Search

Lower Triassic limestones contain giant ooids (>2 mm) along with other precipitated carbonate textures more typical of Precambrian strata. These features appear to have resulted from changes in seawater chemistry associated with the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained challenging. To constrain seawater carbonate saturation state, dissolved inorganic carbon, alkalinity, and pH, we applied a physicochemical model of ooid formation constrained by new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least seven times supersaturated with respect to aragonite and calcite. When combined with independent constraints on atmospheric pCO2 and oceanic [Ca2+], these findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon and alkalinity and a pH near 7.6. Such conditions may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.

The growing digitization of fossil images has vastly improved and broadened the potential application of big data and machine learning, particularly computer vision, in paleontology. Recent studies show that machine learning is capable of approaching human abilities of classifying images, and with the increase in computational power and visual data, it stands to reason that it can match human ability but at much greater efficiency in the near future. Here we demonstrate this potential of using deep learning to identify skeletal grains at different levels of the Linnaean taxonomic hierarchy. Our approach was two-pronged. First, we built a database of skeletal grain images spanning a wide range of animal phyla and classes and used this database to train the model. We used a Python-based method to automate image recognition and extraction from published sources. Second, we developed a deep learning algorithm that can attach multiple labels to a single image. Conventionally, deep learning is used to predict a single class from an image; here, we adopted a Branch Convolutional Neural Network (B-CNN) technique to classify multiple taxonomic levels for a single skeletal grain image. Using this method, we achieved over 90% accuracy for both the coarse, phylum-level recognition and the fine, class-level recognition across diverse skeletal grains (6 phyla and 15 classes). Furthermore, we found that image augmentation improves the overall accuracy. This tool has potential applications in geology ranging from biostratigraphy to paleo-bathymetry, paleoecology, and microfacies analysis. Further improvement of the algorithm and expansion of the training dataset will continue to narrow the efficiency gap between human expertise and machine learning.

A negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end-Permian extinction horizon in South China has been used to argue for an ocean acidification event coincident with mass extinction. This interpretation has proven controversial, both because the excursion has not been demonstrated across multiple, widely separated localities, and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion. Here, we further test potential controls on the Permian-Triassic calcium isotope record by measuring calcium isotope ratios from shallow-marine carbonate successions spanning the Permian-Triassic boundary in Turkey, Italy, and Oman. All measured sections display negative shifts in δ44/40Ca of up to 0.6‰. Consistency in the direction, magnitude, and timing of the calcium isotope excursion across these widely separated localities implies a primary and global δ44/40Ca signature. Based on the results of a coupled box model of the geological carbon and calcium cycles, we interpret the excursion to reflect a series of consequences arising from volcanic CO2 release, including a temporary decrease in seawater δ44/40Ca due to short-lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and, potentially, subsequently elevated carbonate saturation states caused by the persistence of elevated CO2 delivery from volcanism. Locally, changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions, but this effect alone does not explain the globally observed negative excursion in the δ13C values of carbonate sediments and organic matter as well. Only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data. The carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification, warming, and anoxia.

The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed as δ44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ44/40Ca of carbonate rocks is influenced not only by the δ44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from − 2 to + 8‰. Parallel negative δ13C and δ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ44/40Ca of ~ 0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the δ44/40Ca of global seawater. Additionally, δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ44/40Ca values, the relationship between [Sr] and δ44/40Ca holds potential for reconstructing first-order secular changes in seawater δ44/40Ca composition.

The Late Permian to Late Triassic Great Bank of Guizhou (GBG) in southwest China is one of the few isolated carbonate platforms in the world that exposes an essentially complete record of initiation, development, and drowning across multiple platform-to-basin transects. The platform is exceptionally exposed in cross section at the surface by a faulted syncline that rotated strata to a dip angle of approximately 65°. Platform development spanned the end-Permian extinction and Triassic recovery that marks the transition from Paleozoic to Mesozoic styles of carbonate sediment production, providing a rare opportunity to assess the impact of global changes in carbonate factory types at a single locality. In addition, regional basin controls such as differential siliciclastic sediment input and varied antecedent topography provided mechanisms for lateral variability in platform morphology that can be investigated along exposures in several geographic sectors. Consequently, the GBG preserves a record of temporal and spatial variability in platform architecture that offers an unparalleled opportunity to investigate the controls on isolated carbonate platform morphology. A better understanding of these mechanisms is critical for improving predictive geologic models in exploration and field-development settings. The GBG also serves as a key outcrop analog for Early Triassic oolite reservoirs in the Middle East and China, the steep microbial-boundstone slopes of Carboniferous platforms in Kazakhstan, and the Permian platforms of Texas and New Mexico.

Fine-grained low permeability sedimentary rocks, such as shale and mudrock, have drawn attention as unconventional hydrocarbon reservoirs. Fracturing – both natural and induced – is extremely important for increasing permeability in otherwise low-permeability rock. We analyze natural extension fracture networks within a complete measured outcrop section of the Ernst Member of the Boquillas Formation in Big Bend National Park, west Texas. Results of bed-center, dip-parallel scanline surveys demonstrate nearly identical fracture strikes and slight variation in dip between mudrock, chalk, and limestone beds. Fracture spacing tends to increase proportional to bed thickness in limestone and chalk beds; however, dramatic differences in fracture spacing are observed in mudrock. A direct relationship is observed between fracture spacing/thickness ratio and rock competence. Vertical fracture penetrations measured from the middle of chalk and limestone beds generally extend to and often beyond bed boundaries into the vertically adjacent mudrock beds. In contrast, fractures in the mudrock beds rarely penetrate beyond the bed boundaries into the adjacent carbonate beds. Consequently, natural bed-perpendicular fracture connectivity through the mechanically layered sequence generally is poor. Fracture connectivity strongly influences permeability architecture, and fracture prediction should consider thin bed-scale control on fracture heights and the strong lithologic control on fracture spacing.

We appreciate Kershaw et al.'s comment and continued interest in our Permian–Triassic research in the Nanpanjiang Basin. Kershaw et al. comment on aspects of our recent study that contradict results from the study of Collin et al. (2009). Specifically their comments center around two questions: (1) whether the truncation surface at the base of the Permian–Triassic microbialite shows evidence for subaerial exposure, and (2) whether data support the model of genesis of the microbialites by upwelling of anoxic, alkaline waters with elevated carbonate saturation from bacterial sulfate reduction (Kershaw et al. 1999, 2007, 2012). We note that our recent study is much broader in scope than the issues addressed in this comment and reply. Our study presented extensive stratigraphic, geochemical, biostratigraphic and petrographic analyses from multiple sections, introduced new sections that have not been presented in the literature, and evaluated multiple models for the genesis of the microbialite and underlying truncation surface. We welcome this opportunity to respond to Kershaw et al.'s comment and to further illuminate studies of the Permian–Triassic boundary in the Nanpanjiang Basin. ### Two Generations of Fibrous Cement Fringe? Maybe, But There is Still no Evidence for Pendant Geometry Kershaw et al. comment that there are two generations of bladed fringing cement adhered to grains and that only the second generation has a pendant geometry. Collin et al. (2009) made the same interpretation. From the photographs in Collin et al. (2009, fig. 5Aa–5Ab) and from our thin section observations, we interpreted this to be a single generation of fibrous cement fringe adhered to grains, with a faint zonation of clear-cloudy-clear alternation within the fringe, in which the cloudy zone contained a greater concentration of fluid inclusions (fig. 1A, 1B). Note that the photos presented in the comment by Kershaw et al. (figs. 3, 4) are also consistent with this interpretation. The entire bladed cement fringe was used …

Carbonate rocks are important archives of past ocean conditions as well as hosts of economic resources such as hydrocarbons, water, and minerals. Geologists typically perform compositional analysis of grain, matrix, cement and pore types in order to interpret depositional environments, diagenetic modification, and reservoir quality of carbonate strata. Such information can be obtained primarily from petrographic analysis, a task that is costly, labor-intensive, and requires in-depth knowledge of carbonate petrology and micropaleontology. Recent studies have leveraged machine learning-based image analysis, including Deep Convolutional Neural Networks (DCNN), to automate description, classification and interpretation of thin sections, subsurface core images and seismic facies, which would accelerate data acquisition and reproducibility for these tasks. In carbonate rocks, this approach has been applied primarily to recognize carbonate lithofacies, and no attempt has been made to individually identify and quantify various types of carbonate grains, matrix, and cement. In this study, the applicability and performance of DCNN-based object detection and image classification approaches are assessed with respect to carbonate compositional analysis. The training data comprised of more than 13,000 individually labelled objects from nearly 4000 carbonate petrographic images. The dataset is grouped into six and nine different classes for the image classification and object detection tasks, respectively. Even with a small and relatively imbalanced training set, the DCNN was able to achieve an F1 score of 92% for image classification and 84% mean precision for object detection by combining one-cycle policy, class weight, and label mixup-smoothing methods. This study highlights the inefficiency of image classification as an approach to replicating human description and classification of carbonate petrography. By contrast, DCNN-based object detection appears capable of approaching human speed and accuracy in the area of carbonate petrography because it is able to individually locate and identify different carbonate components with greater cost-efficiency, speed, and reproducibility than conventional (human) petrographic analysis.

Carbonate platforms grow through the precipitation, transport, and final deposition of carbonate sediment out of seawater. Quantifying the relative contributions of initial production versus subsequent transport in determining the growth rates and geometries of platforms remains a significant challenge. In this study, stratigraphic forward modeling is used to quantify the roles of sediment production, transport, and deposition during each growth stage of a Permian-Triassic carbonate platform with a complex growth history. Parameter optimization and sensitivity analysis show that, within the range of reasonable values, the morphology of the platform is most sensitive to sediment transport, moderately sensitive to maximum carbonate production rate, and least sensitive to the productivity-depth curve. The ramp-to-high relief, steep-sloped platform transition during Early Triassic time can be explained by any factor that limits sediment transport from shallow water areas of high production to the slope and basin. Reefs may play a role in limiting sediment transport on many platforms but other processes, such as early marine cementation or carbonate production along the slope, may be equally capable of yielding this shift in platform geometry. In this particular case, early lithification of ooid and skeletal shoals on the platform margin, perhaps facilitated by unusually high carbonate saturation state of seawater, may have inhibited sediment transport into the basin prior to the development of a reef on the platform margin. Later, Anisian progradation of the platform margin can be explained by the development of a slope factory rather than requiring increased sediment transport from the platform top. The development of an escarpment margin in the Ladinian is mainly influenced by accommodation in the slope profile created by antecedent Anisian topography. A general implication from the model results is that the growth of steep-sloped carbonate platforms lacking slope factories may often be limited by transport of sediment from the platform top to accommodation on the slope rather than by the intrinsic production capacity of the platform top factory.

Fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure. Abundant fibrous calcite veins containing organic-bearing inclusions occur in faulted Lower Paleozoic through Triassic hydrocarbon source rocks in the Dabashan Foreland Belt (DBF). δ13CPDB and δ18OPDB values of the fibrous calcite range from −4.8‰ to −1.9‰ and −12.8‰ to −8.4‰ respectively, which is lighter than that of associated carbonate host rocks ranging from −1.7‰ to +3.1‰ and −8.7‰ to −4.5‰. A linear relationship between δ13CPDB and δ18OPDB indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids. The fibrous calcite contains a variety of inclusions, such as solid bitumen, methane-bearing all-liquid inclusions, and vapor–liquid aqueous inclusions. Homogenization temperatures of aqueous inclusions range from 140 to 196 °C with an average of 179 °C. Salinities of aqueous inclusions average 9.7 wt% NaCl. Independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane-bearing inclusions were used to determine fluid pressures. Results indicate high pressures, elevated above typical lithostatic confining pressure, from 150 to 200 MPa. The elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions. Instead fluid inclusion P–T data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration. These findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic-bearing fluids from the DBF during the Yanshan orogeny.

Lower Triassic platforms in the Nanpanjiang Basin contain extensive oolites. Interior oolites are stacked in meter-scale cycles arranged into larger coarsening-upward sequences. Oolites thicken toward margins to include grainstones up to 50 m (164 ft) thick and contain giant ooids (up to 1 cm [0.4 in.]) and composite coated grains. Cross-bedding, ripples, and abraded ooids indicate deposition in high-energy shoals. Apparent layer-cake correlation across interiors indicates amalgamation of shoals. Thinner interior lenses represent spillover lobes. Ooids are interpreted to have originally been bimineralic with cortices of radial or micritic fabrics (high-magnesium calcite), alternating with coarse pseudospar or brickwork (originally aragonite). Distorted ooids formed by brittle compaction of micritic cortices around voids are interpreted to have been dissolved aragonite. Abundant potential nuclei indicate that limited supply was not a factor contributing to the large ooid size. High-energy and abnormally high–seawater CaCO3 saturation are interpreted to be causes of the giant ooids. Most previous reports of giant ooids come from the Neoproterozoic, a period of increasing surface-water oxygenation and high CaCO3 saturation caused by a minimal skeletal carbonate precipitation. We interpret similar seawater chemistry in the aftermath of the end-Permian extinction to explain the genesis of the giant ooids in the Early Triassic. The genesis of bimineralic ooids during an Early Triassic period of rapidly increasing pCO2 and low indicates that an increasing Ca/Mg ratio was the primary mechanism driving the change from aragonite to calcite seas. The architecture, textures, and diagenesis of the Lower Triassic oolites of the Nanpanjiang Basin provide useful analogs for coeval reservoirs in Sichuan and the Middle East.

The end-Permian mass extinction horizon is marked by an abrupt shift in style of carbonate sedimentation and a negative excursion in the carbon isotope ( δ 13 C) composition of carbonate minerals. Several extinction scenarios consistent with these observations have been put forward. Secular variation in the calcium isotope ( δ 44/40 Ca) composition of marine sediments provides a tool for distinguishing among these possibilities and thereby constraining the causes of mass extinction. Here we report δ 44/40 Ca across the Permian-Triassic boundary from marine limestone in south China. The δ 44/40 Ca exhibits a transient negative excursion of ∼0.3‰ over a few hundred thousand years or less, which we interpret to reflect a change in the global δ 44/40 Ca composition of seawater. CO 2 -driven ocean acidification best explains the coincidence of the δ 44/40 Ca excursion with negative excursions in the δ 13 C of carbonates and organic matter and the preferential extinction of heavily calcified marine animals. Calcium isotope constraints on carbon cycle calculations suggest that the average δ 13 C of CO 2 released was heavier than -28‰ and more likely near -15‰; these values indicate a source containing substantial amounts of mantle- or carbonate-derived carbon. Collectively, the results point toward Siberian Trap volcanism as the trigger of mass extinction.

On shallow-marine carbonate buildups in south China, Turkey, and Japan, uppermost Permian skeletal limestones are truncated by an erosional surface that exhibits as much as 10 cm of topography, including overhanging relief. Sedimentary facies, microfabrics, carbon isotopes, and cements together suggest that erosion occurred in a submarine setting. Moreover, biostratigraphic data from south China demonstrate that the surface postdates the uppermost Permian sequence boundary at the global stratotype section and truncates strata within the youngest known Permian conodont zone. The occurrences of similar truncation surfaces at the mass-extinction horizon on carbonate platforms across the global tropics, each overlain by microbial buildups, and their association with a large negative excursion in δ 13 C further suggest a causal link between erosion of shallow-marine carbonates and mass extinction. Previously proposed to account for marine extinctions, the hypothesis of rapid carbon release from sedimentary reservoirs or the deep ocean can also explain the petrographic observations. Rapid, unbuffered carbon release would cause submarine carbonate dissolution, accounting for erosion of uppermost Permian skeletal carbonates, and would be followed by a pulse of high carbonate saturation, explaining the precipitation of microbial limestones containing upwardgrowing carbonate crystal fans. Models for other carbon-release events suggest that at least 5 × 10 18 g of carbon, released in

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.

Meter-scale peritidal carbonate strata from an isolated platform in the Nanpanjiang Basin of South China were tested for the presence of periodic or quasi-periodic climatic signals. These signals provide information on factors controlling cyclic sedimentation in the equatorial eastern Tethys during the Early Triassic greenhouse period. Parasequences are composed of shallowing-upward successions of subtidal facies, including skeletal packstone and grainstone, calcimicrobial boundstone, and oolitic packstone and grainstone, intertidal laminated lutite and ribbon rock, and rare supratidal microbial laminite. Parasequence stacking patterns display three orders of cyclicity, suggesting hierarchical stratigraphic relationships. Gamma analysis of Kominz and Bond [Earth Planet. Sci. Lett. 48 (1990) 233–244] was used to estimate facies-dependent thickness–time conversion factors (i.e. γ s), and to construct γ -tuned and γ -untuned time series for two stratigraphic sections. Spectra of these time series indicate the presence of quasi-periodic signals, with prominent short-eccentricity (94.9–131.2 kyr), short-obliquity (35.8 kyr), and long-precessional index (21.2 kyr) peaks, and minor long-eccentricity (412.9 kyr), long-obliquity (45.3 kyr), short-precessional index (17.7 kyr), and constructional-tone (9.7 kyr) peaks when calibrated on the γ -tuned spectra. Thus, Milankovitch climatic forcing probably greatly influenced sedimentation. The calibration indicates that the subtidal facies has a sedimentation rate of 24.6–30.7 cm/kyr and the intertidal–supratidal facies has a rate of 2.7–6.0 cm/kyr. The estimated duration of deposition of the two sections is 1139–1423 kyr, and corresponds to a stratigraphic completeness of 32–92%. The completeness is much greater than that of icehouse stratigraphic records. We speculate that variations in carbonate productivity and environmental conditions driven by Milankovitch climatic forcing, combined with low-amplitude sea-level fluctuations, were likely major controls on cyclic sedimentation. Furthermore, evolutional spectra of the two sections show that dominant Milankovitch climatic forcings varied from short-eccentricity, obliquity, to long-precessional index during the course of sedimentation. Some spectral differences between the two sections suggest variations in depositional/recording mechanisms of Milankovitch climatic signals between platform interior and windward platform margin.

Permian-Triassic boundary (PTB) sections from isolated carbonate platforms in the Nanpanjiang Basin of south China contain Upper Permian skeletal packstones with diverse open-marine fossils overlain by a 7–15 m thick boundary horizon composed of calcimicrobial framestone constructed by globular to tufted, calcified cyanobacteria similar to Renalcis. The framestone contains interbeds of lime-grainstone with abundant thin-shelled bivalves and brachiopods. The overlying Lower Triassic strata contain microgastropod lime-packstones followed by a thick succession of thin-bedded lime-mudstones. The PTB event horizon is interpreted to occur at the top of the packstone containing diverse, open-marine fauna and Palaeofusulina and coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The conformable biostratigraphic boundary occurs at the first appearance of Hindeodus parvus within the basal meter of the calcimicrobial framestone. Intensively studied PTB sections in so...

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.

The Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Mountains, New Mexico, reveals complex relations between long- and short-term relative changes in sea-level, shelf configuration and sedimentation, which interacted to create a distinct toplap geometry. At least five sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and create a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well-sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palaeotopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary unit (updip), merging sandstones contain rare sedimentary structures including evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fabrics and mudcracks. Laterally (downdip), the sandstone-bounding peritidal carbonate facies pass into subtidal carbonate facies (ooid-peloid-fusulinid-dasyclad-mollusc pack- and grainstone) and interbedded sandstones contain sedimentary structures such as ripple marks and trough to planar cross-stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continued in still-submerged areas downdip from the shelf crest, and hence represents depositional toplap. Physical tracing of subaerial exposure surfaces suggests that the shoreline migrated up and down palaeoslope several times. The vertical component of five short-term shoreline migrations decreased during formation of the toplap geometry. Sea-level rose to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high-‘frequency’ relative changes of sea-level that combined to produce the larger-scale geometry. We suggest that changing amplitudes of relative sea-level may play a significant role in the stratigraphic evolution of platforms and that separating ‘short-term’and ‘long-term’relative sea-level may be ambiguous in such instances.