Incised submarine canyons governing new evidence of Early Triassic rifting in East Greenland

Regional studies of the uppermost Permian and lowermost Triassic successions in central East Greenland have shown that sedimentation took place in half-grabens during two phases of active rifting. The Permian-Triassic boundary locally contains submarine canyons, up to several kilometres wide and several tens of metres deep formed in latest Permian or earliest Triassic times. Sandy and conglomeratic turbidites filling the canyons are highly variable in architecture depending on the structural setting and the geometry of the submarine canyons. Bounding surfaces also change character because of an asymmetric subsidence pattern of tilted fault-blocks. The submarine canyons have been studied on Wegener Halvo, where exposures allow tracing of several canyons from the slope to the basin floor setting. The canyons are interpreted to have developed from fluvial incised valleys furthest updip on the fault-block and from turbidity-current erosion further downdip. The canyons have a SE-NW elongation, while palaeocurrents were unimodal towards the NW reflecting the dip of the rotated fault-block. The canyon fill turbidites were deposited by consequent drainage on a NW-dipping, ramp-like hangingwall slope within a half-graben. In areas between the canyons, the Permian-Triassic boundary is onlapped by Triassic mudstones of offshore marine origin. The studied succession is grouped into two facies associations: (1) submarine gravity-flow sandstones and conglomerates; and (2) suspension-deposited shales and mudstones. Traced down-dip, the submarine canyon fills display a change from chaotic, conglomeratic and coarse sandy high-density turbidites proximally to organised sandy turbidites basinwards. Farthest basinward, a remarkable basin-floor fan with well-developed giant-scale foresets is observed. This trend reflects canyon widening and a decrease in gradient at the basin-floor. Progradation of the turbidites was probably controlled by westward tilting of the fault-block causing a steeper slope and subaerial exposure of a larger part of the proximal hangingwall above the fulcrum. This resulted in erosion of the crestal area of the block, that is, Upper Permian and older successions, and led to an increase in sediment supply to the downdip area. Accommodation space was created below fulcrum and downdip transgression governed back-filling of the submarine canyons. The observed eastward onlap of ammonite zones in shales and mudstones between! the canyons was contemporaneous with the filling of the canyons. A later drowning of the updip catchment caused decrease or shut-off of sediment supply leading to deposition of shales and mudstones blanketing the sandy turbidites in the canyons. The tilt of the studied fault-block is interpreted to have caused simultaneous transgression below the fulcrum and regression, with bypass or erosion, above the fulcrum. It is concluded, that the tilted fault-block: setting has important consequences for sequence stratigraphic interpretations due to the asymmetric subsidence causing drastic changes in bounding surface character. Tectonism and sediment supply were the main controls on the stratigraphic architecture of the studied Lower Triassic succession on Wegener Halvo. (C) 2000 Elsevier Science B.V. All rights reserved.