Rob L. Gawthorpe

Linked sequence stratigraphic and structural evolution of propagating normal faults

Two distinct phases in the structural evolution of normal faults can be identified in the Miocene Gulf of Suez rift: (1) an initial growth fold stage when the fault is a buried structure and (2) a subsequent surface faulting stage. During the growth fold stage, strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines. In contrast, once the fault breaks surface, strata form a divergent wedge, which is rotated and thickens into the fault. The two tectono-stratigraphic styles also occur contemporaneously along the length of a single fault segment. Growth folding characterizes deformation around the ends of fault segments where the fault is blind, whereas the center of fault segments are characterized by surface faulting. These observations suggest that marked along-strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting.

Fault-propagation folding in extensional settings: Examples of structural style and synrift sedimentary response from the Suez rift, Sinai, Egypt

Field data from the Oligocene–Miocene Gulf of Suez rift demonstrate that coeval growth faults, folds, and transfer zones exerted a major control on synrift stratigraphic sequence development. Growth folds in the Suez rift are related to steeply dipping normal faults that propagated upward, resulting in broad, upward-widening monoclines in overlying strata. Folding during fault propagation was accommodated by layer-parallel slip and detachment along mudstone horizons as well as by normal and rare reverse secondary faults that propagated away from the master fault. The eventual propagation of the master fault through to the surface left the steep limb of the monocline and most of the secondary faults in the hanging wall. This evolving structural style exerted a marked control on the geometry and stacking patterns of coeval synrift sediments. Synrift sediments display onlap and intraformational unconformities toward the growth monoclines and buried faults, whereas they diverge into broadly synclinal expanded sections away from the growth monocline. Continued movement across buried faults resulted in the progressive rotation of the monoclinal limb and associated synrift sediments, each successively younger sequence dipping basinward at a shallower angle than the previous one. The resulting synrift geometries differ significantly from stratal geometries normally anticipated adjacent to normal faults. Along-strike variations in facies stacking patterns are also commonly associated with decreasing displacement across faults and associated folds toward low-relief transfer zones. Data from other rift basins indicate that fault-propagation folds are not unique to the Gulf of Suez.

Sequence stratigraphy of (?)Pliocene-Quaternary synrift, Gilbert-type fan deltas, northern Peloponnesos, Greece

Abstract Seismic-scale cliff section exposures of exhumed (?)Pliocene-Quaternary, footwall-derived, Gilbert-type fan deltas are located along the faulted southern margin of the Gulf of Corinth rift. The location of these Gilbert-type fan deltas is controlled by antecedent drainage systems and relay zones. Each Gilbert-type fan delta body is made of depositional sequences and systems tracts that can be interpreted within a sequence stratigraphic framework. Sequence architectures were developed against a backdrop of rapid basin deepening due to tectonic subsidence. Sequences are bounded by composite downlap/truncation and toplap surfaces and are dominated by highstand and shelf margin systems tracts. The architecture of systems tracts and key stratal surfaces are interpreted in terms of hangingwall subsidence, sea/lake level changes, hinterland uplift and climatically induced variations in surface runoff. Gross Gilbert-type fan delta architecture is interpreted as the response to a background relative base level rise which was presumably dominated by rates of hangingwall subsidence that generally exceeded other base level control mechanisms.

Ground penetrating radar: application to sandbody geometry and heterogeneity studies

Abstract Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing. Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.

Discrete element modelling of contractional fault-propagation folding above rigid basement fault blocks

Many studies have shown that discrete (blind) faults at depth are commonly linked to more distributed deformation, in particular folding, at higher levels. One category of fault-related folds, forced folds, is common where there is a distinct mechanical contrast between faulted basement and sedimentary cover. Outcrop, numerical and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation (monoclines) above discrete faults at depth. With increasing displacement the folds are often cut by faults as they propagate upwards into the cover. While the trishear kinematic model of fault-propagation folding appears to approximately represent the geometric development of such structures, comparatively little is known of the mechanical controls on their development. Here we present a 2D discrete element model of sedimentary cover deformation above a contractional fault in rigid basement. The elements consist of a series of soft spheres that obey Newtons equations of motion and initially interact with elastic forces under the influence of gravity. Particles are bonded until the separation between them exceeds a defined breaking strain at which time the bond breaks, simulated by the transition from repulsive–attractive forces to solely repulsive forces. The model is used to investigate the influence of basement fault dip and sedimentary cover strength on the geometry of the folds developed and the rate of fault propagation. In all cases an upward widening monocline occurs above the basement fault. We find that shallow basement fault dips produce homogenous thickening of the monocline limb while steeper dips produce contemporaneous thinning and thickening within the monocline. Thinning and thickening within the monocline are accommodated by a combination of small-scale faulting and folding. With decreasing cover strength, the zone of deformation becomes wider, localization does not occur on a single fault and fold geometries resemble trishear fold profiles with low propagation to slip ratios (p/s∼1). In contrast, a stronger cover produces a narrower zone of deformation, localization on a single fault and more rapid fault propagation (similar to trishear fold profiles where p/s∼2–3). The fault propagates into the cover at approximately the same angle as the basement fault. The model reproduces well many of the features observed in analogue modelling and reported from outcrop and seismic studies.

Inversion tectonics of the Variscan foreland of the British Isles

Late Westphalian inversion structures on the Variscan foreland display wide variations in orientation that are the product of the underlying basement grain. The orientation of the major basement lineaments in relation to the direction of maximum shortening also determined the severity of deformation of the inverted basins. NE–SW-trending faults were oriented roughly perpendicular to the NW–SE to NNW–SSE direction of maximum shortening determined from thrust transport directions in the Variscan orogen to the south. As a consequence, NE–SW-trending basins in the Variscan foreland have been strongly inverted, the Silesian post-rift fill has commonly been expelled from the basins and the syn-rift sediments have been deformed by chevron folds (Dublin and Bowland Basins). In contrast, the N–S- and NW–SE-trending faults were oriented more obliquely to the direction of maximum shortening and, as a consequence, display a significant component of oblique slip. This oblique slip component is manifested as en-echelon periclines and flower structures. In general, the N–S- and NW–SE-trending basins were less strongly inverted and the Silesian post-rift fill has been retained.

Growth and linkage of the East Tanka fault zone, Suez rift: structural style and syn-rift stratigraphic response

Abstract: An integrated tectono-stratigraphic analysis of the East Tanka fault zone, Suez rift, indicates fault growth by linkage of initially isolated fault segments that is consistent with fault growth models based on displacement–length (D-L) scaling laws. During the initial 2.4 Ma of rifting, the East Tanka fault zone was composed of two en-echelon fault segments c.1–1.5 km long, separated by a hanging-wall intrabasin high that controlled the geometry of depocentres filled with continental deposits. Alluvial fan conglomerates were fed through the region between the two fault segments, and form a discrete coarse-grained body, preserved in the immediate hanging wall of the fault zone. Subsequent stratigraphic patterns indicate that the two faults hard-linked to form a single fault zone c.3.5 km long. Hard linkage of the segments resulted in migration of the zone of maximum displacement and subsidence into the zone of linkage. Uplift due to the migration of activity caused modification of drainage in the footwall of the fault zone that terminated the growth of the alluvial fan. This study demonstrates the need to integrate structural and stratigraphic data when attempting to reconstruct the temporal and spatial evolution of normal fault zones. Additionally, the fault dynamics illustrated have implications for tectono-stratigraphic models of rift basins, and syn-rift stratigraphic evolution.

Mudstone Lithofacies in the Kimmeridge Clay Formation, Wessex Basin, Southern England: Implications for the Origin and Controls of the Distribution of Mudstones

ABSTRACT Lithofacies analysis has been undertaken on a suite of apparently monotonous organic-rich mudstones from the Upper Jurassic Kimmeridge Clay Formation (KCF) in the Wessex Basin, southern England. Using a combination of hand-specimen description, whole-rock geochemistry, optical petrography, and electron optical petrography, five lithofacies have been identified. These are: clay-rich mudstones, silt-rich mudstones, nannoplankton-rich mudstones, laminated mudstones, and concretionary carbonates. These facies can be distinguished from one another on the basis of physical sedimentary structures and the relative proportions of allochthonous, autochthonous, and diagenetic components in each. All five of the KCF lithofacies studied were deposited in a marine shelf environment below fair-weather wave base but in relatively shallow water close to storm wave base. Deposition occurred in regions where sand and other coarse clastic allochthonous debris were not being supplied and the bottom waters varied from oxic to anoxic. The main factors influencing the development of the five facies were: distance from source area, primary productivity, bottom-water anoxia, absolute bathymetry, clastic dilution, and local sediment accumulation rates. Specifically, the clay-rich mudstones were deposited in regions farthest down the sediment-transport paths, commonly in areas by-passed by the main sediment supply and beneath surface waters of relatively low primary productivity. In contrast, the silt-rich mudstones were deposited in more proximal areas, closer to the source of the sediment, in regions where surface productivity and sediment accumulation rates were much higher. The nannoplankton-rich mudstones accumulated beneath regions where productivity in the surface water layers was fairly high (dominated by coccolithophoroid production) and the supply of clastic sediment was insufficient to significantly dilute the autochthonous fraction of the sediment. Unlike the other facies, laminated mudstones were deposited in areas where the bottom waters were anoxic, surface productivity was enhanced, and local sediment accumulation rates were high. The concretionary carbonates were precipitated by diagenetic processes in areas where sediment accumulation rates were very low.These facies descriptions have enabled a detailed sedimentological study of the Kimmeridge Clay Formation to relate depositional and diagenetic processes directly to the facies present, thereby allowing classical facies analyses to be undertaken on apparently homogeneous mudstone successions.

Cenozoic tectono-sedimentary evolution of the western Iberian margin

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN) and seismic reflection data are used to characterise the Cenozoic tectono-sedimentary evolution of the Porto, Lisbon and Alentejo continental margins, offshore Portugal, where the presence of submarine fans, slope gullies, submarine canyons and seamounts was confirmed. Sediment drifts, some of probable contouritic origin, were recognised over the upper continental slope (500–1000 m) and surrounding the Vigo seamount. Seven echo types offshore Porto (types IA-1, IB, IB-2, IC-2, IIA, IIB, IIIA) and five echo types offshore Lisbon (types IB, IIA, IIB, IB-2, IIIA) were identified on 3.5-kHz profiler data. In addition, 11 Meso–Cenozoic seismic units off Lisbon and Alentejo, plus three post-Turonian seismic packages offshore Porto were interpreted and dated by well, dredge and DSDP/ODP stratigraphic data. During the Cenozoic, the tectono-sedimentary evolution of the studied areas depended on their position in relation to the locus of compression in Iberia. Accommodation space and sediment pathways varied in relation to distinct pulses of uplift or subsidence occurring at different times in the three studied regions. The Porto and Lisbon margins record extensional collapse respectively after the early Eocene and early Chattian, interrupted by short episodes of uplift related to distinct tectonic phases. As a result, gravity flows dictated deposition on these margins in most part of the Cenozoic. Seamounts and halokinetic structures controlled deposition on the Porto margin by inducing topographic barriers to the westward progradation of slope-derived sediment. The relative proximity of the Alentejo margin to the Azores–Gibraltar Fracture Zone resulted in folding and exposure during the middle Oligocene, but subsidence after the early Chattian generated a palaeoslope buried under Neogene units. Oligocene and Burdigalian canyon incision offshore Alentejo preceded the emplacement of modern channels during the Pliocene. These presently transport sediment derived from the shelf and major onshore drainage catchments into abyssal areas.

The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobaca sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobaca sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems.