ABSTRACT: Controls on Stratal Cyclicity in a Shelf to Basin Transition, Middle Eocene, Southern California

Michael S. Clark

The Narizian (middle Eocene) stage in the Topatopa Mountains of southern California is represented by parts of the Matilija, Cozy Dell, and Coldwater formations--siliciclastic strata that filled a marine fore-arc basin. Facies representing shelf, slope, and basin environments record three orders of stratal cyclicity: (1) individual bedsets, (2) progradational units (parasequences) comprised of multiple bedsets and bound by deepening events, and (3) seismic-scale sequences (depositional sequences) comprised of multiple parasequences and bound by unconformities.

Stratal geometries, parasequence stacking patterns, and facies distributions document five depositional sequences: (1) the middle Matilija (Na1), (2) upper Matilija and Cozy Dell (Na2), (3) lower Coldwater (Na3), (4) middle Coldwater (Na4), and (5) upper Coldwater (Na5). Transgresssive surfaces of erosion cap sequences Na2 and Na5 and separate discordant strata or changes in porosity. Regressive surfaces of erosion cap sequences Na1 and Na4. Also, a sand-rich parasequence in the Cozy Dell half-sequence indicates an increase in the sand/shale ratio of sediment entering the basin, even though paleosoils and geohistory diagrams that Narizian deposition in southern California was characterized by decreasing sedimentation rates.

Depositional patterns are attributed to combined tectonic and eustatic controls. Formation of sequences Na2 and Na4 was followed by increased rates of basin subsidence, which created transgressive surfaces of erosion and drowned the sequence capping disconformities. By contrast, uplift of source areas and/or a catastrophic event resulted in deposition of the sand-rich parasequence in the Cozy Dell half-sequence. The regressive surfaces of erosion capping sequences Na1 and Na5 formed during base-level lowering and originated from tectonic and/or eustatic controls. Sequence Na3 was not modified by tectonic processes and may be eustatic in origin. Thus, variable rates of sedimentation, basin subsidence, and/or source area uplift created migrating depocenters that controlled depositional atterns in at least two of the Topatopa sequences. Hence, local controls on sedimentation existed, and no single stratigraphic column is likely to be representative of the depositional and burial history of the entire basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990