Triassic strata of the Yangtze Platform at Guanling contain a dolomitized interior, undolomitized margin, and partially dolomitized slope to basin margin. Dolomitized microbial laminate caps of peritidal cycles and massive dolomite with associated evaporite nodules and solution collapse breccias are consistent with penecontemporaneous tidal flat and evaporative dolomitization in the platform interior. The preferential dolomitization of the slope and basin margin (up to 7 km basinward of the margin), dolomitization along fractures, and selective dolomitization of the matrix in slope breccia that diminishes toward the margin are interpreted to have resulted from the incursion of basin-derived fluids during burial. Integrated analysis of fluid-inclusion microthermometry, oxygen, carbon, and strontium isotopes, trace element geochemistry, U-Pb age dates of carbonate phases, and burial history support the recrystallization of interior dolomite and slope to basin-margin dolomitization by brines at high temperatures during burial. The Yangtze Platform at Guanling provides an excellent example of widespread stratiform dolomitization resulting from the superposition of multiple mechanisms, including penecontemporaneous dolomitization by evaporative seawater brines, high-temperature dolomitization of the slope and basin margin by basinal brines, and high-temperature recrystallization of dolomite by brines during burial. This study provides an example that suggests that widespread stratiform dolomite may result from superposed Earth surface and high-temperature burial dolomitization processes and provides a valuable analog for other carbonate platforms in which the margin remains undolomitized while the interior and basin margin are dolomitized. Similar mechanisms likely contributed to the widespread dolomitization of platforms across the Nanpanjiang and Sichuan basins.

Cretaceous Anacacho Limestone within the Balcones fault system (central Texas) is investigated to understand fracturing of chalk-dominated carbonates in a normal faulting deformation regime. Development of opening-mode fractures is highly sensitive to mineralogy and associated mechanical behavior. Low mechanical rebound beds that have >7 % clay and <90 % carbonate generally lack well-developed opening-mode fracture sets. High-rebound beds with <7 % clay and >90 % carbonate contain opening-mode fracture networks. Away from mapped faults (with similar to 10 m or greater throw), deformation is represented by opening-mode fractures in two orthogonal sets with intensities of <1 fracture/meter. The NE-SW-striking dominant set parallels regional normal-fault strike. Deformation adjacent to a similar to 10-m-throw normal fault includes small-displacement (antithetic and synthetic) normal faults and opening-mode fractures. Near-fault fracture intensity reaches >5 x background levels, and decays to background intensity similar to 30 m from the fault. Fluid inclusion analyses of calcite from dilational crack-seal zones along normal faults and opening-mode fractures within the fault damage zone reveal: (i) liquid hydrocarbon inclusions with similar to 28-36 API gravity oil; (ii) homogenization temperatures from two-phase inclusions used to estimate burial depths of 0.9-1.5 km (oil inclusions) up to 2.4-2.9 km (aqueous inclusions); and (iii) aqueous inclusion ice-melting temperatures indicating basinal brine rather than near-surface meteoric water during vein cementation. U-Pb ages from these veins indicate Paleocene-Eocene faulting and fracturing (57.9 +/- 3.2 to 38.9 +/- 4.1 Ma), and limited Miocene (16.6 +/- 2.2 Ma) activity. This work documents the essential role of small-displacement faults and opening-mode fractures in the migration of hydrocarbons through low-permeability chalk dominated carbonates.

Ecosystem function and its evolution depend on the number of taxa and the amount of biomass. For the oceans, spatial and temporal trends in diversity are well known, and spatial variation in biomass within the modern ocean is increasingly documented. Temporal variation in biomass, by contrast, remains undocumented, leaving a crucial gap in our understanding of how the marine biosphere evolved over geologic time. Here, we compiled compositional data from 7,749 marine limestone samples spanning the past 541 million years that document the proportion of sediment comprising the shells of animals, algae, and protists. The data capture temporal variation in skeletal content that is consistent across geologic settings, water depths, and latitudes. The variation largely parallels long-term trends in taxonomic diversity during intervals of diversification and across the three major mass extinctions with high-resolution compositional data, pointing toward a macroevolutionary coupling between marine biodiversity and biomass.

This study used systematic coring, hyperspectral scans, carbonate/siliciclastic analysis, and stable isotope measurements to evaluate environmental controls on large microbial buildup evolution. Buildups initiated on a discontinuous transgressive lag of flat-pebble conglomerate, controlling their spatial distribution and flat-based geometry. The consistent three-phase buildup architecture indicates allogenic forcing by fluctuations in sea level and siliciclastic influx. The initiation of each phase began in a pure carbonate system, with high-energy skeletal-oolitic grainstone interbuildup deposition. Depositional moats formed in the grainstone adjacent to buildups during phases 1 and 2. Buildups developed up to 5.7 m of synoptic relief during phases 1 and 3 prior to being onlapped by siliciclastic sediment. Phase 2 of the buildups intertongued with heterolithic interbuildup sediment, developing only 30 cm of synoptic relief. Lateral variability in phase 2 architecture indicates autogenic controls causing localized differences in microbialite growth rates versus interbuildup accumulation. Phases 1 and 3 display a dense calcitic calcimicrobial rind that surrounds and encloses the top of the buildups. The interior of buildups is composed of decimeter-scale stromatolitic columns, each encased by a calcimicrite rind and exhibiting mottled internal texture due to bioturbation and dolomitization. Interiors of buildups entrapped more carbonate sediment, creating greater porosity, and thus were destructively dolomitized. In contrast, the calcimicrite rinds of columns and the thick rinds surrounding buildups formed as biochemical carbonate precipitates, with minimal porosity, and, therefore, escaped dolomitization. The δ18O and δ13C values of the column and buildup rinds are consistent with late Cambrian seawater, supporting an origin as a precipitate and age as Sunwaptan.

Recent discoveries at Roland T. Bird's famous 'swimming brontosaur' tracksite on the Mayan Dude Ranch in South Texas have resulted in seven additional footprints in Bird's original sauropod trail, as well as a second manus-dominated sauropod trackway and a single theropod track. Our new study has applied ichnological, photogrammetric, microscopic and geochemical methods. This site has provoked much debate about the preservation of manus-only and manus-dominated tracksites, as well as sauropod locomotion and behaviour. Discussions have focused on whether these trace fossils are a result of a swimming sauropod, or that the manus-dominated trackways could be simply underprints. The results of our study indicate that the second manus-dominated sauropod trackway records partial pes impressions with 4 of the 5 steps preserved. Along with the single theropod track, this indicates that the individuals were not buoyed up by deep water and were not kicking off the bottom with their forelimbs. Depositional evidence demonstrates exceedingly shallow-water conditions (less than one metre) and partial marine lithification of the substrate, hence ruling out a swimming sauropod origin of the tracks.

The Wolfcampian Alta Formation represents 1700 m of deep-water siliciclastic deposits exposed in the Marfa Basin, the southwestern sub-basin of the Permian Basin complex of west Texas. These exposures are important outcrop analogs for the highly productive Wolfcamp Shale oil and gas reservoir of the Delaware and Midland Basins because they are of similar age, lithologies, and depositional environments. We present preliminary field data from outcrops of the Alta Formation in the southeast part of the Chinati Mountains, including lithostratigraphy, fracture characterization, and mineralogical analyses. Mesostructural deformation fabrics are dominated by up to four systematic sets of bed-perpendicular opening- mode fractures but also include rare bedparallel opening-mode veins (beef), and occasional normal faults and thrust faults. Opening-mode fractures are generally bed-restricted and are interpreted to record a complex history reflecting changing extension direction at the time of fracturing in these sandstone and shale strata. Fracture dimensions mapped in a sandstone bedding pavement exposure show that length (parallel to bedding) to height (perpendicular to bedding) ratios for opening-mode fractures range from 0.13 to 38.56, with an average aspect ratio for all mapped opening-mode fractures of 4.84. Scanline surveys of a systematic NE-striking opening-mode fracture set show that fracture spacing is strongly correlated with mineralogy in both sandstone and shale lithologies, with a strong positive correlation for fracture spacing vs. clay content, and very strong negative correlations for fracture spacing vs. quartz, quartz + feldspar, and brittleness index. Bed thickness vs. fracture spacing data from scanlines show marked differences between sandstone and shale beds, with a very strong positive correlation for sandstone beds, a weak negative correlation for shale beds, and a very weak positive correlation - i.e. no correlation - for combined data. These results suggest that composition exerts a first-order control on opening-mode fracture abundance, and that bed thickness is likely a subordinate, or less important, controlling factor. These relationships can potentially be leveraged for mineralogy-based subsurface fracture prediction in comparable siliciclastic deposits.

Carbonate platforms with automicritic boundstone slopes differ from detrital platforms in their architecture and reservoir properties. The controls on the occurrence of automicritic boundstone slopes are poorly constrained, but they have generally been associated with depleted reefs and other metazoan-algal benthic ecosystems. To test this association, we investigated automicritic boundstone occurrence across the end-Permian extinction and Triassic biotic recovery on the Xiliang slope of the Great Bank of Guizhou (GBG), an isolated carbonate platform in southern China. Our findings indicate that automicritic boundstone accumulation was not enhanced by depleted Early Triassic benthic ecosystems. Instead, detrital sediment dominated the GBG slope, and automicritic boundstone was absent until the late Spathian (latest Early Triassic), 4-5 million yr after extinction. Beginning in upper Spathian strata, automicrite forms small (<1.5 cm) and sparse (<15% estimated volume) masses in slope boundstone, coinciding with an early stage of benthic ecosystem recovery. When a metazoan-algal reef subsequently formed during more advanced recovery in the middle Anisian (lower Middle Triassic), automicrite occurrences were more abundant (up to 40% estimated volume) and larger (up to similar to 10 cm). The proportions of automicrite and metazoan-algal fossils within samples are also positively correlated (Spearman's r = 0.70; P < 0.0001). Our findings suggest that the controls on slope automicrite accumulation and metazoan- algal benthic ecosystems are similar or that benthic metazoans and algae enhance, rather than inhibit, automicrite accumulation. Our results imply that concept-driven predictions of automicritic boundstone occurrence should be modified-automicritic slopes are not more likely to occur during geologic intervals with depleted benthic ecosystems.

During the Permo-Triassic time, an extensive epeiric carbonate platform (Khuff Formation) developed on the Arabian Plate. Besides becoming one of the world’s most prolific hydrocarbon reservoirs, the Khuff carbonates also recorded major shifts in physiochemical, biological, and environmental conditions associated with the Permo-Triassic mass extinction. However, recognition and expression of this catastrophic event in the Arabian Plate were unresolved due to ambiguous geochemical signals. Pervasive diagenesis has been interpreted as one of the major causes of creating such mixed geochemical signals, yet the origin and timing of diagenetic processes remain enigmatic. In this study, we used measurements of elemental concentrations and isotope ratios to help distinguish the geochemical signatures of Permo-Triassic environmental change from diagenetic influences related to dolomitization and dissolution-cementation. The geochemical results suggest that most of the primary signals have been altered by meteoric-driven diagenesis as shown from the REE pattern. Dolomitization occurred early, under near-surface conditions, driven primarily by modified Lower Triassic seawater. The δ18OVPDB and δ13CVPDB values fall within the expected range of Triassic seawater, 0.5 to -2.8‰ and 1.6 to 3.1‰, respectively. This interpretation is corroborated by the calculated clumped isotope (Δ47) temperature of around 34oC. The formation of oomoldic porosity and blocky cement in the Khuff carbonates, occurred later, during shallow burial (up to 1 km) and involving evolved and heated meteoric fluids as indicated by negative Eu anomalies, more negative δ18O (up to -8.2‰) and δ13C (up to -2.1‰) isotopes, depleted Sr concentrations and elevated Fe and Mn concentrations. The Δ47-derived temperature of the blocky cement further supports this argument, with values indicating precipitation at ~50oC. Thus, dissolution occurred in burial environments through cryptic dissolution that drove the cementation by increasing the saturation state of the formation waters. Furthermore, a model of fluid-rock interactions shows that meteoric fluids alone cannot explain the observed patterns, and a moderate to high degree of mixing between meteoric fluids and subsurface brines is required. These new constraints on the diagenesis of the Khuff Formation are critical for understanding and predicting the formation of porosity and the relative timing of hydrocarbon charge and migration in the subsurface. The integrated approach should also be applicable to other petroliferous carbonate provinces elsewhere.