Pyroclastic Sediments Research Articles

Cenozoic stratigraphy and sedimentation history of the northern Philippine Sea based on multichannel seismic reflection data

Abstract To clarify the regional distribution and characteristics of the sedimentary deposits in the northern part of the Philippine Sea, multichannel seismic reflection surveys of 26 864 km in total length were performed. The seismic reflection data were interpreted and correlated with available Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) data and a general stratigraphic framework of the area was established. The sedimentary deposits in this area were divided into five layers; Units I, II, III, IV and V in ascending order. Their approximate geological ages are the Early Eocene, Middle to Late Eocene, Oligocene, Miocene and Pliocene‐Pleistocene, respectively. Seismic records were classified into three seismic facies, Facies A, B and C, on the basis of their characteristics. They were judged to represent pelagic and hemipelagic sediments of non‐volcanic origin, fine pyroclastic sediments and coarse pyroclastic or volcanic sediments, respectively, by comparing them with lithological data in the DSDP/ODP holes. From the thickness and facies distributions of these sediments, a sedimentary history in the area was reconstructed as follows. The oldest sediments in the study area, Unit I, interfinger with some parts of the Daito Ridge (acoustic basement) in the Minami Daito Basin. The geological age of the unit is estimated by microfossils in the sediment and supports the idea that this part of the Daito Ridge is composed of the Early Eocene oceanic basalt. Later, a fair amount of sediments were deposited in the Minami Daito Basin in the Middle to Late Eocene age. A large volume of volcanic materials was supplied from the Paleo‐Kyushu‐Palau Ridge in the Kita Daito Basin in the Eocene and Oligocene ages. The eastern part of the Shikoku and Parece Vela basins is characterized by volcanic sediments supplied from the Nishi Shichito and West Mariana Ridges in the Miocene age. However, pelagic and hemipelagic sediments prevail in the northern part of the Shikoku Basin in the Miocene or later. In short, the area of principal sedimentation has generally shifted from west to east through geological time, reflecting the evolution of the island arc systems with the same trend in the northern Philippine Sea.

Change of Moisture and Suction Properties of Volcanic Sand Induced by Shaking Disturbance

Mudflow type failure of a gentle fill slope occurred at Dateshita, Tsukidate town during the 2003 Sanriku-Minami earthquake in Japan. The flowed fill material was pyroclastic sediment that was classified as a volcanic sandy soil with pumice. This study conducted laboratory shaking table tests to examine the change of the water retention nature that is attributable to vibration of this volcanic sand under an unsaturated condition. An electrical moisture sensor measured the change of the apparent volumetric water content of the soil mass with various water contents during and after shaking. The apparent volumetric water content was increased by shaking under the condition of a certain extent of water content. The extent of the water content corresponded to an in-situ one. Suction changes of the soil mass as a consequence of shaking were also tested from various initial suction states for both the drying and wetting processes. The suction change caused by shaking disturbance was observed. The suction change direction depends on the initial suction state. It is considered that the observed nature in two tests results, which is attributable to the peculiarity of the volcanic sandy soil with pumice, may be related to mudflow-type slope failure.

Half graben filling processes in the early phase of continental rifting: The Miocene Namurungule Formation of the Kenya Rift

The Miocene Namurungule Formation crops out on the eastern flank of the Kenya Rift, representing basin fill that developed in association with Miocene rifting. The formation is characterized by large volumes of volcaniclastics supplied passively by pyroclastic fall. Facies analysis reveals that this formation consists mostly of lacustrine delta deposits with minor alluvial fan deposits at its base. The conspicuous occurrence of a flood plain facies with infrequent channel fill deposits in the lower part of this formation suggest that the drainage area was limited in the early stage of deposition. Pyroclastic fall would therefore have been an important source of sediment during the early stages of rift development. The delta deposits are divided into two distinct successions based on lithological characteristics, separated by a thick pyroclastic layer in the middle part of the formation. The stacking pattern of the lower succession is retrogradational, whereas the upper succession is characterized by a pile of prograding bodies. Both delta successions are interpreted to have accumulated in an underfilled basin. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Assuming constant sediment supply, the apparent difference in flooding scale between the lower and upper successions is attributed to the topographical widening of the basin flat. Similar successions also occur in another basin in the East African Rift (Ngorora Formation, central Kenya), in which pyroclastic sediments are dominant and upward decrease in sedimentation rate is recognized. The differing in the stacking patterns between the Namurungule Formation and the Ngorora formation is probably induced by basin width. The narrow Namurungule basin seems to have been sensitive to increased sediment supply owing to the expansion of the catchment area.

Landslides During the Earthquakes on May 26 and July 26, 2003 in Miyagi, Japan

We conducted a site investigation for two major landslides : Dateshita landslide, Tsukidate-cho during the earthquake on May 26, 2003; and Nishisaruta landslide, Kanan-cho during the earthquake on July 26, 2003. In addition, we examined physical and mechanical soil properties and performed preliminary numerical simulations. The subsurface soil of the gentle slope of Dateshita landslide with an angle of about 7° was a fill with pyroclastic sediments. The structure of the fill was very loose, but the unsaturated soil remained stable with high suction. The landslide occurred during or immediately after the principle motion of the earthquake. The slide mass behaved as a mudflow. Then the collapsed soil easily fluidized with cyclic shear. It is likely that the saturated fill liquefied during the earthquake. Moreover, it is possible that the unsaturated fill fluidized, losing the initial shear strength during cyclic shear induced by the earthquake. The Nishisaruta landslide with similar magnitude and configuration to the Dateshita landslide occurred a few minutes after the principle motion of the main shock. Rainfall was an important feature that exacerbated the Nishisaruta landslide, whereas no rainfall had been observed for a week before the Dateshita landslide. The subsurface soil of slope of the Nishisaruta landslide was a fill with fine-graded sand, which originated from sandstone on the hill. The upper portion of the slope that lost shear strength because of liquefaction descended along the slope, and spread with high water content on the lower rice field. Numerical simulations have suggested that the saturated fill liquefied during the main shock. Residual excess pore pressure induced by the foreshock affected the slope's stability.

Tectonostratigraphy of the Khoy Complex, northwestern Iran

Previous studies suggested that only one ophiolite, the “Khoy ophiolite", existed near Khoy, northwestern Iran. This thesis is no longer tenable. Combined investigations (biostratigraphic, chronostratigraphic, geochonometric, geochronologic, and geochemical) demonstrate that there are at least two and perhaps three ophiolite remnants in the Khoy area: (1) A Late Jurassic (early to middle Oxfordian: 156 Ma to 159 Ma 40Ar-39Ar on gabbro) remnant; (2) A Late Cretaceous (early Coniacian: Radiolaria) remnant (~N-MORB geochemistry); and, possibly, (3) A Late Cretaceous (latest Campanian) remnant (E-MORB geochemistry). Because it is impossible to use the term “Khoy ophiolite" in this report, we refer the ophiolitic rocks in the Khoy area to the “Khoy Complex” (sensu International Stratigraphic Guide). The sedimentary contact between Late Cretaceous (early Coniacian) red manganiferous ribbon chert lacking calc-alkaline volcanic contributions and overlying pyroclastics (tuff and tuff breccia) in the far northwestern part of the Khoy complex is of great tectonostratigraphic significance. This interface represents a sudden change from pelagic to pyroclastic sedimentation. Field evidence indicates that the contact is disconformable and is associated with a hiatus of unknown magnitude. Red ribbon chert (lacking calcalkaline contributions) in the same area overlies and is interbedded with N-MORB pillow basalt; early Coniacian Radiolaria were recovered from interpillow siliceous mudstone. We postulate that by the early Coniacian oceanic crust (covered with a veneer of Radiolarian ooze) had moved close enough to an island arc system to receive calc-alkaline pyroclastics. Tectonic melange in the Khoy Complex represents a subduction complex probably associated with the island arc noted above. Micrite (pelagic limestone) knockers in the tectonic melange belt contain Early Cretaceous (late Albian: Vraconian) planktonic foraminifera; Late Cretaceous (early Cenomanian) Radiolaria; Late Cretaceous (early Campanian to early Maastrichtian) planktonic foraminifera; Late Cretaceous (late Maastrichtian) planktonic foraminifera; and early Middle Eocene planktonic foraminifera. The age of the micrite knockers in the tectonic melange, suggests that subduction associated with island arc volcanism continued from the Early Cretaceous (latest Albian) to the Early Tertiary (early middle Eocene).

Formation of analcime and moganite at Lake Lewis, central Australia: significance of groundwater evolution in diagenesis

Abstract Lake Lewis basin in the Northern Territory is a hydrologically closed intermontane basin in which clay-rich palaeolacustrine sediments are infused with highly evolved brines. Acquisition of dissolved silica and sodium by groundwaters from the Proterozoic Arunta granite-gneiss terrain, and from arkosic alluvium and weathering products sourced from these bedrocks, is important to diagenesis at the salt lake. Supersaturation of groundwaters with respect to SiO 2 and the availability of catalytic ions, coupled with high rates of evaporation, favour neoformation of opal and chalcedonic silica from solution near the groundwater discharge zone. Moganite, a distinctive silica polymorph that is commonly associated with evaporites, has been identified amongst precipitated silica species. Down-gradient from the zone of silica precipitation, beneath the playa, brines are SiO 2 -deficient. Long periods of weathering and diagenesis of lacustrine clays immersed in semi-stagnant brines has transformed detrital minerals into substantial proportions of amorphous or gelatinous aluminosilicate material. Aluminosilicate phases have become metastable in the presence of evolved SiO 2 -deficient interstitial brine. Analcime, a sodium-bearing zeolite group mineral, is stable in the presence of the Na-rich brine and is crystallizing authigenically below the watertable from the amorphous aluminosilicate material in the lacustrine sediment. No volcanic rocks or pyroclastic sediments are involved in this diagenetic occurrence of analcime. The appearance of analcime in the evolution of the salt lake system probably commenced within the last 100 000 years and is occurring in the present-day hydrologic and climatic regime of this arid zone area.

Combarbalá advanced argillic alteration zone, Chile: geology, geochemistry, mineralogy and mineralization potential

Central and northern Chile hosts several well-known epithermal precious-metal deposits associated with kaolinite-alunite-quartz alteration, but only Combarbalá district (Coquimbo region) is the altered rock extracted for ornamental purposes. This ornamental rock, called combarbalita is mined from pockets and veins hosted by altered, sub-horizontal, Cretaceous age that crop out around the town of Combarbalá. A study has been undertaken to define the conditions required for combarbalite formation and to evaluate, by analogy with Cu-Au and Cu-Pb deposits in adjacent districts, the significance of silicified bodies within the advanced argillic alteration as possible indicators of metalliferous mineral concentrations in the underlying propylitic structural analysis, microscopy, X-ray diffraction and chemical analysis were the principal methods used.The pyroclastic sediments that host combarbalite are or combarbalite, pyroclastic sedimentary strata of andesites. Field observation, crosscut by cylindrical, massive silica bodies interpreted as vents and considered to be the principal conduits for hydrothermal fluids. The silica bodies in the contain Cu and Pb minerals, have anomalous contents of As, Sb, Hg, Ag and Au and are cut by barite veins. Regional structural control for the silicic and advanced argillic alteration was exerted by faults orientated N 60° W, obliquely to the present north-south graben structure in which the district is located. Malachite and cerussite mineralization roughly follows these northwest-trending fractures, whereas fracture-bound chrysocolla, pyrite and chalcopyrite are found in the underlying propylitic alteration. Sulphide mineral concentrations are likely to be located close to the intersection of the vent structures with the propylitic alteration. As to the existing mining activity, the best, massive varieties of combarbalite are developed in conglomerates, where very fine-grained mineralogy gives the rock workability and preserves its attractive original textures.

Diagenesis of Dioctahedral and Trioctahedral Smectites from Alternating Beds in Miocene to Pleistocene Rocks of the Niigata Basin, Japan

AbstractClay mineral diagenesis in the Niigata basin is documented by mineralogical and chemical analysis of clay minerals from cuttings from the Shinkumoide SK-1D (SSK-1D) well which is characterized by alternating beds containing dioctahedral and trioctahedral smectite minerals with increasing depth. Dioctahedral smectite shows a progressive increase in illite interstratification with increasing depth. The transition of dioctahedral smectite to interstratified illite-smectite (I-S) is supported chemically by an increase in K and Al and a decrease in Si with increasing depth. In contrast, trioctahedral smectite (saponite) reacts to form a 1:1 interstratified chlorite-smectite (C-S) with increasing burial depth and temperature. Considering the geology and the occurrence of smectite, the SSK-1D smectites probably altered diagenetically from two different parent materials: dioctahedral smectite is derived from clastic sediments and transforms to interstratified illite-smectite, whereas trioctahedral smectite is derived from andesitic pyroclastic rocks and transforms to interstratified chlorite-smectite.The C-S occurs at the same depth of ~3200 m as the conversion of randomly interstratified (R = 0) I-S to (R = 1) I-S. Furthermore, the depth is compatible with a Tmax temperature of 430-435°C, which indicates the starting temperature for oil generation from organic matter. The temperature of the conversion of (R = 0) I-S to (R = 1) I-S and the start of corrensite formation is estimated at 110-120°C based on the time-temperature model suggested by others. The clay-mineral diagenesis in the SSK-1D further suggests that I-S and C-S can act as geothermometers in clastic and pyroclastic sediments provided that the effect of time is considered.

Lapilli tuff as evidence of Early Jurassic Strombolian-type volcanism in Scania, southern Sweden

Central Scania, southernmost Sweden, is penetrated by more than one hundred basaltic volcanic necks of Jurassic age. Pyroclastics, related to the volcanic necks, have been investigated to reveal the volcanic environment, and the eruptive character required to produce the pyroclastics. Based on textural and petrographical evidence, a Strombolian-type eruptive character is suggested. The sediment, a lapilli tuff, is clay altered due to early diagenetic processes, but traces of palagonite reveal a former basaltic composition of the lapilli tuff. The volcanoes were part of a monogenetic volcanic field and deposition of the pyroclastic sediment took place on land, probably close to oceanic water.

Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin

Abstract The Ichinohe basin in the western margin of the Kitakami Massif, northeast Japan, consists of the Early to Middle Miocene Yotsuyaku and Keiseitoge formations and represents the incipient stage of Neogene back-arc sedimentation that extends toward the north and northwest. Megaclast breccias derived from Mesozoic basement rocks of the Kitakami Massif were transported from southern areas as debris flows. The debris-flow deposits of the Yotsuyaku Formation grade northwards along the basin axis into clast-supported, braided-stream deposits, where floodplains developed. In the center of the basin voluminous volcanic eruptives of the Keiseitoge Formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries. The deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution. Parallel-laminated, fine-grained sediments intercalated with lignite beds suggest lacustrine deposition, and large cross-beds of volcanic breccias indicate Gilbert-type progradation into the lakes. A number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non-volcanic debris flows derived from the southern uplift. Damming episodes produced by thick lava flows were not recognized in this study. The evolution of the N–S trending, longitudinal Ichinohe Basin indicates that E–W extensional tectonism related to the opening of the Sea of Japan affected not only the Green Tuff Region, to the west of the studied basin, but also even the Mesozoic basement during the Miocene. Although the mingling of volcanic and basement-derived deposits indicates the complexities of sedimentary facies and provenances, our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems.

Preliminary paleomagnetic results from the Upper Carboniferous of Uliastai Block, Inner Mongolia, China

Paleomagnetic samples from five sites from the Upper Carboniferous Baoligemiao Formation of the Uliastai block (117°10′E, 45°30′N) preserve stable normal and reversed magnetizations, with mean D=138.3°, I=−47.8°, a95=9.7°, k=9.9. IRM acquisition and thermal demagnetization of IRM indicate that the magnetization in these pyroclastic sediments is predominantly carried by magnetite, however, higher coercivity minerals contribute to the remnant magnetization as well. The pole position (53.4°N, 14.8°E) derived from these results is near those of Sino‐Korea and central Mongolia but significantly different from that of Siberia, suggesting that the Uliastai was not a Siberian‐related fragment as traditionally proposed. Instead the block, along with the Mongolia and Sonid blocks, may have moved as a series of north‐facing arc terranes (frontal and remnant arcs) of the Sino‐Korean continent during the Late Paleozoic, possibly an ancient analogue of the Southeast Asian archipelago today.

How Has Revegetation Changed Surface Runoff and Sediment Discharge on Pyroclastic Sediments in Unzen Volcano?

火砕流堆積によって荒廃した雲仙普賢岳山腹斜面において, 航空緑化工による斜面安定化が図られている.その効果を調べるために, 緑化工によって草地化した斜面と裸地状態の斜面で, 降雨時の表面流流出と土砂流出を観測した.5回の降雨について観測した結果, 緑化された斜面は裸地斜面に比べて, 表面流の発生量が1/3程度, 流出土砂濃度が1/2以下であり, 従って土砂生産量は1/6以下であることが示唆された.

Early angiosperm fossil leaves in Chubut Group, Cretaceous, Argentina

The oldest angiosperm leaves from Patagonia were reported from Baquero Formation (Barremian-Aptian) in a mesofitic flora of more than one hundred gymnosperm and fern species. The next younger records are from the Chubut Group, a thick assemblage of pyroclastic sediments, with sand and conglomerate facies and frequent paleosols. In the Sierra de San Bernardo area sediments are mainly tufaceous and fluvial, deposited under an arid climate in shallow lakes and swamps of the alluvial plains. The sea was hundreds of kilometers to the West. Four Formations were described: Matasiete (Aptian), Castillo and Bajo Barreal (Senonian) and Laguna Palacios (Campanian-Maastrichtian). We explored in the Senonian Formations, and studied a few outcrops that can be sorted as:a) Monospecific, with only Onychiopsis sp., in palustral sediments. b) Strongly dominated by Eauisetum sp, and few remains of two dicot species with large, entire margined, low rank leaves, in palustral sediments. c) Strongly dominated by two species of angiosperms with small leaves, associated with remains of conifers in tufaceous beds deposited in the alluvial plain. d) Dominated by a few species of dicots, with medium size, entire margined leaves. Also with about 10 species of ferns, gymnosperms and lobate angiosperms. They are in alluvial plains sediments. e) More balanced associations, with several abundant species, including angiosperms with lobate and leaves. They are in fluvial sediments.Although angiosperm radiation and taphonomy undoubtedly accounts for differences of plant composition between outcrops, it seems apparent that the record of early angiosperms in Chubut Group is represented by remains of many different plant associations, with few species, adapted to different environments, that probably coexisted, and developed under an arid climate, far from the sea.

The influence of ephemeral processes on pyroclastic sedimentation in a rift-basin, volcaniclastic-alluvial sequence, Española basin, New Mexico

The Plio-Pleistocene Puye Formation, north-central New Mexico, is a coarse-grained conglomeratic wedge of primary and reworked volcanic deposits that prograded eastward into an actively aggrading floodbasin of the central Rio Grande rift. This alluvial system grew in response to the degradation of dacitic and rhyolitic lava domes in the northeastern part of the Jemez Mountains volcanic field. Analysis of Puye deposits shows that fan sedimentation was governed by the complex interaction of explosive volcanism, rift-basin tectonics, and the highly variable localized ephemeral processes common to arid-region alluvial systems. Preserved in this well exposed sequence are complex associations between primary pyroclastic facies related to explosive eruptions from lava domes, and reworked tuffs that record the rapid emplacement of poorly consolidated pumice and ash. The sporadic availability of water in this semi-arid region, and the episodic flood frequency and runoff that accompanied individual eruptions, largely controlled Puye facies characteristics. Important among the combined influences of explosive eruptions and ephemeral processes were: (1) fluvial transport and reworking of pumice sequences, (2) remobilization of loosely consolidated volcanic talus and tuffs by mass flow, and (3) marked downslope changes in mass-flow deposits and fallout tephra emplaced into lakes.

Location of possibly productive geothermal fracture zones/faults using integrated geophysicals methods over lesvos island geothermal field, Greece

In 1988, Geotest Hellas carried out a geophysical survey on Lesvos island using gravity, vertical electrical soundings and self-potential methods in order to study the geothermal potential of the island. This paper is based on the primary results presented by Geotest Hellas, which were re-examined and presented in a different form to facilitate the selection of the areas of strong geothermal interest. In particular, application of the gravity, vertical electrical sounding (VES) and self-potential (SP) methods has provided information on the geophysical structure of the geothermal target areas of Lesvos island. The gravity survey provided valuable information on the morphology and tectonics of the crystalline basement (and on the peridotitic body and pyroclastic sediments), for the study of the geothermal potential of the area. The electrical survey provided information on the highly conductive zones (usually associated with geothermal fields), the electrical stratigraphy and the tectonics of the main geological units of the target area. The SP survey mapped areas where electrical polarization is observed in the faulting system. An integrated interpretation of the results of all these surveys led to the selection of five target areas considered to be of high geothermal potential. The location of these target areas is shown in Fig. 14, ranked from 1 to 5 in decreasing order of priority.

Discrimination of eolian and pyroclastic-surge processes in the generation of cross-bedded tuffs, Jemez Mountains volcanic field, New Mexico

Discrimination of pyroclastic deposits emplaced by volcanic processes and reworked pyroclastic sediments is a primary objective of studies in volcaniclastic strata. Cross-bedded tuffs of both eolian and pyroclastic-surge origins are identified within the late Miocene Peralta Tuff Member of the Bearhead Rhyolite, in north-central New Mexico. Distribution, paleoflow directions, grain-size characteristics, and depositional structures permit discrimination of these two similar facies. Eolian tuffs exhibit structures diagnostic of eolian sands, are relatively well sorted, and lack both the fine ash and coarse lapilli and bombs typically found in surge deposits. Discrimination between eolian reworked and primary surge pyroclastic deposits is critical for volcanic hazard studies near modern volcanoes and for correct interpretation of proximity to vents in ancient volcaniclastic sequences.

Palaeoceanography and tectonic setting of the Jurassic Coast Range ophiolite, central California: evidence from the extrusive rocks and the volcaniclastic sediment cover

The primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis. Late Jurassic ( c. 169-161 Ma) Coast Range ophiolite remnants are exposed E and W of the San Andreas Fault in central California. Ophiolite volcanic rocks are mainly sub-alkaline tholeiitic basalts, but include subordinate intermediate and silicic flows and breccias. Available ‘immobile’ major and trace element data indicate that the Coast Range ophiolite formed above a subduction zone, possibly in a fore-arc setting, as suggested by the presence of high magnesian andesites and boninites. The ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault-controlled ocean floor. Interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone. Widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water. Local interbeds of arc-derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting. Depositionally overlying deep-water successions up to 600 m thick comprise radiolarian-rich tuffaceous and volcaniclastic sediments of ?Callovian-mid-Tithonian age (165-148 Ma). Depositional processes included airfall, turbidity currents, mass flow and current reworking. Small SiFeMn lenses along the lava-sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation. At one locality (Llanada), the ophiolite is overlain by up to 1.8 km of younger (?Tithonian) basic-intermed, and acid lava flows, submarine lahars and mainly silicic pyroclastic sediments, of inferred volcanic arc origin. The volcaniclastic cover passes conformably upwards into late Tithonian and younger terrigenous deep-water clastic sediments of the Great Valley Group, deposited in a fore-arc setting. The Coast Range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc, possibly developed above an eastward (continent-ward)-dipping subduction zone within the equatorial Pacific Ocean. In this scenario, short-lived (5–8 Ma) supra-subduction zone spreading to form the ophiolite was followed by ?Callovian-Tithonian arc volcanism, shedding tuffaceous debris onto the adjacent fore-arc. Subduction was active separately along the western American continental margin with the result that the Coast Range arc-ophiolite complex eventually collided with North America, triggering the Late Jurassic Nevadan orogeny and initiating Franciscan subduction-accretion. The Coast Range ophiolite remnants, at least those west of the San Andreas Fault, were apparently translated northwards to their present positions after Late Cretaceous.

Fluidization pipes in volcaniclastic mass flows, Volcan Hudson, Southern Chile

ABSTRACTThe presence of fluidization pipes within pyroclastic sediments has been proposed as a criterion for differentiating these volcaniclastic deposits from those of epiclastic mass flows. Observations of pipe structures within low‐temperature epiclastic mudflows produced by the 1971 eruption of Volcan Hudson, Southern Chile, demonstrate that this assumption is invalid and that the presence of pipes may only be utilized to interpret the precise mechanics of the fluidization process itself.

Petrography, inorganic and organic geochemistry of Lower Permian carbonaceous fan sequences (“Brandschiefer Series”) — Federal Republic of Germany: Constraints to their paleogeography and assessment of their source rock potential

Lower Permian clastic and volcaniclastic sediments from the western edge of the Bohemian Massif (northern Bavaria, F.R.G.) were studied petrographically and chemically to decipher their complex history of deposition, their origin of elements (C specifically) and to assess their economic potential (hydrocarbons, Ba, U). The deposit is an alluvial fan sequence, among which pyroclastic sediments are intercalated. Chemical discrimination plots allow these acid to intermediate tuffs to be assigned to the calc-alkaline clan. These volcanites and the bimodal clastic sediments plead for the assumption of an active continental margin. Zr, Ba, Th, Ce and P are delivered by volcanic activity into these basins, while the organic matter originated from plant debris. Postsedimentary redeposition led to the removal of elements from the basinal fine-grained sediments and trapping of Ba, Na and hydrocarbons into the more porous mid-fan sections. The organic matter plays a minor role in U fixation, since it had already lost its sorption capacity at the time of basin evolution. S, C and Fe plots as well as the use of the DOP (=degree of pyritization) led to the conclusion that this pyrite in the more central parts has been syngenetically precipitated under iron-limited conditions. This environment may be considered as an area of U preconcentration and Ba mineralization. Moreover, it shows a good sourcerock potential, mainly for gas.

Pliocene chelonian fossil eggs from Gran Canaria, Canary Islands

ABSTRACT Thirty years ago Macau-Vilar (1958) reported chelonian eggs from Gran Canaria. Analysis of the microstructure of these eggs corroborates this identification. Field studies show that these eggs, which were originally thought to be Miocene in age, are from Pliocene pyroclastic sediments about 3.5–4.5 my old.