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Title: The origin and diagenesis of intruded sandstones
Author: Jonk, Rene
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2003
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Clastic intrusions occur within both sedimentary strata and igneous and metamorphic rocks. They are most commonly composed of sand-sized material and two groups of intruded sandstones may be distinguished: (1) Injected sandstones, and (2) Passively infilled sandstone intrusions. Injected sandstones are becoming increasingly recognized within sedimentary strata and are the focus of investigation here. They form through fluid-suspended transport (fluidization) of grains into pre-existing or newly created hydraulic fractures in the host rock. Fluid-suspended transport is triggered by the development of fluid pressure gradients related to overpressuring in the source sandstone. Since overpressuring may generate reverse pressure gradients, grains suspended into the fluid may travel both upward and downward into the host rocks, thus both upward and downward injected sandstones exist. Sand injection may be caused by a variety of mechanisms, but earthquake shaking and rapid loading seem the most plausible ones. Passively infilled sandstone intrusions are driven by the force of gravity and can thus only intrude downwards. Two mechanisms of passive infilling are distinguished (1) Infilling of a pre-existing (karstified) topography during deposition of the source sandstone, and (2) post-deposition forceful downward intrusion (under the weight of the overburden) of unconsolidated sand into fractures in an underlying competent rock. Passively infilled sandstone intrusions may be difficult to distinguish from downward injected sandstones, but the former generally contains sedimentary stratification that is not observed in injected sandstones. Diagenetic studies of intruded sandstones provide useful insights into their behaviour as fluid conduits. Three important stages of fluid flow may be distinguished: (1) Fluid flow associated with the intrusion process. In the case of injected sandstones, the injection process provides short-lived catastrophic phases of fluid flow and early diagenetic imprints may provide information on the fluids driving injection. (2) Following intrusion, sandstone dykes and sills are typically high permeability structures within (generally) low permeability host rocks and thus interconnected networks of clastic dykes and sills provide important (subvertical) pathways for fluid flow after their formation. However, diagenesis may turn fluid conduits into fluid barriers and early, pervasive cementation (particularly carbonate cementation) is commonly observed. (3) Later phases of tectonic deformation may have a significant impact on reactivating cemented intruded sandstones as fluid conduits if they are hosted by incompetent host rocks (mudstones). Pervasive veins in intruded sandstones are evidence of preferential fracturing and renewed fluid flow in the cemented intruded sandstones.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available