daniel lehrmann

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 m thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone.The cyclic limestone is Olenekian in age, 164 m thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2–7.4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences.The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo–Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic.

The Yangtze platform in south China formed a stable palaeogeographic element from the Late Proterozoic to the end of the Middle Triassic with deposition of shallow-water carbonates during much of this time. A portion of the Yangtze platform in south-central Guizhou drowned at the transition from Permian to Triassic, as the south-adjacent Nanpanjiang basin encroached about 100 km northward, but a new, stable platform margin was established that persisted through the Early and Middle Triassic. This long history as a stable carbonate platform ended at the transition from the Ladinian to the Carnian. The latest Ladinian rocks, the Yangliujing Formation, are 490 m of shoaling-upward carbonate cycles of grapestone and bioclastic grainstone, fenestral limestone, and stromatolitic dolomudstone, commonly overprinted by extensive subaerial diagenesis. The beginning of the Carnian is marked by a rapid transition to medium-dark-grey, nodular lime mudstones containing ammonoids, conodonts and thin-shelled bivalves, the Zhuganpo Formation. The upper part of this thin pelagic limestone contains many muddy intraclasts, some slightly bored and encrusted, indicating incipient cementation. The overlying Wayao Formation is a condensed black shale with thin interbeds of dark-grey, manganiferous lime mudstone near the base. Ammonoids, conodonts, thin-shelled bivalves, and articulated crinoid stems are abundant. Fine-grained greywacke with sole marks forms prominent bundles within grey, calcareous shale in the overlying Laishike Formation. Ammonoids and thin-shelled bivalves occur sporadically in this 810-m-thick unit. Calcareous shale with thicker-shelled bivalves and packages of cleaner, coarser-grained sandstone characterize the Banan Formation, 460 m thick. The sandstone units generally coarsen and thicken upward, with ripples, medium-scale trough cross-beds, and rare U-tube burrows. Quartzose, coal-bearing siliciclastics 690 m thick form the overlying Huobachong Formation. Thick-bedded, cross-stratified sandstone and conglomerates are amalgamated into thinning- and fining-upward intervals separated by blocky mudstones. This fining-upward motif continues into the overlying Erqiao Formation, but coals are lacking. At the beginning of the Late Triassic (Carnian) the previously stable Yangtze platform, on which peritidal limestones were forming, was drowned and covered by dark lime mud that was cemented into intraclasts and nodular lime mudstone. Black shale and manganiferous pelagic limestone formed a condensed interval, recording maximum submergence. Turbidite sandstone and shale of the Laishike flysch filled the accommodation space of 800 m created during drowning of the Yangtze platform, leading to deposition of shoaling-upward shelf and paralic sandstones and shales, but without significant carbonate production. The succeeding fining-upward siliciclastics are interpreted as braided-stream deposits with coals that mark minor marine incursions. The shallow-shelf and braided-stream deposits form a molasse 1500 m thick. It was apparently derived from the west, in contrast to the underlying flysch where palaeocurrent directions are from the north or northeast. The entire Yangtze platform became emergent during the Late Triassic and was never submerged again. Subtle local differences in the drowning sequences indicate differential subsidence and suggest that tectonics played a role in the death of the Yangtze platform.

High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.

A distinct negative δ13C excursion is documented in two Permian–Triassic sections (Heping and Taiping) in shallow marine carbonate platform deposits in the Nanpanjiang Basin, south China. These sections span from the Changhsingian to the Dienerian and are characterized by a distinct marine boundary facies change from massive, skeletal lime packstone in the Changhsingian to distinctive calcimicrobial framestone in the Griesbachian Hindeodus parvus Zone. The δ13Corg and δ13Ccarb excursions occur directly after the onset of the calcimicrobial framestone (herein termed the ‘Permian–Triassic boundary event’) and before the first occurrence of H. parvus. The isotope shifts are associated with a sharp drop in species abundance and diversity and coincide with a decrease in total organic carbon (TOC) content. The shift towards depleted values in δ13Corg and δ13Ccarb at the Permian–Triassic boundary event, together with low TOC contents, persists throughout the Griesbachian H. parvus Zone. These data document a corresponding negative shift of δ13Corg and δ13Ccarb, values and low TOC contents with the onset of growth of calcified microbial framestones (a postextinction ‘disaster facies’) immediately below the base of the Griesbachian H. parvus Zone. Based on paleontological evidence, the first occurrence of the ‘disaster facies’ follows the extinction event, which implies that the 13C-depleted values above this facies postdate the event. This suggests that two separate events had to account for the initiation of the extinction and the δ13C excursion. However, the consequences that led to the negative isotopic shift might be linked to the intriguing recovery lag of Early Triassic ecosystems. Based on data from PTB sections worldwide of a greater δ13C offset in high compared with low latitudes, we propose that methane eruptions from thermal destabilization of high-latitude clathrate deposits may have led to the negative δ13C shift and may have caused long-term adverse ecological conditions.

High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.

A distinct negative δ13C excursion is documented in two Permian–Triassic sections (Heping and Taiping) in shallow marine carbonate platform deposits in the Nanpanjiang Basin, south China. These sections span from the Changhsingian to the Dienerian and are characterized by a distinct marine boundary facies change from massive, skeletal lime packstone in the Changhsingian to distinctive calcimicrobial framestone in the Griesbachian Hindeodus parvus Zone. The δ13Corg and δ13Ccarb excursions occur directly after the onset of the calcimicrobial framestone (herein termed the ‘Permian–Triassic boundary event’) and before the first occurrence of H. parvus. The isotope shifts are associated with a sharp drop in species abundance and diversity and coincide with a decrease in total organic carbon (TOC) content. The shift towards depleted values in δ13Corg and δ13Ccarb at the Permian–Triassic boundary event, together with low TOC contents, persists throughout the Griesbachian H. parvus Zone. These data document a corresponding negative shift of δ13Corg and δ13Ccarb, values and low TOC contents with the onset of growth of calcified microbial framestones (a postextinction ‘disaster facies’) immediately below the base of the Griesbachian H. parvus Zone. Based on paleontological evidence, the first occurrence of the ‘disaster facies’ follows the extinction event, which implies that the 13C-depleted values above this facies postdate the event. This suggests that two separate events had to account for the initiation of the extinction and the δ13C excursion. However, the consequences that led to the negative isotopic shift might be linked to the intriguing recovery lag of Early Triassic ecosystems. Based on data from PTB sections worldwide of a greater δ13C offset in high compared with low latitudes, we propose that methane eruptions from thermal destabilization of high-latitude clathrate deposits may have led to the negative δ13C shift and may have caused long-term adverse ecological conditions.