Abstract #1
Ground-water samples were collected from a section of the upper Texas Gulf Coast to determine the source of salt-water contamination in the Chicot aquifer of the Gulf Coast aquifer system. Two brackish-water areas were sampled in Matagorda County where chloride concentrations are up to 1127mg/L. One site, adjacent to the Gulf Coast, was selected to test an area most likely contaminated with sea water. The other site approximately 25 km inland. A complete chemical analysis was performed on all the samples (Ca, Na, K, Mg, SiO2, Fe, Mn, P, Li, Sr, Ba, B, Cl, Br, I, HCO3, SO4, NO3, HPO4, H2 S, total dissolved solids, temperature, conductivity, pH, Eh), as well as stable isotope analysis for oxygen d18 O and hydrogen dD.
Of the four possible sources for salt-water contamination, mixing with salt-rich water from the overlying Beaumont Formation, halite-dissolution brines, and sea-water intrusion could be discounted based on I-Br-Cl chemistry. Iodide concentrations appear coupled with bromide and chloride concentrations in all the Chicot brackish waters but not chloride. Iodide values in the Chicot brackish water are two to ten times greater than the expected values produced by sea water mixing. Similarly, I/Cl and Br/Cl values in the Chicot brackish water are more than an order of magnitude greater than expected values produced by mixing with halite-dissolution brines. In contrast, I-BR-Cl compositions show that water from the coastal and inland areas have a component of mixing with brines from underlying geopressured sediments. While the two areas do not get brackish enough for the stable isotopes of d18 O and dD to be clear source water tracers, the coastal Chicot water shows a slight isotopic trend towards that of the geopressured fluids.
Abstract #2
The Siwalik Basin in Nepal is a foreland basin that lies between the Himalaya to the north and the Ganges-Terai plain of India to the south. The basin formed as a result of the convergence between India and Asia and the related uplift along two major north-dipping faults in the Himalaya – the Main Central Thrust (MCT) and the Main Boundary Thrust (MBT). The Siwalik Group is a Neogene foreland sequence consisting of sandstones, siltstones and shales whose deposition is directly related to the tectonic evolution of the Himalaya. Several indirect indicators of thrust fault activity observed in a 1740-meter Siwalik section in Tinau Khola provide clues about the timing of deformation in the Himalaya.
The strata in Tinau Khola record an overall coarsening upward fluvial and alluvial sequence. Three separate lithostratigraphic units were identified: the Lower Siwalik (LS), the Lower Middle Siwalik (MS1), and the Upper Middle Siwalik (MS2). The LS rocks were deposited in a well drained fluvial floodplain environment. A transition to coarser sandstone packages above the LS-MS1 boundary indicates the migration of numerous fluvial channels into the study area. The MS2 interval records continuing progradation of a major fluvial system and possibly transition from fluvial to alluvial conditions. Three major coarsening-upward sequences (100’s meters in scale) were observed, and are presumed to have been deposited as a result of some combination of tectonic and geomorphic processes.
Magnetostratigraphic analysis of the Tinau Khola section indicates a depositional age of ~11 Ma to ~7.6 Ma, with the LS/MS1 boundary placed at ~9.1 Ma. Analyses also show three long-term episodes of increased sediment accumulation rates from ~10.6-10.1 Ma, ~9.1-8.8 Ma, and ~8.3-<7.6 Ma. These episodes alternate with the coarsening-upward sequences observed in the measured section. The data suggest a relationship between the episodes of rapid accumulation rates and the migration of coarser facies into the study area. In the simple model proposed the increasing sediment accumulation rates correspond to period of active thrusting, and the intervals of increasing grain size reflect erosion and basinward transport of coarser material during flexural rebound of areas close to the thrusts.
The Siwalik sandstones of Tinau Khola are dominantly very fine- to coarse-grained lithic arenites and sublitharenites composed of low- to medium-grade metamorphic and sedimentary detritus. The metamorphic detritus consists of phyllite, mica schist, gneiss and metaquarzite. Sedimentary detritus is represented by shale, siltstone, and very fine-grained sandstone. The heavy mineral assemblage comprises garnet, zircon, staurolite, tourmaline, epidote, rutile, kyanite, and opaques. In the provenance kyanite bearing rocks are typically found close to the MCT. The first occurrence of kynaite in the Tinau section is interpreted as indicating movement on the MBT and corresponding exhumation of the bedrock close to the MCT between ~8.9 Ma and 8.3 Ma.
Abstract # 3
A reconnaissance investigation to document and interpret the nature of caliche development was performed on the eastern half of the Edwards Plateau, central Texas, using: (1) field analysis, (2) petrographic analysis, and (3) stable carbon and oxygen isotopic analysis.
Within this region, caliche occurs on various Precambrian, Paleozoic, Cretaceous, and Quaternary formations. There is an increase in the variety of caliche horizons, macrofeatures, and biotic macrofeatures as the caliche matures. Caliche forms within outcrops that contain carbonate-rich and basaltic alluvial gravel, marl, indurated limestone, siliciclastic shale, sandstone, terra rossa, and schist. Within this region, variations in caliche development are primarily a function of bedrock lithology and, in some cases, additional characteristics of the host strata; that is, percent gravel content, degree of exposure to the ground surface, fabric of the shales, and degree of brecciation.
Low-magnesium calcite and lesser amounts of quartz are the two dominant minerals present within the non-clay fraction of the caliche, with trace amounts of lithogenic and/or detrital feldspar, hematite, dolomite, and ilmenite. Both abiotic and biotic processes play a role in the development of these caliches, and the combination of both processes produces many of the macrofeatures and caliche horizons (e.g., platy horizons). The degree of biotic activity increases both upward within caliche profiles, and as the degree of caliche development increases. Abiotic features include recrystallized mottled fabrics, desiccation-related fractures, and rod-shaped and filamentous bacteria.
The d13C values reveal a mixed C4 to C3 biomass during caliche development. Hardpans at the ground surfaces show the most positive d13C values when compared with the lower caliche horizons, which could be indicative of either: (A) isotopic mixing with atmospheric CO2, (B) The degassing of 12C-enriched CO2 closer to the ground surfaces, or (C) and increase in the abundance of C4 plant biomass. The d18O values with the caliches, when compared with d18O values within the local waters, show the at uniform temperature conditions occurred during caliche development.