The Role of Carbonate Factories and Sea Water Chemistry on Basin-Wide Ramp to High-Relief Carbonate Platform Evolution: Triassic, Nanpanjiang Basin, South China

The end-Permian extinction and its aftermath altered carbonate factories globally for millions of years, but its impact on platform geometries remains poorly understood. Here, the evolution in architecture and composition of two exceptionally exposed platforms in the Nanpanjiang Basin are constrained and compared with geochemical proxies to evaluate controls on platform geometries. Geochemical proxies indicate elevated siliciclastic and nutrient fluxes in the basal Triassic, at the Induan—Olenekian boundary and in the uppermost Olenekian. Cerium/Ce* shifts from high Ce/Ce* values and a lack of Ce anomaly indicating anoxia during the Lower Triassic to a negative Ce anomaly indicating oxygenation in the latest Olenekian and Anisian. Uranium and Mo depletion represents widespread anoxia in the world\u27s oceans in the Early Triassic with progressive oxygenation in the Anisian. Carbonate factories shifted from skeletal in the Late Permian to abiotic and microbial in the Early Triassic before returning to skeletal systems in the Middle Triassic, Anisian coincident with declining anoxia. Margin facies shifted to oolitic grainstone in the Early Triassic with development of giant ooids and extensive marine cements. Anisian margins, in contrast, are boundstone with a diverse skeletal component. The shift in platform architecture from ramp to steep, high-relief, flat-topped profiles is decoupled from carbonate compositions having occurred in the Olenekian prior to the onset of basin oxygenation and biotic stabilisation of the margins. A basin-wide synchronous shift from ramp to high-relief platforms points to a combination of high subsidence rate and basin starvation coupled with high rates of abiotic and microbial carbonate accumulation and marine cement stabilisation of oolitic margins as the primary causes for margin up-building. High sea water carbonate saturation resulting from a lack of skeletal sinks for precipitation, and basin anoxia promoting an expanded depth of carbonate supersaturation, probably contributed to marine cement stabilisation of margins that stimulated the shift from ramp to high-relief platform architecture

Controls on Carbonate Factory Type (Abiotic, Microbial, Skeletal) on the Hongyan Margin of the Yangtze Platform, South China

At Hongyan Section, the Triassic Yangtze Platform margin architecture is preserved in the western part of the Nanpanjiang Basin. A syncline exposes a continuous two-dimensional cross section through the margin. Stratigraphic correlations with spectral gamma ray logs demonstrated that the Hongyan margin evolved from an Upper Permian platform margin dominated by diverse skeletal grainstone to an Induan, broad ramp profile with ~1.5° slope. The ramp consists of prograded ooid-shoals that change to bioturbated lime mudstone with slump folds and debris flow breccias on the mid ramp and laminated lime mudstones on the outer ramp. In the Olenekian, a more abrupt bank profile developed as the reef steepened, with a barrier of ooid shoals and a restricted lagoon and peritidal interior. During the Middle Triassic (Anisian), the platform developed a progressively steepening Tubiphytes microbial-cement reef-rimmed margin. The margin reached up to 250 m relief with slope clinoforms ~35°. Slope facies changed from debris-flow breccia to talus and calciturbidites, and siliciclastic turbidites of the Xinyuan formation buried the slope. Hongyan section traverses the basin, platform margin, and interior. Analysis of spectral gamma-ray logs and elemental geochemistry (U, Mo, V) shows onset of basin anoxia in the Early Triassic, continuation of an anoxic basin with redox fluctuations at the end of the Early Triassic, and basal Anisian and oxic conditions later in the Middle Triassic. Spectral gamma ray profiles show generally low values and low Uranium (U) content across the Permian-Triassic boundary and in the Lower Triassic (Induan). This notable Uranium depletion probably resulted from a global Oceanic Anoxic Event (OAE) related to the end-Permian mass extinction and depletion of seawater U due to widespread sedimentation of U in the world’s anoxic oceans. Spiky, high gamma ray intervals with elevated U and Potassium (K) within the Lower Triassic (Olenekian) and Middle Triassic (Anisian) reflect locally developed anoxia and siliciclastic influx. Carbonate factory types shift from a relatively high proportion (6%) of skeletal contents in the Upper Permian to abiotic (oolite and micrite) (98%) in the Induan, to microbial and abiotic (11% and 54%, respectively) in the Olenekian, to a return of higher skeletal content (6%) in addition to microbial crusts and cement in the Anisian. This return of biotic contents in the Middle Triassic may reflect the recovery from the end-Permian extinction and seawater anoxia. However, the shift to a more abrupt bank profile in the Olenekian indicates that the change in contents preceded the biotic recovery from the end-Permian extinction, suggesting that seawater redox conditions may have had a greater role on margin architecture than biotic evolution had. These trends mirror those seen in the Great Bank of Guizhou. Previous models predict an increase in marine cement during anoxic conditions; in my study it was found that marine cement levels decreased during periods of anoxia (Induan and Olenekian). I hypothesize that this resulted from greater carbonate saturation during these periods, resulting in greater abiotic micrite production (similar to whitings), which occluded pore space and thus inhibited marine cementation. Future work may evaluate whether seawater redox changes and carbonate saturation state may be able to demonstrate very small-scale secondary porosity or cementation not visible in this study

Triassic Stage Boundary Definition using an Integrated Biostratigraphy, Magnetostratigraphy, Chemostratigraphy and Geochronology in Matine Strata of Guandao Section, Nanpanjiang Basin, South China

The chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic, and it contains abundant conodonts and foraminifers, primary magnetic signature, globally recognized carbon isotope excursions, and several dated volcanic ash horizons. Conodonts enable broad delineation of stages, substages, and numerous biozones from the Changhsingian Stage of the Late Permian through the Carnian Stage of the Late Triassic. We use the following appearances and disappearances of conodont taxa as indicators of stage and substage boundaries: Changhsingian- Griesbachian, first Hindeodus parvus; Griesbachian-Dienerian, last H. parvus, first Neospathodus dieneri; Dienerian-Smithian, last Ns. dieneri, Ns. cristagalli, first Ns. pakistanensis, Novispathodus waageni; Smithian-Spathian, first Ns. crassatus, Spathicuspus spathi, Spathian-Aegean, first Chiosella gondolelloides, Cs. timorensis; Aegean-Bithynian, last Triassospathodus homeri, first Nicoraella germanica, Ni. kockeli, Bithynian-Pelsonian, last Neogondolella regalis, Pelsonian-Illyrian, last Paragondolella bulgarica, first Pg. excelsa; Illyrian-Fassanian, first Budurovignathus trumpyi, Ng. trammeri; Fassanian-Longobardian, first Bv. mungoensis; Longobardian-Cordevolian, first Quadralella polygnathiformis. The section contains numerous magnetic reversals with predominantly normal polarity zones in the Smithian-Spathian Aegean-Bithynian, Longobardian-Cordevolian, and predominantly reversed polarity in the upper Griesbachian-Lower Smithian, Upper Spathian, and Pelsonian. Large amplitude carbon isotope excursions characterize the Lower Triassic and basal Anisian. The overall pattern in magnetic reversals is consistent with that of European sections and the carbon isotope excursions have been correlated globally. Several volcanic tuff horizons bracket the P-T boundary and Spathean-Aegean boundary in Guandao section and across the basin. High resolution U-Pb analysis from zircons in volcanics provide a robust age date of 247.2 Ma for the Spathean-Aegean Triassic boundary

Controls on Carbonate Factories in the Transition from Ramp to Reef-Rimmed Platform in the Hongyan Section of the Triassic Yangtze Platform

At Hongyan the Yangtze Platform margin architecture is preserved in the western part of the Nanpanjiang Basin of south China. A syncline exposes a continuous 2-D cross section through the margin. Stratigraphic correlations with spectral gamma ray logs demonstrated that the margin evolved from an Upper Permian skeletal grainstone shoal to an Induan ramp profile with ~1.5° slope. The ramp consists of prograded ooid-shoals that change to bioturbated lime mudstone with slump folds and debris flow breccias and laminated lime mudstones on the mid to outer ramp. In the Olenekian, a more abrupt profile developed as the barrier of ooid shoals developed with and a restricted lagoon and peritidal interior. During the Middle Triassic (Anisian), the platform developed a progressively steepening Tubiphytes microbial-cement rimmed margin. The margin reached up to 250 m relief with slope clinoforms ~35°. Slope facies changed from debris-flow breccia to talus and calciturbidites, and siliciclastic turbidites of the Xinyuan formation buried the slope. Spectral gamma-ray logs and geochemistry (U, Mo, V) shows evidence of basin anoxia in the Early Triassic and basal Anisian and shift to oxic conditions later in the Middle Triassic. Spectral gamma ray profiles show generally low values and low Uranium (U) content across the Permian-Triassic boundary and in the Lower Triassic (Induan). This notable Uranium depletion probably resulted from a global Oceanic Anoxic Event (OAE) related to the end-Permian mass extinction and depletion of seawater U due to widespread sedimentation of U in the world’s anoxic oceans. Carbonate factories shift from a relatively high proportion (6%) of skeletal contents in Upper Permian to abiotic (oolite and micrite) (98%) in the Induan, to microbial and abiotic (11% and 54%) in the Olenekian, to a return of higher skeletal content (6%) in addition to microbial crusts and cement in the Anisian. This return of skeletal factories in the Middle Triassic may reflect the recovery from the end-Permian extinction and seawater anoxia. However, the shift to a more abrupt bank profile in the Olenekian indicates that the shift preceded the biotic recovery from the end-Permian extinction, suggesting that seawater redox conditions may have had a greater role on margin architecture than biotic evolution

Triassic Stage Boundary Definition using an Integrated Biostratigraphy, Magnetostratigraphy, Chemostratigraphy and Geochronology in Matine Strata of Guandao Section, Nanpanjiang Basin, South China

The chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic, and it contains abundant conodonts and foraminifers, primary magnetic signature, globally recognized carbon isotope excursions, and several dated volcanic ash horizons. Conodonts enable broad delineation of stages, substages, and numerous biozones from the Changhsingian Stage of the Late Permian through the Carnian Stage of the Late Triassic. We use the following appearances and disappearances of conodont taxa as indicators of stage and substage boundaries: Changhsingian- Griesbachian, first Hindeodus parvus; Griesbachian-Dienerian, last H. parvus, first Neospathodus dieneri; Dienerian-Smithian, last Ns. dieneri, Ns. cristagalli, first Ns. pakistanensis, Novispathodus waageni; Smithian-Spathian, first Ns. crassatus, Spathicuspus spathi, Spathian-Aegean, first Chiosella gondolelloides, Cs. timorensis; Aegean-Bithynian, last Triassospathodus homeri, first Nicoraella germanica, Ni. kockeli, Bithynian-Pelsonian, last Neogondolella regalis, Pelsonian-Illyrian, last Paragondolella bulgarica, first Pg. excelsa; Illyrian-Fassanian, first Budurovignathus trumpyi, Ng. trammeri; Fassanian-Longobardian, first Bv. mungoensis; Longobardian-Cordevolian, first Quadralella polygnathiformis. The section contains numerous magnetic reversals with predominantly normal polarity zones in the Smithian-Spathian Aegean-Bithynian, Longobardian-Cordevolian, and predominantly reversed polarity in the upper Griesbachian-Lower Smithian, Upper Spathian, and Pelsonian. Large amplitude carbon isotope excursions characterize the Lower Triassic and basal Anisian. The overall pattern in magnetic reversals is consistent with that of European sections and the carbon isotope excursions have been correlated globally. Several volcanic tuff horizons bracket the P-T boundary and Spathean-Aegean boundary in Guandao section and across the basin. High resolution U-Pb analysis from zircons in volcanics provide a robust age date of 247.2 Ma for the Spathean-Aegean Triassic boundary

Controls on Carbonate Factories in the Transition From Ramp to Reef-Rimmed Platform in the Hongyan Section of the Triassic Yangtze Platform: Preliminary Results

The Nanpanjiang Basin (NPJB) occurs within the south China plate bordered by the Yangtze Platform (YP). The goal of this study is to test whether changes in seawater redox conditions and carbonate saturation state affected variation in carbonate factory distribution and margin architecture across space and time. At Hongyan the YP margin architecture is preserved in the western part of the NPJB. A syncline exposes a continuous two-dimensional cross section through the platform to basin transition. During the Induan the YP developed a broad ramp with ~1.5 o slope. The ramp top consists of interbedded siltstone and lime mudstone with prograded ooid shoals that changes basinward to a lime mudstone-dominated, mid-ramp containing slump folds and debris flow breccia. In the Olenekian a more abrupt bank profile developed with a barrier of ooid shoals, a restricted lagoon, and peritidal interior. During the Middle Triassic, the platform developed a progressively steepening Tubiphytes microbial-cement reef-rimmed margin and upper slope that reached up to 250m relief with slope clinoforms ~35 o. Slope facies changed from debris-flow breccia to talus breccia and calciturbidites. During the late Anisian and Ladinian the platform aggraded as Tubiphytes microbial-skeletal reefs developed at the margin, debris flow breccias and caciturbidites intertongued with siliciclastic turbidites in the basin and peritidal cyclic facies developed across the flat-topped interior. Preliminary analysis of spectral gamma-ray logs and elemental geochemistry (U, Mo, V) show onset of basin anoxia in the Early Triassic and maintenance of an anoxic basin with redox fluctuations to the end of the Early Triassic and oxic conditions in the Anisian. Carbonate factory types shift from skeletal in the Upper Permian to abiotic (oolite and micrite) in the Induan and Olenekian to microbial and abiotic (Tubiphytes, microbial crusts, cement) in the Anisian and microbial, abiotic and skeletal in the Ladinian. The transition to a more abrupt bank profile with oolite barrier and restricted lagoon indicates that the change preceded the biotic “recovery” from the end-Permian extinction, suggesting that seawater redox conditions may have had a greater role on margin architecture than biotic evolution. Quantitative petrographic analysis will allow us to further test whether shifts in basin redox and carbonate saturation affected changes in diagenesis and porosity preservation/development

Change in Seawater Redox and Carbonate Saturation State: A Mechanism for Basin-Wide Shifts in Carbonate Platform Architecture and Carbonate Factories: Examples from the Permian-Triassic Nanpanjiang Basin, South China

The Nanpanjiang Basin occurs within the south China plate bordered by the Yangtze Platform (YP) and contains an isolated platform, the Great Bank of Guizhou (GBG). This study tests whether changes in seawater redox and carbonate saturation affected carbonate factories and margin architecture. The margins of the YP and GBG are preserved in synclines that expose 2-D cross sections. Margin architecture was constrained by outcrop mapping aided with high-res. satellite images and stratigraphic sections. Parallel shifts in architecture of the YP and GBG include: 1) an U. Permian abrupt, high-relief reef rimmed margin, 2) a basal Triassic (Induan) broad ramp with prograded ooid shoals that changes basinward to lime mudstone, 3) a L. Triassic (Olenekian) progressively steepening margin with a barrier of ooid shoals, aggrading up to 500 m relief with slope reaching 31o, and a dolomitized platform interior, and 3) a M. Triassic (Anisian) steep prograding Tubiphytes reef margin which maintained relief of 400-500 m and slopes up to 31o. Gamma-ray logs and elemental geochemistry show onset of basin anoxia in the Induan, redox fluctuations, and a return to oxic conditions in the end of the Olenekian and Anisian. Carbonate factories shift from a relatively high proportion (6%) of skeletal contents in the U. Permian to abiotic (oolite and micrite) (98%) in the Induan, to microbial and abiotic (11% and 54%) in the Olenekian, to a return of higher skeletal content (6%) in addition to microbial crusts and cement in the Anisian. This return of biotics in the Middle Triassic may reflect the recovery from extinction and seawater anoxia. However, the shift to an abrupt aggrading oolite profile in the Olenekian indicates that the change in architecture preceded biotic recovery, suggesting that oxygenated waters and ultra-high carbonate saturation may have spurred the onset of a steep aggraded margin. Oolitic margins occur in ramps with the exception of those that are formed upon antecedent topography. The development of a rapidly aggrading, steep, high-relief oolitic margin may be a new carbonate platform morphotype developed in systems lacking skeletal carbonate production with high carbonate saturation states (e.g. Proterozoic, or episodes following mass extinction). Such systems may have significant potential as hydrocarbon prospects

Influence of Ocean Redox Conditions and Carbonate Saturation State on Carbonate Factories and Platform Architecture: Examples From the Permian and Triassic Nanpanjiang Basin, South China

The Yangtze Platform (YP) and the Great Bank of Guizhou (GBG) formed a prominent shelf and an isolated platform, respectively, in the Nanpanjiang Basin (NPJB) of south China during Late Permian through Middle Triassic time, an interval characterized by large changes in seawater redox conditions. Continuous exposures from platform to basin across the YP and GBG allowed high-resolution mapping of their stratigraphic architecture through time. Parallel histories of platform architecture in the YP and GBG include: 1) an Upper Permian abrupt, high-relief, reef rimmed margin; 2) a basal Triassic (Induan) ramp with prograding ooid shoals that change basinward to a lime-mudstone-dominated outer-ramp; 3) a Lower Triassic (Olenekian) progressively steepening, elevated ooid shoal margin aggrading up to 500 m above the basin with slope angles up to 35o; and 4) a Middle Triassic steep, prograding Tubiphytes reef margin. Spectral gamma-ray logs and elemental geochemistry (U, Mo, V) indicate the onset of protracted basin anoxia during earliest Early Triassic time, subsequent fluctuations in redox conditions, followed by a final return to oxic conditions at the end of the Early Triassic. Carbonate factory types at the platform margins shifted from skeletal animals, foraminifera, and algae in the Upper Permian to abiotic or microbially mediated (oolite and micrite) in the Induan and Olenekian, followed by a return of higher skeletal content in addition to microbially mediated micrite and early marine cements during the Anisian. The shift to an aggraded, steep oolite bank profile in the Olenekian predated the return of the skeletal carbonate factory. Stratigraphic forward modeling of the Lower Triassic architecture of the GBG indicates that a low sediment diffusion rate, presumably resulting from a high carbonate saturation state and pervasive early cementation, is required to explain the Lower Triassic transition to a steep platform margin. As oolites are usually related to ramps or are perched on steep antecedent topography that they did not construct, these Lower Triassic examples appear to reflect a new carbonate platform morphotype that can develop during intervals characterized by high saturation state, even in the absence of skeletal reef builders

Giant Sector-Collapse Structures (Scalloped Margins) of the Yangtze Platform and Great Bank of Guizhou, China: Implications for Genesis of Collapsed Carbonate Platform Margin Systems

Sector-collapse structures ranging up to 27 km wide with up to 7.7 km bankward erosion (scalloped margins) and linear escarpments occur along the east-north-east-trending, south-facing margins of the Yangtze Platform and Great Bank of Guizhou. Exposure of one of the structures on the rotated limb of a syncline displays the geometry in profile view. Declivities range from 65° to 90° in the upper wall and decrease asymptotically to the toe. Catastrophic collapses of the margins in both platforms occurred during the late Ladinian as constrained by the ages of strata truncated along the margins and the siliciclastic turbidites that onlap collapse structures. Middle Triassic Anisian and Ladinian platform-edge reef facies and platform-interior facies were truncated along both the Yangtze and Great Bank of Guizhou margins. Lower Triassic facies were also truncated along the Great Bank of Guizhou margin. Gravity transport during the main episodes of collapse occurred as mud-rich debris-flows and as mud-free hyper-concentrated flows. Clasts, several to tens of metres and, exceptionally, hundreds of metres across, were transported to the basin. Following collapse, talus, carbonate turbidites and periplatform-mud accumulated at the toe of slope. Shedding of skeletal grains and carbonate mud indicates active carbonate factories at the margin. Preserved sections of the margins demonstrate that the platforms evolved high-relief, accretionary escarpments prior to collapse. High-relief, without buttressing by basin-filling sediments, predisposed the margins to collapse by development of tensile strain and fracturing within the margin due to the lack of confining stress. The linear geometry of margins and active tectonics in the region supports tectonic activity triggering the collapse. Collapse is thus interpreted to have been triggered by fault movement and seismic shock. Comparison with other systems indicates that evolution from high-relief accretion to tectonic collapse of largely lithified margins resulted in large sector-collapse structures and deposition of a coarse, generally mud-poor breccia apron

Guano stable isotopic evidence of Anthropocene climate change influence on aridity and vegetation dynamics

A 2.8 m-long core of cave bat guano was obtained from an excavated pit within Bracken Cave (Texas, USA) and analyzed for δ13C and δ15N. Radiocarbon dating of the core indicated that guano deposition occurred between 1974–1989 and 1998–2016 CE. The hiatus between 1989 and 1998 CE was interpreted to be related to documented mining activities during this interval. The average Suess-corrected δ13C values for the entire series (–19.7 ± 1.0 ‰) suggest that insects ingested by the bats had a diet incorporating both C3 and C4 plants. The proportion of each appeared to be dependent on the sensitivity of vegetation to prolonged and frequent droughts. Two documented La Niña events centered around 2008 and 2011 CE occurred contemporaneously with intervals of higher δ13C values. The drought recorded in 2006 may have corresponded with foraging of insects located over C3 vegetation, which resulted in lower δ13C (–22.0 ‰). δ15N values agree well with records of drought and precipitation in Texas, suggesting an aridity control on the local nitrogen cycle. The highest δ15N values occurred during periods of a weaker North American Monsoon (1977 and 1982 CE). Higher δ15N values also occur at 2006, 2009, and 2012 CE, corresponding with years of reduced precipitation locally and throughout the region. This study further demonstrates that guano δ13C and δ15N records can provide paleo-information with respect to climatic influence on the environment in semi-arid regions