Intrabasinal tectonic control on fluvial sandstone bodies in the Cloverly Formation (Early Cretaceous), west‐central Wyoming, USA

Abstract

Temporal 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.

Exhumation of the North American Cordillera revealed by multi-dating of Upper Jurassic–Upper Cretaceous foreland basin deposits

“…In Wyoming, the Cloverly spans the Early Cretaceous (Valanginian) to midAlbian (DeCelles and Burden, 1992;DeCelles et al, 1993;Furer et al, 1997;O'Malley, 1994;Way, 1997;Currie, 1998;Burton et al, 2006). The A interval (Meyers et al, 1992;May et al, 1995;Zaleha, 2006) of the Cloverly Formation near Alcova Reservoir in central Wyoming is Early Cretaceous (Valanginian-Barremian) based on palynomorphs reported in DeCelles and Burden (1992) ( Table 2). …”

Sevier orogenesis and nonmarine basin filling: Implications of new stratigraphic correlations of Lower Cretaceous strata throughout Wyoming, USA

“…The organic-rich mudstones have been interpreted as oxbow lake, abandoned channel, and fl oodplain deposits . In east-central Wyoming (e.g., Alcova and Arminto areas), previous workers have included a thicker section of mudstone below the conglomerate in the Cloverly Formation (DeCelles and Burden, 1992;Meyers et al, 1992;May et al, 1995). However, as will be discussed later, much of that mudstone is likely Morrison Formation.…”

“…The A interval is characterized by fl uvial chert-bearing conglomerates, conglomeratic sandstones, and quartz arenites. The coarse-grained population consisting of granules and pebbles is dominated by phosphatic gray and black chert (Moberly, 1960;Ostrom, 1970;Meyers et al, 1992). The sand-size population is dominantly quartz, with quartz ranging from 46%-97% (mean of 82%; Hooper, 1962a;DeCelles and Burden, 1992).…”

“…Chert grains include spicular (most common), black (phosphatic), and chalcedonic varieties (Hooper, 1962a;Furer, 1970;DeCelles and Burden, 1992). Gray chert and black chert typically constitute less than 10% of the sand-size population (Moberly, 1960;Hooper, 1962a;Meyers et al, 1992). Hence, gray chert and black chert dominate the granule and pebble population, whereas they are typically a minor component of the sand-size population.…”

“…Hence, gray chert and black chert dominate the granule and pebble population, whereas they are typically a minor component of the sand-size population. Paleocurrent indicators in the A interval are dominantly to the northeast (MacKenzie and Ryan, 1962;Meyers et al, 1992).…”