K.O. Omosanya

Combination of geological mapping and geophysical surveys for surface-subsurface structures imaging in Mini-Campus and Methodist Ago-Iwoye NE Areas, Southwestern Nigeria

Electrical resistivity imaging and geological mapping were used to study the geology and delineate geologic structures at the Northeastern part of Ago Iwoye, Southwestern Nigeria. Two areas which include Mini-Campus Olabisi Onabanjo University (MCOOU) and Methodist Comprehensive High School (MCHS), Ago-Iwoye were studied. Electrical resistivity tomography (ERT) survey with Wenner configuration was done to reveal the horizontal and vertical variations in the subsurface geology; Azimuthal resistivity survey (ARS) was used to determine the orientation of the subsurface fractures. Both Wenner and Schlumberger array methods were employed. Measurement for each array were made about a fixed central point at an increment of 45° with respect to the reference axis using maximum electrode expansion of AB/2 at 130 m. Petrographic description and structural mapping were also carried out on three (3) outcrops nearest to the survey area in order to correlate the subsurface and surface geology and geologic structures. Petrographic study of rocks showed that the rock exposures are foliated biotite gneisses with mineralogical composition of quartz, plagioclase feldspar, biotite, and microcline. The orientation of joints (fractures) is dominantly ENE-WSW, E-W, and WNW-ESE. The electrical profiling revealed four geo-electric layers, clayey topsoil, clayey sand, sandy layer and weathered bedrock, the fresh bedrock/basement occurred at depth of 15.9 and 19.8 m subsurface at MCOOU and MCHS, respectively. The ARS showed that there is significant anisotropy between 30 and 65 m with fracture occurrence at 39 and 30 m at Methodist and Mini-campus; these fractures are oriented NW-SE, N-S and NE-SW, respectively. The coefficient of electrical anisotropy indicates that the intensity of fracturing at Methodist opens with depth at one part and later became constant with depth at other parts while that of Mini-campus is constant with depth. Fractures orientation suggests that surface structures are not deep seated and were produced by dissimilar tectonic event relative to subsurface fractures.

Strike-slip tectonics in the SW Barents Sea during North Atlantic rifting [Swaen Graben, Northern Norway]

This study uses high quality, three-dimensional [3-D] seismic data to investigate the occurrence of strike-slip faults in the Swaen Graben, SW Barents Sea. The Swaen Graben is divided into two principal sub-basins:- SSB1 and SSB2. The along-strike and along–dip displacement variations and scale relationships are analyzed for forty-two [42] faults. The displacement profiles for these faults are complex in the Swaen Graben, showing clear evidence for polycyclic fault growth and marked syn-sedimentary activity. The observed variations in the displacement profiles indicate complex along-strike segmentation, linkage and mechanical interactions at distinct structural levels. Along-dip displacement minima indicate fault reactivation by dip-linkage. Importantly, geometric evidence for strike-slip faulting in the Swaen graben includes the presence of extensional horsetail splay faults, positive flower structures and minor transfer faults. This study shows that the faults in the Swaen Graben developed under extensional regimes during Late Jurassic to Early Cretaceous rifting and were reactivated by regional stresses during the Late Cretaceous. The two principal strike-slip faults in the Swaen Graben reveal sinistral movement and are linked at a shallow depth by minor transfer faults at a relay zone. Our work further demonstrates the occurrence of Late Mesozoic strike-slip movements in the SW Barents Sea, which were induced by regional tectonics, halokinesis and fault block rotation. Importantly, strike-slip faulting in the region extends perhaps into the Cenozoic interacting with extension during the North Atlantic rifting.

Detecting Fracture Anisotropy Using Geological Mapping and Vertical Electrical Resistivity Survey

SUMMARY The two areas were studied and have shown different trend of fracture orientations at surface and in the subsurface. Area 1 showed overlap between fractures oriented at shallow subsurface reveal by electrical resistivity anisotropy polygon plot and that of fracture orientation on the surface rock exposures in the study area; suggest that fractures are penetrative and hard-linked. Thus, the fractures at both levels are produced by similar tectonic. Whereas, lack of overlap between fracture orientations at both surface and subsurface at Area 2 suggests that the surface fractures are not deep seated, and that fractures at both scales are thought to be produced by different tectonic events

Provenance of a Blocky Debris Flow Deposit in Mid-continental Slopes (Espírito Santo Basin, SE Brazil)

The provenance of an Oligocene-Miocene Mass Transport Deposit (MTD) in mid continental slope Espirito Santo Basin, SE Brazil was analysed using seismic facies and measurement of preserved blocks. Our method, applied to a high-quality 3D seismic volume from the Espirito Santo Basin (SE Brazil), provides information on the location of source areas of MTDs, their bulk composition and styles of disaggregation in relation to their transporting distances. Whenever blocks of strata are found, their orthogonal axes are measured to estimate parameters such as Maximum Projection Sphericity Index (MPSI), Oblate Prolate Index (OPI), and ratio of short and intermediate axes (ds/di). Eighty two (82) blocks mapped in the MTD have no preferred orientation: they have compact-bladed, bladed, elongate, very-bladed, very-platy and very-elongate shapes. Average block thickness, area coverage and volume are ~128.82m, 0.802 sq. km and ~0.196 cubic km with mean MPSI, OPI, flatness and elongation ratios of 0.398, 0.991, 0.19989 and 0.59861. In addition, the presence of equant blocks with c/a>0.4 and c/b>0.65 ratios are typical of proximal areas in MTDs, reflecting small transporting distances. Thus, the new method used in this research provides an alternative technique to kinematic indicators for determining the provenance of blocky debris flow deposits.

The Significance of Ramps and Flats of Mass Transport Deposits (MTD) as Kinematic Indicators

Ramps and flats of mass transport deposits are commonly studied on seismic profiles as kinematic indicators. In this study, ramps and flats on the margin of salt diapirs are mapped in a high quality 3D seismic volume from SE Brazil (Espirito Santo Basin). Our aim is to test how differently oriented ramps and flats at the basal shear surface of specific MTD interval can be used to infer the direction of mass flow. Statistical analysis of thickness variation of the mass transport deposit within depocentre created by ramps and flats was estimated along slope in order to elucidate the mode of emplacement of the associated MTD. Sliding at the plane of failure is enhanced by seafloor faulting and nearby halokinesis. Ramps significantly contribute to local changes in gradient at the basal shear surfaces, as they are linked to promontories (Type I) and paleo-fault scarps (Type II).