Palaeokarst Characteristics Of Early Permian Carbonate Rocks In Guizhou, South China: Implications For The Late Paleozoic Glaciation

The Permian Period is a critical interval from Devonian greenhouse to Permo-Carboniferous icehouse in Late Paleozoic recording the most important transitions of paleogeography, paleoclimate, paleoceangraphy occurred in the Earth history. The Early Permian stages with glaciers prevailing in the Gondwanan continent and the high-latitude areas of north hemisphere were in the highest developing period of the Late Palaeozoic Ice Age (LPIA) in term of magnitude and duration. The waxing and waning of the Gondwanan icesheet contributed to high-frequency glacioeustatic sea level fluctuations and widely distributed cyclothems in the low-latitudes during the Carboniferous-Permian Period. Recent investigations suggest that the Permo-Carboniferous. Ice Age is composed by eight distinct episodes of glaciation and interglaciation. The glacioeustatic sea-level changes controlled the high frequency sea-level changes have been confirmed in South China during the carboniferous-Permian period, but the high amplitude sea level falls still need pay more attention.Guizhou region was located in a subequatorial position on the Yangtze passive continental margin near the east border of the Paleotethys Ocean during the Early Permian, and in the tectonically stable environment. It represents one of the best study areas for the analyses of sea level changes, characterized by abundant types of sedimentary facies and depositional systems and the continuously exposed Early Permian successions. Based on the relatively fine biological stratigraphic framework, studies on sedimentary characteristics and paleokarst characteristics of the Early Permian deposits from the Puan Longyin section、Ziyun Bianping section and Ziyun Zongdi section in Guizhou region allow us to analyze the evolution of deposit systems in vertical direction, so as to indicate the sea level changes. Combined with the paleokarst characteristics, identidy the the high amplitude sea level falls in this study area, implications for the Early Permian glaciation.The Pu’an Longyin located at the juncture of the carbonate platform and terrigenous clastic sedimentary area, it developed four types of sedimentary facies in the Early Permian, and these are open platform facies, restricted platform facies, shallow marine shelf facies and delta facies. The lower Shazitang formation composed of thin bioclastic micrite limestone, thick bioclastic mudand and wackestone, the limestone identified four layers palaeokarst breccia limestone. The upper part of Shazitang formation maked up of the open platform’s bioclastic micrite limestone. Tthe lower Longyin formation is the deep-water sedimentary of the shallow marine shelf facies, while the upper is the delta’s clastic. The Baomoshan formation consists of open platform limestone facies and shallow shelf facies mudstone which can identified two layers palaeokarst breccia limestone.The Ziyun Bianping section is located at the high-energy carbonate platform margin, the Zisong Stage is mainly the open platform facies which multiple intervaled by platform margin facies, the palaeokarst features have been observed in the platform margin facies Bioclastic grainstone and packstones.The Lower of Longlin Stage is the margin platform facies which developed of solution cavities, upward becomed into reefs facies, front slope facies, shallow marine shelf facies and open platform facies.The Ziyun Zongdi section is located at the carbonate platform, the sediments observed in the Zisong and Longlin Stages are composed entirely of open platform facies and platform margin facies which changes to each other many times, showing cyclically appearing subaerial exposure surfaces. In addition, the Middle Zisong and the Lower Longlin stages had restricted platform facies where the water circulation is blocked.The carbonate units throughout the South Guizhou region show evidence of multiple karsting events, which suggest repeated subaerial exposure during the Early Permian caused by larger amplitudes of relative sea-level change.The strata collapsed to form palaeokarst breccia by the intensively reformed of weathering, erosion and karstification in the surface. In the atmosphere diagenetic zone, Meteoric vadose diagenesis and Meteoric Phreatic diagenesis beneath the subaerial exposure are characterized by the presence of solution vugs, solution cave and cave deposits.Based on the analysis of the characteristics of sedimentary evolution and the recognition of the palaeokarst features, there have two exposure events in the Early Permian carbonate sediments, resulting in the strata repeatedly subaerial exposure and multiple palaeokarst.The first exposure event is during the Asselian to Early Sakmarian. The Shazitang formation of Pu’an Longyin section exposed to the surface repeatedly and long time, which formes collapse breccias.The bioclastic limestone of Zisong Stage in Bianping section widely developed freshwater leaching solution vugs. The interface deposition of Zisong Stage in Zongdi exposured and appear repeatedly palaeokarsts.The second exposure event occurred in the Later Sakmarian to Artinskian The corresponding palaeokarst in Pu’an Longyin section is the clastic rock of Longyin formation and the palaeokarst collapse breccias of Baomoshan formation. The Bianping section in the limestone of upper of Zison Stage is the solution vug filled with isopachous calcite cement and the structure of palaeokarst cavities in the Lower Longlin stage. The Zongdi section is the solution vug and subaerial exposure surfaces in limestone in Zisong stage and the karst collapse breccias in Longlin stage.The stratigraphy records two times of significant high amplitude drop in relative sea level during the Asselian-Early Sakmarian and Later Sakmarian-Artinskian.This consistents with the two times build-up of glacial ice across Gondwana (P1 and P2), and can be separately correlated with the fall of sea level in North America and Europe, the formation of cyclothem deposits, positive shift of carbon and oxygen isotopes and extensively distributed glacial sheet in Gondwana, indicating the EarlyPermian Glaciation.