[1] Dynamic subsidence related to subduction is an important process in retroarc foreland basin systems. Dynamic subsidence in the North American retroarc foreland has been proposed as dominant in the Late Cretaceous; however, questions remain about the nature of the subcrustal load and the basin response to such processes. We present new isopach data using 130 data points covering a large portion of the U.S. and a revised flexural analysis of the Sevier foreland. Higher rigidity associated with the Wyoming craton can best model flexural subsidence directly prior to~81 Ma. We constrain the transition from flexural to nonflexural subsidence to~81 Ma and correlate this with large Late Cretaceous progradation. The locus of subsidence and the location of the subducted Shatsky oceanic plateau suggest that dynamic loading from viscous flow above the subducting plate is the mechanism driving dynamic subsidence in the retroarc foreland basin of the North American Cordillera. Citation: Painter, C. S., and B. Carrapa

New low-temperature thermochronology and geochronology data from Upper Jurassic-Upper Cretaceous strata from the North American Cordilleran foreland basin in Utah, Colorado, Wyoming, and South Dakota document rapid exhumation rates of the adjacent Cordilleran orogenic belt to the west. Both zircon (U-Th-[Sm])/He (zircon He) and apatite fi ssion track (AFT) thermochronology were applied to proximal and distal synorogenic deposits in order to identify a thermochronometer suitable to record source exhumation during the North America Cordilleran orogeny. AFT lag times from Upper Jurassic-Upper Cretaceous deposits are 0-5 m.y. and indicate a relatively steadystate to slightly increasing exhumation rate between 118 Ma and 66 Ma. These lag-time measurements are consistent with active shortening and rapid exhumation rates of ~0.9->1 km/m.y. of the North American Cordillera throughout the Cretaceous.Double dating of the detrital AFT samples was performed on apatites with young AFT cooling ages, in order to test whether or not the young cooling ages represent a signal related to exhumation rather than volcanic activity. Maximum depositional ages using detrital zircon U-Pb geochronology match existing ages on basin stratigraphy. This study indicates that AFT is the most effective thermochronometer to resolve source exhumation from Lower to Upper Cretaceous foreland stratigraphy in the central Cor dilleran foreland, and indicates that source material was exhumed from 4 to 5 km depths but was never buried more than a few kilometers (<4 km) since Cretaceous time. Zircon He dates indicate that the orogenic hinter land could not have been exhumed from depths >8-9 km. Double dating of apatites (with AFT and U-Pb) shows that volcanic contamination is a signifi cant issue that can, however, be addressed by double dating.

Marginal-marine deposits are typically excellent hydrocarbon reservoirs around the world. Certain depositional environments may provide natural, selective, physical processes that produce better reservoir rocks than others depending on associated energy, hydraulic regime, etc. With this paper we present new detailed sedimentological, petrographic, and grainsize data of ancient flood-tidal-delta deposits that provide means to characterize this environment in the ancient as well as to determine reservoir characteristics. In this study, we compare grain size, grain and cement composition, and the ratio of pore space to cement from thin sections between tidal, shoreface, and flood-tidal-delta facies of the Sego Sandstone in northwest Colorado. The intent of this comparison is to provide an evaluation of different physical processes and the degree of diagenesis related to different environments to better understand the relative controls on reservoir characteristics. We find that flood-tidaldeltas show a statistically larger average grain size than either shoreface or other tidal facies. Also, flood-tidal-deltas have a smaller ratio of pore-space to cement. Shoreface facies have a larger ratio of pore space to cement overall than that of floodtidal-delta or other tidal facies. We propose that this may be due to chemistry and higher weathering rates in the backbay environment in contrast to the shoreface environment, where higher mechanical reworking may lead to selective removal of chemically unstable minerals. To test this hypothesis, we conducted statistical analyses of these petrophysical and grain size characteristics. Our results show that flood-tidal-deltas are characterized by clean upper-fine-grained cross-bedded sandstone that is well sorted and subrounded to rounded with a majority of landward paleocurrent directions and a presence of tidal bundles, mud drapes, and reactivation surfaces.

The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wavedominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave-tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.