Late Carboniferous-Early Permian Episodic Tectonics as Recorded in Stratigraphic Sequences

Benoit Beauchamp

The Bashkirian–Kungurian succession of the Sverdrup Basin, Canadian Arctic is a second-order sequence bounded above and below by major unconformities. This sequence recorded peripheral shelf- and shelf-margin sedimentation (Hvitland Peninsula, Nansen, Raanes and Great Bear Cape formations) and slope to basinal deep-water deposition (Otto Fiord, Hare Fiord and Trappers Cove formations). That sequence comprises four third-order sequences \of respectively of Bashkirian–Kasimovian, Gzhelian–Asselian, Sakmarian–Artinskian and Kungurian age. They are bounded by regional subaerial unconformities or ravinement surfaces in the proximal areas and maximum regressive surfaces (MRS) in the distal areas — MRS coincide with a change from a shallowing-upward trend below to a deepening-upward trend above. All four sequences show a similar vertical pattern indicative of an episodic tectonic driver. Basin-wide flooding and retrogradation of the shelf-margin is recorded in the early part of TSTs, contemporaneous with fault-controlled differential subsidence and development of local, and at times restricted, sub-basins. Cessation of faulting was accompanied by basin-wide crustal collapse that led to rapid base level rise. This led to the rapid vertical aggradation of reef mounds and banks that kept pace with rising relative sea level for some time, before eventually being drowned. In contrast, RSTs recorded widespread basinward progradation of biogenic factories and contiguous slopes at a time of tectonic quiescence. The most significant of the episodic collapse events occurred in the Early Moscovian. It marks the end of orthogonal rifting (tectonic transport at 90o to basin axis) in the Sverdrup Basin, cessation of basinal evaporite deposition (Nassichuk and Davies, 1980) and full connection with the global ocean (Embry and Beauchamp, 2008). Keep-up reefs and banks as high as 600 m developed in response to the rapid increase in accommodation. By mid Moscovian, these structures had been drowned and were now towering over the basin floor in a deep and dark environment. With time however, these carbonate bodies started shedding debris in the surrounding deep-water environment, leading to near-vertical erosional escarpments. It is only much later that slope sediments advancing with the prograding shelf caught up with the drowned structures, onlapped their flanks and eroded edges, and ultimately buried them beneath the RST. The above pattern was repeated four times during the Pennsylvanian–Early Permian interval. The only thing that changed from sequence to sequence was the height above the sea floor of keep-up reef mounds and their geographic footprint. These aspects reflect different rates of accommodation increase and the varying ability of warm- and cool-water carbonate factories to keep-up with that increase.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013