Shallow-marine carbonate facies from volcanic-arc settings provide an important, but commonly overlooked, record of relative sea-level change, differential subsidence-uplift, paleoclimate trends, and other environmental changes. Carbonate strata are thin where volcanic eruptions are frequent and voluminous, unless shallow, bathy-metric highs persist for long periods of time and volcaniclastic sediment and erupted materials are trapped in adjacent depocenters. Carbonate platforms and reefs can attain significant thickness, however, if subsidence continues after volcanic activity ceases or the volcanic front migrates. The areal extent of shallow-marine carbonate sedimentation is likewise affected by differential tectonic subsidence, although carbonate platforms are most laterally extensive during transgressive to highstand conditions and when arc depocenters are filled with sediment. Tectonic controls on shallow-marine carbonate sedimentation in arc depocenters include (1) coseismic fault displacements and associated surface deformation; (2) long-wavelength tectonic subsidence related to dynamic mantle flow, flexure, lithospheric thinning, and thermal subsidence; and (3) large-scale plate deformation related to local conditions of subduction. Depositional controls on carbonate sedimentation in arc depocenters include (1) the frequency, volume, and style of volcanic eruptions; (2) accumulation rates for siliciclastic-volcaniclastic sediment; (3) the frequency, volume, and dispersal paths of erupted material; (4) (paleo)wind direction, which influences both carbonate facies development directly and indirectly by controlling the dispersal of volcanic ash and other pyroclastic sediment, which can bury carbonate-producing organisms; (5) the frequency and intensity of tsunami events; and (6) volcanically or seismically triggered mass-wasting events, which can erode or bury carbonate strata. Regarding platform morphologies in arc-related settings, (1) fringing reefs or barrier reef systems with lagoons may develop around volcanic edifices throughout the long-term evolution of volcanic arcs; (2) local reefs and mounds may build on intrabasinal, fault-bounded highs within underfilled forearc, intra-arc, and backarc basins; (3) isolated platforms with variable platform margin-to-basin transitions are common in “underfilled” and tectonically active depocenters; and (4) broad ramps and rimmed carbonate shelves are typically found in tectonically mature and sediment-filled depocenters.

Abstract All scales of tectonic deformation influence the location, sizes, shapes, and internal stratigraphy of carbonate platforms that form in active rift settings. Normal and oblique-slip faults bound the tectono-geomorphologic features that are typically found across rift settings. These fault-bounded structural elements can provide substrates for shallow-water carbonate platforms if they are submerged to shallow water depths. Thus, the incremental and long-term growth of tectonic structures, and interactions between surface processes and carbonate depositional systems that develop around or on top of these structures, determines nearly all stratigraphic aspects of syn-rift carbonate platforms.Isolated carbonate platforms are the most common type of platforms in active rift settings because they form on fault–bounded syn-rift highs. Steeply dipping fault scarps and tilted to flat-topped depositional surfaces control where shallow-marine carbonate deposition is possible. Fault scaling laws and rules for fault growth, spacing, and linkage/interaction are important for understanding the internal stratal patterns within syn-rift carbonate platforms. Footwall highs are common nucleation sites for carbonate platforms, although paleo-wind directions and siliciclastic supply to adjacent depocenters also influence facies distributions, platform morphology, and overall stratigraphic development. Active fault displacements and related surface deformations during platform growth can control platform-margin locations, facies distributions across fault-bounded basement highs, siliciclastic–carbonate interactions (especially in updip fault-bounded depocenters), and the internal growth stratal patterns within syn-rift platforms. Wedge-like growth stratal patterns within syn-rift isolated platforms are characteristic of half-graben structural elements and are well documented in outcrop and subsurface examples. Flexural uplift of footwall margins of large, fault-bounded horsts is also documented by stratal relationships from syn-rift isolated platforms that build on horst highs. Syn-rift thermal subsidence may influence where carbonate facies are distributed across the rift system, as well as timedependent accumulation rates for each platform.Syn-rift carbonate platform strata can form important petroleum reservoirs within rift-basin systems. They also provide critical records for understanding the tectonic evolution and depositional history of rift systems.

Abstract Results of a detailed palaeomagnetic study of Cretaceous-age volcanic, intrusive and sedimentary rock formations from southern Vietnam (24 sites, 163 core samples) are presented. The palaeomagnetic and supplementary rock magnetic studies indicate that magnetite and titanomagnetite are the predominant magnetic carriers in the volcanic and intrusive rock samples, whereas hematite is the principal carrier in the red-beds. The mean palaeomagnetic direction of twenty-one sites from southern Vietnam yields D = 14.5°, I = 33.3°, α 95 = 6.3°, k s / k g = 1.04, which corresponds with a VGP at λ = 74.2°N, φ = 171.1°E, A 95 = 5.9°. Comparison of the pole with the Eurasia mean Cretaceous palaeopole shows that relative to Eurasia southern Vietnam has experienced a southward displacement of 6.5° ± 5.1°, but with insignificant rotation since the Cretaceous. Previously reported Cretaceous palaeomagnetic data, combined with new palaeomagnetic data from this study and analysis of regional structural trends, indicate that Sundaland can be divided into several fault-bounded tectonic domains (Shan–Thai, Indochina, offshore Sundaland), each with a different rotation and/or translation history. Such differential motion might explain, for example, Oligocene transtension and basin formation in the Gulf of Thailand and central onshore Thailand (between the Shan–Thai and Indochina blocks). Our data combined with previously acquired palaeomagnetic data across Southeast Asia, also suggest that, during the Cenozoic, Indochina and parts of Sundaland underwent complex internal deformation and did not behave as a rigid block.

Abstract Stratigraphic Evolution of Foreland Basins - A strong case can be made that foreland basins are where the casual links between sedimentation and tectonic events were first recognized, as evidenced by the interpretations of geologists working in classic foreland areas. This Special Publication was derived from a Research Symposium entitled ?Stratigraphic Sequences in Foreland Basins ?held at the AAPG-SEPM joint annual meeting on June, 1992, in Calgary, Alberta, Canada. This volume provides a well-balanced perspective of current research on foreland basin stratigraphy and also serves as another element in the evolving framework that comprises our understanding of foreland basins. Given that so many of earth?s resources are found in foreland basins and that foreland basin strata often provide the only preserved record of the tectonic events that led to basin development, the impetus for continued studies of foreland basin strata should remain for many generations of geologists to come.

Abstract Clastic foreland basin stratigraphy is primarily determined by the relative rates of first-order basin controlling processes; the rate of mass accretion to an orogen by thrust tectonics, the rate of mass redistribution by surface processes, the rate of fiexural isostatic compensation, and the rate of absolute sea level change. We have developed a composite planform foreland basin model to look for model stratigraphic signatures which reflect either the dominant influence of one of these basin controlling processes or interaction among several processes. The foreland basin model links component models of orogen tectonics, surface processes, lithospheric flexure and eustasy in an internally consistent manner. The tectonic model uses critical wedge principles to construct a doubly-vergent wedge-shaped orogen. The flexural isostasy model uses either an elastic or a thermally-activated linear visco-elastic lithospheric rheology. The surface processes model couples hillslope (mass diffusion) and climate-mediated fluvial (mass advection) transports to erode, redistribute and deposit mass across the orogen, foreland basins and peripheral bulges. Preliminary results are presented from two models which illustrate the terminal stages of ocean closure, the ensuing continent-continent collision, and the kinematic growth of an orogen with two flanking foreland basins. In the first model, there is no significant strike variation in model processes, therefore, cross-sections of any model are sufficient to analyze the basins and compare (hem with previously published results. The contrasting stratigraphic architecture of the basins is controlled by the inherent tectonic asymmetry and by the erosion and sediment flux which become progressively asymmetric as a consequence of the relative positions of the basins on the windward and leeward sides of the growing orogen. The second model demonstrates the complexities that result when there is a significant strike variation in tectonic processes. This model takes the form of a diachronous continent-continent collision between two continental margins inclined at an angle of ~25°. The model collision zone evolves in time and along strike from accretionary prism to orogen. Sediment flux into the windward foreland basin is greatest adjacent to the largest part of the orogen. This region of the basin becomes subaerial first and the drainage network develops a longitudinal trunk river system, similar to those common to many foreland basins. The combination of lateral and longitudinal fluvial transport results in diachronous filling of the marine basin by an assemblage of fluvial and marine facies which prograde down the basin axis.

Abstract: In this paper we investigate the relative effects of intraplate stress level fluctuations and eustatic sea-level changes on foreland basin stratigraphy using forward numerical models. The role played by growth of an evolving orogenic wedge is incorporated. The models show that the effect of stress level fluctuations can be as significant as the effect of eustatic sea-level change. Stress level variations and eustasy can be discriminated in models without orogenic wedge growth because of the asymmetric stratigraphic patterns produced by stress. Models with growth of the orogenic wedge, in contrast, show that asymmetric patterns can also be produced by orogenic wedge growth accompanied by an eustatic sea-level drop. Models adopting a stress level relaxation predict patterns compatible to those produced by isostatic rebound. Therefore, it is suggested that backward reconstructions of generic mechanisms for stratigraphic patterns are preferably not based on these patterns only, and require a combination with sediment provenance, petrological, and structural studies.

Abstract: The Permian Basin of west Texas and southern New Mexico is located in the foreland of the late Paleozoic Marathon-Ouachita orogenic belt. This complex foreland area consists of several sub-basins that are separated by intraforeland uplifts. In an accompanying paper in this volume, regional structural and stratigraphic relationships are used to constrain the tectonic history of the Permian Basin region. A kinematic model for the origin of the Central Basin Platform (CBP), a prominent intraforeland uplift that separates the Midland and Delaware Basins, is also presented. In this paper, we show the results of two-dimensional flexural models for the Permian Basin region. The Marathon orogenic belt is generally considered to be the dominant topographic load that caused flexural subsidence in the Permian Basin region. Basement shortening and uplift associated with the CBP, however, require that the CBP is an additional topographic load that must be considered when modeling the late Paleozoic evolution of the adjacent basinal areas. The CBP was treated as a distributed tectonic load that produced lithospheric flexure of the adjacent basinal areas. To calculate the effects of this load, loading geometries were determined from the excess basement material measured on structural cross sections. These loading geometries were then used to calculate static profiles of the deflected lithosphere using flexural models for elastic lithosphere. Results from flexural analyses compare well with reconstructed synorogenic geometries of the Midland, Delaware, and Val Verde Basins, indicating that subsidence of these basins most likely was produced by the combined topographic loads of the Marathon orogenic belt, the CBP, and probably structures associated with the Diablo Platform and cryptic loads near the Eastern Shelf on the eastern side of the Midland Basin. Best-fit model profiles for the Val Verde and Delaware Basins indicate that values for the flexural rigidity of lithosphere in the Permian Basin region are lowest near the southwest corner of the CBP. The Val Verde Basin is narrowest and the Delaware Basin is deepest near this corner. The apparently low rigidities at this corner also coincide with a prominent salient in the Marathon orogenic belt and with the greatest amount of shortening measured along the boundaries of the CBP. These low calculated rigidities reflect locally weaker lithosphere that might be related to inherent lateral strength variations in the Marathon foreland. Alternatively, high bending stresses produced by the combined loads of the southwest corner of the CBP and the prominent salient of the Marathon orogenic belt may also have weakened the lithosphere in this area.

Abstract: Mudstones from the Taconic and Blountian foreland basins were analyzed for whole-rock chemical composition and clay mineral composition. These foreland basins formed during the middle and late Ordovician when exotic terranes collided with Laurentia. The purpose was to determine if the mudstones record first-order trends in provenance that are related to tectonic history. In both basins, the ratio of chlorite to illite and the ratio of the concentration of three "mafic" elements (Ti, Cr, Ni) to Nb (a "felsic" element) increases with time. However, mudstones from the Taconic foreland basin have a higher proportion of Ti, Cr, and Ni than those from the Blountian foreland basin. Results from the Blountian foreland basin showed the greater amount of scatter. The outboard terranes that collided with Laurentia were the most important sources of siliciclastic sediment because carbonate platforms fringed the continental margin of each basin. Thus, compositional trends in each basin reflect an increase in the proportion of sediment eroded from mafic source rocks within the colliding terrane. In presently accepted tectonic models for the Taconic foreland basin, the colliding terrane is an arc system. The provenance signature in the mudstones suggests the sediment source shifts from a non-magmatic outer arc to the inner volcanic arc during the collision. The comparatively lower concentration of mafic elements in the Blountian foreland basin mudstones may indicate that the colliding terrane was composite or that its angle of convergence was more oblique.

Abstract: The Nanaimo Group of southwest British Columbia overlies the Wrangellia terrain and the western Coast Belt and is in fault contact with the northwestern margin of the Cascades to the southeast. Generally interpreted as deposits of a Late Cretaceous forearc basin, a foreland basin model is preferred for the Nanaimo Group, in large part due to the recent recognition of major, westerly-directed thrust systems in the Coast Belt to the east and northwestern Cascades, coupled with new age constraints which indicate that thrusting in part overlaps with Nanaimo Group sedimentation. As a test of the foreland basin model, U-Pb ages of twenty-two detrital zircons from three formations of the Nanaimo Group provide new evidence about changing source areas with time for Nanaimo Group deposition. Zircons from the lower Campanian Extension and Protection Formations indicate that Coast Belt and San Juan thrust systems were the dominant source areas, and Wrangellia was not a major source of detritus. Submarine fan sandstone of the uppermost Gabriola Formation (Maastrichtian age) contains abundant detrital titanite, epidote, and zircon with ages as follows: Precambrian zircons, zircons of late Mesozoic age with Precambrian inheritance, concordant 87 Ma zircons, and a predominant 72–73 Ma population of zircons. These results indicate derivation from varied sources which may include the eastern Coast Belt (87 Ma), Paleozoic or late Precambrian sedimentary rocks (recycled Precambrian zircons), and a combination of the Idaho Batholith and Omineca Belt plutons (72–73 Ma grains and grains with inheritance). The Idaho Batholith as a possible source is consistent with the position of the basin prior to latest Cretaceous-(?)-Eocene transcurrent dextral faulting. Thus, the source areas for upper Nanaimo Group sediments were considerably more widespread than previously believed, suggesting that major fluvial drainage systems were active in the western Cordillera during the late Maastrichtian.

Abstract: U-Pb geochronology data are presented for single detrital zircon grains from Eifelian, Givetian, and Frasnian sandstones of the Bird Fiord Formation of the central Arctic Islands, Hecla Bay, and Fram Formations of south central Ellesmere Island and the Okse Bay Formation of northern Ellesmere Island. The ranges of 207 Pb/ 206 Pb crystallization ages of the detrital zircons extracted from these sandstones are as follows: eight zircons between 2.62 and 3.0 Ga, four grains between 2.25 and 2.47 Ga, eleven zircons between 1.57 and 2.02 Ga, seven zircons between 1.04 and 1.20 Ga, and one grain of 0.43 Ga. Potential source terrains include the Precambrian shield areas of Canada and Greenland and the mid-Paleozoic Caledonian-Franklinian orogen of Scandinavia, East and North Greenland, and the Canadian Arctic Islands. The East Greenland Caledonian orogen and its unroofed foreland molasse basin are the most probable primary source for the dated detrital zircons of the Devonian clastic wedge. The geology and geographic location of this region also satisfy other aspects of provenance including paleocurrent measurements, mineralogical and petrographic considerations, and the timing and magnitude of provenance area uplift.

Abstract: Intra- and inter-basinal correlations between outcrop and subsurface over most of northern and central Wyoming indicate that chert-bearing conglomerates in the lower Cretaceous Cloverly Formation in the foreland of central Wyoming occupy three distinct stratigraphic levels. The two older conglomerates are in the lower Cloverly Formation in the western Wind River Basin and reflect northerly to northeasterly dispersal. The youngest conglomerate is in the upper Cloverly Formation in the eastern portion of the basin; gravels in this interval also were transported to the north and northeast. The two older conglomerates are separated from the youngest conglomerate by up to 35 m of purple to gray, smectite-rich mudstones that contain distinctive 10 to 90 cm-thick layers of white to dark green devitrified tuff, as well as silica and carbonate nodular beds. Fission-track ages of 125–128 Ma have been obtained from three samples of tuff in the Wind River Basin. These tuffs can be correlated to prominent tuffs further north in the Bighom Basin where a paleomagnetic stratigraphy has been established. Fission-track ages of zircons from devitrified tuff layers and magnetostratigraphy of mudstones suggest that the older two conglomerates in the Wind River Basin were deposited between 133 and 128 Ma and the youngest conglomerate at about 118 to 115 Ma. Three-dimensional, spatially controlled and temporally constrained reconstructions of paleodrainage systems for Cloverly conglomerates illustrate the complexity of fluvial drainage networks within the evolving Early Cretaceous foreland basin. Sand-body geometry and dispersal patterns within these fluvial networks were partially controlled by tectonic activity, which created a series of northeast-oriented horsts and grabens in the Wind River Basin. Location of trunk rivers was controlled by the positions of grabens within the basin.

Abstract: The triangular outline of the late Paleozoic Black Warrior foreland basin on the southern edge of the North American craton in Alabama and Mississippi is framed on the southwest by the northwest-striking Ouachita thrust front and on the southeast by northeast-striking Appalachian thrust-belt structures. The nearly orthogonal intersection of the Ouachita and Appalachian thrust belts implies a composite history of flexural subsidence of the foreland. A long homocline that dips southwest beneath the Ouachita thrust front defines the structure of the basin, and a southwestward-thickening, northeastward-prograding synorogenic clastic wedge of Mississippian and Pennsylvanian rocks fills the basin, indicating a thrust load and sediment source (Ouachita thrust belt) on the southwest. A synorogenic clastic wedge in the Appalachian thrust belt (Cahaba synclinorium) is similar in provenance and dispersal to that in the Black Warrior basin, indicating that the palinspastic site of Appalachian thrust sheets was also part of the original Ouachita foreland. Greater thickness of the clastic wedge in the Cahaba synclinorium reflects partitioning of the Ouachita foreland by reactivation of the down-to-southeast Birmingham basement fault system. Addition of northwest-prograding clastic sediment during the Early Pennsylvanian records initiation of Appalachian orogenesis on the southeast. Subsequently, the southeastern part of the southwest-dipping Black Warrior foreland basin was displaced by northwest-propagating Appalachian thrusts, and part of the older, northeastward-prograding, Ouachita-derived clastic wedge was imbricated.

Abstract: Carbonate platforms and reefs are more common in foreland basins than is generally appreciated and may provide a better record of basin evolution and relative sea-level change than siliciclastic strata. Carbonate platform and reefal facies may develop in the proximal foredeep on a variety of topographic highs, in the distal foreland area far from terrigenous influx, or across the entire foreland basin during tectonically quiescent stages of basin development. Basin geometry, dispersal of siliciclastic sediment, subsidence patterns, and deformation structures across the foreland affect carbonate facies distribution and platform morphology during synorogenic stages of foreland basin evolution. Synorogenic foreland carbonate platforms typically have ramp profiles that mimic the flexural profile produced by tectonic loading. During active convergence, the flexural profile is driven toward the foreland by the advancing orogenic wedge. Synorogenic carbonate ramps are forced to onlap and/or backstep cratonward. Basinward parts of some foreland carbonate platforms may be drowned (sensu stricto) during active convergence, especially when: (1) the underlying lithosphere has low rigidity; (2) the orogenic wedge advances rapidly; (3) a eustatic sea-level rise occurs at the same time as migration of the flexural profile; or (4) some other environmental stress affects carbonate-producing benthos. Two-dimensional forward models show that flexural drowning of some Phanerozoic carbonate platforms, even in the absence of a coeval eustatic sea-level rise or other environmental stress, is possible in less than 250,000 yr. In some foreland areas, complex patterns of synorogenic differential subsidence and foreland deformation can affect carbonate facies tracts many hundreds of kilometers cratonward of the proximal foredeep. These patterns of differential subsidence and deformation probably reflect the response of preexisting basement structures or rheological anisotropies in the foreland area to tectonic loading along the plate margin or the response to sublithospheric processes. Quantitative subsidence analyses from some foreland areas suggest that differential subsidence in the distal foreland is related temporally to tectonic loading along the continental margin, but cratonward limits of the differential subsidence are beyond reasonable limits of flexurally produced subsidence. In addition, patterns of differential subsidence in the distal foreland do not have "normal" flexural wavelengths, amplitudes, or orientations with respect to the orogenic wedge. Therefore, while subsidence in the distal foreland is temporally related to convergence along the plate margin, alternative models for lithospheric deformation are necessary to explain the differential subsidence in the distal foreland. Siliciclastic sediment dispersal is another first-order control on carbonate sedimentation in foreland basins. Coarse-grained, siliciclastic sediment may have less affect on suspension-feeding, carbonate-producing benthic organisms than clay and silt. Hence, bedrock geology, paleoclimate, and depositional gradients in the hinterland and foreland sides of the basin indirectly affect carbonate sedimentation. Effects of siliciclastic sedimentation on foreland carbonates also depend on the evolutionary stage of a foredeep. During "underfilled" stages of basin evolution, siliciclastic sediment is trapped in the proximal foredeep and will not affect carbonate-producing benthos on the distal side of the basin. The distal parts of clastic wedges may fill accommodation during "intermediate" stages of basin evolution and provide a substrate for carbonate platforms that prograde from the peripheral bulge. Progradation of siliciclastics during later "overfilled" stages of basin evolution may terminate carbonate platforms even in the distal foreland.

Abstract: The Permian Basin of West Texas and southern New Mexico is located in the foreland of the Marathon-Ouachita orogenic belt. This complex foreland area consists of several sub-basins that are separated by intraforeland uplifts. This study examined the tectonic, kinematic, and subsidence history of the Permian Basin in order to evaluate how intraforeland deformation affected stratigraphic development. We focused on: (1) the kinematic history of the Central Basin Platform (CBP), an intraforeland uplift that trends at high angles to the frontal thrust of the Marathons and separates the Delaware and Midland Basins; and (2) subsidence and stratigraphic analyses of the Midland, Delaware, and Val Verde Basins. Structure contour maps, seismic profiles, and balanced structural cross sections show that the CBP can be subdivided into two fault-bounded "blucks," the Fort Stockton and Andector Blocks, which are arranged in a left-stepping, en echelon pattern. The distribution of structural features associated with the CBP is best explained by clockwise rotation of these blocks plus an additional component of east-west compression. The rotation model explains: (1) the steeply dipping reverse faults at the SW and NE corners and local extensional faults at NW corners of individual crustal blocks that comprise the CBP; (2) the large structural relief observed at the SW and NE corners of individual blocks; and (3) decreasing amounts of basement shortening away from thrust-faulted corners that can be documented along both sides of the Fort Stockton Block. An additional component of shortening is required to account for imbalances between the amount of shortening versus extension observed along block boundaries. Subsidence analyses from several points in the Midland, Delaware, and Val Verde Basins indicate that the main phase of tectonic activity probably began during middle Pennsylvanian time. Rapid subsidence in each basin began at that time and continued until Early Permian time. Thereafter, subsidence slowed considerably to the end of the Permian. Late Paleozoic unconformities developed across the CBP and locally in adjacent basinal areas at the same time as rapid subsidence in the basins, suggesting that they are related to the same general episode of tectonic activity. Upper Pennsylvanian to Lower Permian stratigraphic cross sections show that synorogenic strata generally thicken toward the CBP. These stratal relationships indicate (hat the CBP acted as an intraforeland load that caused flexure of the adjacent sub-basins that comprise the Permian Basin. The thickest accumulation of upper Pennsylvanian to Lower Permian strata is developed next to the SW and NE corners of the Fort Stockton and Andector Blocks, which also corresponds to the areas where block uplift was greatest. Finally, the variable platform-to-basin relief that was produced during uplift of the CBP resulted in very different patterns of stratigraphic onlap during late Pennsylvanian to Early Permian time. Onlapping upper Pennsylvanian to Wolfcampian strata extend farthest across the top of the CBP where structural relief was least (i.e., at (he NW and SE block corners). Stratal onlap is minimal at (he SW and NE block corners because structural relief was greatest there. This study illustrates how patterns of intraforeland deformation can dramatically affect basin stratigraphy during synorogenic stages of basin development.

Abstract: Development of the Mississippian carbonate platform along the eastern margin of the foreland basin of the Antler Orogen was controlled by subsidence due primarily to emplacement of the Roberts Mountains allochthon, but also to variable rates of carbonate production and eustatic sea-level changes. A series of stratigraphic sections in southern Nevada and eastern California, oriented approximately perpendicular to the original depositional trends, has allowed evaluation of the relative influence of these controlling factors and development of a depositional model that may have wide application. Mississippian carbonate-platform sedimentation began with a rapid transgression in Kinderhookian time, probably due to a sea-level rise and perhaps initial thrust loading of the older Devonian carbonate platform. In early Osagean time, final emplacement of the Roberts Mountains allochthon onto sialic North America depressed the platform, greatly reducing carbonate production. By middle Osagean time, carbonate production rates exceeded the rate of formation of accommodation space, initiating northwestward progradation of a shallow-water carbonate platform. Continued progradation produced relatively steep, unstable slopes, and by early Meramecian time a rimmed platform with coral buildups had formed. These changes in platform morphology are recorded in base-of-slope deposits where slope-derived submarine slides are overlain by sediment-gravity-flow deposits containing debris derived from the platform margin. Carbonate deposition ceased in early Meramecian time when an eustatic sea-level fall exposed the entire platform.