Facies architecture and geometry of landward-stepping shoreface tongues: the Upper Cretaceous Cliff House Sandstone (Mancos Canyon, south-west Colorado)

The Campanian Cliff House Formation represents a series of individually pro gradational shoreface tongues preserved in an overall landward-stepping sys tem. In the Mancos Canyon area, the formation consists of four, 50- to 55-m -thick and 10- to 20-km-wide sandstone tongues, which pinch out landwards i nto lower coastal plain and lagoonal deposits of the Upper Menefee Formatio n and seawards into offshore shales of the :Lewis Shale formation. Photogra mmetric mapping of lithofacies along the steep and well-exposed canyon wall s was combined with sedimentary facies analysis and mapping of the detailed facies architecture. Two major facies associations have been identified, o ne comprising the mostly muddy and organic-rich facies of lagoonal and lowe r coastal plain origin and one comprising the sandstone-dominated facies of shoreface origin. Key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of dep ositional processes. The stratigraphic surfaces (master ravinement surface, shoreface/coastal plain contact, transgressive surface, maximum flooding s urface and the sequence boundary) allow each major sandstone tongue to be d ivided into a simple sequence, consisting of a basal transgressive system t ract (TST) overlain by a highstand system tract (HST). Within each sandston e tongue, a higher frequency cyclicity is evident. The high-frequency cycle s show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion. The comple xities of the Cliff House sandstone tongues are believed to reflect changes in the rate of sea-level rise combined with the responses of the depositio nal system to these changes. Synsedimentary compaction, causing a thickness increase in the sandstone tongues above intervals of previously uncompacte d lagoonal/coastal plain sediments, also played a role. This study of the f acies architecture, geometry and sequence stratigraphy of the Cliff House F ormation highlights the fact that there may be some problems in applying co nventional sequence stratigraphical methods to landward-stepping systems in general. These difficulties stem from the fact that no single stratigraphi c surface can easily be identified and followed from the non-marine to the fully marine realm (i.e. from the landward to the basinward pinch-out of th e sandstone tongues). In addition, the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface.