Browsing by Subject "Petrography"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Ceramic thin-section analysis and Early Postclassic to Middle Postclassic discontinuity at Colha, Belize(2012-08) Reid, Elizabeth Katherine; Valdez, Fred, Jr., 1953-Petrographic and Type:variety analyses of Colha ceramics are used to investigate changes in ceramic production technology and organization during the Early and Middle Postclassic. Postclassic sherds from Colha are grouped by petrofabric, surface treatment, and modes. This dissertation focuses on the petrofabric analysis of the ceramic sherds. The groupings are then compared to locally available raw materials. Changes in technological homogeneity, production specialization, and origin will be examined and related to the general economy of Postclassic Colha.Item Chemical and mechanical diagenetic evolution of deformation bands in sandstone(2016-08) O'Brien, Casey Megan; Eichhubl, Peter; Behr, Whitney; Gulick, SeanDeformation bands are zones of localized compaction, dilation, or shear in porous or granular sediments or sedimentary rock. Deformation bands are generally characterized by reduced permeability for cross-band fluid flow and increased capillary resistance for multi-phase flow, thus acting as barriers or baffles to fluid flow at an outcrop- to reservoir-scale. Deformation bands undergo increased lithification due to enhanced compaction and cementation, despite experiencing the same burial and temperature history as their host sandstone. I test the hypotheses that enhanced cementation in deformation bands is controlled by 1) the time between formation of the deformation band and onset of exhumation; 2) the degree of grain size reduction and thus the amount of fresh surface area available for quartz nucleation; 3) the amount of pore space available for pore-filling quartz cement; or a combination of these. I also evaluate possible links between mechanical deformation and chemical diagenesis in the context of tectonic setting and hostrock composition. To test these hypotheses, I reconstructed the structural and diagenetic evolution of deformation bands formed in the Jurassic Entrada, Navajo, and Wingate Sandstones in a contractional tectonic setting in the San Rafael monocline, and in the Entrada Sandstone in an extensional tectonic setting in the San Rafael Desert, Utah. I combined field-scale observations including band orientation, mutual crosscutting relations, and kinematic interpretations with petrographic point-count analyses using images obtained by high-resolution scanning electron microscopy. Based on structural criteria, I observed as many as six band generations. Younger generations of bands generally experience more cementation and porosity loss than older bands. I have identified that cataclasis is the dominant control over the amount of quartz cementation in bands of a similar lithology and burial history, as opposed to the time between band formation and exhumation, or available pore space. Within the same generation of bands, bands with more slip have more quartz cement. These findings highlight the importance of mechanical deformation in chemical diagenetic processes in sandstone. I find that the degree of cataclasis and quartz cementation does not differ between contractional and extensional tectonic settings, and that the hostrock compositional differences between the sandstones in this study do not cause significant differences in band diagenetic evolution.Item Fault-related fracture systems in the Cambrian Eriboll Formation, Northwest Scotland : a field and petrographic study of a tight gas sandstone analog(2010-12) Hargrove, Peter Gregory; Laubach, Steven E. (Stephen Ernest), 1955-Lower Cambrian Eriboll Formation sandstones of the Ardvreck Group that crop out in the Hebridean foreland west of the Paleozoic Moine Thrust Zone (MTZ) in the Northwest Highlands of Scotland contain five sets of opening-mode fractures with varying degrees of quartz deposits (cement) and topographically prominent but small displacement (mostly less than 10 m) northeast-striking faults. The faults crosscut and in some places displace the MTZ. I interpret these faults to post-date the MTZ and consider them to be late structures (kinematically unrelated to MTZ emplacement). Sparse slip lineations on fault surfaces and offset patterns are evidence for strike-slip to oblique slip. Using geologic mapping I show that relative to their lateral and vertical extents, the faults display small amounts of offset (less than 5 to 10 m). My research documented the patterns and petrology of fractures in a well exposed section of the foreland, documented for the first time fracture patterns adjacent to and within the post-MTZ fault zones, and proposes an account of how fault and fracture patterns developed and their probable effects on fluid flow. Fractures are barren (joints), partially filled (quartz lined), or completely filled (veins). Older fracture sets are typically completely filled, whereas younger sets may be lined with a thin veneer of quartz cement or are barren. Listed in order from oldest to youngest fractures containing quartz strike north, NW to WNW, NE, west, and north (sets A through E respectively). Previously proposed relative ages of the sets were confirmed using crosscutting relationships and preferred orientations of macro- and microfractures (Laubach and Diaz-Tushman, 2009). This study focuses on late northeast-striking fractures (set C) which I interpret to be related to the formation of the small-offset faults. Many of the attributes of late fractures and faults in the Eriboll Formation resemble those found in core from highly quartz cemented sandstone natural gas reservoirs ("tight gas sandstones"). I demonstrate that the well exposed fracture patterns I documented are good analogs for tight gas sandstones, by investigating fracture characteristics such as network configurations and connectivity, fracture intensity (abundance), fracture scaling, fracture length and spacing, and the degree of quartz cement deposits in fractures and cataclastic fault rock. Many of the narrow macroscopic fractures and microfractures I documented using CL methods contain varying amounts of quartz deposits. The excellent preservation of Eriboll outcrops is probably a manifestation of little or no fracture pore space preservation in many of the numerous fractures that are apparent in outcrop. Set C fracture abundance is not distributed in a uniform envelope (or "halo") around the late faults. Using scanlines, I show that set C fracture distribution is heterogeneous and highly variable over short lateral distances (tens of centimeters to meters). I also investigate wing crack assemblages (secondary opening-mode fractures) that are locally associated with set C fractures. The assemblages accommodate small amounts of the distributed displacement (a few millimeters) adjacent to fault zones and are locally responsible for increased amounts of fracture connectivity by linking neighboring fractures. Variations in fracture pattern complexity appears to be related to the presence (or absence) of wing crack assemblages. Localized wing crack development on closely spaced, en echelon set C fractures also leads to precursory development of fragmented lozenges of highly deformed volumes of rock (damage zones) that resemble geometries similar to those seen in preserved Eriboll fault cores. Fault-related deformation in the Eriboll Formation is markedly different than that in the underlying Late Proterozoic Torridonian Applecross Formation (subarkose fluvial sandstone), which is characterized by simple halos fault-related fracture arrays surrounding the same late (post-MTZ) faults. In addition to composition, the Eriboll and Applecross differ in mechanical layer thickness (centimeters versus > tens of meters), mechanical properties (high versus low brittleness), and greater propensity for fractures to be filled with quartz cement in Eriboll sandstones owing to quartz cement growth being impaired by the abundance of non-quartz substrate (feldspar and clay minerals) along fracture walls in the Applecross Formation. Although the Eriboll sandstones are more highly fractured than the older Applecross sandstones, Eriboll fractures are more prone to be filled by quartz cement. In this thesis I also report previously unrecognized early (set A; pre-MTZ) minor normal faults, sandstone petrography and rock mechanical properties of selected Eriboll sandstone samples, and the influence of fractures on the glacial geomorphology of the area. I also describe a previously unmapped igneous dyke. I describe previously unrecognized vugs that are partly strata bound and partly localized along fractures. The attributes of these vugs and a review of the literature suggests that these features could represent evidence of pre-glacial silici-karst in Eriboll quartzites.Item Formation and evolution of Pleistocene (MIS 5e) strandplain grainstones along the leeward margin of West Caicos island, BWI(2016-05) Danger, Nick, M.S. in Geological Sciences; Kerans, C. (Charles), 1954-; Mohrig, David; Bachtel, Steven LThe island of West Caicos is a typical Caribbean Pleistocene-Holocene carbonate strandplain system that developed by accretion of successive dune-foreshore-upper shoreface intraclastic-peloidal-oolitic limestone grainstones. Although strandplains are the most volumetrically significant element of low-latitude carbonate island interglacial highstand deposits, documentation of the 3D facies architecture and accumulation rates for these basic building blocks is sparse. This detailed mapping project documents for the first time on the Caicos platform the timing and origin of a foreshore-upper shoreface ooid grainstone body along the west coast of West Caicos. Timing has been constrained for this grainstone body to the latter portion of the composite Marine Isotope Stage (MIS) 5e signal (5e_2), providing very tight constraints on duration (2-4 ky) and sea level history; there was a rapid rise from near present sea level to +6 m, which lasted from 121-118 kya. Subsequently, deposition ended with a rapid fall to -60m by 5d time (118-110 kya). Using these age constraints and high-resolution airborne LIDAR mapping, this study analyzes vertical and 3D rates of accumulation, discrete facies architecture, provenance, and transport for this MIS 5e_2 strandplain. Dip-oriented 2D cross sectional mapping across the 9 km north to south length of the west coast outcrops was carried out at 3 locations, and several 3D facies models were made. These maps fit into the larger regional map of the west coast MIS 5e and are enhanced with paleocurrent data from longshore-directed trough-cross-strata and foreshore strata. Across the profile, 53 thin sections document grain types, size, and sorting. The profiles consist of 3-6 degree dipping foreshore strata with dip widths of 10-20 m and plunge zone to upper shoreface facies with trough-cross-strata showing a distinct southward transport direction. Along the northern section of the profile, massive bioturbated lower upper shoreface grain-dominated packstones are present down dip from the upper shoreface. These massive grain-dominated packstones accumulated to 1 m thick, occur in greater than 2 meters of water depth, and rest locally on top of corals of the 5e_1 reef system. The rise to a sea level six meters higher than present during MIS 5e_2 time increased ooid production on the Caicos platform and throughout the Caribbean (Kindler and Hearty 1996). Although a large amount of ooids were likely winnowed off the platform top due to prevailing easterlies, many of the ooids were transported from north to south along the leeward coast of West Caicos via longshore drift. This interpretation is supported by a unidirectional southward paleoflow direction, decreasing lateral (along-strike) continuity in 5e_2 grainstone units from north-south, and the poorly sorted nature of grain type in the petrography from north-south. The heterogeneity in grain type suggests a fair amount of grain mixing, indicative of transport over several kilometers; active ooid factories on West Caicos such as Ambergris and Long Bay Beach have >90% ooid compositions, much greater than ooid compositions along the leeward margin of West Caicos. Assuming a 3 ky period of deposition, vertical accumulation rates for this MIS 5e_2 strandplain are as large as 2 m/ky (6 m total), and 3D accumulation is 115,000 m3/ky (345,000 m3 total). Understanding the depositional and accumulation history during sub-Milankovitch sea level fluctuations underscores the stratigraphic complexity of Pleistocene icehouse systems, and shows short-lived rises in sea level can have significant effects on sediment production and carbonate island building.Item Paradox Basin source rock, southeastern Utah : organic geochemical characterization of Gothic and Chimney Rock units, Ismay and Desert Creek zones, within a sequence stratigraphic framework(1995-08) Tischler, Keith Louris; Kirkland, Brenda Lee, 1959-The Chimney Rock and Gothic units of the Pennsylvanian Paradox Formation have long been considered source rocks for the rich hydrocarbon fields of southeastern Utah. Fundamental questions about these units include: source and nature of the organic material, source rock character, and position of the source rocks in the existing sequence stratigraphic framework. The Chimney Rock and Gothic, historically referred to as shales, are composed of calcareous mudstone, dolomudstone, and calcareous sandstone. High total organic carbon (TOC) values are more closely linked to sequence stratigraphic position than lithology. In the Gothic, TOC values decrease upwards. Terrestrial maceral content increases upwards in both the Gothic and the Chimney Rock as determined through point-count and qualitative observation. Pyrolysis indicates that greater than anticipated terrestrial influence is present and is consistent for all wells. No distinct difference in geochemical character exists between the two units. Sequence stratigraphic boundaries appear to be as good as, or better, than traditional lithostratigraphic boundaries for determining high TOC occurrence and source rock location. Within repetitive major sea level transgressions the organic matter that fed the basin evolved from a marine-dominated signature to a terrestrial-dominated signature.Item Provenance of the south Texas Paleocene-Eocene Wilcox Group, western Gulf of Mexico basin : insights from sandstone modal compositions and detrital zircon geochronology(2009-08) Mackey, Glen Nelson; Milliken, Kitty L.; Horton, Brian K.; Steel, Ronald J.Sandstone modal compositions and detrital zircon U-Pb analysis of the Paleocene-Eocene Wilcox Group of the southern Gulf Coast of Texas indicate long-distance sediment transport primarily from volcanic and basement sources to the west, northwest and southwest. The Wilcox Group of south Texas represents the earliest series of major post-Cretaceous pulses of sand deposition along the western margin of the Gulf of Mexico (GoM). Laramide basement uplifts have long been held to be the provenance of the Wilcox Group, implying that initiation of basement uplifts was the driving factor for this transition from carbonate sedimentation to clastic deposition. To determine the provenance of the Wilcox Group and test this conventional hypothesis, 40 thin sections were point-counted using the Gazzi-Dickinson method to determine sandstone composition and 10 detrital zircon samples were analyzed by LA-ICP-MS to determine U-Pb age spectra for each of the sampled areas. Modal data for sand grain populations suggest mixed sources including basement rocks, magmatic arc rocks and subordinate sedimentary rocks for the Wilcox Group. Zircon age spectra for these sandstones reveal a complex grain assemblage derived from older sediments and crystalline rocks ranging in age from Archean to Cenozoic. Sediment was primarily derived from Laramide uplifted crystalline blocks of the central and southern Rocky Mountains, the Cordilleran arc of western North America, and arc related extrusive and intrusive igneous rock of northern Mexico. Comparisons of Upper and Lower Wilcox zircon age spectra show that more arc related material was deposited in the Lower Wilcox, whereas more basement material was deposited in the Upper Wilcox.