Quantitative Comparison of Channel-belt Dimensions, Internal Architecture and Characteristics of Two Fluvial Systems in Sequence 1, Ferron Notom Delta, South-Central Utah, U.S.A.
Montes, Omar A. 1987-
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The geometry, proportion, and spatial distribution of fluvial deposits are critical components in subsurface reservoir analysis, but their prediction can be difficult. This study describes and compares an amalgamated, high sandstone/mudstone interval, channel-belt to underlying isolated, low sandstone/mudstone interval, channel-belts within outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation in south-central Utah, U.S.A. Using an oblique to mean paleocurrent direction cross section, photomosaics, and GPS readings at respective channel-belt margins, the width (W), thickness (T), and width/thickness ratio (W/T) of 12 channel-belts were documented. The amalgamated channel-belt demonstrates a minimum true channel-belt width of ~2.8 km, an average channel-belt thickness of 2.5m, and a minimum W/T ratio of ~1130. The isolated channel-belts range from approximately 40m up to 245m in true channel-belt width, 1.1m up to 5m in channel-belt thickness, and W/T ratios ranging from 25 to 175. The amalgamated channel-belt was formed by low sinuosity braided channels; whereas the isolated channel-belts were most likely laterally migrating meandering distributary channels in the upper delta plain. Using channel-belt dimensions, we determined that there is not a consistent systematic change in channel-belt dimensions relating changes in dimensions to their positioning within a sequence. However, there does appear to be a systematic change in the sandstone/mudstone proportion, where confined valley fill deposits result in high sandstone/mudstone proportions. This is followed by the out-of-valley deposits which are predominantly mudstone with isolated, highly sinuous fluvial deposits, resulting in low sandstone/mudstone proportions. This is then capped by a high sandstone/mudstone interval with amalgamated, low sinuosity deposits. The muddier isolated systems show a tendency for meandering, whereas coarser amalgamated systems show a tendency for braiding. This is consistent with models suggesting that sandstone/mudstone proportions are related to the river plan-form. The variations in the scale of channel-belts are also consistent with stacking patterns predicted in models relating sandstone/mudstone proportions to changes in the rate of aggradation, lateral migration, and avulsion frequency. These factors can be linked back to changes in accommodation, possibly indicating that it may be the determining factor.