ALBERT

Description: During the Late Cretaceous the northeastern margin of the Arabian plate (Zagros-Fars Area) was characterized by significant variations in sedimentary facies, sedimentation patterns and accommodation space, and by shifting depocentres. A succession of events recording the evolution of the region from a passive to an active margin is documented by the study of eight outcrop sections and one well. This new study uses new age dating (benthic and planktonic foraminifers, nannoplankton and radiolarian biozonations and strontium isotope stratigraphy). The new observations provide a detailed overview of the response of the sedimentary system to changes in the tectonic regime related to obduction processes. These changes are very well shown in regional cross-sections and palaeogeographical maps. Three tectono-sedimentary phases are recognized indicating the evolution from a passive to an active margin: Phase I (Late Albian to Cenomanian, before obduction) comprises three depositional third-order sequences comparable with those of the other parts of the Zagros and Arabian plate. This interval is composed of shallow-water platform carbonates and intra-shelf basins. The platform facies consists of rudist and benthic foraminifer-dominated assemblages, whereas the intra-shelf basins contain an Oligostegina' facies. Eustatic sea-level variations and local differential subsidence controlled sediment deposition during this phase. Phase II (Turonian to Late Campanian, obduction phase) is characterized by major changes in depositional environments and sedimentary facies, as a result of obduction and foreland basin creation. It consists of pelagic and platform carbonates in the south, and a foreland basin with obducted radiolarites, ophiolitic and olistoliths or thrust slices in the north. During this phase, large volumes of turbidites and gravity flows with olistoliths were shed from both the SW and NE into the foreland basin. The age of the tectonic slices increases upward through the section, from Early Cretaceous at the base to Permian at the top. Based on various dating methods used on the far-travelled sediments, the depositional age of the radiolarites can be attributed to the Albian-Cenomanian, whereas the planktonic foraminifers are of Santonian to Campanian age. Phase III (Late Campanian to Maastrichtian, after obduction) shows the development of rudist-dominated carbonates in the NE prograding onto the deep basinal facies in the centre of study area. In the extreme NE no sediments of this age have been recorded, suggesting uplift at that time.

Description: This study re-examines large and deep U-shape reflections (2-4 km wide, 100-200 m deep) within the Upper Cretaceous-Danian Chalk Group in the inverted Roar Basin of in the Danish North Sea, previously interpreted as a moat associated with a contour-parallel current system and/or erosive channels by gravity-driven turbidites. Improved 3D seismic data quality and seismic interpretation techniques helped to identify overlooked reflection terminations that suggest that rather than a linear depression, the U-shape reflections outline several bowl-shaped depressions. In addition, vertical high-amplitude columns and vertical discontinuity zones within and below the depressions were recognized and interpreted to indicate the presence of small fluid pipes, suggesting that the formation of the depressions is more complex. Carbon isotope analysis of high acoustic impedance beds within the underlying Lower Cretaceous chalk shows negative δ13C values down to −20‰, and are interpreted to indicate sediments influenced by methane-derived authigenic carbonates. Permo-Triassic half-grabens seem to have been a major source of gas-bearing fluids, as evidenced by hydrocarbon leakage phenomena within Triassic to Lower Cretaceous strata. In areas where Zechstein salt is present, the leakage root lies at salt welds, causing the formation of focussed seismic reflection wipe-out and dim zones. In areas where salt was absent, the leakage root comprises a much more diffuse zone across the fault boundaries of the Permo-Triassic half-grabens, and gas chimneys are characterized seismically as broad vertical dim zones up to 10 km wide. Campanian inversion tectonics caused fault reactivation and several 100s of meters uplift of the Roar Basin, which created an instability of the trapped gas-bearing fluids. Gentle fluid venting through observed pipes caused sediment entrainment, which could be carried away by bottom current activity, causing localized zones of non-deposition and formation of individual depressions. This model thus does not disregard the role of bottom current activity in the formation of the depressions, yet it includes a fluid venting element that fits better with the architecture and overall evidence for fluid venting features in pre-chalk strata as well as in the Chalk Group. Importantly, it shows that prior to the thermogenic maturation of the main source rock (i.e., the Bo Member of the Farsund Formation in the Late Miocene), fluid venting already occurred on the Late Cretaceous seafloor from deeper source rocks that are at present overmature.

Description: The framework of salt tectonics in the Central North Sea was set early in the Triassic. We defined and illustrated five major domains of differing salt tectonic style. The differing structural styles were all interpreted as having evolved under a component of lateral displacement pairing extensional and contractional structures, produced by some combination of decoupled rift extension and gravity sliding. However, the extensional structures are located toward the basin center and the contractional structures near the original updip limits of salt. This suggests a framework driven by gravity sliding of the sediments overlying the Zechstein away from the Central Graben. Possible mechanisms for structural relief away from the Central Graben are the Triassic focus of rifting lying further east at the Norwegian-Danish basin, footwall uplift of a Triassic Central Graben precursor and significant thermal doming occurring much earlier than had previously been thought. The mechanisms are not mutually exclusive and may have acted in concert.

Description: During the Late Cretaceous the northeastern margin of the Arabian plate (Zagros–Fars Area) was characterized by significant variations in sedimentary facies, sedimentation patterns and accommodation space, and by shifting depocentres. A succession of events recording the evolution of the region from a passive to an active margin is documented by the study of eight outcrop sections and one well. This new study uses new age dating (benthic and planktonic foraminifers, nannoplankton and radiolarian biozonations and strontium isotope stratigraphy). The new observations provide a detailed overview of the response of the sedimentary system to changes in the tectonic regime related to obduction processes. These changes are very well shown in regional cross-sections and palaeogeographical maps. Three tectono-sedimentary phases are recognized indicating the evolution from a passive to an active margin: Phase I (Late Albian to Cenomanian, before obduction) comprises three depositional third-order sequences comparable with those of the other parts of the Zagros and Arabian plate. This interval is composed of shallow-water platform carbonates and intra-shelf basins. The platform facies consists of rudist and benthic foraminifer-dominated assemblages, whereas the intra-shelf basins contain an ‘Oligostegina’ facies. Eustatic sea-level variations and local differential subsidence controlled sediment deposition during this phase. Phase II (Turonian to Late Campanian, obduction phase) is characterized by major changes in depositional environments and sedimentary facies, as a result of obduction and foreland basin creation. It consists of pelagic and platform carbonates in the south, and a foreland basin with obducted radiolarites, ophiolitic and olistoliths or thrust slices in the north. During this phase, large volumes of turbidites and gravity flows with olistoliths were shed from both the SW and NE into the foreland basin. The age of the tectonic slices increases upward through the section, from Early Cretaceous at the base to Permian at the top. Based on various dating methods used on the far-travelled sediments, the depositional age of the radiolarites can be attributed to the Albian–Cenomanian, whereas the planktonic foraminifers are of Santonian to Campanian age. Phase III (Late Campanian to Maastrichtian, after obduction) shows the development of rudist-dominated carbonates in the NE prograding onto the deep basinal facies in the centre of study area. In the extreme NE no sediments of this age have been recorded, suggesting uplift at that time.