Abstract
AbstractTemporal and spatial changes in alluvial sand‐body geometry and palaeodispersal patterns of the lower part of the Cloverly Formation (Early Cretaceous) in west‐central Wyoming suggest differential uplift within the developing medial to distal foreland basin and existence of subtle structurally controlled topography nearly 60 Myr before classic Laramide uplift in the Wyoming foreland province. This intraforeland structural topography exerted a significant control on dispersal and deposition of widespread gravels of Neocomian‐Aptian age.Surface studies have established the presence of two conglomeratic sandstones in the Cloverly Formation in the western Wind River Basin. Both conglomeratic sandstones are chert arenites, but the lower conglomeratic sandstone is characterized by dark chert pebbles derived from western and southwestern extrabasinal sources and was deposited by braided rivers, whereas the upper conglomeratic sandstone is characterized by intraformational clasts derived locally from adjacent floodplains and deposited by NE‐flowing rivers of moderate sinuosity which carried a higher proportion of suspended load in stable bank‐confined channels.Surface and subsurface mapping of sandstone‐body geometry in the lower dark chert‐pebble conglomeratic sandstone, together with palaeocurrent analysis of trough cross‐bedding, reveal a major NE‐flowing trunk river system in the area. Lithological correlation of surface sections to adjacent well logs, together with regional format correlation of fine‐grained intervals in well logs up to 120 km E of the outcrop belt, enable detailed mapping of the trunk system. The trunk system was approximately 5–10 km wide, and may have been confined by subtle structural topography developed by recurrent differential uplift along NE‐trending fractures in Archaean basement rocks. The fractures are along strike with diabase dike swarms and faults mapped in Archaean basement rocks of the Laramide Wind River uplift. The lower chert‐pebble sandstone is absent for a lateral distance of at least 30 km between these lineaments, including an area of at least 1000 km2, indicating the existence of a low NE‐SW‐trending interfluve perpendicular to the axis of the modern Wind River Range.Early Cretaceous development of structural topography and partitioning of the medial to distal foreland basin of west‐central Wyoming were most likely controlled by tectonic reactivation and differential uplift along fractures in Archaean basement rocks in response to early thrust loading and intraplate stresses during initial subsidence of the foreland basin.
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