Retroarc Foreland Basin Research Articles

Neoproterozoic to Lower Palaeozoic successions of the Tandilia System in Argentina: implication for the palaeotectonic framework of southwest Gondwana

The Cryogenian to Uppermost Ediacaran successions of the Tandilia System (Sierras Bayas Group and Cerro Negro Formation) in central-eastern Argentina is exceptional because of its unmetamorphosed and nearly undeformed character, its sediment provenance trend and the absence of any identified glacial deposit and the deposition of warm water carbonates. We decipher a dramatic change in the basin evolution from small-scale depositional areas during the Neoproterozoic to a larger basin related to an active continental margin throughout the Uppermost Ediacaran. The base of the succession is represented by immature detritus of alkaline composition (Villa Monica Formation), but towards the top of this formation, the material is sorted and reworked, nonetheless still reflecting in its detritus the local rocks. The clastic deposition is interrupted by diagenetic overprinted dolomites. The unconformable overlying quartz-arenitic Ediacaran Cerro Largo Formation reworked the Cryogenian Villa Monica Formation and contains mainly felsic granitic and metamorphic basement material of slightly wider variety, while the dominant alkaline geochemical signature in rocks of the Villa Monica Formation disappears. Based on diagenetic, petrographic and sedimentological data, we can interpret the unconformity representing a longer time of erosion. The Cerro Largo Formation shows a transition to mudstones and the heterolithic facies of the Olavarria Formation. The top of the Sierras Bayas Group is represented by limestones (Loma Negra Formation), which are discordantly overlain by the Uppermost Ediacaran Cerro Negro Formation. The latter displays detrital material derived from a continental arc, mafic and felsic sources. Several arc-related geochemical proxies (Th/Sc 20) are recorded in the sediment detritus, as are syn-depositional pyroclastites. The absence of volcanic material in the underlying rocks allows proposing that the Cerro Negro Formation is related to an active continental margin fringing Gondwana (“Terra Australis Orogen”) as a retro-arc or retro-arc foreland basin.

Reply to the comment by R. A. Astini and F. M. Dávila on “The West Andean Thrust, the San Ramón Fault, and the seismic hazard for Santiago, Chile”

[1] We have proposed earlier a new tectonic model for the evolution of the Andes mountain belt as a bivergent orogen. Here, to reply to a comment by Astini and Davila [2010], we discuss briefly the protracted diachronic evolution (over tens of million years) by propagating deformation at the large‐ scale (over 10–10 km), its influence on basin formation in the back‐arc region (retroarc foreland basin), and the mechanical implications of the bivergence in the tectonics of the fore‐arc region, particularly the possible effects of the underthrusting of the coastal crustal‐scale rigid block (the Marginal Block) beneath the West Andean Thrust. [2] The comment by Astini and Davila [2010] criticizes our new model presented by Armijo et al. [2010] suggesting that theAndes is a fundamentally bivergent (or doubly vergent) orogen and defends the conventional model of Andean orogeny, which we think obsolete, involving crustal shortening only by development of retroarc thrusts in the back‐arc region (e.g., as discussed, among many others, by Isacks [1988]). Our model is based on the structural study of the San Ramon Fault system and the Principal Cordillera at the front of the western flank of the Andes, which is used to characterize the crustal‐scale West Andean Thrust (WAT), a major fold‐thrust system in the fore‐arc region, synthetic to the subduction zone. Astini and Davila raise cursorily a large number of important questions, which cannot be fully addressed in this reply. [3] To summarize the main argument: (1) Astini and Davila think that according to the critical taper wedge model [e.g., Davis et al., 1983], the dominant thrusting of the Andes cannot have shifted from an initial westward vergence to an eastward vergence, as we propose; (2) they claim that the well‐known eastward (cratonward) migration of thrusting associated with foreland basin formation in Argentina cannot be explained if the initial dominant thrusting of the Andes was in the fore arc, with westward directed vergence; and (3) Astini and Davila argue that no continental block in the fore‐arc region, as the Marginal (or Coastal) Block that we have defined, can be considered as a western foreland of the Andes because there is no well‐ developed western foreland basin. The miscellaneous final remarks by Astini and Davila express their doubts that our model may fit currently accepted models of mass transfer, sediment flow across orogens, and tectonic‐climatic forcing because according to our model, a major topographic slope would have been created to the west of the Andes (a feature that as anyone can check is not hypothetical, but a matter of fact). Last (but not least), the Andes at 33.5°S latitude would not be in an early stage of its evolution (as we claim) because sediments in the Argentinean foreland record its development since more than 20 Ma. Our reply, intended to identify first‐order conflicting issues, is as follows:

The provenance and tectonic setting of the Neoproterozoic Um Hassa Greywacke Member, Wadi Hammamat area, Egypt: Evidence from petrography and geochemistry

We present new petrographic, and major and trace element data for the Um Hassa Greywacke Member, the uppermost unit of the Hammamat Group, at Wadi Hammamat area, the Eastern Desert of Egypt. The Neoproterozoic Hammamat sedimentary rocks are immature molasse-type rocks that were deposited at the end of the Pan-African Orogeny. The Um Hassa Greywacke Member is bedded, weakly deformed and lies conformably on the polymictic conglomerate of the Um Had Member. The Um Hassa greywackes are poorly sorted and composed mainly of quartz, lithic fragments, and feldspar grains, with an average modal composition of QFL = 53:17:30. Quartz is mostly monocrystalline, feldspars are mainly plagioclase, and lithic fragments are predominantly intermediate to felsic volcanic rocks. The Um Hassa greywackes are characterized by low chemical index of alteration (46–65) and SiO 2/Al 2O 3 (4.6–6.1) that indicate limited chemical weathering of the source rocks and deposition close to the source area. Both point-counting modal analyses and geochemical data suggest that the Um Hassa greywackes were derived from mixed sources. In terms of provenance, continental arc volcanic rocks and oceanic island arc-ophiolitic sources are the main contributors to the sediments of the Um Hassa greywackes. These rock types are exposed in the Eastern Desert as Dokhan-type continental arc volcanic rocks and accreted oceanic arc-ophiolitic mélange, respectively. Considering the geodynamic evolution of the Central Eastern Desert, the Um Hassa greywackes in the Wadi Hammamat area appear to have been deposited in a retroarc foreland basin behind the Dokhan continental arc developed over a west-dipping subduction zone.

Tectonic controls on Cenozoic foreland basin development in the north‐eastern Andes, Colombia

ABSTRACTIn order to evaluate the relationship between thrust loading and sedimentary facies evolution, we analyse the progradation of fluvial coarse‐grained deposits in the retroarc foreland basin system of the northern Andes of Colombia. We compare the observed sedimentary facies distribution with the calculated one‐dimensional (1D) Eocene to Quaternary sediment‐accumulation rates in the Medina wedge‐top basin and with a three‐dimensional (3D) sedimentary budget based on the interpretation of ∼1800 km of industry‐style seismic reflection profiles and borehole data. Age constraints are derived from a new chronostratigraphic framework based on extensive fossil palynological assemblages. The sedimentological data from the Medina Basin reveal rapid accumulation of fluvial and lacustrine sediments at rates of up to ∼500 m my−1 during the Miocene. Provenance data based on gravel petrography and paleocurrents reveal that these Miocene fluvial systems were sourced from Upper Cretaceous and Paleocene sedimentary units exposed to the west in the Eastern Cordillera. Peak sediment‐accumulation rates in the upper Carbonera Formation and the Guayabo Group occur during episodes of coarse‐grained facies progradation in the early and late Miocene proximal foredeep. We interpret this positive correlation between sediment accumulation and gravel deposition as the direct consequence of thrust activity along the Servitá–Lengupá faults. This contrasts with one class of models relating gravel progradation in more distal portions of foreland basin systems to episodes of tectonic quiescence.

Silurian collisional suturing onto the southern margin of the North China craton: Detrital zircon geochronology constraints from the Qilian Orogen

The Qilian Orogen of north western China records mid-Paleozoic collisional suturing of arc and continental blocks onto the south western margin of the North China craton. Silurian strata from the retroarc foreland basin mark the transition from ocean closure and northward subduction to the initiation of collision suturing. Detrital zircons were analysed from the western and eastern parts of the basin and show a spectrum of ages from Archean to Paleozoic with major age concentrations at around 2.5 Ga, 1.6 Ga, 1.2 Ga, 0.98 Ga, 0.7 Ga and 0.45 Ga. Archean age grains are derived from the North China craton, whereas the Central Qilian Bloc, which lies to the south provides the likely source for the bulk of the Proterozoic detritus. Paleozoic grains are restricted to Early Silurian samples from the western part of the basin and are considered to have been derived from the magmatic arc related to ocean closure and ultimate collision of the Central Qilian Belt with the North China craton.

Structural architecture and evolution of Kumkuli basin, north Tibet

Utilizing the new data of gravity, magnetic, and magnetotelluric survey, we analyzed the characteristics of the three geophysical attribute (gravity, magnetic, and resistivity) interfaces and the deep architecture and structure of Kumkuli basin. The research results can provide basic data for early basin structural study. From coupled basin and mountain system, analysis of the structure, and evolution of Kumkuli basin, we found that there was zoning from north to south and from west to east. Kumkuli basin has three structural architecture layers including metamorphic crystallization basement, fold basement and sedimentary cover. Kumkuli basin can be divided into three structural units, two depressions, and one uplift. Structural evolution of the Kumkuli basin can be divided into five evolution stages, including Kumkuli microcontinent formed in Sinian-Ordovician, suture around Kumkuli basin formed in Eopaleozoic, retroarc foreland basin formed in Neopaleozoic, rejuvenated foreland basin developed in Mesozoic, and strike slip and compression basin developed in Cenozoic.

The Moín High, East Costa Rica: Seamount, laccolith or contractional structure?

The back-arc area of the southern Central American arc–trench system in East Costa Rica is characterized by a complex basin system. An extensional back-arc area (the North Limón Basin) and a compressional retro-arc foreland basin (the South Limón Basin) are closely related. Both basins are separated by an approximately 50 km long and 30 km wide mound-shaped structure referred to as Moín High, which evolved in Eocene times. The Moín High has previously been interpreted as a basement structure or paleo-high. The modern geothermal gradient is 3 °C/100 m. There is no evidence for thermal anomaly or higher heat flow in that area. A mean heat flow of 56–60 mW/m 2 implies that an origin as a volcanic seamount or magmatic intrusion is unlikely. 3D static models show that the Moín High trends NNE–SSW and has an antiformal shape in cross-section and an elliptic outline in map view. The trend of the Moín High coincides with the orientation of folds in West Costa Rica that formed in response to an Eocene deformation phase. The seismic lines show that Miocene reflectors onlap against the structure. Based on this data set it is likely that the Moín High is an anticline formed due to contraction.

A review of stratigraphic simulation techniques and their applications in sequence stratigraphy and basin analysis

Stratigraphic simulation is a modelling methodology that can be used for exploration purposes to understand the key factors (sea-level change, subsidence, sediment supply rate) that control the stratigraphic geometries and architecture of a basin. Its application offers many advantages to scientists and researchers, as it provides a useful platform for analyzing and unraveling the complexities of sequence stratigraphy and basin development. A stratigraphic simulator models the stratal patterns of basins in various tectonic settings, such as in passive margins, foreland basins, retroarc foreland basins, interarc basins, remnant ocean basins, growth-faulted deltaic basins, and basins with salt diapirs. It enhances biostratigraphic interpretation by providing age constraints on stratal boundaries identified through sequence stratigraphic interpretation. This can lead to a systematic prediction of other geological aspects such as the distribution of source rocks, seals, and reservoirs. Furthermore, it may also lead to the identification of new accumulations or reservoirs within existing oil and gas fields. One type of stratigraphic simulation that is often used is a stratigraphic simulation. This forward modelling method is usually applied to predict sediment distribution of a basin. In this paper, four simulation techniques (CSM, SEDPAK, DIONISOS, and SEDSIM) and their applications are presented. The models are typically applied to verify and infer the potential for hydrocarbon entrapment and accumulation in the basin. For this reason, stratigraphic simulation is useful for petroleum exploration and development. The simulation models are also useful as teaching tools for young geologists.

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.

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.

Tectonic Control on the Sequence Stratigraphy of Nonmarine Retroarc Foreland Basin Fills: Insights from the Upper Jurassic of Central Utah, U.S.A.

Abstract Continental successions of the North American Western Interior retroarc foreland basin provide an excellent opportunity to evaluate the tectonic controls on nonmarine sequence stratigraphy. The transition between the Upper Jurassic Brushy Basin Member anastomosed fluvial system of the Morrison Formation and the gravelly braided-river deposits of the Buckhorn Conglomerate has been studied to assess the dispersal of coarse clastics and the development of associated basin-wide unconformities in a sequence stratigraphic framework. The sharp contact between the two members is interpreted to be conformable based on stratigraphic, sedimentologic, and petrologic data collected at and near Cedar Mountain in central Utah, while a regional, mature paleosol at the top of the Buckhorn Conglomerate indicates the presence of a major sequence boundary. These interpretations are combined with paleoflow data and fluvial architectural analysis to reconstruct the evolution of the alluvial equilibrium profiles that controlled deposition of the succession. The Buckhorn Conglomerate is interpreted as an example of a post-orogenic deposit on the basis of (1) its tabular geometry, (2) distance from the contemporaneous thrust belt, (3) shift to transverse paleoflow direction from the preceding Brushy Basin axial drainage, (4) conformable lower contact, and (5) presence of an overlying sequence boundary with a westward-expanding hiatus eroding the coeval foredeep. The onset of a protracted period of low rate of orthogonal convergence between the Farallon and North American Plates resulted in tectonic quiescence and consequent isostatic rebound of the Middle to Late Jurassic thrust belt and the contiguous contemporaneous foredeep, as well as dynamic uplift. The ensuing basin-wide decrease in subsidence rate induced the progradation of the Buckhorn alluvial plain into the Morrison back-bulge depozone, and the subsequent generation of the overlying pedogenic unconformity, which separates the deposits of the Jurassic flexural event from the overlying Sevier tectonostratigraphic unit.

The Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin

Our understanding of the processes involved in the Indo-Asian collision and the construction of the Tibetan Plateau are, in part, predicated on our understanding of the tectonic setting and crustal conditions of southern Tibet in the time period immediately preceding the Indo-Asian collision (Late Cretaceous). Several hypotheses have been proposed that describe the middle to Late Cretaceous tectonic and paleogeographic evolution of southern Tibet, each with different implications for when and how the Tibetan Plateau was uplifted. We examined the mid-upper Cretaceous Takena Formation of the Lhasa terrane in southern Tibet in order to reconstruct the middle to Late Cretaceous tectono-sedimentary history of this area and test the competing tectonic hypotheses. The Takena Formation consists of >2 km of sedimentary strata that include a lower marine limestone member (Penbo Member) and an upper member of clastic red beds (Lhunzhub Member). The Aptian-Albian Penbo Member consists of ~250 m of orbitolinid-limestone beds that were deposited in a shallow-marine seaway during a time when there was little regional subsidence. The overlying Lhunzhub Member (>1500 m) consists primarily of fl strata that were deposited during a period of increased subsidence. Overall, the Lhunzhub Member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base, to multistory-multilateral braided stream deposits near the top. The fl uvial sandstone units are lithic-rich and contain abundant plagioclase and volcanic grains, and paleocurrent data record northwest-directed transport; these data indicate that the sediment was derived from the Gangdese volcanic arc that developed along the southern margin of the Lhasa terrane. Following deposition, but prior to ca. 70 Ma, the beds of the Takena Formation were folded and partially eroded. The sedimentary and stratigraphic characteristics of the Takena Formation are most consistent with deposition in a retroarc foreland basin setting. The limestone beds of the Penbo Member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the Gangdese volcanic arc. The low subsidence rate recorded in the lowermost strata of the Takena Formation, including the Penbo Member and the paleosol-rich interval of the Lhunzhub Member, is interpreted to be associated with the passage of a fl exural forebulge. The overlying, upward-coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin. The Late Cretaceous folding of the Takena Formation indicates that the foreland basin strata were eventually incorporated into a fold-and-thrust belt. The upper Cretaceous sedimentary strata of the Lhasa terrane of southern Tibet predict that a north-verging fold-and-thrust belt existed along the southern margin of the Lhasa terrane during Late Cretaceous time, prior to the Indo-Asian collision. Collectively, the evidence indicates that the southern margin of the Lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the Indo-Asian collision.

U‐Pb Zircon Dating of the Granitic Conglomerates of the Hutuo Group: Affinities to the Wutai Granitoids and Significance to the Tectonic Evolution of the Trans‐North China Orogen

Abstract: The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and classical cross‐section in the middle segment of the Trans‐North China Orogen, generally divided into Eastern and Western Blocks. Unconformably overlying the Wutai and Fuping Complexes is the Hutuo Group considered as the youngest lithostratigraphic unit in the region and important both for interpreting Precambrian history as well as the overall evolution of the Trans‐North China Orogen. Lack of knowledge about provenance of the sedimentary rocks in this group has hindered understanding of the depositional environments and tectonic significance. LA‐ICP‐MS was applied to obtain U‐Pb zircon ages for the granitic pebbles, the lowest lithostratigraphic rock of the Hutuo Group, which, combined with previous lithostratigraphic, geochronological, structural and metamorphic data, provides new constraints on the sedimentary provenance and tectonic evolution of the region. The sequence of the Hutuo Group ranges upward from lower basal conglomerates and volcaniclastic rocks (Doucun Subgroup), through clastic sediments, slates, dolomites and marbles (Dongye Subgroup), to sandstones and conglomerates at the top (Guojiazhai Subgroup). Zircons from granitic pebbles preserved in the Doucun Subgroup basal conglomerates give weighted mean 207Pb/206Pb ages between 2517 Ma and 2566 Ma, which are the same as those for the late Archean Wutai Granitoids, indicating that the pebbles were derived from the Wutai granitic intrusions (2566‐2515 Ma). Based on the new data and previous studies, combined with an igneous zircon crystallization age of 2087±9 Ma obtained for volcanics in the Hutuo Group, the Hutuo Group was deposited in a subduction‐related retro‐arc foreland basin environment that developed behind the Wutai arc during the eastward‐directed subduction of the Western Block beneath the western margin of the Eastern Block. This basin underwent long‐lived sedimentation and finally closed during the ∼1850 Ma collisional event that resulted in the final amalgamation of the North China Craton.

Mesoproterozoic–Neoproterozoic transition: Geochemistry, provenance and tectonic setting of clastic sedimentary rocks on the SE margin of the Yangtze Block, South China

The SE margin of the Yangtze Block, South China is composed of the Mesoproterozoic Lengjiaxi Group and the Neoproterozoic Banxi Group, with Sinian- and post-Sinian-cover. A geochemical study was undertaken on the Mesoproterozoic–Neoproterozoic clastic sediments in order to delineate the characteristics of the sediment source and to constrain the tectonic development and crustal evolution of South China. Our results show that the Mesoproterozoic clastic sediments have a dominant component derived from a metavolcanic-plutonic terrane, with a large of mafic component. There is a minor contribution of mafic rocks and older upper crustal rocks to the provenance. Strong chemical weathering in the source area occurred before transport and deposition. The provenance for the Neoproterozoic clastic sediments was most likely old upper continental crust composed of tonalite–granodiorite-dominated, tonalite–granodiorite–granite source rocks, which had undergone strong weathering and/or recycling. A minor component of older K-rich granitic plutonic rocks and younger volcanogenic bimodal rocks is also indicated. Based on the regional geology, the geochemical data and the inferred provenance, the Mesoproterozoic Group is interpreted as a successive sedimentary sequence, deposited in an extensional/rifting back-arc basin, adjacent to a >1.80 Ga continental margin arc-terrane. The progressive extension/rifting of the back-arc basin was followed by increasing subsidence and regional uplift during continental marginal arc-continent (the Cathaysian Block) collision at ∼1.0 Ga caused the deposition of the Neoproterozoic Group into back-arc to retro-arc foreland basin. Therefore, the depositional setting of the Proterozoic clastic sediments and associated volcanic rocks within the back-arc basin reflected basin development from an active continental margin (back-arc basin), with extension or rifting of the back-arc basin, to a passive continental margin.

Influence of pre-existing plate-margin structures on foredeep filling: Insights from the Taconian (Blountian) clastic wedge, Southeastern USA

Break up of continents gives rise to conjugate margins with irregular shapes, and with thermal histories and sedimentary records that differ along strike. These elements need to be considered in the integrated study of a foreland-basin/orogenic-belt pair that formed during subsequent contractional events. This paper relates the promontory and embayment configuration of southeastern Laurentia (North American craton) to: (1) along-strike variations in width of the initial depositional profile of the Middle to Upper Ordovician Taconian (Blountian) foredeep; (2) northeastward (toward the embayment) along-strike thickening and migration of siliciclastic depocenters in the foredeep, that favored the resumption of carbonate-ramp deposition on the promontory during the Late Ordovician; and (3) variation in intensity of deformation of intrabasinal structures as recorded by reactivation of across-strike structures located on the transition zone between the promontory and embayment. Geometry and composition of the Blountian foreland strata indicate that the foredeep slope was narrower and steeper on the Alabama promontory than on the southwestern flank of the Tennessee embayment. Differential slope geometries were accommodated by reactivation of transverse basement faults, which controlled local conglomerate deposition in deep-water settings on the transition zone between the promontory and embayment. In contrast, conglomerate beds at the top of coarsening-upward successions on the southwestern flank of the Tennessee embayment record the cratonward progradation of deltaic depositional systems. The results presented here may have relevance to other peripheral foreland basins formed over rifted continental margins or retro-arc foreland basins formed adjacent to inverted intracratonic rifts.

Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia

Foreland sedimentation in the Subandean Zone of south-central Bolivia spans from the Upper Oligocene to present. It records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin, and thereby contains stratigraphic information on the tectonic evolution of the adjacent Andean fold-thrust belt. Within the Neogene orogenic wedge individual siliciclastic-dominated depositional systems formed ahead of an eastward-propagating deformation regime. We defined, described, and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the Neogene foreland basin in the Subandean Zone and the Chaco Plain. These are combined to interpret depositional settings. The up to 7.5 km-thick Neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture: (1) The basal, Oligocene–Miocene, up to 250 m-thick Petaca Formation consists dominantly of calcrete, reworked conglomeratic pedogenic clasts, and fluvial sandstone and mudstone. This unit is interpreted to represent extensive pedogenesis under an arid to semiarid climate with subordinate braided fluvial processes. (2) The overlying, Upper Miocene, up to 350 m thick Yecua Formation records numerous small-scale transgressive–regressive cycles of marginal marine, tidal, and shoreline facies of sandstone, ooid limestones, and varicoloured mudstone. (3) The Upper Miocene, up to 4500 m-thick Tariquia Formation principally consists of sandstone with interbedded sandstone–mudstone couplets representing frequent crevassing in anastomosing streams with an upsection-increasing degree of connectedness. (4) The up to 1500 m-thick Lower Pliocene Guandacay Formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone. (5) The Upper Pliocene, up to 2000 m-thick Emborozu Formation consists predominantly of alluvial-fan-deposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone. The coarsening- and thickening-upward pattern and eastward progradation, coupled with the variable proportions of overbank facies, channel size, and degree of channel abandonment, in the Tariquia, Guandacay, and Emborozu Formations reflect a distal through proximal fluvial megafan environment. This long-lived megafan grew by high sedimentation rates and a northeast-through-southeast radial paleoflow pattern on large, coarse-grained sediment lobes. The marked overall upsection change in pattern and depositional styles indicate fluctuations in accommodation space and sediment supply, regulated by basin subsidence, and are attributable to Andean tectonics and climatic controls.

Mixed sediment deposition in a retro-arc foreland basin: Lower Ellis Group (M. Jurassic), Wyoming and Montana, U.S.A.

The “lower” Ellis Group (M. Jurassic) of northern Wyoming and southern Montana affords an excellent opportunity to examine the influence of tectonics, sea-level change, and incipient topography on facies dynamics and the evolution of mixed sediment ramp deposits. The Sawtooth, Piper, and Gypsum Spring formations (Bajocian to Callovian) represent sedimentation along the forebulge of a retro-arc foreland basin. The “lower” Ellis Group records deposition during two transgressive–regressive cycles, (1) a Bajocian-age cycle dominated by evaporites and red shales, and (2) a Bathonian-age cycle characterized by carbonates, evaporites, and red shales. These cycles are capped by a Callovian-age cycle distinguished by carbonates and red shales that is represented by the “lower” Sundance and Rierdon Formations. Transgressive episodes favored intensified chemical sediment production resulting in thick units deposited in subtidal to peritidal environments. Regressive periods are characterized by supratidal redbed progradation and subsequent shallowing–upward cycles. The depositional cycles in the lower Ellis Group developed due the interplay between sea-level change and tectonic subsidence related to the evolution of a retro-arc foreland basin. Differential subsidence before, during, and after deposition created paleohighs that locally influenced accommodation space and, thereby, complicated depositional and erosional patterns. This paper provides a regional framework for further analysis of the depositional history of the lower Ellis Group by addressing the stratigraphic relationships between the Sawtooth, Piper, and Gypsum Spring Formations.

Patterns of sedimentation in the Precambrian

The principle of uniformitarianism may be applied to Precambrian basin evolution and to the sedimentary record as a whole. The major difference in the Precambrian Eon lay in variability of rates and intensities of processes controlling weathering, erosion, transport, deposition, lithification, and diagenesis. This paper examines Precambrian sedimentation patterns within the larger framework of Earth evolution. Pre-rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis, resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension, all reworked by marine current systems and localised turbidites. From c. 4 to 3.2 Ga, greenstone belts provided the predominant settings for the thin passive margin carbonates, BIF, stromatolitic evaporites, pelites and quartzites, and lesser synorogenic turbidites, conglomerates, and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings. Common palaeoenvironments were high gradient alluvial fans, low sinuosity braided rivers, and relatively shallow marine settings, subject to wave and tidal action, and turbidity currents. Although continental crustal growth continued largely through greenstone belts until c. 2.7 Ga, the Witwatersrand basin (c. 3.0–2.7 Ga; Kaapvaal craton, South Africa) reflects initial stabilisation of the oldest craton, with an epeiric sea accumulating largely fluvial detritus subject to tidal (inland) and storm-wave (craton-marginal) reworking within a retroarc foreland basin setting. Neoarchaean–Palaeoproterozoic sedimentation is discussed within a framework of two global “superevents”, at c. 2.7 Ga and 2.2–1.8 Ga, each encompassing major changes in Earth's evolution related to the supercontinent cycle, mantle superplumes, peaks in crustal growth rates, and significant biochemical changes within the atmosphere–hydrosphere system. Concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed, and deposition of iron-formation peaked globally at c. 2.5 Ga. The first global glaciation at c. 2.4–2.2 Ga provides little support for the “Snowball Earth” theory, but does suggest a negative feedback loop model whereby intraglacial CO 2-related warming and synglacial decreases in weathering alternated up to three times. Models for palaeo-atmospheric and -oceanic evolution are mutually exclusive, but by c.1.8 Ga, the existence of large landmasses and free oxygen in Earth's atmosphere enabled erg development and red bed sedimentation globally; the full spectrum of Phanerozoic–Modern sedimentary environments was thus present on Earth. A third postulated “superevent” at c. 0.8–0.6 Ga essentially recreated conditions experienced at c. 2.2–1.8 Ga, with, additionally, at least three global refrigeration events.

U-Pb geochronology and geochemistry of the McCoy Mountains Formation, southeastern California: A Cretaceous retroarc foreland basin

The timing of deposition of fluvial sediments now forming the >7-km-thick McCoy Mountains Formation is one of the key uncertainties in reconstructing the Mesozoic paleogeography of southern California and western Arizona. Ion-microprobe U-Pb geochronologic data for individual zircons from nine sandstones from the McCoy Mountains type section and six associated igneous rocks provide significant new constraints on the tectonic setting and the timing of deposition within the northwest-trending McCoy basin. U-Pb zircon data from a metavolcanic rock of the underlying Dome Rock sequence in the Palen Mountains confirm that the McCoy Mountains Formation was deposited after regional Middle to Late Jurassic arc magmatism. U-Pb zircon data from a Late Cretaceous granodiorite intruding the formation in the Coxcomb Mountains confirm that the formation was deformed and metamorphosed prior to 73.5 ± 1.3 Ma. Populations of detrital zircons vary systematically with both rock type and stratigraphic height; lithic arkoses predominantly derived from the west have consistently more abundant younger zircons than do litharenite sandstones predominantly derived from the north, and the youngest zircons yield maximum depositional ages that decrease from 116 Ma near the base to 84 Ma near the top of the section. The detrital-zircon data permit a Late Jurassic age for the basal, comparatively quartz-rich sandstone. However, the data further suggest that >90% of the formation was deposited between middle Early and middle Late Cretaceous time. These results are consistent with the hypothesis that most of the McCoy Mountains Formation represents a retroarc foreland basin, deposited behind the active, evolving Cretaceous Cordilleran continental- margin magmatic arc that lay to the west and in the foreland of the actively deforming Cretaceous Maria fold-and-thrust belt.

Late Cretaceous subsidence in Wyoming: Quantifying the dynamic component

The Late Cretaceous Western Interior Basin of North America is generally considered to have been a retroarc foreland basin. Flexural backstripping of the stratigraphic record from 97.2 Ma to 73.4 Ma, along a section perpendicular to the Wyoming-Idaho salient of the Sevier belt, clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads. The simulation demonstrates that the flexural foredeep is only ∼180–120 km wide and shows a forebulge located near the subsequent Laramide Rock Springs uplift. Foredeep strata between the Rock Springs uplift on the east and the thrust front to the west were mostly involved in the eastward- overlapping thrust belt or deeply truncated by inferred rebound after 78.5 Ma. The difference between the observed, decompacted, cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading, the “residual” subsidence, increases from ∼800 m in eastern Wyoming to ∼1.8 km near the thrust belt. The westward-thickening Upper Cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the North American cratonic margin by underplating and mantle flow associated with the shallowly subducted Farallon plate.