Abstract

Fluvial systems are controlled by three major factors — tectonics, climate, and intrinsic controls which are a function of the overall geomorphology and evolution of the drainage basin. The Lower Cretaceous Cloverly Formation of the Wind River Basin is but one example that can be used to evaluate the relative importance of each of these controlling factors on a fluvial system. The vertical and lateral distributions of Cloverly chert-pebble gravels and sands are a consequence of an interaction of all three controlling factors but in varying degrees depending on location in the basin.

Although extrabasinal or basin-margin tectonics associated with the Sevier orogenic event were responsible for supplying gravel and cobble-sized sediment to Cloverly river systems in the foreland basin, intraforeland tectonism also occurred. Intrabasinal tectonism resulted in internal basin partitions, probably manifested in the form of N to NE trending horsts and grabens in the Wind River Basin region. These in turn, controlled the shape and distribution of gravel bodies in the Cloverly Formation.

East of the foredeep, the influence of basin-margin tectonism on controlling thickness of the nonmarine Lower Cretaceous section diminishes. In contrast, the importance of climate and intrinsic or geomorphic threshold controls increases in the medial to distal portion of the foreland basin where Late Jurassic (Morrison) and Early Cretaceous (Cloverly) sediments are relatively thin in comparison to foredeep deposits (Gannett Group) located to the west. Although inferred semi-arid climate was dominant at least throughout deposition of the Cloverly, short-term climatic fluctuations from dry to wet or even shorter-term flashy discharge events are proposed as important processes active in the transportation and deposition of sands and gravels.

Local downcutting of upper Cloverly channels into basal chert-pebble conglomerates near the Jurassic-Cretaceous unconformity may best be explained by very localized uplifts or by simple avulsion of upper meandering systems into the lower braided systems. An overall decrease in coarse sediment supplied up through the Cloverly section is also interpreted to reflect a decrease in extrabasinal tectonics; this change in sediment size presumably forced the fluvial systems to change from braided to meandering through time. Apparent cycles of coarse clastic intervals, however, may be related to episodic isostatic readjustment of the hinterland and thus represent several drainage basin evolutionary cycles rather than three or four separate tectonic pulses during Cloverly deposition.