Siliciclastic Turbidites Research Articles

Una nueva interpretación de la estratigrafía de la Región de Tolimán, Estado de Querétaro

It is proposed in this note that the oldest terrigenous rocks cropping out in the Toliman region in the state of Queretaro belong to two different units: El Chilar complex and the San Juan de la Rosa Formation. The first consists of highly deformed siliciclastic turbidites and the second is a volcano-sedimentary succession deposited in marine conditions. The volcanism of the San Juan de la Rosa Formation is felsic in composition and Late Jurassic-Early Cretaceous in age. We postulate as a working hypothesis that the felsic magmatism of the San Juan de la Rosa Formation are part of the Jurassic volcanic arc in central Mexico that migrated in time and space towards the south.�

Milankovitch forcing of bioturbation intensity in deep-marine thin-bedded siliciclastic turbidites

Trace fossils have been used to infer a Milankovitch-type control on pelagic and hemipelagic sediments, but there have been no comparable studies in deep-marine siliciclastic turbidite successions. Here, we present the results of a quantitative analysis of trace-fossil abundance and intensity in an essentially continuous, Middle Eocene, 230-m-long (Well A6) core, comprising very thin- and thin-bedded siliciclastic turbidites in the deep-marine Ainsa basin, Spanish Pyrenees. After removing the sediment slides and debris flow deposits, as these represent geologically instantaneous events approximately two orders of magnitude thicker than the typical laminated turbiditic sediments in the core, spectral analysis was conducted on the bioturbation intensity data. Spectral analysis of bioturbation intensity in the A6 core suggests, for the first time from a siliciclastic turbidite succession at a tectonically active plate margin (thrust-top basin in a foreland basin), a cyclicity that is interpreted to reflect the 41-kyr and an ~ 112-kyr (possibly an average of the 95- and 125-kyr) Milankovitch frequencies. We propose that this drove environmental changes in bottom-water conditions in the Ainsa basin, most likely leading to critically varying levels of oxygenation (stratification) of bottom waters. Our age model for the Ainsa basin (~ 4 km of deep-marine sediments in ~ 10 Myr) yields an average sediment accumulation rate of ~ 40 cm kyr − 1 , that is consistent with that inferred from the spectral analysis (~ 30 cm kyr − 1 ) for fine-grained sedimentation. An implication of these results is that global environmental change, probably glacio-eustasy, acted as a driver on deep-marine siliciclastic sedimentation patterns during the Middle Eocene.

Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum: Linking sediment nature and accumulation to sea level fluctuations at millennial timescale

Since Last Glacial Maximum (23–19 ka), Earth climate warming and deglaciation occurred in two major steps (Bølling‐Allerød and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5–11.5 ka B.P.). In this study, three cores (MV‐33, MV‐66, and MD‐40) collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bølling‐Allerød and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank‐derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

HP/LT metamorphism in the Volparone Breccia (Northern Corsica, France): evidence for involvement of the Europe/Corsica continental margin in the Alpine subdution zone

AbstractThe Alpine belt in Corsica (France) is characterized by the occurrence of stacked tectonic slices derived from the Corsica/Europe continental margin, which outcrop between two weakly or non‐metamorphic tectonic domains: the ‘autochthonous’ domain of the Hercynian basement to the west and the Balagne Nappe (ophiolitic unit belonging to the ‘Nappes supérieures’) to the east. These slices, including basement rocks (Permian granitoids and their Palaeozoic host rocks), Late Carboniferous–Permian volcano‐sedimentary deposits, coarse‐grained polymict breccias (Volparone Breccia) and Middle Eocene siliciclastic turbidite deposits, were affected by a polyphase deformation history of Alpine age, associated with a well‐developed metamorphic recrystallization. This study provides new quantitative data about the peak of metamorphism and the retrograde P–T path in the Alpine Corsica: the tectonic slices of Volparone Breccia from the Balagne region (previously regarded as unmetamorphosed) were affected by peak metamorphism characterized by the phengite + chlorite + quartz ± albite assemblage. Using the chlorite‐phengite local equilibria method, peak metamorphic P–T conditions coherent with the low‐grade blueschist facies are estimated as 0.60 ± 0.15 GPa and 325 ± 20 °C. Moreover, the retrograde P–T path, characterized by a decrease of pressure and temperature, is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions (0.35 ± 0.06 GPa and 315 ± 20 °C). The occurrence of metamorphic peak at high‐pressure/low‐temperature (HP/LT) conditions is evidence of the fact that these tectonic slices, derived from the Corsica/Europe continental margin, were deformed and metamorphosed in the Alpine subduction zone during their underplating at ∼20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non‐metamorphic oceanic units during a complex exhumation history.

Contrasting sedimentary processes along a convergent margin: the Lesser Antilles arc system

Sedimentation processes occurring in an active convergent setting are well illustrated in the Lesser Antilles island arc. The margin is related to westward subduction of the North and/or the South America plates beneath the Caribbean plate. From east to west, the arc can be subdivided into several tectono-sedimentary depositional domains: the accretionary prism, the fore-arc basin, the arc platform and inter-arc basin, and the Grenada back-arc basin. The Grenada back-arc basin, the fore-arc basin (Tobago Trough) and the accretionary prism on the east side of the volcanic arc constitute traps for particles derived from the arc platform and the South American continent. The arc is volcanically active, and provides large volumes of volcaniclastic sediments which accumulate mainly in the Grenada basin by volcaniclastic gravity flows (volcanic debris avalanches, debris flows, turbiditic flows) and minor amounts by fallout. By contrast, the eastern side of the margin is fed by ash fallout and minor volcaniclastic turbidites. In this area, the dominant component of the sediments is pelagic in origin, or derived from South America (siliciclastic turbidites). Insular shelves are the locations of carbonate sedimentation, such as large platforms which develop in the Limestone Caribbees in the northern part of the margin. Reworking of carbonate material by turbidity currents also delivers lesser amounts to eastern basins of the margin. This contrasting sedimentation on both sides of the arc platform along the margin is controlled by several interacting factors including basin morphology, volcanic productivity, wind and deep-sea current patterns, and sea-level changes. Basin morphology appears to be the most dominant factor. The western slopes of the arc platform are steeper than the eastern ones, thus favouring gravity flow processes.

Achieving True Reservoir Stimulation in Deepwater Horizontal Sandstone Wells

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 95826, "Achieving True Sandstone-Reservoir Stimulation in Deepwater Horizontal Wells," by A. Mendez, SPE, BJ Services Co.; L.F. Neumann, SPE, and E. de Almeida Pinto, Petrobras; and R. Torres, SPE, R. Farias, SPE, and M. Acosta, SPE, BJ Services Co., prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. In offshore horizontally completed sandstone wells, an acid job usually is pumped to remove filter cake. When these wells are acidized with a hydrochloric (HCl)/hydrofluoric (HF) acid mixture, the resulting skin values usually are positive. An improved HF acid system (IHF) was introduced that uses an organophosphonic acid complex to control HF generation and to reduce potentially damaging secondary and tertiary precipitates. The full-length paper describes the results and case histories of field treatments that resulted in truly stimulated wells with negative skin. Introduction Production began in the Campos basin offshore Brazil in the early 1970s. Since then, more than 40 oil fields have been found in water depths between 260 and 7,800 ft. The most-prolific reservoirs are late Albian to early Miocene siliciclastic turbidites. These reservoirs are developed with horizontal and high-angle wells drilled into poorly consolidated reservoirs. The deepwater subsea wells must be designed to allow high production rates, typically more than 10,000 to 15,000 BOPD, with lifetime completions to avoid costly interventions. Campos basin reservoirs were responsible for approximately 1.2 million BOPD in 2002, almost 83% of total Brazilian production, and are expected to produce 1.6 million BOPD by the end of 2005. Sand control is required for wells in sandstone reservoirs that are poorly sorted medium-to-fine sands. These wells need to be completed with a package that includes premium screens in the horizontal section. To pump the gravel throughout the horizontal section effectively, a fluid composed of xanthan gum and calcium carbonate is used to create a filter cake and leave the formation stable. Injector wells are completed in the same way as producers except an acid treatment is required to remove the filter cake and drilling residues remaining in the very-near-wellbore vicinity. Most injector wells are treated with the common HCl/HF mixtures. Laboratory core flow tests were performed with core plugs from two different fields in the Campos basin. Tests compared results of treatments with several different acid blends. Results demonstrated that the IHF is capable not only of removing the filter cake but also of stimulating the formation more than the regular HF acid mixture (RHF). After laboratory tests were concluded, the IHF was field tested in two wells with a stimulation tool run with the gravel-pack tool in a single trip.

The impact of meteoric water on the diagenetic alterations in deep-water, marine siliciclastic turbidites

Meteoric-water flux and formation of kaolinite owing to the dissolution of detrital silicates are common features of continental and paralic sandstones. In deep-water marine sandstones, meteoric-water flux is commonly considered unlikely to occur. However, the study of deep-water, marine sandstones of the Shetland–Faroes Basin on the British continental shelf revealed widespread and extensive dissolution and kaolinitization of mica and feldspar grains, which are attributed to meteoric-water flux during a sea-level lowstand. We suggest that this apparently enigmatic meteoric-water flux mechanism is likely to have occurred by hyperpycnal flow. Hyperpycnal flow occurs when river effluent directly transfers into sediment gravity flow, and enters seawater as a mixture of sediment and fresh water. The likelihood for hyperpycnal flows increases at times when rivers and distributary channels reach the shelf edge, and their flows are delivered directly onto the deepwater slope.

‘Transgressive washing’ concept: a sequence stratigraphic approach for calci- and siliciclastic turbidites

Abstract Carbonate breccias, calciturbidites and sandstone flysch sediments are typically assigned to the lowstand regime, based on the double assumption that these thick sedimentary packages necessitate (1) emergence phases for the production of their terrigenous material, and (2) a relatively lower position of the shoreline, which did not lie too far from the shelf-break line. In strictly regarding these conditions, we can run the risk of neglecting the role played by the transgression-related currents in reworking and exporting basinward bioclastic and siliciclastic sands from shelves. The ‘transgressive washing’ concept proposed here accounts for the latter mechanism, particularly in the case when the stratigraphic record shows that the onset of both the calci- and siliciclastic material was contemporaneous with newly marine-encroached continental source areas. Four examples from the Rifian internal (source) and external zones (basin) are presented to illustrate this phenomenon.

Synthesis of the tectonic-sedimentary evolution of the Mesozoic-Early Cenozoic Pindos ocean: evidence from the NW Peloponnese, Greece

Abstract The tectonic development of the western part of the Pindos ocean in southern Greece is exemplified by the mountainous Pindos thrust belt in the NW Peloponnese. A Late Triassic-Early Cenozoic succession exposed within imbricate thrust sheets records a range of deep-water siliciclastic, redeposited carbonate and siliceous sediments, which in general become more distal oceanwards towards the east. Igneous rocks, locally dated as Triassic, occur within a mélange that is entrained beneath and within the Pindos thrust stack; these igneous rocks and related sediments are interpreted as remnants of a continent-ocean transition zone. ‘Immobile’ element geochemistry is explicable by rifting of a compositionally heterogeneous subcontinental mantle, possibly related to pre-existing Hercynian subduction, although coeval Triassic subduction cannot be excluded based on evidence from this area alone. Localized, ‘enriched’ basalts are interpreted as fragments of oceanic seamounts formed in a relatively distal setting. Late Paleocene-Early Eocene (locally Mid-Eocene) siliciclastic turbidites, derived from the north, record the latest deposition prior to incorporation of the sedimentary succession into a westward-migrating accretionary wedge during post-Early Eocene time in the NW Peloponnese. Structural restoration of the well-ordered thrust stack indicates a minimum of 201 km (55%) of shortening at an average rate of 5.8 mm a −1 . As the Pindos allochthon approached the Apulian continent, the Gavrovo-Tripolitza foreland underwent flexural upwarp during the Mid-Eocene, followed by collapse to create a foreland basin by the Late Eocene. This basin was infilled with generally upward-thickening and -coarsening deep-water turbiditic sediments of Late Eocene-Early Oligocene age. The foreland was, in turn, overthrust by the Pindos accretionary prism during post-Early Paleocene time, and was then imbricated and thrust over the Ionian foreland basin to the west by Pliocene time.

Tectonic and sedimentary evolution of the Lower Pliocene Periadriatic foredeep in Central Italy

The Periadriatic foredeep (Italy) was generated by Neogene downbending of the Adria Plate under the Apennine Chain. The basin is filled with Plio-Pleistocene siliciclastic turbidites. Its substratum consists of the carbonate succession of the southwestern Adria Plate margin. The influence of the basin’s morphology on sedimentation and subsequent tectonic evolution is investigated in the Abruzzo sector of the foredeep (Cellino Basin). The substratum is composed of Messinian evaporites that dip towards the Apennines (W). A NNW component along the depocentral axis is divided into four blocks with different depths. The substratum was also affected by a Messinian extensional fault system, not involving the overlying Pliocene sequence. This morphology controlled the distribution of the turbidites in the lower part of the Cellino Basin. The Plio-Pleistocene compressional deformation of the foredeep produced an inner complex structure (Internal Structure), involving the foredeep substratum and an outer imbricate thrust system (Coastal Structure), detached over the faulted Messinian evaporites. This thrust system is parallel to the extensional faults, suggesting a strong influence of the substratum morphology on the development of the compressional structures. The overall structural setting was validated with a balanced cross-section. Out-of-sequence thrusting and non-coeval deformation within each thrust sheet characterize the local tectonic history.

About the early Late Cretaceous siliciclastic turbidites in Corsica and northern Apennine ophiolitic successions

About the early Late Cretaceous siliciclastic turbidites in Corsica and northern Apennine ophiolitic successions

The Tuzancoa Formation: Evidence of an Early Permian Submarine Continental Arc in East-Central Mexico

Paleozoic rocks exposed in northeast Hidalgo State, Mexico, have traditionally been assigned to the Guacamaya Formation, a orogenic flysch assemblage associated with the collision of South and North America during the formation of Pangea. However, major differences exist in the stratigraphy of these rocks and those of the Guacamaya Formation at its type section. The rocks of Hidalgo are here redefined as the Tuzancoa Formation, and comprise interbedded submarine andesitic-basaltic lava flows, siliciclastic turbidites, volcaniclastic turbidites, calcareous debris-flows, and lenses of conglomerate. The Tuzancoa Formation contains abundant Permian (Wolfcampian-Leonardian) fossils. The lavas and volcaniclastics are andesitic to basaltic in composition, and are related to subduction. Their REE patterns are similar to those of recent island arc magmas, with slight enrichment in LREE, flat HREE, and no Eu anomaly. The εNd(280) of +4.38 in a representative sample is also similar to island arcs and suggests little crustal contamination. However, field evidence indicates that the arc was built upon Precambrian continental crust. Thus, we infer that the "primitive" arc geochemical signatures reflect emplacement on thin crust or under extensional conditions. The Tuzancoa Formation is interpreted to have been deposited in an intra-arc basin, and is probably related to rocks of the Las Delicias Formation in Coahuila and to the Guacamaya Formation in Tamaulipas. All are thought to have formed within a continental arc that extended throughout eastcentral Mexico in the Early Permian.

Deep‐water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines, Italy): did the Mediterranean ever dry out?

AbstractA new genetic facies model for deep‐water clastic evaporites is presented, based on work carried out on the Messinian Gessoso‐solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow‐water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity‐driven deposits ranging from debrisflow to low‐density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep‐water primary deposit, is here reinterpreted as the fine‐grained product of high to low‐density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow‐water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi‐closed thrust‐top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep‐water resedimented deposits rather than shallow‐water to supratidal primary evaporites indicative of a complete basin desiccation.

The Late-Quaternary climatic signal recorded in a deep-sea turbiditic levee (Rhône Neofan, Gulf of Lions, NW Mediterranean): palynological constraints

Siliciclastic turbidites represent huge volumes of sediments, which are of particular significance for (1) petroleum researchers, interested in their potential as oil reservoirs and (2) sedimentologists, who aim at understanding sediment transport processes from continent to deep-basins. An important challenge when studying marine turbidites has been to establish a reliable chronology for the deposits. Indeed, conventional marine proxies applied to hemipelagic sediments are often unreliable in detrital clays. In siliciclastic turbidites, those proxies can be used only in hemipelagic intervals, providing a poor constraint on their chronology. In this study, we have used sediments from the Rhône Neofan (NW Mediterranean Sea) to demonstrate that pollen grains can provide a high-resolution chronostratigraphical framework for detrital clays in turbidites. Vegetation changes occurring from the end of Marine Isotopic Stage 3 to the end of Marine Isotopic Stage 2 (from ∼30 to ∼18 ka cal. BP) are clearly recorded where other proxies have failed previously, mainly because the scarcity of foraminifers in these sediments prevented any continuous Sea Surface Temperature (SST) record and radiocarbon dating to be obtained. We show also that the use of palynology in turbidite deposits is able to contribute to oceanographical and sedimentological purposes: (1) Pinus pollen grains can document the timing of sea-level rise, (2) the ratio between pollen grains transported from the continent via rivers and dinoflagellate cysts (elutriating) allows us to distinguish clearly detrital sediments from pelagic clays. Finally, taken together, all these tools show evidence that the Rhône River disconnected from the canyon during the sea-level rise and thus evidence the subsequent rapid starvation of the neofan at 18.5 ka cal. BP. Younger sediments are hemipelagic: the frequency of foraminifers allowed to date sediments with radiocarbon. First results of Sea Surface Temperature obtained on foraminifers are in good agreement with the dinoflagellate cysts climatic signal. Both provide information on the end of the deglaciation and the Holocene.

Stratigraphy and tectono-sedimentary evolution of the Late Ordovician to Middle Devonian Gaspé Belt in northern New Brunswick: evidence from the Restigouche area

The Gaspé Belt in the Restigouche area comprises three successions separated by a Late Silurian (Salinic) disconformity and an Early Devonian angular unconformity. The lower, Upper Ordovician to Lower Silurian sequence consists of siliciclastic turbidites of the Boland Brook and Whites Brook formations (Grog Brook Group), overlain by calcareous turbidites of the Pabos and White Head formations (Matapédia Group), and slope and shelf deposits of the Upsalquitch and Limestone Point formations (lower Chaleurs Group). Above the Salinic disconformity, the upper Chaleurs Group and the Dalhousie Group record a transgressive–regressive cycle. The former comprises Pridolian carbonate rocks of the West Point Formation and overlying Pridolian to Lochkovian sedimentary rocks of the Indian Point Formation. The Chaleurs Group is conformably overlain by Lochkovian to early Emsian subaerial volcanic rocks of the Dalhousie Group (Val d'Amour Formation), which is unconformably overlain by alluvial–lacustrine deposits of the late Emsian Campbellton Formation. Acadian orogenesis began during the Emsian and is characterized by open to closed folding, heterogenous cleavage development, and reverse and strike-slip faults. The Salinic orogeny is manifested in extensional block faulting, within-plate volcanism, and uplift and deep erosion of Early Silurian strata. Early Devonian high-level intrusion of the Matapédia Group, White Head clasts in Indian Point conglomerate, and thermal maturation data all indicate an extended period of Late Silurian – Early Devonian uplift in parts of the Restigouche area. Thermal maturities of West Point and Indian Point strata are within the oil and condensate windows and suggest potential for hydrocarbons in the study area.

Alternation of ecologic regimes in a deep-marine carbonate basin: calciturbidite trace fossils from the Cretaceous Scaglia Rossa, northeastern Italy

The mostly pelagic Scaglia Rossa limestone exposed in the Vich quarry near Belluno contains numerous light-colored turbidites, consisting of calcareous sandstone and mudstone derived from the adjacent Friuli Platform. The turbidites contain an ichnofauna that records short-lived departures from background conditions in this part of the Belluno Basin in the Late Cretaceous. At least seven distinctive ichnotaxa were recognized, some of which could be the parts or extensions of more complex burrow systems of deposit feeders that exploited the disturbances. Earliest exploitation of the individual turbidites is indicated by nondescript burrow mottling in the lowest sandstone divisions. This is followed by an ensemble of comparatively well preserved sand-filled and typically mud-lined/-mantled burrows produced as the mud fraction began to accumulate gradually from suspension. All of these structures are crosscut by a latest series of mud-filled tunnels. This is not exactly the pattern of siliciclastic turbidites of flysch successions, having post-event ichnoassociations consisting of traces that are more nearly contemporaneous and record either depth of penetration or post-depositional movement of the redox boundary. Because the mud component in the Vich turbidites may have required several months to settle out of suspension, the patterns of bioturbation were extended over months to years. But as in the case of siliciclastic turbidites, emplacement of the calciturbidites suppressed the background fauna and supported a post-event association of different endobenthic animals, by changing temporarily the physical properties of the substrate, providing a windfall of organic particles, and by ventilating the near-bottom water.

Shallowing-Upward Carbonate Cycles in the Belingwe Greenstone Belt, Zimbabwe: A Record of Archean Sea-Level Oscillations

ABSTRACT Shallowing-upward carbonate cycles in the 2650 Ma Cheshire Formation, Belingwe greenstone belt (Zimbabwe), closely resemble their Proterozoic and Phanerozoic counterparts. The cycles form part of a karstified carbonate-ramp sequence that is overlain by, and grades basinward into, siliciclastic turbidites. A single section of 74 cycles (1.5 m average thickness) was studied in detail. Two basic cycle types are recognized, both with an asymmetric facies stacking pattern. One cycle type contains open marine, subtidal shale at the base. Shale is intercalated with storm-generated sandstone and grainstone beds that become more common and thicken upward, indicating progressive shallowing. Wave-rippled ooid-intraclast grainstone beds and bedsets overlie shale or form the base of the second cycle type. Grainstone formed at or above fair-weather wave base as shoreface sand sheets in an agitated, shallow subtidal setting. Microbial laminites constitute the top of both cycle types and are interpreted as peritidal deposits. In the upper part of the studied section, microbial boundstones with aragonite pseudomorphs are intercalated with, or overlie, laminites and formed in a supratidal environment. The vertical facies distribution within a cycle is indicative of rapid submergence followed by gradual shallowing of relative sea level. High-frequency eustatic sea-level changes are favored over an autocyclic mechanism and tectonically induced allocyclicity as the controlling mechanism for the cyclicity. Hierarchies of stratigraphic cyclicity occur on different scales and may be a result of the combined effects of several orders of sea-level oscillations. Cycle recurrence ratios correspond well to the Milankovitch frequencies calculated for the Late Archean, suggesting that orbital climatic forcing may have been in operation in Archean times.

The Cretaceous sedimentary history of the Pindos Basin (Greece)

The Cretaceous sedimentary rocks of the Pindos Zone in western Greece document the evolution of a Tethyan deep-water basin. New sedimentological and micropalaeontological studies reveal a complex basin history. Siliceous sediments with abundant radiolaria and organic-rich facies prevailed up to the early Late Cretaceous. Within the sediment-starved pre-Middle Cenomanian, marked black shale levels appear that are probably linked to oceanic anoxic events. At the change from the late Early to the early Middle Cenomanian, the sedimentary regime altered abruptly. The early Late Cretaceous is characterized by major calcareous redepositional events (orbitoline horizons) and often associated siliciclastic turbidite deposition (submarine-fan environments). In the late Late Cretaceous, carbonate supply increased rapidly, resulting in the evolution of a carbonate slope and basin-plain setting. Pelagic and allodapic limestones recorded basinwide blooms in planktonic foraminifera (elevata event) and a polyphase redepositional history that is interpreted to reflect the sensitivity of the basin to the tectonic evolution of Apulia.

Ordovician‐Silurian accretion tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia contains a remnant ocean basin with Cambrian igneous basement of intra‐arc and backarc boninitic to tholeiitic volcanics and island‐arc calc‐alkaline volcanics overlain by ?Upper Cambrian to dominantly Ordovician siliciclastic turbidite successions derived from Gondwana and deposited in a huge submarine turbidite fan(s). Much of the remnant ocean basin is preserved in accretionary subduction complexes that are characterised by abundant coherent successions with relatively sparse mélanges. The lack of chaotic rocks reflects the accretion of thick (2–5 km) siliciclastic turbidite successions and low rates of underthrusting. Three previously recognised accretionary subduction complexes are located in western Victoria (Stawell and Bendigo Zones), eastern Victoria (Tabberabbera Zone) and along the eastern coastline of southeast Australia (Narooma subduction complex). In addition, a short‐lived west‐dipping subduction zone is proposed to account for the areally restricted Howqua River Zone along the eastern margin of the Melbourne Zone in eastern Victoria. The Howqua River Zone contains gently dipping mélanges and subduction‐related blueschist fragments. The subduction complex in the Tabberabbera Zone is considered to extend throughout the remnant ocean basin succession of the Wagga‐Omeo Zone of the central Lachlan Fold Belt. Apart from the Howqua River Zone, these subduction complexes are an end‐member in the spectrum of accretionary complexes that contrast with more chaotic assemblages as preserved in southwest Japan.

Stratigraphical evidence for the depression of the northern margin of the Menderes–Tauride Block (Turkey) during the Late Cretaceous

Upper Cretaceous units on the Menderes–Tauride Block show a transition from a passive continental margin to a pelagic basin. This transition is related to the emplacement of ophiolitic nappes. The pelagic sediments were studied in the Aydıncık and Arslanköy (Mersin) areas in the central Tauride region, and in the Munzur Mountains in eastern Tauride region. In the Aydıncık area Upper Cretaceous units consist of Upper Campanian carbonate breccias with rudist fragments; Upper Campanian–Lower Maastrichtian slope-to-basinal sediments overlie these carbonate breccias disconformably. In the Arslanköy area the Upper Campanian–Maastrichtian Yavca Formation consists of Globotruncana-bearing biomicrites, siliciclastic turbidites, calciturbidites and debris flows and overlies Jurassic–Cretaceous platform carbonates disconformably. The uppermost levels of the Liassic–Cenomanian platform carbonates in the Munzur Mountains are composed of Cenomanian Hippurites/Orbitolina-bearing reefal limestones and then Turonian–Campanian Globotruncanidae-bearing biomicrites.The stratigraphic sequence in the Menderes–Tauride Block shows that rifting to form a northern Neotethyan basin took place in the Triassic and gave rise to the Inner Tauride Ocean. From Jurassic to Cenomanian times, the northern part of the Menderes–Tauride Platform formed a passive continental margin. In the Late Cretaceous the Inner Tauride Ocean is believed to have commenced intra-oceanic, northward subduction. Inner Tauride oceanic crust-derived nappes were emplaced southward onto the Menderes–Tauride Block. Flexural foreland basins in which pelagic sediments were deposited were formed in the Taurus Mountains related to southward thrusting and the emplacement of the ophiolite nappes.