"Anachronistic Facies" In The Early Triassic Recovery Time

The end-Permian mass extinction happened at 250Ma was the most devastating extinction event in the history of metazoans. Nearly 49% of the marine families experienced reduction, and an estimated 80-90% of the marine species disappeared. The biotic recovery following the end-Permian mass extinction also was the longest one in the Phanerozoic, and the Triassic biotic recovery lasted for 6-7 Ma, and even 10 Ma for the metazoan reef ecosystem. The depleted ecosystems were dominated by cosmopolitan members and opportunists, and this ecological crisis spanned the entire Early Triassic. While the ecosystems were heavily attacked by the end-Permian mass extinction events, the sedimentary systems also had dramatically changed, resulting in the unique "reef gap" on platform,"chert gap" in basin,"coal gap" on land and "anachronistic facies" widespread in the shallow seas of the Early Triassic. "Anachronistic facies" is an distinctive sedimentary record, such as flat-pebble conglomerates, vermicular limestones, subtidal wrinkle structures,microbialites, carbonate seafloor fans,and zebra limestone-mudstones. These sediment characteristic of the Cambrian and Precambrian rocks are regarded as the "unusual sediments" or "anachronistic facies" when they re-occured in the post-Cambrian normal shallow marine deposits, because they have disappeared from the normal shallow marine settings since the Ordovician but occur only in some high-pressure environments devoid of metazoans, such as restricted lagoons.The reappearance of various distinctive sedimentary and correlative structures is characteristic of the Lower Triassic. These sediments characteristic of the Cambrian and Precambrian rocks are fewness in the post-Cambrian normal shallow marine deposits, but reappearecin the Early Triassic normal shallow marine facies. The microbialite is believed to be actively constructed by some benthic microbial communities dominated by cyanobacteria in the environments devoid of the metazoans. The broad synsedimentary submarine cementation and lower bioturbation favored the preservation of primal stratification of sediments and finally made for the formation of the wrinkle structure,the flat-pebble conglomerates, the thin-bedded limestone and zebra limestone. The carbonate seafloor fans should bed formed in a setting of anoxia, oversaturation of calcium carbonate and higher alkalinity sea. Recently, with the wider discovery and more study of "anachronistic facies" including vermicular limestone and their related structures over the world, their origination has been well believed to be closely related to the specific environmental conditions during the Paleozoic-Mesozoic transition. The reappearance of "anachronistic facies" in normal shallow sea has been regarded as a direct symbol of unusual environment that resulted in the delayed recovery following the end-Permian mass extinction。 The distinctive sedimentary records featured by microbialites, vermicular limestones, flat-pebble conglomerates, have been documented in many localities of South China, and distribute at various horizons in the Lower Triassic. Nearly all types of the known distinctive sediments and sedimentary structures have been reported. This provides an excellent opportunity for understanding the Early Triassic environment and its coevolution with the biotic recovery.Microbialites occurred at the Permian-Triassic boundary and in the upper Daye Formation, including stromatolites, oncolited biostromes and bioherms with calci-microbial texture. Microbialites appear spotted, branched or laminated on the weathered outcrops,and they are mainly laminated in the middle and upper parts of the Lower Triassic in South China. The ecological crisis following the end-Permian mass extinction is regarded as the cause of such an environment at the Permian-Triassic boundary and in the Early Triassic that resulted in the microbialites developing into some normal shallow marine facies.Flat-pebble conglomerates occurring in that Daye formation and Jialingjiang formations in the Three Gorge area are the limestone composed of tabular micritic intraclasts and lime-mud matrix. Generally, pebbles are a long axis and an aspect ratio of>3:1 in outcrop. The pebble are in the angular or subangular shapes, and spread in imbricate, bunchy, radiate and irregular arrangement.The conglomerate beds sharply contact with overlying and underlying limestone beds. The flat-pebbles probablly formed in a subtidal environment during the early stage of diagenesis when the unconsolidated thin-bedded limestone was rolled up by storm or gravity flow and then re-deposited. Their formation should profit from broad synsedimentary submarine cementation and lower bioturbation in the Early Triassic, and favored the preservation of primal stratification of sediments.Vermicular limestone is the representative and commonest distinctive sedimentary structure in the Lower Triassic of South China, but has ratively less studied. Integrating the method and means of lithostratigraphy, sequence stratigraphy,carbon isotopes and conodonts biostratigraphy, we have studied the types, distribution of in space and time, causes and formation, rising and demising, matter composition and the cause-effect between the sediments and biotic recovery of these distinctive sediments.The first horizon of vermicular limestone in the Yangtze Gorge area was nearly isochronic, corresponding to the middle-lower part of the Neospathodus waageni zone,i.e, early-middle Smithian, at the boundary between the second member and third members of the Daye formation.Vermicular limestone frequently occurs in the Lower Triassic, and exist in the geological records until the Middle Triassic.According to the occurrence of the vermicular bodies, the vermicular limestones may be grouped into four types,including the stratiform or sub-stratiform type, the irregularly granular type, the deformed columnar type and the ellipsoidal type. The occurrence of the various types of vermicular limestones shows some characteristic associations:the first appeared type is the stratiform or sub-stratiform vermicular limestone or the irregularly granular vermicular limestone, the deformed columnar type ooccurs in the Middle Spathian, corresponding to the middle-upper part of the third member of the Daye Formation. After various type coexistence, the columnar type became the dominant vermicular limestones. The appearance, blooming, type change and disappearance correspond with the beginning, recovery and end of biotic recovery, in other word, the rising and demising of the vermicular limestones can be taken as the mark of the biotic recovery process. There are several hypotheses for the cause and formation of vermicular limestone, including coacervation cause, machinism cause, and bioturbation cause. However, early studies largely ignored the special setting at the Paleozoic-Mesozoic transitional. On basis of newly study, we approve a various cause. The appearance of anachronistic facies implies the natural response of the sedimentary systems and ecosystems to the great Paleozoic-Mesozoic transitional events and their induced harsh environments.The columnar vermicular limestone are the animal burrows or bioturbations, ie a special trace fossils. The granular vermicular limestone might be produced by chemical or microbial coacervation. The stratiform or sub-stratiform vermicular limestone would altered from the differential compaction and dissolution..The vermicular limestone is interbedded with micritic limestone without vermiform structures in the Three Gorge area. Herein we report a micro probe analysis of vermicular limestone from the Lower Triassic Daye Formations, and both vermicular and micritic limestones were sampled and analyzed for the contents of major and trace elements and REE while the "vermicular body" and "matrix" in the vermicular limestone are separately detected in situ by LA-ICP-MS. Results indicated the strong land-weathering and volcanic activity should have resulted in the further deterioration of marine environment represented by the micritic limestone than the vermicular limestone. The unusual environment might have restrained the metazoan recovery. However, the elements are clearly low in the vermicular limestone and the environment thus was conducive to metazoan, which suggests the biogenesis of the vermicular limestone and shows that the biological activity can also rebuild environment.Thin-bedded limestones are quite common in the Lower Triassic of the Three Gorge area. These thin micritic beds differ from the general thin-bedded carbonate rocks in its unique characters that the horizontal bioturbation and small or thin-shelled fossils characteristic of disasters are more or less common on the bedding surfaces but no vertical bioturbation and fossils exist in the inner part of the rocks. The thin-bedded limestones had the same formation mechanism with flat pebble conglomerates. It reflected a lower bioturbated and predator-absent ecosystem.The trace fossils of Three Gorge area have been systematically studied and 17 ichnogenera are recognized, including Arenicolites,Chondrites,Circulichnus,Cochlichnus,Didymaulichnus,Gordia, Megagrapton,Mannillichnus,Planolites,Palaeophycus, Phycodes, Thalassinoides and so on. Didymaulichnus,Palaeophycus,and Planolites are the dominant types, and Chondrites and Phycodes are the second and the, others are rare scale. These ichnofossils are simply structured and all, belong to the Cruziana ichnofacies.During the Early Triassic, the trace fossils in Three Gorge area express a significant evolution, such as increasing of burrows in both diameter and depth, complexing in structures and augmenting in diversity. This suggests that the trace animals had a well development in body size, active space, foraging efficiency and ecological types, and the environments had changed to favor metazoans gradually.A case study from the Three Gorge area indicates that the "anachronistic facies" are widespread in the Lower Triassic but became declining in the latest Early Triassic and disappeared in the early Middle Triassic with the final reconstruction of the Mesozoic ecosystems in the Late Anisian. The dramatic synchronization between the changes of sedimentary systems and the reconstruction of ecosystems further demonstrates the interrelation among the distinctive sediments, specific environmental conditions and development of ecosystems. Unusual environmental events that resulted in the biotic extinctions and the changeover of sedimentary systems, and the deleterious environmental conditions persisting through the Early Triassic led to the abundant distinctive sedimentary records in the Lower Triassic and delayed the biotic recovery following the end-Permian mass extinction. The disappearance of the distinctive sediments can be taken as the mark of the restoration of the ecological environments and the end of the biotic recovery process.Therefore, the ups and downs of the anachronistic facies may act as a proxy for the evolution of ecosystems independent of fossil analyses.