An integrated sequence stratigraphic study based on outcrop, core and wireline log data documents the combined impact of Cretaceous eustacy and oceanic anoxic events on carbonate shelf morphology and facies distributions in the northern Gulf of Mexico. The diverse facies and abundant data of the Comanche platform serve as a nearly complete global reference section and provide a sensitive record of external processes affecting Cretaceous platform development. Regional cross-sections across the shoreline to shelf-margin profile provide a detailed record of mixed carbonate-siliciclastic strata for the Hauterivian to lower Campanian stages (ca 136 to 80 Ma). The study window on the slowly subsiding passive margin allows the stratigraphic response to external forcing mechanisms to be isolated from regional structural processes. Three second-order supersequences comprised of eight composite sequences are recognized in the Valanginian-Barremian, the Aptian-Albian and the Cenomanian-Campanian. The Valanginian-Barremian supersequence transitioned from a siliciclastic ramp to carbonate rimmed shelf and is a product of glacial ice accumulation and melting, as well as variable rates of mid-ocean ridge volcanism. The Aptian-Albian supersequence chronicles the drowning and recovery of the platform surrounding oceanic anoxic events 1a and 1b. The Cenomanian-Campanian supersequence similarly documents shelf drowning following oceanic anoxic event 1d, after which the platform evolved to a deep-subtidal system consisting of anoxic/dysoxic shale and chalk in the time surrounding oceanic anoxic event 2. Each period of oceanic anoxia is associated with composite sequence maximum flooding, termination of carbonate shelf sedimentation and deposition of condensed shale units in distally steepened ramp profiles. Composite sequences unaffected by oceanic anoxic events consist of aggradational to progradational shelves with an abundance of grain-dominated facies and shallow-subtidal to intertidal environments. Because they are products of eustacy and global oceanographic processes, the three supersequences and most composite sequences defined in the south Texas passive margin are recognizable in other carbonate platforms and published eustatic sea-level curves. Maraca Lisure A g u a rd ie n te Apon Rio Negro M o rr o n e d i P a c e n tr o Va lle de ll' In fe rn o T r e G r o t t e Cima Dell Murelle

a b s t r a c tThe ubiquity of carbonate platforms throughout the Cretaceous Period is recognized as a product of high eustatic sea-level and a distinct climatic optimum induced by rapid sea-floor spreading and elevated levels of atmospheric carbon-dioxide. Notably, a series of global oceanic anoxic events (OAEs) punctuate this time-interval and mark periods of significantly reduced free oxygen in the world's oceans. The best records of these events are often from one-dimensional shelf or basin sections where only abrupt shifts between oxygenated carbonates and anoxic shales are recorded. The Comanche Platform of central Texas provides a unique opportunity to study these events within a well-constrained stratigraphic framework in which their up-dip and down-dip sedimentologic effects can be observed and the recovery of the platform to equilibrium states can be timed and understood. Stable isotope data from whole cores in middle Hauterivian through lower Campanian mixed carbonate-siliciclastic strata are used to construct a 52-myr carbon isotope reference profile for the northern Gulf of Mexico. Correlation of this composite curve to numerous global reference profiles permits identification of several anoxic events and allows their impact on platform architecture and facies distribution to be documented. Oceanic anoxic events 1a, 1b, 1d, and 2 occurred immediately before, after, or during shale deposition in the Pine Island Member, Bexar Member, Del Rio Formation, and Eagle Ford Group, respectively. Oceanic anoxic event 3 corresponds to deposition of the Austin Chalk Group. Platform drowning on three occasions more closely coincided with globally recognized anoxic sub-events such as the Fallot, Albian-Cenomanian, and MidCenomanian events. This illustrates that the specific anoxic event most affecting a given carbonate platform varied globally as a function of regional oceanographic circumstances.Using chemo-and sequence-stratigraphic observations, a four-stage model is proposed to describe the changing facies patterns, fauna, sedimentation accumulation rates, platform architectures, and relative sea-level trends of transgressive-regressive composite sequences that developed in response to global carbon-cycle perturbations. The four phases of platform evolution include the equilibrium, crisis, anoxic, and recovery stages. The equilibrium stage is characterized by progradational shelf geometries and coralrudist phototrophic faunal assemblages. Similar phototrophic fauna typify the crisis stage; however, incipient biocalcification crises of this phase led to retrogradational shelf morphologies, transgressive facies patterns, and increased clay mineral proportions. Anoxic stages of the Comanche Platform were coincident with back-ground deposition of organic-rich shale on drowned shelves and heterotrophic fauna dominated by oysters or coccolithophorids. Eustatic peaks of this stage were of moderate amplitude (~30 m), yet relative sea-level rises were greatly enhanced by reduced sedimentation rates. In the recovery sta...

This study highlights three‐dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate–siliciclastic clinoforms in the Permian San Andres Formation. Standard field techniques and mapping using ground‐based lidar reveal a high degree of architectural complexity in channellized, scoured and mounded outer ramp stratigraphy. Development of these features was a function of location along the ramp profile and fluctuations in relative sea‐level. Deposition of coarse‐grained and fine‐grained turbidites in the distal outer ramp occurred through dilute and high‐density turbidity flows and was the result of highstand carbonate shedding within individual cycles. In this setting, high‐frequency cycles of relative sea‐level are interpreted on the basis of turbidite frequency, lateral extent and composition. Submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation. Abundant siliciclastic material is preserved in the basin and distal outer ramp as point‐sourced lowstand wedges and line‐sourced early transgressive blankets. In mounded topography of the outer ramp, siliciclastic preservation is minimal to absent, and rare incised channels offer the best opportunity for recognition of a sequence boundary. Growth of mounded topography in the outer ramp began with scouring, followed by a combination of bioherm construction, fusulinid mound construction and isopachous draping. Intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit. Mound growth was independent of high‐frequency cycles in relative sea‐level but was dependent on available accommodation dictated by low‐frequency cyclicity. Low‐angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence. Mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence. This example represents a ramp‐to‐shelf transition that is the result of forcing by relative sea‐level fluctuations rather than ecologic or tectonic controls.

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Complete understanding of outcrop-scale stratal architectures requires extrapolation of two-dimensional depositional cross sections into three dimensions. This study integrates a high-resolution digital outcrop model with outcrop observations to create a three-dimensional geologic model of distal outer-ramp carbonate stratigraphy. Data were taken from sinuous canyon-wall exposures of the Permian San Andres Formation of Last Chance Canyon, New Mexico. A series of laterally extensive carbonate benches previously interpreted as constructional sponge mud mounds are here modeled as a channel-levee complex that is characterized by an alternating history of aggradation punctuated by erosional sediment bypass. Levee mudstones have an erosional base, contain dolomitized peloidal thin beds, and have limited faunal constituents. Primary levees were initiated as a carbonate apron and transformed into a low-relief channel-levee system as a result of deposition by dilute turbidity currents. Subsequent sediment bypass incised through the pre-established channels and into the underlying facies. The fill consists of skeletal packstone lags and fine peloidal packstones, which are correlated to coral-and sponge-bearing secondary levee deposits that drape the primary levees. Debrites, also found within some channelized areas, are thought to represent channel-margin oversteepening and slumping. Channel widths range from approximately 300 to 800 m, and channel-to-levee relief reaches up to 40 m.Three-dimensional surface models representing architectural elements of the channel-levee complex confirm outcrop observations. Surfaces that were modeled as a best fit to the three-dimensional outcrop tracings reveal development of lowsinuosity channels in previously established bathymetric lows created by earlier phases of sediment bypass and erosion. Sequence stratigraphic interpretations suggest that the channel-levee complex developed during the highstand part of a longterm transgression. The overall transgressive setting led to generation of substantial quantities of carbonate mud in more proximal areas of the ramp. Accumulated mud in the source region was transported down depositional dip during highstand parts of high-frequency cycles and resulted in growth of the levees. Carbonate channel-levee complexes are not likely to form during highstand sequences in which the source sediment is grain-dominated. Leveed channels are a viable component of deepwater carbonate settings and may be more common than has been previously recognized. Complex stratal architectures found in this channel-levee complex could be analogous to apparently gullied slopes of muddy carbonate ramps, especially those developed in transgressive sequences.

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Discussion points raised by Rose () concentrate on late Albian stratigraphic relationships between formations of the East Texas Basin and the San Marcos Arch of the Comanche Platform in the northern Gulf of Mexico. Criticisms of Phelps et al. (2014) regarding stratigraphic nomenclature, palaeogeography and regional lithostratigraphic correlations generally focus on interpretive aspects of the study or do not account for the full scope of published information. Revisions to the top Aptian–Albian Supersequence boundary by Rose are incompatible with the relative location of a subaerial unconformity, as well as deepening lithofacies trends and retrogradational stratigraphic patterns below the interpreted boundary. Rose's placement of the top Aptian–Albian Supersequence boundary precisely at the Albian–Cenomanian stage boundary also implies ca 1·4 Ma of diachroneity in second order sea‐level patterns between the northern Gulf of Mexico and other documented global sedimentary basins.