Basin Research - Volume 1, Issue 2, 1988

Abstract The evolution of a passive margin to a foreland basin is generally assumed to entail early load‐induced up warping of the stable continental platform followed by foreland subsidence. This relatively straightforward elastic response of the continental platform, however, may be complicated if the colliding passive margin is irregular in outline. In a tectonic scenario in which an irregular margin is migrating toward a trench (A‐subduction), those areas of the margin which project seaward, the continental promontories, would be the first to ‘feel’ the approaching thrust terrane by flexing upward and eroding to form shelf unconformities. Those parts of the continental margin that are convex to the craton, the continental re‐entrants, however, would remain subsiding depocentres unaffected by load‐induced uplift at the promontories. Careful analysis of the geographic distribution of shelf unconformities in orogenic belts, then, may help to reveal the pre‐deformation morphology of the passive continental margin. An example of this may be found in the early phases of Ordovician foreland basin development in the central Appalachian orogen. Here, the shelf unconformities are most pronounced (greatest erosional relief) at the inferred Virginia and New York continental promontories. An adjacent inferred continental re‐entrant, the Pennsylvania re‐entrant, is characterized by an uninterrupted Ordovician sequence suggesting that the area of the proto‐North American platform, represented by this segment of the orogen, remained a depocentre during uplift in adjacent areas of the continental margin.

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 100^oC, 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.

Abstract The Hebridean basins are part of a compartmentalized half‐graben developed in the hanging wall of the partially reactivated Outer Isles Fault. The importance of geological inheritance in the development of these basins can be demonstrated clearly using the widespread exposure of metamorphic basement around the basin margins. The basement structures have been analysed using thematically mapped Landsat images in conjunction with selective field studies. Results of such studies have been integrated with maps generated from the interpretation of offshore multichannel seismic reflection profiles to produce an architectural framework for basin development.

It can be demonstrated that the principal basement faults originated in the early Proterozoic as mid‐crustal shear zones and that they have subsequently been partially reactivated during post‐Caledonian basin development beginning in the Carboniferous and probably also during an earlier period of basin development in the late Proterozoic (the Torridonian). It is the geometry of the pre‐existing basement structures that has controlled the three‐dimensional shape of the sedimentary basins and the spatial and temporal distribution of the basin fill.