Marginal Facies Research Articles

Layering and Precious Metals Mineralization in the ;Rincón del Tigre Complex, Eastern Bolivia

Geologic investigations supporting recent, technically successful, platinum exploration of the Rincon del Tigre Complex—a ca. 4.6-km-thick layered sill—have provided significant new understandings of the layering, development, and precious metals mineralization of one of South America’s largest mafic-ultramafic intrusions. Three macrocyclic units are recognized—from the base up, Basal unit, Unit 1, and Unit 2—each most likely the product of a major replenishment with mafic magma of one compositional type. The ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite → granular harzburgite (→ olivine bronzitite) each a few meters thick, with occasional development of bronzitite at the top. Mafic rocks, absent from Unit 1 and present only locally in the Basal unit, form a well-developed norite → gabbro → magnetite gabbro sequence in the upper half of Unit 2. Associated with the base of the magnetite gabbro is the Precious Metals zone, a persistent zone of low-grade (subeconomic) sulfide and precious metals mineralization, 80 to 185 m thick. The Precious Metals zone comprises an upper Cu sulfide-rich portion, its base (the sulfide phase boundary) about 23 to 70 m above the magnetite phase boundary, and a lower precious metals-rich portion, the lower part straddling the magnetite phase boundary. The individual precious metals are concentrated in separate, sulfide-poor subzones, each up to many tens of meters thick, in the stratigraphic order (Rh →)Pt → Pd → Au, with peak offsets at the same scale. Modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic (precious metals) layering of the sulfides in the Precious Metals zone, all at three different superimposed scales from about 3 m to several tens of meters. A possible interpretive framework for the cryptic layering (and perhaps for much of the other layering, including the meter-scale layering of the ultramafic rocks) is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and, in the magnetite gabbro, by repeated convective overturns caused by the onset of magnetite precipitation. The characteristics of the Precious Metals zone fit an orthomagmatic model. The composition, stratigraphic relations, and modal correlation (with magnetite) of the sulfides all imply that S saturation was induced by loss of FeO to magnetite precipitation. This took place after prolonged closed-system fractionation (following the last major magma injection at the base of Unit 2) during which crystallization of olivine, pyroxene, plagioclase, and (finally) magnetite precipitation led to overall S, Cu, and precious metals enrichment, and Ni and (finally) Fe depletion of the magma. The development of the mineralization and its complex layering was controlled by sulfide segregation, Rayleigh fractionation, the different but generally very high partition coefficients (Dsilicate/sulfide) for each precious metal, and repeated convective overturns at three different superimposed scales. The decoupled metals distribution is analogous and similar in origin to that of the Main Sulfide zone of the Great Dyke (Zimbabwe) and of the Main Sulfide layer (offset portion) of the Munni Munni Complex (Western Australia), but the stratigraphic order of metal enrichment peaks is different (Pt → Pd → Au vs. Pd → Pt → Au). This suggests a reversal in the relative magnitudes of the partition coefficients DPt and DPd that was possibly brought about by changes in the temperature and O, S, and Fe activities of the magmas between the middle and late stages of crystallization. Constancy of D values during fractionation of mafic magma should not be assumed. The Precious Metals zone is one of several Skaergaard-type strata-bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions, and the origin of the Skaergaard mineralized zone itself—including the potentially economic Platinova reefs—can be explained in terms of the Precious Metals zone model presented here. The very low grade, great thickness, and wide separation of individual metal subzones in the Precious Metals zone are similar to those of the subeconomic axial facies of the Main Sulfide zone in the wider parts of the Great Dyke (cf. the economic, more compact, marginal facies). On the basis of the lateral and vertical thermal model of the Great Dyke magma chamber, the subeconomic status of the Precious Metals zone may be in large part a function of the specific fluid dynamics of the Rincon del Tigre magma chamber.

Architecture of the Middle Devonian Kvamshesten Group, western Norway: sedimentary response to deformation above a ramp-flat extensional fault

Abstract The Mid-Devonian Kvamshesten basin in western Norway formed during late- to post-Caledonian extensional and strike-slip tectonics. The basin is entirely continental in origin, and was probably a closed basin through most of its history. It was filled by alluvialfluvial deposits belonging to two main terminal fan systems, one sourced in the hanging wall, one sourced in the footwall of the basin-controlling fault(s). Footwall-directed migration of the hanging-wall-sourced fluvial system probably took place during phases of relatively rapid, fault-controlled subsidence. This gave rise to retrogradational stacking of marginal conglomerates and to the footwall-directed migration of the main depocentre(s). During periods of increased sediment influx from footwall catchments and/or decreased subsidence rates, coarse, footwall-sourced material prograded far into the basin. The intercalation of material derived from footwall- and hanging-wall source areas, respectively, gave rise to a pronounced, kilometre-scale rhythmicity that is inferred to reflect fault-block rotation and footwall erosion. The basin fill also displays a pronounced rhythmic organization on the 80–150 m scale. The Kvamshesten basin was deformed during deposition as a result of its position in the hanging wall of a ramp-flat extensional fault and because of extension-parallel shortening. Thus, the pattern of subsidence was non-uniform and the sediment preservation potential varied across the basin. Deposition and preservation of floodbasin fine sediments was enhanced in areas close to the breakaway of the basin-bounding fault and in the hangingwall syncline that developed above the ramp. The crest of the rollover anticline was mainly occupied by high-energy braided fluvial systems represented by amalgamated sandstone complexes with abundant channel scour surfaces. The migration of the rollover anticline towards the breakaway of the principal basin-bounding fault resulted in the footwall-directed migration of facies belts upwards in the stratigraphy. In our interpretation, a kilometresthick, footwall-skewed amalgamated sandstone complex was constructed that marks the trace of the migrating rollover anticline through the basin stratigraphy. On the basis of stacking patterns of marginal facies, large-scale grain-size variations, the geometry of stratigraphic units and the overall lithofacies distribution, we present a sequence stratigraphic model for the basin fill. Within individual systems tracts, lithofacies distribution as well as inferred A/S ratios vary laterally because of the effects of differential subsidence and multidirectional sediment transport. Inferences of A/S ratio from vertical sections in one particular part of the basin thus cannot be used to characterize a systems tract as such. Grain-size turnarounds may, however, be used in the definition of systems tracts and sequences if the above relationships are well constrained and if turnarounds occur in stratigraphic units that are easy to correlate across the basin.

Late Oligocene lacustrine deposition of the Sodmin formation, Abu Hammad Basin, Red Sea, Egypt: sedimentology and factors controlling palustrine carbonates

The Late Oligocene lacustrine sequence of the Sodmin Formation is exposed in the Abu Hammad Basin, 27 km west of Hamrawein. The Sodmin Formation comprises two main lithostratigraphical units that are interpreted to represent the infilling of a closed lake located within a dextral shear setting that developed in association with the earliest stage of rifting of the Red Sea. The lower unit consists of grey lime-mudstone, marl and micritic limestone intercalated towards the top with fine siliciclastics, which reflect two stages of palagolake evolution. Initially, the lake was relatively deep with steep margins, experienced occasional periodic emergence, and had lacustrine carbonates deposited. During the second stage, the lake was generally shallower, had gently sloping margins and had become a palustrine setting in which fluctuations of the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, and modification was very intense in the marginal facies. The upper unit includes dolostone and cherts, which are indicative of lake desiccation and evaporitic conditions. Evidence from geochemical, petrographical and mineralogical analyses clearly indicates a gradual shift from open, freshwater lake conditions during deposition of the lower unit to closed, saline and alkaline waters during deposition of the upper unit. Climate and topography were the main controls on vertical facies variations of the palustrine carbonates. Climate influenced episodic lake water level fluctuations, whereas topography of the basin, which changed through time in response to tectonic uplift of the lake margins, influenced water run-off as well as the frequency and length of subaerial exposure.

Role of unconformity-bounded units in the stratigraphy of the continental record: a case study from the Late Miocene of the western Pannonian Basin, Hungary

Abstract We present an up-to-date stratigraphic framework for the Late Miocene (post-rift) non-marine strata of the western Pannonian Basin, based on unconformity-bounded units as they are derived from seismic interpretation. The data set used for this study consisted of some 1700 km of conventional, multi-channel reflection seismic profiles across western Hungary integrated by 190 km of high-resolution, single-channel seismic profiles acquired on Lake Balaton in June of 1993. Seismic stratigraphic analysis has been constrained by selected geological mapping, well-logs and borehole data. A magnetostratigraphic record was also available from a corehole in the study area, together with recent K/Ar dating of basaltic rocks from the Balaton highland. Five third-order (with 10 6 year periodicities) stratigraphic sequences have been recognized at regional scale in the Late Miocene succession of the western Pannonian Basin. We have designated these sequences, from bottom to top, as Sarmatian-1 (SAR-1) and Pannonian-1 (PAN-1) to Pannonian-4 (PAN-4). Reliable time constraints were only available for the two maximum flooding surfaces of sequences PAN-2 and PAN-3, namely mfs-2 (9.0 Ma) and mfs-3 (7.4 Ma), and the boundary of sequence PAN-2 (PAN-2 SB) which is approximately dated at 8.7 Ma. PAN-2 sequence boundary is associated with evidence of relative water-level drop in the Pannonian Lake and significant exposure of lake margins that is widely recorded in the so-called ‘marginal facies’ of western Hungary. The higher rank unit bounded by PAN-1 SB and PAN-4 SB includes most of the Pannonian s.l. succession of the central Paratethys and approximately correlates with the Tortonian-Messinian of the standard chronostratigraphy. Seemingly, no major palaeoenvironmental impact was perceptible in the western Pannonian Basin during the Messinian salinity crisis of the Mediterranean. However, a significant change in the regional stratigraphic patterns may be observed since earliest Pliocene (after PAN-4 SB), possibly associated with the very beginning of a large-scale tectonic inversion within the intra-Carpathian area. The case of Late Miocene non-marine strata of Pannonian Basin is a textbook example of how single categories of stratigraphic units do not fit (sometimes do not even approximate) chronostratigraphic correlation. The use of unconformity-bounded units offers new insights into the complex and long debated problem of stratigraphic correlation between Late Neogene deposits of the Pannonian Basin and ‘similar’ non-marine strata of the Central Paratethys realm. Our study shows that the so-called ‘Pontian facies’ of western Hungary correspond to an unconformity-bounded unit which is older than the Pontian s. s. facies of the stratotype area (Black Sea basin). Accordingly, we suggest that different stages may be used to discriminate between such similar-in-facies but different-in-age strata. We hence recommend the introduction of a new chronostratigraphic unit (‘Danubian’ or ‘Transdanubian’) in the Late Miocene series of Central Paratethys and a three-fold subdivision of the Pannonian (s.l.) strata into Early Pannonian (Pannonian s.s. ), ‘Middle Pannonian’ (‘Danubian’ or ‘Transdanubian’) and Late Pannonian (Pontian s.s. ) stages.

Impact of salt on the structure of the Central North Sea hydrocarbon fairways

The Central North Sea lies within the North Permian evaporite Basin, which was largely flexural but with locally significant basement faults. While the main evaporite basin fill is halite, seismically reflective basin margin facies delineate the salt basin shape. The marginal Zechstein strata record Permian salt tectonics including listric faulting, slumping of kilometre-scale carbonate rafts and halite diapirism, all related to localized pre-Triassic gravity-driven collapse of the Zechstein basin margin. Triassic thickness in the Central North Sea (CNS) was directly influenced by salt, the most significant phase of salt tectonics in the history of the CNS in terms of volume of salt redistributed and dissolved. Lower–Middle Triassic ‘minibasins’ subsided into the salt over much of the CNS. Different initiating mechanisms operated around the basin–differential loading was important near sediment input points, and thin-skinned extension on the platforms balanced a pulse of basement extension in the Central Graben. Minibasin subsidence was accommodated by salt wall growth; salt was eroded during periods of base-level fall and redeposited locally and in the Southern North Sea. The minibasins touched down on basement progressively from the basin margins towards the Central Graben, the timing being a function of salt thickness. The diachronous halting of accommodation space generation combined with the effects of erosion led to a variety of field relationships between the Middle–Upper Triassic Skagerrak sands and the underlying salt–minibasin system. Regionally, the Skagerrak sands are thickest near input points and in the Central Graben, where accommodation space was supplemented by basement faulting. The platforms were subaerially exposed and eroded for much of the period between the late Triassic and late Jurassic. On these platforms, the distribution of the relatively thin Late Jurassic section was controlled by a combination of regional tilt and pre-existing topography created by differential erosion (including dissolution) of the salt walls versus grounded minibasins. The late Jurassic Fulmar shallow marine sands are thickest on the differentially eroded salt highs, which constituted several hundred foot-deep palaeovalleys. Rifting occurred in the Central Graben where tilts of detachments on top of major basement fault blocks caused thin-skinned extension in the overlying strata, either for the first time, or reactivating Triassic minibasin systems. The Fulmar sands are thickest where basement-controlled accommodation space was accentuated by detached fault systems (e.g. fallen diapirs) near sediment input points in the Central Graben. The Early Cretaceous was a time of tectonic stabilization following Late Jurassic basement rifting, although the perched detachment systems in the major basement graben continued to evolve. Accommodation space for Upper Jurassic and Lower Cretaceous turbidites was influenced by up-dip salt high collapse and down-dip ‘rim syncline’ growth. By the end of the Early Cretaceous these systems had stabilized and the surrounding basement highs were sufficiently blanketed to shut off salt dissolution. Chalk isopach variations pick out subtle regional tilts and local basement fault reactivations. Major flexural subsidence in the Tertiary tilted the basin again and precipitated local cover slip, e.g. extensional faults balanced by down-dip folds on the West Central Shelf and diapir rejuvenation on the graben margin. A variety of inversion structures are present in the CNS and are dominantly Cretaceous and Tertiary in age. Several styles of inversion structure occur including thin-skinned and thick-skinned compression, diapir rejuvenation, compaction and dissolution-related features. Salt governed the geometry of many of these structures and in turn the geometry of the Tertiary sand fairways.

Upper Permian coral reef and colonial rugose corals in northwest Hunan, South China

The roles of Permian colonial corals in forming organic reefs have not been adequately assessed, although they are common fossils in the Permian strata. It is now known that colonial corals were important contributors to reef framework during the middle and late Permian such as those in South China, northeast Japan, Oman and Thailand. A coral reef occurs in Kanjia-ping, Cili County, Hunan, South China. It is formed by erect and unscathed colonies ofWaagenophyllum growing on top of one anotherin situ to form a baffle and framework. Paleontological data of the Cili coral reef indicates a middle to late Changhsing age (Late Permian), corresponding to thePalaeofusulina zone. The coral reef exposure extends along the inner platform margin striking in E-S direction for nearly 4 km laterally and generally 35 to 57 m thick. The Cili coral reef exhibits a lateral differentiation into three main reef facies; reef core facies, fore-reef facies, and marginal slope facies. The major reef-core facies is well exposed in Shenxian-wan and Guanyin-an sections where it rests on the marginal slope facies. Colonial corals are dispersed and preserved in non-living position easward. Sponges become major stabilizing organisms in the eastern part of Changhsing limestone outcrop in Kanjia-ping, but no read sponge reefs were formed. Coral reefs at Cili County in Human are different distinctly from calcisponge reefs in South China in their palaeogeography, lithofacies development, organic constitutuents, palaeoecology and diagenesis. The Cili coral reef also shows differences in age, depositional facies association, reef organisms and diagenesis from coral reefs in South Kitakami of Japan, Khorat Plateau of Thailand, and Saih Hatat of Oman. Although some sponge reefs and mounds can reach up to the unconformable Permian/Triassic boundary, coral reef at Kanjia-ping, Cili County, is the latest Permian reef known. This reef appears to had been formed in a palaeoenvironment that is different from that of the sponge reefs and provides an example of new and unique Permian reef type in South China, and could help us to: 1) understand the significance of colonial corals in Permian carbonate buildups; 2) evaluate the importance of coral community evolution prior to the collapse of reef ecosystems at the Permian/Triassic boundary; 3) better understand the effects of the biotic extinction events in Palaeotethys realm; 4) look for environmental factors that may have controlled reefs through time and space, and 5) provide valuable data for the study of Permian palaeoclimate and global evolutionary changes of Permian reefs and reef community.

Trace fossil analysis of lacustrine facies and basins

Abstract Two ichnofacies are typical of lacustrine depositional systems. The Scoyenia ichnofacies characterizes transitional terrestrial/nonmarine aquatic substrates, periodically inundated or desiccated, and therefore is commonly present in lake margin facies. The Mermia ichnofacies is associated with well oxygenated, permanent subaqueous, fine-grained substrates of hydrologically open, perennial lakes. Bathymetric zonations within the Mermia ichnofacies are complicated by the wide variability of lacustrine systems. Detected proximal–distal trends are useful within particular lake basins, but commonly difficult to extrapolate to other lakes. Other potential ichnofacies include the typically marine Skolithos ichnofacies for high-energy zones of lakes and substrate-controlled, still unnamed ichnofacies, associated to lake margin deposits. Trace fossils are useful for sedimentologic analysis of event beds. Lacustrine turbidites are characterized by low-diversity suites, reflecting colonization by opportunistic organisms after the turbidite event. Underflow current beds record animal activity contemporaneous with nearly continuous sedimentation. Ichnologic studies may also help to distinguish between marine and lacustrine turbidites. Deep-marine turbidites host the Nereites ichnofacies that consists of high diversity of ornate grazing traces and graphoglyptids, recording highly specialized feeding strategies developed to solve the problem of the scarcity of food in the deep sea. Deep lacustrine environments contain the Mermia ichnofacies, which is dominated by unspecialized grazing and feeding traces probably related to the abundance and accessibility of food in lacustrine systems. The lower diversity of lacustrine ichnofaunas in comparison with deep-sea assemblages more likely reflects lower species diversity as a consequence of less stable conditions. Increase of depth and extent of bioturbation through geologic time produced a clear signature in the ichnofabric record of lacustrine facies. Paleozoic lacustrine ichnofaunas are typically dominated by surface trails with little associated bioturbation. During the Mesozoic, bioturbation depth was higher in lake margin facies than in fully lacustrine deposits. While significant degrees of bioturbation were attained in lake margin facies during the Triassic, major biogenic disruption of primary bedding in subaqueous lacustrine deposits did not occur until the Cretaceous.

Non-hydrocarbon components of Carboniferous-sourced gas in the Southern Permian Basin, northwest Europe

Natural gas sourced from Carboniferous coal-bearing strata is produced from Carboniferous, Permian and Triassic reservoirs in the Southern Permian Basin in northwest Europe. The composition of this gas has been reviewed. In all reservoirs a distinct area rich in non-hydrocarbons, principally nitrogen, coincides with the basin depocentre in northern Germany. Furthermore, the average nitrogen concentration increases upwards in the stratigraphy. Carbon dioxide is a minor component which is geographically restricted and occurs mainly in Carboniferous and Zechstein reservoirs. It is generated indigenously in these reservoirs and does not migrate into other reservoirs. Hydrogen sulphide occurs only significantly in the Zechstein and is areally restricted to the basin margin facies of this reservoir. It is formed by thermochemical sulphate reduction. The maturity of the source rock, timing of the trap formation relative to the gas source maturation and reservoir lithology appear to be the dominant controls on the non-hydrocarbon gas composition in the Southern Permian Basin. For very high nitrogen gases (50-100 vol. %), an admixture of inorganic nitrogen to the thermogenic gas from organic sources cannot be excluded.

Evidence for an enriched mantle source for jøtunite (orthopyroxene monzodiorite) associated with the St. Urbain anorthosite, Quebec

Mafic orthopyroxene monzodiorite (jøtunite) lithologies are exposed in the St. Urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks. High concentrations of Ti, Fe, P, K, Ba, Nb, La, Ce, Zn, Ga, Zr and Y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide. Characteristically low concentrations of Ni and Cr, in conjuction with low Mg numbers, have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle-derived magma. In an attempt to distinguish between these competing models, we note that jøtunite display many features that bear a strong resemblance to continental tholeiitic flood basalts, including chemical signatures on normalized multi-element (‘spider’) diagrams. Ratios of incompatible trace elements and patterns on rare earth and ‘spider’ diagrams collectively indicate that the jøtunite rocks were derived from an enriched, rather than depleted, mantle source. Enrichment may have occured by subduction-derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace-element compositions are decoupled. Small amounts of partial melting of mafic granulite has been advanced as an alternative model; we show, however, that the experimental data on which this model is built are not applicable. Our preferred model begins with partial melting of a trace-element enriched mantle source that fractionates olivine at high to moderate pressures. Increasing concentrations of P (and Ti) eventually caused a contraction of the olivine stability field in favor of orthopyroxene. Fractional crystallization may yield the series of rocks from anorthosite, leuconorite, oxide-apatite gabbronorite, to jøtunite. Mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling, which are features common to many models for the genesis of anorthosite and related rocks.

Dasycladales from the Upper Maastrichtian of Salento Peninsula (Puglia, southern Italy)

A rich dasycladalean assemblage, mainly consisting of new taxa, has been discovered in upper Maastrichtian coarse bioclastic limestones of shelf margin facies cropping out along the southeastern coast of Salento peninsula (Puglia, Southern Italy). It consists of 8 species grouped into 6 genera:Cymopolia decastroi Parente 1994;C. barattoloi Parente, 1994;Zittelina fluegeli n.sp;Jodotella koradae (Dieni, Massari & Radoicic, 1983) nov. comb.;Barattoloporella salentina n. gen. n. sp.;Morelletpora dienii n. sp.;Neomeris spp. (two different species). Zittelina fluegeli n. sp. is characterized by an ovoid thallus with calcification made by a calcareous wall enveloping only the proximal part of the branches (except in basal whorls) and by closely packed, and partly coalescent, calcified ampullae arranged all around the median and distal portion of branches. The transferral ofNeomeris (Larvaria) koradae Dieni, Massari & Radoicic, 1983 into the genusJodotella, with the new combinationJodotella koradae, is proposed on the basis of new observations on the number and arrangement of fertile ampullae. The new genusBarattoloporella, type-speciesBarattoloporella salentina n. gen. n. sp., is erected for dasycladalean algae characterized by a segmented thallus. Each segment consists of: a) basal and apical sterile whorls made by primary branches only, b) central fertile whorls made by primary branches bearing in terminal position one fertile ampulla and one or two secondary branches. morelletpora dienii n. sp. is characterized by a segmented thallus with barrel shaped to pear shaped repetitive elements, consisting of simple whorls of first order branches only. Shape of the branches varies from regularly phloiophorous to more or less differentiated in a stalk and a swollen portion, with or without a subterminal constriction. This is by far the most diverse dasycladalean assemblage ever found in the Maastrichtian. Its diversity supports the conclusion that, within the Late Cretaceous, the Maastrichtian represents a maximum in dasycladalean diversity. Its taxonomic composition strenghtens the hypothesis that dasycladaleans were hardly affected by K/T mass extinction.

Climatically driven sedimentary cycles in the Late Miocene sediments of the Pannonian Basin, Hungary

By detailed cycle analysis of marginal, mostly delta plain facies successions of Pannonian sediments the frequency of water-level changes of the Late Miocene Pannonian Lake, Hungary, can be identified. Cycle analysis is presented with the object of defining the orbital record preserved in the strata. We use the bed thickness as input data for our cycle analysis. Statistical analysis revealed a slow, upward increase of both the frequency and thickness of sand beds in the boreholes resulting from infilling of the lake by a basinward migration of the marginal facies. Cycles of different order were detected in the sediments. The higher-order cycles are very close to or are equivalent with the Milankovitch climatic cycles: ∼19 ka caused by precession, ∼50 ka caused by obliquity, and ∼400 ka is the longer period of eccentricity. The longer, >1 m.y., cycle is very poorly developed here. It is probably the analogue of the 3rd-order, global climatic cycle. Cycle analysis suggests, that one of the main controlling factors of the water-level changes of the Pannonian Lake was high-frequency climatic oscillations. The long-term climatic changes only had a faint impact on the sediments. The record of the 3rd-order water-level changes is very subtle. Our results show that the water-level changes followed the rhythm of that of the eustatic changes, but being asynchronous to them, i.e., at high sea-level stage the Pannonian lake-level was low, and vice versa.

A late Devonian (Frasnian) coral-bafflestone reef at Houshan in Guilin, South China

Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian, stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids, receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China with regard to the biotic composition. The reef is composed of fasciculate colonies ofSmithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The species ofSmithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control:Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereasSmithiphyllum occidentaleSorauf, 1972 andSmithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like coral colonies baffled sediments. Algae and stromatoporoids (mainlyStachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods.

Igneous and Metamorphic Enclaves in the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia: Petrographic, Geochemical and Nd-Sr Isotopic Evidence for Crustal Melting and Magma Mixing

The Deddick Granodiorite, a mafic S-type pluton in the Lachlan INTRODUCTION Fold Belt, contains abundant enclaves. Most are derived from Enclaves in granites and volcanic rocks can provide high-grade metasediments of pelitic–psammitic composition, and petrogenetic information not readily available from their migmatites are common. Among these, discrete fragments of host rocks (Didier, 1973; Didier & Barbarin, 1991). For melanosomes rich in cordierite and garnet are restites from partial example, metasedimentary enclaves can place constraints melting. However, compositional and Nd–Sr isotopic data indicate on the age, composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust (Fleming, host magma, but may have formed part of a spectrum of diverse 1991; Steele et al., 1991). Enclaves formed by acmagma source lithologies. Alternatively, they may be accidental cumulation of early phenocrysts (‘autoliths’) document xenoliths. Regardless of their origin, data for these enclaves suggest the early evolution of the host magma, whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three possible role of hybridization (Didier, 1987). types of microgranular enclaves with igneous textures are disEnclave suites in the relatively mafic S-type granitoids tinguished. The most common type are small and rounded mafic of SE Australia are often diverse, including abundant enclaves of tonalitic composition. They carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma, and some contain high-Mg pyroxene derived from enclaves (Phillips et al., 1981; Price, 1983; Chen et al., a mafic magma. Isotopic data form an array with eNd from –6 to 1989; Wyborn et al., 1991). The metasedimentary en–12, and Sr/Sr from 0·7130 to 0·7167 (host rock –10 and claves are lithologically diverse, including gneissic– 0·715, respectively). These enclaves formed as globules of hybrid migmatitic types and biotite-rich ‘surmicaceous’ types. mafic magma commingled with, and contaminated by, the more Apart from their potential as indicators of crustal structure felsic host magma. Other enclave types were derived from disrupted and composition, they are of interest as possible fragments syn-plutonic dykes having a distinct isotopic composition, and from of source material and/or refractory residue from which a cogenetic marginal facies of the host. peraluminous melts were generated and extracted. By

Constraints on the origin of zonation of the granite complexes in the Fichtelgebirge (Germany and Czech Republic): evidence from a gravity and geochemical study

Compositional zoning of granitic plutons is a common feature, but the spatial variation of zoning is rarely studied in detail. It is suggested that important additional information can emerge if the compositional zoning is regarded with relation to the root zones, i.e. with the granite shape at depth. A combined geochemical and gravity investigation was performed on the Hercynian Fichtelgebirge granites, which are composed of an older intrusive complex (OIC) and younger intrusive complex (YIC). The OIC and YIC are distinct from each other with respect to magma origin, differentiation pattern, shape of the granite complexes at depth and the zonation pattern in relation to the root zone. The OIC shows normal compositional zoning, whereas the YIC both reverse (YIC-1) and normal zoning (YIC-2). The general zonation pattern of the OIC and the YIC is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplacement. In addition to the general zonation pattern, local zonation in individual granite bodies of the YIC was caused by sidewall crystallisation producing a less-differentiated marginal facies (G2) and a more-differentiated core facies (G3 and G4). It is suggested that the complex zonation patterns of the Fichtelgebirge granite complexes reflect interactions between the rate of magma output which is partly governed by their rheological properties and the rate of deformation which provides the room for the magma to be emplaced.

Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy

The Tiberino Basin was formed during Plio-Pleistocene extensional tectonic activity in Central Italy. The Pliocene sediments that fill this structure consist predominately of fine-grained clastic lithofacies deposited in a lacustrine system; four facies associations have been distinguished. Facies association A was deposited in a deep-offshore lacustrine environment and consists of massive, laminated, bluish-gray marly clays that lack benthonic fauna but have preserved organic matter (leaves, wood). This facies association is the main lacustrine deposit, whereas the other three represent marginal facies. Facies association B is interpreted as a delta system deposit; it has Gilbert-type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata. Association C corresponds to a coastal environment, where two sub-environments are recognized: (1) a wave-dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross-stratification; and (2) clayey silts and lignites interpreted as a coastal wetland; a spectacular fossil forest is preserved in situ within this unit. Association D, formed on the distal part of a muddy alluvial fan, consists of sheet-flood clayey sandy silts, that alternate with incipient paleosols; rare, ribbon-channel fills are also present. Paleoenvironmental reconstruction indicates that the Tiberino Basin was occupied by a narrow, deep meromictic lake during the Pliocene. The size, shape, depth and sub-environment distribution of this lake were controlled by tectonic phenomena, such as the high subsidence rate, which caused the lake to be large and deep. Only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed.

Abstract: Sequence Stratigraphic Framework of Early Mississippian Mound-Bearing, Ramp Margin Facies, Fort Payne-Warsaw-Salem, Kentucky 

Abstract: Sequence Stratigraphic Framework of Early Mississippian Mound-Bearing, Ramp Margin Facies, Fort Payne-Warsaw-Salem, Kentucky 

Dinosaur tracks and other archosaur footprints from the Triassic of South Wales

Dinosaur tracks and other archosaur footprints from multiple stratigraphic levels in coastal exposures near Cardiff, and a site near Nottage, Porthcawl area, provide evidence of a diverse assemblage of trackmakers from the “marginal facies”; of the Mercia Mudstone Group (Late Triassic), that can be closely compared with other Pangaean ichnofaunas of the same age. The footprints, comprising at least 88 identified trackways from about 10 stratigraphie levels, can be assigned to the ichnogenera Grallator (Grallator), Grallator (cf. Anchisauripus), Chirotherium? or cf. Tetrasauropus, Pseudotetrasauropus? and Otozoum, most of which are new records for the Late Triassic of Great Britain. The ichnofauna indicates a relatively abundant and diverse archosaur‐dominated terrestrial vertebrate fauna in a semi‐arid setting subject to intermittent deposition by ephemeral streams. The numerically dominant track‐makers appear to have been small theropods with a lesser number of larger prosauropods and other archosaurs of...

Norian serpulid and microbial bioconstructions: Implication for the platform evolution in the Lombardy Basin (Southern Alps, Italy)

The development of peculiar margin facies and abundant talus breccias within the Dolomia Principale inner platform is commonly observed in the Lombardy Basin during the Norian. The organisms building these margins are mainly serpulids, benthic microbes, subordinate porostomata and other encrusting forms; typical margin organisms, as sponges or corals, are extremely rare or absent. The build-ups form narrow rims along the borders of tectonic-controlled intraplatform basins. Regional back-stepping and progradation of the margin facies on the talus breccias produced by the erosion of the reef is commonly observed in the uppermost Dolomia Principale depositional system. Widespread occurrence of serpulids and microbial margins in middle-late Norian times is indicative of stressed environmental conditions—fluctuation of salinity and temperature on the inner platform and in the intraplatform basins—controlled by palaeogeographic setting. Physical characteristics allowed the bloom of forms able to develop in a wide range of environmental conditions, such as serpulids.

Mixing between granitic and dioritic crystal mushes, Guernsey, Channel Islands, UK

The contact zone between the Cobo Granite and Bordeaux Diorite Complex of Guernsey (Channel Islands, UK) displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas. Initial interaction resulted in the formation of chilled mafic enclaves in granite magma. As thermal equilibrium was approached, some physical mixing took place to produce a heterogeneous “Marginal Facies”. Continued interaction resulted in incorporation of previously mixed magma into intruding magma. The early mixed material is locally preserved as enclaves, but more commonly underwent disaggregation promoted by its incompletely crystallised nature, the mineral components becoming distributed as xenocrysts and often as microenclaves, or glomeroxenocrysts, into surrounding magma. Further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres. These processes produced geochemical profiles which do not exhibit perfect mixing trends. The petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition, but also provide criteria by which such interaction may be recognised elsewhere. Many features, in particular mineral scale disequilibria and small scale modal heterogeneity, bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent. As such, it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events.

Electrical conductivity and Paleo‐Proterozoic foredeeps

Sedimentary facies associated with Paleo‐Proterozoic foredeeps are apparently highly conductive. The conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies. The shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt. Carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400°C. Although foredeep facies develop diachronously, the most conductive unit is usually found in the core of the fold and thrust belt, suggesting that deformation may be essential for the complete graphitization of the carbon. The foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in Laurentia, and possibly throughout the world. The euxinic facies represent highly specific, tectonic marker horizons that are readily delineated by using electromagnetic methods.