Stable Carbon Isotope Signatures across the Lower to Middle Triassic Boundary Recorded in the Western and Central Panthalassa Shallow-water Carbonates

概要

Carbon isotope (δ13C) stratigraphy of marine carbonates is a useful tool for stratigraphic correlation and potentially for understanding the development of Earth’s climate, evolution of its biota, and CO2 levels in the atmosphere. Currently, the δ13C stratigraphy is well investigated and established for the lowermost and the uppermost Triassic intervals due to the many studies of the Permian–Triassic and the Triassic–Jurassic mass extinction events. The δ13C profiles of the intervening Triassic intervals are inconsistent among different studies and regions. In addition, most research on the Triassic has been undertaken in the Tethyan region, with more limited study of sections on the eastern margin and western margin of the Panthalassic Ocean. The Panthalassic Ocean covered ~60% of the Earth’s surface during the Early Triassic, but the biotic assemblages and sedimentary evolution of these oceanic sediments are poorly documented, and their history remains unresolved.

The remaining marine strata of the Panthalassic Ocean are limited to accretionary terranes in Japan, New Zealand, and British Columbia. Another section, the Middle Triassic carbonate platforms extend westwards from the North-Central Coast region of Vietnam to the northern Laos, located in the western Panthalassa. A thorough understanding of the records preserved in such as Japanese and Vietnamese sections is essential for any analysis of global patterns during the Triassic period. Therefore, the doctoral dissertation focuses on the carbonate succession of Taho Limestone in Southwestern Japan, and Hoang Mai Limestone in the North-Central Coast region of Vietnam. The aims of the study are to (i) interpret the litho-, bio-, and chemostratigraphy of the Middle Triassic carbonate succession in the North-Central Coast region of Vietnam, (ii) provide a new high-resolution Middle Triassic carbon isotope stratigraphy that is constrained by foraminiferal biostratigraphy from the Hoang Mai Limestone, (iii) provide a new high-resolution Spathian to Aegean (upper Lower to lower Middle Triassic) carbon isotope stratigraphy that is constrained by conodont biostratigraphy of the Taho limestone, and (iv) compile Spathian to Aegean δ13C profiles from carbonates in other regions/sites of the Tethys and compare them with those from the Hoang Mai and Taho Limestone to figure out common secular variations in the δ13C values.

In North-Central Coast region of Vietnam area, the Trang An Landscape Complex carbonates, located near the southern margin of the Red River Delta, are designated a UNESCO World Natural Heritage site for their spectacular limestone karst peaks surrounded by steep, almost vertical cliffs. Large volumes of the limestone platforms have been mined for raw materials for cement. In addition, paleontological studies have been conducted on Middle Triassic ammonoids, bivalves, and foraminifera, including species that are age-diagnostic. In this study, the sedimentological, paleontological, and geochemical analyses were conducted on the carbonates of the Hoang Mai Formation of Hoang Mai Limestone in Nghe An province, Vietnam. Drill cores (cores no. 1 and no. 4) were obtained from such carbonate rocks of the Hoang Mai Formation. The cores were 130.0 and 160.0 m long, and recovery was 100% and 99.3%, respectively. A total of 290 bulk carbonate samples were collected from the two cores for geochemical analyses. Additional samples were taken from veins and cements to compare their chemical and isotopic compositions with those of the bulk carbonate samples. The carbonates consist of the following six lithostratigraphic units (from bottom to top): sandy limestone (unit 1), peloidal packstone with a microbialite interlayer (unit 2), peloidal packstone and oncoidal floatstone (unit 3), peloidal–bioclastic packstone (unit 4), peloidal packstone with variable quantities of bioclasts and microbialite (unit 5), and peloidal packstone with variable quantities of bioclasts (unit 6). The sandy nature of unit 1 and of three of the interlayers in unit 2 indicates a supply of terrigenous material during the early stages of formation of the Hoang Mai carbonate platform. The dominance of carbonates with a fine-grained matrix throughout the overlying succession implies a low-energy depositional environment. Common occurrences of dasycladalean algae in units 4–6 indicate a back-reef lagoon environment (Figures 1 and 2).

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