Abstract Comparative analysis of platform evolution recorded along multiple 2D platform-to-basin transects of the Triassic Yangtze carbonate shelf and several isolated platforms in the Triassic Nanpanjiang basin, south China, indicates that laterally variable tectonic subsidence, rate of basinal clastic deposition at the toe of slope, antecedent topography, and changes of carbonate factory type controlled the evolution, large-scale sequence stratigraphic architecture, and geometry of the platform margin and slope. Lateral and temporal changes in these parameters, and their various combinations during the Middle and early Late Triassic, were responsible for the remarkable vertical and along-strike variability in the observed platform architecture and slope profile. Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, back-step geometries, and pinnacle development. Antecedent topography and timing of clastic basin fill dictated differences in platform-margin stability and geometries such as slope angle, relief above basin floor, development of collapse scars, and progradation at basin margins. Changes in slope profile through the Early and Middle Triassic reflect changes in carbonate-factory type and evolving seawater chemistry following the end-Permian extinction. Eustasy, in contrast, had very little influence on platform morphology and large-scale architecture. Process-based depositional models derived from the Nanpanjiang basin of south China present an important analog for understanding, quantifying, and predicting facies distribution and architectural styles at the basin scale in other systems, particularly in areas of active tectonism and temporal variations in oceanic conditions, such as, for example, the prolific Tertiary carbonates reservoir province of southeast Asia.

Abstract The long-lived Yangtze platform (YP) drowned abruptly and was buried by pelagic facies and siliciclastic turbidites in western Guizhou Province during the Late Triassic (Carnian). The uppermost carbonate platform facies are peritidal cyclic limestone and dolostone containing a restricted biota and having fenestral laminate caps. Equivalent margin facies consist of intraclastic, grapestone, oolitic grainstone, and lenses of coral-Tubiphytes algal boundstone indicating high-energy shoals and patch reefs. The drowning horizon is a laterally variable sharp surface or gradational shift to dark, nodular-bedded, pelagic lime mudstone to wackestone. The contact lacks hardgrounds, phosphatized, or glauconitic surfaces that would indicate drowning by excess nutrient flux. Uppermost platform carbonates have a tropical photozoan biota and lack siliciclastic content, indicating neither climate cooling nor clastic flux played a role in drowning. Rare bioturbation and benthic biota in the lower part of the drowning interval indicate dysaerobic conditions with an upward shift to anoxic conditions. Syndepositional faults had a significant impact on the evolution of the western sector of the Yangtze platform and controlled three local accommodation cycles. Faults developed during the last accommodation cycle tip out at the drowning horizon and include a flower structure upon which a pinnacle reef developed as the rest of the platform drowned. Lateral variability in the drowning horizon and thickness of the post-drowning pelagic facies point to differential tectonic subsidence causing sinking of the platform into deep water along faults. Magnetic susceptibility and paleomagnetic reversal correlation demonstrates that the western sector of the platform drowned while shallow marine mixed carbonate-siliciclastic sedimentation continued in the eastern sector to be terminated later in shallow water by increasing rates of clastic flux. Starved black shale horizons in the basin indicate persistent water stratification and bottom water anoxia; elevated trace metal concentrations indicate dysaerobic to anoxic conditions and enhanced preservation of organic matter. Tectonic subsidence likely submerged the western sector into deep, toxic waters of the stratified basin causing the killing of benthic marine carbonate production.

Abstract The Nanpanjiang Basin (NPJB) is a large, complex basin within the south China plate bordered by Precambrian uplifts on the northeast, southeast, and west and by a Triassic suture zone to the south. During the Permian and Triassic, the NPJB formed an embayment in the Yangtze Platform (YP) and contained several isolated carbonate platforms (IPs), including the Great Bank of Guizhou (GBG) and the Chongzuo–Pinnguo Platform. The NPJB presents an exceptional natural laboratory for evaluating controls on carbonate platform margin and slope architecture. Multiple twodimensional transects through the YP and IPs provide exposure along spatial and temporal gradients in tectonic subsidence rate, siliciclastic input, antecedent topography, and oceanography. Platform development across the end-Permian extinction and evolving seawater chemistry allow assessment of the impact of carbonate factory change from a basin-wide perspective.The YP and IPs evolved from ramps and low-relief banks with oolite margins and mud-rich slopes early in the Early Triassic to steepening Tubiphytes-reef rimmed platforms with slopes progressively enriched in clast-supported breccias in the Middle Triassic. Despite differences in the slope angle and windward–leeward differences in grain size at the bank margin, the Early Triassic margin-slope systems have very similar characteristics throughout the basin. During the Middle Triassic, the YP and IPs developed extreme lateral variability in margin architecture due to differences in tectonic subsidence and siliciclastic basin fill at the toe-of-slope. The southwestern sector of the YP and the GBG drowned under pelagic carbonates followed by siliciclastic turbidites in the Late Triassic, Carnian, while the northeastern YP continued shallow-marine deposition until burial by prograding shallow-marine siliciclastics. The southerly IPs have backstepping geometries, terminal pinnacles, and earlier drowning and burial by siliciclastics. Differences in antecedent topography affected margin width and stability, resulting in changes from broad aggrading to prograding margins vs. high-relief and collapsed margins. Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, backstepping geometries, and pinnacle development. Timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and progradation at basin margins. Development of ramp profiles with oolite margins in the Early Triassic and subsequent development of steep-sided margins in the Middle Triassic reflects changes in carbonate factory type following the end-Permian extinction. Process-based depositional models derived from the NPJB can aid in the prediction of facies distribution and architectural styles at the basin scale in other systems, particularly in areas of active tectonism and temporal variations in oceanographic conditions, such as, for example, in the prolific Tertiary carbonates reservoir province of southeast Asia.

Deposition of shallow-water carbonates on the vast Yangtze Platform of south China spanned the late Proterozoic through Middle Triassic, accumulating as much as 4000 m during the Early and Middle Triassic. Deeper-water carbonates and silici-clastics accumulated to comparable thickness in the Nanpanjiang Basin southeast of the platform. After the demise of the platform, an additional 2500 m of mostly silici-clastics spread across the platform in Late Triassic. Deposition of platform carbonates was also widespread in the Permian of south China, but a transgression in perhaps the last 2 m.y. of the Permian combined with differential subsidence to reconfigure the Yangtze Platform in the Triassic. The margin retreated ∼100 km northward in the Guiyang (eastern) sector, and a low-relief ramp developed over the flat top of the Permian platform, whereas the Early Triassic margin mimicked the Permian location in the Zhenfeng sector to the southwest. Nevertheless, deposition was essentially continuous from the Permian into the Triassic in most localities in Guizhou. Triassic deposition began with widespread terrigenous mud bearing thin-shelled bivalves or ammonoids. Renewed carbonate deposition within the Induan produced thin-bedded, laminated, dark-gray lime mudstone with planktonic biota in the basin interspersed with carbonate breccias. Thin-bedded lime mudstones with prominent burrows formed farther updip to the north and west. Thick intervals of oolite and of shallow-water lime mudstone interbedded with terrigenous clastic wedges that thicken and merge to the west mark the updip limit of deposition on a carbonate ramp. Platform-interior carbonates are more than three times as thick as their basinal equivalents. The ramp configuration evolved into a flat-topped platform with a slight rim in the Olenekian, recorded by peritidal carbonate cycles at the platform margin and subtidal lagoonal muds and ultimately evaporites in the interior. Carbonate deposition spread farther westward to cover the terrigenous siliciclastics of the Induan. A major deepening event within the Olenekian is marked by black, ammonoid-bearing, nodular limestone and fissile shale. The patchy distribution of this facies indicates differential warping of the platform rather than purely eustatic causes. The basin received a starvation diet of siliciclastic and carbonate mud with minor silty turbidites and carbonate debris flows. At the end of Early Triassic, platform-interior deposits averaged 1175 m thick and basinal equivalents only 250 m. Acidic volcanic ash spread across the Yangtze Platform at the Olenekian-Anisian transition. Anisian deposits in the Nanpanjiang Basin are dominantly siliciclastics that thicken dramatically in southwestern Guizhou and adjacent Guangxi. Biogenic frame-stones constructed by organisms of questionable origins, Tubiphytes and Plexoramea , assisted by sponges, arborescent corals, skeletal stromatolites, and copious encrusters, rimmed the Anisian platform. This rim collapsed along most of the Guiyang sector to form a basin-margin wedge 175 km long deposited by turbidity currents, debris flows, and rock fall. Collapse led to retreat of the margin by ∼2.7 km; exceptionally by 10 km. In contrast the reef margin advanced slightly in parts of the Zhenfeng sector. Mud-dominated peritidal cycles formed in the lee of the reefs. Deposition in the platform interior was entirely subtidal with alternating episodes of normal marine water, hypersalinity, and siliciclastic influx. An increase in siliciclastic content eastward throughout the Middle Triassic signals the emergence of the Jiangnan Massif, which had been covered throughout the Permian and at least the Induan. Platform sedimentation in the Ladinian features peritidal cycles that extended far into the interior to define a flat-topped platform. High depositional energy is reflected in grainstones and packstones composed of grapestone, bioclasts, and ooids. Barriers other than sand shoals appear to have been absent. Biogenic facies are confined to small outcrops of Tubiphytes and coral boundstone, interpreted as patch reefs. Tepee structures cap many cycles or disrupt successive cycles, indicating extended subaerial exposure. At the beginning of the Ladinian, the platform margin of the Guiyang sector prograded at least 0.6 km. In the Zhenfeng sector the margin retreated slightly, further indication of more rapid subsidence of the western part of the platform. The Nanpanjiang Basin apparently starved early in the Ladinian, but filled to overflowing with siliciclastic turbidites and mud in the later Ladinian or the early Carnian. Transport direction of the very fine sand of the turbidites was from the east, pointing to the Jiangnan Massif as a continued source of sediment, both by land onto the platform and by sea into the basin. East and west sectors of the Yangtze Platform in Guizhou present stark contrasts during the Carnian and Norian. Shallow-water carbonate deposition continued into the Carnian in the Guiyang sector, but tongues of terrigenous mud and sand from the northeast reached to the platform margin and damped out carbonate deposition by the end of the Carnian. Erosion prevailed in the Norian, truncating formations toward the north down to the level of the Anisian. Shallow-water carbonate deposition ended dramatically with the beginning of the Carnian in the Zhenfeng sector. Nodular-bedded, dark-gray lime mudstone with ammonoids overlies peritidal deposits, documenting the drowning of the Yangtze Platform. A very condensed sequence of black shale with concentrations of manganese, reduced iron, and organic carbon followed. Siliciclastic flysch and shallow-water sandstone totaling 1265 m thick filled the accommodation space resulting from the drowning by the end of the Carnian. Norian deposits are cross-bedded sandstones and conglomerates that form thinning- and fining-upward cycles attributed to braided streams that encroached on coastal swamps represented by commercial coal and by mudrocks containing fresh-water, brackish, or marine fossils. The braided streams were rejuvenated and apparently reversed in the Rhaetian to form a coarse-grained clastic wedge that thins and fines toward the north and east across an erosion surface that cuts as deep as Anisian rocks in northern Guizhou. Jurassic and Lower Cretaceous rocks overlie the Rhaetian rocks concordantly, contradicting prevalent interpretations of a major orogeny (Indosinian) in the Late Triassic in Guizhou. The angular unconformity underlying Upper Cretaceous conglomerates dates major deformation in Guizhou as mid-Cretaceous.

Abstract The carboniferous lisburne group in the fourth range, shublik, and sadlerochit mountains of the northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska, U.S.A., consists of carbonate strata of the Alapah and WahooLimestones from the Sadlerochit high to paleotopographically lower areas 65 km to the southwest. The southward paleoslope is documented by onlapping geometry, erosional topography, and lateral facies variation. Twelve surfaces of subaerial exposure were identified in the Alapah Limestone and six in the Wahoo Limestone. Two can be traced over about 100 m of topography; several others disappear southward into subtidal rocks, indicating paleoslope. A major unconformity lies along the top of the Lisburne Group and predates deposition of Permian strata. Crosscutting relationships show that early cathodoluminescence (CL)-banded calcite cement was precipitated during subaerial exposure events; thus it is meteoric in origin. Low Mg and Sr contents and a depleted mean δ 18 O of -6%o support the meteoric interpretation. Correlation of CL zones illustrates that meteoric calcite occluded much of the primary porosity. Cements commonly decrease in abundance downward beneath exposure surfaces. Traces of evaporites and the character of exposure surfaces within the Alapah Limestone indicate an arid climate during deposition, whereas dissolution features within the Wahoo Limestone suggest a humid climate. Variations in cement abundance cannot be related to changes in climate. The proportion of porosity reduced by meteoric calcite is higher in upslope areas than in downslope areas, indicating that duration of exposure was an important control.

Abstract R estricted shelf, bay, or lagoon environments can be reasonably well defined geographically or hydro-graphically in modern settings. In the geologic record, these terms are typically used somewhat more loosely to form a “waste-basket” category for low-energy, shallow-water carbonates, especially if a demonstrable barrier occurs seaward, or if lateral facies equivalents reflect severe restriction, as in the case of evaporitic or euxinic (organic carbon-rich) sequences. Geographically, bays and lagoons are partly enclosed bodies of water. A bay is a shoreline recess or inlet between headlands. Headlands in carbonate settings may be formed by depositional topography from the current depositional cycle as well as by erosional or depositional topography from a previous cycle. Lagoons are more completely enclosed (for example by barrier islands or reefs) so that connection to the sea is distinctly restricted. Lagoons are normally shallow, although lagoons of some larger atolls reach 70 m deep; bays have no particular depth connotation. Restricted shelves may be defined as any part of a continental or island shelf with slow water circulation resulting in abnormal salinity, depleted nutrients, or temperature extremes. Restriction that reduces normal wave or current energy may result from any physical barrier such as reefs, islands, skeletal or oolitic sand shoals, or from the damping effect of vast expanses of shallow water. Abnormal salinities resulting from restriction may be elevated, diluted, or seasonally fluctuating depending on climate and access of terrestrial runoff. Restricted waters are commonly subject to temperature extremes, both high and low, especially if shallow, and to oxygen and nutrient depletion.