Internal architecture and sedimentary evolution of coarse-grained, turbidite channel-levee complexes, Early Eocene Regência Canyon, Espírito Santo Basin, Brazil

Successions of Early Eocene coarse-grained turbidites up to 400 m thick fill fault-controlled canyons along the eastern Brazilian continental margin. They form part of a Late Albian to Early Eocene transgressive succession characterized by onlapping, deepening-upward sedimentation. In the Lagoa Parda oil field (Regência Canyon, Espírito Santo Basin) the turbidite facies consist mostly of unstratified conglomerate and sandstone, with interbedded bioturbated mudstone and thin-bedded, stratified sandstone. Within the main Regência Canyon, the coarser grained facies occur within 38 deeply incised channels. The fills are 9 to >50 m thick, 210 to >1050 m wide and >1 km long. The finer grained facies build asymmetrical levees that are higher and thicker on the left side (looking downstream) of their channels, probably as an effect of the Coriolis force (to the left in the Southern Hemisphere). Nine levee successions up to 50 m thick are associated with the 20 youngest channels. The deposits filling the low-sinuosity Lagoa Parda channels record successive channel abandonment through relatively rapid avulsions. Avulsions of unleveed channels took place randomly, but channels with well-developed levees show preferential avulsion to the right (looking downstream), opposite to the direction of preferential levee growth. Lagoa Parda channels can be grouped into three complexes 20–100 m thick. These complexes have an estimated duration of about 140 000 years. It is suggested that control of the development of individual channel complexes was related to variation in sediment supply, in turn probably related to climatic changes. The deposition of each channel complex would have followed an increase in sediment supply into the Regência Canyon through delta/fan-delta and littoral drift systems, which in turn would have responded to phases of higher denudation rates in the high-relief, ancestral coastal ranges of south-eastern Brazil. Overall, the three Lagoa Parda channel complexes form a turbidite succession characterized by channel fills that become narrower, thinner and finer grained upward. These trends were induced mostly by a longer term (>400 000 years) decrease in sediment supply, which in turn resulted from the combined effects of a long-term (second-order) trend of sea-level rise, and the decreasing fault activity at the basin margin and source area.     

Evolution of Miocene fluvial environments, eastern Potwar plateau, northern Pakistan

The Miocene-Pliocene Siwalik Group records changing fluvial environments in the Himalayan foreland basin. The Nagri and Dhok Pathan Formations of this Group in the eastern Potwar Plateau, northern Pakistan, comprise relatively thick (tens of metres) sandstone bodies and mudstones that contain thinner sandstone bodies (metres thick) and palaeosols. Thick sandstone bodies extend for kilometres normal to palaeoflow, and are composed of large-scale stratasets (storeys) stacked laterally and vertically adjacent to each other. Sandstone bodies represent single or superimposed braided-channel belts, and large-scale stratasets represent channel bars and fills. Channel belts had widths of km, bankfull discharges on the order of 10³ cumecs and braiding parameter up to about 3. Individual channel segments had bankfull widths, maximum depths, and slopes on the order of 10² m, 10¹ m and 10^?4 respectively, and sinuosities around 1-1. These rivers are comparable to many of those flowing over the megafans of the modern Indo-Gangetic basin, and a similar depositional setting is likely. Thin sandstone bodies within mudstone sequences extend laterally for on the order of 10² m and have lobe, wedge, sheet and channel-form geometries: they represent crevasse splays, levees and floodplain channels. Mudstones are relatively bioturbated/disrupted and represent mainly floodbasin and lacustrine deposition. Mudstones and sandstones are extremely disrupted in places, showing evidence of prolonged pedogenesis. These ‘mature’ palaeosols are m thick and extend laterally for km. Lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate. The 500 m-thick Nagri Formation has a greater proportion and thicker sandstone bodies than the overlying 700 m-thick Dhok Pathan Formation. The thick sandstone bodies and their large-scale stratasets thicken and coarsen through the Nagri Formation, then thin and fine at the base of the Dhok Pathan Formation. Compacted deposition rates increase with sandstone proportion (0-53 mm/year for Nagri, 0-24 mm/year for Dhok Pathan), and palaeosols are not as well developed where deposition rates are high. Within both formations there are 100 m-scale variations (representing on the order of 10⁵ years) in the proportion and thickness of thick sandstone bodies, and tens-of-m-scale alternations of thick sandstone bodies and mudstone-sandstone strata that represent on the order of 10⁴ years. Formation-scale stratal variations extend across the Potwar Plateau for at least 100 km, although they may be diachronous: however, 100-m and smaller scale variations can only be traced laterally for up to tens of km. Alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel-belt sizes (mainly), average deposition rate and avulsion frequency on a megafan comparable in size to modern examples. 100-m-scale variations in thick sandstone-body proportion and thickness could result from ‘regional’ shifts in the position of major channels, possibly associated with ‘fan lobes’on a single megafan or with separate megafans. However, such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system. Formation-scale and 100-m-scale stratal variations are probably associated with interelated changes in tectonic uplift, sediment supply and basin subsidence. Increased rates of hinterland uplift, sediment supply and basin subsidence, recorded by the Nagri Formation, may have resulted in diversion of a relatively large river to the area. Alternatively, changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland (possibly linked to tectonic uplift). Climate during deposition of the Siwalik Group was monsoonal. Although the deposits contain no direct evidence for climate change, independent evidence indicates global cooling throughout the Miocene, and the possibility of glacial periods (e.g. around 10-8 Ma, corresponding to base of Nagri Formation). If the higher Himalayas were periodically glaciated, a mechanism would exist for varying sediment supply to megafans on time scales of 10⁴-10⁵ years. Although eustatic sea-level changes are related to global climatic change, they are not directly related to Siwalik stratigraphic changes, because the shoreline was many 100 km away during the Miocene.     

Preservation of planar laminae due to migration of low-relief bed waves over aggrading upper-stage plane beds: comparison of experimental data with theory

Experimental studies of the formation of planar laminae by migration of low-relief bed waves over aggrading upper-stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point. In general, the preserved laminae represent only a small proportion (generally less than 50%) of the height of the largest bed waves in the population. The theory developed by Paola & Borgman (1991), relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation, was adapted for aggrading conditions and shown to agree well with data. These results suggest that the theory can be used to estimate, from the distribution of thickness of planar laminae, either the distribution of bed wave heights, mean aggradation rate, mean bed wave length or mean bed wave celerity, provided the other parameters can be estimated.     

Deep-sea avulsion and morphosedimentary evolution of the Rhône Fan Valley and Neofan during the Late Quaternary (north-western Mediterranean Sea)

The Petit-Rhône Fan Valley (north-western Mediterranean) is a broad, sinuous, filled valley that is deeply incised by a narrow, sinuous thalweg. The valley fill is differentiated into three seismic subunits on high-resolution seismic-reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high-amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass-flow deposits resulting from the disintegration of the fan-valley flanks. The upper bedded subunit shows an overall lens-shaped geometry and the seismic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhône levees. Piston core data show that the upper few meters of this upper subunit consist of thin turbidites, probably deposited by overflow processes. The few available ¹⁴C ages suggest that the upper stratified subunit filled the Petit-Rhône Fan Valley between 21 and 11 kyr BP. The upper bedded subunit is deposited within the Petit-Rhône Fan Valley downslope of a major decrease in slope gradient. This upper subunit and the thalweg are genetically related and represent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature resulting from a recent avulsion of the major channel course. Our interpretation implies that the thalweg is not a purely erosional feature but a depositional/erosional channel. This small channel/levee system is superimposed on a large muddy channel/levee system after the sediment supply changed from thick muddy flows during the main phase of aggradation of the Rhône Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (～16–18 ka BP). The pre-existing morphology of the Petit-Rhône Fan Valley played a determinant role in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow stripping resulting from the changes in the slope gradient along the thalweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit-Rhône Fan Valley. Coarser channelled sediment remaining in the thalweg were deposited as a ‘sandy’lobe (Neofan). As indicated by ¹⁴C dating, sedimentation on this lobe continued until very recently, suggesting a further evolution of the turbidity flows from small mixed flows to small sandy flows. the deposition of this study lobe and the sedimentary fill of the Petit-Rhône Fan Valley may be related to widespread shelf edge and canyon wall failures with a resulting downslope evolution of failed sediment into turbidity currents.     

Spillover models for axial rivers in regions of continental extension: the Rio Mimbres and Rio Grande in the southern Rio Grande rift, USA

The Pliocene-early Pleistocene history of the ancestral Rio Grande and Quaternary history of the Rio Mimbres in the southern Rio Grande rift, New Mexico, illustrate how axial rivers may alternately spill into and subsequently abandon extensional basins. Three types of spillover basins are recognized, based on the angle at which the axial river enters the basin and whether it descends the hanging wall dip slope or footwall scarp to reach the basin floor. In the Mimbres basin type, the axial river enters and flows through the spillover basin nearly parallel to the footwall scarp, resulting in a narrow belt of basin-axis-parallel channel sand bodies located near the footwall scarp. In contrast, an axial river may enter a spillover basin at a high angle to its axis, either descending the hanging wall dip slope (Columbus basin type) or footwall scarp (Tularosa basin type), and construct a fluvial fan, consisting of radiating distributary channels orientated nearly perpendicular to the basin axis. Faulting exerts significant control on river spillover by creating the topographic gaps through which the axial river moves and by terminating spillover by subsequently uplifting or tilting the gap. Spillover may also be autocyclic in origin as a result of aggradation to the level of a pre-existing gap, headward erosion creating and/or intersecting a gap, or simple river avulsion upstream of a gap. Predicting facies architecture in the three types of spillover basins is critical to successful subsurface exploration for hydrocarbon reservoirs, groundwater aquifers or placer mineral deposits.     

Microspar development during early marine burial diagenesis: a comparison of Pliocene carbonates from the Bahamas with Silurian limestones from Gotland (Sweden)

Comparison of ultrastructures in Pliocene periplatform carbonates from the Bahamas with Silurian limestones from Gotland (Sweden) reveals that despite the differences in primary sediment composition and age, they reflect a similar mechanism of lithification. In both sequences calcite microspar was formed as a primary cement at an early stage of marine burial diagenesis. Neither significant compression nor meteoric influence are necessary for the formation of calcite microspar. A model is proposed for the process of microsparitic cementation of fine-grained aragonite needle muds comprising four stages: (1) unconsolidated, aragonite-dominated carbonate mud; (2) precipitation of microspar that engulfs aragonite needles; (3) dissolution of aragonite, resulting in pitted surfaces of the microspar crystals; and (4) slight recrystallization. Our results contradict the widespread opinion that microspar necessarily is a product of secondary recrystallization of a previously lithified micrite.     

Partial melting during tectonic exhumation of a granulite terrane: an example from the Larsemann Hills, East Antarctica

Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.     

Organic pollutants associated with macromolecular soil organic matter: Mode of binding

A study of ether-linked moieties in macromolecular bound residues of polycyclic aromatic hydrocarbons (PAH) generated in bioremediation experiments was performed using high temperature hydrolysis degradation with subsequent analysis of the products by GC-MS. This hydrolysis reaction was specifically designed to cleave ether bonds including relatively stable diarylether structures. Among the reaction products, aromatic alcohols representing typical microbiologically derived metabolites of PAH were found in addition to natural compounds. Thus, parts of the bound residues appeared to be linked within the macromolecular material by ether bonds. Model experiments with an oxidoreductase enzyme and aromatic alcohols indicate the formation of these ether bonds to be an enzyme-catalysed process.     

Comparison of source rock geochemistry of selected rocks from the Schei Point group and Ringnes formation, Sverdrup basin, arctic Canada

Organic-rich from the Schei Point group (middle to late Triassic in age) and the Ringnes formation (late Jurassic) from the Sverdrup basin of the Canadian arctic archipelago have been geochemically evaluated for source rock characterization. Most samples from the Schei Point group are organic-rich (> 2% TOC and are considered as immature to mature oil-prone source rocks [kerogen types I, I–II (IIA) and II (IIA)]. These kerogen types contain abundant AOM1, AOM2 and alginite (Tasmanales, Nostocopsis, Leiosphaeridia, acritarch and dinoflagellate) with variable amounts of vitrinite, inertinite and exinite. Samples from the Ringnes formation contain dominant vitrinite and inertinite with partially oxidized AOM2, alginite and exinite forming mostly immature to mature condensate- and gas-prone source rocks [kerogen type II–III (IIB), III and a few II (IIA)]. Schei Point samples contain higher bitumen extract, saturate hydrocarbons and saturate/ aromatic ratio than the Ringnes samples. Triterpane and sterane (dominant C₃₀) distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the Schei Point group are at the onset of oil generation and contain a mixture of sapropelic (algal) and minor terrestrial humic organic matter. Sterane carbon number distributions in the Ringnes formation also suggest a mixed algal and terrestrial organic matter type. There are some variations in hopane carbon number distributions, but these are apparently a function of thermal maturity rather than significant genetic differences among samples. Pyrolysis-gas chromatography/mass spectrometry of the two samples with similar maturity shows that the Schei Point sample generates three times more pyrolyzate than the Ringnes sample. Both samples have a dominant aliphatic character, although the Ringnes sample contains phenol and an aromaticity that is higher than that of the Schei Point sample.     

Are assemblages of black bream (Acanthopagrus butcheri) in different estuaries genetically distinct?

Samples of the estuarine-spawning teleostAcanthopagrus butcheri were collected from nine estuaries and a coastal lake, located in the Pilbara and South-western drainage divisions of Western Australia and distributed along a coastline covering a distance of nearly 2,000 km. The patterns of allozyme variation in these samples were used to explore the extent to which there was variation in the genetic compositions of black bream assemblages in geographically-isolated estuarine systems, and whether or not any such variation could be related to the geographical location or type of estuary. Although only three of 36 scorable loci (Gpi-1, Ldh andMdh-2) exhibited variation that could be used for analysis, there was considerable variation in allele frequencies at these loci among the different samples (mean F_ST=0.166). Much of the detected variation was attributable to differences between the samples collected from the two drainage divisions, which are located in very different climatic regions. Furthermore, the genetic compositions of samples from neighbouring estuaries were typically more similar to each other than to those of samples collected from more distantly-located systems. However, the assemblages in one west coast and two south coast estuaries, that are closed to the ocean for extensive periods of time during the year, all showed very similar genetic compositions. Nevertheless, it is crucial to recognise that, pairwise comparisons of samples collected from the different estuaries, both within and between the two drainage divisions, almost invariably showed statistically significant differences in allele frequencies at one or more loci. Thus, our results indicate that the local populations of black bream in individual estuaries are genetically distinct, which is probably a consequence of both a limited movement by individuals between estuaries and the effects of differences in regional and local environmental conditions.