A giant sand injection complex: Processes and implications for basin evolution and subsurface fluid flow

Abstract

Giant sand injection complexes form, intricate, basin-scale fluid plumbing systems and document the remobilisation and intrusion of several tens of cubic kilometres of sand within the shallow crust in stratigraphic units 100's metres thick. This is the first detailed and extensive account of the Panoche Giant Injection Complex (PGIG), a regionally significant outcrop (>300 km2) and part of a larger subsurface development (>4000 km2) identified in boreholes and on seismic reflection data. Magnificent exposure of the PGIC occurs along the north western margin of the San Joaquin Valley and presents the opportunity to examine the regional geological significance of a giant sand injection complex and its origin in the context of a late Cretaceous – early Paleocene forearc basin. Between 25 and 49 km3 of sand were remobilised and injected, at least 0.35 km3 of which extruded onto the paleo-seafloor. Large sandstone intrusions often >10 m thick and laterally extensive on a kilometer scale formed saucer-shaped intrusions, wing-like intrusions and a variety of sill geometries along with volumetrically smaller randomly oriented dikes in a 200–300 m thick interval. Dikes prevail below and above this interval, some reaching the paleo seafloor and extruding sand. Networks of propagating hydrofractures form intensely brecciated host strata, some of which were intruded by sand. All intrusions formed in a single pulsed event in which the most intense hydrofracturing caused by supra-lithostatic fluid pressure occurred approximately 600 to 800 m below the paleo seafloor. A crudely orthogonal arrangement of dikes is preserved with most oriented normal, and less commonly oriented parallel to the oceanic trench associated with the late Mesozoic to early Tertiary North Pacific subduction. Dikes orthogonal to the trench opened against the minimum horizontal stress, which was parallel to the trench. Dikes parallel to the trench opened against the regional maximum horizontal stress along minor faults formed in extension caused by shallow crustal deformation. There is no evidence that compressional tectonics influenced the onset of elevated pore fluid pressure necessary to promote sand injection. However, tectonic compression was responsible for creating the basin physiography that locally increased subsidence and accelerated chemical diagenesis in the basin centre. PGIC outcrop, located along the basin margins, was unlikely to have experienced heating above 70 °C, equivalent about 2 km burial, so the effects of chemical diagenesis in the host strata of the injection complex had negligible potential to evolve significant pore water volume. In a deeper part of the basin approximately 150 km to the south, lateral equivalents of the host strata were subjected to heating >100 °C and would expel significant volumes of water displaced by quartz cementation and clay dehydration that caused lateral pressure transfer to the north and western margin of the basin where the PGIC formed. Estimates of the total volume of water expelled from the deep basin suggest that a fluid volume equivalent to a gross rock volume reduction <1% would have provided a fluid budget sufficient to fluidise and inject the sand that forms the PGIC. In terms of areal and vertical extent, volume and architecture the PGIC shares strong similarity with the regionally developed giant injectite systems of Tertiary age in the North Sea basin. In both cases regional sand injection is genetically linked to pressure transfer toward the basin margin from more rapidly subsiding basin centres. Aqueous fluid is derived from thermally driven chemical diagenesis of thick deep water clastic sandstone and smectitic mudstone or from deeper, stratigraphically older, aquifers.