morphodynamic model New (Fig. 2D) revealed that: (1) the rate of channel- Orleans RK 410 RK 130 bed aggradation (r) continuously decreased °W RK 165 RK 0 89 across the lower 410 km reach of the system, Baton Rouge °N and (2) dune celerity (c), calculated based on 2011 survey RK 410-165 H` ead of Passes 29 modeled sediment flux (Simons et al., 1965; see 2002 survey RK 165-130 Supplemental Material), decreased rapidly be- tween RK 410 and RK 390, but then decreased 30°N 91° W 2003 survey RK 130-0 90°W 29°N more gently toward the river outlet. From these Figure 1. Lowermost Mississippi River (MR), from river kilometer (RK) 410 to Head of Passes assessments, it is noted that the ratio r/c in- (i.e., RK 0). River centerline is color coded by bathymetric survey dates. creased between RK 410 and 320 but then de- creased toward the river outlet. belts), arising over hundreds of meters to tens measured upstream of the outlet at Head of of kilometers, and time scales that include mul- Passes) were collected during low-water dis- tiple flood events (i.e., decades to centuries). charge (8300–9900 m–3 s–1) from three surveys The systematic variability expressed by small- conducted during 2002, 2003, and 2011 (Fig. 1). er-scale features, such as dunes and associated The data were used to evaluate dune geometry A cross-strata, due to spatially varying backwa- (e.g., dune height; Supplemental Material Figs. ter hydrodynamics has not been investigated S1–S31). Because the surveys were conducted at to our knowledge. This is a critical oversight, low-discharge conditions following prolonged as dune-scale cross-strata are ubiquitous in flu- periods (months) of relatively stable flow, the vial sediments (Best and Fielding, 2020) and dunes were expected to be in morphological thus are characteristic outcrop-scale features equilibrium with the water discharge (Martin that comprise significant proportions of fluvio- and Jerolmack, 2013). Anthropogenic influences deltaic channel stratigraphy. Importantly, such (e.g., construction of levees and revetments, de- paleoenvironmental reconstructions often rely velopment of spillways and Old River station, B upon inverting bed-form strata to assess channel dredging; see Supplemental Material) on the paleoflow depths (Paola and Borgman, 1991; river hydrodynamics and bed forms in the river Leclair and Bridge, 2001; Bradley and Vendit- length examined herein were limited because ti, 2017). These models assume that sediment the backwater hydrodynamics persist, especially transport and dune geometry are equilibrated during low discharge (Nittrouer et al., 2012), in time and space. However, sediment transport and no apparent dredged regions were identified capacity varies with nonuniform flow (Nittrouer from the MBES data set. et al., 2012; Lamb et al., 2012), and the result- A one-dimensional morphodynamic model ing gradients in sediment flux can be expected (Parker et al., 2008) was developed and ap- to affect the size and celerity (c) of the dune and plied to the lowermost MR for a low-discharge C the vertical aggradation rate (r), and thus the (∼10,000 m–3 s–1) condition to simulate reach-av- geometry of the stratal architecture (Jerolmack eraged trends in annual channel-bed