Dissertations / Theses on the topic 'Petrology Petrology Geology Geology'

The origin of the Mule Ear Spring Tuff (Tmet) member of the Chisos Formation of the Big Bend Group in Big Bend National Park (BBNP) (Maxwell et al., 1967) and its relationship to Sierra Quemada (SQ) is debated (a caldera versus a ring dike complex without associated collapse), as well as how the exact age of the Tmet relates to volcanic material present in and around the SQ structure. The main objectives were to identify main types of clasts present within the lithic-rich tuff and to determine the relative age of the lithic-rich tuff within SQ in order to help identify the type of structure and the type of activity—caldera with collapse or simply a ring dike.

Detailed lithologic and petrologic descriptions of hand samples and thin sections were performed to determine relationships of the clasts within the lithic-rich tuff to units outside of SQ. The identification and comparison of the samples produced a relative age of approximately 30.3 Ma to 33.7 Ma for the activity within SQ, which is comparable to published ages of Tmet.

Measurements of the clasts, along with the apparent thickness of the lithic-rich tuff, were compared with studies done on lithic-rich accumulations within modern and ancient calderas. The concentration and sizes of the clasts within the lithic-rich tuff from SQ are comparable to, or larger than, calderas with similar diameters to the SQ structure. The results are compatible with the formation of a typical resurgent caldera. The lack of lithic fragments younger than Tmet within the tuff is compatible with the age of Tmet. Therefore, the age of activity and formation of SQ occurred approximately 34 Ma.

The study of terrestrial meteorite impact craters and of impacted meteorites expands our understanding of cratered rocky surfaces throughout the solar system. Terrestrial craters uniquely expand upon data from remote imaging and planetary surface exploration by providing analogs for understanding the buried sub-surface portions of impact structures, while impacted meteorites provide examples of a much wider range of surface and subsurface impactite materials than we can directly sample thus far through solar system exploration.

This report examines three facets of the impact record preserved in terrestrial impact craters and in meteorites. First, it looks at the macroscopic structure of the Sutters Mill meteorite, a brecciated regolithic CM chondrite that preserves a three-dimensional record of the one of the most primitive known impact gardened surfaces in the solar system. The report details distinct lithologies preserved in the meteorite and the ways in which these lithologies reflect impact and alteration processes, with the intention of contextualizing and illuminating the wider body of recently published instrumental work on the stone by the current authors and others. Second, this dissertation presents a detailed analysis of the origin and nature of unique sub-spherical `round rocks' commonly associated with the surface exposed sediments at the proposed Weaubleau impact structure, in west-central Missouri. Third, and finally, the dissertation looks at the nature of impact evidence for small impact pits and craters on earth. Unambiguously proving the impact origin of sub-kilometer terrestrial impact craters has presented significant historical challenges. A systematic analysis of field reports for all widely recognized sub-km terrestrial craters addresses both the nature of compelling evidence for impact origin for structures in this size range and the adequacy of the existing record of evidence for currently recognized structures.

Lakes formed in the Aysén region of southern Chile after the retreat of mountain glaciers, beginning by at least ~17,900 cal yrs BP, contain numerous late-glacial and Holocene tephra layers derived from >70 eruptions of the volcanoes in the region, including Hudson, the southernmost in the Andean Southern Volcanic Zone (SVZ). Sediment cores from six of these lakes each contain an unusually thick late-glacial age tephra layer, which based on its distribution and bulk trace-element composition was derived from a large explosive eruption of the Hudson volcano between 17,300 and 17,440 cal yrs BP, and is termed Ho. In these cores, located ~100 km northeast of Hudson, the Ho tephra layers range between 35 to 88 cm in thickness. Comparison with three previously documented large explosive Holocene Hudson eruptions (H1, H2, H3 1991 AD) suggests that Ho was larger, with an estimated tephra volume of >20 km3, the largest post-glacial eruption documented for any volcano in the southern Andes. In total, Hudson has erupted ≥45 km³ of pyroclastic material in the last ~17,500 years, making it the most active volcano in the southern Andes in terms of the total volume of pyroclastic material erupted since the beginning of deglaciation in the region. Chemical stratification is not seen in the Ho deposits, but this eruption was bi-modal, with a much greater proportion of dark glassy basaltic-andesite dense fragments and pumice, which range between 55 to 59 wt % SiO₂, and volumetrically less significant lighter colored dacite pumice with 66 wt % SiO₂. In contrast, H1 was andesitic in composition, H2 was more felsic than H1, being composed essentially of dacite, and although H3 in 1991 AD was again bi-modal, it erupted a much smaller proportion of mafic compared to felsic material than Ho. Thus, the repetitive large explosive eruptions of Hudson volcano have evolved to progressively less mafic overall compositions from late-glacial to historic times, and their volumes have decreased. All analyzed phases of different Hudson eruptions, have similar Sr-isotopic composition (0.70444 ± 0.00007), indicating that crystal-liquid fractionation rather than crustal assimilation was the main process responsible for these chemical variations.

The objective of this research was to determine the size, shape, activity of dunes, petrological characteristics, and provenance of sand in the Winnemucca Dune Complex (WDC). Methods and procedures included the extraction of weather records from meteorological stations, generating surficial landform maps, measuring dune advancement from historical aerial imagery, and field sampling of sand for laboratory inspection of grain size and mineralogical composition. Grain size parameters and textural classification of dune sand were determined using a Laser Granulometer and GRADISTAT v.8 (Blott & Pye 2001). The mineralogical composition and physical classification of dune sand was analyzed using fine powder X-ray Diffractometry and stained standard thin sections. Results were plotted on ternary diagrams with Quartz-Feldspar-Lithic (Folk 1974) and Quartz-Alkali feldspar-Plagioclase (Streckeisen 1976, 1978) overlays.

Measurements from surficial landform maps estimate wind-blown deposits are distributed on 472.2 square kilometers of terrain. Active dunes are universally dominated by unique configurations of intermediate shaped barchan and parabolic dunes. For the purpose of this study these features were termed as barchanbolic. WDC is primarily covered by 6 crescentic complexes, 1 large sand sheet, and discontinuous sets of compound barchanbolic-parabolic dune fields. The crescentic complexes are composed of closely spaced barchanoidal and transverse ridges with occasional star dunes. Between the complexes are repetitive sequences of compound and individual barchanbolic-parabolic dunes that laterally radiate towards the bounding perimeter of WDC. Sand sheets, ramps, climbing, descending, cliff-top, and lee dunes are also present along mountain crests and hillsides. Sand sheets (56.3 square kilometers) and active dunes (162 square kilometers) extend across 218.3 square kilometers which constitutes 46.2% of the wind-blown deposits in WDC. Since the year 1980 sand dunes have been advancing at maximum rates from 1.6 to 6.9 meters per year on an azimuth of 35-130 degrees. Rose diagrams and historical wind records verify the sand dunes reach peak advancement rates during the warm season months of April to the middle of July. During this time of year the strongest winds prevail from west-southwest when the daily maximum wind speed is near 7 meters per second. Measurements of sand dune advancement rates from the years 1980-2012 show eolian activity has spatiotemporally fluctuated within the complex.

WDC sand was observed to have distinguishing textural attributes. Sediments from active dunes were mesokurtic, symmetrical, and trended towards moderately well sorted medium sand. Sediments from stable dunes were mesokurtic and trended towards moderately sorted fine sand but varied in skew from symmetrical to fine. Micro-stereoscopic inspection of bulk samples, thin sections, and the QFL ternary diagram revealed that sand traveling down the sediment transport corridor will physically weather from a White to Grey & Very Pale Brown Litharenite into a Very Dark Grey to Light Yellowish Brown & Pale Brown Feldspathic litharenite sand. The QAP ternary analysis and X-ray Diffractometry demonstrated that during the processes of dune stabilization and mineralogical maturation of sand the relative weight percent of total Quartz will increase (20 to 68%) and the percent relative abundance of lithic material will decrease (100 to 45%). Feldspar minerals were plentiful and ranged from 32 to 80 relative weight percent. The mineralogical maturity of sand when interpreted by the ratio of Quartz to Feldspar grades the maturation as low to fractionally intermediate. The QAP ternary diagram demonstrates there are distinct mineralogical differences within the sand and that mixing of sediments from various supply sources have contributed to its composition. Similar to findings from the Mojave Desert (Zimbelman & Williams 2002) the abundance of Feldspar and lack of Quartz enrichment in WDC dune sand may imply the mineralogical maturity is directly inherited from the parent material. The lack of Quartz enrichment also indicates WDC is geologically young and most likely has not endured extended periods of inactivity. Prominent angular to subangular grains in WDC sediments suggest dune sand has not been transported over extremely long distances. Potential sediment supply sources for dune sand may include the Jungo terrane, Comforter Basin Formation, McDermitt-Santa Rose volcanic field, and sedimentary deposits from Lake Lahontan.

The fluid history of serpentinites from three locations in the Franciscan Complex, San Rafael Mountains, California is evaluated with petrologic and stable isotope data that allow interpretation of the serpentinization history and tectonic origin of these rocks. Petrologic evidence shows that most samples were originally serpentinized in a relatively low temperature seafloor hydrothermal environment, but some rocks underwent subsequent recrystallization. Data obtained from serpentine-magnetite geothermometry indicate that the serpentinization temperatures ranged from 168°C to 306°C. Oxygen isotopic values suggest that the serpentinites may have originated in a forearc setting. Hydrogen isotopic values obtained do not reflect the original conditions of serpentinization, but indicate that the rocks subsequently underwent isotopic exchange with meteoric water once they were emplaced onto the continent.

The Sierra Blanca metamorphic core complex (SBMCC), located 90 miles west of Tucson, is part of the southern belt of metamorphic core complexes that stretches across southern Arizona. The SBMCC exposes Jurassic age sedimentary rocks that have been metamorphosed by intruding Late Cretaceous peraluminous granites and pegmatites. Evidence of this magmatic episode includes polysythetic twinning in plagioclase, albite exsolution of alkali feldspar resulting in myrmekitic texture, and garnet, mica and feldspar assemblages. The magmatic fabric is overprinted by a Tertiary (Miocene?) tectonic fabric, associated with the exhumation of the Sierra Blanca metamorphic core along a low-angle detachment fault, forming the SBMCC. The NW-SE elongated dome of metamorphic rocks forms the footwall of the detachment shear zone, and is separated from the hanging wall, composed of Paleozoic and Mesozoic metasedimentary rocks, by a low-angle detachment shear zone. Foliation is defined by gneissic layering and aligned muscovite, and is generally sub-horizontal, defining the dome. The NNW-SSE mineral stretching lineation is expressed by plagioclase and K-feldspar porphyroclasts, and various shear sense indicators consistent with a top-to the-NNW shear sense. Lineation trends in a NNW-SSE orientation; however, plunge changes across the domiform shape of the MCC. Much of the deformation is preserved in the blastomylonitic gneiss derived from the peraluminous granite, including epidote porphyroclasts, grain boundary migration in quartz, lozenged amphiboles, mica fish, and retrograde mineral alterations. Detailed petrologic observation and microstructural analysis indicate deformation temperatures of 450-575 ? ?C presented here provide thermomechanical constraints on the evolution of the SBMCC.

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host.

I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na₂O-K₂O-CaO-FeO-O₂-MnO-MgO-Al₂O ₃-SiO₂-TiO₂-H₂O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase.

SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.