3-D Facies Architecture of a Fluvial-Dominated Deltaic System in the Cretaceous Ferron Notom Delta Complex,

Three dimensional facies architectural studies of ancient deltaic systems are critical for developing 3-D models for subsurface reservoir characterization. Variably-oriented exposures of clinoforms within fluvial dominated parasequences of the Cretaceous Notom Delta complex of the Ferron Sandsotne Member, of the Mancos Shale Formation, Utah, allow a 3D reconstruction. The Notom delta complex has been divided into 6 sequences, and 17 parasequence sets. This study focuses on Parasequence 6, which is the youngest parasequence within the strongly seaward-stepping Parasequence Set 6. The outcrop consists of three well-exposed cliff faces. Photomosaic interpretation were integrated with detailed sedimentological measured sequences. Each of the seven measured sections shows a similar coasening upward facies succession. Sandstone beds in the distal delta front and proximal show hummocky cross stratification (HCS) and climbing wave ripples, suggesting rapid deposition during storms. Several beds show abundant Bergaueria (sea anemone resting traces) suggesting periods of fully marine conditions. The low bioturbation, and abundance of Bouma sequences suggest river-fed, possibly hyperpycnal delta front turbidites. The locally high levels of fully marine bioturbation on bed tops suggest a return to fully marine conditions . The presence of HCS and aggrading wave rippled beds indicates that the river-flood deposits were at times associated with major oceanic storms. Rapid deposition caused sediment liquefaction and failure, resulting in the abundant soft-sediment deformation features. We this interpret this systems at a river-flood, storm-dominated deltaic succession. Externally, the parasequence consists of clinoforms beds. Beds dip at a maximum of about 6°. In strike view they show a mounded, lens-shaped geometry. These are interpreted as mouth bar deposits. Bed boundaries show little evidence of undulation and scours, despite the overall lens-shaped external geometry. Individual mouth bar sands pass into frontal splay deposits, and may extend for several 100 meters offshore. Along-strike, lenses are less than a few hundred meters in width. Beds may also be draped with silty mudstones, which may represent flow barriers or baffles indicating a complex reservoir architecture.