John R. Underhill

Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence

Although the ‘Mid-Cimmerian event’ or unconformity has been recognized over much of Europe, its exact stratigraphic relations and causal mechanism have remained unclear. Application of a genetic sequence stratigraphic approach (using 17 marine condensed sections and maximum flooding surfaces) to Jurassic sequences across NW Europe allows the stratigraphic succession to be subdivided into a series of time-slices (genetic stratigraphic sequences) and allows the true nature of the unconformity to be determined. They indicate that the main event’s correlative conformity falls in the Aalenian near the break between the opalinum and murchisonae ammonite biochronozones. Further study of the associated spatial and temporal variation indicates that systematic truncation of stratigraphy occurred throughout the North Sea domain (the oldest stratigraphies subcrop in areas adjacent to the triple junction) with subsequent progressive onlap towards the same area. When integrated with igneous evidence, these observations are interpreted to confirm regional (Toarcian–Aalenian) domal uplift, resulting from the impingement of a broad-based (> 1250 km diameter), transient plume head or ‘blob’ at the base of the lithosphere. Progressive pre-rift, Aalenian–early Bathonian marine onlap records differential subsidence in response to the initial deflation of the dome while central regions may have continued to rise. Subsequent subsidence post-dated Bathonian–Callovian volcanism but still pre-dated the timing of most significant (Kimmeridgian–Volgian) rifting. Such temporal relations demonstrate that North Sea volcanism is inconsistent with a classic ‘passive’ rift model. Instead, it seems more appropriate to equate Mid–Late Jurassic North Sea development with an ‘active’ rift model following mantle-driven thermal doming. Integration of sedimentation patterns with basin development suggests that the early Toarcian–early Kimmeridgian succession records a long-term, second-order regressive–transgressive episode related to regional tectonism. Comparison with the current chart of coastal onlap and global sea-level change highlights the correlation of the Intra-Aalenian event with one of the most significant regressions (the 177 Ma event separating the Absaroka and Zuni first-order megacycles). The knowledge that this part of the curve appears to be based exclusively on sections from Dorset and Yorkshire, within and adjacent to the region affected by regional doming, suggests that there remains a need to test this part of the chart using sections from outside the uplifted area and emphasizes the impracticality of using just two relatively closely spaced sections in trying to define a truly global signal. Clearly, sections should be taken from several areas and preferably from different plates as tectonically uncoupled as possible. Until then, the worry will remain that regional tectonic events could overprint any global signal and be erroneously interpreted as abrupt changes in global eustasy. The fact that doubts such as these can be cast upon parts of the eustatic sea-level chart suggests that it is still someway off being a valid global standard with true predictive capabilities.

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.

A mechanism to explain rift-basin subsidence and stratigraphic patterns through fault-array evolution

Rift-basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate. The physical basis for this transition is not well understood, although an increase in extension rate is commonly implied. Here, a numerical fault-growth model is used to investigate the influence of segment linkage on fault-displacement-rate patterns along an evolving normal fault array. The linkage process we describe is controlled by a stress feedback mechanism, which leads to enhanced growth of optimally positioned faults. Model results indicate that, even with constant extension rates, slow displacement rates prevail during an initial phase of distributed extension, followed by an increase in displacement rates as strain becomes localized on linked fault arrays. This is due to the dynamics of fault interactions rather than mechanical weakening. Comparison of model simulations with rift-basin subsidence and stratigraphic patterns in the Gulf of Suez and North Sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault-zone evolution, without invoking a change in regional extension rates.

Late Cenozoic deformation of the Hellenide foreland, western Greece

The late Cenozoic structural evolution of western Greece involved the shortening of Mesozoic passive continental-margin and Cenozoic foreland basin sequences in a foreland-propagating, fold-and-thrust system and superimposed neotectonic deformation related to the development of an active island arc. Although the Pre-Apulian zone has been regarded as the undeformed, western foreland to the thrust belt, three-dimensional exposures in the Ionian islands of Kephalonia and Zakinthos afford evidence of significant late Neogene and Quaternary shortening, including thrusting on reactivated normal faults. Time relations and fault kinematics show that these structures are compatible with the westward migration of the locus of Hellenide thrusting beyond its previously recognized limit and outer-arc compression related to active subduction. The presence of the Pre-Apulian zone in the west contrasts with other Aegean foreland areas, where no such intermediate block lay between the thrust pile and old (?Mesozoic) oceanic crust. Consequently, the Pre-Apulian block took up continued Hellenide shortening in the late Miocene while oceanic crust was being overridden (subducted) in southern parts of the arc to the east. The Pre-Apulian zone subsequently overrode the ocean-floored Ionian Sea during the late Neogene. The confusion in determining driving mechanisms for late Cenozoic deformation in western Greece and the apparent conflict between evidence for the age of initial subduction in different parts of the arc are explained by this model, which is consistent with regional considerations, including structuration in adjacent areas and important paleomagnetic rotations in the arc.

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.

Biostratigraphic calibration of genetic stratigraphic sequences in the Jurassic–lowermost Cretaceous (Hettangian to Ryazanian) of the North Sea and adjacent areas

Thirty-three regionally correlatable marine condensed sections containing maximum flooding surfaces have been recognized in the area allowing the North Sea Jurassic succession to be subdivided into 32 genetic stratigraphic sequences ( sensu Galloway). Each event is biostratigraphically calibrated using microfossils (dinoflagellate cysts, radiolaria, ostracoda and foraminifera). The new scheme provides the basis for a basin-wide stratigraphic framework for the Jurassic of the North Sea basin.

Displacement rates of normal faults

Previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less. These estimates have been highly variable, which has led to a belief that longer-term displacement rates on individual faults are likely to be variable as well. Here we report estimates of long-term normal-fault displacement rates averaged over time intervals ranging from 1 to 40 Myr, and based on observed decreases in displacement of progressively younger horizons intersected by syn-sedimentary faults. We find that displacement rates are remarkably stable over these longer time periods, and within a given fault system the rates are strongly dependent on the relative size of the fault (as measured by cumulative vertical displacement). Taken together, these results indicate that faults become large relative to nearby faults by having higher displacement rates, even when small, rather than as a consequence of having been active for longer. Our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities.

The Role of Fault Interaction and Linkage in Controlling Synrift Stratigraphic Sequences: Late Jurassic, Statfjord East Area, Northern North Sea

Examination of well-constrained three-dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the Statfjord East fault, a typical Late Jurassic structure in the northern North Sea Brent province. Al though now a single fault, the Statfjord East fault originally consisted of several en echelon segments, each of which defined individual subbasins. Structural and stratigraphic evidence, both along and across fault strike, indicates that the fault resulted from segment propagation, interaction, and linkage. Facies architecture, thickness variations, and the internal character of synrift formations are temporally and spatially related to the subbasin geometry. Variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults, including anomalous displacement gradients in regions of segment overlap. We attribute the observed shifts in depocenters to local enhancement of displacement rates, resulting from the interaction of neighboring fault segments. The results have far-reaching consequences for synrift plays in the northern North Sea because they imply that only from the perspective of fault growth and linkage can the Late Jurassic structure and stratigraphy be fully understood.

Faulting mechanisms in high-porosity sandstones; New Red Sandstone, Arran, Scotland

Summary Faults in the ‘New Red’ aeolian sandstones of Arran are unusual, firstly for occurring as closely-spaced (less than 1 m) often conjugate sets affecting large volumes of rock, and secondly for forming upstanding fault zones with numerous anastomosing strands of granulated rock, each preserving a small increment of slip. Anisotropy, such as bedding and cross-bed sets, has no discernible effect on fault behaviour. In contrast to the underlying Carboniferous rocks, large displacements are rarely concentrated on a single fault plane within the high-porosity sandstones. The proposed cause is slip-hardening of each fault after a very small displacement (less than 10 mm) causing the next slip increment to be taken up through undeformed rock rather than on the original plane. The common factor in recent records of similar faults elsewhere is their occurrence in high-porosity sandstones. Because of the low grain-contact strength, these rocks are partly analogous to unconsolidated sediment. The high porosity promotes high grain-contact stresses which induce rapid cataclasis during initial slip. Grain fracture and spalling of iron oxide coatings and quartz overgrowths produce a seam with reduced grain size, poorer sorting, higher angularity and lower porosity than the unfaulted rock. These factors collectively strengthen the seam because the coefficient of friction is increased, even though cohesion is reduced. This results in a Mohr failure envelope that lies outside the envelope of the undeformed rock for most stress states. A transient pore pressure increase in the fault seam may be important during slip. Rocks deformed by this slip-hardened faulting preserve a record of each increment of strain. If the displacement on each individual fault seam is the same, the geometry of the total fault systems is directly related to the bulk strain. Quadrimodal systems observed by us in Arran, and by others elsewhere, are probably a response to triaxial strain and show that bimodal ‘Andersonian’ fault systems are only special plane strain cases. If the bulk strain is irrotational, both the orientation and relative magnitude of the principal strains might be estimated.

Implications of Mesozoic-Recent basin development in the western Inner Moray Firth, UK

Abstract A new interpretation of a comprehensive seismic database, consisting of over 5000 km of seismic lines, contradicts the notion that strike-slip motion on the Great Glen Fault (GGF) was the dominant control on Mesozoic basin development in the Inner Moray Firth, UK. It is suggested that the GGF, which may have been active during Palaeozoic times, lay dormant during the Mesozoic and that normal fault systems (such as the Helmsdale-Wick system), formed during two extensional events (pre-Jurassic and Late Jurassic), were the dominant controls on Mesozoic basin evolution. The present expression of the GGF as a discrete, sub-vertical through-going fault attests to its subsequent activity as a divergent wrench structure as part of limited oblique-slip motion after the Early Cretaceous. It seems most likely that such movements occurred in the Tertiary and may have been contemporaneous with regional uplift of the Scottish Highlands during Palaeocene-Eocene (Thulean) events, in response to north-east Atlantic rifting or subsequent Oligo-Miocene (Alpine) tectonism. The new model aids the understanding of the areas general hydrocarbon prospectivity. It can now be demonstrated that the most likely kitchen area for oil resident in the Beatrice Field was centred in the Sutherland Terrace area to the north-west when it formed the site of an active half-graben. Maturation in, and direct up-dip migration from, that area probably took place at the time of maximum burial during the Late Jurassic and Cretaceous, below the half-graben depocentre adjacent to the Helmsdale Fault. Subsequent regional uplift and dissection of the Sutherland Terrace may have served to take the kitchen area out of the maturation window and led to reactivation of several pre-existing structures. Breach and remigration of hydrocarbons is likely to have occurred as a result of the later motions.