Basin Research - Volume 17, Issue 4, 2005

Fission‐track, U–Pb and Pb–Pb analyses of detrital heavy mineral populations in depositional basins and modern river sediments are widely used to infer the exhumational history of mountain belts. However, relatively few studies address the underlying assumption that detrital mineral populations provide an accurate representation of their entire source region. Implicit in this assumption is the idea that all units have equal potential to contribute heavy minerals in proportion to their exposure area in the source region. In reality, the detrital mineral population may be biased by variable concentrations of minerals in bedrock and differential erosion rates within the source region. This study evaluates the relative importance of these two variables by using mixing of U–Pb zircon ages to trace zircon populations from source units, through the fluvial system, and into the foreland.

The first part of the study focuses on the Marsyandi drainage in central Nepal, using tributaries that drain single formations to define the U–Pb age distributions of individual units and using trunk river samples to evaluate the relative contributions from each lithology. Observed mixing proportions are compared with proportions predicted by a simple model incorporating lithologic exposure area and zircon concentration. The relative erosion rates that account for the discrepancy between the observed and predicted mixing proportions are then modelled and compared with independent erosional proxies. The study also compares U–Pb age distributions from four adjacent drainages spanning ∼250 km along the Himalayan front using the Kolmogorov–Smirnov statistic and statistical estimates of the proportion of zircon derived from each upstream lithology. Results show that, along this broad swath of rugged mountains, the U–Pb age distributions are remarkably similar, thereby allowing data from more localized sources to be extrapolated along strike.

Deposition and subsidence analysis, coupled with previous structural studies of the Sevier thrust belt, provide a means of reconstructing the detailed kinematic history of depositional response to episodic thrusting in the Cordilleran foreland basin of southern Wyoming, western interior USA. The Upper Cretaceous basin fill is divided into five megasequences bounded by unconformities. The Sevier thrust belt in northern Utah and southwestern Wyoming deformed in an eastward progression of episodic thrusting. Three major episodes of displacement on the Willard‐Meade, Crawford and ‘early’ Absaroka thrusts occurred from Aptian to early Campanian, and the thrust wedge gradually became supercritically tapered. The Frontier Formation conglomerate, Echo Canyon and Weber Canyon Conglomerates and Little Muddy Creek Conglomerate were deposited in response to these major thrusting events. Corresponding to these proximal conglomerates within the thrust belt, Megasequences 1, 2 and 3 were developed in the distal foreland of southern Wyoming. Two‐dimensional (2‐D) subsidence analyses show that the basin was divided into foredeep, forebulge and backbulge depozones. Foredeep subsidence in Megasequences 1, 2 and 3, resulting from Willard‐Meade, Crawford and ‘early’ Absaroka thrust loading, were confined to a narrow zone in the western part of the basin. Subsidence in the broad region east of the forebulge was dominantly controlled by sediment loading and inferred dynamic subsidence. Individual subsidence curves are characterized by three stages from rapid to slow. Controlled by relationships between accommodation and sediment supply, the basin was filled with retrogradational shales during periods of rapid subsidence, followed by progradational coarse clastic wedges during periods of slow subsidence. During middle Campanian time (ca. 78.5–73.4 Ma), the thrust wedge was stalled because of wedge‐top erosion and became subcritical, and the foredeep zone eroded and rebounded because of isostasy. The eroded sediments were transported far from the thrust belt, and constitute Megasequence 4 that was mostly composed of fluvial and coastal plain depositional systems. Subsidence rates were very slow, because of post‐thrusting rebound, and the resulting 2‐D subsidence was lenticular in an east–west direction. During late Campanian to early Maastrichtian time, widespread deposits of coarse sediment (the Hams Fork Conglomerate) aggraded the top of the thrust wedge after it stalled, prior to initiation of ‘late’ Absaroka thrusting. Meanwhile Megasequence 5 was deposited in the Wyoming foreland under the influence of both the intraforeland Wind River basement uplift and the Sevier thrust belt.

A two‐dimensional, discrete‐element modelling technique is used to investigate the initiation and growth of detachment folds in sedimentary rocks above a weak décollement level. The model depicts the sedimentary rocks as an assemblage of spheres that obey Newton's equations of motion and that interact with elastic forces under the influence of gravity. Faulting or fracturing between neighbouring elements is represented by a transition from repulsive–attractive forces to solely repulsive forces. The sedimentary sequence is mechanically heterogeneous, consisting of intercalated layers of markedly different strengths and thicknesses. The interlayering of weak and strong layers within the sedimentary rocks promotes the localization of flexural flow deformation within the weak layers. Even with simple displacement boundary conditions, and straightforward interlayering of weak and strong layers, the structural geometries that develop are complex, with a combination of box, lift‐off and disharmonic detachment fold styles forming above the décollement. In detail, it is found that the modelled folds grow by both limb rotation and limb lengthening. The combination of these two mechanisms results in uplift patterns above the folds that are difficult, or misleading, to interpret in terms of simple kinematic models. Comparison of modelling results with natural examples and with kinematic models highlights the complexities of structural interpretation in such settings.

Early Eocene successions from Spitsbergen and offshore Ireland, showing well‐developed shelf‐margin clinoforms and a variety of deepwater sands, are used to develop models to predict the presence or absence of turbidite sands in clinoform strata without significant slope disturbance/ponding by salt or mud diapers. The studied clinoforms formed in front of narrow to moderate width (10–60 km) shelves and have slopes, 2–4°, that are typical of accreting shelf margins. The clinoforms are evaluated in terms of both shelf‐transiting sediment‐delivery systems and the resultant partitioning of the sand and mud budget along their different segments. Although this sediment‐budget partitioning is controlled by sediment type and flux, shelf width and gradient, process regime on the shelf and relative sea‐level behaviour, the most tell‐tale or predictive signs in the stratigraphic record appear to be (1) sediment‐delivery system type, (2) degree of shelf‐edge channelling and (3) character of shelf‐edge trajectory through time. The clinoform data sets from the Porcupine Basin (wells and 3‐D seismic) and from the Central Basin on Spitsbergen (outcrops) suggest that river‐dominated deltas are the most efficient delivery systems for dispersing sand into deep water beyond the shelf‐slope break. In addition, low‐angle or flat, channelled shelf‐edge trajectories associate with co‐eval deepwater slope and basin‐floor sands, whereas rising trajectories tend to associate with muddy slopes and basin floors. Characteristic features of the shelf‐edge, slope and basin‐floor segments of clinoforms for these trajectory types are documented. Seismic lines along the slope to basin‐floor transects tend to show apparent up‐dip sandstone pinchouts, but most of these are likely to be simply sidelap features. Dip lines aligned along the axes of sandy fairways show that stratigraphic traps are unlikely, unless slope channels become mud‐filled or are structurally partitioned. Another feature that is prominent in the data sets examined is the lack of slope onlap. During the relative rise of sea level back up to the shelf, the clinoform slopes are generally mud‐prone and they are characteristically aggradational.

The Central Graben in the Danish North Sea sector consists of a series of N–S to NW–SE trending, eastward‐tilted half‐grabens, bound to the east by the Coffee Soil Fault zone. This fault zone has a complex Jurassic history that encompasses at least two fault populations; N–S to NNW–SSE striking faults active in the Late Aalenian–Early Oxfordian, and NNW–SSE to WNW–ESE striking faults forming in Late Kimmeridgian time (sensu gallico), following a short period of tectonic quiescence. Sediment transport across the Coffee Soil Fault zone was controlled by fault array evolution, and in particular the development of relay ramps that formed potential entry points for antecedent drainage systems from the Ringkøbing–Fyn High east of the rift. Fault and isochore trends of the Upper Kimmeridgian–Lower Volgian succession in the northeast Danish Central Graben show that accommodation space was initially generated close to several minor, isolated or overlapping faults. Subsidence became focused along a few master faults in the Early Volgian through progressive linkage of selected faults. Seismic time isochore geometries, seismic facies, amplitude trends and well ties indicate the presence of coarse clastic lithologies locally along the fault zone. The deposits probably represent submarine mass flow deposits supplied from footwall degradation and possibly also from the graben hinterland via a relay ramp. The latter source appears to have been cut off as the relay ramp was breached and the footwall block are uplifted. Fault growth and linkage processes thus controlled the spatial and temporal trends of accommodation space generation and sediment supply to the rift basin.

Analysis of accommodation space variation during deposition of the Cretaceous Qingshankou Formation in the Songliao Basin, NE China, indicates that accommodation space changed both through time and across the basin as a seesaw movement. The mid‐upper Qingshankou Formation is divided into three units. In each unit, changes of accommodation space differ in the southern and northern part of the basin. Increasing accommodation in the southern part is accompanied by a decrease in the northern part, and vice versa. Between the northern and southern basin, there was a neutral belt that is like a fulcrum, called the transformation belt here, where the accommodation did not change to any significant degree. We call this response ‘accommodation transformation’, whose characteristics are defined by tectonic subsidence analysis, palaeontological and sedimentary analyses. The accommodation increasing belt, decreasing belt, transformation belt and accommodation transformation boundary together constitute the accommodation transformation system. The recognition of accommodation transformation in the Songliao Basin provides a new insight into sequence stratigraphy and might be widely applicable.

A cap rock's capacity to seal hydrocarbons depends on its wettability and the sizes of the pore throats within the interconnected pore system that the leaking hydrocarbons must penetrate. These critical pore throat sizes are often poorly constrained in hydrocarbon exploration, partly because measurements of pore throat sizes have not been performed, and partly because pore throat measurements on a few individual samples in the cap rock may not be representative for the seal capacity of the top seal as a whole.

To the contrary, the presence of formation overpressure can normally be estimated in drilled exploration targets. The presence of overpressure in reservoirs testifies to small pore throats in the cap rocks, as large pore throats will result in sufficiently high cap rock permeability to bleed off the overpressure.

We suggest a stepwise procedure that enables quantification of top seal capacities of overpressured traps, based on subsurface pressure information. This procedure starts with the estimation of cap rock permeabilities, which are consistent with observed overpressure gradients across the top seals. Knowledge of burial histories is essential for these estimations. Relationships between pore throat size and permeability from laboratory experiments are then applied to estimate critical pore throat diameters in cap rocks. These critical pore throat diameters, combined with information of the physical properties of the pore fluids, are then used to calculate membrane seal capacity of cap rocks.

Estimates of top seal capacity based on this procedure are rather sensitive to the vertical fluid velocity, but they are also to some extent sensitive to inaccuracies of the pore throat/permeability relationship, overpressure gradient, interfacial tensions between pore fluids, hydrocarbon density and water viscosity values. Despite these uncertainties, applications of the above‐mentioned procedure demonstrated that the mere presence of reservoir overpressures testifies to sufficient membrane seal capacity of cap rocks for most geological histories. Exempt from this statement are basins with rapid and substantial sediment compaction in the recent past.