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

Abstract

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 55° to the San Gregorio Fault and about 70° to the San Andreas Fault during the late Miocene. This stress field is similar to the modern stress field and suggests moderate fault weakness.

Fluid migration and fluid seepage in the Connemara Field, Porcupine Basin interpreted from industrial 3D seismic and well data combined with high-resolution site survey data

“…The crest of the horst block is a very leaky trap and fluid injection features are abundant. Pore fluid pressures can rise by lateral flow towards the crest of the horst block and exceed the minimal principal stress in the overlying Paleocene seal (Stump and Flemings 2000;Boehm and Moore 2002). Where the tops of the fault blocks occur at deeper levels the Paleocene and Cretaceous seals become more effective.…”

Rapid Sedimentation, Overpressure, and Focused Fluid Flow, Gulf of Mexico Continental Margin

“…The lower part of the basin contains mostly thin-bedded, muddy turbidites. The upper part contains muddy slump deposits derived from the basin margin and sand-rich turbidites that form several packets of very fi ne to fi ne sand beds that are ultimately at the seafl oor (Boehm and Moore, 2002;Davies et al, 2002;Seldon and Flemings, 2005). This is a potentially important mechanism for the transfer of fl uids from the solid earth to the hydrosphere and the atmosphere.…”

Unusually large clastic dykes formed by elutriation of a poorly sorted, coarser-grained source

“…In both areas, the distributions of widths are approximately log-normal, with many narrow dykes and only a few large ones (some narrow dykes were not measured at Coombs Hills). The widest dyke at Coombs Hills (75 m) can be followed over nearly 1.5 km; its large size is apparent by comparison with Boehm and Moore's (2002) description of ≤150 m-wide Miocene dykes in California as "the largest subaerial exposures of clastic intrusions on earth".…”