Massive Facies Research Articles

Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

Fil: Biel, C.. Universidad de Zaragoza; Espana. Productos Minerales para la Industria S.A.; Espana

Stratigrafia fisica della successione sedimentaria miocenica del settore Nord-orientale della Stretta di Catanzaro (Calabria centro-orientale)

Physical Stratigraphy of the upper Miocene sedimentary succession in the northeastern Catanzaro Through (Central Calabria, Italy).In the northeastern sector of the Catanzaro Trough, a terrigenous, evaporitic and carbonate mio-pliocenic sedimentary succession overlies the igneous-metamorphic basement of the Sila Unit. These deposits are strongly controlled by NW- strike-slip system, arranged in a right-hand en echelon pattern (Tansi et alii, 2007). Coarse-grained fluvial deposits of the Corace conglomerate Unit represent the onset of the Serravallian – Tortonian sedimentation. Overlying shallow marine arenites and calcarenites, including Porites bodies, pass upward to offshore marly-clays deposits of the Gagliano Unit, evolving upsection to diatomite and organic-rich laminites (Tripoli Fm.) at the Tortonian/Messinian boundary. Evaporitic conditions characterized the sedimentation within the Mediterranean basin during the Messinian as demonstrated by the wide distribution of selenite, micrite and halite deposits.In the northeastern sector of the Catanzaro Trough such conditions are recorded by fi ne grained carbonate deposits, of assumed bacterial origin (Guido et alii, 2007), and can be correlated to CdB type 2 by Manzi et alii (2010), developed during the fi rst MSC stage, from 5,96 Ma to 5,6 Ma. Chaotic deposits of selenitic blocks, gypsrudites and gypsarenites (resedimented evaporites; sub-unit 1 of Riato Conglomerate) and limestone breccias (CdB-2) overlie the Tripoli Fm. along a paraconcordance surface and characterize the depocentral area. The chaotic deposits pass upward to sub-unit 2 of Riato Conglomerate and Arieste Fm characterized by channels and bars facies. They show a fi ning and thinning upward trend represented by a drastic decrease of the intrabasinal evaporitic and carbonate alimentation in favour of metamorphic and plutonic detritus supplied by the Calabrian-Peloritan Arc units.The development of a coarse grained fl uvial-deltaic depositional system characterizes the MSC fi nal stage. Towards the depocentre continental sediments pass to clayey-sandy deposits, containing biofacies with ostracods (Lago Mare) and selenitic bodies with banded and massive facies, referred to Upper Evaporites (Costa, pers. Comm.) The sedimentary succession exposed in the northeastern sector of the Catanzaro Through defi nes a depositional architecture that may be included in the evolution model proposed by Roveri et alii (2008). In this contest the chaotic and terrigenous-evaporitic-carbonate deposits (sub-units 1-2 of the Riato Conglomerate, CdB-2 and Arieste Fm.), originated by the cannibalism of the selenite and carbonates deposits probably between 5.6 and 5.55 Ma, record a gradual return to a extrabasinal alimentation originated by the erosion of the Serravallian and Tortonian sedimentary units and crystalline rocks of the Sila Unit.

Pyroclastic flow hazard assessment at Somma–Vesuvius based on the geological record

During the past 22 ka of activity at Somma–Vesuvius, catastrophic pyroclastic density currents (PDCs) have been generated repeatedly. Examples are those that destroyed the towns of Pompeii and Ercolano in AD 79, as well as Torre del Greco and several circum-Vesuvian villages in AD 1631. Using new field data and data available from the literature, we delineate the area impacted by PDCs at Somma–Vesuvius to improve the related hazard assessment. We mainly focus on the dispersal, thickness, and extent of the PDC deposits generated during seven plinian and sub-plinian eruptions, namely, the Pomici di Base, Greenish Pumice, Pomici di Mercato, Pomici di Avellino, Pompeii Pumice, AD 472 Pollena, and AD 1631 eruptions. We present maps of the total thickness of the PDC deposits for each eruption. Five out of seven eruptions dispersed PDCs radially, sometimes showing a preferred direction controlled by the position of the vent and the paleotopography. Only the PDCs from AD 1631 eruption were influenced by the presence of the Mt Somma caldera wall which stopped their advance in a northerly direction. Most PDC deposits are located downslope of the pronounced break-in slope that marks the base of the Somma–Vesuvius cone. PDCs from the Pomici di Avellino and Pompeii Pumice eruptions have the most dispersed deposits (extending more than 20 km from the inferred vent). These deposits are relatively thin, normally graded, and stratified. In contrast, thick, massive, lithic-rich deposits are only dispersed within 7 to 8 km of the vent. Isopach maps and the deposit features reveal that PDC dispersal was strongly controlled by the intensity of the eruption (in terms of magma discharge rate), the position of the vent area with respect to the Mt Somma caldera wall, and the pre-existing topography. Facies characteristics of the PDC deposits appear to correlate with dispersal; the stratified facies are consistently dispersed more widely than the massive facies.

Climate and tectonic controls on Pleistocene sequence development and river evolution in the Southern Vienna Basin (Austria)

The impact of climate and tectonism on sedimentation and lithofacies development is demonstrated for Austria's largest Pleistocene sedimentary basin situated within the Vienna Basin. The coarse grained, massive facies is associated with high-energetic flood events reflecting a distinct seasonality effect of discharge during glacial times. In contrast, fine grained, well stratified sediments mark periods of rather high discharge to sediment supply ratios during warmer periods. Preservation of depositional sequences was facilitated by subsidence rates of approx. 0.5–1 mm/a. High subsidence rates in the basin preserve a relatively large record of sequences compared to fluvial terraces which reflect mainly sediments deposited during and shortly after glacials. 14C and relative ages demonstrate the strong impact of oscillations in climate on fluvial stratigraphy covering times from the upper period of MIS 3 to MIS 1. Abundant sediment supply and the associated increase in accumulation space as well as tectonic factors affect not only the sequence development but also the geomorphology of two mountain front alluvial fans which toe out into the Mitterndorf Basin. High discharge to sediment supply ratios leads to fan incision and to abandonment of fan surfaces. During phases of reduced sediment supply the effect of subsidence results in headcut erosion, and, if sufficient time is available, may lead to complete through trenching. Abandoned fan surfaces are exposed to widespread soil formation which mark sequence boundaries to the following coarse grained, massive facies reflecting high sediment supply to discharge ratios.

The complex facies architecture and emplacement sequence of a Miocene submarine mega-pillow lava flow system, Muriwai, North Island, New Zealand

The early Miocene Waiatarua Formation at Maori Bay, Muriwai, North Island, New Zealand consists of a complex association of basaltic andesite volcanic facies including entablature-jointed thick massive facies, colonnade-jointed thin massive sheet facies, mega-pillow facies, normal pillow facies and minor associated fragmental facies, including vitric sandstone and breccia interpreted as hyaloclastite and peperite. Field observations suggest that the facies, which form the Waiatarua Formation lava, were emplaced as multiple flow lobes in a single lava flow from one sustained eruption. Magma discharge rate at the vent was high; however magma supply rate to the more distal and lateral portions of the flow, as its surface area increased, varied considerably. Higher magma supply rates produced thick, massive and thin sheet facies, whereas mega-pillow facies and normal pillow facies were produced contemporaneously with the thick and thin massive sheet facies but were restricted to portions of the distal flow subjected to lower magma supply rates. The evolution of the Waiatarua formation lava flow at Maori Bay has been reconstructed from the complex facies architecture. This suggests that the propagation of the lava involved 7 discrete lobes that were emplaced successively.

Basaltic pyroclastic flows of Fuji volcano, Japan: characteristics of the deposits and their origin

Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.

Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska

In the Aleutian volcanic chain (USA), the 2,050±50 bp collapse of Okmok caldera generated pyroclasts that spread over 1,000 km2 on Umnak Island. After expelling up to 0.25 km3 DRE of rhyodacitic Plinian air fall and 0.35 km3 DRE of andesitic phreatomagmatic tephra, the caldera collapsed and produced the 29 km3 DRE Okmok II scoria deposit, which is composed of valley-ponding, poorly sorted, massive facies and over-bank, stratified facies with planar and cross bedding. Geological and sedimentological data suggest that a single density current produced the Okmok II deposits by segregating into a highly concentrated base and an overriding dilute cloud. The dense base deposited massive facies, whereas the dilute cloud sedimented preferentially on hills as stratified deposits. The pyroclastic current spread around Okmok in an axisymmetric fashion, encountering topographic barriers on the southwest, reaching Unalaska Island across an 8-km strait on the east, and reaching the shoreline of Umnak in the other directions. A kinematic model (Burgisser and Bergantz, Earth Planet Sci Lett 202:405–418, 2002) was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography. In the former case, the dense part of the current and the lithics transported by the dilute cloud went underwater. In the latter case, topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking. The resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift-off.

Sedimentation and pedogenic features in a clay deposit in Quadrilátero Ferrífero, Minas Gerais, Brazil

The clay deposit of Caxambu Hill occurs in a NW/SE oriented graben originated by syntectonic sedimentation during Cenozoic. Four facies were identified (fragmentary, nodular, massive and friable) and their differentiation is related to gravity mass-flow processes. The fragmentary facies is composed of extraclast fragments of the local Paleoproterozoic basement and sand size quartz-grains dispersed in a kaolinite-muscovite-goethitehematite matrix. The nodular facies is constituted by lithorelictal and pedorelictal nodules dispersed in a similar matrix as in the fragmentary facies. The massive facies is characterized by quartz grains dispersed in a kaolinite, hematite and goethite matrix with minor amounts of muscovite. The friable facies differs from the massive facies by its channel morphology, higher quartz and kaolinite content and the presence of millimetric clay-balls. The fragmentary facies is considered as deposited during the early stage of opening of the basin. After that, under sub-arid conditions, the slumping of lateritic materials from the surrounding regolith led to the formation of the nodular and massive facies. The friable facies was originated by the action of unidirectional flow that reworking the clayey sediments. The increasing of kaolinite content towards the top is related to the chemical weathering action after the deposition of the sediments.

The Meillers Autunian hydrothermal chalcedony: first evidence of a ∼295 Ma auriferous epithermal sinter in the French Massif Central

The Meillers “quartzite” deposit represents a major hydrothermal siliceous sinter, some 50 m thick and covering an area of some 30 ha within Autunian sandstone of the northern Massif Central. This siliceous sinter comprises three facies of microcrystalline quartz: (i) a dark-colored facies (black chalcedony), locally rich in pyrite, at the base, (ii) a gray-brown fairly massive facies (gray-brown chalcedony) in the middle; and (iii) a white, finely banded, facies (white chalcedony) at the top. Orientation measurements of the banding have revealed the paleo-flow channels of the silica-saturated fluids. The geochemistry of the deposits shows modest metal values, in particular for gold (average of 0.58 g/t Au, giving a gold metal content of some 20 t). The hydrothermal event is stratigraphically well correlated with the basal Autunian (around 295 Ma); an age that has been confirmed through radiochronological determination (300±21 Ma; SHRIMP on hydrothermal zircon). The lead isotopic composition is not very radiogenic ( 206Pb/ 204Pb=18.20) and is similar to that of the Late Hercynian gold lodes in the Massif Central. The mineralized sinter appears to have derived from a geyser-type hot springs. This hydrothermal activity coincides with the auriferous metallogenic peak that occurred in the Hercynides around 300 Ma. This is the first known epithermal-type surface manifestation described for the Hercynian gold event.

Morphology and propagation styles of Miocene submarine basanite lavas at Stanley, northwestern Tasmania, Australia

Miocene submarine basanite pillows, lava lobes, megapillows and sheet lavas in the Stanley Peninsula, northwestern Tasmania, Australia, are well-preserved in three dimensions. The pillows have ropy wrinkles, transverse wrinkles, symmetrical wrinkles, contraction cracks and three types of spreading cracks on their surfaces, and concentric and radial joints in the interior. The lava lobes have ropy wrinkles and contraction cracks on their surfaces. The megapillows are cylindrical with a smoothly curved upper surface and steep sides, and are characterized by distinct radial columnar joints in the interior. They are connected to pillows that propagate radially from its basal margin. The sheet lavas are tabular and have vertical columnar joints in the interior. The largest sheet lava shows a remarkable gradation from a lower 5-m-thick pillow facies to an upper massive facies. The pillows, lava lobes, megapillows and sheet lavas are inferred to have been emplaced completely below sea level but in a shallow marine environment. Their morphological features suggest that the pillows grew by episodic rupture of a near-solid crust and emergence of hot lava, whereas the lava lobes propagated by continuous stretching of the outer skin at the flow front. The megapillows and sheet lavas were master feeder channels by which molten lava was conveyed to the advancing pillows. The sheet lavas propagated by repeated processes of pillow formation and overriding by an upper massive part. Alternating pillow and massive facies commonly found in ocean-floor drill cores and exposed in cross-section in many subaqueous volcanic successions may have formed by propagation of pillows from the basal margins of advancing sheet lavas.

The Quaternary pyroclastic succession of southeast Tenerife, Canary Islands: explosive eruptions, related caldera subsidence, and sector collapse

A much-revised Quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the Bandas del Sur, in southern Tenerife. New 40Ar/39Ar data obtained for the Arico, Granadilla, Fasnia, Poris, La Caleta and Abrigo formations are presented, allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments. We also provide a minimum age of 287±7 ka for a major sector collapse event at the Güimar valley. The Bandas del Sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics, including widespread basal Plinian layers, predominantly phonolite composition, ignimbrites with similar extensive geographic distributions, thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns, lateral gradations into localized massive facies within palaeo-wadis, and widespread lithic breccia layers that probably record caldera-forming eruptions. Each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean. The succession indicates that Las Cañadas volcano underwent a series of major explosive eruptions, each starting with a Plinian phase followed by emplacement of ignimbrites and thin ash layers, some of co-ignimbrite origin. Several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption. Because passage of each pyroclastic density current was characterized by phases of non-deposition and erosion, the entire course of each eruption is incompletely recorded at any one location, accounting for some previously perceived differences between the units. Because each current passed into the ocean, estimating eruption volumes is virtually impossible. Nevertheless, the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the Bandas del Sur succession and probably formed a complex, nested collapse structure. Detailed field relationships show that extensive ignimbrite sheets (e.g. the Arico, Poris and La Caleta formations) relate to previously unrecognized caldera collapse events. We envisage that the evolution of the nested Las Cañadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite-related caldera collapse events, and large sector collapse events, interspersed with edifice-building phases.

Formation and evolution processes of the Salanfe W–Au–As-skarns (Aiguilles Rouges Massif, western Swiss Alps)

At Salanfe (Aiguilles Rouges Massif, Swiss Alps) several stratabound W–Au–As-skarn bodies were formed at the contact between pre-Ordovician marble lenses and paragneiss in close proximity to an anatectic leucogranite of Late Palaeozoic age. Anatexis and leucogranite formation occurred at peak metamorphic conditions of P=0.45 GPa and T~650–700 °C. This metamorphic event, dated at 317 Ma in the adjacent Mont Blanc Massif, was related to dextral transpression following continental collision. Structural evidence indicates that the initial stages of skarn formation also occurred under the above P–T conditions. The main stratiform skarn body (Robert) consists of a massive facies close to the marble and of a banded facies on the paragneiss side. The massive facies resulted from the replacement of former dolomite marble, whereas the banded facies resulted mostly from the replacement of thin carbonate layers intercalated in the paragneiss. In the massive facies, the prograde stage skarn is composed by hedenbergite with increasing Fe concentrations through skarn evolutionary stages, whereas in the banded facies it consists of diopside–plagioclase–quartz. The imprint of the retrograde stage is minor and unevenly distributed, with replacement of the pyroxene by magnetite/haematite–calcite–quartz close to the marble and by chlorite–epidote close to the paragneiss. The economic ore minerals are scheelite, arsenopyrite, loellingite and gold and are mainly associated with the prograde skarn stage. Most of the scheelite mineralization occurs during the early prograde stage, probably at temperatures higher than 600 °C, and is associated with pyroxene (hedenbergite 55). Arsenopyrite and loellingite precipitated after scheelite at temperatures between 600 and 350 °C and are associated with hedenbergite>80. Gold occurs both in the final prograde stage and in the retrograde stage. Minor second generation scheelite is associated with epidote and chlorite of the retrograde stage. Lead isotopes indicate that skarn ore minerals were precipitated by a low Th/U fluid like that in equilibrium with the anatectic leucogranite. Oxygen isotopes support the contention that the prograde skarn was formed by a fluid in equilibrium with the leucogranite, mixed with increasing amounts of CO2 approaching the skarn–marble contact. Oxygen and hydrogen isotopes suggest the intervention of a non-magmatic fluid (meteoric, connate?) during the retrograde stage. The Salanfe skarns were formed at deep crustal levels by fluids in equilibrium with an anatectic leucogranite, possibly channelled along permeable paragneiss during regional-scale transpression, and could be the deep expression of a widespread W–Au hydrothermal ore event that occurred along the European Variscides at 310–305 Ma.

Cretaceous Nonmarine Trace Fossils from the Hasandong and Jinju Formations of the Namhae Area, Kyongsangnamdo, Southeast Korea

Nonmarine strata of the Cretaceous Hasandong and Jinju formations of the Sindong Group of the Namhae area, Kyongsangnamdo, southeast Korea, contain relatively diverse and abundant ichnofaunas including domichnia, fodinichnia, pascichnia, and repichnia. These ichnofaunas are systematically described herein and three ichnocoenoses, namely the Skolithos, Palaeophycus, and Chondrites ichnocoenoses, are defined based on their paleoenvironmental occurrence.The Skolithos ichnocoenose characterizes tabular laminated sandstone facies deposited on fluvial channel levées. It is dominated by infaunally produced ichnotaxa, particularly Skolithos magnus Howell, Planolites montanus Richter and Chondrites isp. type A, with reduced numbers of Skolithos verticalis (Hall) Taenidium barretti (Bradshaw) and rare examples of Circulichnus montanus (Vyalov), Cochlichnus anguineus Hitchcock, Helminthopsis abeli Książkiewicz, Helimthopsis hieroglyphica Wetzel and Bromley, Laevicyclus isp., Planolites beverleyensis (Billings), Skolithos isp., Spirodesmos isp., Thalassinoides suevicus (Rieth), and rhizoliths.The Palaeophycus ichnocoenose characterizes floodplain environments and occurs in purple mudstones and associated massive sandstone facies of crevasse-splay deposits and dark gray to green shale facies of abandoned channels. Palaeophycus tubularis Hall and Planolites montanus Richter are the most characteristic ichnotaxa, while Beaconites coronus (Frey, Pemberton and Fagerstrom), Planolites annularius Walcott, Skolithos magnus Howell, Thalassinoides paradoxicus (Woodward), and sauropod and ornithopod tracks also occur but in reduced numbers.The Chondrites ichnocoenose occurs in dark gray to black mudstones deposited in marginal lacustrine environments. It is dominated by Chondrites isp. type B. with associated Beaconites antarcticus Vyalov, Helminthopsis hieroglyphica, Palaeophycus tubularis, Skolithos magnus, Taenidium barretti, and Torrowangea rosei Webby.The three ichnocoenoses recognized herein cannot be satisfactorily assigned to currently defined nonmarine ichnofacies, suggesting that existing models of such ichnofacies are in need of additional refinement.

Timing of lateritization on the Guiana Shield: synthesis of paleomagnetic results from French Guiana and Suriname

A paleomagnetic study was carried out on representative duricrust paleosurfaces of the Guiana Shield in French Guiana (Cayenne region and Kaw Mountain) and Suriname (Moengo and Bakhuis Mountains). The age of weathering of ferruginous and bauxitic duricrusts was estimated from comparison of paleomagnetic poles determined from these duricrust horizons with the South American apparent polar wander path (APWP). In most of the studied profiles, hematite is the dominant magnetic carrier, as indicated by unblocking temperatures that exceed 580°C for about 20–60% of the natural remanent magnetization (NRM). Magnetite and goethite may also be present and contribute to the overall NRM, but contribute little or nothing to the characteristic remanent magnetization, which is estimated from principal component analysis of the medium to high unblocking temperature component. Paleomagnetic directions from the duricrust profiles generally cluster well (dispersion parameters >30) and give paleomagnetic poles that fall between the Cenozoic and the present-day portion of the APWP. Recent paleomagnetic directions are usually detected in the uppermost part of the duricrust profiles which may have sustained some recent reworking. In contrast, the middle part of the duricrust, of massive facies, usually shows ancient directions. The bauxitic surfaces of Bakhuis Mountains at 400 m elevation, Kaw Mountain (upper surface, 300 m elevation) and probably the Cayenne Unit 1 (75–200 m elevation) can be attributed to the ‘Sul Americano’ lateritization cycle of Paleocene–Eocene age, with respectively average relative ages of 60, 50 and 40 Ma. The bauxitic paleosurface of Kaw Mountain (lower surface, 220 m), the coastal Moengo deposit (40 m) and the ferruginous surface of Cayenne Unit 2 (20–50 m) may be related to the ‘Late Velhas’ lateritization cycle of Miocene age with relative average ages of 13 and 10 Ma. The Fe-duricrust of Cayenne Unit 3, close to sea level and occupying most of the coastal plain, is geomorphologically the youngest duricrust paleosurface, but also seems related to a Late Miocene lateritization event (5–13 Ma). The Oligocene appears to represent a major break in the development of weathering profiles, with erosion as the predominant process which stripped 50–80 m from the landscape between the two main lateritization phases of Paleocene–Eocene and Miocene.

Formation of Subaqueous Felsic Domes and Accompanying Pyroclastic Deposits on the Foça Peninsula (Izmir, Turkey)

In western Anatolia, a thick volcanic succession of andesitic to rhyolitic lavas and volcaniclastic rocks crops out extensively. On Foça Peninsula, the westernmost part of the region, a dominantly rhyolitic sequence is exposed where massive rhyolites occur as dome or domelike stubby lava flows. These rhyolite domes vertically and laterally pass into blanketing volcaniclastic sequences. The gradational boundary relations and the facies characteristics of the surrounding volcaniclastic sequences indicate that the silicic domes directly intruded a subaqueous environment and were shattered upon sudden contact with water to form hyaloclastic blankets. In and around these rhyolite domes, we have defined six different volcanic and volcaniclastic facies, consisting of: (1) massive rhyolite; (2) massive perlite; (3) hyaloclastic breccias; (4) rhyolite pumice and lithic fragment-bearing volcaniclastic rocks; (5) subaqueous welded ignimbrites; and (6) brecciated perlite. The massive rhyolite facies have distinct structures from the centers to the peripheries of the domes and stubby lava flows. Massive lava facies gradually pass into hyaloclastic breccias and massive perlite facies, indicating water-magma interaction during the emplacement. Phreatomagmatic explosive activity and doming caused the subaqueous pyroclastic flows on the flanks of the volcanic center. Welding in the upper parts of these pyroclastic flow deposits indicates the high-temperature emplacement of the pyroclastic material and relatively slow cooling caused by the cushioning effect of the gas-vapor mixture and rapid deposition of younger pyroclastic units.

Holocene glacimarine sedimentation, inner Scoresby Sund, East Greenland: the influence of fast-flowing ice-sheet outlet glaciers

Holocene glacimarine sedimentation associated with fast-flowing outlet glaciers draining the Greenland Ice Sheet is investigated using sedimentary and acoustic data from inner Scoresby Sund, East Greenland. Sedimentation in inner Scoresby Sund is dominated by three processes which are influenced by differences in proximity to fast-flowing outlet glaciers, extent of glacier-ice cover and fjord bathymetry: (1) sediment-gravity flow, principally in the form of turbidity currents and debris flows; (2) suspension sedimentation from turbid meltwater plumes; and (3) iceberg rafting. These processes result in texturally and sedimentologically heterogeneous lithofacies. Proportionally, fine-grained muds (laminated, stratified and massive facies) dominate cores recovered from inner Scoresby Sund, accounting for 80% of the total, whereas diamict facies account for only 15%. Abundant fine-grained muds demonstrate that meltwater flux and sedimentation is significant in this high Arctic glacimarine environment, in settings proximal to fast-flowing outlet glaciers. With increasing distance from these glacier termini, muds are replaced progressively by iceberg-rafted, coarse-grained sediment. The dominance of this iceberg-rafted sediment in outer Scoresby Sund reflects both its more distal location from fast-flowing glacier termini, and the high calving flux associated with these ice masses. Laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner Scoresby Sund are similar to lithofacies produced in temperate and subpolar glacimarine systems. This implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically-, glaciologically- (fast-flowing and non fast-flowing ice-masses) and oceanographically-variable glacimarine settings. Recognition of laminated, fine-grained facies in the geological record therefore does not necessarily indicate a temperate palaeo-glacial setting. However, the predominance of iceberg-rafted diamict facies in ice-distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg-sedimentation played a dominant role.

Seismic stratigraphy of Waterton Lake, a sediment-starved glaciated basin in the Rocky Mountains of Alberta, Canada and Montana, USA

Abstract Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km2) watershed in the eastern Front Ranges of the Rocky Mountains of southern Alberta, Canada and northern Montana, U.S.A. The stratigraphic infill of the lake has been imaged with 123 km of single-channel FM sonar (`chirp') reflection profiles. Offshore sonar data are combined with more than 2.5 km of multi-channel, land-based seismic reflection profiles collected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal reflector (bedrock) that reaches a maximum depth of about 250 m below lake level. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transparent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750–2100 m/s) typical of compact till or outwash. A second stratigraphic succession (SSS II, up to 50 m thick), occurs throughout the lake basin and is characterised by continuous, closely spaced reflectors typical of repetitively bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent seismic facies, up to 5 m thick, are present within SSS II. At the northern end of Upper Waterton Lake, SSS II has a hummocky surface underlain by collapse structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massive facies in SSS II. A thin Holocene succession (SSS III,

Paleomagnetism applied to lateritic profiles to assess saprolite and duricrust formation processes: the example of Mont Baduel profile (French Guiana)

Based on the fact that a chemical remanent magnetization (CRM) can be acquired during the lateritization, the paleomagnetic signal was used to interpret the succession of horizons, from the weathering front to the top of the duricrust surface of a thick bauxitic profile in French Guiana. The 54-m-thick profile is composed of a saprolite capped by a duricrust comprising successively a mottled zone, a massive facies and a pisolitic facies at the top. At the base of the saprolite the natural remanent magnetization (NRM) is composed of a superimposed magnetite and hematite contribution, the magnetite showing an apparently inherited Proterozoic direction. Within the profile, the main magnetic carrier is hematite; goethite does not provide any significant contribution. Five magnetic reversals have been identified within the saprolite and through comparing the reversal sequence with the reference geomagnetic polarity time scale (GPTS) it has been possible to deduce the saprolitization rate; thus the Bruhnes/Matuyama (780 ka) transition is observed at 8.8 m above the weathering front, giving a 11.3±0.5 m/Ma saprolitization rate. The duricrust is significantly divided into two levels within the massive facies; the base of the duricrust shows paleomagnetic directions that fit with the present-day situation, whereas the upper part records directions of the South American apparent pole wander path (APWP) older than 10 Ma. The study shows that hematite precipitation was not uniform within the duricrust during lateritization, and seems to preserve traces of successive and distinct lateritization phases; thus the lower massive duricrust formed during a recent lateritization period, whereas the upper duricrust has preserved a paleomagnetic signal of an older lateritization phase. Our duricrust paleomagnetic interpretation is supported by SEM observations of changes in hematite morphology.

The effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites: the WTT Cupa (Roccamonfina volcano, Italy)

The distribution of lithic clasts within two trachytic, small volume, pumiceous ignimbrites are described from the Quaternary `White Trachytic Tuff Cupa' formation of Roccamonfina volcano, Italy. The ignimbrites show a downslope grading of lithics, with a maximum size where there is a major break in the volcano's slope, rather than at proximal locations. This is also the location where ignimbrites are thickest and most massive. The break in slope is interpreted to have reduced flow capacity and velocity, increasing the sedimentation rate, so that massive ignimbrite formed by hindered settling sedimentation. Ignimbrite Cc, exhibits no vertical grading of lithics, though it does show downslope grading with maximum size at the major break in slope and a rapid decrease further downslope. Ignimbrite Cc thins away from the break in slope, and shows an upward fining of the grain size within the topmost few decimeters of the unit. The ignimbrite is stratified proximally, and grades to massive facies at the break in slope, and distally to stratified facies with numerous inverse-graded beds. The simplest mechanism accounting for these downslope variations is progressive aggradation from a quasi-steady, nonuniform pyroclastic density current. The changes in deposit thickness and facies are interpreted to record downcurrent changes in sedimentation rate. The upward fining reflects waning flow. Inversely graded, bedded depositional facies in distal areas is interpreted to reflect flow unsteadiness and a decrease in suspended sediment load. Ignimbrite Cd shows vertical, as well as downslope grading of lithics. This characteristic, coupled with the widespread massive facies of the deposit and the tabular unit geometry are features that can be reconciled with both the debris flow/plug analogy for pyroclastic flows ( Sparks, 1976) and the progressive aggradation model ( Branney and Kokelaar, 1992). However, none of them appears to satisfy completely the field evidences, implying that when dealing with massive ignimbrites, other evidence than lithic grading needs to be presented to better understand the related transport and depositional processes.

The Middle Jurassic Oseberg Delta, Northern North Sea:  A Sedimentological and Sequence Stratigraphic Interpretation

The Aalenian Oseberg Formation (0-80 m thick) is an important reservoir unit in the Middle Jurassic Brent Group in the northern North Sea, consisting of multiple sets of sandy Gilbert-type deltas. Small-scale (1.5-10 m) fining-upward units seen in the gamma-ray log correspond with individual delta sets, as independently confirmed by steepening-upward trends seen in the dip log. Within each set, the steep foreset slopes typically show thinly bedded sandstone facies (avalanche grain flows), whereas the lower foreset slopes, toesets, and bottomsets are formed largely by massive sandstone facies (sandy debris flows). On an intermediate scale (up to 40 m), the gamma-ray logs show both fining-upward and coarsening-upward trends through stacked delta sets, and these trends, traceable between wells, are interpreted in terms of decelerating and accelerating rates of relative sea level rise, respectively. The relative abundance of the sandy debris-flow deposits reflects a periodic and significant instability of the delta's upper foreset slope, probably during times of increased water depth in front of the delta. The normal progradation of individual Gilbert-type sets, however, is likely to have been along a subhorizontal topography during periods of little or no change in water depth. The long-term change to produce the observed vertical stacking of deltaic sets was one of a generally rising relative sea level. Modeling of the sea level rise in a steplike manner can account for the rarity of topsets. The progressive eastward truncation of the underlying Drake Formation on the Horda Platform by the Oseberg Formation suggests tectonic uplift and a lowstand of sea level immediately prior to the development of the Oseberg deltas. The Oseberg Formation is thus seen as a lowstand prograding wedge that developed during an interval of relatively high sediment supply and a variably rising sea level, culminating in the latest Aalenian flooding seen in the base of the overlying Rannoch Formation. The dip-log data, integrated with cores and other logs, are critical to correctly interpreting the reservoir's internal architecture. The slope-readjustment model predicts that the development of stacked massive reservoir sandstones of Oseberg type will occur preferentially in local areas of high subsidence or syndepositional faulting, or where there is deeper water in the lee of already existing fault scarps.