Foreland Basin Development Research Articles

A Damara orogen perspective on the assembly of southwestern Gondwana

Abstract The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo–São Francisco and Río de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari–Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga–Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Araçuaí Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580–550 Ma in the Kaoko Belt and at 545–530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530–500 Ma, with thrusting onto the Kalahari Craton from 495 Ma through to 480 Ma. Coupling of the Congo and Río de la Plata cratons occurred before final closure of the Mozambique and Khomas (Damara Belt) oceans with the consequence that the Kuunga suture extends into Africa as the Damara Belt, and the Lufilian Arc and Zambezi Belt of Zambia. Palaeomagnetic data indicate that the Gondwana cratonic components were in close proximity by c. 550 Ma, so the last stages of the Damara–Kuunga orogeny were intracratonic, and led to eventual out-stepping of deformation/metamorphism to the Ross–Delamerian orogen ( c. 520–500 Ma) along the leading edge of the Gondwana supercontinental margin.

Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros

The regionally extensive, coarse-grained Bakhtiyari Formation represents the youngest synorogenic fill in the Zagros foreland basin of Iran. The Bakhtiyari is present throughout the Zagros fold-thrust belt and consists of conglomerate with subordinate sandstone and marl. The formation is up to 3000 m thick and was deposited in foredeep and wedge-top depocenters flanked by fold-thrust structures. Although the Bakhtiyari concordantly overlies Miocene deposits in foreland regions, an angular unconformity above tilted Paleozoic to Miocene rocks is expressed in the hinterland (High Zagros). The Bakhtiyari Formation has been widely considered to be a regional sheet of Pliocene–Pleistocene conglomerate deposited during and after major late Miocene–Pliocene shortening. It is further believed that rapid fold growth and Bakhtiyari deposition commenced simultaneously across the fold-thrust belt, with limited migration from hinterland (NE) to foreland (SW). Thus, the Bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the Zagros. However, new structural and stratigraphic data show that the most-proximal Bakhtiyari exposures, in the High Zagros south of Shahr-kord, were deposited during the early Miocene and probably Oligocene. In this locality, a coarse-grained Bakhtiyari succession several hundred meters thick contains gray marl, limestone, and sandstone with diagnostic marine pelecypod, gastropod, coral, and coralline algae fossils. Foraminiferal and palynological species indicate deposition during early Miocene time. However, the lower Miocene marine interval lies in angular unconformity above ~ 150 m of Bakhtiyari conglomerate that, in turn, unconformably caps an Oligocene marine sequence. These relationships attest to syndepositional deformation and suggest that the oldest Bakhtiyari conglomerate could be Oligocene in age. The new age information constrains the timing of initial foreland-basin development and proximal Bakhtiyari deposition in the Zagros hinterland. These findings reveal that structural evolution of the High Zagros was underway by early Miocene and probably Oligocene time, earlier than commonly envisioned. The age of the Bakhtiyari Formation in the High Zagros contrasts significantly with the Pliocene–Quaternary Bakhtiyari deposits near the modern deformation front, suggesting a long-term (> 20 Myr) advance of deformation toward the foreland.

Miocene forebulge development previous to broken foreland partitioning in the southern Central Andes, west‐central Argentina

In the distal region of the modern flat‐slab segment in the southern Central Andes, an unusual stack of middle Miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west. These features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system. Sedimentary thickness farther east and far from the Cordilleran tectonic loads suggests accommodation and preservation driven by “nonisostatic” dynamic subsidence. Regional overlapping relationships and basin modeling suggest that the Modern broken foreland (present Sierras Pampeanas) can be interpreted as a reactivation of a formerly partitioned broad forebulge.

Tectonics of the Vosges (NE France) and the Schwarzwald (SW Germany): evidence from Devonian-Carboniferous active margin basins and their deformation

Devonian-Carboniferous sedimentary and volcanic rocks are exposed in the Badenweiler-Lenzkirch Zone (BLZ) of the Southern Schwarzwald and in the Southern Vosges (Central Europe). Several lithostratigraphic units are recognized. Sedimentary or tectonic relations between some of the units can be realized by comparing sedimentary and volcanic facies associations as well as petrographic and geochemical datasets. During the Variscan orogenesis the sediments were deposited in different kinds of basins along an active margin setting. Their deformation was studied in the Markstein area of the Southern Vosges and is presented here in detail. Some arguments favour a northward underthrusting of a narrow and oceanic pre-Upper Devonian back-arc basin beneath the crystalline units of the Central Vosges and Schwarzwald. Accretion of continental crust continued from the Upper Devonian to the late Lower Carboniferous along with a characteristical evolution of a deep marine retro-arc foreland basin. It has accumulated about 3500 m of turbidite sandstones, shales and conglomerates of volcano-plutonic origin, assembling now the Markstein Group in the Southern Vosges. To the south of the Markstein Group, deep marine mud-rich turbidite systems and a shallowing-upward succession of fluvio-deltaic sediments filled in the Oderen Basin, which subsided in the late Tournaisian and early Visean. A subduction type volcanism occurred in form of tholeiitic basalts, K-calcalkaline island arc andesites and continental arc sandstones. In contrast to former models, a genetical relationship between the Markstein and the Oderen Group is rejected here. Instead, different subduction systems in space and time have caused the development of the active margin basins. The Variscan deformation of the exposed sedimentary units started in the upper Visean. Structural analysis of kinematic indicators and macroscopic fabrics in the Markstein Group give evidence for thrusting and dextrally transpressive shearing, forming a positive flower structure along the southern border of the Central Vosges crystalline units. This was broadly synchronous with dextrally transpressive deformation along the Lalaye-Lubine/Baden Baden fault at the northern border of the Central Vosges and Schwarzwald and along the Transition Complex between the Central and the Southern Schwarzwald. The high-K-calcalkaline Metzeral granite intruded postkinematically into the principal displacement zone and has a position similar to the dextrally sheared Randgranite association along the northern border of the BLZ. A juxtaposition between the Oderen and the Markstein Groups was achieved in the upper Visean due to the dextral transpressive motion along the ESE-trending Markstein Fault. The exhumation of high grade metamorphic rocks of the Central Vosges and Schwarzwald during the upper Visean is discussed in the context of long lasting accretionary tectonics and the two-sided dextral transpression, favouring crustal extrusion as a main exhumation process. A thorough examination of the supracrustal rocks and their mode of deformation in the Southern Vosges helps us to determine the relationships with the BLZ and contributes also to the paleogeographic reconstruction of the Armorica - Proto-Alpine - Gondwana connection.

Fragmentation of a foreland basin in response to out-of-sequence basement uplifts and structural reactivation: El Cajon-Campo del Arenal basin, NW Argentina

The style and mechanisms by which a foreland region is incorporated into an orogen depends on the tectonic style, effectiveness of uplift, and dynamic subsidence. Classical foreland-basin models reflect a self-similar propagation of deformation into the foreland in a thin-skinned thrust-belt setting governed by wedge mechanics. Thick-skinned foreland regions, which are characterized by high-angle reverse-fault–bounded basement uplifts and intervening basins, however, do not fit this idealized model. Unlike thin-skinned tectonic provinces, deformation and uplift in these regions may be highly variable in time and space. Furthermore, deformation patterns may be complicated by the presence of preexisting structures, particularly those which lie at orientations that enable them to be reactivated and utilized to accommodated deformation under compression. The Neogene El Cajon–Campo del Are-nal basin is one of a series of basins located along the eastern margin of the Puna Plateau within the Sierras Pampeanas, a region that is composed of a thick-skinned foreland fragmented by reverse-fault–bounded basement uplifts that regionally characterize an eastward-younging trend. This region is superimposed onto the Cretaceous Salta Rift province, which provides a series of pre-existing structures that may potentially be reactivated. The basin is located along the eastern margin of the Puna Plateau, an integral component of the Andean orogen, which includes several filled, uplifted, and internally drained Cenozoic intraplateau basins. Structural and sedimentological similarities exist between basins along the margin of the Puna and those within it. Understanding the evolution of foreland basins, such as the El Cajon–Campo del Arenal basin, provides possible mechanisms for the development and incorporation of marginal basins into orogenic belts, and in the case of the Andean orogen, the potential for these basins to be incorporated into the plateau. Our analysis, which integrates seismic, sedimentary, and thermochronological data, characterizes the evolution of this basin and surrounding ranges. The appearance in the sedimentary section of a distinct grain-age population derived from the basement erosion surface constrains the uplift and erosion of an out-of-sequence intrabasin high to ca. 6 Ma. The basin fill, therefore, records an evolution from an undeformed foreland to one that is compartmentalized by basement uplifts and that is incorporated into the greater orogenic structure. The data reveal the importance of the reactivation of preexisting structures along the basin margin in creating east-dipping structures in a generally west-dipping domain. These opposing faults on the basin margin consequently caused the out-of-sequence uplift of the intrabasin range, the Sierra de Quilmes. The Sierra de Quilmes fragments the foreland and, because its position is locked by loads to the west and east, creates increased deformation within the basin, basin fill, uplift, and incorporation into the orogen. Unlike basins within the plateau, however, the El Cajon–Campo del Arenal basin has been re-excavated and integrated once more into the foreland drainage network.

Upper Cretaceous to Paleogene Forbulge Unconformity Associated With Foreland Basin Evolution (Kras, Matarsko Podolje and Istria; SW Slovenia and NW Croatia)

A regional unconformity separates the Cretaceous passive margin shallow-marine carbonate sequence of Adriatic Carbonate Platform from the Upper Cretaceous and/or Paleogene shallow-marine sequences of synorogenic carbonate platform in southwestern Slovenia and Istria (a part of southwestern Slovenia and northwestern Croatia). The unconformity is expressed by irregular paleokarstic surface, locally marked by bauxite deposits. Distinctive subsurface paleokarstic features occur below the surface (e.g. filled phreatic caves, spongework horizons…). The age of the limestones that immediately underlie the unconformity and the extent of the chronostratigraphic gap in southwestern Slovenia and Istria systematically increase from northeast towards southwest, while the age of the overlying limestones decreases in this direction. Similarly, the deposits of synorogenic carbonate platform, pelagic marls and flysch (i.e. underfilled trinity), deposits typical of underfilled peripheral foreland basin, are also diachronous over the area and had been advancing from northeast towards southwest from Campanian to Eocene. Systematic trends of isochrones of the carbonate rocks that immediately under- and overlie the paleokarstic surface, and consequently, of the extent of the chronostratigraphic gap can be explained mainly by the evolution and topography of peripheral foreland bulge (the forebulge). The advancing flexural foreland profile was the result of vertical loading of the foreland lithospheric plate (Adria microplate) by the evolving orogenic wedge. Because of syn- and post-orogenic tectonic processes, and time discrepancy between adjacent foreland basin deposits and tectonic (“orogenic”) phases it is difficult to define the exact tectonic phase responsible for the evolution of the foreland complex. According to position and migration of the subaerially exposed forebulge, distribution of the foreland related macrofacies and orientation of tectonic structures, especially of Dinaric nappes, and Dinaric mountain chain I suggest that the foreland basin complex in western Slovenia and Istria was formed during mesoalpine (“Dinaric”) tectonic phase due to oblique collision between Austroalpine terrane/Tisia microplate and Adria microplate when probably also a segmentation of the foreland plate (Adria microplate) occurred.

The Miocene of the Hatay area, S Turkey: Transition from the Arabian passive margin to an underfilled foreland basin related to closure of the Southern Neotethys Ocean

Miocene sediments of the Hatay area document the distal, southerly, part of an “underfilled” peripheral foreland basin related to the closure of the Southern Neotethys. The basin is underlain by latest Cretaceous and Eocene shallow-marine carbonates, interpreted as the youngest part of the Arabian passive margin succession. Unconformably overlying Lower Miocene conglomerates, sandstones and palaeosols (up to 250 m thick) represent braided river deposits derived from an uplifted area to the south. During the Middle Miocene, carbonate sediments accumulated in a variety of shallow-marine environments dominated by a northward-sloping ramp, with non-marine, peritidal, lagoonal and coral reef deposition in local areas. The Upper Miocene succession is mainly deeper-water hemipelagic marl with clastic interbeds. Messinian evaporites were deposited near the depocentre. The Early Miocene coarse fluvial sediments above the regional unconformity reflect the development of a flexural bulge, or re-activated basement structure. The Middle Miocene carbonates and Upper Miocene marls are interpreted as the lower and middle units of an “underfilled trinity”, considered typical of peripheral foreland basins. Similar Lower Miocene coarse clastic sediments and Mid-Miocene shallow-water carbonates unconformably overlie the Arabian platform succession c. 75 km to the north, where the succession continues upwards into thick shallow-marine to non-marine terrigenous sediments (upper unit of the “trinity”), sourced from the overthrust load. Comparable coarse clastic sediments are absent from the distal foreland basin in the Hatay area, probably reflecting its more southerly position, structural barriers in the foreland and the westward bypassing of sediment towards the Mediterranean Sea. Regional convergence halted by the end of the Miocene and was followed by strike–slip and westward “tectonic escape”. As a result the Miocene foreland basin was dismembered to form the Plio-Quaternary transtensional Hatay Graben. This like many other foreland basins shows some features which are not accommodated by ideal models of foreland basin evolution.

An Early Ordovician (Finnmarkian?) foreland basin and related lithospheric flexure in the Scandinavian Caledonides

Early Ordovician (479 - 455 Ma) graywackes overlying Cambrian passive margin successions in the Baltica palaeocontinent-related Lower AUochthon of the Scandinavian Caledonides document early Caledonian tectonic activity. Such rocks occur from the Jamtland area (Sweden) northwards along the eastern Caledonian margin, and imply a wide regional extent of a foreland basin in Early Ordovician times. This foreland basin was subsequently incorporated into a fold-and-thrust belt during the final Caledonian, Scandian tectonic episode (c. 425 - 390 Ma). Based on published cross section data, this late deformation is restored. The results and other available information are compiled into a palaeogeographic map and sections on the geometry and evolution of the Early Ordovician foreland basin. Graywacke/turbidite sedimentation (Follinge Formation) started in western (internal) areas already in Early Arenig times with a main phase from Llanvirn to Late Caradoc (c. 472 - 461 Ma). There, the Follinge Formation rests with an erosional unconformity on older beds. Towards east, however, it overlies successively younger beds related to a carbonate domain. It is suggested that the areas of the sub-turbidite erosional unconformity represent the early location of a flexural forebulge, which subsequently migrated eastwards towards the margin of the carbonate domain of palaeocontinent Baltica. The restored foreland-basin geometry is compared with numerical models in order to derive some characteristics of the flexure of the foreland lithosphere. Time and lithospheric constraints make it possible to test the viability of the restoration and of foreland basin models. Relative to the available lithospheric strength data, the width of the basin is too large, the depth relatively small, and the large wavelength of the flexure is difficult to explain with simple orogenic loading. Graywacke sedimentation occurred after Finnmarkian (515 - 475 Ma) HP metamorphism, perhaps as a consequence of exhumation and consequent loading of the Baltican margin. Graywacke sedimentation ended at the time of Jamtland phase (460 -440 Ma) HP metamorphism and relatively deep water conditions. It is speculated that eclogitization of crustal material produced an additional load, which caused a relatively large down flexure of the lithosphere.

Coelobiontic communities in neptunian fissures of synsedimentary tectonic origin in Permian reef, southern Urals, Russia

Abstract Sedimentary dykes in the Permian reef complexes of the Russian platform are well preserved and important in providing information about reef growth, the reef biota and, particularly, cavity-dwelling organisms and sediment sources. Two main fissure assemblages are recognized with N80° and N170° (late Asselian-early Sakmarian) and N130°–140° and N60° (Sakmarian-Artinskian) orientations. These contemporaneous orthogonal dyke sets present orientations corresponding to the regional tectonic fabric and a tectonic origin for fracturing associated with the foreland basin development. The largest dykes record eight lithofacies and several stages of fracture opening. Stromatoids and centimetre-thick deposits of peloidal grainstones-packstones (thromboids), which form in situ within microbial laminae on the fissure walls, preceded filling by skeletal and terrigenous sediments. The fissures contain a well-preserved biota similar to the Lower Permian cavity-dwelling organisms observed within the reef. The coelobiontic habitat was episodically enlarged by successive synsedimentary fracturing episodes reflecting several phases of encrustation and infill of recurrent lithofacies. The pioneer microbialites grew when nothing else was deposited in the fissures immediately after fracturing. Gastropods and ostracods occur as dense clusters, probably indicating in situ growth, or are considered as reworked material from the subsurface. Others organisms, such as crinoids, bryozoans, Tubiphytes , conodonts, brachiopods, ammonoids and spiculate sponges, are also found within fractures as reworked shells and skeletons, indicating that they were washed in from the overlying sea floor. The biota in the dykes is mainly represented by the dweller guilds living on the reef surface, whereas members of the binding and frame-building guilds are poorly represented. The dykes formed during reef development and the last stage of argillaceous wackestone sedimentation within the fractures is related to a major flooding event corresponding to upper Artinskian drowning of the Russian platform. Biostratigraphically significant fossils in the dykes date the ages of the host rock and of the fissure formation and filling from late Asselian to upper Artinskian. The Tratau reef thus provides an instructive example of the interaction between carbonate accumulation and tectonic events.

Erosion-rate determination from foreland basin geometry

The geometry of foreland basins is controlled by a dynamic balance between thrust-belt migration, flexural subsidence, and fluvial deposition. To improve our ability to quantify the relationships between tectonics, climate, and foreland basin geometry, I developed analytic solutions for basin topography and stratigraphy using a classic two-dimensional mathematical model of foreland basin evolution. Model predictions for basin topography are successfully tested against observed profiles along a humid-to-arid climatic gradient in the central Andes. Using published estimates for the thrust-belt migration rate, flexural parameter, and thrust-front basin depth in this region, I show that basin topographic profiles can be used to estimate the upstream erosion rate at any point along a foreland basin. Basin-averaged erosion rates estimated in this way vary from 0.025 to 0.045 mm/yr in the central Andes, increasing from semiarid to humid climates.

Structural geometry and development of the Witwatersrand Basin, South Africa

Abstract The Witwatersrand Basin is an Archaean basin situated on the Kaapvaal Craton of Southern Africa. The results presented here focus on the structural geometry and development of the basin. Detailed structural sections across the western and northwestern parts of the basin are presented, using seismic data integrated with borehole, mine and outcrop information. The structural development of the basin can be expressed in simple terms, and in a manner that is standard practice within the oil industry. There are several clearly identified stages in basin evolution. The basin was initiated as a rift during Dominion Group times ( c. 3074 Ma), with post-rift thermal subsidence during the early part of West Rand Group times. Thermal subsidence would have been completed by late West Rand Group times ( c. 2900 Ma). Minor volcanic interludes within the West Rand Group sequence may testify to the existence of phases of extension during West Rand Group times. Onset of compression and thrusting outside the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during late West Rand Group times ( c. 2950 Ma). Foreland basin development culminated in Central Rand Group times, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during Klipriviersberg Group times (2714 Ma) by the accumulation of basic volcanic rocks. Further thrusting occurred at the end of Klipriviersberg Group times. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2709 Ma), a major rift event across much of the area. Thermal subsidence related to this rift phase occurred during late Platberg Group times. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, and it is difficult to elucidate the earlier compressional thrust-fold structures. Careful integration of seismic, borehole, mine and outcrop data allows structures from the thrusting event be recognized and more complete sections drawn. These present a larger-scale view of the structure of the basin than has previously been obtained.

Tectonic and sedimentary evolution of basins in the northeast of Qinghai-Tibet Plateau and their implication for the northward growth of the Plateau

The northeast of the Qinghai-Tibet Plateau, from the Hoh Xil Basin to the Hexi Corridor Basin, demonstrates a basin-ridge geomorphy that is the result of the long-term uplift of the Qinghai-Tibet Plateau. The geological evolution of the Qinghai-Tibet Plateau is recorded in its associated basins. From the Cenozoic sedimentary filling pattern in the Hoh Xil, Qaidam and Jiuquan Basins, we find that the evolution of these basins is similar: they began as strike-slip basins, evolved into foreland basins, and then to intermontane basins. Foreland basins are the direct result of orogenic activity in the northeast of Qinghai-Tibet Plateau, and can be documented to have formed over the following time periods: 49–23 Ma for the Hoh Xil foreland basin, 46–2.45 Ma for the Qaidam foreland basin, and 29.5–0 Ma for the Jiuquan foreland basin. This northward migration in the timing of foreland basin development suggests that the northeastern part of Tibet grew at least in part by accretion of Cenozoic sedimentary basins to its northern margin.

Stratigraphy and Detrital Modes of Upper Messinian Post-evaporitic Sandstones of the Southern Apennines, Italy: Evidence of Foreland-Basin Evolution during the Messinian Mediterranean Salinity Crisis

During the Messinian, the southern Apennines thrust belt experienced a period of strong tectonic rearrangement and accretion, activation of overthrusts, and consequent migration of depocenters. The upper Miocene successions cropping out in the northern segment of the southern Apennine thrust belt have good potential for improving our understanding of the interplay between Messinian salinity-crisis events and foreland-basin evolution. The local Messinian stratigraphy includes: (1) pre-evaporitic thin-bedded euxinic marly clay, interbedded with diatomaceous marls; (2) evaporitic limestone, crystalline gypsum, and reworked gypsum; (3) post-evaporitic deposits subdivided into two main units: the Torrente Fiumarella unit and the Anzano Molasse Formation that grade upward into ostracod-rich deposits (Lago-Mare facies). The evaporitic and post-evaporitic sequences are separated by an angular unconformity. This paper deals with the stratigraphic and petrographic study of the post-evaporitic deposits. The Torrente Fiumarella unit includes lacustrine and alluvial conglomerates, quartzolithic sandstones containing abundant carbonate detritus, shale, and reworked clastic gypsum. The Anzano Molasse Formation includes thick-bedded deltaic to turbiditic conglomerates and sandstones passing upward to thin-bedded turbidite sandstones and marlyclayey siltstones. Sandstones are quartzofeldspathic with variable proportions of sedimentary (both carbonate and siliciclastic) and plutonic detritus. In particular, two populations are present, plutonic-rich and mixed plutonic-sedimentary. Volcaniclastic layers, composed of dominantly vitric particles (shards and pumice), are also interbedded within Anzano Molasse sandstones. The Anzano succession includes rare freshwater ostracods that increase in abundance in the uppermost Lago-Mare facies. The Lago-Mare facies deposits are represented by silty-marly clay with abundant Ostracoda shells (Ilyocypris gibba, Cyprideis torosa and Candona sp.) and intrarenite having abundant intrabasinal carbonate particles (ooids, peloids, and bioclasts) and subordinate extrabasinal noncarbonate and carbonate particles. The post-evaporitic sequences represent an infilled foredeep basin, with a lacustrine environment progressively deepening and experiencing gravity resedimentation. Detrital modes document complex provenance relations from upper Messinian accreted terranes of the southern Apennines thrust belt. Post-evaporitic sandstones in the Irpinia-Daunia sector of the southern Apennines foreland-basin system record both the effects of the foreland tectonic evolution and the Messinian Mediterranean salinity crisis. They may represent alternative models for foreland-basin evolution during a restricted time in late Messinian, which can be applicable also in other portions of the circum-Mediterranean orogen.

Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia

ABSTRACTEastward Andean orogenic growth since the late Oligocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation in the Chaco basin. To understand the interaction between Andean tectonics and contemporaneous foreland development, we analyse stratigraphic, sedimentologic and seismic data from the Subandean Belt and the Chaco Basin. The structural features provide a mechanism for transferring zones of deposition, subsidence and uplift. These can be reconstructed based on regional distribution of clastic sequences. Isopach maps, combined with sedimentary architecture analysis, establish systematic thickness variations, facies changes and depositional styles. The foreland basin consists of five stratigraphic successions controlled by Andean orogenic episodes and climate: (1) the foreland basin sequence commences between ∼27 and 14 Ma with the regionally unconformable, thin, easterly sourced fluvial Petaca strata. It represents a significant time interval of low sediment accumulation in a forebulge‐backbulge depocentre. (2) The overlying ∼14–7 Ma‐old Yecua Formation, deposited in marine, fluvial and lacustrine settings, represents increased subsidence rates from thrust‐belt loading outpacing sedimentation rates. It marks the onset of active deformation and the underfilled stage of the foreland basin in a distal foredeep. (3) The overlying ∼7–6 Ma‐old, westerly sourced Tariquia Formation indicates a relatively high accommodation and sediment supply concomitant with the onset of deposition of Andean‐derived sediment in the medial‐foredeep depocentre on a distal fluvial megafan. Progradation of syntectonic, wedge‐shaped, westerly sourced, thickening‐ and coarsening‐upward clastics of the (4) ∼6–2.1 Ma‐old Guandacay and (5) ∼2.1 Ma‐to‐Recent Emborozú Formations represent the propagation of the deformation front in the present Subandean Zone, thereby indicating selective trapping of coarse sediments in the proximal foredeep and wedge‐top depocentres, respectively.Overall, the late Cenozoic stratigraphic intervals record the easterly propagation of the deformation front and foreland depocentre in response to loading and flexure by the growing Intra‐ and Subandean fold‐and‐thrust belt.

Modelling interactions between fold–thrust belt deformation, foreland flexure and surface mass transport

ABSTRACTInteractions between fold and thrust belt deformation, foreland flexure and surface mass transport are investigated using a newly developed mathematical model incorporating fully dynamic coupling between mechanics and surface processes. The mechanical model is two dimensional (plane strain) and includes an elasto‐visco‐plastic rheology. The evolving model is flexurally compensated using an elastic beam formulation. Erosion and deposition at the surface are treated in a simple manner using a linear diffusion equation. The model is solved with the finite element method using a Lagrangian scheme with marker particles. Because the model is particle based, it enables straightforward tracking of stratigraphy and exhumation paths and it can sustain very large strain. It is thus ideally suited to study deformation, erosion and sedimentation in fold–thrust belts and foreland basins.The model is used to investigate how fold–thrust deformation and foreland basin development is influenced by the non‐dimensional parameter , which can be interpreted as the ratio of the deformation time scale to the time scale for surface processes. Large values of imply that the rate of surface mass transport is significantly greater than the rate of deformation. When , the rates of surface processes are so slow that one observes a classic propagating fold–thrust belt with well‐developed wedge top basins and a largely underfilled foreland flexural depression. Increasing causes (1) deposition to shift progressively from the wedge top into the foredeep, which deepens and may eventually become filled, (2) widespread exhumation of the fold–thrust belt, (3) reduced rates of frontal thrust propagation and possible attainment of a steady‐state orogen width and (4) change in the style and dynamics of deformation. Together, these effects indicate that erosion and sedimentation, rather than passively responding to tectonics, play an active and dynamic role in the development of fold–thrust belts and foreland basins. Results demonstrate that regional differences in the relative rates of surface processes (e.g. because of different climatic settings) may lead to fold–thrust belts and foreland basins with markedly different characteristics. Results also imply that variations in the efficiency of surface processes through time (e.g., because of climate change or the emergence of orogens above sea level) may cause major temporal changes in orogen and basin dynamics.

Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India

Recent work on Himalayan tectonics indicates that prior to the Cenozoic collision of India and Asia, an enigmatic Cambrian– Ordovician event may have strongly infl uenced the regional geology of the Himalaya. Stratigraphic and sedimentological analyses of well-preserved Cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the Himalaya. The Parahio Formation, defi ned herein, of the Parahio Valley, Spiti region, in the Tethyan Himalaya of India, is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya. This formation consists of >1350 m of dominantly siliciclastic deltaic deposits. The formation ranges from uppermost Lower Cambrian (Lungwangmiaoan Stage) to middle Middle Cambrian (Hsuchuangian Stage), representing a time span of ~5–10 m.y. It contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross-bedded fl uvial facies. Many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits. The cycles are interpreted to have resulted from delta-lobe switching, based on a lack of systematic stratigraphic changes in cycle or facies thicknesses. This paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep-sea fl ysch to shallow-marine tidalites. In addition, our paleoenvironmental analysis and paleocurrent data suggest that the uppermost Lower to Middle Cambrian deposits of the Lesser and Tethyan Himalaya are parts of the same ancient northward-prograding, fl uvial-deltaic depositional system of the paleo-Tethys margin of India. An angular unconformity with overlying Ordovician conglomeratic rocks has considerable local relief, with meter-scale scours and a valley fi ll >100 m thick. The scours have northeast-southwest orientations, which parallel both the paleocurrents in the underlying Parahio Formation as well as published paleocurrent readings from the coarse red beds of the overlying Ordovician strata. The Cambrian–Ordovician unconformity is of regional extent, and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in Spiti is ~15 m.y. Our sedimentological analysis and associated paleocurrent data from the Parahio Formation, along with additional data from units both above and below the unconformity, indicate that published models portraying foreland basin development at this time with southward-directed thrusting are problematic. An alternate possibility— that uplift took place south of the Tethyan Himalaya—is also problematic, because no published stratigraphic or structural evidence exists for such an uplift to the south for either the Greater or the Lesser Himalaya lithotectonic zones.

Cambrian Depositional History of the Zanskar Valley Region of the Indian Himalaya: Tectonic Implications

A well-preserved Cambrian section in the Zanskar Valley of northern India has previously been interpreted to record the transition from a passive to an active tectonic margin related to Cambrian-Ordovician orogenesis. This interpretation has been used to support the tectonostratigraphic interpretation of other successions across the Tethyan Himalaya. Our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these Cambrian deposits: no definitive record of impending Cambrian-Ordovician orogenesis is recorded in these late Middle Cambrian rocks. A critical transition from an , 125-m-thick, stromatolite-bearing carbonate deposit, the Karsha Formation, into shale and sandstone of the Kurgiakh Formation, was interpreted to represent tectonically induced drowning of a carbonate platform. Siliciclastic strata of the Kurgiakh Formation were thought to record deep-water flysch deposition in a tectonically active foreland basin next to an arc-trench system. This interpretation was based on sandstone beds with classic Bouma sequences. We show that these event beds in the Kurgiakh instead contain hummocky cross-stratification, quasi-planar lamination, and combined-flow ripple stratification, all of which reflect deposition in shallow-marine, storm-influenced environments. Thus, although the Karsha carbonate platform may have been drowned, it did not culminate in deep-sea flysch deposition, and this in turn eliminates a major line of evidence linking Kurgiakh deposition to the onset of Cambrian-Ordovician orogenesis. Other aspects of Cambrian-Ordovician deposits of northern India also shed doubt on the proposed link between Kurgiakh sedimentation and the Cambrian-Ordovician orogenic event. First, our improved biostratigraphic database suggests that the transition from the Karsha carbonate to the Kurgiakh Formation may have predated the main phase of Cambrian-Ordovician orogenesis, as recorded by overlying Ordovician molasse, by as much as 20-30 My. Second, published data from the Ordovician molasse indicate northward paleocurrents, which are parallel to those recorded by siliciclastic deposits of the Parahio Formation below the Karsha, and thus are at odds with standard models of foreland basin development for the Cambrian-Ordovician event. Our sedimentological analysis of depositional cycles of the Parahio Formation indicates that these strata record storm- influenced environments from offshore marine to shoreface to fluvial settings. This is at odds with previous paleoenvironmental interpretations that ranged from deep-sea flysch to intertidal deposits. Paleocurrent data for marine and fluvial facies of the Parahio Formation in both Zanskar and the Spiti Valley to the south indicate northeast sediment transport. This supports the view that the Parahio and overlying carbonate of the Karsha Formation record the ancient northern passive margin of India during the Cambrian and that these strata may be distal equivalents of the younger Cambrian deposits of the Lesser Himalaya.

Dalton Wells: Geology and significance of debris-flow-hosted dinosaur bonebeds in the Cedar Mountain Formation (Lower Cretaceous) of eastern Utah, USA

The Lower Cretaceous Dalton Wells dinosaur locality, north of Moab, Utah, consists of a 2 m thick, stacked succession of four fossil-rich bonebeds with an estimated collective lateral extent of more than 4000 m 2. To date, 215 m 2 of the bonebeds have been excavated and more than 4200 vertebrate fossil field specimens have been collected. The site occurs at the base of the Yellow Cat Member of the Cedar Mountain Formation, and lies unconformably on Upper Jurassic strata of the Morrison Formation. The age of the assemblage is tentatively accepted as Barremian. Nine dinosaur taxa are recognized, making Dalton Wells one of the richest (abundance and diversity) Lower Cretaceous dinosaur localities in the world. The bonebeds are rare examples of fossiliferous subaerial debris flow deposits. At least four flows originated an undetermined distance up-slope from an Early Cretaceous site of dinosaur bone concentration and flowed through the site carrying bones short distances toward an adjacent area of lakes, ponds and wetlands. Host sediments most likely accumulated on the eastward-dipping slopes of a backbulge in a foreland basin. The overall depositional setting was alluvial–lacustrine in a warm-to-hot and seasonally wet-and-dry climate. The consistent expression and location of lacustrine facies in both the Brushy Basin Member of the Morrison Formation and the Yellow Cat Member of the Cedar Mountain Formation indicate that high rates of subsidence typified the Dalton Wells area both before and after the J–K transition, and further suggest that there was some form of long-term to episodic and localized influence on subsidence. The depositional slope required to generate debris flows may have been established by forebulge crustal flexure, but could also have been established or amplified by the high rates of subsidence that maintained the lacustrine paleoenvironments in the field area. Concretionary deposits in the Yellow Cat Member at Dalton Wells are diagenetic and apparently resulted from groundwater precipitation, likely enhanced by the presence of organics (bones) and lacustrine carbonates. The regional and cross-facies expression of calcretes in the Yellow Cat Member combined with the rarity of pedogenic indicators confirms that they are not paleosols. Mineral precipitation likely occurred as groundwater flow was established in response to foreland basin development. It remains unknown under what specific circumstances animals died and carcasses accumulated prior to being reworked by debris flows. However, the stratigraphic association of seasonally wet-and-dry facies throughout the Yellow Cat Member suggests that drought-induced mortality events were probably common. In a seasonally wet-and-dry environment, debris flows may have been triggered by intense rainfall or seismic events.

Tectonic-sedimentary evolution of the western margin of the Mesozoic Vardar Ocean: evidence from the Pelagonian and Almopias zones, northern Greece

Abstract The Vardar Zone documents the Mesozoic-Early Cenozoic evolution of several small oceanic basins and a complex history of terrane assembly. Following a Hercynian phase of deformation and granitic intrusion within the Pelagonian Zone to the west, the Vardar Zone rifted in Permian-Triassic time, with the creation of an oceanic basin (Almopias Ocean) during the Late Triassic-Early Jurassic. During the Mid-Jurassic, this ocean subducted northeastwards beneath the Paikon Zone and the Serbo-Macedonian Zone, giving rise to arc volcanism and back-arc rifting. A second ocean basin, the Pindos Ocean, opened to the west of a Pelagonian microcontinent, also during Late Triassic-Early Jurassic time. During the Mid-Late Jurassic, ophiolites were emplaced northeastwards (in present co-ordinates) from the Pindos Ocean onto the Pelagonian microcontinent, forming the Pelagonian ophiolitic mélange within a flexural foredeep. This emplacement is dated at pre-Late Oxfordian-Early Kimmeridgian from the evidence of corals within neritic carbonates that depositionally overlie the emplaced ophiolitic rocks in several areas. Related greenschist- or amphibolite-facies metamorphism is attributed to deep burial following trench-margin collision and the attempted subduction of the Pelagonian continent. An inferred phase of NNW-SSE displacement, also of pre-latest Jurassic age, imparted a regionally persistent stretching lineation and related ductile fabric, apparently related to post-collisional strike-slip. The Pelagonian Zone and its emplaced ophiolitic rocks then underwent extensional exhumation during Late Jurassic-Early Cretaceous time. The western margin of the Vardar Zone experienced extensional (or transtensional) faulting, neritic carbonate and terrigenous clastic deposition, and intermediate-silicic magmatism during Late Jurassic-Early Cretaceous time. Oceanic crust (Meglenitsa Ophiolite) formed further east in the Vardar Zone during Late Jurassic-Early Cretaceous time, possibly above a subduction zone. A near-margin setting is suggested by the presence of a deep-water terrigenous cover, probably derived from the Paikon continental unit to the east. The Vardar Zone as a whole finally closed related to eastward subduction beneath Eurasia, culminating in collision with the Pelagonian microcontinent during latest Cretaceous-Eocene time, as recorded in foreland basin development, HP-LT metamorphism, ophiolite emplacement and large-scale westward thrusting. In contrast to models that suggest closure of the Vardar Ocean in the Mid-Late Jurassic, followed by reopening of a Cretaceous ocean, we believe that the Vardar Ocean remained partly open from Triassic to Late Cretaceous-Early Cenozoic time.

Ganges basin geometry records a pre-15 Ma isostatic rebound of Himalaya

The Tertiary continental strata of the Himalayan foreland basin are subdivided into two groups, but the meaning of this subdivision was previously unclear. From the analysis of drill holes, seismic lines, dated sections, outcrops, and balanced cross sections, we find that the southward migration rate of the depositional pinch-out of the younger group is 19 ± 5 mm/yr and equals the Himalayan shortening rate. This equality shows that the flexural foreland basin development is mainly controlled by the motion of the thrust load. The long-term pinch-out migration rate was slower for the older synorogenic group. Erosion locally occurred at the end of its deposition, due to tectonic reactivation of lineaments of the Indian shield. We suggest that this change in the basin development is linked to the detachment of the subducted Indian lithosphere that decreased the slab pull and increased the mean compressive stress within the Indian plate, whereas the plate motion remained constant. The most important implication of our work is that the associated isostatic rebound could have increased the Himalayan elevation prior to 15 Ma.