Cretaceous mixed siliciclastic and carbonate sedimentation patterns: Examples from northern Mexico, Texas, and Montana

Four study areas in northern Mexico, Texas and Montana were selected to study siliciclastic and carbonate sedimentation, fossil assemblages, facies associations, and their distribution. The results of the studies contribute relevant sedimentologic and paleontologic data for interpretation of Cretaceous depositional environments along North American basin margins. All major features of the Middle Albian Fredericksburg Group are exposed in a condensed section of 10 m of profile in an outcrop at Lake Texoma, Grayson County, Texas. The general depositional trend ranges from older deltaic to younger shallow open marine conditions. The development of a terrestrial hardground at the top of the Paluxy Sand Formation points to the presence of a sequence boundary. This sequence boundary can be correlated to the one between UPPER ZUNI A 1.2 and 1.3 in the Mesozoic and Cenozoic Cycle Chart of Haq et al. (1988). The Middle Albian Finlay Formation was studied in the Rimrock escarpment in Hudspeth County, Texas. A stratotype profile was stratigraphically documented and compared to other areas along the escarpment. The Lower Finlay Formation contains a succession of nodular weathering limestone layers intercalated with marls, which were deposited in a ramp depositional system. The Upper Finlay Formation includes massive, cliff-forming biowackestones to biopackstones, which contain caprinid rudists in the upper part and deposited on a carbonate platform. Two sequence boundaries can be recognized that are correlative with the major A5 and minor A6 sequence boundary of the new chronostratigraphy chart of Hardenbol et al. (1998) and the UPPER ZUNI A 1.2 and 1.3 in the Mesozoic and Cenozoic cycle chart of Haq et al. (1988). An outcrop of Late Aptian age was sampled for bivalves and gastropods in the Lampazos Sequence, central Sonora, Mexico. Fossils were collected from the upper black shale beds of the Agua Salada Formation. The lower part of the formation includes thick chert beds, which are rich in ammonites, whereas the upper part mainly consists of medium gray to dark black shale beds. Six different gastropod species can be recognized, mainly belonging to the mesogastropod subclass. In addition, six different bivalve species were observed. An open shelf environment is proposed for the Agua Salada Formation. The lower Campanian (Upper Cretaceous) Virgelle Formation was studied in northwestern Montana in front of the Rocky Mountain foothills near the Augusta syncline. The Virgelle Formation includes 84 m (276 ft) of coarsening upward siltstone to sandstones, deposited during an overall regressive phase along the western coast of the Western Interior seaway. After deposition, the facies associations were transported to the east during thin-skinned thrusting. During tectonic activity, post-depositionally cemented sandstone layers were pervasively fractured. Facies stacking patterns were used to establish a sequence stratigraphic framework in the study area