ALBERT

Description: The deep-water subsurface offshore Angola is characterized by many linear, high-gradient submarine channels typically only tens of metres wide and deep. Larger channel systems (C. 3-5 km wide, 〉300 m deep) with highly sinuous channels at their bases are also common, although they appear to have evolved from small, linear, high gradient systems. Generally, such small linear channels become enlarged by sediment gravity flows and therefore are rarely preserved except in examples where avulsion occurs. These small linear systems are often associated with relatively continuous levees 1-3 km wide flanking the channel. Results presented here suggest that small, linear channels evolve from erosional lineations on the slope generated by large, infrequent turbidity currents. Results also indicate that linear, high-gradient channels also exhibit the most significant and distinctive geometry changes where there is complex topography, such as near salt structures. Sedimentary bodies associated with linear, high-gradient channels often deposit within slope depressions as discreet J- or S-shaped structures in plan view. The dominant control on these sedimentary bodies is interpreted to be seafloor gradient and topography. This paper examines a number of these relatively young channels in terms of their geometry, gradient, levee development and seismic facies. The results improve our ability to predict subsurface channel geometries and recognise key evolutionary trends.

Description: Forced folds typically develop above the tips of propagating normal faults in rifts that contain thick, prerift salt or mudstone sequences. This structural style is associated with the deposition of wedge-shaped synrift deposits that thin and onlap toward monoclinal growth folds overlying the vertically restricted fault tips. Subtle stratigraphic traps may develop on the flanks of these folds although, because of limited seismic resolution and sparse well data, the architecture, thickness, and distribution of these early synrift reservoirs are difficult to predict. To improve our understanding of early synrift reservoir development on the flanks of forced folds, we focus on seismic-scale outcrop analogs along the Hadahid fault system, Suez rift, Egypt. Our data indicate that forced folding dominated during early rifting and that the onset of folding was diachronous along strike. Fluvial systems incised the rotating monocline limbs, leading to the formation of valley-like erosional relief along the base synrift unconformity. Reservoir-prone fluvial facies are only locally developed along the forced-fold flank, with their distribution related to the degree of sediment bypass downdip into the adjacent basin. Early synrift relief not filled by fluvial strata was backfilled by transgressive, tidally influenced, reservoir-prone facies, with carbonates being locally developed in areas of low clastic sediment supply. Further extension and fault-tip propagation led to amplification of the forced folds, and deposition of shallow marine-to-shelf parasequences that became thinner toward the growing folds. Although displaying greater strike continuity than the underlying fluvial or tidal reservoirs, shoreface sandstone reservoirs amalgamate onto the flanks of the forced folds and may be absent toward the fold crest. This seismic-scale outcrop analog helps us better understand the subseismic stratigraphic architecture and facies distributions of early synrift reservoirs on the flanks of extensional forced folds. Observations from this and other well-exposed outcrop analogs should help reduce subsurface uncertainty and risk when exploring for hitherto under-explored, subtle, early synrift stratigraphic traps.

Description: Fault-segment boundaries initiate, evolve and die as a result of the propagation, interaction and linkage of normal faults during crustal extension. However, little is known about the distribution, evolution and controls on the development of relay ramps, which are the key structures developed at synthetic segment boundaries. In this study, we use a series of scaled physical models (wet clay) to investigate the distribution and evolution of fault-segment boundaries within an evolving normal-fault population during orthogonal extension. From the models, we can establish a simple geometrical classification for segment boundaries, analyse their spatial and temporal evolution, and identify key factors that influence their variability. Development of overlapping fault tips is a prerequisite for fault growth via segment linkage. Synthetic segment boundaries are the most common segment boundary type developed in the models. The proportion of synthetic segment boundaries in the total fault population increases with increasing strain, whereas conjugate (antithetic) segment boundaries are very rare. Hanging-wall-breached relay ramps are the most common type (〉70%) of breached-segment boundary, followed by footwall-breached relay ramps (〈25%). Transfer faults are uncommon in our models. The type of breached segment boundary that develops cannot be predicted based on fault overlap to fault spacing aspect ratio alone. Instead, we show that fault linkage occurs in a range of styles across a wide range of fault overlap to fault spacing ratios (1:1–7:1). Furthermore, we show that fault spacing is constrained by stress-reduction shadows at the time of fault nucleation, whereas fault overlap changes during fault growth and interaction. Our study thus shows that scaled physical models are a powerful tool to assess the style, distribution and controls on the evolution of synthetic segment boundaries developing in rifts. Predictions from these models must now be assessed with data from natural examples exposed in the field or imaged in the subsurface.

Description: Recent developments in workflows and techniques for the integration and analysis of terrestrial LiDAR (Light Detection And Ranging) and conventional outcrop datasets are demonstrated through three case studies. The first study shows the power of three-dimensional (3D) data visualization, in association with an innovative surface-modelling technique, for establishing large-scale 3D stratigraphical frameworks. The second presents an approach to derive reliable geometrical data on sediment-body geometries, whereas the third presents a new technique to quantify the proportions, distributions and variability of sedimentary facies directly from outcrop. In combination, these techniques provide essential conditioning data for geocellular and stochastic facies modelling. Built upon robust, reproducible and quantitative data, the resultant models combine realistic 3D geological architectures with sufficient quantities of reliable numerical data required for stable statistical analysis and establishing uncertainty. Together this new information provides detailed understanding and quantification of the 3D complexity of the sedimentary systems in question, thus offering insights of value for predicting the subsurface anatomy of analogous petroleum systems. As such, use of LiDAR, when combined with conventional field geology, offers a powerful tool for quantitative outcrop analysis, tightly constraining 3D structural and stratigraphical interpretations, and effectively increasing the statistical significance of outcrop analogues for reservoir characterization.

Description: Continental rift deposits contain critical clues concerning the evolution of extensional tectonics, yet such evidence is often obscure due to poor geochronology, burial by younger deposits, or later tectonic overprinting. We revisit Corinth rift development, which began as distributed extension created synrift depocenters with rivers flowing into shallow (〈50 m) lakes. Subsequent focused deformation initiated a "Great Deepening" event, evidenced by fan deltas prograding into 300–600-m-deep water. A chronology is provided for the event from 40 Ar/ 39 Ar dating of the Xylocastro ash by single-crystal CO 2 laser fusion, yielding a precise age of 2.550 ± 0.007 Ma (1, full error propagation). Sedimentological data indicate that the ash-bearing sediments were deposited as turbidites and hemipelagites on sublacustrine fans fed from the Mavro fan delta at the faulted south-central rift margin. The ash age and turbidite provenance data enable stratigraphic constraints for an estimate of central rift climax occurring between 3.2 and 3.0 Ma. This is some 0.8–1.0 m.y. earlier than radioisotopic- and magnetostratigraphic-constrained estimates for the eastern Corinth rift. Central rift climax was probably triggered by initial counterclockwise rotation of the Peloponnesus block with respect to central Greece. The rotation pole of this block subsequently migrated to its present position as rift climax moved eastward in an "unzipping" action, with the southern active margin also migrating northward. These events are unlikely to be due to local or regional fault kinematics, but rather to the consequences of deep-seated interactions between the rapidly southward-moving Aegean continental forearc and the slowly northward-subducting African oceanic plate. A possible scenario involves forearc "pushback" with décollement on a low-angle subducting lower plate. This causes acceleration and counterclockwise rotation of Peloponnesus with respect to central Greece and strain localization across the boundary; the Corinth rift.

Description: New, high-resolution lithofacies data from hanging-wall Miocene synrift (Rudeis Formation) exposures of the eastern Suez Rift margin, Egypt, reveal a submarine slope depositional system dominated by coarse-grained (pebble), heterogeneous, lenticular beds, formed by coalescing turbidity currents, slumps, and debris flows deposited on deforming submarine substrate. Flows include prerift clasts and contemporaneous shallow-marine fossil fragments originating from an uplifted eastern hinterland. Multiple terrestrial drainages debouched onto faulted offshore slopes or fed small fan deltas on narrow shelves (〈0.5 km width). Steep, subaerial rift-flank topography also shed rock avalanche and landslide material offshore. The 〉360 m Rudeis Formation is divided into stratigraphic units R1 and R2, which exhibit upward-coarsening and unordered vertical motifs. Ongoing faulting influenced synrift deposition by controlling the locus of subsidence and gravity base level. Mesoscale faults became inactive during Rudeis Formation times, with strain localized on the large rift border fault system, leading to a wider basin with time. We compare 16 subaqueous rift-margin basin fills from various tectonic and geographic settings and show they generally represent proximal gravity-flow deposits dominated by nongraded beds. We find little commonality in vertical grain-size trends, highlighting the diversity of stratal architectures. Most basin fills show an inverse relationship between maximum clast size and shelf width. We propose a new model to capture the spectrum of sedimentary responses to rifting within rift-margin basins, varying as a function of shelf width, slope gradient, maximum grain size, and textural maturity. Rudeis Formation strata at north Wadi Baba represent a particularly coarse-grained end member, deposited on steep slopes, with a narrow shelf.