Abstract

A profound down-basin change in alluvial architecture is coincident in time and space with the early phase of uplift of the San Rafael Swell along the Book Cliffs of central Utah. Marked thinning in the lower part of the Price River Formation (Campanian) approaching the crest of the swell demonstrates that the swell was first active as a region of reduced subsidence rate before it developed topographic relief. A comparison of total unit thickness, percentage of sandstone, and relative abundance of sheet vs. isolated sand bodies in vertical sections within the Price River Formation and laterally equivalent Farrer Formation shows changes in these measures across the swell crest. The abundance of isolated lenticular fluvial sand bodies increases in the area of reduced aggradation at the expense of sheetlike bodies. This change in alluvial architecture over the crest of the swell is found only in units laid down when the swell was active, not in overlying or underlying units. This coincidence strongly suggests a link between early growth of the swell and alluvial response. However, the sense of change—i.e., fewer connected sand bodies in the region of reduced aggradation—is opposite to that predicted by most alluvial-architecture models. After comparison to other field and experimental studies, it seems likely that different river systems may reveal different avulsion frequencies that ultimately produce varied, but not necessarily unpredictable, alluvial architectures.

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