3-D Seismic and Well Log Analyses of the Victoria Island Structure, a Potential Buried Impact Crater, San Joaquin County, California

Introduction: 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 (Figs. 1–3). 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. Description: The Victoria Island structure is characterized by a concentric, annular, terraced rim and trough surrounding a structurally uplifted central peak (Figs. 3–4). 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 [1]. 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 (Figs. 1–3). A disturbed stratigraphic sequence under the structure includes upper to lower Domengine and underlying lower Eocene Capay Formation and CretaceousPaleocene Mokelumne River Formation siliciclastic units. Characterized by discontinuous seismic reflectors (Fig. 3), the central peak is estimated to be ~600 m in 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 (Fig. 3). The 3-D data further suggest the presence of a series of discontinuous, inwarddipping, 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. Ongoing Work: The observations above, including the seismic expression, complex morphology with central uplift, high circularity, depth-to-diameter ratio, and anomalous setting of the structure, are most consistent with documented, diagnostic characteristics of impact craters [2–4]. Ongoing petrographic work is examining drill cuttings from wells within and around the structure, to seek such additional impact indicators as an impactite layer, shocked mineral grains, glass fragments, or melt particles, and to assess the feasibility of future geochemical analyses of the structure. Supplemental cuttings and well log data may also further constrain the stratigraphic age of the structure within the Domengine-Nortonville interval. Together with the previously proposed, 1.3-km-diameter, Miocene-age Cowell structure [5], the Victoria Island structure represents the second potential buried impact crater from California’s Central Valley region. Acknowledgments: John Spray, Richard Pike, and Raymond Sullivan gave helpful input during the early phases of the project. ABA Energy, Bakersfield, CA, and Rising Star Energy, L.L. C. , Dallas, TX, are gratefully acknowledged for providing access to seismic data, well logs, and well cuttings used in the project. The California Well Sample Repository, Bakersfield, CA, is also thanked for providing additional well log data and cuttings currently under study. References: [1] Sullivan M. D. et al. (2003) Pac. Sec. SEPM Guidebook 94, 51 p. [2] Melosh H. J. (1989) Impact cratering, a geological process, Oxford Univ. Press, 245 p. [3] Therriault A. M. et al. (2002) Bull. Czech Geol. Survey 77(4), 253–263. [4] Stewart S. A. (2003) Geology 31(11), 929–932. [5] Blake R. G. (1998) AAPG Bull. 82(5A), 842.