Jean Marcoux

The Oman Exotics: a key to the understanding of the Neotethyan geodynamic evolution

AbstractThe study of the exotic blocks of the Hawasina Nappes (Sultanate of Oman) leads to give apposit data that allow us to propose a new paleogeographic evolution of the Oman margin in time and space. A revised classification of exotic blocks into different paleogeographical units is presented. Two newly introduced stratigraphic groups, the Ramaq Group (Ordovician to Triassic) and the Al Buda’ah Group (upper Permian to Jurassic) are interpreted as tilted blocks related to the Oman continental margin. The Kawr Group (middle Triassic to Cretaceous) is redefined and interpreted as an atoll-type seamount. The paleogeography and paleoenvironments of these units are integrated into a new scheme of the Neotethyan rifting history. Brecciae and olistoliths of the Hawasina series are interpreted to have originated from tectonic movements affecting the Oman margin and the Neotethyan ocean floor. The breccias of late Permian age were generated by the extension processes affecting the margin, and by the creation of...

Paleomagnetic results from Saudi Arabia and the Permo-Triassic Pangea configuration

The Pangea configuration for the interval of Late Carboniferous to Early Jurassic is still debated: was it similar to the well known Wegenerian Pangea or did Gondwanaland move with respect to Laurussia along a major right-lateral lineament of several thousand kilometres during the Triassic, as suggested by Irving? We try here to answer this important question with the paleomagnetic study of well dated Permo-Triassic sediments from a borehole in Saudi Arabia. Sediments of the same age have also been sampled at the surface, but proved to be remagnetized, probably by a strong desert alteration. The paleomagnetic study of the bore-hole core yielded a high temperature component magnetization thought to be of primary origin carried either by magnetite (Late Permian limestones) or hematite (Early Triassic red shales) with antipodal normal and reversed inclinations. Its co-latitude and pole directions compare well with those from a selection of South American, African, Madagascar, and Moroccan poles. A new reliable pole west Gondwana with a mean age of 244 ± 11 m.y. is derived (λ = 53.4°N, Φ = 259.4°E, A95 = 3.6 in West African coordinates), and its comparison with the Laurussian poles of same age window strongly suggests a Pangea in which the northern part of South America was facing the east of North America (Pangea B of Irving) at the Permo-Triassic boundary. A large right-lateral strike-slip movement of some 3500 km during the Triassic is thus required to reconstruct Pangea to its Jurassic pre-Atlantic opening position.

Integrated Upper Carnian to Lower Norian biochronology and implications for the Upper Triassic magnetic polarity time scale

Abstract We summarize the ammonoid, conodont and halobiid biochronology of the Upper Carnian to Lower Norian, based on a discussion of data in the Alps, Sicily, Balkans, Turkey, Himalayas and Timor. With this integrated biostratigraphic scale, the Pizzo Mondello section (Sicily) can be recalibrated and the Carnian–Norian boundary more precisely located there. As a result, the magnetostratigraphy of this section is now in good agreement with previous results from Turkey, although the latter series are more condensed. Cross-correlation of available magnetostratigraphic data from marine Tethyan sections allow us to construct a composite Upper Carnian to Upper Norian geomagnetic polarity time scale (GPTS). This GPTS leads us to question previously proposed magnetobiostratigraphic and chronostratigraphic correlations within the Upper Triassic Newark non-marine sedimentary sequence.

Improving the Upper Triassic numerical time scale from cross-correlation between Tethyan marine sections and the continental Newark basin sequence

Abstract Magnetostratigraphic cross-correlations between biochronologically well-dated Tethyan marine sections and the astronomically tuned sedimentary sequence from the continental Newark basin (eastern North America) help to improve the Upper Triassic time scale. We show here that the most recent radiometric results constraining the age of the base and top of the Upper Triassic (∼237 and ∼200 Ma, respectively) strengthen the chronostratigraphic calibration of the Newark sedimentary sequence proposed by Krystyn et al. [Earth Planet. Sci. Lett. 203 (2002) 343–351]; these results allow one to reconcile the Milankovitch-based cyclostratigraphy from the Newark sequence, the magnetostratigraphic data from both marine and continental environments, and Upper Triassic biostratigraphy (including data on Tethyan marine faunas and on terrestrial vertebrates). According to this calibration, the successive Upper Triassic stages would have very different durations: ∼10 Myr for the Carnian (237–227 Ma), ∼25 Myr for the Norian (227–202 Ma) and ∼2 Myr for the Rhaetian (202–200 Ma, but only if the entire Rhaetian is recorded in the Newark sequence). We further show that the biozone durations in the Upper Triassic would have been highly variable and, as a whole, would have lasted much longer than in the Lower and Middle Triassic.