3-D seismic, well log, and petrographic analyses of the Victoria Island structure, a potential buried impact crater, San Joaquin county, California
Jared Morrow Department of Geological Sciences
San Diego State University
Bennett Spevack ABA Energy Corporation
Bakersfield
Analyses of a 3-D seismic survey and well logs in the southwestern Sacramento basin, San Joaquin County, California, have revealed a subsurface, circular, ~5.5-km-diameter anomaly that may represent a previously unrecognized complex impact crater. This unique anomaly, buried 1,490–1,600 m below sea level under the southwestern part of the Sacramento-San Joaquin Delta, is provisionally named the Victoria Island structure for an overlying surface geographic feature.
The Victoria Island structure is characterized by a concentric, annular, terraced rim and trough surrounding a structurally uplifted central peak. Well logs tied to seismic data show that the upper surface of the structure occurs stratigraphically near the top of the siliciclastic, continental to shallow-marine Domengine Formation, indicating a middle Eocene age. Overlying fill material, which reaches an estimated thickness of at least 80 m in the trough, is primarily deep-marine, middle Eocene Nortonville Shale. Both well and seismic data indicate thinned Domengine and thickened Nortonville sections across the center of the feature. A disturbed stratigraphic sequence under the structure includes upper to lower Domengine and underlying lower Eocene Capay Formation and Cretaceous-Paleocene Mokelumne River Formation siliciclastic units. Characterized by discontinuous seismic reflectors, the central peak is estimated to be ~600 min diameter with at least 35 m of structural uplift. The seismic data demonstrate that the feature is ‘rootless’, being underlain by gently dipping, relatively undeformed strata. The 3-D data further suggest the presence of a series of discontinuous, inward dipping, concentric normal faults with minor offset surrounding the trough and outer rim areas. Estimates of the dimensions of the structure indicate a circularity ratio (short-to-long axes) of 0.91 and a depth-todiameter ratio of ~0.02.
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