Abstract:
We compare a middle Miocene sequence (Light Blue, approximately 14.7-14.3 Ma, Poulsen et al., in press) and a late Miocene/Pliocene sequence (pp4(s), approximately 10.0-1.7 Ma) using ODP Leg 174A log/core data, local industry wells, and closely spaced ( approximately 300-500 m) 1995 2-D seismic data. The Light Blue sequence is dominated by a thick ( approximately 400 m) lowstand wedge that onlaps the underlying breakpoint, downlaps successively seaward, and has a shale-prone, coarsening-upward log signature. It is overlain by a thin highstand deposit ( approximately 40 m) that also has a coarsening-upward log signature. The highstand deposit constitutes the entire sequence landward of the breakpoint. In contrast, the pp4(s) sequence has no evidence of onlap onto the underlying shelf. A landward-thickening ( approximately 50-150 m) sand-prone facies comprises the entire sequence 10 km landward of the breakpoint. Seaward of the breakpoint, a thick ( approximately 200 m), shale-prone, deepening-upward deposit migrates and pinches landward. It overlies the sand-prone facies on the shelf. The pp5(s) reflector separates these facies and records the transition from inner to middle neritic paleodepths. This facies succession represents a deepening-upward transgressive deposit. This is overlain by a seaward-thickening, progradational highstand deposit. On the shelf, this highstand deposit is thin ( approximately 20 m), shale-prone, and coarsens upward. It thickens 10 km seaward to approximately 150 m and is sand-dominated. During the middle Miocene, high regional sedimentation rates (Poag and Sevon, 1989) and a falling, fluctuating, eustatic sea level (Haq et al., 1987) preserved a highly progradational, relatively short duration sequence. During the late Miocene/Pliocene, high regional sedimentation rates and a large amplitude, rising eustatic sea level preserved a transgressive-dominated sequence. These differences show that sea level falls superimposed on an overall falling sea level and sea level falls superimposed on an overall rising sea level create sequences with fundamentally different internal stacking patterns.