Geology of the Labrador Shelf

The Cretaceous-Tertiary sequences penetrated by drilling operations on the Labrador Shelf have been divided into four formations for which formal names are proposed in this paper. The BerriasianValanginian Alexis Formation, up to 260 m thick, consists of subaerially extruded basalt flows and associated sediments which overlie Precambrian basement containing outliers of Paleozoic rocks. The Alexis Formation is overlain by the Bjarni Formation, a series of predominantly continental coarse clastic rocks up to 1130 m thick of Hauterivian to Early Neocomian age. The unconformably overlying Cartwright Formation, up to 1200 m thick, consists of shallow water marine turbiditic shales of Turonian to Early Eocene age. Locally, paralic to shallow water marine sand members occur in the Cartwright Formation. The conformably overlying Early Eocene and younger Saglek Formation, over 1700 m thick, comprises a series of deeper water marine turbiditic mudstones overstepping the older formations. Within the Saglek Formation, shallower water and locally developed paralic sediments were deposited during the Late Eocene and Oligocene, a period of reduced basin subsidence. Structural sections across the Labrador margin show that two superimposed basins are present. The earlier, forming in the Berriasian, is bound by en echelon faults, trends approximately parallel to the present day Labrador coast and terminates on the northern flank of the Cartwright Arch. This inshore basin is overstepped by Early Eocene and younger sediments which form a simple clastic wedge on the margin of the present day Labrador Sea, a basin apparently formed in the Late Cretaceous or Paleocene. While the Labrador Sea is presumed by others to have resulted from seafloor spreading, this theory does not satisfactorily explain some important geological features. An alternative origin for the Labrador Sea proposed by the author is the collapse of a minor undation in lowermost Cretaceous times followed by the collapse of a major undation in late Cretaceous times. Three gas and condensate discoveries on the Labrador Shelf show similarities in structure and source though the reservoirs are different. In each discovery, closure is associated with upthrown faulted basement blocks within the inshore basin and the hydrocarbons appear to originate from the Cartwright Formation. Insufficient data exist to assess the hydrocarbon potential of the Labrador Shelf but there are indications the area could prove to be an important hydrocarbon province.