Carbonate sequence stratigraphy - a summary and perspective with case history, Neogene, Papua New Guinea

Abstract

Abstract Sequence stratigraphic analysis subdivides stratal packages into chronostratigraphic units composed of genetically related facies. Sequences are composed of stratal units that develop in response to changes in shelfal accommodation. Carbonate platform successions develop a hierarchy of sequences and cycles ranging from high (10,000 yrs.) to low (10-20 My supersequences) frequency. This stratigraphic hierarchy controls the facies distribution during long-term platform development. Due to the dominant organic origin of sediments, important differences exist between carbonates and siliciclastics that must be taken into account in a sequence analysis. Eustasy and tectonic subsidence control accommodation, and the health of the carbonate controls sediment supply. Generally, the bulk of deposition occurs during sea level highstands. Early marine cementation is not equal for all platforms. Platforms with steep foreslopes or platforms facing restricted basins will have long residence time in an active cementation environment, and are cement-rich. Highly productive platforms facing shallow basins will fill accommodation space rapidly, have a short residence time, and will be cement-poor. In general, sequence boundaries are regional onlap surfaces. Submarine erosional truncation commonly occurs at platform margins and on the slope. Abrupt facies truncation or dislocation is commonly present at sequence boundaries. The degree of subaerial alteration during sequence boundary formation will vary depending on the climate, the original mineralogy of the underlying highstand platform, and time. If the original mineralogy of a platform is dominantly aragonite and hi-Mg calcite the degree of alteration can be extensive. If the lowstand climate is relatively humid, platform-wide solution occurs that may extend deep into the highstand depending on the magnitude and duration of the sea level fall. If the climate is arid to semi-arid, only relatively minor karstification is predicted to occur. Low-Mg calcite-dominated systems show extensive karstification only during major multi-million year periods of subaerial exposure (i.e., 2nd-order sea level falls). Depositional slope angle and the degree of early cementation play a critical role in the development of lowstand systems tracts. In-situ, lowstand platforms and banks can develop in Type-1 sequences (1) in down ramp positions, and (2) on the slope and toe-of-slope of low-angle platforms. Where platform margins are steeper and well-cemented, lowstand deposition is characterized by abundant coarse debris eroded from the platform margin and slope areas. Steep, by-pass margins present a special case, as both transgressive and lowstand sedimentation may develop toe-of-slope onlap geometries. Significant transgressive carbonates will develop where paleoceanographic conditions permit the factory to keep-up with sea level rise. Well-circulated, shallow water conditions over a wide area (i.e., low slope or broad shelf) will allow a significant transgressive systems tract to develop. Integration of seismic, well log, and core data, paleontology, strontium age dating, and velocity analysis enabled construction of a sequence stratigraphic framework for the Gulf of Papua case study area. Rapid subsidence during early foreland development in the Gulf allowed development of thick Miocene platforms on isolated paleo-highs and hinged to the basin margin. Onlap above and erosional truncation below define sequence boundaries. Pervasive dolomitization and subaerial vuggy to cavernous porosity, extends below sequence boundaries for tens to hundreds of meters. Highstand systems tracts are characterized by mounded to chaotic seismic facies at platform margins. Platform interior seismic facies vary between parallel, concordant and mounded facies. Mounded facies are interpreted to be reefal buildups and parallel facies are interpreted to be flat lying lagoonal sediments. Platform foreslopes have gentle to steep dips depending on platform margin relief at basin margin positions. Lowstand systems tracts are observed at platform margins and comprise in-situ shallow water platform facies, and allochthonous debris. The Neogene comprises three 2nd-order supersequences, Lower Miocene, upper Lower to Middle Miocene, and Upper Miocene-Recent. These supersequences are divided into six Miocene and two Pliocene 3rd-order sequences. The Lower Miocene 2nd-order sea-level rise, combined with rapid subsidence, initiated platform growth. Reefal platforms kept pace with sea level rise and comprise the bulk of growth over isolated paleohighs. Although the western basin margin, Borabi platform trend kept pace with subsequent relative rises in sea level, rapid subsidence due to initial thrust sheet loading combined with an early Middle Miocene 2nd-order sea level rise drowned isolated platform growth centers in the Gulf basin area. Later forebulge development in the Middle-Late Miocene resulted in prolonged subaerial exposure of these drowned platforms. Renewed thrust sheet emplacement combined with the 2nd-order eustatic sea level rise during the latest Miocene and early Pliocene, drowned platforms throughout the Gulf and caused a major backstepping of the Borabi trend platforms. During the Pliocene to Recent, rapid influx of siliciclastics from the north and west onlapped and covered most reef platforms and deposition became restricted to the southern portion of the Borabi trend.