BASIN-MARGIN SEDIMENTATION: EOCENE LA JOLLA GROUP, SAN DIEGO COUNTY, CALIFORNIA
MAY, JEFFREY ALLYN
Doctor of Philosophy thesis
Continental to deep-marine facies transitions, eustatic versus tectonic controls on basin-margin stratigraphy, shelf-edge unconformities, and depositional mechanisms along basin margins were investigated for Middle Eocene strata, San Diego County, California. Coeval fan delta, nearshore, offshore, shelf, slope, submarine canyon, and proximal submarine fan facies indicate steep paleobathymetric gradients. Mass-transport processes dominated the canyon-fan system: sandy and muddy debris flows, fluidized and liquefied flows, grain flows, high- and low-density turbidity currents, slumps, and rockfalls. The submarine canyon fill is tripartite and fining-upward, representing progressive detachment from a nearshore source. Planar- to convolute-laminated sandstone overlies a basal amalgamated pebbly sandstone. Lithologically variegated cross-cutting channels to 100's of meters wide cap the sequence. A qualitative sand budget indicates the pebbly sandstone bypassed the wave zone, directly tapping an unsorted fluvial source. Residual lag deposition predominated. The coarsest fraction (0 to 3 (phi)) was also trapped and deposited by traction in the paralic zone, whereas intermittent suspension removed the 3 to 4 (phi) component onto the shelf. Size-sorting occurred downcanyon. Traction and intermittent suspension characterized inner-fan channel deposition. Lag plus traction and suspension constituents distinguish mid-fan channels. Eustacy primarily controlled stratigraphic development. A depositional "hemicycle" of 9-10 m.y. corresponds to Vail et al.'s (1977) supercycle Tb. Punctuation by marine progradation was concurrent with an intervening eustatic fluctuation. Subaerial notching of the shelf edge coincided with the Late Penutian sea level drop. During the subsequent rise, canyons eroded headward and a thin, retrogradational sequence was deposited. Coarse-grained, nearshore accumulations of the Early Ulatisian highstand were flushed basinward, responding to a slight sea level fall; submarine fan progradation resulted. After minor retrogradation, a Late Ulatisian to Early Narizian highstand induced thick, progradational development. Similar stratigraphic sequences developed simultaneously in other Pacific margin coastal basins. This suggests primary eustatic control on sedimentation and/or simultaneous continental-margin uplift and subsidence. Variations in rates of and absolute paleodepth changes indicate local tectonics. Combining global sea level fluctuations and resultant depositional patterns can provide a powerful tool in frontier exploration.