Interplay Of Tectonics Research Articles

Novel 3D sequence stratigraphic numerical model for syn-rift basins: Analysing architectural responses to eustasy, sedimentation and tectonics

Syn-rift clastic sedimentary systems preserve a complicated stratigraphic architecture that records the interplay of tectonics, eustatic sea level and storage and routing of sediments. Previous conceptual models describe and explain changes in depositional stacking patterns along a fault segment. However, stacking patterns, and the nature of key stratigraphic surfaces, is challenging to predict accurately with conventional sequence stratigraphic models that do not consider the three-dimensional interplay of subsidence, sedimentation, and eustasy. We present a novel, geometric, 3D sequence stratigraphic model (‘Syn-Strat’), which applies temporally- and spatially-variable, fault-scale tectonic constraints to stratigraphic forward modelling, as well as allowing flexibility in the other controls in time and space. Syn-Strat generates a 3D graphical surface that represents accommodation. Although the model has the capacity to model footwall variation, here we present model results from the hangingwall of a normal fault, with temporal and spatial (dip and strike) predictions made of stacking patterns and systems tracts for a given set of controls. Sensitivity tests are tied to the depositional architecture of field-based examples from the Loreto Basin, Gulf of California and Alkyonides Basin, Gulf of Corinth. Here, the relative influence of major sedimentary controls, different subsidence histories, varying sedimentation distribution, including along-strike variation in stacking patterns, are assessed and demonstrate the potential of Syn-Strat for reducing subsurface uncertainties by resolving multiple scenarios. In addition, the model demonstrates the nature of diachroneity of key stratigraphic surfaces that can arise in syn-rift settings, which could be represented by a bypass surface (sequence boundary) or reservoir seal (maximum flooding surface) in the rock record. Enabling a quantitative assessment of these surfaces is critical for prospect analysis in hangingwall half-graben-fills, where these surfaces are heavily relied upon for well correlations that are used for hydrocarbon volume and production rate predictions.

Interplay of tectonism and eustasy during the Early Permian icehouse: Southern Sydney Basin, southeast Australia

This investigation presents an outcrop‐based integrated study of sedimentological analysis and sequence stratigraphy applied to the Lower Permian sedimentary succession in the southern Sydney Basin, Australia. This succession accumulated in several depositional environments and sub‐environments that range from non‐marine (fluvial) to marine (outer shelf), representing the fill of a sedimentary basin that resembles a fault‐bounded, rift basin. The stratigraphic analysis indicates a deepening‐upward trend, and the sequence stratigraphic approach has established that these sediments can be attributed to the lowstand, transgressive, and highstand systems tracts. Lowstand sediments can be defined by fluvial facies that are located between the subaerial unconformity and the maximum regressive surface. Transgressive facies correspond to estuarine, upper and lower shoreface, and inner and outer shelf depositional environments and are located between the maximum regressive and the maximum flooding surfaces. Highstand bottomset sediments are accumulated above the maximum flooding surface and are represented by outer shelf facies. The stratigraphic architecture indicates the development of an almost complete depositional sequence, mainly developed under the control of tectonically induced subsidence, but also influenced by the high sedimentation rates and the high gradient of the inherited topography. Eustatic sea‐level fluctuations were of minor importance during the deposition of the examined sediments.

Tidal dynamics and mangrove carbon sequestration during the Oligo\u2013Miocene in the South China Sea

Modern mangroves are among the most carbon-rich biomes on Earth, but their long-term (≥106 years) impact on the global carbon cycle is unknown. The extent, productivity and preservation of mangroves are controlled by the interplay of tectonics, global sea level and sedimentation, including tide, wave and fluvial processes. The impact of these processes on mangrove-bearing successions in the Oligo–Miocene of the South China Sea (SCS) is evaluated herein. Palaeogeographic reconstructions, palaeotidal modelling and facies analysis suggest that elevated tidal range and bed shear stress optimized mangrove development along tide-influenced tropical coastlines. Preservation of mangrove organic carbon (OC) was promoted by high tectonic subsidence and fluvial sediment supply. Lithospheric storage of OC in peripheral SCS basins potentially exceeded 4,000 Gt (equivalent to 2,000 p.p.m. of atmospheric CO2). These results highlight the crucial impact of tectonic and oceanographic processes on mangrove OC sequestration within the global carbon cycle on geological timescales.

The Cretaceous of the Swiss Jura Mountains: an improved lithostratigraphic scheme

In the course of the HARMOS project of the Swiss Geological Survey, the lithostratigraphic subdivisions of the Cretaceous sedimentary rocks outcropping in the Swiss Jura Mountains were revisited. New formation names are proposed where only inadequate facies terms existed so far. As in some cases outcrop conditions in the Swiss Jura do not allow for logging complete sections to characterise the formations, type localities in neighbouring France have been chosen. The following formations (Fm.) are defined to describe the Cretaceous lithological units (from bottom to top): Goldberg Fm., Pierre-Châtel Fm., Vions Fm., Chambotte Fm., Vuache Fm., Grand Essert Fm., Gorges de l’Orbe Fm., Vallorbe Fm., Perte du Rhone Fm., Narlay Fm. Dating of the formations is based on biostratigraphy (ammonites, echinids, dasycladalean algae, foraminifera, calpionellids, dinocysts, nannofossils). The fossils indicate Berriasian through to Coniacian ages. The lithostratigraphic units describe the general evolution from a shallow, peritidal platform to deeper-water shelf environments, then the installation of a carbonate platform, and finally the drowning of this platform followed by the predominance of pelagic conditions. The common lateral and vertical changes in facies and sedimentation rates as well as numerous hiatuses within the formations testify to a complex interplay of tectonics, climate, and sea level that controlled the Swiss Jura realm during the Cretaceous.

Mixed carbonate–siliciclastic sedimentation in forebulge grabens: An example from the Ediacaran Bambuí Group, São Francisco Basin, Brazil

Mixed carbonate–siliciclastic successions have been described in multiple Phanerozoic sedimentary settings recording the dynamic interplay of tectonics, eustasy, climate, in situ carbonate production, and variations in siliciclastic sediment supply. The Ediacaran Bambuí 1st-order sequence (i.e., Bambuí Group) covers most of the intracratonic São Francisco basin (southeast Brazil) and encompasses thick packages of carbonate and fine- to coarse-grained siliciclastic strata. Recording a marine foreland basin stage that developed in the São Francisco plate during the Neoproterozoic–early Paleozoic West Gondwana assembly, the Ediacaran deposits unconformably overlie Archean–Paleoproterozoic metamorphic assemblages of the Sete Lagoas basement high and fill a series of kilometer-long grabens in the southern São Francisco basin. Seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the Sete Lagoas high, in the early evolutionary stages of the Bambuí basin cycle. Based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns, we recognized two transgressive–regressive 2nd-order sequences preserved within one of the focused grabens: (i) Sequence 1 includes the glaciogenic deposits of the basal Carrancas Formation that grade upward into the carbonate ramp successions of the Sete Lagoas Formation; (ii) Sequence 2 contains the siliciclastic-dominated and deep water to deltaic strata of the Serra de Santa Helena Formation and passes upward into peritidal carbonates of the Lagoa do Jacaré Formation. These sedimentary successions encompass suites of retrogradational, aggradational, and progradational lower-rank cycles and are bounded by erosional surfaces. Regional seismic interpretation, well data, and the available literature indicate that most of these deposits and their correlatives are continuous over most of the Sete Lagoas basement high, showing remarkable variations within the studied forebulge grabens. Our analysis demonstrates that the depositional architecture of Sequence 1 and Sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes. The latter are mainly recorded by the lower second-order sequence (Sequence 1), which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late Neoproterozoic. An additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses. As indicated by lower- to higher-rank cycles, it has been responsible for the deposition of thicker transgressive to early regressive successions, a few gravitational flow deposits, and the development of the deltaic to shallow marine carbonate cycles of Sequence 2. The overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the Phanerozoic were also present in the late Neoproterozoic. The close association between organic-rich post-glacial shales, potential reservoir facies, and rift-related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns.

Middle–late Miocene palaeoenvironments, palynological data and a fossil fish Lagerstätte from the Central Kenya Rift (East Africa)

AbstractThe Miocene epoch was a time of major change in the East African Rift System (EARS) as forest habitats were transformed into grasslands and hominids appeared in the landscape. Here we provide new sedimentological and palynological data on the middle–upper Miocene Ngorora Formation (Tugen Hills, Central Kenya Rift, EARS), together with clay mineral characterizations, mammal finds and a description of the Ngorora fish Lagerstätte. Furthermore, we introduce a revised age ofc. 13.3 Ma for the onset of the Ngorora Formation. The older part of the Ngorora Formation (c. 13.3–12 Ma) records low-energy settings of lakes, floodplains and palaeosols, and evidence of analcime indicates that lakes were alkaline. The palynomorph spectrum consists of tree pollen (Juniperus, Podocarpus), Euphorbiaceae pollen (Acalypha, Croton) and herbaceous pollen of Poaceae and Asteraceae, suggestive of wooded grasslands or grassy woodlands. Alkaline lakes, floodplains and palaeosols continue upsection (c. 12–9 Ma), but environmental fluctuations become more dynamic. Paucity of palynomorphs and the presence of an equid may point to progressively drier conditions. A total of about 500 articulated fish fossils were recovered from distinctive layers of almost all sections studied and represent different lineages of the Haplotilapiines (Pseudocrenilabrinae, Cichlidae). Some of the fish kills may be attributable to rapid water acidification and/or asphyxiation by episodic ash falls. Repeated instances of abrupt change in water depth in many sections are more likely to be due to synsedimentary tectonic activity of the Central Kenya Rift than to climatic variation. Overall, the preservation of the Ngorora fish Lagerstätte resulted from the interplay of tectonics, formation of alkaline lakes and explosive volcanism. As records of grasslands that pre-date late Miocene time are rare, our finding of middle Miocene (12–13 Ma) grassy savannah in the Central Kenya Rift is also relevant to models of human evolution in East Africa.

Human–climate interactions in the central Mediterranean region during the last millennia: The laminated record of Lake Butrint (Albania)

Lake Butrint (39°47 N, 20°1 E) is a ca. 21 m deep, coastal lagoon located in SW Albania where finely-laminated sediments have been continuously deposited during the last millennia. The multi-proxy analysis (sedimentology, high-resolution elemental geochemistry and pollen) of a 12 m long sediment core, supported by seven AMS radiocarbon dates and 137Cs dating, enable a precise reconstruction of the environmental change that occurred in the central Mediterranean region during the last ∼4.5 cal kyrs BP. Sediments consist of triplets of authigenic carbonates, organic matter and clayey laminae. Fluctuations in the thickness and/or presence of these different types of seasonal laminae indicate variations in water salinity, organic productivity and runoff in the lake's catchment, as a result of the complex interplay of tectonics, anthropogenic forcing and climate variability. The progradation of the Pavllo river delta, favoured by variable human activity from the nearby ancient city of Butrint, led to the progressive isolation of this hydrological system from the Ionian Sea. The system evolved from an open bay to a restricted lagoon, which is consistent with archaeological data. An abrupt increase in mass-wasting activity between 1515 and 1450 BC, likely caused by nearby seismic activity, led to the accumulation of 24 homogenites, up to 17 cm thick. They have been deposited during the onset of finely laminated sedimentation, which indicates restricted, anoxic bottom water conditions and higher salinity. Periods of maximum water salinity, biological productivity, and carbonate precipitation coincide with warmer intervals, such as the early Roman Warm Period (RWP) (500 BC–0 AD), the Medieval Climate Anomaly (MCA) (800–1400 AD) and recent times (after 1800 AD). Conversely, lower salinity and more oxic conditions, with higher clastic input were recorded during 1400–500 BC, the Late Roman and the Early Medieval periods (0–800 AD) and during the Little Ice Age (1400–1800 AD). Hydrological fluctuations recorded in Butrint are in phase with most central and western Mediterranean records and correlate with NAO variability. In contrast, opposite hydrological patterns have been recorded in the Eastern Balkans and the Levant during the last millennium, emphasizing a complex spatial variability in the region. Phases of maximum settlement intensity in Butrint (Roman-Late Antique) coincide with warmer and/or stable climate periods (0–800 AD and MCA, respectively), indicating a long-term influence of climatic conditions on human activities. The Late Holocene sedimentary record of Lake Butrint demonstrates the complex interplay of climate variability, tectonics and human impact in the recent evolution of coastal Mediterranean regions.

Effects of tectonic deformation and sea level on river path selection: Theory and application to the Ganges-Brahmaputra-Meghna River Delta

The set of active rivers of the Ganges-Brahmaputra-Meghna (GBM) Delta in Bangladesh overlies an active plate boundary that continually modifies the landscape of the delta by deformation. The response of rivers to spatially variable subsidence, from tectonic tilting or other causes, has been thought to include preferred occupation of regions of higher subsidence. In this paper, we develop further the theoretical framework for analysis of the interplay of tectonics and river dynamics, and apply this model to conditions in the GBM Delta. First, we examine the overall competition between variable subsidence and channel dynamics, and find that tilting in Bangladesh should be strong enough to influence river path selection. We then present new theory for the effect of subsidence that is spatially (not temporally) variable. We find a constant residence timescale on different parts of the delta, and differing frequencies of avulsion to these locations, and describe the effects of incision or floodplain deposition on these quantities. We present estimates of the channel residence timescale of the Jamuna (Brahmaputra) River reconstructed from the lithology, provenance, and dating of sediment cores. We apply our framework to a map of regional subsidence to predict the effects on avulsion for the Jamuna River. Comparison between our predicted (2150 years) and our stratigraphically based estimates of avulsion timescale (1800 years) shows encouraging consistency.

Lithofacies Superimposition in a Shallow Basin - Interplay of Tectonics and Sedimentation: Evidences from Kolhan Basin, Eastern India

The 2.2-2.1 Ga pear shaped Kolhan basin show the development of a time transgressive group in a half-graben setting developed during the fragmentation of the Rodinia supercontinent. The overall style of sedimentation reflect a switchover from low-sinuous avulsed channels developed within a braided-fluvial-ephemeral streams to a lacustrine fan-delta complex during the later part of the sedimentation history. The fan-delta facies indicate sediment dispersal by hyperconcentrated flows in the form of sheetfloods and channelized flows. These different-scale cycles are interpreted as the sedimentary response to pulses of deformation of the basin margin at variable frequencies, related to the contemporary thrusts (ca. 20 km away from the basin). The episodes of tectonism downwarped the basin margin sediments and made the basin shift periodically toward the margin, and created progressive lithological changes in the sedimentary succession. The immediate effects of a tectonic pulse included lake transgression and accentuation of the structural hinge of the basin margin, causing a decline of sediment supply from the source rock. As the basin margin was subsequently reduced by denudation, the fans prograded and fan deltas were formed in normal conditions of graben subsidence. The sediment geometries and the climate exerted a major control on the processes of sediment transfer. Our results show that the fluvio-lacustrine strata show on lap-pinch out relationship at the centre of the basin but only on lap relationship along the lateral edges. This transition can be best explained by the fault growth models.

To what extent can intracrater layered deposits that lack clear sedimentary textures be used to infer depositional environments?

Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism—can provide insight into potentially changeable depositional conditions of early Mars.

Dynamic stratigraphy of composite peripheral unconformity in a foredeep basin

AbstractThis study employs facies analysis and basic principles of sequence stratigraphy to correlate isolated outcrop sections and reveal the depositional history of the Chmielnik Formation – a prominent mid‐Serravalian clastic wedge formed on the basinward forebulge flank of the Polish Carpathian Foredeep. The coarse‐grained clastic wedge, up to 30 m thick and spanningca1·1 Ma within biozoneNN6, consists of fluvio‐deltaic, foreshore and shoreface deposits with a range of large littoral sand bars, all enveloped in muddy offshore‐transition deposits. Its dynamic stratigraphy indicates rapid shoreline shifts and environmental changes due to the interplay of forebulge tectonism, sediment supply and third‐order eustatic cycles. A similar interplay of tectonism and eustasy is recognizable in the whole middle Miocene sedimentary succession deposited on the forebulge flank, demonstrating an extreme case of an accommodation‐controlled shelf and indicating tectonic cycles of the forebulge uplift and subsidence spanningca800 to 900 ka. The episodes of forebulge uplift correlate with the main pulses of orogen thrusting. The resulting composite peripheral unconformity differs markedly from the idealized model of a ‘steady‐state’ stepwise onlap driven by forebulge continuous retreat. It is concluded that the foredeep peripheral unconformities, instead of being simplified in accordance with this idealized model, should rather be studied in detail because they bear a valuable high‐resolution record of regional events and give unique insights into the local role of tectonics, eustasy and sediment supply.

Sequence Stratigraphy and Incised Valley Architecture of the Domengine Formation, Black Diamond Mines Regional Preserve and the Southern Sacramento Basin, California, U.S.A

Abstract The middle Eocene Domengine Formation crops out in the Coast Ranges along the structurally complex western margin of the basin and forms an economically important gas reservoir in the Sacramento basin, California. Previous studies had interpreted the Domengine Formation as a conformable succession of barrier-island sandstone with tidal channels and coastal-plain deposits or a delta, tidal shelf, marsh complex located along a north–south-trending shoreline bordering the shelf. Integration of outcrop and subsurface data, however, indicate that the Domengine Formation can be regionally subdivided into two sequences. The bases of each sequence consist of fluvial and estuarine sandstones that were deposited in northeast–southwest-trending incised-valley systems that are interpreted to have been formed by fluvial incision. Shelfal shales and shoreline sandstones overlie the incised-valley fills and reflect flooding of the shelf. This cyclicity is interpreted to have been produced by the interplay of tectonism and eustasy. Tectonism is interpreted to have controlled the location of the incised submarine canyons and the fluvial estuarine systems throughout the early Tertiary as they stack vertically and trend southwest toward the structurally controlled depocenter. Eustasy appeared to have controlled the timing of the transgressive–regressive depositional cyclicity present in the lower Tertiary succession due to the strong correlation of available biostratigraphic data constraining the timing of these major periods of incision to the global coastal-onlap curves. Thickness trends observed in the Domengine Formation are interpreted as a product of variable incision, and not the result of depositional thinning in a northwesterly direction suggested by the previous models. The importance of this reinterpretation of the Domengine Formation, within a sequence stratigraphic framework, is that it provides a predictive model for both understanding the thickness trends and facies distributions of the Domengine Formation. It also potentially provides a more accurate depositional model for exploration and development of this important hydrocarbon reservoir.

Sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin and its response to tectonism and sea‐level change

AbstractThe sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin,China, were formed in response to the interplay of tectonism and sea‐level change, their history being documented by the integrated analysis of many seismic lines, drilling and outcrop data. TheOrdovician carbonate system in the basin is divided into four composite sequences defined by major unconformities. Each sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into 10 third‐order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early‐riftedManjiaer aulacogen, the carbonate platform margins of the Lower and MiddleOrdovician that prograded eastward in an arcuate belt extending generally north‐south across the northern part of the basin. The development of theTazhong uplift due to compression resulted in an extensive paleokarst hiatus between the Middle and the UpperOrdovician in the south‐central basin, and subsequently constrained the formation of a peninsula‐shaped carbonate platform whose margins were controlled by marginal thrust‐fault belts of the paleo‐uplift during the LateOrdovician. In the northern basin, the LateOrdovician carbonate platform margin developed around the marginal slope of theTabei paleouplift. The transgressive–regressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. Stacking patterns of the composite sequences varied due to the interplay between the accommodation produced by tectonism and sea‐level change, and the carbonate production rate. The reef–shoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.

Pre-1.8 Ga tectono-magmatic evolution of the Kalkadoon–Leichhardt Belt: implications for the crustal architecture and metallogeny of the Mount Isa Inlier, northwest Queensland, Australia

New geochemical, U–Pb zircon and Nd–Hf isotope data for felsic and mafic components of the Kalkadoon–Leichardt Belt (KLB) in the Mount Isa Inlier of northwest Queensland confirm that the evolution and tectonic make-up of this belt prior to 1.8 Ga was closely aligned with that of the Western Fold Belt. Like pre-1.8 Ga magmatic rocks of the Western Fold Belt, those in the KLB are characterised by late Archean to Paleoproterozoic crustal residence ages (TDM ca 2.6–2.3 Ga) but the geochemical data suggest stronger within-plate affinities, implying that the KLB intrusions underwent a greater degree of crustal assimilation and/or were emplaced further inboard of the active subduction margin. A new SHRIMP U–Pb zircon age of ca 1.86 Ga from a felsic intrusion in the northern KLB confirms earlier age constraints for the main phase of the Kalkadoon Granite. Nd isotope patterns for intrusions in the Western Fold Belt and the KLB are distinct from those of ca 1.7–1.5 Ga intrusions in the Eastern Fold Belt. This isotopic discrepancy suggests the Eastern Fold Belt underwent a separate evolutionary history and was accreted to the WFB/KLB at some stage prior to, or during, the ca 1.86 Ga Barramundi Orogeny. The occurrence of widespread, isotopically homogeneous ca 1.72 Ga magmatism (Argylla Event) across the entire Mount Isa Inlier implies that amalgamation of the allochthonous Eastern Fold Belt with the North Australian Craton was completed by that time. The significantly elevated metallogenic potential of the Eastern Fold Belt for ca 1540–1500 Ma Fe-oxide–copper–gold, relatively low endowment of the Kalkadoon–Leichhardt Belt, and predominance of ca 1650–1550 Ma base metal occurrences in the Western Fold Belt, are all consequences of the tectonic interplay and lithospheric processes that controlled the evolution of the Mt Isa Inlier prior to ca 1.8 Ga.

Geological map of the Pliocene succession of the Northern Siena Basin (Tuscany, Italy)

Please click here to download the map associated with this article. The late Miocene-Pliocene sedimentary fill of the Siena Basin (Tuscany, Italy) consists dominantly of clastics and has internal architecture that reflects the interplay of tectonics, relative sea-level changes and climate variations. Pliocene sediments are extensively exposed and overlay both late Miocene deposits and pre-Neogene bedrock. Specifically, Pliocene basin margin sediments consist largely of sand with gravel and mud intercalations, deposited mainly in nearshore settings with minor fluvial depositional episodes. They grade basinward to dominantly offshore fines with intervening turbiditic sand bodies. New fieldwork revealed that basin margin deposits, notwithstanding lithologically rather homogeneous, are made of a variety of sedimentary facies and bear several unconformities. They have been traditionally described and mapped using lithostratigraphic criteria, that have proven to be unfit to represent such complex stratigraphic architectures. The aim of this paper is to describe the allostratigraphic architecture of the Pliocene deposits exposed in a marginal key-area (45 km) of the northern Siena Basin by means of a 1:10,000 scale geological map. The recognized succession of allostratigraphic units and their bounding discontinuities, along with new biostratigraphic data from calcareous plankton, provides new insights into the geological history of the Siena Basin and represents valuable constraints for long-distance correlation.

An integrated study of Permo-Triassic basins along the North Atlantic passive margin: implication for future exploration

Abstract Permo-Triassic rift basins offer important hydrocarbon targets along the Atlantic margins. Their fill is dominated by continental red beds, comprising braided fluvial, alluvial fan, aeolian, floodplain and lacustrine facies. These relatively lightly explored basins span both the Atlantic and Tethyan domains and developed above a complex basement with inherited structural fabrics. Sparse data in offshore regions constrain understanding of depositional geometries and sedimentary architecture, further impeded by their deep burial beneath younger strata, combined with the effects of later deformation during continental breakup. This paper provides results from a multidisciplinary analysis of basins along the Atlantic margin. Regional seismic and well data, combined with geochemical provenance analysis from the European North Atlantic margins, are integrated with detailed outcrop studies in Morocco and Nova Scotia. The research provides new insights into regional basin tectonostratigraphic evolution, sediment fill, and reservoir distribution, architecture and quality at a range of scales. Regional seismic profiles, supported by key well data, indicate the presence of post-orogenic collapse basins, focused narrow rifts and low-magnitude multiple extensional depocentres. Significantly, Permo-Triassic basin geometries are different and more varied than the overlying Jurassic and younger basins. Provenance analysis using Pb isotopic composition of detrital K-feldspar yields new and robust controls on the sediment dispersal patterns of Triassic sandstones in the NE Atlantic margin. The evolving sedimentary architecture is characterized by detailed sedimentological studies of key outcrops of age equivalent Permian–Triassic rifts in Morocco and Nova Scotia. The interplay of tectonics and climate is observed to influence sedimentation, which has significant implications for reservoir distribution in analogue basins. New digital outcrop techniques are providing improved reservoir models, and identification of key marker horizons and sequence boundaries offers a potential subsurface correlation tool. Future work will address source and seal distribution within the potentially petroliferous basins.

Unsteady evolution of the Bolivian Subandean thrust belt: The role of enhanced erosion and clastic wedge progradation

The Subandean fold and thrust belt of Bolivia constitutes the easternmost part of the Andean orogen that reflects thin-skinned shortening and eastward propagation of the Andean deformation front. The exact interplay of tectonics, climate, and erosion in the deposition of up to 7.5 km of late Cenozoic strata exposed in the Subandes remains unclear. To better constrain these relationships, we use four W–E industry seismic reflection profiles, eight new zircon U–Pb ages from Mio-Pliocene sedimentary strata, and cross-section balancing to evaluate the rates of thrust propagation, shortening, and deposition pinch-out migration. Eastward thrusting arrived in the Subandean belt at ~ 12.4 ± 0.5 Ma and propagated rapidly toward the foreland unit approximately 6 Ma. This was followed by out-of-sequence deformation from ca. 4 to 2.1 Ma and by renewed eastward propagation thereafter. Our results show that the thrust-front propagation- and deposition pinch-out migration rates mimic the sediment accumulation rate. The rates of deposition pinch-out migration and thrust propagation increased three- and two fold, respectively (8 mm/a; 3.3 mm/a) at 8–6 Ma. The three-fold increase in deposition pinch-out migration rate at this time is an indication of enhanced erosional efficiency in the hinterland, probably coupled with flexural rebound of the basin. Following the pulse of pinch-out migration, the Subandean belt witnessed rapid ~ 80 km eastward propagation of thrusting to the La Vertiente structure at 6 Ma. As there is no evidence for this event of thrust front migration being linked to an increase in shortening rate, the enhanced frontal accretion suggests a shift to supercritical wedge taper conditions. We propose that the supercritical state was due to a drop in basal strength, caused by sediment loading and pore fluid overpressure. This scenario implies that climate-controlled variation in erosional efficiency was the driver of late Miocene mass redistribution, which induced flexural rebound of the Subandean thrust belt, spreading of a large clastic wedge across the basin, and subsequent thrust-front propagation.

The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof

AbstractThe Southern Mountains Zone of the Rum Central Complex lies inside a major ring fault and comprises an intricate association of country-rock outcrops, breccias and rhyodacite. The breccias and rhyodacite were long thought to be products of subterranean explosion and intrusion, respectively. Here, we report new observations that support re-interpretation of these units as mass movement deposits and ignimbrites. The most abundant breccias (Coire Dubh-type) consist mainly of country-rock clasts <1 m in diameter in a sand or silt matrix. Internally bedded and graded, and interlayered with sandstones and lithic tuffs, these breccias are interpreted as debris flow and stream flow deposits. Rhyodacite sheets show gradational or sharp, concordant contacts with Coire Dubh-type breccias, and display graded basal lithic tuffs and graded fiamme swarms. These sheets are interpreted as moderately to densely welded rhyodacite ignimbrites (25–100 m thick). A steep body of fragmented (fiamme-bearing) rhyodacite with intrusive non-fragmented contacts is interpreted as an ignimbrite vent system. The rhyodacite and breccia succession is over 200 m thick and unconformably overlies a structurally uplifted Precambrian basement, within which there is also evidence of later subsidence. Outcrops of potential caldera-collapse ‘megabreccia’ are more structurally consistent than previously thought, and are re-interpreted here as coherent segments of Precambrian country rock (caldera floor). The Southern Mountains Zone breccias and rhyodacites respectively reflect sedimentary and pyroclastic processes acting in response to a complex tectonic interplay of intrusion-related uplift and caldera subsidence.

Late Quaternary landscape development along the Rancho Marino coastal range front (south‐central Pacific Coast Ranges, California, USA)

AbstractLate Quaternary landscape development along the Rancho Marino coastal range front in the central‐southern Pacific Coast Ranges of California has been documented using field mapping, surveying, sedimentary facies analysis and a luminescence age determination. Late Quaternary sediments along the base of the range front form a single composite marine terrace buried by alluvial fans. Marine terrace sediments overlie two palaeoshore platforms at 5 m and 0 m altitude. Correlation with the nearby Cayucos and San Simeon sites links platform and marine terrace development to the 125 ka and 105 ka sea‐level highstands. Uplift rate estimates based on the 125 ka shoreline angle are 0.01–0.09 m ka−1 (mean 0.04 m ka−1), and suggest an increase in regional uplift along the coast towards the NW where the San Simeon fault zone intersects the coastline. Furthermore, such low rates suggest that pre‐125 ka uplift was responsible for most of the relief generation at Rancho Marino. The coastal range front landscape development is, thus, primarily controlled by post 125 ka climatic and sea‐level changes. Post 125 ka sea‐level lowering expanded the range front piedmont area to a width of 7.5 km by the 18 ka Last Glacial Maximum lowstand. This sea‐level lowering created space for alluvial fan building along the range front. A 45 ± 3 ka optically stimulated luminescence (OSL) age provides a basal age for alluvial fan building or marks the time by which distal alluvial fan sedimentation has reached 300 m from the range front slope. Fan sedimentation is related to climatic change, with increased sediment supply to the range front occurring during (1) glacial period cold stage maxima and/or (2) the Late Pleistocene–Holocene transition, when respective increases in precipitation and/or storminess resulted in hillslope erosion. Sea‐level rise after the 18 ka lowstand resulted in range front erosion, with elevated localised erosion linked to the higher relief and steeper slopes in the SE. This study demonstrates that late Quaternary coastal range front landscape development is driven by interplay of tectonics, climatic and sea‐level change. In areas of low tectonic activity, climatic and sea‐level changes dominate coastal landscape development. When the sea‐level controlled shoreline is in close proximity to the coastal range front, localised patterns of sedimentation and erosion are passively influenced by the pre‐125 ka topography. Copyright © 2008 John Wiley & Sons, Ltd.

Morpho‐sedimentary records at the Brahmaputra River exit, NE Himalaya: climate–tectonic interplay during the Late Pleistocene–Holocene

AbstractMorphological and sedimentary records at the exit of Brahmaputra River at Pasighat in the NE Himalaya inform about the climate–tectonic interplay during the past ca. 15 ka. The geomorphology of the area comprises (1) fan terrace T3, (2) a high‐angle fan (3) terrace T2, (4) terrace T1 and (5) a low‐angle fan. Geomorphic consideration suggests that the fan terrace T3 and high‐angle fans are the oldest units and were coeval. The low‐angle fan is the youngest geomorphic unit. Sedimentological studies and optically stimulated luminescence chronology suggest that (i) fan terrace T3 formed between 13 and 10.5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment; (ii) the high‐angle fan formed during 15–10 ka and comprises channel bar aggradation in braided river conditions; (iii) terrace T2 formed during 10–8 ka due to aggradation in a braided channel environment with lesser events of debris flows; (iv) terrace T1 formed during <7 and 3 ka took place as bars of the braided river. Sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka; and (v) the low‐angle fans dated to <3 ka formed due to aggradation in a small tributary joining the Brahmaputra River. This implies a phase when the main channel of the Brahmaputra did not flood regularly and the tributaries were actively aggrading. The sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the Siwalik range along the Himalayan Frontal Fault. Copyright © 2008 John Wiley & Sons, Ltd.