Sequence stratigraphy of the Upper Cretaceous Castlegate condensed section through Iles Formation, Piceance Basin, northwestern Colorado.

摘要

The Piceance Basin of northwestern Colorado is a Laramide age intermontane basin within the Rocky Mountain region. Fifteen Campanian age formal and informal stratigraphic units of the uppermost Mancos Shale and lower Mesaverde Group were correlated across the Piceance Basin study area using 2,747 electronic well-logs. The fifteen units include: the Castlegate condensed section, the Morapos Sandstone, the Castlegate Sandstone, the Buck Tongue of the Mancos Shale, the Loyd Sandstone, the lower Sego Sandstone, the Anchor Mine Tongue of the Mancos Shale, the upper Sego Sandstone, the Corcoran Member of the Iles Formation, the Anchor coal zone, the Palisade coal zone, the lower Cozzette Member of the Iles Formation, the Chesterfield coal zone, the upper Cozzette Member of the Iles Formation, and the Rollins Member of the Iles Formation.;Ten different log facies were identified within the study interval and were grouped into five different depositional environments: offshore marine, lower shoreface, upper shoreface-deltaic, coastal plain, and estuarine complex. Marine deposits were correlated (when possible) landward to their corresponding coastal plain time equivalents.;Eight sequence boundaries and four major flooding surfaces were identified in the Castlegate condensed section through Rollins Member interval. Sequence boundaries were recognized by facies dislocation. Typically, incisions are overlain by tidally influenced strata.;Eight regressive clastic units within the Morapos Sandstone through Rollins Member interval were identified. The regressive units represent the progradational shoreline evolution within the Piceance Basin. Each unit has a corresponding underlying tongue of Mancos Shale. The mapped maximum regressive and transgressive extents reflect both the overall regression of the Western Interior Seaway and alterations in basin subsidence rates due to pre-Laramide tectonic activity.