No abstract

The compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, was determined. Two types of gravity deposit were present in the cores of ODP Leg 101, Site 632A, i.e., calciturbidites and calcidebrites. In analogy with earlier studies, the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e., platform interior, platform edge, and platform slope. Calciturbidites deposited during interglacial, sealevel highstands show a dominance of non-skeletal grains, largely derived from the platform interior, while calciturbidites of glacial, sea-level lowstands, show a dominance of skeletal platform-edge to platform-slope-derived grains. Thus, the calciturbidite composition can be used to reconstruct the position of absolute sea level. In addition, the mud content of the calciturbidites increased after Marine Isotope Stage 11. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform-edge-derived sediments during varying sea-level positions. The Bahamian carbonate platform is located in a tectonically stable passive-margin setting and the gravity-flow deposits were laid down in an environment exclusively controlled by eustatic sea-level fluctuations. This study shows that all types of gravity-induced carbonate deposits, calciturbidites, and calcidebrites, were deposited in response to global eustatic sea-level variations. The sediment composition could be linked directly to sediment input from specific facies realms along the carbonate platform margin. Hence, sediment composition analysis is a strong tool that may be used to discriminate between gravity-induced deposition triggered by eustatic sea-level changes and that related to tectonic events, when analyzing resedimentation processes in sedimentary basins.

Because of outstanding outcrops, Spitsbergen (Svalbard archipelago) provides unique opportunities to investigate the whole Upper Palaeozoic succession in great detail. This study can help to interpret the stratigraphic history and depositional evolution at other locations exposing coeval shelf strata along the northern margin of Pangea, e.g., the southern Barents Sea and Arctic Canada. Bed-scale outcrop observations are combined with microfacies studies to interpret the sedimentary settings and depositional environment of the Upper Palaeozoic strata. A sequencestratigraphic analysis has been carried out to evaluate the relative timing of sediment facies deposition in response to sea-level changes. The Early Artinskian to Kazanian successions of the Templet Member and the Kapp Starostin Formation were divided into five parasequences that are superimposed on a long-term, second-order, sea-level fall. These parasequences record a fundamental change of the sedimentary setting, from a restricted-marine, warm-water carbonate platform to an open-marine, cold-water, biosiliceous-carbonate ramp system. A cross-section across Svalbard comprising nine onshore sections shows that during deposition of the Kapp Starostin Formation a major depocentre marked by thick parasequences and a higher proportion of deep-water facies (bedded cherts) is located in the southwest of Spitsbergen (at Akseløya), whereas northeastern Svalbard records shallow-water microfacies. Svalbard was tectonically passive during the Permian; the local differences in accommodation space and facies were most likely linked to the rejuvenation of pre-existing structural elements, inherited from the Carboniferous. A deepening of the depositional environment combined with cold-water climatic conditions as recorded in our study area has also been documented in other Upper Palaeozoic successions around the Arctic, such as the Finnmark Platform (Norwegian Barents Sea) and the Sverdrup Basin (Arctic Canada). This transition in the depositional environment along the northern margin of Pangea is the result of large-scale changes in oceanic circulation patterns and local palaeogeographic changes during the northward movement of Pangea.

-Based on seven measured sections from Svalbard, the marine strata of the Permian Kapp Starostin Formation are arranged into seven transgressive-regressive sequences (TR1-TR7) of c. 4-5 Ma average duration, each bound by a maximum regressive surface. Facies, including heterozoandominated limestones, spiculitic cherts, sandstones, siltstones and shales, record deposition within inner, middle and outer shelf areas. The lowermost sequence, TR1, comprises most of the basal Vøringen Member, which records a transgression across the Gipshuken Formation following a hiatus of unknown duration. Temperate to cold, storm-dominated facies established in inner to middle shelf areas between the latest Artinskian and Kungurian. Prolonged deepening during sequences TR2 and TR3 was succeeded by a long-term shallowing-upward trend that lasted until the latest Permian (TR4-TR7). A major depocentre existed in central and western Spitsbergen while to the north, Dickson Land remained a shallow platform, leading to a shallow homoclinal ramp in NE Spitsbergen and Nordaustlandet. The Middle Permian extinction (late Capitanian) is recorded near the base of TR6 in deeper parts of the basin only; elsewhere this sequence is not recorded. Likewise the youngest sequence, TR7, extending to the upper formational contact of latest Permian age, is found only in the basin depocentre. Comparison with age-equivalent strata in the Sverdrup Basin of Canada reveals a remarkably similar depositional history, with, for example, two (third-order) sea-level cycles recorded in the Late Permian of both regions, in keeping with the global record. Sequence stratigraphy may therefore be a powerful correlative tool for onshore and offshore Permian deposits across NW Pangaea.