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  • 1
    Electronic Resource
    Electronic Resource
    Small-scale hummocky cross-stratification in turbidites: a form of antidune stratification? (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Sedimentology 37 (1990), S. 0 
    Details
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Small-scale hummocky cross-stratification occurs in Upper Cretaceous calciclastic turbidites exposed in the western Basque Pyrenees; facies associations and microfossil assemblages indicate slope to base-of-slope (bathyal) depositional environments. It is developed in the fine-grained portion of beds and displays spacings mostly between 0.2 and 0.7 m. The beds fine upward with no sharp grain size breaks or mud partings, suggesting that deposition occurred during a single flow event.Hummocky intervals are 0.1–0.8 m thick and consistently grade laterally and vertically into flat, planar laminations of Bouma B divisions suggesting that deposition occurred under upper-flow-regime conditions. They have wave-like geometries with laminae continuous across ‘crests’ and ‘troughs’ and display a ratio of ‘wavelength’ to estimated underflow thickness of 11.3–12.8.Combining the above observations and inferences, these examples of small-scale hummocky cross-stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner, denser underflow (main body/tail of the turbidity current) and an overlying thicker, low-density layer. This occurrence is further evidence that small-scale hummocky cross-stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3091.1990.tb00152.x
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  • 2
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    Oxygenated conditions in the aftermath of the Lomagundi-Jatuli Event: The carbon isotope and rare earth element signatures of the Paleoproterozoic Zaonega Formation, Russia (2020)
    Elsevier
    Details
    Publication Date: 2020-09-01
    Print ISSN: 0301-9268
    Electronic ISSN: 1872-7433
    Topics: Geosciences
    Published by Elsevier
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  • 3
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    Duration and nature of the end-Cryogenian (Marinoan) glaciation (2016)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2016-07-22
    Description: The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 4
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    Sedimentology of the early Neoproterozoic Morar Group in northern Scotland: implications for basin models and tectonic setting (2012)
    Geological Society of London
    Details
    Publication Date: 2012-01-03
    Description: The metasedimentary rocks of the Morar Group in northern Scotland form part of the early Neoproterozoic Moine Supergroup. The upper part of the group is c . 2–3 km thick and contains two large kilometre-scale facies successions: a coarsening-upwards marine-to-fluvial regression overlain by a fining-upwards fluvial-to-marine transgression. Fluvial facies make up less than a third of the total thickness; shallow-marine lithofacies make up the remainder. Combining these new findings with previously published data indicates that the Morar Group represents, overall, a transgressive stratigraphic succession c. 6–9 km thick, in which there is both an upward and eastward predominance of shallow-marine deposits, and a concomitant loss of fluvial facies. Smaller-scale (hundreds of metres thick) transgressive–regressive cycles are superimposed on this transgressive trend. Collectively, the characteristics of the succession are consistent with deposition in a foreland basin located adjacent to the Grenville orogen, and possibly linked to the peri-Rodinian ocean. Subsidence and progressive deepening of the Morar basin may have, at least in part, been driven by loading of thrust sheets emplaced during the Grenville orogeny, and aided by sediment loading. However, the relative contributions of thrust loading versus plate boundary effects and/or eustatic sea-level rise on basin evolution remain speculative.
    Print ISSN: 0016-7649
    Topics: Geosciences
    Published by Geological Society of London
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  • 5
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    Ocean acidification in the aftermath of the Marinoan glaciation (2014)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2014-11-25
    Description: Boron isotope patterns preserved in cap carbonates deposited in the aftermath of the younger Cryogenian (Marinoan, ca. 635 Ma) glaciation confirm a temporary ocean acidification event on the continental margin of the southern Congo craton, Namibia. To test the significance of this acidification event and reconstruct Earth’s global seawater pH states at the Cryogenian-Ediacaran transition, we present a new boron isotope data set recorded in cap carbonates deposited on the Yangtze Platform in south China and on the Karatau microcontinent in Kazakhstan. Our compiled 11 B data reveal similar ocean pH patterns for all investigated cratons and confirm the presence of a global and synchronous ocean acidification event during the Marinoan deglacial period, compatible with elevated postglacial p CO 2 concentrations. Differences in the details of the ocean acidification event point to regional distinctions in the buffering capacity of Ediacaran seawater.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 6
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    Neoproterozoic ice sheets and olistoliths: multiple glacial cycles in the Kingston Peak Formation, California (2014)
    Geological Society of London
    Details
    Publication Date: 2014-07-01
    Description: The Kingston Peak Formation is a diamictite-bearing succession that crops out in the Death Valley region, California, USA. An exceptionally thick (〉1.5 km) outcrop belt in its type area (the Kingston Range) provides clear insights into the dynamics of mid-Cryogenian (‘Sturtian’) ice sheets in Laurentia. Seven detailed logs allow the lateral and vertical distribution of facies associations to be assessed. We recognize (1) diamictite facies association (ice-proximal glacigenic debris flows), (2) lonestone-bearing facies association (ice-marginal hemipelagic deposits and low-density gravity flows with iceberg rafting), (3) pebble to boulder conglomerate facies association (ice-proximal cogenetic glacigenic debris flows and high-density turbidites), (4) megaclast facies association (olistostrome and hemipelagic sediments subject to ice-rafting), and (5) interbedded heterolithics facies association (low-density turbidites and hemipelagic deposits). The stratigraphic motif allows three glacial cycles to be inferred across the range. Ice-minimum conditions interrupting the Kingston Peak succession are associated with the development of an olistostrome complex, succeeded by a thick accumulation of boulder conglomerates deposited during ice readvance. The data testify to a strong glacial influence on sedimentation within this ancient subaqueous succession, and to highly dynamic ice sheet behaviour with clear glacial cycles during the Sturtian glaciation.
    Print ISSN: 0016-7649
    Topics: Geosciences
    Published by Geological Society of London
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  • 7
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    Proterozoic onset of crustal reworking and collisional tectonics: Reappraisal of the zircon oxygen isotope record (2014)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2014-04-19
    Description: A global U-Pb and 18 O zircon database shows temporal changes in the magmatic record related to changes in the degree of crustal reworking. The 18 O composition of bulk sediment remains relatively constant through geologic time, with a mean value of 14.9. In contrast, the 18 O values in magmatic zircons vary from relatively low values averaging ~6 in the Archean to increasingly higher and scattered values defining a series of peaks and troughs in post-Archean data. The degree of crustal reworking increases at times of supercontinent assembly. Therefore we attribute the pattern of post-Archean 18 O values recorded by magmatic zircons to a significant increase in the incorporation of high 18 O sediment in response to enhanced crustal thickening and reworking associated with the onset of collisional tectonics, especially during formation of supercontinents.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 8
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    Two-billion-year-old evaporites capture Earths great oxidation (2018)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2018-04-20
    Description: Major changes in atmospheric and ocean chemistry occurred in the Paleoproterozoic era (2.5 to 1.6 billion years ago). Increasing oxidation dramatically changed Earth’s surface, but few quantitative constraints exist on this important transition. This study describes the sedimentology, mineralogy, and geochemistry of a 2-billion-year-old, ~800-meter-thick evaporite succession from the Onega Basin in Russian Karelia. The deposit consists of a basal unit dominated by halite (~100 meters) followed by units dominated by anhydrite-magnesite (~500 meters) and dolomite-magnesite (~200 meters). The evaporite minerals robustly constrain marine sulfate concentrations to at least 10 millimoles per kilogram of water, representing an oxidant reservoir equivalent to more than 20% of the modern ocean-atmosphere oxidizing capacity. These results show that substantial amounts of surface oxidant accumulated during this critical transition in Earth’s oxygenation.
    Keywords: Geochemistry, Geophysics
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 9
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    Detrital zircon geochronology of the Grenville/Llano foreland and basal Sauk Sequence in west Texas, USA (2014)
    Geological Society of America (GSA)
    Details
    Publication Date: 2014-06-28
    Description: U-Pb detrital zircon ages from Mesoproterozoic and Cambrian siliciclastic units in west Texas (USA) constrain the depositional setting, provenance, and tectonic history of the region within a late Mesoproterozoic Grenville foreland basin and the early Paleozoic Sauk transgressive sequence. Two key units, the Hazel and Lanoria Formations, have detrital zircon age spectra dominated by detritus derived from the Grenville orogen (the Llano uplift and eroded equivalents), the ca. 1.4 Ga Granite-Rhyolite, and the ca. 1.7–1.6 Ga Yavapai/Mazatzal provinces. These data, combined with sedimentological data, permit interpreting those formations as the proximal and distal deposits, respectively, of a molasse shed into the Grenvillian foreland basin. Detrital zircons as young as ca. 520 Ma show that the Van Horn Formation, previously considered to be Precambrian in age, is no older than middle Cambrian. Further, the overall detrital zircon age spectrum of the Van Horn Formation is similar to that of the overlying Cambro-Ordovician Bliss Formation: both indicate derivation from sources that included the Colorado-Oklahoma aulacogen, Grenville, Granite-Rhyolite, and Yavapai/Mazatzal provinces. The similarities between the depositional history of the Van Horn and Bliss Formations lead us to conclude that the base of the Sauk Sequence in west Texas occurs at the base of the Van Horn Formation. Base-level rise associated with the Sauk transgression affected drainage patterns and sediment deposition along southwestern Laurentia some 20 m.y. earlier than previously assumed.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 10
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    The Neoproterozoic Noonday Formation, Death Valley region, California: Reply (2013)
    Geological Society of America (GSA)
    Details
    Publication Date: 2013-01-03
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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