TY - JOUR
T1 - Field evidence for disequilibrium dynamics in preserved fluvial cross-strata
T2 - A record of discharge variability or morphodynamic hierarchy?
AU - Lyster, Sinéad J.
AU - Whittaker, Alexander C.
AU - Hajek, Elizabeth A.
AU - Ganti, Vamsi
N1 - Funding Information:
This research was primarily funded by a Natural Environment Research Council (NERC) Science and Solutions for a Changing Planet Doctoral Training Partnership (DTP) grant to SJL, with additional funding by The Geological Society of London and The British Sedimentological Research Group to SJL. We thank George Hedley and Bailey Lathrop for support with field data collection, and we thank the Safiya Alpheus for useful discussions. We would also like to thank Luca Colombera and one anonymous reviewer for their constructive reviews, which have greatly improved this manuscript.
Funding Information:
This research was primarily funded by a Natural Environment Research Council (NERC) Science and Solutions for a Changing Planet Doctoral Training Partnership (DTP) grant to SJL, with additional funding by The Geological Society of London and The British Sedimentological Research Group to SJL. We thank George Hedley and Bailey Lathrop for support with field data collection, and we thank the Safiya Alpheus for useful discussions. We would also like to thank Luca Colombera and one anonymous reviewer for their constructive reviews, which have greatly improved this manuscript.
Publisher Copyright:
© 2021 Elsevier B.V.
PY - 2022/2/1
Y1 - 2022/2/1
N2 - Bedforms preserved in the rock record can provide detailed information on the morphologies and hydrodynamics of ancient fluvial systems on Earth and other planets. Existing process–product relations for bedform preservation assume that fluvial cross strata reflect conditions under which bedforms were equilibrated with the prevailing flow, i.e., steady-state conditions. However, recent theoretical and experimental observations indicate that enhanced bedform preservation can occur in non-steady state, or disequilibrium, conditions, and it is currently unclear how prevalent disequilibrium dynamics are in preserved fluvial strata at outcrop scale. Here we explore whether steady-state assumptions are appropriate for ancient fluvial systems by evaluating the nature of bedform preservation in well studied fluvial deposits of three Upper Cretaceous (Turonian and Campanian) geologic formations in central Utah, USA: the Blackhawk Formation, Castlegate Sandstone, and Ferron Sandstone. In the field, we made systematic measurements of dune-scale cross-strata to quantify the extent to which preserved cross-sets reflect dune preservation in steady-state conditions. Across the three formations, consistently low coefficients of variation in preserved cross-set thicknesses of 0.25–0.5 are inconsistent with bedform preservation in steady-state conditions, and instead point to fluvial systems in which enhanced bedform preservation occurred in disequilibrium conditions. Enhanced bedform preservation in dune-scale cross-stratification can be explained by two independent hypotheses: the effect of flashy flood hydrographs on bedform preservation (flood hypothesis) or bedform preservation in the presence of larger migrating barforms (hierarchy hypothesis). We estimated bedform turnover timescales to quantitatively assess these competing hypotheses and contextualize their implications. Under the flood hypothesis, field measurements are consistent with enhanced bedform preservation driven by flashy flood hydrographs with flood durations ranging on the order of hours to a few days, which are consistent with perennial fluvial systems subject to heavy rains and tropical storms. Alternatively, under the hierarchy hypothesis, field measurements are consistent with bedform climb angles that range from 10−2 to 10−1, reflecting rapid bar migration. Our work provides a novel way of investigating fluvial discharge variability in the geologic past, and we outline the potential next steps to disentangle the relative controls of flow variability and morphodynamic hierarchy in controlling bedform preservation in ancient fluvial systems.
AB - Bedforms preserved in the rock record can provide detailed information on the morphologies and hydrodynamics of ancient fluvial systems on Earth and other planets. Existing process–product relations for bedform preservation assume that fluvial cross strata reflect conditions under which bedforms were equilibrated with the prevailing flow, i.e., steady-state conditions. However, recent theoretical and experimental observations indicate that enhanced bedform preservation can occur in non-steady state, or disequilibrium, conditions, and it is currently unclear how prevalent disequilibrium dynamics are in preserved fluvial strata at outcrop scale. Here we explore whether steady-state assumptions are appropriate for ancient fluvial systems by evaluating the nature of bedform preservation in well studied fluvial deposits of three Upper Cretaceous (Turonian and Campanian) geologic formations in central Utah, USA: the Blackhawk Formation, Castlegate Sandstone, and Ferron Sandstone. In the field, we made systematic measurements of dune-scale cross-strata to quantify the extent to which preserved cross-sets reflect dune preservation in steady-state conditions. Across the three formations, consistently low coefficients of variation in preserved cross-set thicknesses of 0.25–0.5 are inconsistent with bedform preservation in steady-state conditions, and instead point to fluvial systems in which enhanced bedform preservation occurred in disequilibrium conditions. Enhanced bedform preservation in dune-scale cross-stratification can be explained by two independent hypotheses: the effect of flashy flood hydrographs on bedform preservation (flood hypothesis) or bedform preservation in the presence of larger migrating barforms (hierarchy hypothesis). We estimated bedform turnover timescales to quantitatively assess these competing hypotheses and contextualize their implications. Under the flood hypothesis, field measurements are consistent with enhanced bedform preservation driven by flashy flood hydrographs with flood durations ranging on the order of hours to a few days, which are consistent with perennial fluvial systems subject to heavy rains and tropical storms. Alternatively, under the hierarchy hypothesis, field measurements are consistent with bedform climb angles that range from 10−2 to 10−1, reflecting rapid bar migration. Our work provides a novel way of investigating fluvial discharge variability in the geologic past, and we outline the potential next steps to disentangle the relative controls of flow variability and morphodynamic hierarchy in controlling bedform preservation in ancient fluvial systems.
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U2 - 10.1016/j.epsl.2021.117355
DO - 10.1016/j.epsl.2021.117355
M3 - Article
AN - SCOPUS:85122366390
SN - 0012-821X
VL - 579
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
M1 - 117355
ER -