Abstract

We analyze a compound, piedmont-linked, coastal-plain incised-valley system from the Turonian Ferron sandstone, Utah, to evaluate valley-fill facies models in the context of bayline and backwater effects, as well as presenting quantitative estimates of paleohydraulic parameters, such as slope and discharge. Three photomosaics and eight measured sections along variably oriented cliffs show three terraces formed by a single-thread meandering trunk river that was about 7 m deep, forming a compound valley with up to 28 m of erosional relief. Channel fills in some of the terrace deposits show double mudstone drapes, tidal reworking, and brackish-water to marine trace fossils, suggesting deposition in both the tidal backwater and bayline regions. Paleoslope estimates are on the order of 2.6 to 5.3 × 10–4, and corresponding backwater lengths are estimated to range from 3 to 25 km. Assuming microtidal conditions, the bayline limit was likely less than 7 km and the shoreline was likely positioned within 20 km of the study area. Each successive valley-fill terrace is punctuated by a transgressive then regressive episode of cut and fill thus forming stepped forced-regressive fluvially dominated, tidally affected valley fills. There is a lack of extensive transgressive marine or estuarine fill, consistent with an ever-wet tropical climate and a steep gradient that resulted in a high-sediment supply system, rather different from the transgressive, estuarine to marine simple valley-fill facies models that dominate much of the literature. Width-to-thickness ratios are 10:1, < 100:1, and about 1000:1 for channels, channel belts, and valleys respectively. These are consistent with ratios from modern systems, and the lower channel-belt widths are also consistent with deposition in the lower backwater, which favors aggradation versus lateral migration of channels.

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