Devonian to Mississippian strata of the Shine Jinst region revisited: Facies development and stratigraphy in southern Mongolia (Gobi Altai Terrane)

Abstract

This report provides new stratigraphical and facies data from Devonian and Carboniferous rocks in the Shine Jinst region (Trans Altai Zone, southern Mongolia) with a special focus on the Lower Devonian Chuluun Formation, the Middle Devonian Tsagaankhalga Formation, and the Upper Devonian to Mississippian Heermorit Member of the Indert Formation. Facies development in the Shine Jinst region exhibits a fundamental break in the carbonate platform evolution in the Lower Devonian as reef building organisms were affected by a major regression and deposition of several metres-thick conglomerates at the base of the Tsakhir Formation (Lower Devonian). The overlying Hurenboom Member of the Chuluun Formation is composed of fossiliferous limestones. Reef building organisms, such as colonial corals and stromatoporoids show low diversity and exhibit limited vertical growth and lateral extension of individuals. Thus, they do not represent a real reef as proposed in previous publications but biostromal limestones instead. One reason might be the isolated position of the Shine Jinst region between an unknown continent and a volcanic arc in the early Middle Devonian that hampered the successful colonization in shallow-water areas. Bivalves of the Alatoconchid family were once grouped into reef builders or biostrome builders and they are known only from Permian rocks. The found bivalve biostomes in Mongolia may represent precursors, which would document the oldest record of Alatoconchids found in the world. Remarkable thicknesses of massive crinoidal grainstones (“encrinites”) are documented in many parts of the succession, which suggest rather stable environmental conditions of a carbonate ramp setting at different times. The occurrence of thick-bedded conglomerates in the Shine Jinst section is not restricted to the Lochkovian to Pragian interval (Tsakhir Formation), but also occurs in the Eifelian. A thick-bedded conglomerate, which is interpreted to represent braided fluvial or fan-delta to shallow-marine deposits occurs at the base of the Tsagaankhaalga Formation. A steep relief associated with uplift and volcanism seems to be a realistic scenario for deposition of these sediments. This succession points to a remarkable tectonic uplift or sea-level fall in the Middle Devonian. Conodont findings of the studied section confirm the occurrence of time-equivalent strata of the Choteč Event, the Dasberg Crisis, and the Hangenberg Event found elsewhere in the world, which are described from Mongolia for the first time. Sedimentological descriptions, revised biostratigraphical data, and U-Pb dating by LA ICP-MS of some volcaniclastic rocks from the Chuluun Formation are presented in this report. The studied section records a complex interaction of sedimentation, regional tectonics, sea-level changes and coeval volcanism, which is very similar to other regions in Mongolia. The new data provide the background for further scientific studies in this region. This is a contribution to the Special Series on “The Central Asian Orogenic Belt (CAOB) during Late Devonian: New insights from southern Mongolia”, published in this journal.