Provenance and distribution of tethyan pelagic and hemipelagic siliceous sediments,pindos mountains,Greece

The broad range of time over which ribbon bedded cherts were deposited does not extend into the present marine environment, and no ribbon cherts have been recovered from the sea floor by the Deep Sea Drilling Project. The depositional environment of bedded cherts is difficult to determine, but extra-silicic impurities in the rock may offer clues about the provenance of the non-biogenic component. To test the usefulness of relative abundances of the extra silicic components in extracting information on the depositional environment of the chert, I analyzed the major element chemistry of chert samples from a broad range of environments including ophiolite-associated chert from the Franciscan Formation of California, deep-sea chert and porcellanite from the northwest Pacific (DSDP Leg 32), shallow pelagic shelf chert nodules from the Chalk of Britain, continental marginal basin chert from the Monterey Formation of California, and continental marginal basin chert from the Pindos Zone of Greece. The ratios FeO/A1₂O₃, TiO₂/A1₂O₃ and A1/A1+Fe+Mn were considered in detail. The interpretative logic is simple but empirically supported by observations of these ratio values at different depositional environments in the Pacific: A1 is concentrated most highly in continental material while Fe and Mn are more concentrated in pelagic sediments. FeO/A1₂O₃ can be used to differentiate between ophiolite associated chert and chert associated only with other sediments. TiO₂/A1₂O₃ is not a useful indicator, possibly because of the equalizing effect of widespread eolian transport. The A1/A1+Fe+Mn ratio was measured in detail in one stratigraphic section in central continental Greece. This ratio varied with the type of sediment admixture, decreasing in value after the influx of ophiolite debris-bearing sediments, even when their presence was undetectable in hand sample or under petrographic microscope.To help clarify the paleogeography of the main study area, the Pindos Zone, and to identify sources and dispersal patterns of extra-basinal materials, isopach maps of sedimentary facies of the Pindos were constructed. Superimposed directly upon the series of imbricated thrust slices that comprise the Pindos Zone, the maps are at best compressed pictures of the Pindos Sea Floor. Persistent regional variation of facies thicknesses over time suggests the existence of several smaller depressions surrounded by submarine highs in the Pindos Basin.     

Carbon-13 NMR spectra of macerals separated from individual coals

The density gradient centrifugation technique has recently been applied to the separation of macerals from whole coals. Sufficient quantities have been separated to permit examination of pure exinite, vitrinite and inertinite fractions by combined cross polarization and magic angle sample spinning (CP/MASS) carbon-13 NMR techniques. The similarities and differences observed in the maceral groups of two coals are discussed. Diversity in precursors or in differential chemical maturation or a combination of both of these factors can be used to account for subtle spectral differences in the two coals and macerals studied even though the gross spectral features are very similar for a given maceral type. The data show that CP/MASS is a technique that can address such interesting geochemical questions.     

Geophysical observations bearing upon the origin of the newfoundland ridge

Multichannel reflection seismic profiles extending southward from the Grand Banks show gently dipping reflectors within “basement” features underlying the Newfoundland Ridge. These reflections appear to be from sedimentary strata, indicating that the Newfoundland Ridge is a remnant of a former sedimentary basin, rather than a ridge of oceanic crust as prescribed by plate tectonic models. Probably this feature is underlain, and to some extent surrounded by, continental crust.     

Iron-formation and glaciogenic rocks of the Rapitan Group,Northwest Territories,Canada

In northwestern Canada, iron-formation occurs as part of the Rapitan Group, a dominantly sedimentary succession of probable Late Precambrian age. The Rapitan Group contains abundant evidence of glaciogenic deposition. It includes massive mixtites which contain numerous faceted and striated clasts. Finely bedded and laminated sedimentary rocks of the Lower Rapitan contain many large isolated (ice-rafted?) intra- and extra-basinal clasts. The Lower and Middle Rapitan are interpreted as products of a glacial marine regime. The iron-formation is interbedded with thin mixtite beds and contains large exotic clasts which are probably indicative of the existence of floating ice at the time of deposition of at least part of the iron-formation. If the apparently low paleolatitudes are confirmed, then glacial marine interpretation of the Rapitan, and the probably correlative Toby Conglomerate of southern British Columbia, support the postulate of a very extensive Late Precambrian ice sheet in North America.Similar iron-formations of similar age are present in South America (Jacadigo Series), in South-West Africa (Damara Supergroup) and in South Australia (Yudnamutana Sub-Group). All of these iron-formations are associated with glaciogenic rocks. In addition to the iron-formations, dolostones, limestones and evaporites (?) are intimately associated with Late Precambrian mixtites, considered by many to be glaciogenic.Huronian (Early Proterozoic) and correlative sequences of North America, and rocks of similar age in South Africa also contain closely juxtaposed undoubted glaciogenic rocks, iron-formations, dolostones and aluminous quartzites. The dolostones and aluminous sedimentary rocks have been interpreted as having formed under warm climatic conditions, but might also be explained by invoking higher P_CO2 levels in the Early Proterozoic atmosphere. By analogy with the Huronian succession, preservation of “warm climate” indicators in mixtite-bearing Late Precambrian sequences does not preclude a glacial origin for the mixtites.     

Evidence for extensive diagenesis,Madagascar Basin,Deep Sea Drilling Site 245

Deep Sea Drilling Hole 245 (31°32′S, 52° 18′E) in the southwest Indian Ocean shows pronounced linear concentration-depth gradients in interstitial dissolved Ca, Mg and Sr. Electrical conductivity tests enable us to make the estimate of a constant diffusion coefficient with depth of about 2 × 10^?6 cm²/sec. The shapes of the concentration-depth gradients suggest that the major reaction sites in this hole are situated in the basal sediments and/or underlying basalts. It is proposed that observed interstitial water concentration changes in Ca and Mg are related to alteration of basaltic material, whereas those in Sr are due to calcium carbonate recrystallization processes. Support for the basaltic material alteration hypothesis comes from petrochemical and mineralogical data. Geochemical data also indicate that the high contents in Fe and Mn of the basal sediments can be related to low temperature alteration of basaltic glass and not necessarily to ‘hydrothermal’ activity.     

The petrogenesis of komatiites and related rocks as evidence for a layered upper mantle

Although the CaO/Al₂O₃ ratio of komatiites has been regarded as one of the distinguishing features of these rocks, a comparison of various komatiite and oceanic tholeiite analyses suggests that there is a continuum of ratios between the two. The extremely high MgO values of peridotitic komatiites suggest that they are the result of high degrees of partial melting of the mantle, leaving a harzburgitic residuum depleted in CaO and Al₂O₃, and hence preserving in the melt the original CaO/Al₂O₃ ratio of the parental material. Available chemical models of the mantle have CaO/Al₂O₃ ratios too low to explain the origin of komatiite by such a process. Shallow-level melting of a layered mantle in which clinopyroxene content decreases and garnet content increases with depth, may explain the chemistry of komatiites and related ultrabasic lavas.     

The magnetic effects of brecciation and shock in meteorites: I. The Ll-chondrites

The complex brecciation and shock history of amphoterite (LL-) chondrites is well reflected in their diverse natural magnetic remanence (NRM) behavior: Most LL-chondrites have a multicomponent, undemagnetizable NRM, analogous to that of lunar breccias. Only one meteorite among those studied, namely Dhurmsala (LL6), meets the criteria of NRM stability and directional coherence with progressive AF cleaning, indicative of a useful paleoremanence. Ancient field paleointensity determinations for Dhurmsala (LL6) of 0.03 and 0.1 Oe, agree well with our earlier estimates of 0.01 and 0.08 Oe for the LL6 Jelica and Vavilovka, respectively. In light of their petrographic structure, cooling rates, radiometric ages and shock indicators, it appears likely that the NRM may have been thermally imprinted, during cooling following shock-metamorphism. The closely similar saturation remanence (IRM_s) behavior for LL-chondrites, in contrast to the intragroup scatter in NRM characteristics, implies that - although formed by similar process from the same starting material, - the LL-chondrites have suffered widely different degrees of shock/metamorphic reheating.     

Using geostatistics to elucidate temporal change in the spatial variation of aeolian sediment transport

Little is known about the spatial and temporal scales of variation in aeolian processes. Studies that aim to investigate surface erodibility often sample aeolian sediment transport at the nodes of a regular grid of arbitrary size. Few aeolian transport investigations have the resources to obtain sufficient samples to produce reliable models for mapping the spatial variation of transport. This study reports the use of an innovative nested strategy for sampling multiple spatial scales simultaneously using 40 sediment samplers. Reliable models of the spatial variation in aeolian sediment transport were produced and used for ordinary punctual kriging and stochastic simulated annealing to produce maps for several wind erosion events over a 25 km² playa in western Queensland, Australia. The results support the existence of a highly dynamic wind erosion system that was responding to possibly cyclic variation in the availability of material and fluctuations in wind energy. The spatial scale of transport was considerably larger than the small scale expected of the factors controlling surface erodibility. Thus, it appears that transport cannot be used as a surrogate of erodibility at the scale of this investigation. Simulation maps of transport provided considerably more information than those from kriging about the variability in aeolian sediment transport and its possible controlling factors. The proposed optimal sampling strategy involves a nested approach using ca 50 samplers. Copyright © 2003 John Wiley & Sons, Ltd.     

A theoretical model for reverse water-level fluctuations induced by transient permeability in thrust fault zones

A numerical model was applied to simulate the poroelastic response to changes in fault permeability as a result of earthquakes. The ‘fault valve’ model describes faults as impermeable barriers for fluids except immediately after earthquakes, when fault zones are damaged and transient pathways for fluids are created. In this case the fault is viewed as a discharging well, draining fluids from the surrounding rock. The reverse water-level effect is characterized by the increase of water level in adjacent aquifers and aquitards, resulting from withdrawing fluids through a well. Theoretical calculations suggest that the reverse water-level effect exists also in earthquake cycling and is in the same order of magnitude as the co-seismic hydraulic head change. A significant rise of the hydraulic head (>1 m) occurs within the country rock from both sides of the fault. The rise of the water level takes months to years to occur, and perhaps that is why it cannot be easily distinguished from seasonal hydrologic changes observed in the field. The reverse water-level effect also propagates away from the fault at a rate of hundreds of meters per year, depending on the permeability of the country rock. In deep formations where the permeability is low, the propagation takes years. The magnitude of the reverse water-level effect is greater when the fault efficiently drains fluids, when it is highly permeable and slow to reseal.