Early history of the Izu–Bonin – Mariana arc system: Evidence from Belau and the Palau Trench

Volcanic rocks of the Kyushu–Palau Ridge (KPR) from Deep Sea Drilling Project (DSDP) site 448 and from Belau comprise a low-to-medium-K arc tholeiitic series. Belau rocks include (probable) Mid-Eocene low-Ca type-3 boninite and pre-Early Oligocene–Early Miocene low-K arc tholeiitic basalt, basaltic andesite, andesite and dacite. Palau Trench samples include sparsely phyric high-Mg, -Cr and -Ni rocks which resemble the Belau boninite and Izu–Bonin – Mariana (IBM) system boninites. The high-Mg Palau Trench samples also resemble other primitive arc lavas (e.g. arc picrites). Their chemistry suggests an origin involving steep thermal gradients in multiply depleted mantle. Subduction of hot, young lithosphere under a young hot upper plate is postulated to explain this occurrence. The KPR is inferred to be the source of Eocene boninite and arc tholeiitic terranes presently in forearc regions of the IBM system. A model is presented here showing how many IBM boninites may have originated in a small area near Belau. These have migrated eastward by episodic back-arc opening accompanying eastward migration of arcs and trenches. Oldest known KPR rocks ( ca 47.5 Ma at DSDP site 296), and presumed KPR-derived exotic terranes of Guam ( ca 43.8 Ma), presage the postulated Eocene ( ca 42–43 Ma) change in Pacific plate motion invoked as the cause of subduction initiation at the KPR. The KPR has been rotated more than 40° clockwise since the Eocene, thus the age mismatch may indicate a different tectonic style, for example transtension or transpression, in earliest KPR history.     

Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.     

Catchment infiltration I: Distribution of variables

Environmental modelling usually requires spatially distributed inputs for model operation. We propose that such inputs are best obtained from field measured data. Geographic information systems (GIS) provide a logical framework to distribute measured inputs spatially, to manipulate ensuing data fields during analysis, and to display the results. This paper describes a study conducted on a 123 km² catchment in Pennsylvania. The purpose was to evaluate how spatial variability of macroporosity affects distribution of other infiltration-related parameters. We measured sorptivity, conductivity and macroporosity at specific points within a catchment, and interpolated their spatial distributions by kriging. The measurements were made with ring and disk infiltrometers, sampling locations were geo-referenced with a global positioning system (GPS), and data were analysed using geostatistical techniques in a GIS context. Field values ( hard data ) were supplemented by soft data derived from cumulative distribution functions (cdfs) and available soil maps. Results showed that, when spatial variability associated with macroporosity was removed, infiltration parameters became less variable. Observed correlation among measured parameters suggested a form of potential transfer functions. We conclude that infiltration can be modelled at either the farm or catchment scale if macroporosity and spatial variability of infiltration parameters are adequately defined, and we suggest approaches which can be used in a GIS context to attain that goal.     

Sediment diffusion during overbank flows

Distinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post-settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion. If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post-settlement period, the model may be used to estimate palaeohydraulic characteristics of post-settlement floods. Calculations indicate that 212 post-settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel.     

Petrology of the Marginal Border Series of the Skaergaard Intrusion

The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An₇₂) andolivine (Fo₈₄) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber.     

Surging of the southwestern part of the Laurentide Ice Sheet

The southwestern part of the Laurentidc Ice Sheet, in central North America, repeatedly surged during the last part of the Wisconsin Glaciation. Evidence includes the extreme lobation of the ice margin, the gentle slopes of lateral moraines and other marginal features, a radiocarbon chronology indicating extremely rapid marginal advance and retreat, and the abundance of supraglacial flow till. Rapid ice movement was caused by subglacial water and was probably limited to areas of slowly permeable substrate, which slowed the escape of the water.     

The prodelta environment of a fjord: suspended particle dynamics

The dynamics of suspended particles within a fjord's estuarine circulation are investigated and the results compared with larger non-enclosed prodelta environments. In the upper prodelta, the seaward-flowing river plume flows over the ambient marine water depositing much of the initial riverine suspended load. Sedimentation is dominated by coarse silt and fine-grained sand particles with coarseness determined by the tidal and fluvial stage. Particles less than 10 μn have similar settling velocities regardless of size because they settle in flocs: the settling velocity at a water depth of 5 m is 30 m day^-1 and increases with depth so that at 30 m the particles settle at 100 m day^-1. For larger particles, the downward settling velocity enhancement due to flocculation decreases with increasing grain size. Hydraulic sorting allows the preferential settling of feldspar and quartz over mica. Particle dynamics in the lower prodelta are dependent on the character of the freshwater wedge that thins seaward of the upper prodelta. The vertical flux of particles is controlled by biogeochemical interactions such as pelletization of fine particles and flocculation (which occurs within rather than below the surface layer in contrast to the upper prodelta). The pellets are produced by indiscriminate filter feeding zooplankton. Across the lower prodelta the suspensate character, recognized in the composition of both flocs and pellets, changes from a dominance of mineral grains to that of autochthonous organic matter. The interaction of bacteria with the suspended particles increases with depth and seaward distance. At depth, the mucoid filaments form stable interconnecting webs. Particle concentration in the surface layer decreases at a rate proportional to the negative one-half and three-halves power of the distance seaward over the upper and lower prodelta, respectively. This relationship is hypothesized as being universal for large marine deltas dominated by buoyancy flow dynamics, regardless of the levels of initial riverine particle concentration or their composition.     

Discrimination between depositional environments using grain-size analyses

Grain-size analyses were performed on 84 samples from six environments in the Burdekin River Delta, NE Queensland, Australia. The data were subjected to a stepwise discriminant function analysis and canonical analysis which showed that the grain-size characteristics of the sediments are sufficiently distinctive to distinguish the six different environments of deposition. The resulting ‘reference set’ was then tested by plotting the grain-size characteristics of 66 samples from different environments of the Mississippi River Delta, Louisiana and Galveston Island, Texas. Seventy-four per cent of the North American samples were correctly classified.     

Assimilation of Crustal Material by Basaltic Magma: Strontium Isotopic and Trace Element Data from the Edgecumbe Volcanic Field, SE Alaska

The Edgecumbe volcanics, which range from basalt through rhyodacite,have Sr contents between 125 and 370 p.p.m., Rb contents of1 to 70 p.p.m., Ba ranging from 50–550 p.p.m. and initial⁸⁷Sr/⁸⁶Sr ratios between 0.70291 and 0.70404. No simple correlationexists between these components and silica. The highest ⁸⁷Sr/⁸⁶Srvalues occur in a group of intermediate lavas (55–60 wt.per cent SiO₂) while the rhyodacites have initial ratios between0.7035 and 0.7038. With increasing silica, Sr increases to amaximum in the andesites and then steadily decreases; Ba andRb increase over the same compositional range. The highest ⁸⁷Sr/⁸⁶Srlavas have major and trace element concentrations which departfrom trends defined by most of the lavas. The variation in strontiumisotopic compositions suggests interaction between parentalbasaltic magma and crustal material. Attempts to model the assimilationprocess using fixed end-member assimilation and assimilation-fractionalcrystallization models have failed to produce the observed chemicaltrends. Because the parental basaltic liquid underwent littlefractionation, the variability in hybrid lavas is attributedto variation in contaminant composition. Initial melts werelow in CaO, A1₂O₃, MgO and Sr and enriched in SiO₂. K₂O, Na₂O,Rb and Ba. As melting progressed, melts became enriched in themore refractory components. Because hybrid strontium isotopiccomposition is a function of Sr concentration as well as isotopiccompositions, the Sr content of the assimilant strongly influencesresultant isotopic systematics. The development of the assimilantssuggests plagioclase was a residual phase during early melting.This model of crustal assimilation represents one end-memberin the spectrum of processes responsible for the generationof continental volcanic suites.     

Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co-precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non-ferroan calcite cements indicates that both are original precipitates that formed during pre-Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ¹⁸Os of megaquartz (mean =+27.0^0/00 SMOW; range =+ 24.8 to + 28.9^0/00) and calcite (mean =+ 28.0^0/00 SMOW; range =+ 27.3 to + 28.4^0/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre-Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre-Pennsylvanian opal-CT precursors, and therefore probably re-equilibrated during recrystallization in late or post-Mississippian time. We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ¹⁸Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground-water system, the seaward terminus of which moved southward during lowering of pre-Pennsylvanian sea level. The calcite cements and some of the opal-CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ¹⁸Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.