Journal of Paleontology, 90(1), 2016, p. 78–91 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.18
Lower Silurian conodonts from the Shine Jinst region, southwestern Mongolia
Erika M. Danielsen* and D. Jeffrey Over
Department of Geological Sciences, State University of New York College at Geneseo, Geneseo, New York 14454, USA ⟨
edanielsen027@gmail.com⟩, ⟨
over@geneseo.edu⟩
Abstract.—Lower Silurian conodonts were recovered from carbonate beds in the upper Zalaa and lower Sharchuluut formations at Yamaan Us in the Shine Jinst region, Gobi-Altai Zone, of southwestern Mongolia. Conodonts are described from the Zalaa Formation for the first time; key taxa recovered include Distomodus kentuckyensis Branson and Branson, 1947, Ozarkodina hassi (Pollock, Rexroad, and Nicoll, 1970), Pranognathus siluricus (Pollock, Rexroad, and Nicoll, 1970), and Pseudolonchodina expansa (Armstrong, 1990). These species constrain the onset of the Silurian carbonate platform, upper Zalaa to lower Sharchuluut formations, to the Pranog- nathus tenuis Zone, middle Aeronian. The presence of a cosmopolitan conodont fauna places these strata from the Gobi-Altai Zone in a basin-shelf marine setting with connection and circulation to the open ocean during the Llandovery.
Introduction
The Shine Jinst region of southwestern Mongolia has been the focus of various tectonic, stratigraphic, and paleontological studies (Minjin et al., 2001; Wang et al., 2005; Lamb et al., 2008; Gibson et al., 2013), although investigation of age diag- nostic microfauna is in its early stages. In-depth studies of the conodont succession in the Shine Jinst region have potential to greatly increase the understanding of the geologic history of the area. Conodonts have previously been observed in several for- mations in the area, aiding in age determination for Ordovician strata (Minjin et al., 2001) and Devonian and Carboniferous strata (Aristov and Nyamsuren, 1998; Minjin et al., 2001; Wang et al., 2005). Little has been reported on conodonts from the Upper Ordovician–Silurian. Two formations, the Zalaa and Sharchuluut, have been generally assigned to this age (Minjin et al., 2001). Conodonts have not previously been reported from the Zalaa formation, and a slightly younger Silurian conodont fauna than the one described herein has been observed from the Sharchuluut Formation at Sharchuluut, although no detailed record or images of the fauna were provided (Minjin et al., 2001). Herein, Silurian conodonts from the Shine Jinst region at Yamaan Us (Fig. 1) are described and constrain the age of the upper Zalaa and lower Sharchuluut formations to the middle Llandovery. This establishes the timing of onset of the Silurian carbonate platform and supports a dynamic, open, basin-shelf marine environment for the Gobi-Altai Zone of southern Mongolia.
* Current address: Department of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa 52242, USA
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Geologic setting
Central Asia formed as an amalgamation of various geologic units that accreted throughout the Paleozoic and Mesozoic (Badarch et al., 2002; Cocks and Torsvik, 2007; Lamb et al., 2008; Krӧner et al., 2010; Guy et al., 2014). These units are typically defined as geologically unique terranes or larger tectonic zones. Badarch et al. (2002) divided Mongolia into 44 terranes, differentiable by rock type and other geologic characteristics, and bounded by fault zones. The Zalaa and Sharchuluut formations are within the Gobi Altai and Mandalovoo terranes. The Gobi Altai Terrane was classified as a back-arc basin that formed adjacent to the island arc of the Mandalovoo Terrane, which accreted to the Siberian Craton by the end of the Devonian. A more general tectonic scheme was proposed by Krӧner et al. (2010) in which four tectonic zones based on lithostratigraphic and paleontological descriptions were recognized. The study area is located in the Gobi-Altai Zone, an early Paleozoic passive margin sequence, which is bounded by the Main Mongolian Lineament to the north and the Trans-Altai Fault to the south. Gibson et al. (2013) classified the Gobi-Altai Zone as a cratonic Ordovician-Carboniferous basin. The Zalaa Formation consists of thin, trilobite-rich,
carbonate beds in shale, grading into crinoidal carbonates interbedded with green shale. The carbonate beds contain abundant crinoids, trilobites, ostracodes, brachiopods, and orthocone nautiloids, as well as corals and bryozoans (Minjin et al., 2001). Based on the ages of surrounding units, the Zalaa Formation is considered Ordovician to early Silurian. The Sharchuluut Formation mainly consists of massive carbonates that, at Yamaan Us, conformably overlie the Zalaa
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