Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A.
Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A.
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交汇冲刷与切开山谷：来自美国犹他州东南部白垩纪费伦诺托姆三角洲的例子
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发表时间：
2015
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影响因子：
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通讯作者：
W. Dupré
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作者：
M. Ullah;J. Bhattacharya;W. Dupré
摘要 辫状河中的汇流冲刷发生在河道线连接在一起的地方，产生比平均河道深度深得多的侵蚀起伏。根据对大陆尺度恒河-雅鲁藏布江水系的研究，观察到自循环过程汇流冲刷的最大深度可达入河河道平均深度的四到五倍。考虑到古代河流系统中可能发生如此大规模的冲刷，有人认为，只有当侵蚀起伏超过平均河道深度的五倍以上时，才应在古代系统中正确定义层序边界处异体产生的切谷。根据横截面几何形状，对犹他州费伦砂岩段诺托姆三角洲杂岩中白垩纪复合切谷系统中暴露良好的河流露头上的许多交汇冲刷充填进行了解释。露头的横截面观察以及与现代河流的比较表明，交汇冲刷具有诊断性充填相（单组大型陡峭前积层），并且不会产生多层砂体。汇合冲刷充填物产生单层主体，其中五阶冲刷被单元坝前积层填充，而单位坝前积层又被复合坝沉积物覆盖的四阶表面所覆盖。汇合冲刷中的层厚度并不代表平均河道深度，因为汇合冲刷可以保留河道最深部分以及完全保存异常厚的层。因此，我们认为，对与层序边界相关的古代系统中切谷的解释不应基于侵蚀面的深度与平均保存的河道层数的关系。相反，它应该由侵蚀浮雕来定义，该浮雕比最厚的完全保存的层要深得多，在辫状溪流中，这些层可能代表汇合冲刷填充物。
Abstract Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges–Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth. Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a single-story body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.