Triassic synthems of southern South America (southwestern Gondwana) and the Western Caucasus (the northern Neotethys), and global tracing of their boundaries

Abstract

Global tracing of the key surfaces of Triassic deposits may contribute significantly to the understanding of the common patterns in their accumulation. We attempt to define synthems – disconformity-bounded sedimentary complexes – in the Triassic successions of southern South America (southwestern Gondwana, Brazil and Argentina) and the Western Caucasus (the northern Neotethys, Russia), and then to trace their boundaries in the adjacent regions and globally. In southern South America, a number of synthems have been recognized – the Cuyo Basin: the Río Mendoza–Cerro de las Cabras Synthem (Olenekian–Ladinian) and the Potrerillos–Cacheuta–Río Blanco Synthem (Carnian–Rhaetian); the Ischigualasto Basin: the Ischichuca-Los Rastros Synthem (Anisian–Ladinian) and the Ischigualasto–Los Colorados Synthem (Carnian–Rhaetian); the Chaco–Paraná Basin: the Sanga do Cabral Synthem (Induan), the Santa Maria 1 Synthem (Ladinian), the Santa Maria 2 Synthem (Carnian), and the Caturrita Synthem (Norian); western Argentina: the Talampaya Synthem (Lower Triassic) and the Tarjados Synthem (Olenekian?). In the Western Caucasus, three common synthems have been distinguished: WC-1 (Induan–Anisian), WC-2 (uppermost Anisian–Carnian), and WC-3 (Norian–lower Rhaetian). The lower boundary of WC-1 corresponds to a hiatus whose duration seems to be shorter than that previously postulated. The synthem boundaries that are common to southwestern Gondwana and the Western Caucasus lie close to the base and top of the Triassic. The Lower Triassic, Ladinian, and Upper Triassic disconformities are traced within the studied basins of southern South America, and the first two are also established in South Africa. The Upper Triassic disconformity is only traced within the entire Caucasus, whereas all synthem boundaries established in the Western Caucasus are traced partly within Europe. In general, the synthem boundaries recognized in southern South America and the Western Caucasus are correlated to the global Triassic sequence boundaries and sea-level falls. Although regional peculiarities are superimposed on the appearance of global events in the Triassic synthem architecture, the successful global tracing suggests that planetary-scale mechanisms of synthem formation existed and that they were active in regions dominated by both marine and non-marine sedimentation.