Robert J. Hooper

The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa

Abstract The Niger Delta has a distinctive structural and stratigraphic zonation. Regional and counter-regional growth faults, developed in an outer-shelf and upper-slope setting, are linked, via a translational zone containing shale diapirs, to a contractional zone defined by a fold-thrust belt that developed in a toe-of-slope setting. Structural and depositional systems have migrated with the progradation of the delta. A paleo fold belt is buried under the modern upper/middle slope. The structural system in this paleo fold belt is complex and comprises a series of en echelon thrust-cored folds and associated ponded slope-basins, shale diapirs, and extensional growth faults. Analysis of the growth sections filling the ponded slope-basins provides a record of how this accommodation was created and subsequently filled and how the individual structural elements interact to create and modify the available space. The depositional systems initially exploit primary accommodation on the slope created by structural movement—the synchronous growth of the fold, the extensional faults and the shale diapir. As the pond is progressively filled, the previously deposited strata modify the accommodation and subsequent depositional systems compensate accordingly.

Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA

The southern Appalachian Pine Mountain window exposes 1.1 Ga Grenvillian basement and its metasedimentary Paleozoic(?) cover through the allochthonous Inner Piedmont. The issue of whether the crustal block inside the window was either transported above the master Appalachian (late Alleghanian) decollement or is an autochthonous block that was overridden by the decollement has been debated for some time. New detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from Gondwana but docked with Laurentia before the Alleghanian event. Reprocessed deep seismic reflection data from west-central Georgia (pr- and poststack noise reduction, amplitude variation analysis, and prestack depth migration) indicate that a significant band of subhorizontal reflections occurs almost continuously beneath the window collinear with the originally recognized decollement reflections north of the window. A marked variation in the decollement image, from strong and coherent north of the window to more diffuse directly beneath the window, is likely a partial consequence of the different geology between the Inner Piedmont and the window. The more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover (i.e., signal-to-noise ratio). Two alternative tectonic models for the Pine Mountain window can partially account for the observed variation in the decollement reflectivity. (1) The Pine Mountain block could be truncated below by a relatively smooth continuation of the decollement. The window would thus expose an allochthonous basement duplex or hors-block thrust upward from the south along the Late Proterozoic rifted continental margin. (2) The window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window. Either model is viable if only reflector geometry is considered; model (1) is favored if both geometry and kinematics of Blue Ridge–Piedmont thrust sheet emplacement are incorporated. In either model, the southern margin of the window merges to the west with the Iapetan early Alleghanian Central Piedmont suture, which juxtaposes North American–affinity Piedmont rocks to the north and exotic Panafrican rocks of the Carolina (Avalon) terrane to the south. Immediately south of the window, this suture dips southward and merges in the lower crust with the late Alleghanian suture joining the Appalachians with Gondwana.

ABSTRACT: On how the interaction of tectonics and sedimentation within the Niger Delta has created a complete petroleum system

The Niger Delta comprises a highly integrated tectonostratigraphic system where all elements of the deltaic system work in concert to form the source, reservoirs, seals, migration pathways and traps that are key to a working petroleum system. The delta was initially confined within the Benue Trough. Sediment mass was low and relatively few structures were developed. In this setting much of the sediment load bypassed the slope and was deposited on the basin floor. These strata were subsequently buried by the advancing delta front. Continued sediment loading during progradation of the delta created gravitational instability that in turn lead to the development of linked systems of extensional, diapiric and contractional structures. These structures modified accommodation on the slope and the sedimentary systems compensated accordingly. Deformation of the delta caused restricted bypass of the sediment mass to the basin floor, creating the slope centered depositional profile observed today. The increasing sediment load had another important effect -- it generated hydrocarbons, which in turn helped create the structural form. The generation of hydrocarbons within shales of the lower deltaic sequences created overpressured conditions over a wide region. This facilitated the generation of the huge detachment surface that underlies the linked extensional and contractional regions. To complete the linkage, the structural and stratigraphic systems themselves not only interact to create traps and seals within the delta system but also serve as conduits for hydrocarbon migration. The Niger Delta is thus a complete petroleum system in which all elements are contained within the deltaic succession.