Browsing by Author "Droxler, André W"

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    Anomalous widespread arid events in Asia over the past 550,000 years
    (Oxford University Press, 2023) Carrasqueira, Igor Gustavo da Fonseca; Jovane, Luigi; Droxler, André W; Alvarez Zarikian, Carlos A; Lanci, Luca; Alonso-Garcia, Montserrat; Laya, Juan Carlos; Kroon, Dick
    Records of element ratios obtained from the Maldives Inner Sea sediments provide a detailed view on how the Indian Monsoon System has varied at high-resolution time scales. Here, we present records from International Ocean Discovery Program (IODP) Site U1471 based on a refined chronology through the past 550,000 years. The record's high resolution and a proper approach to set the chronology allowed us to reconstruct changes in the Indian Monsoon System on a scale of anomalies and to verify their relationships with established records from the East Asian Monsoon System. On the basis of Fe/sum and Fe/Si records, it can be demonstrated that the Asia continental aridity tracks sea-level changes, while the intensity of winter monsoon winds responds to changes in Northern Hemisphere summer insolation. Furthermore, the anomalies of continental aridity and intensity of winter monsoon winds at millennial-scale events exhibit power in the precession band, nearly in antiphase with Northern Hemisphere summer insolation. These observations indicate that the insolation drove the anomalies in the Indian Summer Monsoon. The good correspondence between our record and the East Asian monsoon anomaly records suggests the occurrence of anomalous widespread arid events in Asia.
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    Detailed Characterization of Upper Cambrian Microbial Buildups (Mason County, Texas, USA)
    (2018-04-20) Hopson, Heath Hilton; Droxler, André W
    Field studies, extensive shallow coring, thin section petrography, and hyperspectral scanning are used to characterize a series of Upper Cambrian (Wilberns Formation) subtidal microbial reefs outcropping in Mason County, central Texas. Results document details of microbial textures that were responsible for growth of the buildups, thereby providing an understanding of the environmental conditions that influenced varied stages in their evolution. Thrombolitic fabrics of dominantly microbial micrite with fossil fragments evidence carbonate precipitation induced by microbial colonies that produced a tight framework hindering diagenesis. Stromatolitic fabrics show alternating laminations of microbially precipitated calcite and amalgamated trapped grainstones exhibiting ferroan dolomite replacement and subsequent oxidation of these grains. Initial characterization of the microbial reefs identified successive growth stages, repeatedly identifiable across several separate outcrops, from which a three-phase growth model was formulated. Within the context of the three phases, the core-based geologic characterization of the buildups and coeval sediments refines the growth model into a more robust depositional model. Nucleating on lenses of flat pebbles, an initial “colonizing” Phase 1 growth results in 3-4 m high buildups defined by their distinct and early-cemented outer margins of a thick thrombolitic rind. Buildup interiors exhibit amalgamated microbial heads with poorly preserved internal structure enveloped by cm-thin thrombolitic rinds. The buildups grew in high-energy conditions, but without interacting with coeval interbuildup oolitic-bioclastic grainstones. Phase 1 terminated with onlapped terrigenous and calcareous silts (~35% CaCO3). The overlying Phase 2 growth produced buildups up to 8 m thick, characterized by a mutual interaction with interbuildup high-energy oolitic-bioclastic grainstones and packstones and generally lacking an external rind. Internally, Phase 2 growth consists of vertically aggrading and laterally expanding stromatolitic columns, each exhibiting cm-thick thrombolitic rinds directly interacting with intercolumn bioclastic grainstones. Phase 3 of growth produces a well-defined thrombolitic 2-3 m thick rind atop Phase 2, and in some instances on top of the interbuildup grainstones.
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    Evolution of North Malé Atoll Rim during the Last Full Glacial Cycle (Malé Island, Republic of Maldives)
    (2014-07-31) Koksal, Tugba; Droxler, André W; Sawyer, Dale S; Nittrouer, Jeffrey A; Gischler, Eberhard
    My study focuses on the evolution of Malé Island to elucidate the late Pleistocene-Holocene evolution of the discontinuous North Malé Atoll rim. Sample analyses from two boreholes, published information from additional boreholes, and a multi-beam bathymetric map for Malé Island deep surroundings, are available for this study. Facies analyses of the lower sedimentary unit reveal an overall deepening coralgal reef that accumulated probably during the previous interglacial and subsequently was altered by meteoric diagenesis during a 100 ky-long exposure. The upper Holocene unconsolidated coralgal reef, overlying the karstified coralgal MIS 5e lower unit, was initiated at ~8200 kyr BP and vertically grew 25 m high until 6510 kyr BP, protected behind a karstified late Pleistocene reef. A small, though 30-35 m deep, newly formed lagoon started to fill up only at ~5500 kyr BP, when a reef initiated on top of the highest elevated Pleistocene karstified reef and sea level rise stalled.
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    Examining Spatial and Temporal Variations in Neogene Carbonate Margin Development, Browse Basin, Northwest Shelf, Australia
    (2019-02-01) Sudhakar, Rahul; Droxler, André W
    The Neogene Global Stratigraphic Signature (NGSS) is a late Oligocene to Pliocene depositional architecture thought to be controlled by eustatic variation; although the signature is defined geometrically by stratal relationships that exist in seemingly contemporaneous carbonate and siliciclastic systems worldwide, details of its presence have not been documented regionally. Here, we investigate the Neogene carbonate system of the Browse Basin (Northwest Shelf of Australia) to test for the presence of the NGSS; subsequent to its identification, we characterize accumulations in three dimensions, drawing particular attention to their variability. From our analysis, which is based on two- and three-dimensional seismic reflection surveys, drilled industry and research wells, and published datasets, we show that four carbonate platform phases (CPPs) display marked asymmetric growth through time. When compared to the NGSS, CPPs display a similar stratigraphic architecture, but variations exist that are attributed to non-eustatic controls (i.e., subsidence, oceanographic, and climatic-related phenomena). By characterizing the three-dimensional stratigraphic evolution of the CPPs, we provide a robust tool for predicting near-shore facies, which offer new insight into the role of global signals on the local development of Neogene carbonate architecture (NGSS).
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    Upscaling Lithology and Porosity Types from Microscopic to Core Domain with Thin Section Petrography, Dual Energy CT, and Rock Types: Creation of Diagenesis and Porosity Type Logs
    (2018-02-20) Proctor, Jacob; Droxler, André W
    Upscaling Lithology and Porosity Types from the Microscope to the Core with Thin Section Petrography, Dual Energy CT, and Rock Types: Creation of Diagenesis and Porosity Type Logs Authors: Jacob M. Proctor, André W. Droxler, Naum Derzhi, Heath H. Hopson, Paul (Mitch) Harris, Pankaj Khanna, and Daniel J. Lehrmann ABSTRACT The primary objective of this study was to develop and validate a new approach to upscale lithology, diagenesis, and porosity from thin sections to cores using dual energy and multi-scale CT. A new rock typing approach (Genetic Rock Typing) is proposed to upscale diagenetic minerals and pore types, from thin sections to the core domain, to create a diagenesis log. A newly acquired large collection of Late Cambrian short cores from Mason County (Texas) was used for the development and validation of this new rock typing approach. Among the outcropping microbial buildups and their inter buildup sediments, the three main depositional facies, which underwent a wide range of diagenetic alteration pathways, was an adequate sample set to develop and test this new approach. Fractions of diagenetic calcite and diagenetic porosity were upscaled to the core scale using Genetic Rock Types (GRT). From this, we derived a Diagenesis Log at 0.5mm resolution that factors in depositional facies and the diagenetic overprint. The diagenesis log was subdivided based on the number of pore throat size classes within each GRT which provided a framework to distribute porosity types from thin sections to log form. When the predicted extent of diagenetic alteration from the diagenesis log is compared to that quantified for each thin section, a high correlation coefficient is observed (R2 = 0.918). While this study was restricted to the core domain, whole core dual energy computed tomography (CT) (0.5 mm resolution) mineralogy and total porosity logs were integrated into a multi-scale CT scanning framework. This links directly to the photo electric effect (PE) and bulk density wireline logs because both are x-ray generating instruments and are governed by the same physical laws governing x-ray attenuation. This approach thus has the ability to span 6 orders of magnitude (500 mm to 0.0005 mm). The diagenesis log can be used to extrapolate porosity types from thin sections to logs, from which will generally transform qualitative geologic interpretations into quantitative numbers that petrophysicists, petroleum engineers, reservoir engineers, and geologists can use and rely upon.