Abstract: Models Using Subsurface Data for Interpretation of Paleoenvironments of Western Interior Cretaceous Marine and Coastal Deposits

Charles W. Spencer

Previous classifications of Cretaceous depositional environments in the Western Interior have been based chiefly on sedimentary and biologic features that can be seen on outcrops. The models have been established using criteria and terminology developed by geologists who are well versed in marine geology and geomorphology. Many exploration geologists not specializing in these fields have difficulty applying the available models to practical subsurface mapping where only core and electric-log data are available. In an attempt to bridge this gap, a study was made of Upper and Lower Cretaceous outcrops and cores to simplify terminology and to identify criteria for environmental interpretation that can be recognized in unoriented, slabbed cores. Trace fossils such as Rhizocorallium, Arthrophycus, Teichichnus, and Nereites are very useful for determining environments from outcrops but almost never are seen in cores. However, Arenicolites, Asterosoma, Ophiomorpha, and nondescript bioturbation are common in cores and are helpful guides for environmental interpretation.

A core can be placed within an energy profile ranging from open-marine to coastal environment using sedimentary structures, rock type, and trace fossils. The areal geometry of a particular clastic body then can be determined from regional electric-log studies, and such features as offshore bars, deltas, and beaches can be identified. Most Cretaceous oil production in the Rocky Mountains comes from marginal-marine sandstone reservoirs. Refined environmental mapping will permit improved prediction of stratigraphic traps.

Several minor changes in marginal-marine terminology are suggested. Foreshore should be called tidal beach and shoreface should be modified to submarine shoreface. Such changes in terminology will provide a more graphic description of these particular environments.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA