Abstract Integrated surface and subsurface stratigraphic and sediraentologic analysis of the nonmarine Lower Cretaceous rocks along the eastern margin of the Rocky Mountain foreland demonstrates the crilical role of intrabasinal and cratonic deformation in controlling alluvial architecture in a distal foreland-basin setting. Three coarse-grained intervals within the Lakota Formation can be recognized and correlated throughout the Black Hills and into the subsurface of the Powder River Basin. We informally designate these intervals from oldest to youngest as L1, L2 and L3. Detritus in the L1 interval was deposited in northward-flowing high-sinuosity rivers. The L2 interval lies atop of a regional intraformational angular unconformity which truncates rocks as old as the Middle Jurassic Sundance Formation. L2 detritus was deposited by northeastward-flowing braided rivers. The coincidence of location of these rivers and lineaments reflecting recurrent movement of basement-rooted structures is evidence of intrabasinal tectonic control. A variety of evidence suggests that this movement was associated with transpression along steeply dipping basement-rooted faults. As much as 65 m of stratigraphic throw may have occurred on one of these faults in the study area. Provenance analysis indicates that sediment deposited by L3 estuarine systems was derived from uplift of the Transcontinental Arch located within the craton, approximately 600-km southeast of the study area. Preliminary chronostratigraphic correlation of Early Cretaceous alluvial deposits throughout Wyoming indicates that intrabasinal deformation may have started as early as 134 Ma in the western Wind River Basin and progressively advanced eastward through the foreland over approximately the next 25 my, culminating slightly less than 110 Ma with the uplift of the Transcontinental Arch. This uplift resulted in a 120°change in direction of paleoslope in the distal Rocky Mountain foreland.

Abstract: Intra- and inter-basinal correlations between outcrop and subsurface over most of northern and central Wyoming indicate that chert-bearing conglomerates in the lower Cretaceous Cloverly Formation in the foreland of central Wyoming occupy three distinct stratigraphic levels. The two older conglomerates are in the lower Cloverly Formation in the western Wind River Basin and reflect northerly to northeasterly dispersal. The youngest conglomerate is in the upper Cloverly Formation in the eastern portion of the basin; gravels in this interval also were transported to the north and northeast. The two older conglomerates are separated from the youngest conglomerate by up to 35 m of purple to gray, smectite-rich mudstones that contain distinctive 10 to 90 cm-thick layers of white to dark green devitrified tuff, as well as silica and carbonate nodular beds. Fission-track ages of 125–128 Ma have been obtained from three samples of tuff in the Wind River Basin. These tuffs can be correlated to prominent tuffs further north in the Bighom Basin where a paleomagnetic stratigraphy has been established. Fission-track ages of zircons from devitrified tuff layers and magnetostratigraphy of mudstones suggest that the older two conglomerates in the Wind River Basin were deposited between 133 and 128 Ma and the youngest conglomerate at about 118 to 115 Ma. Three-dimensional, spatially controlled and temporally constrained reconstructions of paleodrainage systems for Cloverly conglomerates illustrate the complexity of fluvial drainage networks within the evolving Early Cretaceous foreland basin. Sand-body geometry and dispersal patterns within these fluvial networks were partially controlled by tectonic activity, which created a series of northeast-oriented horsts and grabens in the Wind River Basin. Location of trunk rivers was controlled by the positions of grabens within the basin.

Differences in framework composition of first-cycle sandstones within coarse-grained delta systems of the Fountain and Minturn Formations (Pennsylvanian) near Colorado Springs and McCoy, Colorado, largely are a function of variable mechanical disaggregation and hydrodynamic sorting characteristic of different depositional environments within the deltas. Modification of composition occurred in spite of deposition in a tectonically active setting where rates of sediment supply and burial were relatively high. Within a wave-dominated delta in the Fountain Formation near Colorado Springs foreshore sandstones are most mature (Q 69 F 28 R 3 ) and offshore/transition sandstones are most immature (Q 50 F 48 R 2 ). Differences in maturity reflect shoreline reworking processes. Feldspar is mechanically broken and abraded in the foreshore due to swash/backwash processes. Smaller grains of feldspar are winnowed from the foreshore and transported in suspension to the offshore during storms. The average composition of shoreface sandstone (Q 62 F 34 R 4 ) closely resembles the composition of its precursor alluvial sandstone (Q 61 F 35 R 4 ), suggesting most shoreface sand was derived directly from the alluvial channels and underwent little or no compositional change. However, the overall variability in the composition of shoreface sandstones is greater than that of alluvial sandstones, suggesting a more complex derivational history. Some samples of shoreface sandstone are enriched in feldspar, presumably derived directly by winnowing from the foreshore; other samples are enriched in quartz, indicating total reworking of coarser mature sand from the foreshore. The Minturn Formation near McCoy contains facies of river-dominated deltas, fed by both meandering- and braided-river systems. Differences in depositional processes between the two systems caused distinctly different modification of the framework composition of sandstones deposited in the two systems. Sand delivered by meandering-fluvial systems presumably formed under more intense weathering conditions and contains up to 8% fewer rock fragments and as much as 12% more feldspar than braided-fluvial sandstones. Compositions of fluvial sandstones were subsequently modified by marine processes and the compositions of the sandstones in the braided and meandering fluviodeltaic systems diverged even further. The primary variation in composition is reflected by a decrease in abundance of rock fragments, with resulting enrichment of quartz and feldspar. In both systems, beach facies are very similar in composition to their parent alluvial sand, suggesting that sand in the Minturn deltas had low residence time in the beach and that the beaches experienced low wave activity. Overall, the marine-influenced fades of meandering-fluviodeltaic origin are more distinct in composition than those of braided-fluviodeltaic origin. Although variation in grain size between fades accounts for most of the observed difference in composition of sandstones of the meandering-fluviodeltaic system, weak correlation between grain size and QFR composition indicates that grain size by itself cannot explain all of the compositional variation in sandstones of the braided-fluviodeltaic system. Differences in compositional modification between the wave-dominated (Fountain) and river-dominated (Minturn) coarse-grained deltas were due largely to subtle differences in the composition and grain size of alluvial sand brought to the shoreline and in both the rigor and duration of reworking in the marine environments of the two types of deltas. Largely as a consequence of the latter, the composition of beach sandstones of the Fountain was more distinctly modified relative to other facies in the Fountain. Also, more modification occurred in Fountain beach sandstones than in shoreline sandstones of the Minturn Formation where full-scale development of beaches and their long-term existence were prevented because of the dominance of fluvial processes on the delta plains and delta fronts.

Abstract Abstract—Mineralogic provinces are compositionally distinctive three-dimensional bodies of rock constituting natural units in terms of age, origin and distribution. More than one province likely will be present within the same sedimentary basin. Unconformities and not necessarily group, formation or member boundaries are the sharpest stratigraphic boundaries of provinces. Lateral boundaries are frequently gradational; depending on the scale of the basin analysis these areas of gradation can be separately defined as hybrid provinces. Over-generalization and over-simplification of interpretation of mineralogic provinces has resulted from inadequate appreciation and evaluation of the relative influence of the four principal factors controlling composition of a province: provenance, transportation, depositional environment and diagenesis. Each of these factors is in turn a dependent variable. A total of 13 immediate processes controlling province composition are identified. Improved ability to interpret ancient mineralogic provinces will develop from i) better documentation of starting or parent detritus, ii) quantitative estimation of the effects of transportation and environment on sediment composition through study of Holocene sediment, iii) more precise characterization of mineralogic provinces and iv) refinement of current techniques used in interpreting the origin of detrital quartz, feldspar, and accessory minerals.