Abstract Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.
New bulk and molecular organic geochemical analyses of source rock and oil samples from Mongolia indicate the presence of lacustrine-sourced petroleum systems in this frontier region. Samples of potential source rocks include carbonate, coal, and lacustrine-mudstone lithologies that range from Paleozoic to Mesozoic in age, and represent a variety of tectonic settings and depositional environments. Rock-Eval and total organic carbon data from these samples reflect generally high-quality source rocks, including both oil- and gas-prone kerogen types, mainly in the early stages of generation. Bulk geochemical and biomarker data indicate that Lower Cretaceous lacustrine mudstone found in core from the Zuunbayan field is the most likely source facies for the East Gobi basin of southeastern Mongolia. Oil and selected source rock samples from the Zuunbayan and Tsagan Els fields (both in the East Gobi basin) demonstrate geochemical characteristics consistent with nonmarine source environments and indicate strong evidence for algal input into fresh- to brackish-water source facies, including elevated concentrations of unusual hexacyclic and heptacyclic polyprenoids. Despite similarities between Zuunbayan and Tsagan Els oil samples, biomarker parameters suggest higher algal input in facies sourcing Zuunbayan oil compared to Tsagan Els oil. Tsagan Els oil samples are also generated by distinctly more mature source rocks than oil from the Zuunbayan field, based on sterane and hopane isomerization ratios.
Research Article| December 01, 2001 Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia S.A. Graham; S.A. Graham 1School of Earth Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar M.S. Hendrix; M.S. Hendrix 2Department of Geology, University of Montana, Missoula, Montana 59812, USA Search for other works by this author on: GSW Google Scholar C.L. Johnson; C.L. Johnson 3School of Earth Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar D. Badamgarav; D. Badamgarav 4Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 Search for other works by this author on: GSW Google Scholar G. Badarch; G. Badarch 5Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 Search for other works by this author on: GSW Google Scholar J. Amory; J. Amory 6Shell International Exploration and Production, Box 60, 2280 AB Rijzwijk-ZH, Netherlands Search for other works by this author on: GSW Google Scholar M. Porter; M. Porter 7Brigham Exploration, Inc., Austin, Texas 78736, USA Search for other works by this author on: GSW Google Scholar R. Barsbold; R. Barsbold 8Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 Search for other works by this author on: GSW Google Scholar L.E. Webb; L.E. Webb 9Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA Search for other works by this author on: GSW Google Scholar B.R. Hacker B.R. Hacker 10Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Author and Article Information S.A. Graham 1School of Earth Sciences, Stanford University, Stanford, California 94305, USA M.S. Hendrix 2Department of Geology, University of Montana, Missoula, Montana 59812, USA C.L. Johnson 3School of Earth Sciences, Stanford University, Stanford, California 94305, USA D. Badamgarav 4Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 G. Badarch 5Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 J. Amory 6Shell International Exploration and Production, Box 60, 2280 AB Rijzwijk-ZH, Netherlands M. Porter 7Brigham Exploration, Inc., Austin, Texas 78736, USA R. Barsbold 8Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351 L.E. Webb 9Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA B.R. Hacker 10Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA Publisher: Geological Society of America Received: 05 Sep 2000 Revision Received: 02 May 2001 Accepted: 18 Jun 2001 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2001) 113 (12): 1560–1579. https://doi.org/10.1130/0016-7606(2001)113<1560:SRATIO>2.0.CO;2 Article history Received: 05 Sep 2000 Revision Received: 02 May 2001 Accepted: 18 Jun 2001 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S.A. Graham, M.S. Hendrix, C.L. Johnson, D. Badamgarav, G. Badarch, J. Amory, M. Porter, R. Barsbold, L.E. Webb, B.R. Hacker; Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia. GSA Bulletin 2001;; 113 (12): 1560–1579. doi: https://doi.org/10.1130/0016-7606(2001)113<1560:SRATIO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence.The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Research Article| February 01, 1999 Occurrence, age, and implications of the Yagan–Onch Hayrhan metamorphic core complex, southern Mongolia L. E. Webb; L. E. Webb 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar S. A. Graham; S. A. Graham 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar C. L. Johnson; C. L. Johnson 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar G. Badarch; G. Badarch 2Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, 63 Peace Avenue, Ulaanbaatar, Mongolia 210357 Search for other works by this author on: GSW Google Scholar M. S. Hendrix M. S. Hendrix 3Department of Geology, University of Montana, Missoula, Montana 59812, USA Search for other works by this author on: GSW Google Scholar Author and Article Information L. E. Webb 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA S. A. Graham 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA C. L. Johnson 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA G. Badarch 2Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, 63 Peace Avenue, Ulaanbaatar, Mongolia 210357 M. S. Hendrix 3Department of Geology, University of Montana, Missoula, Montana 59812, USA Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (2): 143–146. https://doi.org/10.1130/0091-7613(1999)027<0143:OAAIOT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation L. E. Webb, S. A. Graham, C. L. Johnson, G. Badarch, M. S. Hendrix; Occurrence, age, and implications of the Yagan–Onch Hayrhan metamorphic core complex, southern Mongolia. Geology 1999;; 27 (2): 143–146. doi: https://doi.org/10.1130/0091-7613(1999)027<0143:OAAIOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mylonitic rocks associated with the south-dipping detachment fault of the Yagan–Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast–directed extension in the Early Cretaceous; synkinematic biotites give 40Ar/39Ar ages of 129 to 126 Ma. The Yagan–Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. The presence of Mesozoic metamorphism at Onch Hayrhan, previously presumed to be Precambrian, brings into question the existence of the South Gobi microcontinent. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Ancient barrier islands are poorly understood relative to other clastic depositional environments, despite being prominent features along modern coastlines and important for understanding transgressive shoreline deposits. A new dataset of ancient barrier island dimensions (n=83 examples) addresses this knowledge gap with a quantitative analysis of barrier island sand body dimensions including thickness (vertical), length (shore-parallel direction), and width (shore-perpendicular direction). This dataset of barrier island deposits was compared to planform measurements made for modern islands (n=274), to investigate possible scaling relationships and other aspects of modern to ancient linkages. These measurements are nuanced and challenging to perform, and first-pass comparisons show that modern barrier islands should not be used as direct analogs for ancient systems. Nevertheless, results emphasize key depositional and preservation processes, and the dimensional differences between deposits formed over geologic versus modern time scales. Using the methods outlined herein, barrier island deposits appear to be 2-5x longer (p50 modern = 10.7 km; p50 ancient = 20.0 km), and 6-15x wider (p50 modern = 1.2 km; p50 ancient = 7.3 km) than modern barrier islands. We interpret the results to indicate that ancient barrier islands are time-transgressive deposits recording vertical amalgamation, and barrier island growth by lateral accretion, and progradation. When comparing single barrier islands, thickness measurements do not vary systemically between modern and ancient examples, suggesting that local accommodation dictates barrier island thickness as a preservation control. Gross length, width, and thickness measurements are too coarse for robust paleomorphodynamic calculations, therefore more detailed sub-environment analysis (e.g., upper shoreface delineation), with improved facies models, is required before rigorous quantifications can be generated. However, these initial comparisons do show scaling trends between length and width which could be leveraged, with caution, in the interim. As sea levels continue to rise, understanding barrier island motion and preservation will be central to predicting coastal change. Keywords: paleomorphodynamics, barrier island, scaling relationships, accommodation, shallow marine, dimension prediction, modern analog, reservoir, transgressive
