New petrographic observations and stable isotope data help reinterpret the complex sedimentology of the Albian–Cenomanian boundary exposed in the famous red and white chalks of the cliff section at Hunstanton in Norfolk, UK. Thin-section analysis of a prominent crust at the top of the Hunstanton Red Chalk Formation reveals fossil microstructures attributable to the microbe Frutexites. These Red Chalk microstructures are less bushy that Frutexites sensu stricto, and poor preservation, in part caused by later diagenetic iron migration, means they are identified only tentatively as Frutexites. Stable oxygen isotope values from the crust are similar to those from early diagenetic nodular chalks immediately below the crust, and to partially altered chalks elsewhere in Norfolk. The δ18O data are interpreted as Albian seafloor depositional values albeit slightly altered by subsequent meteoric diagenesis. The microbial affinities of Frutexites are not yet proven; thus, the presence of Frutexites alone is not diagnostic of either photic zone or deep-water sedimentation. However, the presence of Frutexites(?) suggests that the red colour of the Hunstanton Red Chalk is due, at least in part, to the mediation of iron-fixing microbes in the accumulating chalk sediment at a dysoxic–anoxic interface. Centimetre-scale columnar and nodular structures above the ‘Frutexites crust’ that project upwards into the basal Paradoxica Bed of the overlying Ferriby Chalk Formation are sites of localized syndepositional iron staining. These nodules are not stromatolitic or microbial and are not evidence for deposition in shallow-water or intertidal settings.

40Ar/39Ar in situ UV laser ablation of white mica, Rb–Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by 40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb–Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle–Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The 40Ar/39Ar spot age spectra also display relict age peaks at 477–465 Ma, 439 Ma and 420–428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician–Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.

The Carboniferous-Triassic Trinity Peninsula Group is a metasedimentary sequence that crops out widely in the northern Antarctic Peninsula. These are some of the most extensive outcrops in the area and hold the key to evaluating the connections of the Antarctic Peninsula in Gondwana; however, they are still poorly understood. Here we present our provenance study of the Trinity Peninsula Group using petrographic and geochemical approaches in combination with cathodoluminescence of detrital quartz in order to constrain its source characteristics and tectonic setting. Using differences in modal composition and quartz cathodoluminescence characteristics, we define three petrofacies derived from the progressive uplift and erosion of a volcano-plutonic continental arc, which exposed the plutonic-metamorphic roots. As indicated by major and trace elements, the source is felsic with a composition ranging from tonalitic to granodioritic. The relatively unweathered condition of the source area points to a dry and cold climate at the time of deposition, but this does not necessarily mean that it was glaciated. Deposition of the sediments occurred within an active continental margin, relatively close to the source area, probably along the south Patagonia–Antarctic Peninsula sector of Gondwana. Strong chronological, petrological and chemical similarities with the sediments of the Duque the York Complex in Patagonia suggest that they were derived from the same source.

Deep-marine muds were collected from two boreholes (Crati II and Neto VI) along the Ionian Calabrian Basin. The samples from the Crati II and the Neto VI boreholes show a similar mineralogical distribution; the marine muds contain mostly phyllosilicates, quartz, calcite, feldspars and dolomite. Traces of gypsum are present in a few samples. The Neto muds show higher concentrations of carbonates than the Crati muds; these contents are mainly related to recycling of the Neogene–Quaternary carbonate-rich marine deposits of the Crotone Basin, which mostly influences the composition of the Neto muds. The geochemical signatures of the muds mainly reflect a provenance characterized by felsic rocks with a minor, but not negligible, mafic supply. In particular, the hinterland composition of the Crati drainage area is on average more mafic in composition than the Neto drainage area. The higher mafic concentration of the Crati sample muds is probably related to the ophiolitiferous units that are exposed in the Crati drainage basin. The degree of source area weathering was most probably of low–moderate intensity because the Chemical Index of Alteration values for the studied muds range from 67 to 69. Furthermore, the low and constant Al/K and Rb/K ratios suggest low–moderate weathering without important fluctuations in weathering intensity. The Al2O3–TiO2–Zr ternary diagram and the values of the Index of Compositional Variability indicate that both the Neto and Crati muds are first-cycle, compositionally immature sediments, related to a tectonically active (collision) setting such as the Calabria–Peloritani Arc, where chemical weathering plays a minor role.

The Mingshui–Jilasitai–Suolun area, located in the central part of the Great Xing’an Range, is characterized by large volumes of alkali feldspar granites. However, the formation time and tectonic setting of these rocks remains controversial owing to a lack of precise geochronological and detailed geochemical data. In this paper, we report new SIMS U–Pb zircon ages and mineralogical, petrographical and geochemical data for Lower Cretaceous alkali feldspar granites from the Mingshui–Jilasitai–Suolun area. The SIMS zircon dating results indicate that these granites formed at 133.6–135.9 Ma. The mineralogical, petrographical and geochemical data show that these granitic rocks belong to highly fractionated I-type granites. Combined with the regional geology data, we propose that the formation of the Lower Cretaceous alkali feldspar granitic rocks was related to an extension induced by delamination of the lithosphere that arose from subduction of the Palaeo-Pacific plate.

The Calcare di Base Formation is a part of the Rossano Basin characterizing the Foreland Basin System of northeastern Calabria. Messinian argillaceous marls from the Calcare di Base Formation have been studied to characterize the sedimentary evolution of this formation during the post-orogenic phases of the Calabria–Peloritani Arc. The mineralogical assemblage of the argillaceous marls is dominated by phyllosilicates (illite, chlorite, illite/smectite mixed layers and traces of kaolinite), carbonate minerals (calcite, aragonite and dolomite), quartz and traces of feldspars (both K-feldspars and plagioclase), gypsum and celestine. The palaeoweathering index records changes at the source, reflecting variations in the tectonic regime as shown in the A–CN–K plot, where the studied samples describe a trend typical of a source area in which active tectonism allows erosion of all zones within weathering profiles developed on source rocks. The studied samples are derived from an environment in which non-steady-state weathering conditions prevailed. This trend could record deformational events that affected the Mediterranean area during the Miocene. The Th/Sc versus Zr/Sc ratios and Al–Zr–Ti plot suggest that the samples likely record a recycling effect from their basement rocks. The geochemical proxies of these samples suggest a provenance from a mainly felsic source. The Messinian argillaceous marls record that deposition probably occurred in a semi-closed marine environment mainly subject to hypersalinity with local episodes of meteoric water influx, during a period characterized by persistent dry and warm/arid conditions alternating with relatively wet conditions.

Altered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3 enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob's Linn bentonites from southern Scotland suggest a common volcanic source in Katian times.

The common belief that organic-rich mudstones formed in quiescent, distal settings is further weakened by study of an upper Cambrian (Leptoplastus – lower Peltura superzones) succession in the Chesley Drive Group in Avalonian Cape Breton Island that is comparable to Alum Shale successions in Baltica. Dramatic sea-level (likely eustatic) changes are now recognized by punctuation of deposition of shallow, wave-influenced black mudstone with brachiopod (Orusia lenticularis) and olenid trilobite-bearing limestones by offlap and formation of a subaerially cemented calcrete-clast conglomerate. Subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with Leptoplastus Superzone (L. ovatus Zone) trilobite sclerites. Dynamic processes are shown by wave ripples in the mudstone and limestone, sorting and winnowing of fossil rudstones, and pre-compactional fracture of the conglomerate and rudstones. Orusia rudstones in the succession below the conglomerate are regarded as analogues of Eoorthis and Billingsella rudstones in the ‘biomere’ extinction intervals of the Laurentian basal Sunwaptan. The lowest Orusia-rich beds are no older than the P. spinulosa Zone but, as elsewhere in Avalonia, they range into the higher Leptoplastus (Cape Breton) and even the Peltura (Britain, New Brunswick) superzones. Rare agnostoid sclerites in lower Peltura Superzone (Ctenopyge tumida Zone) olenid rudstone resemble those traditionally assigned to Lotagnostus trisectus in Avalonian Britain and Sweden, and are distinct from Laurentian L. americanus. An L. americanus Zone cannot be identified in Avalonia or Baltica, and the first appearance datum (FAD) of purported ‘L. americanus’ is not suitable as a standard for the base of the highest Cambrian stage.

The lower bathyal Ocean Drilling Program Hole 1261A was sampled near an upper Quaternary oxygen minimum zone (OMZ). Glauconite, the percentage of the foraminiferal assemblage as benthic specimens and assemblage composition were used to investigate the behaviour of the OMZ. Benthic foraminifera and glauconite were comparable with the upper margin of the modern OMZ off California. The percentage abundances of U. peregrina and C. laevigata were on the Demerara Rise negatively correlated, the proportional abundance of U. peregrina increasing upwards through the section. This reflects variations in proximity to the upper margin of the OMZ. This might reflect either crustal subsidence or long-term shallowing of the OMZ during the earlier late Quaternary. Neither hypothesis can be accepted unequivocally. The purported subsidence can be ascribed to crustal loading by the Amazon and Orinoco deep-sea fans, but this would require that the palaeodepth to the top of the OMZ remains constant across several glacial–interglacial cycles. In contrast, it is difficult to envisage any mechanism that could have caused progressive shallowing of the OMZ across several glacial–interglacial cycles. The epifaunal Planulina wuellerstorfi is related to more oxic waters and enhanced current action. This suggests that intervals with more abundant P. wuellerstorfi were somewhat less dysoxic than those with few. These intervals approximate to those with more abundant C. laevigata. Superimposed on this low-frequency signal is a higher-frequency signal, indicated by a between-sample assemblage turnover index (ATIs) that might prove useful for long-range sequence stratigraphic correlation at bathyal depths.

The Early Palaeozoic Shuanggou ophiolite is the best-preserved part of the Ailaoshan ophiolite belt. The microgabbros (basaltic dykes) and basalts (basaltic lavas) show distinct characteristics in geochemistry, implying that their genetic mechanisms are different. With Al2O3 contents ranging from 14.7% to 17.0%, the microgabbros belong to low-alumina type. They exhibit normal mid-ocean-ridge basalt (N-MORB) -like trace elemental characteristics with positive εNd(t) values (9.7–11.6) and slightly variable (87Sr/86Sr)i ratios (0.7036–0.7046). In contrast, the basalts have high Al2O3 contents (19.5–23.2%), therefore belonging to high-alumina type. A plagioclase-accumulation model is used to account for the high alumina contents. Moreover, the basalts have enriched MORB (E-MORB) -like trace element characteristics with lower εNd(t) values (6.4–8.0) and (87Sr/86Sr)i ratios (0.7032–0.7036). Their incompatible element ratios exhibit linear correlation with the isotopic data, which is probably related to the contribution of a mixed lithosphere–asthenosphere source. In summary, a two-stage model is proposed to explain the formation of the Shuanggou ophiolite: (1) at the continent–ocean transition stage, the basalts were generated by low-degree partial melting of the mixed mantle near a slow-spreading embryonic centre; and (2) at the mature stage of the Ailaoshan Ocean, the microgabbros were produced by moderate-degree partial melting of the depleted asthenospheric mantle.

Systematic lithofacies, palaeocurrent, palaeomorphological and palaeohydrological analyses have provided detailed information about a hitherto unstudied river system of the Siwalik foreland basin of the Himalaya. Three distinct lithofacies associations, each representing a specific depositional setting, have been identified and named as ‘Facies Association A’, ‘Facies Association B’ and ‘Facies Association C’. The ‘Facies Association A’ comprises pebbly sandstone, cross-bedded sandstone, ripple-laminated sandy siltstone and bioturbated mudstone lithofacies and represents deposits of a braided channel. The ‘Facies Association B’ comprises cross-bedded sandstone, bioturbated mudstone, fine sandstone–mudstone alternation and lensoid to prismatic sandstone lithofacies and represents deposits of a channel, natural levee, crevasse-splay and flood plain of a meandering stream. The ‘Facies Association C’ comprises mottled siltstone–mudstone heterolith and fine sandstone lithofacies and represents deposits of the upland interfluve region. The braided stream had a maximum depth of 4.15 m, maximum width of 305 m and maximum discharge of 7045 cumec, whereas the meandering stream had a sinuosity of 1.26, maximum depth of 3.71 m, maximum width of 180 m and maximum discharge of 4070 cumec. The area had a regional radial outward flow pattern, but dominantly towards the SSW. However, the braided river had a bimodal flow pattern due to an active basement-high-induced bend along its course. A comparison of the sediment characters and morphological and hydrological parameters of these streams with those of the modern rivers of the Ganga (Gangetic) basin has enabled us to infer that this river system was located in the medial-distal megafan-interfan setting of the basin.

The uppermost Famennian – lowermost Tournaisian interval has been analysed in detail using biostratigraphy, sedimentology, magnetic susceptibility and geochemistry in a reference section of the relatively shallow carbonate ramp environment within the Pomeranian Basin. High-resolution biostratigraphic study, based on miospores, allows recognition of the standard western European lepidophyta–nitidus (LN) and verrucosus–incohatus (VI) zones, as well as the Convolutispora major Zone, a local Pomeranian equivalent of the European standard hibernicus–distinctus (HD) Zone. The sedimentary succession and specific phenomena recognized close to the Devonian/Carboniferous boundary, such as fluctuations in water column euxinia, wildfire evidence, relative sea-level changes and perturbations of the carbon cycle reflected by positive carbon excursions, display a pattern partly similar to that observed in many areas in Europe during the Hangenberg Event, although the Hangenberg Black Shale horizon is not developed here. These important microscale environmental perturbations were observed not only within the Famennian LN miospore Zone but in a wide interval between the LN and the lowermost local Convolutispora major miospore zones ( = lower part of HD standard miospore Zone). It is still uncertain whether the recognized event(s) were connected solely with the Hangenberg Event, which was possibly complex and multi-phased as is sometimes suggested, or whether they represent a succession of regionally limited, post-Hangenberg events. This question needs to be further investigated on broader stratigraphic and geographical scales.

A volcanic tuff 1.0 m above the base of the Triebenreuth Formation in the Franconian Forest provides the first precise and biostratigraphically bracketed date within the traditional Middle Cambrian. The first illustration of fossils from the Triebenreuth Formation in this report and their discussion allow a more highly refined correlation within the Middle Cambrian. A weighted mean 206Pb–238U date of 503.14±0.13/0.25/0.59 Ma on zircons from this subaerial pyroclastic tuff was determined by U–Pb chemical abrasion isotope dilution mass spectrometry (CA-TIMS) techniques. At c. 6.0–7.0 Ma younger than the base of the traditional Middle Cambrian in Avalonia, the new West Gondwanan date from east-central Germany suggests that estimates of 500 Ma for the base of the traditional Upper Cambrian and 497 Ma on the base of the Furongian Series may prove to be too ‘old’. Biostratigraphically well-bracketed dates through most of the Middle Cambrian/Series 3 and below the upper Upper Cambrian/upper Furongian Series do not exist. An earlier determined 494.4±3.8 Ma date from the Southwell Group of Tasmania may actually prove to be a reasonable estimate for the age of the base of the traditional Upper Cambrian. Until high precision dates are determined on the base of the traditional Upper Cambrian and base of the Furongian Series, the rates of biotic replacements and geological developments and the durations of biotic zones in the Middle/Series 3 and Upper Cambrian/Furongian Series remain as ‘best guesses’.

The Upper Ordovician (Katian–Hirnantian) brachiopods of Tafilalt, eastern Anti-Atlas, are locally abundant, diverse and well preserved, providing a near-continuous record of faunal change on a high-latitude siliciclastic-dominated platform. A chronostratigraphic framework, based on brachiopod distribution and preservation in shell accumulation events and integrated with sequence stratigraphy, has been generated for the Katian interval, which has allowed correlation with the chitinozoan-based chronostratigraphic and sequence-stratigraphic framework erected for the central Anti-Atlas. In Tafilalt, two Katian (transgressive–regressive) composite depositional sequences, c. 60 and 170 m thick and related to third-order fluctuations in sea level, were unaffected by Hirnantian glaciogenic erosion. They were deposited on a mixed platform with a bryonoderm association dominated by brachiopods, bryozoans and echinoderms. Brachiopods developed in high-energy inner shelf areas, whereas bryozoans (mainly trepostomates and fenestrates) and pelmatozoans (cystoids and crinoids) dominated in low-energy outer shelf areas. Brachiopod accumulations mark distinct event surfaces, such as lag and event concentrations, hydraulic simple and composite concentrations related to transgressive surfaces, and hiatal condensed concentrations marking maximum flooding surfaces. The taphonomic condensation displayed by the Hirnantian Alnif Member, which onlaps the erosive base of glaciogenic tunnel channels, is explained as reworking and resedimentation of allochthonous, robust, biogenic hard parts sourced from the underlying (Katian) Ktaoua Group.

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons (c. 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

The circular 625 km2 alkaline Pilanesberg Complex, South Africa, contains coeval eruptive and several distinctive intrusive syenitic and foyaitic components, concentrically arranged at the surface. However, owing to poor outcrop the relationships between the different intrusive rocks, and their shape in the third dimension cannot be convincingly determined in the field. The original interpretation was a laccolith, whereas later models suggested a funnel shape, and appealed to ring-dyke and cone-sheet emplacement mechanisms. However, the radial widths of these coarse-grained bodies are over 1 km and so cannot have been emplaced as ring dykes or cone sheets, which are usually quite thin and fine grained. Creating the space for emplacement and removal of pre-existing country rocks for each postulated subsequent intrusive event presents a major challenge to this latter hypothesis. Extensive previously published and new field relationships are re-evaluated here to suggest that the body is a gently inward-dipping sheet and that subsequent injections of magma merely pumped up an existing and evolving magma chamber rather than intruded into solid rocks. A Bouguer gravity anomaly model is presented that supports the concept of a shallow, flat-bottomed body rather than one that continues to significant depth. There are many analogies with the Kangerlussuaq Intrusion, Greenland.

We amend the taxonomy and provide new anatomical information on the hadrosaurid dinosaur Saurolophus morrisi (upper Maastrichtian Moreno Formation, central California, USA) derived from full preparation of the referred skull roof. The cranial morphology of this species is distinct enough to justify the new combination Augustynolophus morrisi gen. nov. The morphology of the nasals and surrounding cranial bones indicates that A. morrisi sported a solid nasal crest ending in an elongate triangular plate that extended above the skull roof. Autapomorphies include a crescentic base of the frontal caudodorsal process and extension of the process caudal to the frontal ‘dome’; distal end of nasal crest with knob-like process inflected rostrally; circumnarial depression lightly incised and weakly emarginated, adjacent to caudolateral margin of nasal and occupying two-thirds the width of lateral surface of distal region of crest; and caudal surface of distal nasal crest subrectangular. We formally establish the new tribe Saurolophini consisting of Prosaurolophus, Augustynolophus and Saurolophus. Saurolophin synapomorphies include a premaxilla with broad arcuate contour of rostrolateral region of thin everted oral margin and flat and steeply inclined occlusal surface of dentary dental battery, among other characters. Saurolophin crests evolved towards increasing caudodorsal length, along with caudal extension of the circumnarial fossa and involvement into the crest of adjacent facial elements. Augustynolophus is the second described genus of North American late Maastrichtian hadrosaurids. Its recognition implies a greater diversity among late Maastrichtian dinosaur faunas than previously recognized and is congruent with hypotheses of endemism and/or provinciality during Late Cretaceous time.

Nearly 50 new Nd isotope analyses are presented for the Shawanaga region of Georgian Bay, Ontario, to study crustal evolution in the Grenvillian Central Gneiss Belt. Depleted mantle (TDM) Nd model ages are used to map a major Grenvillian tectonic boundary, the Allochthon Boundary Thrust (ABT), which in the Shawanaga area separates gneisses with TDM ages above and below 1.65 Ga. This is lower than the 1.8 Ga age cut-off observed further north, and is attributed to a southward increase in Mesoproterozoic magmatic reworking of an original Palaeoproterozoic continental margin, causing a progressive southward decrease in Nd model ages. Between Shawanaga Island and Franklin Island, Nd isotope mapping yields an ABT trajectory that closely matches published geological mapping, and passes within 100 m of four retrogressed eclogite bodies typically associated with the thrust boundary. This validation of the method gives confidence in the mapped trajectory south of Snake Island, where sparse outcrop inhibits lithological mapping. The new results suggest that published 1.7–1.9 Ga TDM ages in the Lower Go Home domain of the Central Gneiss Belt further south are also indicative of parautochthonous crust. Hence, we propose that the main ramp of the ABT is located in the immediate hangingwall of the Go Home domain, much further south than generally recognized. This has important implications for the large-scale crustal structure of the SW Grenville Province, suggesting that the ABT ramp has a similar curved trajectory to the Grenville Front and the Central Metasedimentary Belt boundary thrust.

The Palaeozoic rocks of the Dingle Peninsula provide a record of the evolution of the Caledonides, Acadides and Variscides. The succession ranges from Early Ordovician deep-water sediments, through Silurian shallow marine to non-marine sediments and volcanic rocks to an Old Red Sandstone (ORS) succession topped by Carboniferous marine shales. Comparison of structural styles in the unconformity-bounded groups, together with a detailed analysis of fault zones, allows the tectonic history to be deduced. The older rocks record Caledonian processes on the margin of Avalonia during Early Ordovician time and convergence then soft collision with Laurentia during Silurian time. The Dingle Basin was developed during the late Silurian – Early Devonian transtension in the Iapetus suture zone and was inverted in the latest Emsian Acadian orogenic episode. Post-Dingle Group ORS groups in the north of the peninsula were deposited in a syn-rift footwall block to the main Munster Basin. The Acadian transpressional and Munster Basin extensional structures were reactivated or overprinted in the Variscan deformation such that Acadian folds are transected by Variscan cleavage in both plan and vertical views. After Iapetus closure, changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the Rheic Ocean.

A conodont-graptolite biostratigraphic study was carried out on the top strata of the San Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera of San Juan in western Argentina. Significant conodont records in the San Juan and Las Chacritas formations allow for the recognition of the Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (Microzarkodina hagetiana and M. ozarkodella subzones) and Eoplacognathus suecicus zones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of the Nemagraptus gracilis and the Pygodus anserinus zones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to the Pygodus serra Zone and the Pterograptus elegans and Hustedograptus teretiusculus zones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows that Periodon and Paroistodus are the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such as Histiodella, Periodon, Microzarkodina, Eoplacognathus and Baltoniodus, allow a precise global correlation with other regions such as south-central China, Baltoscandia, North America, Great Britain, Southern Australia and New Zealand. The graptolite fauna identified here are recognized worldwide in equivalent strata in the Baltic region, Great Britain, North America, China, southern Australia and New Zealand. The presence of graptolites in the ribbon limestones of the Las Chacritas Formation is documented for the first time.