Abstract Three-dimensional seismic data from the Torosa Field (Torosa 3D Survey) in the Browse Basin of the Northwest Shelf, Australia, display a series of well imaged isolated carbonate platforms that allow the documentation of platform evolution from initiation to terminal drowning. The application of seismic geomorphology allows recognition of various carbonate geomorphic elements (depositional and diagenetic) including prograding margins, slope deposits, platform interior patch reefs, and karst fabrics. A variety of techniques integrating seismic geomorphology and seismic stratigraphy have been applied to assist in visualization of these deposits. Examples of workflows and techniques including horizon slicing, optical stacking, and facies mapping, along with the integration of seismic section views ( i.e. , stratigraphy) and plan views ( i.e. , geomorphology) will be illustrated. This presentation will focus on the lower part of the carbonate succession. The carbonate factory is initiated through nucleation on a subtle pre-existing structural high. Initially, the carbonate system is characterized by a platform characterized by linear margins associated with progradation marked by clinoform architecture in both directions, suggesting an isolated platform with no direct connection to the Australian continent. An initial ramp style platform gradually gives way to a platform bounded by progradational slope physiography. The platform margin is characterized by reefal buildup repeatedly crosscut by channels that connect the open ocean with the interior platform. Within the interior of the platform small patch reefs and occasional seaways are common. Several flooding events punctuate the platform evolution, with each drowning event immediately preceded by deposition of clusters of small patch reefs, less than 300m wide.

Abstract Three-dimensional seismic data from the Torosa Field (Torosa 3D Survey) in the Browse basin of the North West Shelf, Australia display a series of well imaged isolated carbonate platforms that evolve from platform initiation to terminal drowning. The application of seismic geomorphology allows recognition of various carbonate geomorphic elements (depositional and diagenetic) including prograding margins, slope fan and gullied slope deposits, platform interior patch reefs, and karst fabrics. Mapping these features in successive horizon-slices and comparing the spatial changes with changes in seismic geometries (cross-sectional view) allow a more detailed understanding of platform response to relative changes in sea level and the relative complexity of the systems through time. Platform growth initiates as a series of kilometerscale buildups characterized by a circular to elliptical morphology (in plan view). These small-scale buildups quickly coalesce into a series of build-up complexes (2-6 km) with progradational margins. These complexes display either elliptical or rhombohedral shapes. Individual patch reefs can be observed in the platform interiors of the build-up complexes. A second phase of coalescence occurs and the build-up complexes combine into a larger platform (25 × 10 km). This large platform then backsteps and is characterized by a strongly elongate and elliptical shape. The backstepped platform progrades up to 3 km prior to final drowning and burial. The growth of elliptical and rhombohedral buildup shapes appear to be influenced by focused currents between the platforms. A distinct windward-leeward asymmetry has not been recognized in the platforms. The rhombic platform complexes are characterized by a unique “pointed promontory” morphology that develops (perpendicular to the long dimension) as the elliptical complexes evolve into more rhombic shapes through time. Sand waves are also observed in the bottom of the interplatform seaways. The backstepped platforms display prograding seismic reflectors that terminate laterally and are confined to areas of the platform margin protected from influence of currents. Coalescence of the platforms followed the growth of small buildups in the mouths of the interplatform seaways, effectively blocking the seaways and allowing sediment to accumulate in the previously current swept passage. This dataset provides an alternative growth model for platforms strongly influenced by currents, rather than the more common model of progradation with sediment flux dominated by windward-leeward sediment transport.

Abstract Ahigh-resolution, two-dimensional seismic survey covering 7500 km 2 provides an unprecedented view of the evolution of a Miocene-Pliocene carbonate platform in the East Natuna-Sarawak Sea, Indonesia. The Segitiga Platform (1400 km 2 ) contains Terumbu Formation carbonate strata as much as 1800 m thick that were deposited in platform interior, reef and shoal margin, and slope to basin environments. The Segitiga Platform was subdivided into 12 seismic sequences that demonstrate a history of (1) initial isolation, (2) progradation and coalescence, (3) backstepping and shrinkage, and (4) terminal drowning. Interpretations of seismic facies maps for each sequence were used to help illustrate platform history. These seismic facies maps indicate that the Segitiga Platform originated as three smaller platforms on extensional fault-block highs. Deep intraplatform seaways separated these smaller platforms. Progradation of shallow-water carbonates filled the seaways during a phase of coalescence and the three platforms were amalgamated to form a merged composite platform (1400 km 2 ; middle-upper Miocene). A rapid relative rise in sea level at the end of Miocene time caused a major backstepping of the carbonate margins (and a concomitant drowning of the adjacent Natuna field carbonate platform to the east) resulting in a platform of greatly reduced size (600 km 2 ) during the lower Pliocene. Rapid subsidence, combined with an eustatic rise at the end of the early Pliocene, caused terminal drowning of the Segitiga Platform. The platform was buried by younger siliciclastics of the Muda Formation. Eustatic sea level change controlled the timing of sequence-boundary formation, but structural movements modified internal sequence character and facies distribution. Faulting created topography that acted as templates for the initiation of carbonate platform deposition and provided pedestals for the localizationofbackstepped platforms. Cessation of faulting may have instigated progradation of the platform resulting from the deceleration of accommodation-space production. Regional subsidence may have controlled the location and extent of platform backstepping. Geographic variability in sequence stacking of coeval platform margins is observed over relatively short distances. Progradation is most strongly developed on the leeward side of the platform, but increased accommodation resulting from the rapid local subsidence or changing ocean-ographic currents also influenced the direction and magnitude of progradation.

Abstract The strata] architecture of the Holder Formation (Pennsylvanian, Virgilian) in the Sacramento Mountains, New Mexico, reveals the complexities of mixed carbonate-siliciclastic depositional systems. Thirty-three measured sections, correlated by physical tracing, and three research cores and well logs provide an extensive database for examination of two- and three-dimensional stratigraphic architecture. In the Holder Formation, carbonate and siliciclastic facies are arranged into 22 shallowing-upward high-frequency sequences, each bounded by either a surface of subaerial exposure developed on subtidal strata (an abnormal subaerial-exposure surface) or its correlative conformity. Vertical facies successions within individual high-frequency sequences vary significantly across the narrow shelf, as shown in the late 1960s by James Lee Wilson. High-frequency sequence boundaries cross lateral facies transitions and extend down paleoslope; such boundaries are the highest-resolution, correlatable chrono-straligraphic surfaces in this succession. Subaerial exposure of subtidal strata, fluvial incision through marine strata, and major downward and basinward facies shifts indicate diat relative changes in sea-level significantly influenced stratal architecture. Measured minimum amplitudes of relative falls in sea level are as great as 32 m and exceed measured thicknesses of high-frequency sequences, suggesting that accommodation space was not completely filled. Stratigraphic response to relative changes in sea level was controlled by spatial and temporal changes in sediment supply and production and shelf position. Relative lows in sea level and the early parts of relative rises in sea level were characterized by siliciclastic sediment bypass of the emergent shelf and deposition and onlap in more downdip positions. Flooding of the shelf is indicated by a vertical facies transition from alluvial siliciclastics to marine carbonates and shales. During relative highs in sea level, shelf-wide carbonate sedimentation dominated, although localized deltaic siliciclastics were deposited on the shelf during the early part of the relative high. Responses to relative changes in sea level were mediated by shelf position, and strata at different shelf positions reflect distinct parts of the sea-level curve. Syndepositional structural deformation affected stratigraphic architecture at several scales through its influence on siliciclastic sediment supply. subsidence, and uplift; such deformation accounts for some of the considerable vertical and lateral complexity in the system as first described by J. L. Wilson. A relatively steep depositional gradient and depositional topography during deposition of the lower part of the Holder Formation (partly controlled by La Luz anticline) led to distinct lateral changes in facies and laterally compressed facies belts; the less pronounced topography and gradient that existed during deposition of the upper Holder Formation, because of in-filled topography and the waning influence of La Luz anticline, resulted in more laterally continuous sheetlike deposits.