SEQUENCE STRATIGRAPHY OF AN INTERBEDDED BIOGENIC-CLASTIC RESERVOIR, BELRIDGE DIATOMITE AT LOST HILLS FIELD, SAN JOAQUIN BASIN, CALIFORNIA

Shallowing-upward cycles (parasequences) of diatomite and sandstone in the upper Miocene Belridge diatomite (Monterey Formation) at Lost Hills field, San Joaquin basin, California, record a different depositional response to accommodation change (subsidence + sea-level) than the diatomite itself. These reservoir-scale (>100 feet thick) cycles compose three seismic-scale (150-500 feet thick) depositional sequences evidenced by progradational stacking patterns. Lithology trends, stratigraphic thinning and Pliocene onlap indicate deposition on a faulted anticlinal high analogous to the modern Lost Hills structure.

Late Miocene cooling associated with glacial-induced global sea-level fall resulted in marine upwelling that stimulated diatom blooms. Sediment-gravity flows deflected around the Lost Hills paleo-structure deposited sand and clay on the flanks, whereas “clean”, biogenic diatomite accumulated on the crest. Thus, the 670- to 1060-feet thick Belridge diatomite (0.5-0.8 my duration) at Lost Hills represents a depositional response to accommodation decrease (relative shallowing). However, diatomites at the bases of parasequences composing this interval record a different response. These parasequences are coarsening-upward packages of diatomite (bases) and sandstone (tops) bound by flooding surfaces. Because Helminthopsis burrows in the diatomites indicate slow pelagic sedimentation, whereas Teichichnus burrows in the sandstones indicate rapid, land-derived sedimentation, individual diatomite beds represent responses to accommodation increase (relative deepening).

Basinward thickening of diatomites and contrasting basinward thinning of sandstones further characterize the reservoir architecture. Also, thickening of strata on the downthrown sides of faults, and thinning over flexures, indicate deformation concurrent with diatomite deposition. Thus, although cyclic deposition evidences eustatic controls, tectonics and paleotopography were important as well.