Stages Of Basin Evolution Research Articles

Sedimentary and tectonic environment of the Ordovician Föllinge Greywacke, Storsjön area, Swedish Caledonides

The Middle to Upper Ordovician Föllinge Greywacke occupies about 4 500 km2 of the easternmost overthrust terrain in the Caledonides of central Sweden. Its outcrop area around lake Storsjön can be divided into two realms in which turbidites are preserved from the two principal stages of development of a basin that was characterized by a western facies of sand-rich fans and an eastern basin plain with dark mud. The eastern, Föllinge Realm, represents an early stage of the basin, with lithic greywackes, with feldspar and detrital carbonate among the clasts. The western, Mörsil Realm is characterized by quartz wackes to lithic greywackes with a high clay content and little carbonate or feldspar, and transport from the southwest and west. The heavy-mineral suite of both realms is dominated by zircon and minerals derived either from old granitic terrains or reworked sediments derived from the erosion of such terrains. The Cr and Ni content is relatively high and increased in late stages of basin evolution, as did the chlorite content. The basin was at first situated between a western tectonic rise, deformed between the Middle Cambrian and the Middle Ordovician, and an eastern starved carbonate facies to the east. Subsidence of the rise in late Middle Ordovician to early Late Ordovician led to westward migration of the deepest part of the basin, which was subsequently fed from an elevated terrain still farther to the west, but also from rises to the north and south.

Contribution of erosion and intrusive-hydrothermal activity to the depth profile of organic matter maturation in sedimentary basins

Sharp variations in organic matter (OM) maturity with depth in sedimentary basins reflected in an abrupt increase in vitrinite reflectance (%Ro) can be caused by both the thermal effect of intrusions accelerating OM maturation in adjacent rocks and removal of a thick sedimentary layer during the extensive erosion of the basin. It is traditionally believed that the erosion of a 1.5–3.0-km-thick sedimentary layer must significantly affect the depth profile of OM maturity, resulting in a drastic increase in %Ro in the present-day sedimentary basin. Some authors used such jumps (ΔRo) to estimate the amplitude of erosion. However, our analysis showed that the depth profile of %Ro is controlled not only by the amplitude of erosion but also by sedimentation history during the posterosion stage of basin evolution. Eastern Algerian basins are good examples to examine this problem, because their evolution involved both the stage of extensive Hercynian erosion and subsequent strong hydrothermal and intrusive activity. Numerous measurements of %Ro in core samples from eastern Algerian sedimentary basins were used to analyze this problem. The calculations of %Ro(z) distributions on the basis of numerical reconstructions of the thermal history of sedimentary sections in more than 60 boreholes of the area showed that the Hercynian erosion could account for only a minor part of the observed amplitude of Δ%Ro, in spite of the significant amplitude of erosion (1.5–3.5 km) typical of the basins of the Algerian Sahara. At the same time, the intrusive and associated hydrothermal activity of the Triassic and Jurassic adequately explains the amplitude of a stepwise increase in OM maturation with depth in the present-day sedimentary sections of these basins. Thus, the reliability of erosion amplitudes estimated from jumps in depth profiles of %Ro can be assessed only after the detailed examination of the posterosion history of the basin.

Unravelling the multi‐stage burial history of the Swiss Molasse Basin: integration of apatite fission track, vitrinite reflectance and biomarker isomerisation analysis

ABSTRACTA complex basin evolution was studied using various methods, including thermal constraints based on apatite fission‐track (AFT) analysis, vitrinite reflectance (VR) and biomarker isomerisation, in addition to a detailed analysis of the regional stratigraphic record and of the lithological properties. The study indicates that (1) given the substantial amount of data, the distinction and characterisation of successive stages of heating and burial in the same area are feasible, and (2) the three thermal indicators (AFT, VR and biomarkers) yield internally consistent thermal histories, which supports the validity of the underlying kinetic algorithms and their applicability to natural basins. All data pertaining to burial and thermal evolution were integrated in a basin model, which provides constraints on the thickness of eroded sections and on heat flow over geologic time.Three stages of basin evolution occurred in northern Switzerland. The Permo‐Carboniferous strike–slip basin was characterised by high geothermal gradients (80–100°C km−1) and maximum temperature up to 160°C. After the erosion of a few hundreds of metres in the Permian, the post‐orogenic, epicontinental Mesozoic basin developed in Central Europe, with subsidence triggered by several stages of rifting. Geothermal gradients in northern Switzerland during Cretaceous burial were relatively high (35–40°C km−1), and maximum temperature typically reached 75°C (top middle Jurassic) to 100°C (base Mesozoic). At least in the early Cretaceous, a stage of increased heat flow is needed to explain the observed maturity level. After erosion of 600–700 m of Cretaceous and late Jurassic strata during the Paleocene, the wedge‐shaped Molasse Foreland Basin developed. Geothermal gradients were low at this time (≤20°C km−1). Maximum temperature of Miocene burial exceeded that of Cretaceous burial in proximal parts (<35 km from the Alpine front), but was lower in more distal parts (>45 km). Thus, maximum temperature as well as maximum burial depth ever reached in Mesozoic strata occurred at different times in different regions. Since the Miocene, 750–1050 m were eroded, a process that still continues in the proximal parts of the basin. Current average geothermal gradients in the uppermost 2500 m are elevated (32–47°C km−1). They are due to a Quaternary increase of heat flow, most probably triggered by limited advective heat transport along Paleozoic faults in the crystalline basement.

Lower Triassic sequence stratigraphy of the western part of the Germanic Basin (west of Black Forest): Fluvial system evolution through time and space

The aim of this paper is to analyse the fluvial evolution of the Lower Triassic in the western part of the Germanic Basin through time and space, as well as the impact of the geodynamic and climatic setting on the preservation of fluvial deposits. The Lower Triassic crops out only in the Vosges Massif and the Black Forest, so well-log studies are required to realise sequence stratigraphy correlations and establish comparisons with others parts of the Germanic Basin. In a first step, we use well-log data analyses to characterise the electrofacies associations in the Triassic and then define the well-log signatures of each formation. In a second step, the characterisation and recognition of genetic sequences and their stacking pattern allow us to define seven minor cycles integrated into two major cycles. Finally, the quantification of the lithologies at different stages of basin evolution leads to the reconstruction of paleoenvironmental maps to illustrate facies evolution through space and time. A comparison with cycles defined in the Germanic Basin allows us to propose correlations of the Lower Triassic on either side of the Rhine Graben and leads to a discussion of the evolution of fluvial systems through time and space. During the Scythian, the fluvial style is characterised by braided fluvial systems evolving laterally into lake deposits towards the central part of the Germanic Basin. During this stage, the basin was a huge depression with very few marine connections in its extreme eastern part. The stratigraphic cycles represent rhythmic fluctuations in relative lake level that could be attributed to sediment supply and/or lake level variations in an arid setting. Four minor stratigraphic cycles are observed that are integrated within a single major stratigraphic cycle. During the period of the stratigraphic base-level rise of the major cycle, a maximum of 233 m of sediment would represent a duration of sedimentation in the Paris Basin of at least 1.8 m.y. During the period of the stratigraphic base-level fall of the major cycle, a maximum of 65 m of sediment would have accumulated over 2.5 m.y. On the western edge of the Germanic Basin, the top of the Scythian is marked by a major sedimentary break characterised by a planation surface, with preservation of the first paleosols, followed by the Hardegsen unconformity. This unconformity is tectonically deformed, leading to the development of a new sedimentation area in the west of the basin. The fluvial sedimentation above this discontinuity shows a trend towards enhanced development of floodplain or lacustrine-type environments at its western margin, with the formation of paleosols. The fluvial systems are linked with sabkhas, and then with a shallow sea connected to the Tethys Ocean. In this context, the stratigraphic cycles are caused by variations in relative sea level and/or sediment supply. The fluvial deposits are preserved in an exoreic basin.

Geometry and structural evolution of the Mesohellenic Trough (Greece): a new approach

Abstract The Mesohellenic Trough (MHT) is an elongate basin parallel to the Hellenide isopic zones that extends from southern Albania through northern Greece. The basin developed from Mid-Late Eocene to Mid-Late Miocene time related to Alpine orogenic processes. Structural and kinematic evidence shows that the MHT developed in response to successive tectonic events, involving isostatic crustal flexure, strike-slip and normal faulting, all related to inferred oblique convergence of the Apulian and Pelagonian microcontinents. The Mesohellenic Trough evolved as a piggyback basin above westward-emplacing ophiolites and higher Pelagonian units. This differs from previous interpretations that envisaged foreland flexure related to backthrusting, or subsidence associated with asymmetrical flexure, or normal faulting. The first stage of basin development during the Mid-Late Eocene was contemporaneous with the final emplacement of Pindos oceanic units and culminated in deformation and uplift of Eocene strata. The second phase was dominated by strikeslip faulting during Oligocene-Early Miocene time. The third stage was characterized by low-angle normal faulting at the eastern boundary of the MHT during the Early-Late Miocene. The evolution of the sedimentary basin ended around Late Miocene time, followed by rapid uplift and marine regression. A compressional event occurred during the latest Miocene. Finally, extensional tectonics affected the area from the Late Miocene to the present.

A connection between the Neoproterozoic Dom Feliciano (Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts – evidence from a reconnaissance provenance study

A provenance study of Neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the Pan-African Gariep Belt (Stinkfontein Subgroup and Oranjemund Group, respectively) in southwestern Africa, as well as in the Rocha Group of the Punta del Este Terrane (Dom Feliciano Belt) in Uruguay, revealed that the Oranjemund and Rocha Groups can be correlated and most likely formed in the same basin. Thus, the Rocha Group is considered to represent the fill of the westernmost part of a re-activated Ediacaran Gariep Basin. The lower parts of the Oranjemund and Rocha Groups reflect erosion of mafic rocks, whereas the upper parts are derived from a predominantly felsic source area. First major and trace element geochemical data are consistent with oceanic islands of within-plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the Oranjemund Group, whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper Oranjemund Group. Age spectra obtained by SHRIMP U–Pb analyses of detrital zircon grains from the Stinkfontein Subgroup (Port Nolloth Group), the Oranjemund Group, and the Rocha Group are very similar, except for a lack of the youngest age group around 600 Ma in the Stinkfontein Subgroup. In all three units, zircon grains of 1000–1200 Ma dominate, with a further peak in the age distribution between 1700 and 2000 Ma. These ages compare well with the pre-Gariep basement geology in southwestern Africa, where the former age range corresponds to magmatic and high-grade metamorphic activity in the Mesoproterozoic Namaqua-Natal Belt and the latter to an extensive Palaeoproterozoic Andean-type volcanic arc (Richtersveld Terrane). Comparable ages are conspicuously absent in the basement of the Rio de la Plata Craton in South America. Derivation of the Rocha Group sediments from a similar source as the contemporaneous Oranjemund Group sediments is therefore suggested. The most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640–590 Ma magmatic arc of the Dom Feliciano Belt. This interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the Gariep Belt. Two stages of basin evolution are distinguished for the Gariep Basin, an early failed continental rift that subsequently became re-activated as back-arc basin related to the Cryogenian Dom Feliciano arc. Consequently, the Gariep Basin does not represent the main suture between the Rio de la Plata and the Kalahari Cratons. This suture is now suspected to be west of the Dom Feliciano volcanic arc, and might be connected with the Sierra Ballena – Major Gercino Lineament.

Sedimentary facies, mineralogy, and geochemistry of the sulphate-bearing Miocene Dam Formation in Qatar

The Miocene deposits of the Dam Formation were deposited in a narrow seaway stretching along the western edge of the Qatar Arch. During the initial stages of basin evolution the rising Zagros Mts. delivered debris in this fore deep basin. The paleocurrent and paleogeographic zonation are reflected by the heavy mineral assemblage, by the spatial distribution of phyllosilicates and the various types of sulphate. From NW towards the SE, the contents of smectite and palygorskite increase, whereas the illite and kaolinite contents decrease. Mega crystals of gypsum are found in the NW and massive fine-grained gypsum in the SE of the basin. During the waning stages of basin subsidence, the Arabian Shield became more and more important as a source for the Miocene sediments. In this study, the Dam Formation was subdivided into 7 members/lithofacies associations (lower, middle, upper Salwa, and Al Nakhsh Members, Abu Samrah Member). The Salwa Members at the base of the Dam Formation consists of heterolithic siliciclastic-calcareous sediments which were laid down under meso- to microtidal conditions. The Al Nakhsh Members formed under macrotidal conditions with sub- to supratidal depositional environments passing into continental ones. Celestite, gypsum, and microbial mats (stromatolites) are very widespread in these sabkha sediments. Crystals of gypsum and the thickness of stromatolites tremendously increase towards younger sediments indicating thereby a close genetic link between growth of microbial domes and gypsum precipitation. Throughout the Abu Samrah Member marine calcareous sediments were deposited in a microtidal wave-dominated environment. Dissolution of Eocene evaporites at depth governed the lithofacies differentiation in the Miocene Dam Formation.

Fluid systems and mineralization in the north German and Polish basin

AbstractThe North European Basin hosts mineral deposits like the Kupferschiefer and the Mississippi Valley Type deposits in the Silesian sub‐basin in Poland. The basement to this basin, exposed in the Harz Mts and in the Flechtingen and Calvörde Blocks, contains Mesozoic Pb–Zn vein mineralization and barite–fluorite deposits as well as massive hematite veins in the Rotliegend volcanics. A comparison of the mineralizing models of these deposits with results from a basin‐wide petrographic, fluid inclusion and stable isotope study shows that the genesis of the mineral deposits can be explained by fluid systems that were active during different stages of basin evolution. These comprise syn‐ to post‐magmatic fluids derived from or mobilized in the course of the Rotliegend magmatism, fluids convecting in the Rotliegend units during the extensional basin subsidence in the Permo‐Triassic and originating from progressive devolatilization of the basin sequence and fluids derived from the overlying Zechstein evaporites. Deep‐reaching fault systems developing during the Cretaceous tectonic reactivation enhanced fluid percolation from the surface to the deep sections of the basin sequence. Identification and correlation of these fluids across the basin and in the mineralizations provide the base for a basin‐wide metallogenetic model.

Quantitative Meso-/Cenozoic development of the eastern Central Atlantic continental shelf, western High Atlas, Morocco

The well exposed Late Paleozoic to Cenozoic succession in the western High Atlas (Morocco) documents the early rift to mature drift development of the eastern Central Atlantic continental shelf basin. Vertical sections, depositional geometries and unconformities have been used to reconstruct the basin architecture prior to Atlasian inversion. Two-dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest Paleozoic to Early Cenozoic. Basin evolution stages include (i) early rift, Late Permian to Anisian; (ii) rift climax, Ladinian to Carnian; (iii) sag, Norian to Early Pliensbachian; (iv) early drift, Late Pliensbachian to Tithonian; (v) mature drift, Berriasian to Cenomanian; (vi) mature drift with initial Atlasian deformation, Turonian to Late Eocene; (vii) Atlasian deformation; Late Eocene to Early Miocene; (viii) Atlasian uplift and basin inversion, Early Miocene to Recent. The Late Permian to Late Cretaceous basin development comprises eight subsidence trends of 10–35 Myr duration. Trends were initiated by changes in thermo-tectonic subsidence, which in turn triggered positive and negative feedback processes between sediment flux, flexural and compaction-induced subsidence. Plate-tectonic reconfigurations in the Atlantic domain controlled the thermo-tectonic subsidence history and the basin development of the Agadir segment of the northwest African passive continental margin: (1) major shifts in the sea-floor spreading axis; (2) significant decreases in sea-floor spreading rates; (3) the stepwise migration of crustal separation and sea-floor spreading in and beyond the Central Atlantic; (4) African–Eurasian relative plate motions and convergence rates. During the Early Pliensbachian/Toarcian to Cenomanian the first three plate-tectonic reconfigurations triggered changes in ridge-push forces and modulated the extensional stress field of Central Atlantic plate drifting. Since the Turonian, African–Eurasian relative plate motions and convergence rates represented the dominant control on the thermo-tectonic subsidence history in the Agadir Basin. Major variations in sediment flux and total subsidence characterize the development of the northwest African passive continental margin. The explanation of typical stratigraphic sequences as caused predominantly by sea-level fluctuations, and rough assumptions on sediment input/production and subsidence, is not necessarily applicable to passive continental margins. The methodology applied in this study, including the newly developed tool of ‘Compositional Accommodation Analysis’, allows to develop more rigorous genetic models for the development of continental shelf basins.

Detrital Fission-Track Analysis and Sedimentary Petrofacies as Keys of Alpine Exhumation: The Example of the Venetian Foreland (European Southern Alps, Italy)

Sedimentary petrography and detrital apatite fission-track analysis of sediments from the Oligo-Miocene Venetian foreland succession (NE Italy) reveal a multi-stage evolution of the Alpine source areas. During the first stage of sedimentation (Chattian to Langhian), the foreland basin was filled by sediments derived from erosion of mainly crystalline Austroalpine units exhumed north of the Periadriatic Lineament. These sediments are characterized by litharenites with mean quartz content of about 61% and by a mean detrital apatite fission-track peak of about 30 Ma. The Permian-Paleogene sedimentary cover of the Southalpine crystalline basement (Dolomite region) was bypassed by the sediments and made only a minor input. From the Serravallian onwards, the foreland basin was incorporated into the Southalpine thrust belt. The compressional events of the Valsugana phase resulted in thrusting and uplift of the Southalpine domain, as documented by Tortonian fission-track ages in the hanging wall of the major regional thrust (Valsugana thrust fault). The Southalpine crystalline basement, unaffected by Alpine metamorphism, and its sedimentary cover became the main source of sediments, as revealed by the increase in extrabasinal carbonate grains and detrital apatite grains with Late Triassic-Jurassic fission-track ages. During this final stage of basin evolution, about 3000 m of sediments were deposited. These strata subsequently have been uplifted and eroded since the Pliocene, indicating a southward migration of the Alpine deformation front.

Tectonic controls on spatio‐temporal development of depositional systems and generation of fining‐upward basin fills in a strike‐slip setting: Kyokpori Formation (Cretaceous), south‐west Korea

Abstract The Kyokpori Formation (Cretaceous), south‐west Korea, represents a small‐scale lacustrine strike‐slip basin and consists of an ≈ 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep‐gradient large‐scale delta slope; (IV) base of delta slope to prodelta; (V) small‐scale nested Gilbert‐type delta; (VI) small‐scale delta‐lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large‐scale steep‐sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening‐ to fining‐upward succession (up to 215 m thick) and the upper fining‐upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. Stage 1 is represented by thick coarse‐grained deposits in the lower succession that form subaqueous breccia talus and steep‐sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests a half‐graben structure for the basin, and the thick accumulation of coarse‐grained deposits most likely reflects rapid subsidence of the basin floor during the transtensional opening of the basin. Stage 2 is marked by sandy black shale deposits in the upper part of the lower succession. The black shale is readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large‐scale dip slip suppressing the lateral slip component on basin‐bounding faults. Stage 3 is characterized by gravelly delta‐lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip‐slip displacement on basin‐margin faults can be regarded as the prime control for determining stacking patterns of such basin fills. The resultant basinwide fining‐upward sequences deviate from the coarsening‐upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike‐slip basins controlled by the changes in relative sense and magnitude of fault movements at the basin margins.

Lower Liassic Formations of the Central High-Atlas near Rich (Morocco) : lithostratigraphic specification and basin evolution

Lower Liassic Formations of the Central High-Atlas near Rich (Morocco) : lithostratigraphic specification and basin evolution

Stratigraphic architecture, magnetostratigraphy, and incised-valley systems of the Pliocene-Pleistocene collisional marine foreland basin of Taiwan

Lithofacies analysis, magnetostratigraphy, and seismic profiles of Pliocene-Pleistocene foreland basin deposits of Taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin. We have recognized several scales of stratigraphic packages and unconformities in deposits of the Taiwan foreland basin. Small-scale (20 to 150 m thick) stratigraphic sequences contain upward-shallowing, marine lithofacies successions that are bracketed by thin coquina sandstones. We interpret the small-scale stratigraphic packages as “parasequences” in the traditional sequence stratigraphy model, the thin coquina sandstones representing marine-flooding intervals. The average duration of individual small-scale packages was in the range of 37.5 k.y., on the basis of our magnetostratigraphy. These sequences are interpreted as the product of eustatic sea- level change possibly related to the orbital time series of obliquity. Intermediate-scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections. The unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin. One of the unconformities is locally an angular one that we interpret as representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca. 1.25 Ma. Across this angular unconformity, there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin. Other major unconformities that bound intermediate-scale stratigraphic sequences are high-relief disconformities. These unconformities may be the product of eustatic changes, because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities. The duration of individual, intermediate-scale packages ranges from ∼100 000 to 700 000 yr, on the basis of magnetostratigraphy and biostratigraphy. We interpret the intermediate-scale sequences as “sequences” in the traditional sequence stratigraphy model. Our analysis of the Pliocene-Pleistocene deposits of the Taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin. (1) Deposition in the Taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision. Large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development. (2) The presence of high-relief unconformities and growth structures in the Pliocene-Pleistocene foreland basin deposits suggests a well-developed wedge-top depozone in the foreland basin system. (3) The Pliocene- Pleistocene strata of the foreland basin of Taiwan record ∼2.3 m.y. of deposition, on the basis of our magnetostratigraphy. Sediment accumulation rate was on the order of ∼950 m/m.y. during the earlier stages of basin development. During the later stages of basin development, sediment accumulation rate increased to ∼1900 m/m.y. Sediment accumulation rates in the collisional marine foreland basin of Taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins.

A coupled petrological–tectonic model for sedimentary basin evolution: the influence of metamorphic reactions on basin subsidence

The lithosphere is subject to fluctuations in temperature and pressure during the formation of sedimentary basins. These fluctuations cause metamorphic reactions that change the density of the lithosphere, which, in turn, influences basin subsidence. This contribution develops a model for sedimentary basin formation to assess the importance of this coupling. The model shows that basin subsidence is significantly affected by metamorphic densification. Compared to results obtained with cruder density models, metamorphic densification accelerates subsidence in the initial post‐rifting stages as garnet becomes stable over an increasing depth interval within the mantle, an effect that amplifies the crust–mantle density contrast. For models with an extraordinarily cold lithosphere, uplift is generated as a late stage of basin evolution. In general, subsidence is not smooth but occurs instead in small steps reflecting periods of accelerated/decelerated subsidence. For typical crustal thicknesses, subsidence is controlled largely by reactions in the mantle, and particularly those determining garnet stability.

Overview of the meso- to neoproterozoic evolution of the Vindhyan basin, central India

Abstract Despite its origin in a stable intracratonic basin, the Vindhyan Supergroup, central India, embodies tectonic-driven depositional cycles of various orders. A marked change in sedimentation pattern was coupled with a transient plate-margin compression in the otherwise extensional regime. An unconformity laterally correlatable with a conformity, that divides the Supergroup in two sequences arose in consequence. Maximum flooding surfaces (MFS) within the sequences can generally be correlated with intrabasinal volcanism that drastically diminished through the initial rift to later sag stage of basin evolution. Concomitantly with the change in tectonic milieu, paleoseismic deformation structures and related deposits also became drastically reduced in scale. Nevertheless, metre- and decametre-scale depositional cycles correlate well with the subtle record of paleoseismicity. Intermittent tilting of the depositional substratum is demonstrable in systematic bed-dip changes in metre-scale fining upward depositional cycles in the rift stage. In the sag stage, it is reflected merely in consistent deflection of paleocurrent directions at the top of the decametre-scale parasequence sets. Towards the top of the Supergroup, progressive thickening and decreasing frequency of parasequences indicates a declining rate of basin subsidence. Progradation, despite punctuation, thus resulted in basin-filling.

DOLOMITIZATION OF THE EARLY EOCENE JIRANI DOLOMITE FORMATION, GABES‐TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

Dolomitization in the early Eocene Jirani Formation in the Gabes‐Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near‐surface seawater composition under oxidising conditions modified by evaporation. Non‐luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf.Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea‐level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic‐free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore‐filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre‐dolomitization and syn‐dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.

Tectonics and sedimentation in a paleo/mesoproterozoic rift-sag basin (Espinhaço basin, southeastern Brazil)

Sedimentologic, paleogeographic, stratigraphic, structural and tectonic studies in a paleo/mesoproterozoic metasedimentary succession (Espinhaço Megasequence, southeastern Brazil) indicates deposition in a rift-sag basin. Four basin evolution stages are recognized (prerift, rift, transitional and flexural). The four stages can be represented by six unconformity-bounded tectonosequences. The unconformities are recognized in the field and mappable even on a regional scale. The prerift and rift stages of the Espinhaço basin were filled by products of continental depositional systems. The prerift stage probably represents the first product of the rifting process, before the development of the half-grabens that characterize the rift stage. During the rift stage, mechanical subsidence due to lithospheric stretching was predominant and led to episodic rising of the depositional base level. As a result, the basin fill is characterized by coarsening-upward intervals. Paleocurrent patterns indicate that block tilting and half-graben subsidence/uplift controlled sediment dispersion. The first marine incursion within the Espinhaço basin marks the change in the subsidence regime of the basin. The evolution of the transitional and flexural stages was probably controlled by thermal subsidence due to thermal contraction of the lithosphere during cooling. The transitional stage was characterized by relatively low subsidence rates. Higher subsidence rates and a consequent sea-level rise characterize the flexural stage of the Espinhaço basin, in which three second-order transgressive-progradational sequences can be recognized.

The Andrelândia basin, a Neoproterozoic intraplate continental margin, southern Brasília Belt, Brazil

The Andrelândia succession comprises six lithofacies associations, from base to top, represented by: Al- paragneiss; A2- paragneiss, quartzite and schist; A3- quartzite; A4- grey phyllite/schist; A5- biotite schist/gneiss; A6- paragneiss, metachert and calc-silicate rocks. Amphibolite bodies are common in the Al, A2 and A6 associations. They represent continental and oceanic tholeiitic magmatism generated during the basin evolution. The vertical and horizontal distribution of these lithofacies suggest transgressive and regressive depositional systems of common intraplate continental margin paleoenvironments, probably including continental (Al), coastal plain-platform (A2, A3, A4, A6), submarine fan and basin plain deposits (A5.A6). The lithofacies associations and the mafic magmatism also reflect different stages of basin evolution from rifting (Al) to proto-oceanic (A2, A6) and mature continental margin stages (A3, A4, A5, A6). This continental margin had its paleocoastline oriented NW-SE and depocenter to WSW as indicated by the regional distribution of the quartzitic and politic lithofacies. The basin opening is related to a world-wide taphrogenic event that started at circa 1,000 Ma. The Andrelândia Basin comprises the intraplate continental margins between the Sao Francisco plate and the Parana Block. The basin closure took place in two subsequent collisional stages during the Brasiliano Orogeny that resulted in the Brasilia Belt and the Central Segment of the Ribeira Belt.

Tertiary foreland sedimentation in the Southern Subalpine Chains, SE France: a geodynamic appraisal

Tertiary foreland sedimentation in SE France occurred along the western sidewall of the Alpine orogen during collision of the Apulian indentor with the European passive margin. A detailed reappraisal of the stratigraphy and structure of the Southern Subalpine Chains (SSC) in SE France shows that Tertiary depocentres of differing character developed progressively toward the foreland during ongoing SW‐directed shortening. The geodynamic controls on each of four stages of basin development are evaluated using a flexural isostatic modelling package of thrust sheet emplacement and foreland basin formation. (1) The initial stage (mid to late Eocene) can be explained as a flexural basin that migrated toward the NW, closing off to the SW against the uplifting Maures–Esterel block. This broad, shallow basin can be reproduced in forward modelling by loading a lower lithospheric plate with an effective elastic thickness of 20 km. (2) The end of detectable flexural subsidence in the early Oligocene coincides with the emplacement of the internally derived Embrunais–Ubaye (E‐U) nappes, which caused 11 km of SW‐directed shortening in the underlying SSC. The lack of Oligocene flexural subsidence dictates that the E‐U units were emplaced as gravitational nappes. Within the SSC, Oligocene sedimentation was restricted to small thrust‐sheet‐top basins recording mainly continental conditions and ongoing folding. Further west, Oligocene to Aquitanian NNW–SSE extension generated the Manosque half‐graben as part of the European graben system that affected an area from the Gulf of Lion to the Rhine graben. (3) Following the Burdigalian breakup of the Gulf of Lion rift, a marine transgression migrated northward along the European graben system. Subsequent thermal subsidence allowed 1 km of marine sediments to be deposited across the Valensole and Manosque blocks, west of the active SSC thrust belt. (4) Mio‐Pliocene conglomeratic deposits (2 km thick) were trapped within the Valensole basin by the uplifting Vaucluse block to the west and the advancing Alpine thrust sheets to the east. Late Pliocene thrusting of the SSC across the Valensole basin (approx. 10.5 km) can be linked along a Triassic detachment to the hinterland uplift of the Argentera basement massif.

Evolution of the Northeast German Basin — inferences from a 3D structural model and subsidence analysis

A 3D structural model of the Northeast German Basin was evaluated with special emphasis on its evolution as an intracontinental depression. The study includes investigations on subsidence history and structural setting of the basin. Thickness evolution and calculated tectonic subsidence volumes of Permian to Quaternary sediments in the Northeast German Basin indicate that the subsidence history was related to five stages of basin evolution which differ in their subsidence mechanisms. For the initial rift phase in the Late Carboniferous to Early Permian, a dominant thermal event and subordinate horizontal stresses were indicated by thickness variation evolution and by structural evidence. The main part of basin subsidence occurred in a NW–SE-oriented basin in the subsequent phase of thermal relaxation with maximum subsidence from Early Permian (Rotliegend) to Middle Triassic (Muschelkalk). From Middle Triassic the thermal subsidence pattern was superposed by further tectonic events. In the Middle Triassic regional extension led to a reconfiguration of the southern part of the basin, where new NNE–SSW-trending troughs (Rheinsberg and Gifhorn Troughs) developed. In the Jurassic the northwestern part of the basin was uplifted while in the south the Keuper subsiding areas continued to sink and NW–SE-trending depressions, related to salt margins, became important. Differentiation continued into Cretaceous times when regional compression caused uplift of the southeastern part of the basin and basin margins. A final subsidence phase occurred in the Cenozoic. This was accompanied by intensive salt movement. Recent basin configuration reflects the superposition of structural elements resulting from different evolution stages. The main structural characteristics of the basin are: (1) a vertical tectonic zonation in a pre-Zechstein succession, which lacks significant internal structures, and a strongly deformed post-Zechstein succession, which was decoupled due to the thick Zechstein salt; and (2) a marked asymmetry of the basin with a shallow northern slope and a steep bounding fault at the southern margin (Elbe Fault System). The northwestern part of the basin shows the structural properties of an intracratonic sag basin with persisting subsidence and with minor salt mobilisation. In contrast, initial structures in the southeastern part are strongly overprinted by younger tectonic events including Middle Triassic to Jurassic extension, Late Cretaceous inversion and Late- to post-Cretaceous salt movements. Tectonic elements that deform the whole sedimentary succession are restricted to the basin's southern, eastern and northeastern margins where salt thickness decreases. Combined volumetric and backstripping investigations show that 2/3 of the total subsidence was induced by the sediment load and 1/3 was caused by tectonics. The tectonic subsidence history varies laterally across the basin. While tectonic subsidence curves in the northwestern part of the basin show fast subsidence during the Permian, and Early Triassic decreasing exponentially with time, subsidence curves in the southeastern part indicate repeated tectonic activity. However, the tectonic subsidence volume created during Permian to Late Triassic is significantly higher than the tectonic subsidence during younger phases of basin history.