Field evidence for disequilibrium dynamics in preserved fluvial cross-strata: A record of discharge variability or morphodynamic hierarchy?
Field evidence for disequilibrium dynamics in preserved fluvial cross-strata: A record of discharge variability or morphodynamic hierarchy?
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DOI：
10.1016/j.epsl.2021.117355
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发表时间：
2022-02
影响因子：
5.3
通讯作者：
S. Lyster;A. Whittaker;E. Hajek;V. Ganti
中科院分区：
文献类型：
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作者：
S. Lyster;A. Whittaker;E. Hajek;V. Ganti
保存在岩石记录中的底形可以提供关于地球和其他行星上古代河流系统的形态和流体动力学的详细信息。现有的底形保存过程-产品关系假定河流交错地层反映了底形与主流水流平衡的条件，即，稳态条件。然而，最近的理论和实验观察表明，增强的底形保存可以发生在非稳态，或不平衡，条件下，目前还不清楚如何普遍不平衡动力学保存河流地层在露头规模。在这里，我们探讨是否稳定状态的假设是适当的古代河流系统，通过评估的性质，研究了三个上白垩统（Turonian和Campanian）在美国中部犹他州的地质地层的河流沉积物的床形保存：黑鹰组，Castlegate砂岩，和Ferron砂岩。在实地，我们对沙丘尺度的交叉地层进行了系统的测量，以量化保存的交叉集在多大程度上反映了稳态条件下沙丘的保存。在这三个地层中，0.25-0.5的保存交错层厚度的变异系数始终较低，这与稳定状态条件下的底形保存不一致，而是指向河流系统，在该系统中，在不平衡条件下发生了增强的底形保存。沙丘尺度交错分层中增强的底形保存可以通过两个独立的假设来解释：暴洪过程线对底形保存的影响（洪水假说）或在存在较大迁移沙坝时底形保存的影响（层次假说）。我们估计了床型周转时间尺度，以定量评估这些相互竞争的假设，并将其含义联系起来。根据洪水假说，实地测量是一致的，加强底形保存驱动的洪水持续时间范围在几个小时到几天的顺序，这是符合常年河流系统的暴雨和热带风暴的洪水过程。或者，在层次假设下，野外测量结果与10 - 2 ~ 10 - 1的底形爬升角一致，反映了沙洲的快速迁移。我们的工作提供了一种新的方式来调查河流流量的变化在地质过去，我们概述了潜在的下一步解开的相对控制流的变化和形态动力学层次结构，在控制古河流系统的床形保存。
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 10−2to 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.