Print this pageAbstract: Petroleum Accumulations in Santa Margarita Formation, Western Santa Cruz Mountains, California
The Santa Margarita Formation in the western Santa Cruz Mountains, California, locally contains discontinuous and patchy accumulations of heavy petroleum tar sands. The history of petroleum accumulation and trapping reflects the depositional environment of the sandstone and the Neogene structural development of the region.
The Santa Margarita Formation, here of middle and late Miocene age, represents a transgressive, tide-influenced deposit. A siliceous shale, the Santa Cruz Mudstone of Clark, conformably overlies this formation. The tar sands occur within breached exposures of a west-dipping homocline of both formations.
A broad tidal-channel system, up to 8 km wide, dominated much of the depositional environment of the Santa Margarita. The channel-fill sediments contain sandsize to gravel-size material that was deposited in west to southwest-migrating, large-scale, cross-stratified beds. These coarse channel sediments provided a conduit for the initial migration of petroleum, and differential compaction between the thick channel sandstones and the overlying shales provided a structure for the initial accumulation of petroleum. Later folding, on a northeast-southwest trend, further reproduced structures in which petroleum has accumulated.
Faulting related to strike-slip movement of the San Gregorio fault system fractured the capping shales and possibly allowed some of the petroleum to leak from the structures. Horizontal and vertical migration of the tar sands occurred along faulted and brecciated, northwest-to northeast-trending shear zones.
Stratigraphic and structural petroleum traps in the Santa Margarita Formation are present along the main tidal-channel axis, within pinchouts of sand on topographic highs, in fault-bounded blocks in homoclinal folds, and in clastic dikes of fault zones.
AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California