Award Date

8-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Ganqing Jiang

Second Committee Member

Matthew Lachniet

Third Committee Member

Michael Nicholl

Fourth Committee Member

Minghua Ren

Fifth Committee Member

Jennifer Rennels

Number of Pages

93

Abstract

Carbonates are mostly produced in shallow-marine environments and their deposition is sensitive to water depth changes on carbonate platforms. The water depth in depositional environments of a particular carbonate platform is controlled by the interplay of eustatic sea-level change, tectonic subsidence, platform morphology, and depositional rate. Due to the morphological variations of carbonate platforms in different tectonic settings, facies distribution across carbonate platforms varies significantly even during a single eustatic sea-level cycle. Carbonate platforms developed on passive continental margins are thought to be the most stable platforms and their facies distribution is commonly taken as examples in textbook depositional models. Some studies, however, have documented significant variations in facies and cycles across the passive-margin carbonate platforms, implying that the existing depositional models are overly simplified. To better understand the facies and geochemical change across passive-margin carbonate platforms, this dissertation applies three-dimensional (3D) modeling techniques to visualize the internal facies and isotope change of the Darriwilian (Middle Ordovician; ca. 467– 458 Ma) carbonate platform in the southern Great Basin.

Lithological, sedimentological, and carbon and oxygen isotope (δ13Ccarb and δ18Ocarb) data used for modeling were collected from nine sections including the Nopah Range (NR), Arrow Canyon Range (AC), Meiklejohn (MJ), Pahranagat Range (PR), Shingle Pass (SP), Lone Mountain (LM), Monitor Valley (MV) and Hot Creek Range (HC). Among these, the isotope data from the Ibex Hills (IH) were analyzed in this study and those from the AC, PR, LM, MV, and HC sections were unpublished data from previous students of Dr. Jiang’s Group. Isotope data from the MJ and SP sections were from literature. In all these sections, the Darriwilian carbonate rocks are unconformably overlain by the Eureka Quartzite, the base of which serves as a datum for stratigraphic correlation across the platform. The stratigraphic units belong to the upper Pogonip Group and the age of these units is constrained by biostratigraphy of conodonts, brachiopod, bivalves, gastropods, trilobites and calcareous algae.

The 3D modeling proceeded with data preparation, grid creation, data interpolation, and quality check on modeling outputs. Data preparation involved extracting data from literature and fieldwork examination in some of the sections. Grid creation was conducted using the Petrel software with grids of 170 cells in the x-direction, 120 cells in the y-direction and 251 cells in the z direction. The interpolation was conducted on all lithological, sedimentological, and δ13Ccarb and δ18Ocarb data using the moving average geostatistical method. The modeling outputs were visually checked with input data in each section to test the validity of the 3D modeling procedure.

Chemostratigraphic modeling used the global Darriwilian δ13Ccarb and δ18Ocarb curves documented from Baltoscandia (BC) and summarized from well-preserved brachiopods, respectively. The δ13Ccarb curve of the Darriwilian strata is divided into five isotope intervals in Baltoscandia (BC1, BC2, BC3, BC4 and BC5; BC = Baltoscandia carbon isotope interval) that are roughly correlatable across the Darriwilian platform in the southern Great Basin. In general, both δ13Ccarb and δ18Ocarb values in the southern Great Basin are lower than those of the standardized Darriwilian curves and the modeling results show large variations in both δ13Ccarb and δ18Ocarb across the carbonate platform. The lower values in δ13Ccarb and δ18Ocarb in the Great Basin may have been exerted by intensive bioturbation and organic matter respiration, along with potential diagenetic alterations beneath the basal Eureka Quartzite unconformity. The spatial variations in δ13Ccarb and δ18Ocarb may record isotope changes in locally restricted depositional environments on the Darriwilian carbonate platform.

Within the chemostratigraphic framework, facies modeling results show significant variations in facies association and cyclicity across the Darriwilian carbonate platform. In general, the northern and eastern areas are dominated by shallow-water facies (mostly intertidal to supratidal), while the southern regions show frequent facies changes from BC1 to BC5. During BC1 and BC2, facies show a general pattern that deepened from the north-northeast towards the southern region. During BC3 and BC4, however, the southern area was dominated by shallowwater facies (supratidal and intertidal). During BC5, facies changed back to the southwarddeepening trend similar to that of BC1 and BC2 but had more inter-platform facies variations.

The modeled results also show temporal and spatial variations in meter-scale cycles. The thicknesses of cycles in BC3 to BC4 intervals (10–15 m) are larger than those of BC1, BC2, and BC5 (6–10 m). In the north and northwest regions, meter-scale cycles are absent (most likely amalgamated) and only large-scale cycles (sequences) with a thickness of 20–50 m are recorded. Based on the thickness of the Darriwilian strata and duration of this interval, the 20–50-m-thick cycles recorded in the northern areas are most likely third-order cycles (sequences) correspondent with major eustatic sea-level changes (>1-million-year duration) during the Darriwilian. The temporal and spatial changes in facies and cyclicity suggest that even in a carbonate platform that was developed in a passive continental margin, sedimentary facies distribution and platform morphology may not have been as stable as previously thought. The morphological change of the Darriwilian carbonate platform may record a local tectonic event or fast growth of a shelf margin shoal complex in the southern part of the platform, which needs to be tested in future studies.

Keywords

Basin dynamics; Characterization; Middle Ordoovician; Petrel; The Southern Great Basin; Three-dimensional (3D)

Disciplines

Geology | Geotechnical Engineering | Sedimentology

File Format

pdf

File Size

2200 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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