Visual description and Radiolarian abundance estimates of ODP Hole 200-1223A sediments

Among the groups of oceanic microfossils, only Radiolaria occur in abundances and preservation states sufficient to provide biostratigraphic control for restricted intervals within sediments recovered in Hole 1223A. The distribution of these microfossils has been divided into four major intervals, A-D. Radiolaria distribution Interval A occupies the depth range 0-3.0 meters below seafloor (mbsf), where the abundance of specimens is very low and preservation is poor. Radiolaria distribution Interval B occupies the depth range 3.02-7.1 mbsf. Radiolaria in Interval B are locally rare to abundant and well preserved, and assemblages range in age from pure early Eocene to early Eocene admixed with late Neogene taxa. Radiolaria distribution Interval C occupies the depth range 7.1-36.99 mbsf and is characterized by sediments either barren of microfossils or containing extremely rare early Eocene specimens. Radiolaria distribution Interval D occupies the depth range 36.99-38.7 mbsf (base of the recovered sedimentary section), where early Eocene Radiolaria are present in rare to common frequencies, but opal-A to opal-CT recrystallization has degraded the preservation state. The late Neogene assemblage of Radiolaria distribution Interval B is dated at 1.55-2.0 Ma, based on occurrences of Eucyrtidium matuyamai, Lamprocyclas heteroporos, and Theocorythium trachelium trachelium. The early Eocene assemblage of Radiolaria distribution Intervals B and D is somewhat problematically assigned to the Buryella clinata Zone.

Clay mineralogy of ODP Site 131-808 sediments

Drill core recovered at Ocean Drilling Program Site 808 (Leg 131) proves that the wedge of trench sediment within the central region of the Nankai Trough comprises approximately 600 m of hemipelagic mud, sandy turbidites, and silty turbidites. The stratigraphic succession thickens and coarsens upward, with hemipelagic muds and volcanic-ash layers of the Shikoku Basin overlain by silty and sandy trench-wedge deposits. Past investigations of clay mineralogy and sand petrography within this region have led to the hypothesis that most of the detritus in the Nankai Trough was derived from the Izu-Honshu collision zone and transported southwestward via axial turbidity currents. Shipboard analyses of paleocurrent indicators, on the other hand, show that most of the ripple cross-laminae within silty turbidites of the outer marginal trench-wedge facies are inclined to the north and northwest; thus, many of the turbidity currents reflected off the seaward slope of the trench rather than moving straight down the trench axis. Shore-based analyses of detrital clay minerals demonstrate that the hemipelagic muds and matrix materials within sandy and silty turbidites are all enriched in illite; chlorite is the second-most abundant clay mineral, followed by smectite. In general, the relative mineral percentages change relatively little as a function of depth, and the hemipelagic clay-mineral population is virtually identical to the turbidite-matrix population. Comparisons between different size fractions (<2 µm and 2-6 µm) show modest amounts of mineral partitioning, with chlorite content increasing in the coarser fraction and smectite increasing in the finer fraction. Values of illite crystallinity index are consistent with conditions of advanced anchimetamorphism and epimetamorphism within the source region. Of the three mica polytypes detected, the 2M1 variety dominates over the 1M and 1Md polytypes; these data are consistent with values of illite crystallinity. Measurements of mica bo lattice spacing show that the detrital illite particles were eroded from a zone of intermediate-pressure metamorphism. Collectively, these data provide an excellent match with the lithologic and metamorphic character of the Izu-Honshu collision zone. Data from Leg 131, therefore, confirm the earlier interpretations of detrital provenance. The regional pattern of sediment dispersal is dominated by a combination of southwest-directed axial turbidity currents, radial expansion of the axial flows, oblique movement of suspended clouds onto and beyond the seaward slope of the Nankai Trough, and flow reflection back toward the trench axis.

Grain size distribution analysis of prominent beds on Southern Central Chilean seamounts

Gravity cores obtained from isolated seamounts located within, and rising up to 300 m from the sediment-filled Peru-Chile Trench off Southern Central Chile (36°S-39°S) contain numerous turbidite layers which are much coarser than the hemipelagic background sedimentation. The mineralogical composition of some of the beds indicates a mixed origin from various source terrains while the faunal assemblage of benthic foraminifera in one of the turbidite layers shows a mixed origin from upper shelfal to middle-lower bathyal depths which could indicate a multi-source origin and therefore indicate an earthquake triggering of the causing turbidity currents. The bathymetric setting and the grain size distribution of the sampled layers, together with swath echosounder and sediment echosounder data which monitor the distribution of turbidites on the elevated Nazca Plate allow some estimates on the flow direction, flow velocity and height of the causing turbidity currents. We discuss two alternative models of deposition, both of which imply high (175-450 m) turbidity currents and we suggest a channelized transport process as the general mode of turbidite deposition. Whether these turbidites are suspension fallout products of thick turbiditic flows or bedload deposits from sheet-like turbidity currents overwhelming elevated structures cannot be decided upon using our sedimentological data, but the specific morphology of the seamounts rather argues for the first option. Oxygen isotope stratigraphy of one of the cores indicates that the turbiditic sequences were deposited during the last Glacial period and during the following transition period and turbiditic deposition stopped during the Holocene. This climatic coupling seems to be dominant, while the occurrence of megathrust earthquakes provides a trigger mechanism. This seismic triggering takes effect only during times of very high sediment supply to the shelf and slope.

Age model and stable isotopes of sediment cores from north-central Chile and south-central Chile

Terrigenous sediment supply, marine transport, and depositional processes along tectonically active margins are key to decoding turbidite successions as potential archives of climatic and seismic forcings. Sequence stratigraphic models predict coarse-grained sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine siliciclastic deposition during transgressions and highstands has been attributed to sustained connectivity between terrigenous sources and marine sinks facilitated by narrow shelves. To decipher the controls on Holocene highstand turbidite deposition, we analyzed 12 sediment cores from spatially discrete, coeval turbidite systems along the Chile margin (29° - 40°S) with changing climatic and geomorphic characteristics but uniform changes in sea level. Sediment cores from intraslope basins in north-central Chile (29° - 33°S) offshore a narrow to absent shelf record a shut-off of turbidite deposition during the Holocene due to postglacial aridification. In contrast, core sites in south-central Chile (36° - 40°S) offshore a wide shelf record frequent turbidite deposition during highstand conditions. Two core sites are linked to the Biobío river-canyon system and receive sediment directly from the river mouth. However, intraslope basins are not connected via canyons to fluvial systems but yield even higher turbidite frequencies. High sediment supply combined with a wide shelf and an undercurrent moving sediment toward the shelf edge appear to control Holocene turbidite sedimentation and distribution. Shelf undercurrents may play an important role in lateral sediment transport and supply to the deep sea and need to be accounted for in sediment-mass balances.

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks, Bowen Basin, Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay. minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite-smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic (R greater than or equal to 3) illite-smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic (R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less. illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu)(sn) ratios show negative correlations with the illite content in illite-smectite and positive correlations with the delta(18)O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu)(sn) ratios and illite content in illite-smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/ rock ratio conditions. Increasing HREE enrichment with delta(18)O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin. Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins. (C) 2003 Elsevier Science B.V. All rights reserved.

Stratigrafia fisica ed analisi di facies dei depositi torbiditici della Formazione delle Arenarie del Monte Cervarola tra la Val Secchia e la Val Scoltenna (Aquitaniano-Burdigaliano, Appennino Settentrionale)

The Cervarola Sandstones Formation (CSF), Aquitanian-Burdigalian in age, was deposited in an elongate, NW-stretched foredeep basin formed in front of the growing Northern Apennines orogenic wedge. The stratigraphic succession of the CSF, in the same way of other Apennine foredeep deposits, records the progressive closure of the basin due to the propagation of thrust fronts toward north-east, i.e. toward the outer and shallower foreland ramp. This process produce a complex foredeep characterized by synsedimentary structural highs and depocenters that can strongly influence the lateral and vertical turbidite facies distribution. Of consequence the main aim of this work is to describe and discuss this influence on the basis of a new high-resolution stratigraphic framework performed by measuring ten stratigraphic logs, for a total thickness of about 2000m, between the Secchia and Scoltenna Valleys (30km apart). In particular, the relationship between the turbidite sedimentation and the ongoing tectonic activity during the foredeep evolution has been describe through various stratigraphic cross sections oriented parallel and perpendicular to the main tectonic structures. On the basis of the high resolution physical stratigraphy of the studied succession, we propose a facies tract and an evolutionary model for the Cervarola Sandstones in the studied area. Thanks to these results and the analogies with others foredeep deposits of the northern Apennines, such as the Marnoso-arenacea Formation, the Cervarola basin has been interpreted as a highly confined foredeep controlled by an intense synsedimentary tectonic activity. The most important evidences supporting this hypothesis are: 1) the upward increase, in the studied stratigraphic succession (about 1000m thick), of sandstone/mudstone ratio, grain sizes and Ophiomorpha-type trace fossils testifying the high degree of flow deceleration related to the progressive closure and uplift of the foredeep. 2) the occurrence in the upper part of the stratigraphic succession of coarse-grained massive sandstones overlain by tractive structures such as megaripples and traction carpets passing downcurrent into fine-grained laminated contained-reflected beds. This facies tract is interpreted as related to deceleration and decoupling of bipartite flows with the deposition of the basal dense flows and bypass of the upper turbulent flows. 3) the widespread occurrence of contained reflected beds related to morphological obstacles created by tectonic structures parallel and perpendicular to the basin axis (see for example the Pievepelago line). 4) occurrence of intra-formational slumps, constituted by highly deformed portion of fine-grained succession, indicating a syn-sedimentary tectonic activity of the tectonic structures able to destabilize the margins of the basin. These types of deposits increase towards the upper part of the stratigraphic succession (see points 1 and 2) 5) the impressive lateral facies changes between intrabasinal topographic highs characterized by fine-grained and thin sandstone beds and marlstones and depocenters characterized by thick to very thick coarse-grained massive sandstones. 6) the common occurrence of amalgamation surfaces, flow impact structures and mud-draped scours related to sudden deceleration of the turbidite flows induced by the structurally-controlled confinement and morphological irregularities. In conclusion, the CSF has many analogies with the facies associations occurring in other tectonically-controlled foredeeps such as those of Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (southern France) showing how thrust fronts and transversal structures moving towards the foreland, were able to produce a segmented foredeep that can strongly influence the turbidity current deposition.

Petrology of weathered lower lias clays

In weak argillaceous rocks the unweathered strength may be barely sufficient to meet civil engineering reguirements and any reductjon due to weathering will be critical. This study investigates the weathering of the Lower Lias clays with particular reference to their petrography and engineering properties. Investigations revealed the Midland Basin of deposition to contain reasonable thicknesses of clay, relatively uniform in nature with a well developed weathered zone, From the available exposures, the weathering zone of the Blockley Clay pit was selected and sampled for laboratory investigations of; Structure, Mineralogy and Chemistry and Engineering Properties. The nature and orientation of the fissures in the unweathered clay were analysed. A close relationship was found to exist between the major joint set and the ground surface, with stress release due to excavation being almost negligible. Thin sections of the clay, examined for structural data, suggested that there exist layers or areas that have been disturbed as a result of density differences. Shear planes were found in both the unweathered and weathered clay, in the latter case often associated with remoulding of the material. A direct measure of remoulding was obtained from the birefringence ratio. The fabric was examined in closer detail using the scanning electron microscope. Mineralogy, as revealed by X-ray and optical techniques indicated illite as the dominant clay mineral, with kaolinite subsidiary; quartz, calcite, pyrite, chlorite/vermiculite are present as accessory minerals. Weathering changes this relationship, calcite and pyrite being removed early in the process, with illite being degraded. The cementing action of calcite and iron oxides was investigated however, this was shown to be negligible. Quantitative measurements of both fixed (with minerals) and free (oxide coatings) iron were obtained by atomic absorption, with the Fe 3+/ Fe2+ ratio obtained by Mossbauer spectroscopy, Evidence indicates that free iron oxide coatings only become important as a result of weathering with the maximum concentration in the very highly weathered material. Engineering index properties and shear strength values were taken throughout the profile, Relationships between moisture content and strength, liquid limit and iron (Fe) were obtained and a correlation between the weathering zomes and the shear strength/depth curve has been established.

The sedimentology and diagenesis of the Pendleside limestone group in the Craven Basin of Northern England. Available in 2 volumes.

This thesis describes the stratigraphy, sedimentology and diagenesis of the Pendleside Limestone (Asbian age), a sequence of limestones, shales and dolostones in the Clitheroe area of N. W. England. Field study of 19 measured sections indicates that it was deposited in a rhythmically subsiding basin (Craven Basin) because of movements on the Mid-Craven Fault which was active in Dinantian times. The sequence is up to 190m thick and consists mostly of distal turbidite deposits which have been reworked at horizons when sediment accumulation built up to the wave base. The original depositional fabric and mineralogy of the Pendleside Limestone Group has been extensively modified by diagenetic processes including cementation, authi­genesis, dolomitization and silicification. These processes have been studied using a wide variety of laboratory techniques. The carbonate cements of the PendIeside Limestone consist predominantly of ferroan calcite and non-ferroan calcite with microdolomite incIusions. The former is probably a stable replacement of original-high-magnesian calcite. Cementation was accompanied by the formation of authigenic albite and quartz. Much of the upper part of the Pendleside Limestone has been extensively dolomitized and chertified. Several distinct zones of dolomitization are found which increase in thickness and intensity towards the top of the Pendleside Limestone Group. The dolostone horizons correspond to coarser-grained lithologies deposited during periods of shallow water sedimentation. The composition of the dolomites changes from ferroan dolomite in the lower part of the Group to non-ferroan dolomite in the upper part. The low strontium and sodium content of the dolostones in association with the other evidence suggests that the dolomitization was brought about in an open system by the mixing of marine and fresh water in phreatic lens which were established at periodic intervals. The dolomitization was closely associated with chertification although this was initiated by the dissolution of siliceous spicules which provided the necessary source of silica.

Age determination of sediment cores of the Golf of Lions

Siliciclastic turbidites represent huge volumes of sediments, which are of particular significance for (1) petroleum researchers, interested in their potential as oil reservoirs and (2) sedimentologists, who aim at understanding sediment transport processes from continent to deep-basins. An important challenge when studying marine turbidites has been to establish a reliable chronology for the deposits. Indeed, conventional marine proxies applied to hemipelagic sediments are often unreliable in detrital clays. In siliciclastic turbidites, those proxies can be used only in hemipelagic intervals, providing a poor constraint on their chronology. In this study, we have used sediments from the Rhône Neofan (NW Mediterranean Sea) to demonstrate that pollen grains can provide a high-resolution chronostratigraphical framework for detrital clays in turbidites. Vegetation changes occurring from the end of Marine Isotopic Stage 3 to the end of Marine Isotopic Stage 2 (from ~30 to ~18 ka cal. BP) are clearly recorded where other proxies have failed previously, mainly because the scarcity of foraminifers in these sediments prevented any continuous Sea Surface Temperature (SST) record and radiocarbon dating to be obtained. We show also that the use of palynology in turbidite deposits is able to contribute to oceanographical and sedimentological purposes: (1) Pinus pollen grains can document the timing of sea-level rise, (2) the ratio between pollen grains transported from the continent via rivers and dinoflagellate cysts (elutriating) allows us to distinguish clearly detrital sediments from pelagic clays. Finally, taken together, all these tools show evidence that the Rhône River disconnected from the canyon during the sea-level rise and thus evidence the subsequent rapid starvation of the neofan at 18.5 ka cal. BP. Younger sediments are hemipelagic: the frequency of foraminifers allowed to date sediments with radiocarbon. First results of Sea Surface Temperature obtained on foraminifers are in good agreement with the dinoflagellate cysts climatic signal. Both provide information on the end of the deglaciation and the Holocene.

(Table 1) TOC, d13C, concentrations of C37 alkenones, crenarchaeol and GDGTs of turbidites

One of the primary prerequisites for the application of organic proxies is that they should not be substantially affected by diagenesis. However, studies have shown that oxic degradation of biomarker lipids can affect their relative distribution. We tested the diagenetic stability of the UK'37 and TEX86 palaeothermometers upon long term oxygen exposure. For this purpose, we studied the distributions of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in different sections of turbidites at the Madeira Abyssal Plain (MAP) that experienced different degrees of oxygen exposure. Sediments were deposited anoxically on the shelf and then transported by turbidity currents to the MAP, which has oxic bottom water. This resulted in partial degradation of the turbidite organic matter as a result of long term exposure to oxic bottom water. Concentrations of GDGTs and alkenones were reduced by one to two orders of magnitude in the oxidized parts of the turbidites compared to the unoxidized parts, indicating substantial degradation. High-resolution analysis of the Pleistocene F-turbidite showed that the UK'37 index of long chain alkenones increased only slightly (0.01, corresponding to <0.5 °C) in the oxidized part of the turbidite, suggesting minor preferential degradation of the C37:3 alkenone, in agreement with previous studies. TEX86 values showed a small increase (0.02, corresponding to ~2 °C) in the F-turbidite, like UK'37 , while for other Pliocene/Miocene turbidites it either remained unchanged or decreased substantially (up to 0.06, corresponding to ~6 °C). Previous observations showed that the BIT index, a proxy for the contribution of soil organic matter to total organic carbon, was always substantially higher in the oxidized part in all the turbidites, as a result of preferential degradation of marine-derived GDGTs. This relative increase in soil-derived GDGTs affects TEX86, as the isoprenoid GDGT distribution on the continent can be quite different from that in the marine environment. Our results indicate that the organic proxies are affected by long term oxic degradation to different extents; this should be taken into account when applying these proxies in palaeoceanographic studies of sediments which have been exposed to prolonged oxic degradation.

Análise estratigráfica de subsuperfície do Devoniano Inferior da Bacia do Rio do Peixe, Nordeste do Brasil

Subsurface stratigraphic analysis of Devonian strata from the Rio do Peixe Basin, newly recognized by palynological studies, has resulted in the identification of two new lithostratigraphic units assembled in the Santa Helena Group. The Pilões Formation, the lower unit, is composed mainly of dark mudstones and medium-tovery fine-grained sandstones, with minor conglomerates and breccias. The Triunfo Formation, the upper unit, comprises whitish grey, kaolinitic, coarse-grained to conglomeratic, cross stratified sandstones and conglomerates, with interbedded mudstones and fine-grained sandstones. These units were characterized using cores, sidewall and cuttings samples, conventional logs and image log, from three wells drilled by PETROBRAS, and 3D seismic data. The Pilões Formation is interpreted as prodeltaic facies, with lesser associated subaqueous talus, debrite and sandy turbidite lobe facies, distal part of fandelta and braided fluviodeltaic facies of Triunfo Formation. The Santa Helena Group corresponds to the Lower Devonian tectono-sequence deposited in a NW-SE-trending graben during a transgressiveregressive cycle. With 343 meters of thickness (isochore) in well 1-PIL-1-PB (Pilões 1), this sequence has a non-conformity at the lower boundary and its upper boundary is an unconformity with the Lower Cretaceous tectono-sequence (Rio do Peixe Group), that represents a hiatus of about 265 million years. Ignimbrites and coignimbrite breccias (Poço da Jurema volcanic breccia), related to an unknown pyroclastic volcanic event, were recognized at the northern margin of the Sousa halfgraben. Evidence from well data suggests that this event is coeval with the Devonian graben filling. The present study indicates a polyhistorical tectono-volcanosedimentary evolution of the basin. This lithostratigraphic update brings new perspectives for geological research in the Rio do Peixe Basin, as well as in other inland basins of the Northeastern of Brazil. The results of the research also contribute to the kwnoledge of the Borborema Province and western Gondwana paleogeography during the Early Devonian.

Investigations on sediment coring site PS1823 in the Riiser-Larsen Sea, Southern Ocean

Physical and sedimentological investigations were carried out on a 14 m long gravity core and a 0.5 m long box core from 4440 m water depth off Queen Maud Land, East-Antarctica. Strongly bioturbated hemipelagic muds of predominantly terrigenous origin and a very small biogenic part build up the 'Normal-Facies'. Several sandy to silty layers are inserted in the 'Normal-Facies'. These layers are seperated by lithology, structure and the investigated parameters of this study and are interpreted as turbidites. The source area for the turbidity currents is supposed to be at the uppermost continental margin, close to the shelf break and there is evidenee for this gravity transport within the erosive Ritscher-Canyon, which extends close to the core position. The distribution of biogenic components indicates an age of 1.3 million years or more, with an average sedimentation rate of about 1 cm/1000 years. Early diagenetic proeesses caused water loss by compaction, errosion and dissolution of biogenic components and precipitation and recrystallization of manganese micronodules. Cyclic fluctuations of the sediment-parameters within the 'Normal-Facies' enable the distinction of a 'Glazial'- and an 'Interglazial'-Facies. The 'Glazial'-Facies reflects glacial sedimentary conditions and shows a dark olive gray colour, high susceptibility, low silt/clay-ratios, only a few biogenic components and the regular occurence of interrelated turbidite layers. In contrast, the 'Interglazial'-Facies is dominated by a light olive or olive-brown colour, low susceptibility, high silt/clay-ratios and an increased number of biogenic components. This facies corresponds to interglacial conditions. Three main processes are supposed to have been responsible for the observed facies changes: (1) the bottom water mass circulation, (2) the gravity transport by turbidity currents and (3) the biogenic surface production. These processes are related to the quaternary climatic changes. The extension of the ice shelves directed the gravity transport to the deep sea and the formation of Antarctic Bottom Water, which in turn influenced the silt/clay-ratios in the sediment record. Fluctuations in sea ice coverage controlled the biogenic surface production.

Biomarkers in turbidites at ODP Sites 157-951 and 157-952

Free and ester-bound lipid biomarkers were analysed in oxidised and unoxidised parts of four distinct turbidites from the Madeira Abyssal Plain (MAP), which contained 1 to 2% organic carbon homogeneously distributed throughout the turbidites at the time they were deposited. These turbidites are well suited to study the effects of oxic degradation on lipid biomarkers without the complicating influence of varying organic matter sources, sedimentation rates, or bioturbation. One sample from the oxidised turbidite was compared with two samples from the unoxidised part of each turbidite. Postdepositional oxic degradation decreased concentrations of biomarkers by several orders of magnitude. The ester-bound lipids were degraded to a far lesser extent than their free counterparts were. The extent of degradation of different compounds differed substantially. Within a specific class of biomarkers, degradation also took place to a different extent, altering their distributions. This study shows that oxic degradation of the organic matter may have a profound effect on the biomarker fingerprint and may result in a severe bias in, for example, the interpretation of organic matter sources and the estimation of the palaeoproductivity of specific groups of phytoplankton.