Geometry and Fluid Flow Mechanisms of the Bituminous Sandstone Intrusion at Yellow Bank Creek, Northern Santa Cruz County, California
Brian J. Thompson
Clastic intrusions of Late Tertiary age crop out along the southwestern flank of Ben Lomond Mountain, central California. The intrusion at Yellow Bank Creek is anomalous among these intrusions because of its large size and internal complexity. Documentation of this intrusion constrains a source for the intrusive sandstone, reveals the mechanics of its emplacement, and suggests that fluidization occurred near the edge of a petroleum reservoir.
Fluidized sands intruded fractures within the local Santa Cruz Mudstone. Characteristics of these intrusions correlate with depositional facies (cross-bedded or bioturbated) in the underlying Santa Margarita Sandstone. Volcanic clastic grains within the intrusion at Yellow Bank Creek suggest the overlying Purisima sandstone as a possible source, but geometry, geothermal history, and the overall petrography of the intrusion indicate the underlying Santa Margarita Sandstone was also a source.
Sea cliffs at Yellow Bank Creek expose a 300 m wide intrusion which displays a complex internal configuration consisting of fluidization structures (flow banding and bituminous heave structures) and intricate patterns created by secondary limonite staining. Fluidization structures and geometry of the intrusion show flow pathways and expansion directions of the fluidized sands during emplacement. Structure within the intrusion also suggests that injection of bituminous sands followed fluidization of water-saturated sands. A model is proposed where fluidization began in water-saturated sediments of a finer-grained bioturbated facies and expansion of the intrusion during fluidization tapped underlying coarser-grained sediments of the cross-bedded facies that were at least partially satur ted by petroleum.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California