Browsing by Subject "subsidence"
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Item An experimental method to increase sediment supply to a salt marsh in subsidence dominated environments(Texas A&M University, 2007-09-17) Thomas, Robert C.This thesis examines the environmental conditions which led to the loss of 90% of the natural salt marsh in Galveston Island State Park since 1930 and analyzes one potential method to reduce future loss. Available data and recent studies suggest that the primary factor responsible for the historic loss was the lack of a sufficient supply of sediment to keep up with relative sea level rise. The average rate of sediment accretion for the period from 1963 to 2006 was measured to be 0.25 cm/year based on 137Cs and 239,240Pu nuclides. This rate is about 0.4 cm/year less than the relative sea level rise of approximately 0.65 cm/year during the same period. The marsh restoration project, constructed in 1999 at the Galveston Island State Park, focused on reduction of wave induced erosion and direct replacement of marsh substrate through terracing. The restoration project did not address the potential for marsh lost to submergence. As an alternative to geotubes or more permanent breakwater methods, a submerged sacrificial berm constructed around the marsh is a possible approach to address ongoing submergence. The sacrificial berm increases the available sediment supply by allowing partial transmission of waves to create a net transport of sediment into the marsh. In addition, the berm is designed to limit wave height in the marsh to reduce wave induced erosion. The proposed method involves iteratively adjusting the width and elevation of the berm top to maximize sediment transport from the berm into the marsh. A sediment transport model is developed to quantify the increased transport into the marsh, estimate a nourishment interval and qualitatively judge the expected berm evolution. The Galveston Island State Park marsh was used for demonstration purposes; however, the restoration concept and method of analysis is applicable to other marshes in Galveston Bay.Item Tectonostratigraphic evolution of the northeastern Maturin foreland basin, Venezuela(2009-08) Taboada, Gustavo Adolfo; Fisher, W. L. (William Lawrence), 1932-; Mann, Paul, 1956-; Steel, Ronald J.; Escalona, AlejandroThe study uses subsidence analysis of three deep wells to basement combined with sequence stratigraphic mapping to show that a 85,000 km² area of the Eastern Venezuelan foreland basin in the region of the Orinoco Delta underwent three main stages of foreland-related subsidence that followed a protracted Cretaceous - late Oligocene period of precollisional, passive margin formation. Phase 1 consists of increased foreland basin subsidence in the late Oligocene to middle Miocene (23 - 13 Ma) at average sedimentation rates of 0.14 mm/yr. Clastic rocks of Phase 1 include the Freites Formation, a 1.2 km-thick section of greenish-gray fissile shale and shaly sandstone deposited in shallow marine- neritic environments. Seismic facies show progradation of Phase 1 clastic rocks as a wedge from the NE and NNE. Clastic rocks deposited during the accelerated Phase 2 in the middle to late Miocene (13 -11 Ma at sedimentation rates of 1.45 mm/yr) include the La Pica Formation, a 2.7 km-thick section of gray silt and fine-grained sandstone deposited in shallow marine/coastal proximal environments. Seismic facies show progradation of Phase 2 clastic rocks as a wedge to the northeast. Phase 3 consists of decelerating foreland basin subsidence in the period of late Miocene-mid Pliocene (11-6 Ma at average sedimentation rates of 0.86 mm/yr). Sedimentary rocks deposited during this period include the Las Piedras Formation, a 1.45 km-thick section of sandstone, carbonaceous siltstone and shale deposited in deltaic environments. Seismic facies show a progradation of Phase 3 clastic rocks as a wedge to the northeast and east-northeast. Deeper marine environments and more rapid subsidence rates of Phases 1 and 2 are interpreted as an underfilled foreland basin controlled by active thrusting along the Serrania del Interior at the northern flank of the basin. Deltaic environments and slower rates of Phase 3 are interpreted as an overfilled foreland related to rapid seaward progradation of the Orinoco Delta and its filling of the former, dynamically- maintained interior seaway. Paleogeographic maps constrained by wells and seismic lines show a large regression of the Orinoco River towards the west across the Columbus basin and Eastern Venezuelan basin during the late Miocene and the Paleocene. In this foreland basin setting, the effects of thrust-related tectonic subsidence and early deposition of the Orinoco Delta play a larger role in the early Miocene-Pleistocene sequences than eustatic effects.