Seismic Structure of the San Fernando and Antelope Valleys, Southern California: Results From LARSE II Refraction, Industry Reflection, and Oil-Test Well Data
Abstract
The second phase of the Los Angeles Region Seismic Experiment (LARSE II) recorded refraction and low-fold reflection data from the Santa Monica Mts. to the southern Sierra Nevada, in southern California. The LARSE II corridor crossed two sedimentary basins, underlying the San Fernando and Antelope Valleys, for which we have both industry reflection and oil-test well data to aid in interpretation of geologic structure. In the San Fernando Valley (SFV), refraction modeling of the main line and three cross lines reveal five primary layers. Layer 1 (1.3 km/s), up to 0.1 km thick, represents unsaturated, unconsolidated Quaternary (Q) sediments, and Layer 2 (1.8-2.3 km/s), up to 0.4 km thick, represents saturated, unconsolidated Q sediments and weathered Tertiary (T) sedimentary rocks. Using industry reflection sections and oil-test well data, we observe the following for deeper layers: South of the Northridge Hills fault (NHF), located in the northern SFV, Layer 3 (2.3-3.2 km/s), up to 1.6 km thick, corresponds chiefly to reflective T rocks, including Modelo through Saugus Formations, and Layer 4 (3.6-4.8 km/s), up to 2.2 km thick, corresponds chiefly to non-reflective Topanga Formation and older rocks. North of the NHF, the relationship of velocity layering to stratigraphy becomes more complex, and velocities appear to increase somewhat throughout the stratigraphic section. Layer 5 (5.0-6.0 km/s) is interpreted as basement rocks and may include Mesozoic igneous and metamorphic rocks. The top of layer 5 is deepest (4 km) just north of the surface trace of the NHF. Layers 3-5 thicken markedly northward in the southern SFV, and the boundary between layers 3 and 4 is offset, up to the north, along the north-dipping NHF. In the Antelope Valley (AV), located in the western Mojave Desert, refraction modeling has identified four basic layers. Preliminary interpretations are as follows: Layer 1 (0.1 km/s or less), up to 0.2 km thick, is unsaturated, unconsolidated Q sediments. Layers 2 and 3 (2.0-3.9 km/s), which total 1.5 km in the central AV, are Q and T sedimentary rocks. An oil-test well in the southern AV bottoms at the base of Layer 3, at 1.04-km depth, in T rocks. An oil-test well in the northern AV bottoms in Layer 4 (4.4 km/s and up), at 1.01-km depth, in quartz diorite. Layer 4, thus, is interpreted as basement. The top of basement reaches a maximum depth of 1.7 km in the central AV, and upward steps in the top of this layer can be seen on both the north and south sides of the AV, at or near mapped faults.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.S21F0391F
- Keywords:
-
- 0905 Continental structures (8109;
- 8110);
- 7205 Continental crust (1242);
- 8105 Continental margins and sedimentary basins;
- 8107 Continental neotectonics