FACIES ARCHITECTURAL STUDY OF INCISED VALLEYS, DISTRIBUTARY CHANNELS, AND MOUTH BARS IN THE CRETACEOUS FERRON NOTOM DELTA, SOUTHERN CENTRAL UTAH, USA.
This dissertation is focused on analysing architectural elements to understand the formative processes in linked fluvial and deltaic depositional systems in an ancient system. A compound incised valley system (IVS) and distributary channel and crevasse delta system are documented in successively more distal positions within outcrops of the Cretaceous Ferron Notom Delta in central Utah. The compound IVS is composed of three simple IVS systems, IVS3, IVS2 and IVS1 (oldest to youngest). IVS3 consists of tidally influenced deposits that form a terrace cut into lower shoreface deposits. IVS2 consists of multi-storey fluvial deposits with minor tidally influenced fluvial deposits in the upper 10%. IVS1 consists entirely of medium-grained fluvial deposits. The composite valley fill records generally increasing fluvial-dominance and decreasing tide-influence during successive cut-and-fill episodes associated with each simple valley fill. These changes are interpreted to correlate with a longer term, stepped relative fall of sea level, punctuated by stillstands, or minor rises of sea level. A lower delta-plain distributary channel system is mapped in 3-D outcrop exposures in Parasequence 5a, Sequence 2. A main channel belt about 250 m wide narrows to 200 m downstream of the branching point. The subordinate channel belt is 80 m wide. Water discharge from the main distributary channel, upstream of the branching point, is estimated to be 85-170 m3/s. Compared to paleodischarge of trunk rivers mapped in previous studies in the Notom Delta, the branching is estimated to be a 4th order split. A crevasse delta that prograded toward the west is mapped in Parasequence 6a Sequence 2, while the regional delta prograded toward the east. The crevasse delta was protected from marine influence by a wave-dominated barrier system. The proximal delta-front facies consists of planar beds which pass upward into meter-scale low-angle cross beds, which decrease in dimensions upward and finally change to decimeter-scale cross beds. Planar-stratified sandstones are interpreted to be deposited in an inertia-dominated environment and cross-stratified sandstones are interpreted to be deposits in a friction-dominated environment. The upward decrease of cross set dimensions is mainly due to the filling of accommodation and shallowing of the water.