Early Paleozoic Ocean Research Articles

Early Paleozoic paleogeography of Laurentia and western Gondwana: Evidence from tectonic subsidence analysis

Research Article| August 01, 1997 Early Paleozoic paleogeography of Laurentia and western Gondwana: Evidence from tectonic subsidence analysis Kenneth E. Williams Kenneth E. Williams 1Texaco Inc., Exploration and Production Technology Department, 3901 Briarpark, Houston, Texas 77042 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (8): 747–750. https://doi.org/10.1130/0091-7613(1997)025<0747:EPPOLA>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 MailTo Tools Icon Tools Get Permissions Search Site Citation Kenneth E. Williams; Early Paleozoic paleogeography of Laurentia and western Gondwana: Evidence from tectonic subsidence analysis. Geology 1997;; 25 (8): 747–750. doi: https://doi.org/10.1130/0091-7613(1997)025<0747:EPPOLA>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 A number of terranes were present in the early Paleozoic ocean between Laurentia and Gondwana following rifting. The precise location of these microcontinents and island arcs is unclear. We present tectonic subsidence curves from selected basins that may provide some constraints on the possible paleogeographic interpretations for this time interval. Among these tectonic blocks were the Precordillera terrane that originated in the Ouachita area of Laurentia and the composite Avalon-Carolina terrane that was derived from the Gondwana margin. Also present were the island-arc terranes of Famatina, Mixteca, and Zapoteca. The Precordillera and Famatina terranes are currently located in southern South America. The Avalon-Carolina terrane is in eastern North America. The Mixteca and Zapoteca terranes are in Central America. A review of previous work augmented by new tectonic subsidence analysis indicates that these terranes were swept up by early Paleozoic plate movements and were translated to approximately their present relative positions by the Early Carboniferous. This content is PDF only. Please click on the PDF icon to access. 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.

Carbonate-Slope Failures as Indicators of Sea-Level Lowerings

Occasionally, carbonate-slope failures of such a magnitude occur that immense volumes of material move downslope as submarine slides and boulder-bearing debris flows. These spectacular deposits can be triggered by earthquakes or tsunamis. However, when such deposits are regionally widespread or are on separate lithospheric plates, at times of sea-level lowering, trigger mechanism is most likely eustatic sea-level fluctuations. The authors propose that during the initial phases of a sea-level lowering, slope and/or platform-margin collapse can happen, owing to gravitational instability of partially cemented carbonates. Fragments of the margins of the early Paleozoic proto-Pacific Ocean are found in widely separated terranes, including western North America and southern Kazakhstan (USSR). Coeval carbonate-slope and platform-margin failures occurred in both areas during the Late Cambrian and Early Ordovician. Up to 75-km-long segments of these carbonate slopes and platform margins collapsed and were transported seaward as submarine slides and megabreccia debris flows. These catastrophic events contributed to the development of 500-m-thick carbonate submarine fans and aprons. Slope and platform-margin failures also correlate with solution breccia and faunal disconformities in platform-interior sites. They interpret these widely separated yet coeval mass-transport processes to have happened during rapid oceanward progradation of their respective carbonate margins, in combination withmore » several eustatic sea-level lowerings.« less

The oceanic non-sulfidic oxygen minimum zone: a habitat for graptolites?

The denitrified low oxygen zone in Early Paleozoic oceans is proposed as a potential habitat of planktic graptolites. Modern analogs of this zone are found in the eastern tropical Pacific (ETP) and in the north­ern Arabian Sea as shallow regions, up to a 100 meters thick, at the top of the pycnocline. There, oxygen is low or undetected and hydrogen sulfide has not been found. In modem oceans, denitrification regions are limited vertically and horizontally as oxygen is replenished from below by ventilated deep waters. In the Early Paleozoic ocean, the denitrification layer would be global due to poor deep ventilation. It would be transitional between oxygenated surface waters and toxic sulfide-rich water. Many branched graptolites could have evolved when the denitrified waters were in or close to the photic zone, feeding on the abun­dant phytoplankton attracted to both light and nutrients. As this zone sank below the photic zone, grapto­lites who developed planktic mode of life could have migrated daily toward the food supply, similar to euphausiids in the modern ETP. Thus the changes in graptolite rhabdosomes from pendent to scandent and from many branched to biserial and uniserial are suggested as adaptations to assist vertical migration and feeding. With the continued ventilation of the oceans and the shrinking of the denitrified layer, grap­tolite extinction could have resulted as a combination of reduction in food supply and living space, in­creased predation from the then evolving fish and ammonites, and competition in the zoop!ankton niche from smaller (less visible) and more motile forms.

Nd and Sr isotopic variations of Early Paleozoic oceans

We report 143Nd/ 144Nd and 87Sr/ 86Sr isotopic data for Lower Paleozoic phosphatic brachiopod and conodont fossils. The data appear to represent the isotopic values of Early Paleozoic seawaters. We show that different paleoceanic water masses can be distinguished on the basis of their ε Nd signatures. Two sides of what is classically considered one circulating Iapetus Ocean have different ε Nd signatures from at least the Middle Cambrian until the Late Middle Ordovician. We infer two ocean basins between North America and Baltica separated by an island and/or shoal circulation barrier. Thus, it appears necessary to redefine the area of the Iapetus Ocean. The ε Nd signature of the redefined smaller Iapetus Ocean ranges from −5 to −9 and the ε Nd signature of the larger, coeval Panthalassa Ocean, including part of what was formerly called the Iapetus Ocean, ranges from −10 to −20. The first time that the ε Nd values are identical in these two water masses is coincident with the onset of the Taconic Orogeny of North America. The paleogeographic geometry we infer from this work is consistent with paleogeographic reconstructions having the Baltica continent at very high latitudes in the Early/Middle Ordovician. The ε Nd and faunal distribution support temperature-controlled conodont faunal provinciality. A rough mean age for exposed continental crust in the Early Paleozoic can be obtained from the average ε Nd value of Early Paleozoic Oceans. The data suggest that the mean age of the crust as a function of time has remained essentially constant or even decreased during the past 500 Ma, and suggest substantial additions of new crust to the continents through the Phanerozoic.