Sedimentology and sequence stratigraphy of fan-delta and river-delta deposystems, Pennsylvanian Minturn Formation, Colorado

Abstract

Fan-delta and river-delta strata of the middle Pennsylvanian Minturn Formation were deposited in the Central Colorado trough and are well exposed in the Sangre de Cristo Mountains. Proximal fan-delta strata were deposited in lowstand, transgressive, and highstand systems tracts, whereas distal fan-delta strata were deposited in lowstand, highstand, and forced regressive systems tracts. Fan-delta strata were deposited adjacent to coeval westward-dipping thrust faults along the western margin of the Central Colorado trough. Inferred high subsidence rates caused by thrust loading along the western margin of the basin created accommodation space closer to the basin margin and resulted in localized aggradation of fan-delta deposits.Proximal river-delta strata were deposited in transgressive, highstand, and early regressive systems tracts, whereas distal river-delta strata were deposited during all stages of sea level change. River-delta deposystems are interpreted to have formed in parts of the basin that experienced relatively less subsidence associated with Pennsylvanian thrust loading. Lower subsidence rates associated with river-delta deposystems resulted in progradation into more distal parts of the basin.Results of our study point out that lateral changes in depositional systems, related to local variation in tectonic subsidence, may produce significant along-strike differences in the sequence stratigraphic framework of flexural basins. Our analysis also shows that potential reservoir facies in coeval fan-delta and river-delta deposystems form at different times and in different parts of the basin during sea level fluctuation.