Fluidized sandstone intrusions as an indicator of Paleostress orientation, Santa Cruz, California

The late Miocene sandstone intrusions of northern Santa Cruz County, California, are the largest subaerial exposures of clastic intrusions on earth. The intrusions are sourced from a sandstone, underlying mudstone, accumulated in an outer shelf to upper slope environment. Dikes are the most frequent intrusion type, reach the greatest thickness and tend to strike north-east and dip steeply. One giant dike is more than 150 m wide. Sills are least frequent, locally >8 m thick and have no clear preferred geographical distribution. Clustered intrusions are commonly <10 cm thick and mostly composed of dikes of various attitudes. The majority of the intrusions probably were injected shallowly as some extrude onto the seafloor. The local seafloor extrusion also indicates injection during the deposition of the Santa Cruz Mudstone (7–9 Ma). The intrusions are concentrated at the basin margin. Fluid pressure at the centre of the basin and perhaps hydrocarbons were communicated to the basin margin through the then sand, causing fluid overpressures that contributed to the fluidization and intrusion into the overlying mudstone. Primarily north-east-striking, steeply dipping dikes and secondarily, shallowly dipping sills are most significant in terms of regional connectivity of intrusions and physical dilation of the formation. The orientation of the dikes and sills indicates a regional stress field with a horizontal NE–SW maximum and a NW–SE minimum compressive direction. The simultaneous development of dikes and sills suggests similar magnitudes of the minimum and intermediate principal stresses. Preferential weakness along bedding contributed to the development of sills. Palaeomagnetic data indicate no significant block rotation around a vertical axis. The maximum principal stress direction indicated by the intrusions is about 558 to the San Gregorio Fault and about 708 to the San Andreas Fault during the late Miocene. This stress field is similar to the modern stress field and suggests moderate fault weakness. Key-words: contour plots, high-permeability conduit for the transmission of fluid pressure, paleostress field from preferred intrusions orientation, regional strain, sandstone fluidization and intrusion Received 23 June 2001; accepted 20 December 2001 Corresponding author: Dr Anja Boehm, IIS – ETH Zürich, Gloriastr. 35, 8092 Zürich, Switzerland. E-mail: [email protected]. Tel: þ41-1-6324664. Fax: þ41-1-6321194. Geofluids (2002) 2, 147–161