100 research outputs found

    Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic paleoclimate

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    Several recent analyses of paleomagnetic data support the concept of Pangea, an assemblage of most of the world‘s continents that was mobile in terms of large-scale internal deformation and with respect to paleolatitude. The main feature of internal deformation involved the transformation from a Pangea B—type configuration in the late Paleozoic, with northwestern South America adjacent to eastern North America, to a more traditional Pangea A—type configuration in the early Mesozoic, with northwestern Africa adjacent to eastern North America. Pangea B thus seems to coincide in time with extensive low-latitude coal deposition and high southern-latitude Gondwana glaciations, whereas Pangea A coincides with generally drier conditions over the continents and no polar ice sheets. Although the configuration of Pangea may have been more stable as an A-type configuration in the Early and Middle Jurassic prior to breakup, the paleomagnetic evidence suggests that there was appreciable latitudinal change of the assembly. Such changing tectonic boundary conditions emphasize the practical importance of age registry of paleoclimate data in making valid comparisons with model results. A simple zonal climate model coupled with the geocentric axial dipole hypothesis for establishing paleolatitudes in precisely controlled paleogeographic reconstructions can explain many of the climate patterns in both the late Paleozoic and the early Mesozoic, but it cannot explain the presence or absence of continental ice sheets

    Widespread formation of cherts during the early Eocene climate optimum

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    Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones

    ADRIA AS PROMONTORY OF AFRICA AND ITS CONCEPTUAL ROLE IN THE TETHYS TWIST AND PANGEA B TO PANGEA A TRANSFORMATION IN THE PERMIAN

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    It has been almost 60 years since the first results from the Early Permian Bolzano Quartz Porphyries from the Trento Plateau of northern Italy (Southern Alps) showed paleomagnetic inclinations steeper than inclinations from broadly coeval units from central Europe. This experimental discrepancy, confirmed ever since at varying levels of magnitude and certitude, implied that northern Italy had paleolatitudes too northerly relative to Europe to be considered part of the European continent. On the other hand, it became progressively more apparent that paleomagnetic data from northern Italy were more compatible with data from Africa than with data from Europe, and this observation revived and complemented Argand’s original concept of Adria as a promontory of Africa. But if Adria was part of Africa, then the paleolatitude anomaly of Adria relative to Europe translated into a huge crustal misfit of Gondwana relative to Laurasia when these landmasses were forced into a classic Wegenerian Pangea as typified by the Bullard fit of the circum-Atlantic continents. This crustal misfit between Gondwana and Laurasia was shown to persist in the ever-growing paleomagnetic database even when data from Adria were provisionally excluded as non-cratonic in nature. Various solutions were offered that ultimately involved placing Gondwana to the east (allowing it to be more northerly) relative to Laurasia and envisaging a dextral shear occurring in the Tethys (Mediterranean) realm between these supercontinental landmasses. This shear or transformation was initially thought to occur as a continuum over the course of the Mesozoic–Cenozoic (the so-called ‘Tethys Twist’) but soon afterwards when plate tectonics came into play and limited the younger extent, as a discrete event during the post-Triassic, Triassic or most probably – as in the latest and preferred reconstructions – the Permian between a configuration of Pangea termed B – with the northwestern margin of Africa against southern Europe – to a configuration termed Pangea A-2, with the northwestern margin of Africa against eastern North America, that is more proximal in shape to the classic Pangea A-1 that started fragmenting in the Jurassic with the opening of the Atlantic Ocean. The Permian timing and presumed locus of the ~2300 km dextral shear is supported by rotated tectonic domains in Sardinia and elsewhere along the interface between Lauarasia and Gondwana. The concept of Pangea B and its transformation into Pangea A developed therefore in close conjunction with the concept and paleomagnetic support of Adria as a promontory of Africa, and has ramifications to many aspects of tectonics, climate change and biogeography yet to be explored

    A Novel Plate Tectonic Scenario for the Genesis and Sealing of Some Major Mesozoic Oil Fields

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    Recent research reveals evidence of a major event of global plate motion during the Jurassic, with a magnitude and tempo hitherto not fully appreciated. Yet, its legacy persists today as the potent Jurassic source-reservoir-seal oil systems in the Persian Gulf region. We suggest that these formed as a consequence of a sweeping tectonic movement whereby Arabia drifted rapidly from the oil- forming Intertropical Convergence Zone along the equator to the arid tropics of the southern hemisphere with rapid deposition of relatively uncemented carbonate reservoirs and anhydrite seals, all during as little as 15 m.y. in the Late Jurassic. The Ghawar super- giant oil field of Saudi Arabia was one of the results. Rapid latitu- dinal change may have influenced the development of some source-reservoir-seal oil systems elsewhere as well

    Lower and Middle Triassic foraminifera from the Eros Limestone, Hydra Island, Greece

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    Abstract. The systematics and stratigraphic ranges (constrained by conodont dating) of abundant and well preserved foraminiferal faunas from six sections in the Lower and Middle Triassic Eros Limestone of central and western Hydra (Argolis Peninsula, Greece) are described. A joint analysis of the conodonts, foraminifera and bivalves has enabled the Scythian and Anisian stages to be recognized with some certainty within the Eros Limestone carbonate platform. The foraminifera have affinities with those of many other Tethyan localities, in particular the Dinarides, Balkans, Carpathians and the Southern Alps

    Human migration into Europe during the late Early Pleistocene climate transition

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    A critical assesment of the available magnetostratigraphic and/or radiometric age constraints on key sites bearing hominin remains and/or lithic industries from southern Europe (Italy, France, Spain) leads us to propose that the main window of early hominin presence in southern Europe is broadly comprised between the Jaramillo subchron and the Brunhes–Matuyama boundary (i.e., subchron C1r.1r, 0.99–0.78 Ma). Within the dating uncertainties, this ~ 200 ky time window broadly coincides with the late Early Pleistocene global climate transition that contains marine isotope stage (MIS) 22 (~ 0.87 Ma), the first prominent cold stage of the Pleistocene. We suggest that aridification in North Africa and Eastern Europe, particularly harsh during MIS 22 times, triggered migration pulses of large herbivores, particularly elephants, from these regions into southern European refugia, and that hominins migrated with them. Finally, we speculate on common pathways of late Early Pleistocene dispersal of elephants and hominins from their home in savannah Africa to southern Europe, elephant and hominin buen retiro. In particular, we stress the importance of the Po Valley of northern Italy that became largely and permanently exposed only since MIS 22, thus allowing possibly for the first time in the Pleistocene viable new migration routes for large mammals and hominins across northern Italy to southern France and Spain in the west

    Magnetostratigraphy of a Lower-Middle Triassic boundary section from Chios (Greece)

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    The Marmarotrapeza Formation at Chios Island (northern Aegean Sea, Greece) is renowned for its Lower-Middle Triassic boundary sections in a marine Tethyan setting. Two sections have been sampled bed by bed to develop a magnetostratigraphic framework for the ammonoid and conodont biostratigraphy. The boundary sections occur within a lower normal (A+)-reverse (B−)-upper normal (C+) polarity sequence. The Lower-Middle Triassic boundary, placed at the first occurrence of the ammonoid genera Aegeiceras ugra Diener, Paracrochordiceras spp., Paradanubites depressus Fantini Sestini and Japonites sp., and close to the first appearance of the conodont species Gondolella timorensis Nogami, occurs in normal polarity zone Chios C+. The overall mean direction of the reversal-bearing characteristic component, whose early acquisition is suggested by a tilt test, is D = 271.2°, I = 33.2° (α95 = 11.7°, k = 112.5, N = 3). The inferred paleolatitude of the sampling sites is about 18°N, consistent with either an African or stable European affinity, although the declinations suggest large-scale counter-clockwise rotations with respect to Africa or stable Europe since the Early-Middle Triassic

    Astrochronostratigraphic polarity time scale (APTS) for the Late Triassic and Early Jurassic from continental sediments and correlation with standard marine stages

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    Paleomagnetic and cycle stratigraphic analyses of nearly 7000 m of section from continuous cores in the Newark basin and an overlapping 2500 meter-thick composite outcrop and core section in the nearby Hartford basin provide an astrochronostratigraphic polarity time-scale (APTS) for practically the entire Late Triassic (Carnian, Norian and Rhaetian) and the Hettangian and early Sinemurian stages of the Early Jurassic (233 to 199 Ma in toto). Aperiodic magnetic polarity reversals make a distinctive pattern of normal and reverse chrons for correlation, ideally paced by the periodic timing of orbital climate cycles, and anchored to million years ago (Ma) by high-precision U-Pb zircon dates from stratigraphically-constrained basalts of the Central Atlantic Magmatic Province (CAMP). Pinned by the CAMP dates, the Newark-Hartford APTS is calibrated by sixty-six McLaughlin cycles, each a reflection of climate forcing by the long astronomical eccentricity variation with the stable 405 kyr period, from 199.5 to 225.8 Ma and encompassing fifty-one magnetic polarity intervals, making it one of the longest continuous astrochronostratigraphic polarity time-scales available in the Mesozoic and Cenozoic. Extrapolation of sediment accumulation rates in fluvial sediments in the basal Newark section extends the sequence an additional fifteen polarity intervals to 232.7 Ma. The lengths of the 66 polarity chrons vary from 0.011 Myr (Chron E23r) to 1.63 Myr (Chron H24n) with an overall mean duration of 0.53 Myr. The oldest CAMP basalts provide a zircon U-Pb-based estimated age of 201.5 Ma for the base of the stratigraphically superjacent McLaughlin cycle 61 and 201.6 Ma using cycle stratigraphy for the onset of the immediately subjacent Chron E23r. The calibration age of 201.5 Ma for the base of McLaughlin cycle 61 is remarkably consistent with the calculated phase of the 498th long eccentricity cycle counting back using a period of 405 kyr from the most recent peak at 0.216 Ma. Accordingly, we suggest a nomenclature (Ecc405:k, where k is the cycle number or fraction thereof) to unambiguously assign ages from the astrochronostratigraphy. Magnetostratigraphic correlation of key Tethyan sections with diagnostic marine biostratigraphic elements to the Newark-Hartford APTS allows determination of numerical ages of standard marine stages, as follows: 227 Ma for the Carnian/Norian boundary, 205.5 Ma for the Norian/Rhaetian boundary (using a chemostratigraphic criterion, or about 4 Myr older for alternative criteria), 201.4 Ma for the Triassic/Jurassic boundary, and 199.5 Ma for the Hettangian/Sinemurian boundary. These age estimates are in excellent agreement with available constraints from high-precision U-Pb zircon dating from the Pucara Basin of Peru and along with the presence of the short Chron E23r in several basins argue strongly against suggestions that millions of years of Rhaetian time is missing in a cryptic hiatus or unconformity that supposedly occurs just above Chron E23r in the Newark Supergroup basins. It is more parsimonious to explain the apparent temporal delays in appearances and disappearances of palynoflora, conchostracans, and other endemic taxa in continental deposits as a reflection of demonstrated continental drift across climate belts and the misinterpretation of ecostratigraphy as chronostratigraphy. The Newark-Hartford APTS provides a chronostratigraphic template for continuing efforts at correlation of Late Triassic and Early Jurassic continental and marine sections throughout the world, including integration with atmospheric pCO2 measurements from paleosol carbonates and carbon isotopic measurements from marine carbonates to better understand the global carbon cycle as well as understanding the causes of and recovery from the end-Triassic mass extinction

    Integrated Anisian–Ladinian boundary chronology

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    We report magnetostratigraphic and biostratigraphic data from the Seceda core and the correlative outcrop section from the Dolomites of northern Italy. The Seceda rock succession consists of Tethyan marine limestones and radiometrically dated volcaniclastic layers of the Buchenstein Beds of Middle Triassic age (∼238–242 Ma). The Seceda outcrop section was correlated to coeval sections from the literature using magnetic polarity reversals and a selection of laterally traceable and isochronous lithostratigraphic marker beds. This allowed us to import the distribution of age-diagnostic conodonts, ammonoids, and daonellas from these sections into a Seceda reference stratigraphy for the construction of an integrated biochronology extending across a consistent portion of the Anisian–Ladinian boundary interval. Among the three options selected by the Subcommission for Triassic Stratigraphy to establish the Ladinian Global Stratigraphic Section and Point, we propose to adopt the level containing the base of the Curionii ammonoid Zone at Bagolino (Southern Alps, Italy) because this level is closely associated with a global means of correlation represented by the base of polarity submagnetozone SC2r.2r. The first occurrence of Neogondolella praehungarica in the Dolomites predates slightly the base of the Curionii Zone and can be used to approximate the Anisian–Ladinian boundary in the absence of ammonoids
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