Paleochannel hydraulics,geometries, and associated alluvial architecture of Early Cretaceous rivers,Sevier Foreland Basin,Wyoming, USA

Early Cretaceous, retro-foreland basin fluvial deposits throughout Wyoming record interactions between orogenesis, subsidence, sediment accumulation, basin physiography, and syndepositional structural deformation associated with the early stages of the Sevier Orogeny. Quantitative paleochannel reconstructions presented here are important for understanding these interactions, evaluating controls on alluvial architecture, and can be applied to basin-modeling studies. Most paleochannel sandstones and conglomerates represent point bars associated with meandering rivers, although some rivers may have been braided. Paleoflow of earliest Cretaceous Cloverly A-interval paleochannels (forebulge depozone, central WY) was generally to the north, northeast, and east, which suggests that most are deposits of basin-axial rivers. Discharges of overlying B-interval paleochannels are less than most of those of the A interval, possibly reflecting a temporal decrease in water supply related to the eastward expansion through time of an orographic rain shadow caused by progressive rising of the Sevier Orogen to the west. The Bechler (western WY), Cloverly B (central WY), and Lakota L2 (eastern WY) intervals are correlative and record deposition throughout the basin in the foredeep, forebulge, and backbulge depozones, respectively. Paleocurrents suggest that Bechler paleochannels are deposits of basin-transverse rivers that flowed to the east, whereas B and L2 paleochannels are deposits of basin-axial rivers that flowed dominantly to the north and northeast. The scales and discharges of most L2 paleochannels are much greater than those of the Bechler and B-interval. This eastward increase in discharge may reflect an eastward increase in precipitation related to the spatially decreasing effects of an orographic rain shadow caused by the Sevier Orogen to the west. Additionally, or alternatively, the higher discharges of most L2 rivers may indicate that they represent a more distal part of a tributary fluvial system than B-interval rivers (consistent with some lower slopes of L2 paleochannels).The alluvial architecture of thick foredeep deposits contrasts markedly with that of stratigraphically equivalent, much thinner deposits farther east that were associated with the forebulge and backbulge depozones. Foredeep deposits are dominated by overbank and lacustrine mudstones, and channel deposits tend to be isolated with limited lateral extents typically on the order of 10's of meters. Forebulge and backbulge channel deposits tend to be laterally and vertically connected forming sandstones and conglomerates with lateral extents on the order of 10's of km to >100 km. Long-term compacted sediment accumulation rates for the foredeep (generally 10⁻² mm year⁻¹) are an order of magnitude greater than those for the forebulge and backbulge depozones (10⁻³ mm year⁻¹). Quantitative simulations of channel-deposit proportions indicate that basin-wide differences in alluvial architecture are attributable to differences in sediment accumulation rates, which, in turn, reflect variable subsidence rates of the different depozones. Additionally, in some areas of the fore- and backbulge depozones, alluvial architecture was controlled by local syndepositional structures. However, the alluvial architecture in areas influenced by syndepositional structures is broadly similar to that in areas where such structures were absent, both reflecting the same general tectonic setting that experienced limited regional subsidence. Hence, the two cases are not easily distinguished solely on the basis of alluvial architecture.     

Tectonically induced climate and its control on the distribution of depositional systems in a continental foreland basin, Cloverly and Lakota Formations (Lower Cretaceous) of Wyoming, U.S.A.

Continental sediments of the Cloverly and Lakota Formations (Lower Cretaceous) in Wyoming are subdivided into three depositional systems: perennial to intermittent alluvial, intermittent to ephemeral alluvial, and playa. Chert-bearing sandstones, conglomerates, carbonaceous mudrocks, blocky mudrocks, and skeletal limestones were deposited by perennial to intermittent alluvial systems. Carbonaceous mudrocks contain abundant wood fragments, cuticle and cortical debris, and other vascular plant remains representing deposition in oxbow lakes, abandoned channels, and on floodplains under humid to seasonal conditions. Intraformational conglomerates, sandstones, bioturbated and blocky mudrocks with caliche nodules, and bioturbated limestones characterize deposition in intermittent to ephemeral alluvial systems. Bioturbated limestones are encased in bioturbated mudrocks with abundant pseudo-slickensides. The presence of caliche nodules in some of the blocky to bioturbated mudrocks is consistent with supersaturation and precipitation of calcium carbonate from groundwater under semi-arid conditions. Caliche nodules, pseudo-slickensides, and carbonate-rich floodplain sediments are interpreted to have been deposited by intermittent to ephemeral alluvial systems under seasonal to semi-arid climatic conditions. Laminated mudrocks, siltstones, vuggy carbonates, bedded to nodular evaporites, pebbly mudrocks, and diamictites were deposited in evaporative alkaline lakes or playas. Pebbly mudrocks and diamictites are interpreted to represent deposition from channelized and unchannelized hyperconcentrated flows on a playa, resulting from intense rain events within the basin.The areal abundance and distribution of these depositional systems change systematically across the overfilled portion of the Early Cretaceous Cordilleran foreland basin in Wyoming. The lower part (A-interval) of the Cloverly and Lakota Formations is characterized by deposits of perennial to intermittent rivers that existed 300 to 1000 km east of the Sevier fold-and-thrust belt. Proximal to the Sevier fold-and-thrust belt, the A-interval of the Cloverly Formation and upper Ephraim Formation of the Gannett Group are typified by deposits of intermittent to ephemeral rivers and their associated floodplains. In the middle part (B-interval) of the Cloverly Formation, intermittent to ephemeral alluvial systems expand to 600 km into the basin. The upper part (C-interval) of the Cloverly Formation is characterized by playa deposits in the Bighorn and Wind River Basins and intermittent to ephemeral alluvial deposits along the front of the ancestral Sevier Mountains. Deposits of perennial to intermittent alluvial systems in the C-interval of the Cloverly and Lakota Formations are restricted to the Black Hills region, almost 900 km to the east of the Sevier Mountains. The change in the areal distribution of depositional systems through time within this continental foreland basin may be attributed to the development of a rain shadow associated with the uplift of the Sevier Mountains in the Early Cretaceous.