Masters Thesis

Marine geology of the western extension of the transverse ranges : Point Conception to Point Arguello

The study area lies offshore of the western Santa Ynez Mountains and contains the northwestern corner of the Santa Barbara Basin and the offshore southern-most portion of the Santa Maria Basin. Two major geologic-geomorphic units, the western Transverse Ranges and the Coast Ranges Provinces meet within this area. The junction of these provinces and their evolution during Cenozoic and Quaternary time were studied by using and correlating geological and geophysical data sets. These data sets include: 1. Seismic reflection data, including digitally processed common depth point, 4 second profiles. 2. Surbsurface information from onshore and offshore core holes, including the stratigraphic test hole OCS-CAL-78-164- No. 1. 3. Onshore mapping and stratigraphic analyses by Dibblee (1950, 1978), Link (1975), and other workers. Methods used in this study include: shallow-bedrock geologic mapping of structural trends and stratigraphic units; correlation of seismic stratigraphic horizons; and structure contour mapping on key horizons (top mid-upper Miocene, near top Monterey, and top pre-Vaqueros unconformity). In addition, Quaternary fault activity was evaluated using high resolution seismic profiles. In contrast to the results of previous studies, the transition or junction of the western Transverse Ranges and Coast Ranges Provinces was found to be abrupt, not gradual. This junction is mapped along a northerly continuation of the F-1 fault zone west of Point Conception. East of Point Conception the F-1 fault zone, trends east-west as is typical of the Transverse Ranges Province. To the west of the point, the fault swings abruptly to the northwest-southeast. Structures west of this area include the Hueso anticlinal trend, which is the location of a recently discovered major oilfield. The Paleogene evolution of the study area is recorded by an onlapping regressive sequence of marine Paleocene-Eocene-Oligocene rocks. These rocks onlap a northeasterly trending erosional surface of Cretaceous rocks. However, details of this trend are not clear due to the tectonic overprinting of Neogene trends. In particular, the onland location of the eastern extension of the Paleogene trend is unknown. Neogene trends within the area are typical of their respective geomorphic province. The Neogene structures in the westernmost Transverse Ranges Province are orientated east-west, while the Neogene structures in the offshore Santa Maria Basin trend northwest-southeast. Many of the east-west trends are active today. These faults display sea-floor offsets, while folds show thickening of Quaternary sediments along synclinal troughs and a thinning over anticlinal crests.

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