Equivalent Facies Research Articles

Contourite depositional system after the exit of a strait: Case study from the late Miocene South Rifian Corridor, Morocco

AbstractIdealized facies of bottom current deposits (contourites) have been established for fine‐grained contourite drifts in modern deep‐marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine‐grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non‐existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo‐Mediterranean Outflow Water. Foraminifera assemblages were used for age‐constraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain‐size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo‐Mediterranean Outflow Water, which is mainly driven by precession and millennial‐scale climate variations. Tides substantially modulated the palaeo‐Mediterranean Outflow Water on a sub‐annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process‐based interpretation of deep‐water bottom current deposits.

Biostratigraphic indices for Lower Jurassic carbonate-platform deposits (Perdido Group), Overthrust Terrane Association, Timor-Leste

Lower Jurassic carbonate-platform deposits in Timor-Leste have been subject to much controversy. The Perdido Group contains the Bahamian-type facies shallow-water limestone of the Lower Jurassic that had been mixed with an upper Oligocene–lowest Miocene carbonate unit in Audley-Charles's Cablac Limestone. Foraminiferal assemblages from five sites in the Perdido Group are used to confirm a Sinemurian–Pliensbachian age. Species recorded for the first time from Timor and from the broader region, are Biokovina sp., Bosniella oenensis Gušić, Endotriadella ifranensis Bassoullet, Boutakiout, Vachard, Lituosepta recoarensis Cati, Palaeocyclammina complanata Bassoullet, Boutakiout and Echarfaoui, Planisepta compressa (Hottinger), and Radoicicina ciarapicae Gale, Barattolo and Rettori. Previously Siphovalvulina spp. and Meandrovoluta asiagoensis Fugagnoli and Rettori were described from the group. These species allow distinction between the Perdido Group (part of the Overthrust Terrane Association in Timor-Leste) and the carbonate-platform Upper Triassic Bandeira Group (part of the East Gondwana Interior Rift Association). No equivalent facies coeval with the Perdido Group and with similar foraminiferal assemblages have been recognized in the Indonesian, Malayasian, Timor-Leste, or north-west Australian regions. The closest known Sinemurian–Pliensbachian occurrence containing some of the species found in the Perdido Group, is in Western Thailand, with a fauna including Lituosepta cf. recoarensis.

Unravelling depositional setting, age and provenance of the Simlipal volcano-sedimentary complex, Singhbhum craton: Evidence for Hadean crust and Mesoarchean marginal marine sedimentation

The Singhbhum craton of eastern India preserves an extensive record of basin formation spanning the Paleoarchean to Neoarchean. Although spatially extensive and well exposed, the absolute age, depositional environment, and regional correlations of many of the purportedly Archean basins of this craton remain poorly resolved. We present a detailed lithostratigraphic analysis of siliciclastic strata in the (lower) Simlipal volcano-sedimentary succession and report the first detrital zircon U-Pb ages from this succession. The studied section is dominated by fine- to medium-grained, quartz-rich sandstones, which preserve herringbone stratification, tidal bundles, soft sediment deformation structures along with ubiquitous trough cross-stratification. Petrographic study confirms these sandstones as quartz arenites and reveal their high textural and mineralogical maturity. These features are consistent with deposition of the lower siliciclastic succession of the Simlipal volcano-sedimentary complex within a tidally-influenced marginal marine setting. The new detrital zircon data support a ~3.08 Ga maximum depositional age for the succession and reveal a provenance with age peaks at ca. 3.55–3.45 Ga, 3.38–3.24 Ga, and 3.10–3.08 Ga. These ages likely correspond to local basement sources including the Older Metamorphic Tonalite Gneisses, the Singhbhum Granitoid Complex, and the Mayurbhanj Granite Suite. Two detrital zircons from our dataset have concordant ages of ~4.02 Ga and ~4.03 Ga. They represent the first Hadean detrital zircons recovered from any Archean strata in the Singhbhum craton, documenting the involvement of Hadean crust in the early development of the craton. The detrital zircons show prominent Pb-loss at ~1.2–1.0 Ga that exemplifies the tectonothermal imprint of a late Mesoproterozoic to early Neoproterozoic orogeny (likely related to the assembly of Rodinia) on the Singhbhum craton. Our findings further support the interpretation that the lower siliciclastic strata in the Simlipal succession are the lateral facies equivalents of alluvial fan deposits at the base of the Dhanjori Formation, exposed to the north.

Geochemistry and petrogenesis of Paleoproterozoic rhyolite-hosted zinc-rich metamorphosed volcanogenic massive sulfide deposits in the eastern Betul Belt, central India

The Palaeoproterozoic Betul Belt in central India contains several small base metal (Zn-Cu and Zn-Pb-Cu) deposits that are hosted by felsic volcanics which form part of a bimodal volcanic suite. Although a volcanogenic massive sulfide (VMS) origin has been attributed to these deposits, they are not well-defined based on modern classification criteria. Regionally, the volcanic facies association around the Bhuyari zinc prospect includes massive rhyolite, pillowed basalt, rhyolitic autobreccia, hyaloclastite and peperite that indicate volcanism and sedimentation in relatively deep water. At the deposit-scale, the mineralization is hosted by intensely altered massive quartz-porphyritic rhyolite and associated tremolite-carbonate rock. Alteration facies mapping show a zoned alteration system with an inner phlogopite-biotite-chlorite-garnet-rich core (BPG schist; interpreted as having a chlorite-rich protolith) and an outer quartz-muscovite-K-feldspar zone (QMK schist; sericite-rich protolith). Metamorphism was largely isochemical and the mineral assemblages are amphibolite facies equivalents of hydrothermally altered protoliths. Tremolite-carbonate rocks are spatially close to mineralization and are interpreted either as metaexhalites or intensely altered rhyolite and probably had talc-dolomite protoliths. Carbon and oxygen isotope signatures of carbonate show derivation from seawater-dominant hydrothermal solutions. Tremolite-carbonate and some BPG schist show enrichment in HREE indicating hydrothermal mobilization of REE. Geochemically, the least-altered rhyolite has transitional magmatic affinities with moderate negative Nb anomaly and strong negative Sr, and Ti anomalies. Mantle upwelling and the associated high-heat source generated high-temperature, high-silica rhyolites with elevated HSFE and REE contents. Emplacement of felsic melts at depth provided heat that drove hydrothermal fluid circulation and formation of zinc-rich mineralization and associated hydrothermal alteration zones. Pb-model ages of galena give ~1800 Ma using the Stacey and Kramers (1975) model for VMS mineralization in Bhuyari. The Bhuyari deposit possibly formed in a continental back-arc rift environment during the amalgamation of Bastar and Bundelkhand cratons along the Central Indian Tectonic Zone. The high HFSE-REE contents of Bhuyari rhyolites are similar to other highly prospective VMS districts formed by rifting of evolved continental crust. The recognition of metamorphosed VMS deposit at Bhuyari has implications for further exploration of such deposits along the Paleoproterozoic Betul Belt and similar-aged deformed terrains in India and elsewhere.

Biostratigraphy of the Hareelv Formation (Upper Jurassic) in the Blokelv-1 core, Jameson Land, central East Greenland

The Hareelv Formation in the Blokelv-1 core is biostratigraphically subdivided by means of ammonite and dinoflagellate cyst stratigraphy. The succession ranges from the Oxfordian C. densiplicatum Chronozone to the Volgian P. elegans Chronozone. The mudstones of the Blokelv-1 core are characterised by large amounts of amorphous organic matter. This hampers the preparation and identification of dinoflagellate cysts, which are also commonly degraded and corroded. Ammonites, on the other hand, are common and well-preserved in the core, contrasting with that observed in the equivalent facies and stratigraphic interval at outcrop. Integration of the ammonite and dinoflagellate cyst biostratigraphical data yields a robust chronostratigraphic subdivision of the middle Oxfordian – lowermost Volgian cored section.

Using neural networks and the Markov Chain approach for facies analysis and prediction from well logs in the Precipice Sandstone and Evergreen Formation, Surat Basin, Australia

Abstract Facies analysis is crucial for reservoir evaluation because the distribution of facies has significant impact on reservoir properties. Artificial Neural Networks (ANN) are a powerful way to use facies interpretations from core to determine equivalent facies from wireline logs in uncored wells. However, ANN do not incorporate information that relates to facies successions. This has limited the ability to effectively use facies information derived from logs alone in reservoir modelling, especially at the regional scale where data is often sparse, clustered, or incomplete. In this study, based on observations of 8 cored wells with a total thickness of ∼2000 m, 20 core facies were defined that range from 0.22 m to 11.56 m thick. Facies were based on grain size, sedimentary structures, and ichnological characteristics; Each facies corresponds to a distinct depositional sub-environment within the broader context of a large nearshore to shallow marine system. It was essential that these facies were incorporated into reservoir models to accurately map the distribution of reservoir and seal geobodies for CO2 storage assessment in the Surat Basin, Australia. However, core data are few and far between in the Surat Basin. To use core-defined facies in the absence of core, six wireline log parameters – gamma ray, density, sonic, neutron, photoelectric factor, and deep resistivity were plotted in multidimensional space and examined using Linear Discriminator Analysis. Combined with model recognition and Fisher Canonical Discriminance, the 20 core facies were simplified into 10 representative wireline log facies (WLF) with unique petrophysical parameters. We then used the Markov Chains Approach (MCA) to determine the significance of vertical facies transitions, which supported the interpretation that facies group into 5 distinct associations: (1) channel-levee complex; (2) lower delta plain; (3) subaqueous delta; (4) shoreface and; (5) tidal flats and channels. Based on the facies analysis and statistical classification, Multilayer Perceptron Classifier, a type of neural network method was applied using a training set of three cored wells that had all 6 wireline log data and using the facies successions determined from the MCA. Results show that the accuracy of WLF prediction ranges from 66% to 99% (ca. 83%). The accuracy of facies recognition decreased step wise with a decreasing number of logs as input data, such that when only gamma ray, density, deep resistivity, and sonic were used to train neural networks the accuracy dropped to between 45 and 98% (ca. 67%), depending upon the facies. This was considered the lowest acceptable threshold of accuracy for facies determination for input into reservoir models for carbon capture and storage. The results of this study show that sedimentary facies can be accurately predicted for uncored intervals in the Precipice Sandstone and Evergreen Formation to improve facies mapping and static reservoir modelling. Additionally, wireline log facies are helpful for interpreting Lower Jurassic stratigraphy, depositional setting, and basin evolution in the Mesozoic of Eastern Australia.

Modern rather than Mesoarchaean oxidative weathering responsible for the heavy stable Cr isotopic signatures of the 2.95 Ga old Ijzermijn iron formation (South Africa)

Previously reported stable Cr isotopic fractionation in Archaean paleosols and iron formations (IFs) have been interpreted as a signature of oxidative weathering of Cr(III) to Cr(VI) in soils, and delivery of isotopically heavy Cr(VI) into the oceans. One of the oldest reported fingerprints of this process is isotopically heavy Cr preserved in the 2.95 Ga old Ijzermijn IF, Sinqeni Formation of the Mozaan Group (Pongola Supergroup), South Africa and could suggest that atmospheric free oxygen was present ca. 600 million years earlier than the Great Oxidation Event (GOE). However, fractionated stable Cr isotopic signatures have only been found to date in surface outcrop samples of the White Mfolozi Inlier exposed along the White Mfolozi River Gorge. In this study, the latter outcrop was resampled along with two drill cores of the Ijzermijn IF and a drill core of the Scotts Hill IF to represent multiple exposures of Mozaan Group IFs with different states of preservation. A detailed geochemical comparison on bulk samples of different units was undertaken using stable Cr isotopes coupled with trace and major elements. Outcrop iron-rich mudstones (Fe - lutites) show very low LOI [wt] %, and very low Fe(II)/Fetot ratios, and lower Ca and Mg relative to equivalent facies in drill cores, indicating the effects that oxidative recent surface weathering had on Fe/Mn-rich carbonate minerals of the IF. Overall rare earth element and yttrium (REE + Y) mixing models agree well with previous studies, confirming that they were minimally disturbed by weathering and are consistent with a high magnitude of continental solutes delivered in a near–shore depositional environment, with a minor contribution of hydrothermally derived fluids that upwelled into shallower depositional setting. Importantly, all drill core samples of this study revealed δ53/52Cr values within the igneous inventory, despite variable amounts of detrital Cr input that includes nearly detritus-free, chert/jasper-rich units. By contrast, a specific group of Fe-lutite samples near the base of White Mfolozi River outcrop bear fractionated Cr isotopic signatures with δ53/52Cr values up to 0.418‰. These outcrop samples also display unusually high U/Th ratios (max. 12.6) as well as enrichments of other elements (W, Tl, As, MREE) that far exceed that observed in correlative drill core units. These observations together with the lack of Cr isotopic fractionation in drill core samples lead us to propose that the heavy δ53/52Cr values of Fe-lutites from outcrop Ijzermijn IF samples reported here and in a previous study are the product of modern oxidative weathering rather than an indicator for Mesoarchaean oxidative weathering at ca. 2.95 Ga.

New data on the age and stratigraphic relationships of the Czajakowa Radiolarite Formation in the Pieniny Klippen Belt (Carpathians) based on the radiolarian biostratigraphy in the stratotype section

The radiolarian biostratigraphy of the Middle–Upper Jurassic pelagic siliceous sediments (Czajakowa Radiolarite Formation) in the Niedzica succession of the Pieniny Klippen Belt (Carpathians) is interpreted in terms of their age in a stratotype section, and facies equivalents in other tectonic-facies units of this region. The siliceous sediments are represented by radiolarian cherts and silicified limestones which are underlain and overlain by red nodular limestones, equivalents of the Rosso Ammonitico facies. The radiolarian association includes thirty-seven taxa belonging to twenty one genera which represent the Northern Tethyan Palaeogeographic Province. Key radiolarians recorded provide a means of correlation with zonation schemes based on Unitary Associations defined for the Jurassic Tethyan sediments. The age of the Czajakowa Radiolarite Formation in the stratotype section is determined as U.A.Z.9 to U.A.Z.11 corresponding to middle Oxfordian up to Kimmeridgian. Comparison of radiolarian biozones from the stratotype section with other facial equivalent sections in the Pieniny Klippen Belt reveals a significant diachronism for both the lower and the upper limits of the Jurassic pelagic siliceous facies.

Whence the Gosport Sand (Upper Middle Eocene, Alabama)? The Origin of Glauconitic Shell Beds In the Paleogene of the U.S. Gulf Coastal Plain

ABSTRACT: An explanation for the depositional history of the Gosport Sand has proven to be as puzzling as its diverse mollusk fossils is beguiling. We propose that shifting delta lobes produced patchy environments which accumulated to form the Gosport Sand and likely other glauconite- and fossil-rich shell beds of the Gulf Coastal Plain of the Paleogene. We present facies and mineralogical data to suggest glauconite in the Gosport Sand was deposited during relatively stable environmental conditions. Increased glauconite maturity and taphonomic alteration in the Gosport Sand are attributed to eustatic transgression, resulting in decreased deposition, enhanced winnowing, and increased exposure at the sediment–water interface. Increased fossil diversity and abundance likely result from time-averaging and accumulation of shells from ancient patchy habitats analogous to modern “inner shallow shelf” and “open sound” environments. Delta progradation and subsidence would have determined the migration of these environments. We evoke a similar depositional model for glauconite-rich shell beds distributed throughout the Paleocene and Eocene of the Gulf Coastal Plain based on their transgressive nature and roughly equivalent facies. These shifting environmental conditions may have played a partial role in generating the high molluscan diversity of the Gulf Coastal Plain of the Paleogene.

Fold-thrust Style and Fluid Reservoir Potential of Eocene Sakesar Limestone: Sothern Surghar Range, Trans-Indus Ranges, North Pakistan

The Surghar Range western extension of the Trans-Indus ranges constitutes the southeastern frontal fold-andthrust belt of the Kohat Plateau. This structural province is comprised of various local to regional scale anticlines right from Serkia-Mitha Khattak to Kutki areas. The existing range-front anticlinal trend is well-built along the east-west trending segment of the Surghar Range. These anticlinal features reveal infantile tendency from east to west and unearthing the platform rock sequences ranging from Permian to Eocene which is unconformably overlain by the Mitha Khattak Formation equivalent facies to the Rawalpindi Group. This facies in turn has overlain by the fluvial sediments of Siwalik Group. Overall three major anticlines have been mapped from west to east as the Mitha Khattak, Makarwal and Malla Khel Anticline. Differential stratigraphic levels are exposed in cores of these anticlines which proved excellent prospect to be potential hydrocarbon reservoirs horizons. The Eocene Sakesar Limestone has been selected for details studies of his fractures and joints analysis. The Sakesar Limestone exposed along the range front making fraction of the frontal limbs of different anticlines. Various fractures networks and joints pattern has been observed in the Sakesar Limestone at different localities reveal high secondary porosity and permeability. Most of the secondary tectonically induced and primary diagenetic opening and ruptures planes are interconnected and tenders proficient conduit lattice for munificent circulation of fluids in the Sakesar horizon. Origin of fractures and joints growth is mainly associated to force folding in response to the compressional, transpressional and trans-tensional deformation being observed in the region. The studied anticlines reveal that they are the product of fault-bend and fault-propagation folding tender excellent structural fluid trapping philosophy. The range frontal flanks reveal that different level of strata thrust against the foredeep showing inconsistency in the subsurface level of basal detachment horizon which is too hopeful for the construction of structural traps at various levels. Blending of the structural style of the area with the sedimentary structural features of the Sakesar Limestone of Surghar Range urges that this structural province is significantly associated to make hydrocarbon reservoir potential at the stratigraphic level of Sakesar Limestone.

COMMENT TO IBARRA ET AL. MICROFACIES OF THE COTHAM MARBLE: A TUBESTONE MICROBIALITE FROM THE UPPER TRIASSIC, SOUTHWESTERN U.K

Ibarra et al. (2014) have presented an interesting paper on the Cotham Marble, an enigmatic microbialite from the Rhaetian Penarth Group of SW England, and an iconic piece of British stratigraphy. They have re-interpreted some aspects of our earlier work (Wright and Mayall 1981) in which we tried to be amongst the first researchers to apply an ethological approach to decipher an ancient microbial texture. That paper (Wright and Mayall 1981) was restricted editorially with respect to its content requiring a second one to illustrate the microbial microtextures we felt were critical to understanding the morphogenesis of the limestone (Mayall and Wright 1981). In addition to challenging previously published work, Ibarra et al. (2014) use the Cotham Marble to identify evidence for the late Triassic mass extinction event, thus substantially broadening the importance of this unit for the wider readership. However we would like to take this opportunity to question a number of their conclusions, in particular using the Cotham Marble as an indicator of a mass extinction event and the role of Microtubus communis in forming the Cotham Marble. ### Regional Distribution of the Cotham Marble Ibarra at al. (2014) claim that the Cotham Marble as a carbonate microbialite is aerially extensive for approximately 2000 km2 in western Britain. They use this claim as evidence that the Cotham Marble represents the end-Triassic mass extinction event. This is a potentially misleading statement and we wish to clarify the details of its distribution. The ‘classical’ Cotham Marble with the ‘Landscape’ form that was described by Hamilton (1961) and Wright and Mayall (1981), and which Ibarra et al. (2014) focus on, simply does not extend over such a large area. The characteristics of the Cotham Marble and equivalent facies can be summarized by the following. First, at all localities in southwestern England there is a hard, blue-gray …

Chlorine isotope behavior during prograde metamorphism of sedimentary rocks

Chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n=25) were collected from a 50 m section of Triassic fluvial and playa-lake strata and Jurassic (Liassic) marine black shales in a well-characterized quarry. Low greenschist to middle amphibolite facies equivalents (n>80) were collected from the Glarus Alps, Urseren Zone, and Lucomagno region. Bulk δ37Cl values are constant within individual sedimentary layers, but vary from −2.0 to +2.4‰ in Triassic rocks and from −3.0 to 0‰ in the black shales. Dolomitic and gypsiferous samples have positive δ37Cl values, but marls and shales are isotopically negative. Bulk Cl contents show only small declines during the earliest stages of metamorphism. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in δ37Cl as their protoliths. Samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in δ37Cl. These data lead to the following conclusions: (1) Terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition. As a result, an unambiguous “sedimentary signature” does not exist in the chlorine stable isotope system. (2) No isotopic fractionation is discernable during metamorphic devolatilization, even at low temperatures. Alpine-style metamorphism thus has little to no effect on bulk chlorine isotopic compositions, despite significant devolatilization. (3) Cl is largely retained in the rocks during devolatilization, contrary to the normally assumed hydrophilic behavior of chlorine. Continuous release of mixed-volatile C–O–H fluids likely affected Cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks. (4) There is no evidence for fluid communication across (meta)sedimentary layers except within the Urseren shear zone. These data contribute to a relatively closed-system view of metamorphic devolatilization from anchizone through mid-amphibolite facies conditions.

Ichnology of shallow marine clastic facies in the Belait Formation, Brunei Darussalam

Trace fossil assemblages from a variety of outcropping tidal and shoreface facies in the Belait Formation of Brunei Darussalam were characterized to determine their value in distinguishing shallow marine sedimentary environments on a low energy margin. Most trace fossil assemblages in tide-dominated environments have a proximal Cruziana ichnofacies, although higher energy tidal channels have a distal Skolithos ichnofacies. Tidal sand flats tend to have more diverse and abundant traces than tidal channels. Trace fossils are more abundant in tidal mud flat facies than in tidal channels and sand flats, and whilst mud flat assemblages are more diverse than tidal channel assemblages, they are less diverse than sand flat assemblages. Generally, these assemblages are similar to those in equivalent depositional environments in higher tidal range settings. W ave-dominated environments also have Cruziana assemblages, except for the upper shoreface that has a distal Skolithos assemblage. Lower shoreface assemblages are more abundant and diverse than upper shoreface assemblages and belong to the proximal Cruziana ichnofacies. Trace fossils are more abundant and diverse in offshore transition facies than in either the upper or lower shoreface and belong to the distal Cruziana ichnofacies; assemblages in interbedded storm sands often include a minor component of the Skolithos ichnofacies. These assemblages are comparable to those from wave-dominated deposits from higher energy settings, except for the upper shoreface environment which contains a lower energy assemblage than in equivalent facies elsewhere. T race fossils tend to be less diverse, less abundant and individual traces generally are smaller in tide-dominated depositional environments when compared to wave-dominated depositional environments. Also, Ophiomorpha form complex burrow systems in wave-dominated environments in contrast to simple burrows in tide-dominated environments.

Local δ34S variability in ∼580 Ma carbonates of northwestern Mexico and the Neoproterozoic marine sulfate reservoir

Many δ34S records have been produced from carbonate-associated sulfate (CAS) in order to understand the oxidation state of the Neoproterozoic oceans, but interregional correlation is complicated by the absence of robust chronostratigraphic markers. Here, a globally correlatable stratigraphic interval containing the Wonoka–Shuram (W–S) δ13C excursion was analyzed to explore variability in the sulfur isotope record. In the excursion-containing units, the local δ34S record from multiple, closely spaced sections in Sonora, Mexico, was examined to explore potential heterogeneities, and then these were compared to more distant sections elsewhere.In Sonora, the inception and isotopic minimum of the W–S excursion is located in the Clemente Formation and coincides with an extensive oolite marker bed. Five sections containing this marker bed span ∼25km of lateral distance, exhibit significant variability in δ34SCAS (range: +18.6 to +27.6‰ VCDT) and high variability in CAS concentration (range: <30 to >1200ppm). Unlike the δ34SCAS values, CAS concentrations show strong negative correlation with Mn/Sr ratios and Fecarb concentrations, consistent with CAS removal upon diagenetic recrystallization and an absence of an accompanying sulfur isotopic fractionation. Indeed, samples containing low CAS concentrations exhibit petrographic characteristics consistent with diagenetic recrystallization including fabric destructive neomorphism and relatively coarse-crystalline textures.Coeval W–S strata in Death Valley, Oman and the least altered Sonora samples record a decrease in δ34SCAS and an increase in CAS concentration; however, the magnitude of the changes are distinct, perhaps reflecting differential response to a transient oxidation event. In contrast, time equivalent facies of South China do not exhibit a decrease in δ34SCAS nor a relative increase in CAS concentrations. The variability in geochemical characteristics likely developed from local and/or regional marine δ34Ssulfate heterogeneity.Although Neoproterozoic oceanic heterogeneity in δ34Ssulfate has been proposed, it has not been reported on such close spatial scales. In addition, the existence of variability among the Sonora sections in which strata were all deposited at similar depth indicates that heterogeneity could have occurred laterally. Finally, these findings demonstrate that while CAS concentrations may be drastically affected by diagenesis, δ34SCAS may retain primary signatures.

Outcrop gamma-ray logging of siliciclastic turbidites: Separating the detrital provenance signal from facies in the foreland-basin turbidites of the Moravo-Silesian basin, Czech Republic

Standard and spectral gamma-ray (GRS) logs are widely used as indicators of facies in the subsurface. In siliciclastics, however, the gamma-ray signal is often influenced by changes in the provenance of the K, U and Th-bearing detrital minerals. In this paper, we have compared outcrop and laboratory GRS with modal and chemical sandstone and mudstone composition and facies in an approximately 2.5km-thick siliciclastic turbidite system of the Moravo-Silesian Culm Basin (Lower Carboniferous), Czech Republic. The aim was to separate the facies signal from the detrital provenance one. The siliciclastics have moderately high outcrop gamma-ray values (174 API on average) and slightly lower laboratory values (127 API). Both the outcrop and laboratory data show low sensitivity to facies, which is demonstrated by the low contrast between the K, U and Th concentrations in the seven facies types ranging from proximal to distal turbidites. Markedly higher GRS variability is observed between equivalent facies at different stratigraphic levels. Major carriers of the GRS signal include K-feldspars, muscovite, sericite, biotite and albite for K, zircon, apatite, monazite and xenotime for U and monazite, thorite, REE secondary minerals, xenotime, apatite and zircon for Th. With the effect of facies filtered out, the GRS values reveal a stratigraphic variability, which coincide with the changes in the sandstone modal composition. A shift from the low-grade metamorphic and volcano-sedimentary provenance to predominantly magmatic sources with ultrapotassic plutonites in the early Late Viséan is associated with a marked increase in U and Th concentrations and generally higher sandstone radioactivity compared to mudstones. Another provenance shift to high-grade metamorphic sources with granulites in the latest Viséan is associated with a rapid decrease in Th, U and partly K concentrations and an increase in the GRS contrast between sandstone and mudstone facies. The GRS data sensitively reflect the extremely rapid exhumation of mid-crustal and deep-crustal rocks in the major source area, the Moldanubian Zone of the Bohemian Massif.

The callovian (Middle Jurassic) crinoids from northern Lithuania

Callovian strata in northern Lithuania (Papilė area) have yielded some crinoid taxa[Chariocrinus andreae (Desor),Balanocrinus berchteni Hess —Pugin,B. subteres (Munster inGoldfuss) andIsocrinus nicoleti (Desor)], which have not been described from this area. The only form known previously wasBalanocrinus pentagonalis (Goldfuss).Palaeocomaster jaegeri n. sp. is the first free-living crinoid recorded from the Callovian of eastern Europe. It is characterized by possessing a very low, strongly flattened and narrow radial cavity. A very similar crinoid assemblage is known from the Callovian glacially-derived clays exposed in the Łukow region (eastern Poland), which were originally located near the Baltic Sea during the Middle Jurassic. It is considered that the allochthonous Callovian deposits from Poland are facies equivalents of the deposits known from Lithuania. The sole significant exception is the domination of free-moving comatulids (Comatulida) in the sediments known from the Papilė region, which may suggest that the Lithuanian sea basin was a little shallower than that located farther west in the Baltic area.

Massive generation of atypical ferrosilicic magmas along the Gondwana active margin: Implications for cold plumes and back-arc magma generation

One of the most intriguing characteristics of the northern (Iberia) and southern (Puna) Gondwana margins is the presence of large volumes of Late Cambrian–Early Ordovician magmatic rocks with ferrosilicic composition, i.e., rocks with high iron and silica contents (FeO > 4.0 wt.%, SiO > 63 wt.%) for very low contents in calcium (CaO < 1.5 wt.%). Geological and geochemical features, as well as experimental results, show that ferrosilicic magmas resulted from near-total melting (80–90%) of crustal sources of metagreywacke and charnockite affinities, possibly derived from Neoproterozoic volcanoclastic sediments and/or their granulite facies equivalents, under very high temperatures (1000 °C–1200 °C) and at pressures of 1.0 to 2.0 GPa. A plausible tectonic setting for this peculiar magmatism is a back-arc region subjected to extension, with the ferrosilicic magmas ascending from a deep cold diapir or mantle wedge plume. Rifting in the back-arc progressed until the aperture of an ocean basin (the Rheic ocean) in the northern margin of Gondwana, but became aborted in Argentina.

Provenance of sediments during subduction of Palaeotethys: Detrital zircon ages and olistolith analysis in Palaeozoic sediments from Chios Island, Greece

Detrital zircon geochronology and analysis of fossiliferous olistoliths from Chios Island, Greece, are used here to constrain terrane accretion processes and the provenance of crustal sources for sediments during the subduction of (a branch of) the Palaeotethys Ocean. U/Pb ages obtained by ion microprobe (SHRIMP-II) analyses of detrital zircons from a Carboniferous greywacke belonging to the tectonostratigraphic Lower Unit of Chios gave major age groups of 2150–1890 Ma, 640–540 Ma, 505–475 Ma and 365–322 Ma. Detrital zircons from a Permian–Triassic sandstone yielded prominent age clusters of 2200–1840 Ma, 1100–910 Ma, 625–560 Ma and 385–370 Ma. The lack of zircon ages between 1.8 and 1.1 Ga in both samples, coupled with the occurrence of ca. 2 Ga-old zircons, imply a northern Gondwana (NW Africa) source. The conodont fauna recovered from an ‘ Orthoceras’-bearing limestone lens from the Carboniferous succession of the Lower Unit indicates a Late Silurian age. The fauna is typical of the Ludfordian Polygnathoides siluricus conodont zone. The material has a conodont colour alteration index (CAI) of about 1–2, indicating very low-grade thermal alteration of less than 100 °C. The closest localities with similar conodont-bearing limestones are to be found in the Balkan region and in the Istanbul Zone of northern Turkey. The occurrence of ‘ Orthoceras Limestone’ can be used as an indicator of palaeosource reconstruction. Our new zircon ages in conjunction with provenance analysis of Silurian to Lower Carboniferous olistoliths strongly suggest that the clastic succession of Chios received its detritus from basement rocks of the Sakarya microcontinent in western Turkey and time and facies equivalents of Palaeozoic units from the Istanbul Zone in northern Turkey and the Balkan region due to subduction of a branch of Palaeotethys close to the southern active margin of Eurasia in Late Palaeozoic times. The multidisciplinary approach of this study underlines the importance of terrane accretion during stepwise closure of Palaeotethys.

The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes

The chronostratigraphic and magmatic framework for the Proterozoic Mount Isa Inlier, northwest Queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years. This material and existing chronometric constraints is used to construct a new time–space plot for the Inlier and to compare Eastern Succession volcanosedimentary accumulations with temporally equivalent units in the Western Fold Belt. The accumulation of volcanosedimentary sequences in the Eastern Succession occurred in four main periods: namely 1890–1850 Ma, 1800–1750 Ma, 1710–1690 Ma, and 1680–1610 Ma. Temporally equivalent units can be identified in the Western Fold Belt. Periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the Inlier. The locus of bimodal volcanism migrates from west to east with time. Intracontinental rifting is the most likely tectonic setting for the sedimentary basins. The majority of the Soldiers Cap Group was deposited between ca. 1675 and 1650 Ma with the exception of units in the Boomarra Horst now known to belong to older sequences (deposited ca. 1775–1750 Ma), and the Llewellyn Creek Formation at Snake Creek which may be as old as ca. 1690 Ma. Upper amphibolite facies rock units in the southwest (e.g. the Gandry Dam Gneiss) are likely to be higher metamorphic grade, temporal equivalents of the Soldiers Cap Group. Five rock units previously assigned to the Cover Sequence 2 Mary Kathleen Group; namely, the Answer Slate, Kuridala Formation, Staveley Formation, Marimo Slate and Agate Downs Siltstone are now known to have been deposited between ca. 1675 and 1610 Ma. These units (bar the Kuridala Formation) have been removed from the Cover Sequence 2 Mary Kathleen Group and given the informal name ‘Young Australia Group’. The Kuridala Formation has been included in the Soldiers Cap Group. The rock units within the Young Australia Group are likely to be lateral facies equivalents of the younger units within the Soldiers Cap Group. Rock units within the Mount Albert Group (although undated) are considered likely to be temporal equivalents of the Soldiers Cap Group and the Young Australia Group. The Roxmere Quartzite is included here in the Young Australia Group that it tectonically overlays. The Ballara Quartzite and Mitakoodi Quartzite represent a single regionally extensive sequence deposited at ca. 1755 Ma. The Double Crossing Metamorphics (previously mapped as basement) are likely to correlate with the Argylla Formation and Mitakoodi Quartzite. Most of the Eastern Succession to the west of the Pilgrim Fault is likely to have been a topographic high between ca. 1780 and 1755 Ma, prior to the Ballara Quartzite unconformably overlying the ca. 1780 Ma Argylla Formation. Detrital zircons of ‘Argylla’ and ‘Wonga’ ages indicate that the Mary Kathleen Belt is likely to have been a topographic high at least periodically during accumulation of the Soldiers Cap and Young Australia Groups. Soldiers Cap Group units in the southeast contain numerous detrital zircons giving ages between ca. 1730 and 1710 indicating a possible siliciclastic source to the east. The host rocks to the Pb–Zn–Ag deposits are restricted to Cover Sequence 3 units deposited between ca. 1680 and 1590 Ma and, with the exception of Century, young from the southeast towards the northwest. Cu ± Au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation. However, many of the most significant deposits are found within the younger sequences.

Geochemical characteristics of whole rock and minerals from the Late Cretaceous granitic rocks of the Rokko Mountains

The Rokko Mountains consist primarily of late Cretaceous granitic rocks. These are recognized as Nunobiki Granodiorite, Yahatadani Granite, Dobashi Quartz Diorite, and Rokko Granite. Based on grain size and texture, the Nunobiki Granodiorite is subdivided into a porphyritic and equigranular facies, whereas the vertically zoned Rokko Granite pluton is differentiated into fine-grained, medium -to coarse-grained, and micrographic facies. The fine-grained facies occurs at high altitude, and the medium -to coarse-grained granite is found at an intermediate to low altitude within the Rokko Mountains. The micrographic facies intrudes both of these varieties. The granitic rocks were analyzed for trace and rare earth elements and mineral composition. The fine and micrographic facies of the Rokko Granite have higher Rb, Nb, and Y, and lower Sr and Zr when compared with the medium -to coarse-grained facies equivalents and have strong negative Eu anomalies in chondrite normalized REE patterns. In the Rokko Granite the Rb content decreases with an increase in Sr. Whole-rock chemistry indicates that the Rokko granitic magma formed a zoned magma chamber due to plagioclase crystal fractionation. Results of the least-square mass balance calculation for the major elements are in good agreement with the analysis data for Eu. The Rokko granitic magma was rich in the volatile components, SiO2, and K2O at the top of the magma chamber, whereas the base was more intermediate in chemical composition. The fine-grained facies has a low An content, and biotite in the fine and micrographic facies is rich in Fe and poor in Mg. These chemical characteristics indicate that two facies are formed from a more differentiated magma than the medium -to coarse-grained facies.