Ooid factories operating under hothouse conditions in the earliest Triassic of South China

Abstract

Abstract The concept of “Carbonate Factory” was introduced to describe areas of carbonate production and accumulation. In this paper, the “Ooid Factories” is used to analyze extensive Griesbachian (Triassic) oolite deposits in South China that, along with microbialites and lime-mud deposits from different carbonate depositional environments. Late Permian tropical shallow-water skeletal factories (dominant by calcareous algae, sponge, and other associated organisms) collapsed in pace with the most severe mass extinction event, and were rapidly replaced by mud-mound factories (microbialite and lime-mud deposits) and subsequent ooid factories in shallow-water circumstances of Griesbachian. Meantime, temporal nutrient-rich and high-temperature conditions probably delayed re-establishment of tropical shallow-water skeletal factories. As an elementary unit of ooid factories, Lower Triassic oolites have diagnostic characteristics of ooid fabrics (alternatively dark- and light-colored laminae), size ranges (1–5 mm), and grain compositions (pure ooids, and only a few gastropod and bivalve fragments). Continuous aggradational and progradational oolite sequences caused extensive oolite accumulations that significantly contribute to the development of platform architecture with the collaboration of mud-mound factories (mainly line-mud deposits) in South China, through the evolution of low-angle or homoclinal ramps in the early Griesbachian to distally steepened ramps or flat-topped shelves in the late Griesbachian. Moreover, ooid factories also developed in Western Tethyan, Cimmerian, and Arabian carbonate platforms within low latitudes of the Tethyan Ocean during the Griesbachian. It is suggested that extreme hothouse climate with (seasonal) dry conditions in low latitudes significantly facilitated the extensive carbonate production of ooid factories at that time due to the strong linear relationship between surface seawater temperature and carbonate saturation state.