Abstract: The Cretaceous Ferron Sandstone of East-Central Utah: A Tale of Three Different Morphologies for Deltaic Parasequences
R. D. Adams, F. W. Stapor
The Ferron Sandstone Member of the Mancos Shale is composed of fluvial and fluvial-dominated-deltaic lithofacies. Deltaic lithofacies may be used as models for deltaic reservoirs. The lithofacies are arranged into parasequences and parasequence sets that change in stacking arrangement from progradational (oldest) to aggradational to retrogradational (youngest). Because of the stacking patterns, the Ferron is interpreted as a single sequence with shelf margin and transgressive systems tracts. Subsurface correlations indicate the Ferron prograded across a pre-existing foredeep, over its eastern margin, and into shallower waters of the Cretaceous Interior Seaway.
The deltaic morphologies can be related to changes in basin-floor topography and to the different stacking patterns: (1) Thick (65-210 ft, 20-65 m), but laterally restricted (<= 3 mi, 5 km), Gilbert-type deltas are present in the southern part of the study area and stack in a strongly progradational arrangement The delta morphology resulted from deposition into relatively deep (^approx250 ft, 75 m) waters during the early shelf margin systems tract (2) Thin (15-80 ft, 5-25 m), but laterally extensive (<= 13 mi, 20 km), mouth-bar deltas are present in the central and northern parts of the study area. The stacking arrangement of these deltas is more complex and changes from progradational to aggradational to retrogradational. This delta morphology resulted from deposition into sha lower (^approx100 ft, 30 m) waters during the later shelf margin and early transgressive systems tracts. (3) Thin (15-40 ft, 5-12 m) and laterally restricted (1-3 mi, 1.5-5 km), strongly wave-influenced, mouth-bar deltas are the youngest parasequences and are found across the Ferron. They stack in a strongly retrogradational pattern. This delta morphology resulted from deposition into shallower (^approx50 ft, 15 m) waters during the later transgressive systems tract The small size of the deltas may have resulted from either low sediment influx or short duration of the parasequence.
Each delta morphology produces different reservoir geometries: (1) Thick Gilbert-type deltas form thick, sheet-like sand bodies with few shale baffles and barriers within and between parasequences. (2) Thin, laterally extensive, mouth-bar deltas form multi-story sand bodies with numerous shale baffles and barriers within and between parasequences. (3) Thin, laterally restricted mouth-bar deltas form small isolated sand bodies.
AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada