Incised submarine canyons governing new evidence of Early Triassic riftingin East Greenland

Regional studies of the uppermost Permian and lowermost Triassic succession s in central East Greenland have shown that sedimentation took place in hal f-grabens during two phases of active rifting. The Permian-Triassic boundar y locally contains submarine canyons, up to several kilometres wide and sev eral tens of metres deep formed in latest Permian or earliest Triassic time s. Sandy and conglomeratic turbidites filling the canyons are highly variab le in architecture depending on the structural setting and the geometry of the submarine canyons. Bounding surfaces also change character because of a n asymmetric subsidence pattern of tilted fault-blocks. The submarine canyo ns have been studied on Wegener Halvo, where exposures allow tracing of sev eral canyons from the slope to the basin floor setting. The canyons are int erpreted to have developed from fluvial incised valleys furthest updip on t he fault-block and from turbidity-current erosion further downdip. The cany ons have a SE-NW elongation, while palaeocurrents were unimodal towards the NW reflecting the dip of the rotated fault-block. The canyon fill turbidit es were deposited by consequent drainage on a NW-dipping, ramp-like hanging wall slope within a half-graben. In areas between the canyons, the Permian- Triassic boundary is onlapped by Triassic mudstones of offshore marine orig in. The studied succession is grouped into two facies associations: (1) sub marine gravity-flow sandstones and conglomerates; and (2) suspension-deposi ted shales and mudstones. Traced down-dip, the submarine canyon fills displ ay a change from chaotic, conglomeratic and coarse sandy high-density turbi dites proximally to organised sandy turbidites basinwards. Farthest basinwa rd, a remarkable basin-floor fan with well-developed giant-scale foresets i s 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 subaeria l 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 P ermian 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 cany ons was contemporaneous with the filling of the canyons. A later drowning o f the updip catchment caused decrease or shut-off of sediment supply leadin g to deposition of shales and mudstones blanketing the sandy turbidites in the canyons. The tilt of the studied fault-block is interpreted to have cau sed simultaneous transgression below the fulcrum and regression, with bypas s or erosion, above the fulcrum. It is concluded, that the tilted fault-blo ck: setting has important consequences for sequence stratigraphic interpret ations 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.