Browsing by Subject "OAE"

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Carbonate factory response and recovery after Ocean Anoxic Event 1a, Pearsall Formation, Central Texas
(2020-08-13) Pedersen, Esben Skjold; Kerans, C. (Charles), 1954-; Larson, Toti Erik
Ocean Anoxic Events (OAEs) are major carbon cycle perturbations that occurred several times in the Mesozoic. OAEs are commonly found to have been caused by a combination of climatic warming and increased surface weathering delivering surface nutrients to the oceans. This feedback loop leads to the expansion of the oxygen minimum zone of the waterbody and increased influx of terrigenous material. The resultant dysoxic to euxinic conditions are thought to have played a prominent role in the suppression of the benthic carbonate factory and deposition of organic-rich mudstones. The establishment of these oceanographic conditions are postulated to have imparted a lasting effect on the deposition of stressed-carbonate facies during the recovery phase of OAEs. Major questions regarding OAE events remain, including the degree of variability in the impact that OAEs have on carbonate factories and the drivers for this variability, on both global and regional scales. This study builds upon previous work and further investigates the regional Early Cretaceous (Aptian) OAE-1a signal that is recorded in the Pearsall Formation in Central Texas, with a particular focus on the record of carbonate factory recovery observed in transects from the San Marcos Arch to the Pearsall Arch. Shoreline-proximal data include outcrops and 8 cores with 1530 ft of coverage. Distal cores include 7 subsurface exploration wells (total 1745 ft core) from the San Marcos Arch to the Pearsall Arch, a strike-parallel distance of 210 km. Physical characterization of stratigraphic data was paired with the multivariate statistical analysis of 10 pXRF datasets, involving Principal Component Analysis (PCA) segmentation, which led to the establishment of five end member chemofacies. These chemofacies allow for high-resolution identification of mineralogic variability across OAE-1a, including the documentation of pulses of terrigenous input as well as cycles of dysoxic to euxinic oceanographic conditions at a sub-lithofacies scale. When paired with the development and application of a deep learning neural network trained by a type-pXRF training dataset, this study outlines a new methodology that allows for the direct comparison of pXRF data across core control through a unified chemofacies schema. The oceanographic conditions identified with this workflow are then used to delineate oceanographic variability and pulses of terrigenous enrichment in association with the recovery from OAE-1a. The characterization of these geochemical processes is particularly relevant in the mudrock component of depositional systems, where biologic productivity, bottom-water redox conditions, and any subsequent diagenesis are critical determinants for the ultimate preservation of TOC in organic-rich shales. TOC rich shale intervals then create potential for an economical petroleum source rock and successive charge of either conventional or unconventional reservoirs. The incidence of OAE-1a is found to be a fundamental driver of facies evolution and faunal composition in the three composite sequences studied: the James (Aptian) composite sequence, the Bexar (Aptian-Albian) composite sequence, and the Glen Rose composite sequence (Albian) (cf. Phelps et al., 2014). OAE-1 is coincident with the drowning of the antecedent Sligo reef margin and deposition of the Pine Island Shale. This drowning event was a result of environmental stressors posed by the OAE and the resultant suppression of sedimentation rates on the platform as the carbonate factory was substantially weakened. Partial recovery of the carbonate factory from OAE-1a is expressed in the deposition of the Cow Creek Member before punctuation of deposition due to the subaerial exposure event at the top-James composite sequence boundary. A second phase of recovery is documented in the Bexar and Glen Rose composite sequences, including reef systems in the platform interior that are coeval with transgression and deposition of the Hensel Formation, as well as the progradation of Lower Glen Rose carbonates and the aggradation of microbial-coral-rudist bioherms in highstand depositional sequences of the Glen Rose Formation. Recovery of the carbonate factory was fundamentally different between the San Marcos Arch and Pearsall Arch areas. Earliest recovery fauna in the Cow Creek Member is comprised of monospecific echinoid-mollusk packstones-grainstones in shoreline proximal settings and oyster-oncoid rudstones distally. Combined observations from pXRF data and the heightened prevalence of pyrite in oncoid cortices on the San Marcos Arch compared to the Pearsall area is interpreted to represent a higher degree of dysoxic and/or euxinic conditions on the San Marcos Arch. During later stages of recovery, the Cow Creek in the Pearsall Arch area is shown to have maintained healthier carbonate deposition in comparison to the San Marcos Arch, including the sustained deposition of reefal assemblages, such as the sequence of stromatoporoid boundstone present in the Tenneco Sirianni well. This combined core-outcrop framework demonstrates the superimposed regional variability inherent even in global carbon cycle perturbations such as OAE-1a, driven by the degree of shelf restriction, oceanographic circulation patterns, basin geometry, and the degree of terrigenous influx. The documented differences in oceanographic conditions and carbonate factory recovery on the regional scale of OAE-1a will aid in better understanding the multi-scaled geochemical and environmental evolution associated with these events, and ultimately pushes towards the development of predictive concepts for future studies.
Middle-Hauterivian to Lower-Campanian sequence stratigraphy and stable isotope geochemistry of the Comanche platform, south Texas
(2011-05) Phelps, Ryan Matthew, 1982-; Kerans, C. (Charles), 1954-; Loucks, Robert G.; Janson, Xavier; Scott, Robert W.; Fisher, William L.; Quinn, Terrence M.
Carbonate platforms contain a wealth of information regarding the changing biota, sea level, ocean-chemistry, and climate of the Cretaceous Period. The Comanche platform of the northern Gulf of Mexico represents a vast, long-lived carbonate system that extended from west Texas through the Florida panhandle. In central and south Texas, excellent outcrops and an extensive suite of subsurface data provide an opportunity to document the evolution of this system, from the shoreline to the shelf-margin and slope. This study examines the changing facies, platform morphologies, and shelf-margin architectures of the mixed carbonate-siliciclastic, middle-Hauterivian to lower-Campanian interval. Stratigraphic results are integrated with stable-isotope geochemistry to document the detrimental effects of oceanic anoxic events on the carbonate platform. Seven second-order, transgressive-regressive supersequences of 3-14 Myr duration are defined in south Texas using sequence stratigraphic analysis of shelf-interior facies successions. Second-order supersequences are subdivided into several third-order depositional sequences of 1-3 Myr duration. In these sequences, facies proportions and stratal geometries of the shelf-interior are found to be the result of changing platform morphology and temporal evolution from distally-steepened ramp to rimmed-shelf depositional profiles. Shelf-margin trajectories, stratigraphic architectures, and facies proportions are a function of long-term accommodation trends expressed in second-order supersequences. These characteristics are modified by lateral variability in the underlying structural/tectonic setting and localized syndepositional faulting. The stratigraphic equivalents of oceanic anoxic events 1a, 1b, 1d, 2, and 3 are documented in the Cretaceous section of south Texas. These oceanic anoxic events coincided with maximum flooding zones of supersequences and are linked to carbonate platform drowning events on four separate occasions. The occurrence of oceanic anoxic events is found to be a fundamental driver of carbonate platform morphology, faunal composition, and facies evolution in transgressive-regressive supersequences of the northern Gulf of Mexico.
Stratigraphy, depositional history, and pore network of the Lower Cretaceous Sunniland carbonates in the South Florida basin
(2016-05) Liu, Xinggang Christopher; Kerans, C. (Charles), 1954-; Loucks, R. G.; Fisher, William
The South Florida Basin of the eastern Gulf of Mexico represents a vast, undisturbed carbonate system that extended from the Florida Keys through the Tampa-Sarasota Arch. In South Florida, extensive subsurface data and analogous modern environments provide an opportunity to unravel the evolution of this system from shoreline to shelf-margin. This study examines the changing facies and the pore network of the Latest Aptian-Early Albian Sunniland interval. Stratigraphic results are closely comparable with contemporary carbonate platform studies in the northern Gulf of Mexico. The Sunniland Formation was deposited during a major transgressive-regressive sequence. The Sunniland interval is divided into five third-fourth order, transgress-regressive depositional cycles (S-1 to S-5) in south Florida using sequence analysis of shelf-interior facies succession. In these sequences, facies proportion, faunal composition, and stratal geometries of the shelf-interior are found to be the result of the changing accommodation trends and ocean chemistry. As in the Comanche Platform in South Texas, the detrimental effects of oceanic anoxic event 1B may fundamentally drive the evolution of platform morphology in the eastern Gulf of Mexico as: • Rimmed shelf (crisis phase: S1) • Distally steepened ramp (anoxic/dysoxic phase: S2, recovery phase: S3, S4) • High-angle rimmed shelf (recovery to equilibrium phase: S5). Within this hydrocarbon-producing trend, the lowered sea level at the end of S4 enhances the reservoir quality in the high-energy settings including back-reef debris aprons, tidal shoal-complex and carbonate beach by dissolution. The tight sabkha-tidal flat facies in S5 forms the reservoir seal, whereas the medium-fine crystalline dolomites in S3 may not adversely affect and likely facilitate the migration of hydrocarbon self-sourced from the high TOC, argillaceous mudstone in S2.