Basin Research - Volume 4, Issue 3‐4, 1992

The elastic thickness of the continental lithosphere is strongly ‘bi‐modal’. Foreland basins reflect this bi‐modality with narrow, deep basins (e.g. Apennines) and wide, shallow ones (e.g. Ganges). The bi‐modal distribution cannot be explained by thermal models which describe the relationship between elastic thickness and plate age in the oceans, suggesting the involvement of factors other than secular cooling. The high values (80–90 km) are consistent with present‐day temperature gradients of the cratons and the scatter within cratons may be explained by changes in the radiogenic heat production. The low values (10–20 km), however, are more difficult to explain. Foreland basins develop by flexure in front of thrust/fold loads as they advance over former passive margins and onto the cratons. Recent studies suggest that passive margins are underlain by highly attenuated crust and lithosphere which has a low elastic thickness that remains low for long periods (> 10⁸ yr) of time. Foreland basins may inherit these low values as they migrate over a passive margin. Stratigraphic modelling suggests that the low elastic thicknesses would have a profound effect on the development of foreland basins predicting as they do the asymmetry, the pattern of onlap and, the transition from the ‘underfilled’ to ‘overfilled’ phase. Why stretched crust and lithosphere is so weak on long time‐scales is enigmatic. Rifting, however, seems to proceed in such a way that the strong uppermost part of the crust is effectively ‘de‐coupled’ from any support that it might otherwise receive from the strong underlying mantle.

Lateral strength variations in foreland lithosphere in front of an advancing subduction zone (or thrust belt) can exert strong controls on the spatial and temporal evolution of the foredeep basin because local weak zones within the foreland serve to localize the position of the outer flexural bulge (with respect to the foreland) for extended periods of time. This occurs because the weak segment bends more easily than the surrounding, suffer plate, so that plate bending is concentrated within the weak zone. With time, the dip of the down‐going (foreland) plate becomes steeper, narrowing and/or deepening the foredeep basin. If several weak zones are present within the foreland, basin development can become episodic, with protracted periods of time during which the basin remains fixed with respect to the foreland, interspersed by brief periods of time during which the basin advances rapidly toward the foreland. The flexural behaviour will be directly reflected in the stratigraphic record, producing facies belts which migrate toward the foreland in an episodic fashion. If the foreland lithosphere behaves as an elastic sheet, the weak zones must have a flexural rigidity about an order of magnitude lower than (or an elastic plate thickness about half of) the surrounding, stiffer plate in order to induce strongly episodic behaviour. When the effects of inelastic yielding in a lithosphere of brittle‐elastic‐ductile rheology are considered, the strength contrast required to produce episodic behaviour is reduced to about a factor of two in flexural rigidity (or 10–15 % in initial elastic plate thickness). Examples taken from two foredeep basins show evidence for episodic foredeep basin development: in both the early Proterozoic Kilohigok basin of northern Canada and the Pliocene‐Quaternary Apennine foredeep basin beneath the Adriatic Sea, the outer flexural bulge appears to have remained fixed with respect to the foreland while the inner part of the basin deepened through time. In addition, the flexural geometry of the Pliocene‐Quaternary Apennine foredeep basin is well modelled by an elastic plate varying laterally in strength of thickness Telastic=8.5 km with a 20 km‐long weak zone of thickness Tweak= 4.25 km.

South Wanganui Basin is a Plio‐Pleistocene basin formed behind the Hikurangi subduction zone of the North Island, New Zealand. The active Benioffzone lies about 30–40 km beneath the centre of Wanganui Basin. We attribute the development of this basin primarily to frictional shear between the subducted Pacific plate and overriding Australian plate. This frictional interaction harnesses part of the ‘slab pull’ force of plate tectonics to deform the overriding plate and create the basin. The overall shape of the basin is modified to a lesser extent by the effects of uplift around the basin margin and by in‐plane compressional stress. A quantitative three‐dimensional model, based on flexure of a thin elastic sheet with spatially varying rigidity, is used to test the implications of our hypothesis. A maximum vertical loading stress of about 85 MPa is required to produce the vertical deformation in Wanganui Basin, and that in turn implies about 170 MPa of shear stress at the inclined subduction thrust. A vertical stress of 85 MPa applied over a 40‐km‐wide subduction thrust represents about 50% of the magnitude theoretically present in the ‘slab pull’ force. The effective elastic thickness (Te) of the flexed plate that provides the best fit to the observed deformation is 10 ± 5 km; such a low value of 7“e is attributed to the high plate curvature and high strain rates associated with Wanganui Basin. Constraints on our analysis are provided by seismic reflection, seismicity, gravity, geodetic and geological data. A similar situation to Wanganui Basin, in which a high‐friction subduction thrust induces shortening and flexure of the overlying crust, may be found in the Puget Sound area of Washington State, USA.

The long‐term (50 Myr) development of orogenic thrust wedges and their neighbouring foreland basins are inextricably linked. Foreland basins, such as occur nonh of the Alps, flanking the Pyrenees, east of the Apennines and south of the Himalayas, are characterized by an early underfilled (deep‐water) phase, followed by a filled (shallow‐marine) or overfilled (continental) phase. The extent to which a foreland basin is filled to sea‐level can be understood by comparing, over time, the rate at which sediment is delivered to the basin with the rate at which accommodation space is generated within the basin. This approach is applied to the Alpine thrust wedge/North Alpine Foreland Basin (NAFB) system. Using simple geometrical calculations assuming a critically tapered thrust wedge sliding over a foreland plate, preceded by a flexurally induced trough (foreland basin), it is possible to develop some general parameter relationships for the system. The thrust wedge is described firstly in terms of the history of exhumation which is directly linked to denudation and hence sediment generation, and secondly to thrust front advance rates. The accommodation space can be approximated by the product of the thrust front advance rate and the deflection of the basin at the thrust front. The extent to which a basin is, at one instant, being undersupplied or oversupplied (F) can then be described by the ratio of sediment supply to accommodation space generated:

F=exhumation rate. width of thrust wedgeladvance rate. deflection at thrust front

When F> I, the basin is on a trend towards overfilling, and when F< 1 the basin is on a trend towards underfilling. The value for the width of the thrust wedge undergoing exhumation is held as a constant for most of the evolution of the NAFB.

The development of the Alpine thrust wedge/NAFB system is characterized by an initial submarine phase (Cenomanian to mid‐Oligocene) of rapid thrust front advance and slow exhumation rates; this resulted in deep‐water and shelfal sedimentation in the foreland basin (F< 1), and is described as the occretionary wedge phase. By the mid‐Oligocene, exhumation rates were accelerating associated with major backthrusting, and frontal advance rates were slowing down; this resulted in the foreland basin being filled to sea‐level (F← 1), accumulating shallow‐marine and continental sediments. This period is described as the continental wedge phase. Although F‐values probably fell back to close to 1 following the period of major Oligocene exhumation, the basin remained in its filled state for the remainder of its geological history.

Modern studies indicate that rapid exhumation rates lead to increased local relief and denudation. The increased relief causes increased maximum elevations, so enhancing orographic precipitation and glaciation leading to further increases in denudation. This positive feedback loop between denudation, exhumation and climate may have enhanced the rapid inversion of the core of the orogenic wedge during the latter part of its growth.

The occurrence of cyclic patterns of sedimentation on a large scale, or abrupt changes in lithology or facies patterns in foreland basins, are most commonly attributed to tectonism. Climatic controls are invoked much less often, and geomorphic controls are rarely considered except for small‐scale features. Tectonism is the first‐order control on sedimentation at mountain fronts by providing accommodation space for sediment accumulation, and the requisite energy for the system to operate. However, geomorphic controls on sediment yield from source areas, transformation of sediment yield in transfer systems, and feedback mechanisms between source areas and depositional basins may be the secondary controls on sediment dispersal and accumulation near mountain fronts.

Drainage basins pass through a series of stages during their evolution. Sediment flux is large during initial stages of basin evolution, allowing fans to prograde rapidly. But as drainage nets in the source area expand, and valleys increase their capacity to store sediment eroded from interfluves, the quantity and caliber of the sediment load at the outlet diminishes, and fan sequences begin to fine upward. Eventually the source area drainage network will expand to its maximum size. Relief is greatest in the source area at this time, and the quantity and calibre of the sediment eroded from valley walls reaches a maximum. Dynamic equilibrium between fan and source area is attained and a period during which spontaneous incision of source area valleys, fanhead entrenchment, and depositional lobe progradation occurs The amount and size of sediment supplied to the fan reaches a maximum at this time, and fan deposits coarsen upward through this period. However, feedback relationships between fan and depositional basin limit the ability of fans to prograde basinward. Fans begin to retrograde as relief in the source area declines and storage capacity increases. Fining‐upward sequences arc deposited and basinal facies encroach on the fan during this final phase.

Ancient alluvial fans commonly consist of coarsening‐ then fining‐upward stratigraphic sequences consistent with this evolutionary model, and cross‐sections of alluvial fan deposits normally show that fan facies stack vertically near their upland sources. A typical first‐order fan deposit ranges in thickness between 100 m and 250 m, but those in foreland basins near thrust margins are considerably thicker, possibly in response to increased accommodation space provided by basin subsidence, extended periods of downcutting caused by continued movement on the thrust, and to basinward translation of the source area.

The Burnside Formation records the transition from marine shelf to largely alluvial conditions in the palaeo‐Proterozoic (about 1.9 Ga) Kilohigok foreland basin in the Slave Province of the Canadian Shield. The Burnside Formation forms a thick (up to 3.5 km), N W‐tapering siliciclastic wedge, representing a braided alluvial system that prograded transversely across the Slave Province. The stratigraphic architecture of this unfossiliferous, predominantly alluvial succession documents the location and distribution of unconformities caused by lithospheric flexure in a Proterozoic foreland basin. The locations of these unconformities demonstrate the changing locations of tectonic and sedimentary loads on both sides of the Silave craton.

Associations of sedimentary facies and palaeocurrents indicate that initially a braid delta system drained longitudinally, and was restricted to the proximal part of the basin between the orogenic hinterland (the Thelon Orogen) and a syn‐sedimentary flexural arch. Palaeocurrents for the lowermost braid delta facies in the proximal foreland were parallel to palaeocurrents in underlying foreland basin turbidites, parallel to the flexural arch closest to the Thelon Orogen, parallel to the strike of the orogen, and perpendicular to the transport direction of thrust‐nappes directed out of the orogenic belt. An abrupt shift to a transverse palaeocurrent mode accompanied the onset of higher‐gradient braided stream conditions across the entire foreland basin, showing that from this time, sediment supply exceeded subsidence in the proximal foreland. The long‐lived, laterally extensive transverse dispersal and the presence of sedimentary facies lacking interbedded mud or sedimentary structures indicative of desiccation suggest that fluvial discharge was probably perennial.

The sharp superposition of medial braid plain facies directly on marine shelf and deltaic facies in the distal foreland indicates that the transition to non‐marine conditions marks an erosional unconformity over two coeval flexural arches. The eastern flexural arch relates to covergence on the eastern margin of the Slave Province and to thrust loading in the Thelon Orogen that created the Kilohigok foreland basin. The evolution of a passive margin on the western side of the Slave Province caused the other flexural arch. In the centre of Kilohigok Basin a profound unconformity exists at the top of the Burnside alluvial succession. This unconformity suggests that a single flexural arch existed at the end of alluvial sedimentation that may signify the migration and yoking together of the two previously independent flexural arches. This migration may have been caused by the combined mass of the thick alluvial sedimentary wedge over the eastern edge of the Slave craton, coupled with initial thrust loading and foreland basin development in Wopmay Orogen.

The transition within a foreland basin from largely marine to largely alluvial deposition reflects changing subsidence patterns across the basin due to changes in rates of convergence and changing subsidence and sedimentation patterns due to erosional mass redistribution from the orogenic hinterland into the foreland basin. This transition appears to separate two distinct phases in the evolution of the Kilohigok foreland basin. In the first phase, active thrust loading and asymmetric flexural subsidence restricted alluvial facies near the orog;en. In the second phase, convergence slowed or ceased, and erosional unloading and uplift of the hinterland caused subsidence patterns to become more uniform across the foreland basin, promoting rapid progradation of alluvial facies across the basin. Palaeomagnetic and sedimentological data indicate that the basin lay in the trade wind belt and that humid or monsoonal climatic conditions existed at the time. This suggests that moist trade winds blew onto the Thelon Orogen on the side closest to the Kilohigok Basin. Such an orographic effect would have caused rapid erosion, localized the position of the Thelon orogenic front, increased the flux of material through the orogenic belt, and caused more extensive progradation of the alluvial system.

The stratigraphic record in the San Rafael Swell region of central Utah suggests that thrust loading and attendant foreland sedimentation began in adjacent parts of the Sevier orogenic belt during Albian time. Regional stratigraphy, isopachs, gravel composition and grain‐size trends suggest that the Morrison Formation of Late Jurassic age and Buckhorn Conglomerate Member of the Cedar Mountain Formation of Early Cretaceous age were not derived from the adjacent thrust belt as they neither thicken nor coarsen significantly towards the west. In contrast, the upper member of the Cedar Mountain Formation of Albian age and Dakota Sandstone of early Cenomanian age rapidly coarsen and thicken westward towards the thrust belt, suggesting that these units were deposited during a time of rapid flexural subsidence early in the history of thrust loading by the Sevier belt. Hence, in the case of the foreland basin in central Utah, basin subsidence rates and asymmetry provides the strongest evidence of the initiation of thrust‐belt loading by the adjacent Sevier orogenic belt.

Lithostratigraphic, chronostratigraphic, sedimentological and penological data from the Upper Jurassic‐Lower Cretaceous Morrison and Cloverly Formations in central Wyoming allow detailed characterization of the early history of the central part of the Cordilleran foreland basin. The Morrison is divisible into three informal members: (1) a lower sandstone, deposited by a complex coastal dune‐foreshore–fluvial system during retreat of the Sundance sea; (2) a middle mudstone, deposited by muddy fluvial and ephemeral lacustrine systems during a period of regional, seasonal aridity; and (3) an upper sandstone, deposited by a sandy fluvial system of variable sinuosity. The overlying Cloverly Formation is divisible into two informal members: (1) a lower mudstone (previously considered as part of the Morrison Formation), deposited by muddy fluvial and lacustrine systems; and (2) an upper chert‐pebble conglomerate and sandstone, deposited primarily by gravel‐dominant braided rivers. Palynological data and a single fission‐track date indicate that the lower part of the middle Morrison mudstone is early to middle Oxfordian and the upper part of the lower Cloverly mudstone is Valanginian.

Morrison sandstones are subarkosic, with average %QFL = 91,6, 3 and %QmFLt = 83, 6, 11. Cloverly sandstones are cherty litharenites and sublitharenites, with average %QFL = 99.6, 0,0.4 and %QmFLt = 82,0,18 (Gazzi‐Dickinson point‐counting method). Palaeocurrent data and sandstone compositions indicate a complex provenance including exirrabasinal sources in lower Mesozoic and upper Palaeozoic sedimentary and volcanic rocks of the Cordillera and intrabasinal sources of Proterozoic clasts in south‐central Wyoming. Cloverly sandstone compositions in the eastern part of the study area were influenced by short‐term fluvial reworking within the basin.

The thickness of the composite Morrison‐Cloverly succession is practically constant over a distance of several hundred km east of the Idaho‐Wyoming thrust belt, and its internal chronostratigraphic zones are subparallel. On the other hand, equivalent strata in the Gannett Group of the thrust belt are at least three times thicker. This indicates that the Morrison and Cloverly in central Wyoming were deposited within the overfilled part of the foreland basin. Preliminary regional correlation indicates that coarse‐grained lithofacies in these rocks are significantly time‐transgressive, generally becoming younger toward the E and NE. Overfilling of the early Cordilleran foreland basin in central Wyoming was accomplished by progradation from the W and S. In spite of their three‐dimensional (3D) complexity, the Morrison and Cloverly Formations generally confirm theoretical model predictions for overfilled foreland basins.

Temporal and spatial changes in alluvial sand‐body geometry and palaeodispersal patterns of the lower part of the Cloverly Formation (Early Cretaceous) in west‐central Wyoming suggest differential uplift within the developing medial to distal foreland basin and existence of subtle structurally controlled topography nearly 60 Myr before classic Laramide uplift in the Wyoming foreland province. This intraforeland structural topography exerted a significant control on dispersal and deposition of widespread gravels of Neocomian‐Aptian age.

Surface studies have established the presence of two conglomeratic sandstones in the Cloverly Formation in the western Wind River Basin. Both conglomeratic sandstones are chert arenites, but the lower conglomeratic sandstone is characterized by dark chert pebbles derived from western and southwestern extrabasinal sources and was deposited by braided rivers, whereas the upper conglomeratic sandstone is characterized by intraformational clasts derived locally from adjacent floodplains and deposited by NE‐flowing rivers of moderate sinuosity which carried a higher proportion of suspended load in stable bank‐confined channels.

Surface and subsurface mapping of sandstone‐body geometry in the lower dark chert‐pebble conglomeratic sandstone, together with palaeocurrent analysis of trough cross‐bedding, reveal a major NE‐flowing trunk river system in the area. Lithological correlation of surface sections to adjacent well logs, together with regional format correlation of fine‐grained intervals in well logs up to 120 km E of the outcrop belt, enable detailed mapping of the trunk system. The trunk system was approximately 5–10 km wide, and may have been confined by subtle structural topography developed by recurrent differential uplift along NE‐trending fractures in Archaean basement rocks. The fractures are along strike with diabase dike swarms and faults mapped in Archaean basement rocks of the Laramide Wind River uplift. The lower chert‐pebble sandstone is absent for a lateral distance of at least 30 km between these lineaments, including an area of at least 1000 km², indicating the existence of a low NE‐SW‐trending interfluve perpendicular to the axis of the modern Wind River Range.

Early Cretaceous development of structural topography and partitioning of the medial to distal foreland basin of west‐central Wyoming were most likely controlled by tectonic reactivation and differential uplift along fractures in Archaean basement rocks in response to early thrust loading and intraplate stresses during initial subsidence of the foreland basin.

In young or currently active foreland basins of the world, along‐orogen variations in structural deformation and/or depositional environments are common elements of the later phases of basin development. The late Eocene Escanilla Formation of the South‐Central Pyrenean foreland basin represents an ancient drainage system in which such variability can be studied in detail using high‐resolution magnetostratigraphy combined with a more traditional field‐based approach. Downstream changes in the nature of the alluvial system were strongly influenced by the on‐going Eocene structural partitioning of the foreland basin as it began to become incorporated into the southward‐advancing South Pyrenean thrust system. Lower subsidence rates within these allochthonous ‘piggy‐back’ sub‐basins served to increase channel‐body interconnectedness of sheet‐like alluvial conglomerates, to inhibit the preservation of significant volumes of fine‐grained overbank material, and to promote the extensive development of pedogenic calcretes.

During the phase of coastal progradation along the subsiding basin axis, a number of N‐S‐trending anticlines impeded the westward progradation of the alluvial system, producing a strong diachrony in the age of a Lutetian‐Priabonian‐aged deltaic system along the orogen. Fold growth across the western oblique ramp of the South‐Central Unit thrust system dramatically influenced middle to late Eocene drainage patterns and lithofacies distributions. Within the portions of the drainage system upstream of these active folds, the alluvial deposits were periodically ponded, allowing the deposition of micritic lacustrine limestones. Fluctuations in regional base‐level exerted control on the drainage system upstream into the alluvial drainage basin. Base‐level rises caused short reversals in the longer‐term westward regression of marine environments across the foreland basin, whereas base‐level falls produced widespread sheet conglomerate deposition.