Radiolarites Research Articles

The geological setting of volcanogenic sulphide orebodies in Albanian ophiolites

Cu–Fe sulphides with minor Zn, Au and Ag occur as massive, disseminated and vein mineralizations in volcanosedimentary rocks, volcanics and occasionally associated intrusions of a sheeted dyke complex in ophiolites in Albania. The ophiolites form two sub-parallel units, the Western Ophiolite and Eastern Ophiolite Belts, stretching across the country from north-west to south-east, and have mid-ocean ridge and island arc (boninite supra-subduction type) affinities, respectively. The ores have been extensively prospected and were mined until the 1990s.The Western Ophiolite Belt comprises a volcanosedimentary series, pillow basalts, ultramafic rocks, gabbros and plagiogranites. The most significant orebodies are in volcanosedimentary rocks at Rubik, Palaj-Karme and Porave. The orebodies with massive chalcopyrite–pyrite ores vary in shape occurring between hematised radiolarian chert and tholeiitic basalt pillow lavas. Mapping the shape of the main orebody at Rubik shows a series of mounds comparable to the collapse of 'black smoker' chimneys. Massive and disseminated sulphides are hosted by MORB affinity basalts at Kachinar, Derven and Rehova. A geophysical induced polarisation survey has indicated further resources at depth at West Kachinar.The Eastern Ophiolite Belt consists of a basalt–andesite–rhyolite series cut by a sheeted dyke complex and underlain by quartz diorite–plagiogranite, gabbro, gabbronorite and harzburgite. It includes a 25-km long belt of a significant longitudinal continuity of mineralization between Chafe Mal and Reps. Massive sulphide ores underlain by disseminated veins and stockworks are hosted by a Middle–Upper Jurassic lower basalt–andesite succession at Reps, Spach, Maja e Made, Tuch, Lak Rosch, Paluce and Fushe Arres. Alteration includes chloritization, epidotization, silicification, argillisation, zeolitization and carbonatisation. The Upper Jurassic, upper dacite volcanoclastic, andesite–dacite, boninite succession hosts massive pyrite–chalcopyrite–sphalerite ore underlain by few or no disseminated veins or stockworks at Gurth Spach, Munelle and Chafe Bar. At Perlat, massive ores are in an andesite pillow lava. Induced polarisation anomalies indicate further orebodies at depth in this highly thrusted area. Sulphide veins and other quartz–lode sulphides occur in some gabbros. An unusual orebody at Gjegjan occurs in a tectonic mélange of volcanics, greenschists and amphibolites.Similarities between Albania and other ophiolitehosted volcanogenic deposits at Cyprus and Oman include a clustering of orebodies along the general strike of the ophiolite, the occurrence of massive ores at successive horizons, and the presences of vein, stockwork and disseminated mineralization surrounding and beneath the massive sulphide zones; but, in Albania, ochres and umbers are not found, the age is older and the age-span greater. The Albanian orebodies range in size from 0·3–10·0 Mt. Those in the Western Ophiolite Belt are comparable in size to modern seafloor deposits at ocean ridges. Eastern Ophiolite Belt orebodies are similar in size to others interpreted as having formed in a suprasubduction environment. The Eastern Ophiolite Belt offers the greatest potential for locating future orebodies, possibly beneath thrust planes.

The Devonian–Lower Carboniferous succession in Northwest Peninsular Malaysia

A new stratigraphic nomenclature is proposed for the approximately 600 m thick, mainly clastic transitional sequence between the underlying Mempelam Limestone and overlying Kubang Pasu/Singa Formation in northwest Peninsular Malaysia. This sequence represents shallow marine deposits of the continental margin of the Sibumasu Terrane during the Middle Palaeozoic (Devonian–Carboniferous). It is separated into several formations. The Timah Tasoh Formation is an approximately 76 m sequence consisting of 40 m of laminated tentaculitid shales at the base, containing Monograptus yukonensis Jackson and Lenz and Nowakia (Turkestanella) acuaria Alberti, giving an Early Devonian (Pragian–Emsian) age, and about 36 m of rhythmically interbedded, light coloured argillo-arenites. The Chepor Formation is about 90 m thick and consists mainly of thick red mudstone interbedded with sandstone beds, of Middle to Late Devonian age. A new limestone unit is recognized and named the Sanai Limestone, which contains conodonts of Famennian age. The Binjal Formation consists of red and white mudstone interbedded with sandstone beds showing Bouma sequences. The Telaga Jatoh Formation is 9 m thick and consists mainly of radiolarian chert. The Wang Kelian Formation is composed of thick red mudstone beds interbedded with silty sandstone, and contain fossils indicative of an Early Carboniferous (Visean) age. The succession was deposited on the outer shelf, with depositional environments vertically fluctuating from prodelta to basinal marine. The Devonian–Carboniferous boundary is exposed at Hutan Aji and Kampung Guar Jentik, and indicates a major regressive event during the latest Devonian.

Ocean Plate Stratigraphy in East and Southeast Asia

Ancient accretionary wedges have been recognised by the presence of glaucophane schist, radiolarian chert and mélange. Recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy, provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating. Reconstructed ocean floor sequences found in ancient accretionary complexes in Japan comprise, from oldest to youngest, pillow basalt, limestone, radiolarian chert, siliceous shale, and shale and sandstone. Similar lithologies also occur in the mélange complexes of the Philippines, Indonesia, Thailand and other regions. This succession is called ‘Ocean Plate Stratigraphy’ (OPS), and it represents the following sequence of processes: birth of the oceanic plate at the oceanic ridge; formation of volcanic islands near the ridge, covered by calcareous reefs; sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited; deposition of radiolarian skeletons on the oceanic plate in a pelagic setting, and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting; and sedimentation of coarse-grained sandstone and shale at or near the trench of the convergent margin. Radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction. The ages of detrital sediments becomes younger oceanward as younger packages of OPS are scraped off the downgoing plate. OPS reconstructed from ancient accretionary complexes give us the age of subduction and accretion, direction of subduction, and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo-oceans now represented only in suture zones and orogenic belts.

Sedimentary environments of the Middle Permian phosphorite–chert complex from the northeastern Yangtze platform, China; the Gufeng Formation: a continental shelf radiolarian chert

Radiolarian chert deposited on a continental shelf occurs in the Middle Permian Gufeng Formation on the northeastern Yangtze platform, China. The sedimentary environments of radiolarian cherts from accretionary complexes have been well studied; however, there are few studies about radiolarian chert deposited on continental shelves. Therefore we have completed a sedimentological and geochemical study of the Gufeng Formation. The Gufeng Formation is subdivided into the Phosphate Nodule-bearing Mudstone Member (PNMM) and the Siliceous Rock Member (SRM) in ascending order. The basal PNMM consists of glauconite-bearing mudstone, which indicates deposition under aerobic shallow-marine conditions, whereas the upper PNMM is composed of mudstone including abundant phosphate nodules, suggesting deposition near the outer shelf in suboxic conditions. The SRM consists mainly of alternating beds of black chert, mudstone, and siliceous mudstone, with minor tuffaceous mudstone and porous chert beds. The black chert contains abundant radiolarians, sponge spicules, and organic matter. Framboidal pyrite occurs in the black chert. Porous chert in the SRM includes abundant rhombohedral cavities, which are dolomite moulds surrounded by quartz, suggesting dolomitization before silicification during early diagenesis. The Gufeng Chert is characterized by high Si, Mo, Ni, Cu, and Zn contents, and extremely low Mn content. The Gufeng Chert has high normal paraffin concentrations with petroleum-like markers. These geochemical features suggest that the Gufeng Chert was deposited under sulfate-reducing conditions; and is not hydrothermal, but rather biogenic in origin. The sedimentological and geochemical data suggest that the Gufeng Formation was deposited mainly on the outer shelf in suboxic–anoxic conditions caused by organic matter produced during upwelling. The upwelling probably led to high radiolarian productivity. Abundant silica and organic matter were deposited in the Gufeng basin. Compared with chert from other sites, the Gufeng Chert shows many similarities with chert from platformal basins, but not with chert from accretionary complexes. Especially, with respect to age, rock type, stratigraphy, and geochemical features, the chert of the Gufeng Formation is similar to that of the Phosphoria Formation in the western U.S.A., which is a world-class phosphorite giant. These similarities suggest that upwelling occurred around the eastern margins of Panthalassa and the Paleotethys concurrently during the Middle Permian. In general, shelf-type radiolarian chert, represented by the Gufeng Chert, usually shows some of the following features: dark color, lack of rhythmical bedding, association with phosphorite, platformal limestone, felsic tuff, abundant organic matter, and sulfides. Shelf-type radiolarian chert is deposited in a poorly aerated restricted basin, or in an oxygen-minimum zone.

Upper Cretaceous oceanic red beds in southern Tibet: a major change from anoxic to oxic, deep-sea environments

Red marine mudstones intercalated with pelagic marlstones, limestones and radiolarian cherts comprise the Chuangde Formation, which overlies mid-Cretaceous dark grey shales in the northern subzone of the Himalayan Tethys of southern Tibet. The red mudstones reflect deposition below the carbonate compensation depth (CCD) in a deep oceanic basin. The intercalated, thin-bedded marlstones represent fine-grained turbidites derived from the upper slope and transported into the adjacent deep basin. Planktonic foraminifera and nannofossils in the marlstones indicate a Santonian–early Campanian age for the formation. The red beds of the Chuangde Formation were deposited in a highly oxygenated deep-sea environment. This is confirmed by the high iron-trioxide content, the negative Cerium anomaly at the bottom of the red sequence, and the very low total organic carbon content (TOC 0.01–ca. 0.14%). Deposition of the red beds coincided with a Santonian–early Campanian diversity peak of planktonic foraminifera. The environmental interpretation is supported by the bulk carbon isotope δ 13C value of carbonate turbidites that becomes more positive stratigraphically up from the base of the formation. The highly oxygenated bottom-water conditions were not restricted to a deep ocean basin, but extended up to the continental rise/slope, as indicated by synsedimentary red marls incorporated in slumps and olistoliths near the top of the formation. This indicates that not only the bottom waters but also intermediate waters were highly oxygenated. Deposits of the Chuangde Formation are similar to Upper Cretaceous oceanic red beds (CORBs) in Western Europe, such as the Scaglia Rossa in Italy and the Capas Rojas in southern Spain. They represent the easternmost occurrence of Upper Cretaceous pelagic red beds of the Tethys. Late Cretaceous oxic oceanic conditions extended into the southern Tethys because the formation was deposited in southern palaeolatitudes. Therefore, the change from a dysoxic/anoxic to an oxic bottom ocean environment during the Late Cretaceous was at least hemiglobally if not globally wide. The change to oxic bottom conditions occurred later in the southern Tethys than in the western Tethys, where it began during the late Turonian. Several different earth processes could have caused such change. We suggest that the oxidized character of these deep-sea deposits was a result of a combination of various processes, among which very low sedimentation rates and changes in bottom ocean circulation may have been the most important.

GEOLOGICAL EVOLUTION OF THE KOZIAKAS OPHIOLITIC COMPLEX (WESTERN THESSALY, GREECE)

The Koziakas ophiolitic complex is located in Western Thessaly and is interpreted as a dismembered ophiolite sequence, which overthrust the Western Thessaly Unit (WTU). The Koziakas basal Unit consists of sedimentary rocks and occupies the stratigraphically lowermost members of the WTU. The Ophiolitic Unit overthrust the WTU and consists, from bottom to top, of a well-developed ophiolitic melange, a sub-ophiolitic metamorphic sole, serpentinites, serpentinized peridotites, peridotites (harzburgite, lherzolite, plagioclase lherzolite), massive dolerites, massive basalts and pillow lavas interlayered with Mnrich radiolarites. The ophiolitic melange comprises a chaotic multicoloured complex of various rock fragments. A sparse network of gabbroic (locally rodingitized), plagiogranitic and doleritic dykes intrudes the mantle tectonites along two main cross-cutting directions: the first strikes NE-SW and dips 50o-70o to the NW, the second strikes N-S to NNW-SSE with a dip of about 65o-80o to the west. The dominant tectonic fabrics such as shear zones, fold axes, foliation planes and mineral streching lineations, imprinted mainly on the Koziakas mantle tectonites trend NW to NE and are consistent with a NE-SW emplacement direction. The similar radiometric ages and the comparable geological characteristics of the Koziakas ophiolitic complex with its adjacent ophiolite suites of Pindos, Vourinos and Othris imply that they probably developed in the same marginal basin represented by the Pindos ocean basin.

Tehama-Colusa Serpentinite Mélange: A Remnant of Franciscan Jurassic Oceanic Lithosphere, Northern California

The Coast Range ultramafic belt that borders the Sacramento Valley is largely a serpentinitematrix mélange, called the Tehama-Colusa serpentinite mélange (TCSM) after the counties it spans. It is bordered on the west across the Coast Range fault by exhumed high-P/T metamorphic rocks of the Franciscan subduction complex, and on the east, across the steep Stony Creek fault, by the Jura-Cretaceous terriginous clastic strata of the Great Valley Group (GVG). Remnants of the Coast Range ophiolite (CRO) that lie stratigraphically beneath the basal GVG are exposed only at the northern end of the Tehama-Colusa serpentinite mélange belt. The serpentinite protolith was peridotite tectonite (harzburgite > dunite), comprising oceanic upper mantle. Non-native mélange blocks in the serpentinite are chiefly basaltic submarine lava and sparse radiolarian ribbon chert (Middle and early Late Jurassic), plus rare plutonic rocks and slaty argillite. The serpentinite mélange protolith was basaltic oceanic crust above uppermost mantle, overlain by radiolarian chert and thin argillite (deep-sea clay?). An open-ocean setting, far from a continent margin or volcanic arc, is inferred from the absence of terriginous clastic or volcaniclastic sediments. The TCSM wraps around the Stonyford volcanic complex, a basaltic seamount that grew atop the basaltic oceanic crust during the late Middle and Late Jurassic. A widespread deepsea tectonic event in the Late Jurassic disrupted the proto-TCSM upper mantle and crust. Pervasive hydration turned peridotite into serpentinite, which invaded and mixed with the basalt/chert oceanic crust, creating mélange. Possible causes are discussed. The TCSM basaltic lavas and radiolarian chert closely resemble Franciscan oceanic crustal (not seamount) rocks, and are within the Franciscan basalt/chert age span; they are unlike the CRO igneous assemblage and its Late Jurassic tuffaceous chert. The TCSM was a segment of Franciscan oceanic lithosphere that records only Jurassic open-ocean depositional and tectonic history. It escaped latest Jurassic-Cretaceous continent-margin and trench sedimentation, subduction, subduction accretion, high-P/T metamorphism, and exhumation of deeply subducted rocks that characterize the Franciscan Complex farther west in the Coast Ranges. The TCSM and CRO terranes are remnants of two different mid Jurassic paleoequatorial oceanic plates, separated now by the Stony Creek-Beehive Flat fault system. TCSM evolution involves four successive stages of Mesozoic tectonic history: (1) Late Jurassic, when the CRO oceanic plate was being drawn NNE toward the Great Valley subduction zone in front of the oceanic Nevadan arc, trailed by TCSM (eastern Franciscan) lithosphere; (2) the latest Jurassic-earliest Cretaceous, when (a) N-S dextral transform faulting replaced earlier dextral oblique subduction, bringing the Jurassic TCSM/Franciscan oceanic plate northward alongside the unsubducted CRO plate remnant, and (b) terriginous sediments from the new Nevadan orogen spread progressively seaward across the CRO and adjacent TCSM/Franciscan Jurassic ocean floor; (3) resumption of eastward subduction in the Early Cretaceous, from a new trench lying farther west, caused (a) disruption, mélanging, and metamorphism of the Tithonian-Valanginian clastic strata and underlying Jurassic (Franciscan) oceanic crust, and (b) underthrusting, uplift, and eastward tilting of the unbroken TCSM/CRO oceanic basement, forming a new forearc ridge; and (4) a post-Valanginian Cretaceous era when Franciscan subduction operated west of the TCSM/CRO submarine ridge, and GVG forearc basin sedimentation progressed behind the ridge on the east. The Stony Creek-Beehive Flat composite fault sytem between the TCSM and CRO was a dextral transform fault during stage 2 (Tithonian-Valanginian) and a W-vergent reverse fault during stage 3 (Valanginian), during underthrusting and tilting of TCSM/CRO basement. The extensional Coast Range fault, bounding TCSM on the west, brought up deeply subducted Franciscan Cretaceous high-P/T metamorphic rocks in the late Cretaceous (Jayko et al., 1987).

Stratigraphic evolution of Triassic arc-backarc system in northwestern Croatia

Abstract Middle Triassic arc-related extensional tectonics in the western Tethys generated a complex pattern of intra-and backarc basins. We studied volcano-sedimentary successions of subsided continental-margin blocks (Mts. Žumberak and Ivanščica) and of dismembered incomplete ophiolite sequences interpreted as remnants of a backarc basin (Mts. Medvednica and Kalnik) in northwestern Croatia. We dated the successions with radiolarians, conodonts, foraminifers, algae, and sponges. The continental margin experienced a phase of accelerated subsidence in the late Anisian that was approximately coincident with the onset of intermediate and acidic volcanism; pelagic sediments with volcaniclastics accumulated atop subsided carbonate platforms. These relatively shallow basins were later infilled completely by prograding platforms in the late Ladinian-Carnian. In the backarc basin, sea-floor spreading initiated near the Anisian-Ladinian boundary and continued into the late Carnian. Pillow basalts were erupted and interlayered with radiolarian cherts and shales. The studied area was a part of a larger Triassic arc-backarc system preserved in the southern Alps, Alpine-Carpathian Belt, Dinarides, and Hellenides. Volcano-sedimentary successions of Mts. Medvednica and Kalnik are relics of the Meliata-Maliak backarc basin. In comparison to other previously dated oceanic remnants of this system, the longest continuous sea-floor spreading is now documented in one restricted tectonic unit.

Lower Cretaceous radiolarian bedded chert from the Mineoka Belt, Boso Peninsula, Japan

A meter-scale, bedded red radiolarian chert block was found on the Yo-oka Beach at the eastern edge of the Mineoka Belt, Boso Peninsula, central Japan. The block is included in the area composed of fragments of the Mineoka ophiolite and other sedimentary and igneous rocks, including a Paleocene to Miocene pelagic limestone/chert sequence. The radiolarian fauna indicates an assemblage zone of middle to late Albian (Early Cretaceous), partly corresponding to the age of the Tethyan fauna. The tectonic significance of this Cretaceous radiolarian chert block is not clear: it could be from either the Mineoka ophiolite and related oceanic plate stratigraphy or from the Shimanto Supergroup.

High-Ti type N-MORB parentage of basalts from the south Andaman ophiolite suite, India

A complete dismembered sequence of ophiolite is well exposed in the south Andaman region that mainly comprises ultramafic cumulates, serpentinite mafic plutonic and dyke rocks, pillow lava, radiolarian chert, and plagiogranite. Pillow lavas of basaltic composition occupy a major part of the Andaman ophiolite suite (AOS). These basalts are well exposed all along the east coast of southern part of the south AOS. Although these basalts are altered due to low-grade metamorphism and late hydrothermal processes, their igneous textures are still preserved. These basalts are mostly either aphyric or phyric in nature. Aphyric type exhibits intersertal or variolitic textures, whereas phyric variety shows porphyritic or sub-ophitic textures. The content of alkalies and silica classify these basalts as sub-alkaline basalts and alkaline basalts. A few samples show basaltic andesite, trachy-basalt, or basanitic chemical composition. High-field strength element (HFSE) geochemistry suggests that studied basalt samples are probably derived from similar parental magmas. Al2O3/TiO2 and CaO/TiO2 ratios classify these basalts as high-Ti type basalt. On the basis of these ratios and many discriminant functions and diagrams, it is suggested that the studied basalts, associated with Andaman ophiolite suite, were derived from magma similar to N-MORB and emplaced in the mid-oceanic ridge tectonic setting.

Middle Permian palaeobiogeography study in East Kunlun, A’nyêmaqên and Bayan Har

Three regions can easily be identified in the study area according to the Middle Permian palaeobiogeographic distribution of biota, they are the southern slope of East Kunlun, A’nyemaqen and Bayan Har. Biotic constitution and ecology in the southern slope of East Kunlun and Bayan Har are very similar. Both the diversity and abundance of organisms in these two areas are very high and reefs are widely developed. However, biotic diversity and abundance in A’nyemaqen which is between the above two areas are obviously low. Differentiation of palaeobiogeographic distribution in these areas should be due to the baring of A’nyemaqen ocean in the time of Middle Permian. Middle Permian radiolarian chert and thick abyssal red ooze are widely spread in A’nyemaqen, implying that the A’nyemaqen ocean had a great scale in size. Vast scale of deep ocean basin became an impassable gulf for some of the benthos, and as a result, only part of the organisms could have the chance to get to the isolated islands situated in ocean basin. Small living space and hard conditions in the islands further limited the abundance and diversity of biota. Tectonic background reflected by the geochemical study of basalt in the three areas is coupling well enough with the palaeobiogeographic division.

The Geology of the Darien, Panama, and the late Miocene-Pliocene collision of the Panama arc with northwestern South America

The geology of the Darien province of eastern Panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its Upper Cretaceous to upper Miocene sediments. The sequence of events inferred from the stratigraphic record includes the collision of the Panama arc (the southwestern margin of the Caribbean plate) and South American continent. Three tectonostratigraphic units underlie the Darien region: (1) Precollisional Upper Cretaceous–Eocene crystalline basement rocks of the San Blas Complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the Darien province. These rocks formed part of a >20 m.y. submarine volcanic arc developed in a Pacific setting distant from the continental margin of northwestern South America. The northerly basement rocks are quartz diorites, granodiorites, and basaltic andesites, through dacites to rhyolites, indicating the presence of a magmatic arc. The southerly basement rocks are an accreted suite of diabase, pillow basalt, and radiolarian chert deposited at abyssal depths. Precollisional arc-related rocks, of Eocene to lower Miocene age, consist of 4000 m of pillow basalts and volcaniclastics, and biogenic calcareous and siliceous deep-water sediments. They consist of the Eocene-Oligocene Darien Formation, the Oligocene Porcona Formation and the lower-middle Miocene Clarita Formation. Postcollisional deposits are mostly coarse- to fine-grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest Miocene age. This 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central Chucunaque-Tuira Basin of the Darien. The sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12.8−7.1 Ma collision of the Panama arc with South America. The sediments are divided into the upper middle Miocene Tapaliza Formation, the lower upper Miocene Tuira and Membrillo Formations, the middle upper Miocene Yaviza Formation, and the middle to upper Miocene Chucunaque Formation. The precollisional open marine units of Late Cretaceous–middle Miocene age are separated from the overlying postcollisional sequence of middle to late Miocene age by a regional unconformity at 14.8−12.8 Ma. This unconformity marks the disappearance of radiolarians, the changeover of predominantly silica deposition from the Atlantic to the Pacific, the initiation of the uplift of the isthmus of Panama, and the onset of shallowing upward, coarser clastic deposition. This pattern is also recorded from the southern Limon Basin of Caribbean Costa Rica to the Atrato Basin of northwestern Colombia. By the middle late Miocene, neritic depths were widespread throughout the Darien region, and a regional unconformity suggests completion of the Central American arc collision with South America by 7.1 Ma. No Pliocene deposits are recorded from either the Darien or the Panama Canal Basin, and no sediments younger than 4.8 Ma have been identified in the Atrato Basin of Colombia, suggesting rapid uplift and extensive emergence of the Central American isthmus in the latest Miocene. Northward movement of the eastern segment of the Panama arc along a now quiescent Panama Canal Zone fault during Eocene-Oligocene time may have dislocated the pre-collision arc. Since collision, the portion west of this fault (Chorotega Block) has remained stable, without rotation; to the east, in the Darien region, compression has been accommodated through formation of a Panama microplate with convergent boundaries to the north (North Panama deformed belt) and south (South Panama deformed belt), and suturing with South America along the Atrato Valley. Deformation within the microplate has been accommodated in the Darien province by several major left-lateral strike-slip faults that were active until the early Pliocene, since when the plate has behaved rigidly.

Conodont biostratigraphy and faunal assemblages in radiolarian ribbon-banded cherts of the Burubaital Formation, West Balkhash Region, Kazakhstan

Biostratigraphical study of the early to mid-Ordovician conodont fauna from ribbon-banded radiolarian cherts of the middle Burubaital Formation in Central Kazakhstan reveals an almost complete succession of conodont biozones from the late Tremadocian to the early Darriwilian. During this interval, biosiliceous sediments were deposited in basinal environments, inhabited by lingulate brachiopods, sponges, pterobranchs and caryocaridids in conditions of high fertility and primary productivity of surface water. The community structure of taxonomically diverse conodont assemblages typifying open oceanic environments is not significantly different from that of epicratonic basins of the North Atlantic conodont province. The regional increase of oxygenated bottom waters at the base of the Oepikodus evae Biozone is possibly related to considerable changes in palaeo-oceanographical circulation patterns. The finds of three natural clusters of Prioniodus oepiki (McTavish) enable us to propose an emended diagnosis of this species.

Geochemical characteristics, 40Ar– 39Ar ages and original tectonic setting of the Band-e-Zeyarat/Dar Anar ophiolite, Makran accretionary prism, S.E. Iran

The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three 40Ar– 39Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K–Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic–Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (∼150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90–100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).

Late Miocene remagnetization within the internal sector of the Northern Apennines, Italy

Paleomagnetic and geologic evidence indicates that Upper Jurassic radiolarian cherts of both the Tuscan Cherts Formation (continental margin, Tuscan Units) and the Monte Alpe Cherts Formation (oceanic crust, Ligurian Units) were remagnetized during Miocene orogenesis of the Northern Apennines of Italy. Characteristic overprint magnetizations with reversed polarities have been found over a large area within the internal sector of the Northern Apennines, including eastern Liguria, Elba Island and the Thyrrenian margin, and west of the Middle Tuscan Ridge. The reversed-polarity overprint (average direction: D=177°, I=−52°, α 95=15°) was most likely acquired during Late Miocene uplift and denudation of the orogenic chain, and thermochemical remagnetization was a probable consequence of increased circulation of orogenic fluids. Similarly, mostly reversed-polarity directions of magnetization have been found by other workers in overlying post-orogenic Messinian sediments ( D=177°, I=−57°, α 95=3°), which show little counterclockwise (CCW) vertical-axis rotation with respect to stable Europe (−8±5°). The Monte Alpe Cherts sampled at sites in the external sector of the Northern Apennines, close to major tectonic features, have normal-polarity overprint directions with in situ W–SW declinations. Since the overlying post-orogenic Messinian sediments have not been substantially rotated about vertical axes, the evidence points to an earlier, pre-Late Miocene remagnetization in the external parts of the orogenic chain.

Stratigraphic architecture of the northern Oman continental margin - Mesozoic Hamrat Duru Group, Hawasina complex, Oman

ABSTRACT The Triassic to Late Cretaceous deep-marine sediments of the Hamrat Duru Group, Oman Mountains, represent a subunit of the Hawasina nappe-complex which was deposited in a deep marine basin. During the Late Cretaceous SSW-directed obduction of the Semail Ophiolite, the Hawasina complex was emplaced onto the autochthonous cover of the Arabian basement, while the original configuration of the basin was destroyed. New lithostratigraphic results and high-resolution radiolarian and conodont biostratigraphy lead to a revised stratigraphic scheme of the Hamrat Duru Group which conforms with the standard stratigraphical nomenclature. The Hamrat Duru Group is divided into six formations: (1) The Early Triassic (Olenekian) to Late Triassic (Upper Norian) Zulla Formation (Limestone and Shale Member, Sandstone and Shale Member, Radiolarian Chert Member and Halobia Limestone Member); (2) The Late Triassic (late Norian to Rhaetian) Al Ayn Formation; (3) The Early Jurassic (late Pliensbachian) to Middle Jurassic (early Callovian) Guwayza Formation (Tawi Sadh Member and Oolitic Limestone Member); (4) Middle Jurassic (Callovian) to Late Cretaceous (Cenomanian?) Sid’r Formation (Lower Member, Upper Member); (5) Late Cretaceous (Cenomanian? to Santonian?) Nayid Formation; and (6) Late Jurassic (early Callovian) to Early (Late?) Cretaceous Wahrah Formation. Most of the lithostratigraphic units (formations and members) show isochronous boundaries between the different outcrop areas. The stratigraphic architecture of the Hamrat Duru Group megasequence is controlled by alternating siliciclastic and carbonate sedimentation possibly related to the second-order sea-level variations. The sediments accumulated on the continental rise of the Arabian margin mostly by submarine sediment-gravity flows and hemipelagic to pelagic rainout. A close relationship of the evolution of the Arabian Platform and the adjoining slope and basinal environments is evident. Changes in carbonate supply, oceanographic circulation and/or variations in silica productivity resulted in two distinct phases of radiolarian sedimentation. The first phase corresponds to the Triassic late Anisian-early Norian time interval; the second started in the Early Jurassic late Pliensbachian and lasted, with some interruptions, up to the Late Cretaceous Coniacian. The litho- and biostratigraphic similarities between the Mesozoic Hamrat Duru Basin of the northern/central Oman Mountains and the Mesozoic Batain Basin of northeastern Oman are seen as related to Neo-Tethys-wide palaeoceanographic changes and suggest a strong interdependence of the two basins with the evolution of the Arabian Platform.

JURASSIC PALEOGEOGRAPHY OF THE PIENINY AND OUTER CARPATHIAN BASINS

The Jurassic history of the Pieniny/Outer Carpathian basins reflects the evolution of the Circum-Tethyan area, especially its Alpine Tethys part. The Alpine Tethys that is Ligurian, Penninic Oceans and Pieniny/Magura Basin constitute the extension of the Central Atlantic system. The synrift stage lasted in the Pieniny/Magura Basin from late Early Jurassic to Tithonian (the Magura Unit constitutes the southernmost part of the Outer Flysch Carpathians). The Pieniny rift opened during Pliensbachian – Aalenian. The central Atlantic and Alpine Tethys went into a drifting stage during the Middle Jurassic. The Late Jurassic (Oxfordian-Kimmeridgian) history of the Pieniny/Magura Basin reflects strongest facial differentiation within sedimentary basin where mixed siliceous-carbonate sedimentation took place. Greatest deepening effect is indicated by widespread Oxfordian radiolarites, which occur in the all basinal successions, whereas the shallowest zone is completely devoid of siliceous intercalations at that time (sedimentation from Ammonitico Rosso facies up to coral reef limestone). The southern part of the North European Platform, north from the Pieniny/Magura realm, started to be rifted during Late Jurassic time and Silesian Basin in the Outer Western Carpathians and Sinaia Basin in the Eastern Carpathians, with black, mainly redeposited marls have been created. The outer sub-basins were differentiated during the latest (Hauterivian-Barremian) phase of basinal development. The connection of Silesian Basin with Sinaia and Southern Carpathian Severin areas suggests the NW-SE direction of the basinal axis while the orientation of the Pieniny Klippen Belt/Magura Basin was SW-NE so, two Outer Carpathian perpendicular directions are possible within the basins. Major reorganization happened during the Tithonian-Berriasian time. It was reflected by both paleoceanographical and paleoclimatical changes. The Neo-Cimmerian tectonic events as well as main phase of the Outer Carpathian basins opening is connected with this reorganization.

JURASSIC PALEONTOLOGICAL HERITAGE OF MURCIA (BETIC CORDILLERA, SOUTH-EASTERN SPAIN)

Jurassic rocks of the External and Internal Zones of the Betic Cordillera are widespread in the province of Murcia. Four areas are considered of special interest for stratigraphical and paleontological analysis: a) Sierra Quipar and b) Sierras Lugar-Corque (External Subbetic), c) Sierra Ricote (Median Subbetic) and d) Sierra Espuna (Malaguide Complex). The first two contain Jurassic sections including Sinemurian-Tithonian deposits, and major stratigraphic discontinuities, containing significant cephalopod concentrations of taphonomic and taxonomic interest, occuring in the Lower-Upper Pliensbachian, Lower/Middle Jurassic and Middle/Upper Jurassic boundaries. These areas are also relevant for biostratigraphical analysis of the Middle-Upper Jurassic interval. In the Sierra de Ricote, the Mahoma section is of especial interest for the study of Lias/Dogger transition. Casa Chimeneas section constitutes the best Subbetic site for the analysis of the Lower/Upper Bajocian boundary. In the La Bermeja-Casas de Vite area, the Bajocian-Tithonian interval is well-represented, including a parastratotype of the Radiolarite Jarropa Formation. Finally, the Malvariche section in Sierra Espuna represents the best Jurassic succession of Internal Zones of the Betic Cordillera and could be considered as a reference section for this Betic Domain. In this paper a heritage evaluation has been carried out for these classical jurassic sections with the object of protecting these sites according to the legal framework prevailing in the province of Murcia.

TRANSITION FROM CARBONATE PLATFORM TO PELAGIC DEPOSITION (MID JURASSIC- LATE CRETACEOUS), VOURINOS MASSIF, NORTHERN GREECE

A Jurassic- Cretaceous carbonate succession crops out along the Zyghosti Rema, Kozani (Northern Greece). The substratum consists of the ophiolitic succession of the Vourinos Massif (Pelagonian Domain): serpentinites tectonically overlain by basalts, with thin lenses of radiolarian cherts of middle Bathonian age. The contact with the overlying Jurassic limestones is tectonic. Eight informal units have been distinguished within the Mesozoic limestones, from the base upwards. (A) bioclastic, intraclastic and oolitic packstone (Callovian- Oxfordian). (B) bioclastic packstone and coral boundstone (Oxfordian ). (C) bioclastic and oncoidal wackestone with Clypeina jurassica (Oxfordian- Upper Kimmeridgian). (D) (Upper Kimmeridgian- Portlandian): oncoidal packstone and rudstone (facies D1); intraclastic and bioclastic grainstone and packstone (facies D2); neptunian dykes with intraclastic and bioclastic wackestone and packstone filling (facies D3); neptunian dykes with Fe-Mn rich laterite filling and with pink silty filling of early Late Cretaceous age. An unconformity surface, due to emersion and erosion of the platform during the latest Jurassic- Early Cretaceous, is overlain by (E) intraclastic, bioclastic packstone and grainstone (Cenomanian). (F) massive body of debrites with coral, echinoderm, algae and rudist large clasts (facies F1) (Cenomanian); turbiditic beds of bioclastic, intraclastic and lithoclastic rudstone and grainstone (facies F2). (G) thin bedded bioclastic mudstone and wackestone with planktonic foraminifers and radiolarians, alternating with turbiditic beds of bioclastic, intraclastic packstone and rudstone and with conglomeratic levels and slumped beds of the previous turbidites (upper Santonian- lower Campanian). (H): bioclastic packstone with planktonic foraminifers (facies H1) (lower Campanian - ?Maastrichtian); amalgamated turbiditic beds of bioclastic wackestone and packstone with planktonic foraminifers (facies H2); turbiditic beds of bioclastic packstone and rudstone (facies H3). These features allow to recognise the following sequence of events: 1) development of a carbonate platform in the Middle and Late Jurassic; 2) its overthrusting onto the ophiolites and its emersion starting from latest Jurassic time, with erosion and deposition of laterites; 3) marine transgression on the Jurassic platform and on the ophiolites during the early Late Cretaceous, and 4) extensional tectonism and platform demise starting in the Cenomanian, with sedimentation of gravity flows and turbidity currents deposits from the Cenomanian to the Campanian- ?Maastrichtian.

The depth of pelagic deposits in the Tethyan Jurassic and the use of corals: an example from the Apennines

Assessing the palaeobathymery of pelagic deposits is rather speculative, as proof through lithology or fossils significant for depth estimates is sparse. This is unfortunate as the bathymetric history of pelagic successions allows to conceive the evolution of continental margins and oceanic basins. Discoveries in coral biology bring an unexpected impact on basin analysis. Evidence strongly suggests that pennular corals, fossil and modern, constitute a zooxanthellate group with an outstanding specialization in colonizing deeper parts of the marine photic zone. This adaptation includes light amplification by autofluorescent pigmented cells, and particular feeding, witnessed by peculiar gastric ducts and skeletal features. Such corals occur in the Umbria–Marche and Sabina Apennines on top of Late Jurassic submarine highs and at basin margins. Values of palaeodepth relative to pelagic deposits are provided by corals and other environmental data. Because depth reconstruction involves classical Tethyan facies, such as Ammonitico Rosso, Aptychus limestone and radiolarian cherts, we must note that these results do not meet with actualistic models relying on carbonate dissolution for estimating depth. Deposits viewed as bathyal to abyssal could also have accumulated within, or just below, the photic zone. Thus, a new insight opens on Mesozoic bathymetries, regarding vast areas (Middle East to Caribbean) and on subjects ranging from platform drowning to regional extension styles.