Long runout distance of a block breccia density flow along a half-graben axis: latest Jurassic rift climax in East Greenland

A basinally-isolated breccia–pebbly sandstone couplet occurs in the axial part of the latest Jurassic rift-climax halfgraben in Wollaston Forland, East Greenland. Synrift breccias are otherwise restricted to a narrow zone along the scarp of the basin-margin Dombjerg Fault. Towards the north the Dombjerg Fault sidesteps en echelon towards the east where it continues in the Thomsen Land Fault. The breccia part of the couplet was transported by a noncohesive debris flow and the pebbly sandstone by a high-density turbidity. The couplet is located about 15 km east of the Dombjerg scarp and flowed southwards along the halfgraben axis. It is interpreted to have been derived from the southern end of the Thomsen Land Fault. This gives a run-out distance of at least 25 km. Other isolated breccias have been reported towards the south along the halfgraben axis, giving a run-out distance of up to 40 km along the roughly horizontal halfgraben axis. The angularity of the clasts is remarkable and reflects derivation directly from the fault scarp during a single catastrophic event. The occurrence of breccias thus cannot be taken as evidence of short transport and deposition at the foot of a scarp or steep slope.