Reflection-Seismic Definition of a Late Cretaceous East-Vergent Thrust on the Western Flank of the Northern San Joaquin Basin and Its Influence on Deep-Water Deposition

Harrison, Paul J.¹, Stephan Graham², Denise H. Harrison¹, David H. Suek³, Steven D. Jones⁴, David B. Codding⁵ (1) Fall-Line Exploration, Dillon, CO (2) Stanford University, Stanford, CA (3) Black Coral LLC, Centennial, CO (4) Lariat Exploration, Centennial, CO (5) Yates Petroleum, Artesia, NM

The San Joaquin and Sacramento Basins beneath the Central Valley of California were parts of the larger, unsegmented Great Valley forearc basin in the Cretaceous. An exploration program in the northern San Joaquin basin utilizing regional gravity and magnetic data, 4,300 miles of 2D seismic, 300 square miles of 3D seismic data, and well data, led to drilling of three deep holes, but no commercial success.

Basement rocks of the Mesozoic Sierra Nevada magmatic arc underlie the gentle eastern limb of the asymmetric San Joaquin basin, whereas its western flank is truncated in outcrop due to Cenozoic uplift and erosion. Reflection seismic data reveal a steep slope from Sierran shelf to basin floor during Turonian-Campian time (E-H foraminferal zones of Goudkoff). The data image thick turbidite sequences in the basin axis, ponded oceanward (westward) by a sea-floor bathymetric feature. This feature is defined by high amplitude reflection packages with eastward-vergent (arcward) thrust geometry, which are overlapped by unfaulted younger Cretaceous strata. Drilling demonstrated that the ponded turbidite sequences comprise several stacked submarine fans consisting of low-quality reservoir, separated by flooding surface shale units. One well bottomed in the uppermost thrusted sheet, encountering either Great Valley Group or Franciscan Complex rocks.

Although accretionary wedge thrusting is typically ocean-vergent, arcward backthrusting as in the northern San Joaquin basin occurs in some modern convergent plate settings (e.g., Barbados forearc). Arcward thrusting in the San Joaquin basin fostered accommodation space for stacked fans through creation of seafloor topography, as well as thrust-load subsidence.