PALEOGEOGRAPHIC EVOLUTION OF THE CRETACEOUS IN SE COLORADO

Cretaceous strata of SE Colorado record a largely uninterrupted sedimentary succession, deposited under a range of depositional environments, occurring between the well-defined Jurassic-Cretaceous (J-K) and Cretaceous-Tertiary (K-T) boundary layers. This section is typically 1200m thick, defined by a basal unconformity, where Jurassic redbeds are incised by Cretaceous mixed clastics; while the uppermost Cretaceous is defined by air-fall deposits associated with the Chicxulub impactor. Using outcrop (sedimentology and ichnology), core, and subsurface petrophysical log datasets a complete high-resolution Cretaceous ‘strato-type’ section is constructed.

Cretaceous sequences afford an unparalleled insight into basin scale controls, both allogenic and autogenic in origin. Five major depositional systems are identified: a) the Western Interior Seaway with deposition of cyclic marginal marine, coastal plain and fluvio-lacustrine sandstones and associated organic rich shales (Dakota Group); b) shallow marine epeiric sheet-form carbonates, marls and organic shales (Greenhorn and Niobrara Groups) with localized storm-deposits, shelly coquinas and near-shore transgressive sands (e.g. Codell Sandstone Member); c) restricted basin mudstones and shales (Pierre Group) with regional “hot-shale” highstand markers and localized thin siliciclastics reflecting detached lowstand shoreface deposits; d) establishment of the Rocky Mountain retroarc foreland basin and progradation of extensive linear clastic shoreline sandstones (Trinidad Sandstone Formation) and trailing alluvial floodbasins that accommodated extensive coal swamps (Vermejo Formation); e) initiation of Laramide orogenesis marked by an erosive, valley-fill sequence (Raton Conglomerate) composed of extraformational clasts reflecting basement unroofing and a switch to an intramontane setting.

Collectively, this Cretaceous lithostratigraphic framework and the paleogeographic interpretations allow a comprehensive evaluation of the total petroleum system of SE Colorado, enabling a more predictive method for reservoir distribution patterns.