Basin architecture and thermal maturation in the strike‐slip Deer Lake Basin, Carboniferous of Newfoundland

Abstract

Abstract The Deer Lake Basin is an entirely non‐marine basin associated with the Cabot fault zone. Structural and stratigraphic evidence strongly suggest dextral strike‐slip movements along the fault zone during Tournaisian‐Visean time. Two elongated, end‐on structural blocks (probable positive flower structures) contain fold axes and second‐order faults oriented obliquely to fault traces bounding the blocks, in a manner implying dextral movements. In one part of the basin, the stratigraphic thickness of a long homoclinal section of later basin‐fill sediment (Deer Lake Group) greatly exceeds the suggested depth to basement based on gravity measurements, a situation common to strike‐slip basins. Formations representing basin fill can be arranged into megasequences (from oldest to youngest: Anguille Group, Wetstone Point and Wigwam Brook Formations, Deer Lake Group, Howley Formation) corresponding to lateral growth stages of the basin.Gravity, magnetic, and seismic data show that depths to basement on either side of the end‐on flower structures are comparable, so that the youngest strata in the basin (Howley Formation) are not underlain by earlier basin fill. These geophysical data, therefore, corroborate the geological conclusion of onlapping stratigraphic relations. The geophysical data suggest participation of basement in Carboniferous gravity faulting and show the location of the subsurface extension of the Taylors Brook Fault in the western part of the Deer Lake Basin.Thermal maturation of the Anguille and Deer Lake Groups, as measured by vitrinite reflectance, clay mineral assemblages, illite crystallinity, and Rock‐Eval pyrolysis, indicate a much higher level of maturation for the Anguille than for the Deer Lake Group. Palaeotemperatures for the Anguille and Deer Lake Groups are estimated to be around 200 and 100oC, respectively, suggesting that Anguille Group rocks are overmature whereas Deer Lake Group strata are within the oil‐generating window. Onlapping stratigraphic relations and areally homogeneous time/temperature effects, however, have created a situation in which the Deer Lake Group and Howley Formation have similar maturation levels.