Block Facies Research Articles

Simultaneous fall and flow during pyroclastic eruptions: A novel proximal hybrid facies

Abstract The deposits of Plinian and subplinian eruptions provide critical insights into past volcanic events and inform numerical models that aim to mitigate against future hazards. However, pyroclastic deposits are often considered from either a fallout or pyroclastic density current (PDC) perspective, with little attention given to facies exhibiting characteristics of both processes. Such hybrid units may be created where fallout and PDCs act simultaneously, where a transitional phase between the two occurs, and/or due to reworking. This study presents analysis of a novel hybrid pyroclastic lithofacies found on Tenerife (Canary Islands) and Pantelleria (Italy). The coarse pumice block facies has an openwork texture and correlates with distal Plinian units, but it is cross-stratified and relatively poorly sorted with an erosional base. The facies is proposed to record the simultaneous interaction of very proximal fallout and turbulent PDCs, and it reveals a fuller spectrum of hybrid deposition than previously reported. This work highlights the importance of recognizing hybrid deposition both in the rock record and in hazard modeling.

Nyexon Rock Avalanches: A Special Intrusion Restraint Mechanism, Tibet, China

The Nyexon Rock Avalanches in the southern Qinghai-Tibet Plateau is a huge scale earthquake-induced slope disaster in the Holocene, the accumulation area has distinct sedimentological characteristics, which is of great significance for studying the intrusion and restraint mechanism during long-distance transportation of large rock avalanches or debris avalanche. This long-distance transportation induced a series of landform types, such as ridges, hills, and ravines; they are widely distributed in all areas and extensively developed shear zones, jigsaw cracks, and other structures within the sedimentary structure. With the analysis of DEM data and geological survey, two main types of basement structures and their transition relationships are distinguished; they play an essential role in the restraining bottom during rock avalanches. In the sedimentary structure, the block facies and mixing facies occupy the main body of the deposition from the center to the distal area. Under the basement restriction, mixing facies are formed between the bottom of the sedimentary layer and the basement sedimentary structure; the shear band is mainly developed along with the mixing facies and basement facies, which is accompanied by basement liquefaction and rheology. A sedimentary facies model is established based on the sedimentary structure sequence of the Nyexon Rock Avalanches transportation. After analyzing the failure mechanism of the rock avalanches, it is believed that in the initial stage of failure, the rock avalanches is transformed into a particle flow that is similar to the debris avalanche, which is restrained by the basement structure and lateral bound; then, an accumulated obstacle highland is formed in the central area after deceleration, making the transportation of the main fluid to deflect quickly.

Geological and geotechnical characterization of the debris avalanche and pyroclastic deposits of Cotopaxi Volcano (Ecuador). A contribute to instability-related hazard studies

The huge volcanic debris avalanche occurred at 4.5ka is a major event in the evolution of the Cotopaxi volcano, Ecuador. The present volcanic hazard in the Cotopaxi region is related to lahars generated by volcanic eruptions and concurrent ice melting. This paper presents the geological and geotechnical field and laboratory characterization of the 4.5ka Cotopaxi debris avalanche deposit and of the younger unconsolidated pyroclastic deposits, representing the probable source of future shallow landslides. The debris avalanche formed a deposit with a well-developed hummocky topography, and climbed a difference in height of about 260m along the slopes of the adjacent Sincholagua volcano. The debris avalanche deposit includes four lithofacies (megablock, block, mixed, and sheared facies) that represent different flow regimes and degrees of substratum involvement. The facies distribution suggests that, in the proximal area, the debris avalanche slid predominantly confined to the valleys along the N and NE flank of the volcanic cone, emplacing a stack of megablocks. When the flow reached the break in slope at the base of the edifice, it became unconfined and spread laterally over most of the area of the Rio Pita valley. A dynamic block fragmentation and dilation occurred during the debris avalanche transport, emplacing the block facies. The incorporation of the older Chalupas Ignimbrite is responsible for the mixed facies and the sheared facies. Geotechnical results include a full-range grain size characterization, which enabled to make broader considerations on possible variability among the sampled facies. Consolidated drained triaxial compression tests, carried out on the fine fraction <4.76mm, point out that shear strength for cohesionless sandy materials is only due to effective friction angle, and show a quite homogeneous behaviour over the set of tested samples.The investigated post-4.5 pyroclastic deposits constitute a 5–12m thick sequence of poorly consolidated materials that are interlayered with lava flows. Their geotechnical analyses have evidenced a strong variability in grain size distribution, reflecting the depositional processes, and a generally high porosity. Consolidated drained triaxial compression tests delineated a similar shear stress-strain behaviour among the different units, where shear strength is only due to friction angle. Failure surfaces are always well developed, indicating that the poorly consolidated pyroclastic cover could undergo failure leading to the formation of a gravity driven instability phenomena, like granular or debris flows, which are mainly controlled by the fine fraction.This work underlies the general necessity for a site-specific, and interdisciplinary approach in the characterization of volcanic successions to provide reliable data for gravitational instability studies.

Submarine landslide deposits of the historical lateral collapse of Ritter Island, Papua New Guinea

The March 13th 1888 collapse of Ritter Island in Papua New Guinea is the largest known sector collapse of an island volcano in historical times. One single event removed most of the island and its western submarine flank, and produced a landslide deposit that extends at least 70 km from the headwall of the collapse scar. We have mapped and described the deposits of the debris avalanche left by the collapse using full-coverage multibeam bathymetry, side-scan sonar backscatter intensity mapping, chirp seismic-reflection profiles, TowCam photographs of the seafloor and samples from a single dredge. Applying concepts originally developed on the 1980 Mount St. Helens collapse landslide deposits, we find that the Ritter landslide deposits show three distinct morphological facies: large block debris avalanche, matrix-rich debris avalanche and distal debris flow facies. Restoring the island's land and submarine topography we obtained a volume of 4.2 km3 for the initial collapse, about 75% of which is now forming the large block facies at distances less than 12 km from the collapse scar. The matrix-rich facies volume is unknown, but large scale erosion of the marine sediment substrate yielded a minimum total volume of 6.4 km3 in the distal debris flow and/or turbidite deposits, highlighting the efficiency of substrate erosion during the later history of the landslide movement. Although studying submarine landslide deposits we can never have the same confidence that subaerial observations provide, our analysis shows that well-exposed submarine landslide deposits can be interpreted in a similar way to subaerial volcano collapse deposits, and that they can in turn be used to interpret older, incompletely exposed submarine landslide deposits. Studying the deposits from a facies perspective provides the basis for reconstructing the kinematics of a collapse event landslide; understanding the mechanisms involved in its movement and deposition; and so providing key inputs to tsunami models.

Volcanic facies of the debris avalanche deposit of Tata Sabaya Volcano, Central Andes.

Debris avalanches associated with partial sector collapse of volcanic edifices are common phenomena in the evolution of a volcano. These flows are formed by edifice instability, which could be due to several factors such as the presence of hydrothermally altered areas, climatic changes, earthquakes, shallow magma intrusions (cryptodomes and/or dykes) and/or fault activity beneath or close to the volcano. The final product of this avalanche flow, known as debris avalanche deposit (DAD) shows typical hummocky and closed drainage morphologies. More than 14 volcanic centers of the Central Andes have volcanic DAD&rsquo;s, being Tata Sabaya (Bolivia) one of them. The collapse that has originated the deposit could have been generated by a combination of magmatic and seismic activity in the volcano. The deposit associated to the partial collapse of Tata Sabaya volcano is distributed on its lower southern flank and partially fills the north-western part of the Salar de Coipasa basin. The deposit covers a minimum area of 230 km2 and has an estimated minimum volume of 6&plusmn;1 km3. The deposit is formed by 6 different types of hummocks, according to their compositions: lavic, pyroclastic, sedimentary, mixed, pyroclastic breccia and basaltic-andesitic hummocks. Based on the predominant hummock type and the spatial distribution, the deposit has been divided into 6 different facies (Toreva Block Facies, Volcanic Hummocks Facies, Central Facies, Sedimentary Hummocks Facies, Mixed Facies and Pyroclastic Breccia Hummocks Facies). Taking into account the facies distribution and their characteristics, we infer the pre-collapse structure of the volcano.

Large eruption-triggered ocean-island landslide at Tenerife: Onshore record and long-term effects on hazardous pyroclastic dispersal

An extensive debris-avalanche deposit has been discovered on Canadas volcano, Tenerife (Canary Islands). The onshore component of the 733 ± 3 ka Abona landslide deposit exposes classic block facies and mixed facies across 90 km 2 . Three lines of evidence together show that the avalanche was triggered by an ignimbrite-forming explosive eruption: (1) the deposit is enclosed by phonolitic ignimbrites and is draped by a Plinian fallout layer, all within a single eruption unit; (2) it contains prismatic-jointed pumice blocks that were hot during landslide emplacement, indicated by chilled rims and breadcrust surfaces; (3) these blocks yield the same 40 Ar/ 39 Ar date as the associated ignimbrite and fall deposit. Landslide hummocks dammed surface water, forming ephemeral lakes perched on the volcano flank. Phonolite dome growth destabilized the southeast sector of a mid-Pleistocene Canadas caldera wall, and created a major breach that affected the passage of destructive pyroclastic density currents on Tenerife for 0.5 m.y., showing that landslides can have enduring consequences for pyroclastic dispersal and hazards.

The Tancitaro Debris Avalanche: Characterization, propagation and modeling

The Tancitaro volcano (3860 m) is an andesitic–dacitic stratovolcano located in the western portion of the Trans-Mexican Volcanic Belt within the state of Michoacán (Mexico). The tectonic activity of this area has likely contributed to a large sector collapse of the volcano. The first findings of a multidisciplinary investigation into this debris avalanche are presented here. Geomorphological analyses, based on the interpretation of orthophotos, satellite imagery and on GIS elaborations, had the objective of determining the main morphometric features of the landslide. The collapse structure is an east-facing horseshoe-shaped crater (4 km wide and 5.3 km long), while the deposit forms a large fan that is 66 km long, covers an area of approximately 1155 km 2 and has an estimated volume of 18 km 3. Event volume was established by reconstructing the paleo-edifice in a GIS and taking into account volumetric expansion. Cross sections measured in the field were also used for this purpose. Field investigations also highlighted the presence of two texturally distinct units, which are referred to as the “block facies” and the “matrix facies”, respectively. The first is responsible for the typical hummock morphologies found in the proximal area. A transitional zone contains a “mixed block and matrix facies” while in the distal portion blocks and megablocks, some of which have a jigsaw puzzle texture, gradually decrease in size until they disappear entirely. A number of matrix samples were collected to conduct direct shear tests, granulometric analyses and classification of the materials. The data and analyses described above were used to discuss the mechanism controlling the long runout of the avalanche. Based on the comparison between the Tancitaro debris avalanche and similar events we propose that mechanical fluidization was the mechanism responsible for the remarkable mobility of the landslide. The predisposing factors leading to the collapse were also considered. Field observations suggest that these are mainly related to weakening processes operating both in volcanoes and in non-volcanic areas. The runout of the Tancitaro debris avalanche was numerically modeled using DAN-W. The geotechnical parameters determined in the field and from the laboratory analyses were used as input. The DAN-W code models longitudinal spreading and the thickness and velocity of the failed mass by opportunely selecting a specific rheological model (Voellmy, Frictional, Bingham). Thus, it was determined that the two-parameter “Voellmy model” provided the best simulation of the Tancitaro debris avalanche movement, and the best fitting rheological parameters have been found through back analysis.

Allochthonous blocks misidentified as the basement: Implication for petroleum exploration in the SW Thrace Basin (Turkey)

The sedimentary sequence of the SW Thrace Basin (NW Turkey) comprises of three major sedimentary packages: a) Cretaceous (Meastrichtien)–Early Eocene package formed as pelagic carbonate, turbidite, shelf and nearshore-fluvial deposits, b) Middle Eocene–Early Oligocene package formed as shallow marine, slope apron and turbidite sediments, c) Middle Miocene–Early Pliocene package formed as alluvial fan-fluvial and nearshore deposits. This basin is structurally complex and its southern margin is represented by a fold-thrust zone. The Westward Propagation of the North Anatolian Fault (WPNAF) intersects the Thrace Basin along its southwestern flank. In the SW Thrace Basin, there are ophiolite and limestone blocks in the sedimentary succession, which previously were interpreted as suture of the Intra-Pontide Ocean, Cretaceous aged Yeniköy Mélange or Olistostromal Unit in the Eocene sediments. In this study these rocks have been defined as allochthonous blocks facies of the Gaziköy Formation aged Middle–Late Eocene. In previous studies the Karaağaç Formation (Early Eocene) was considered to be within oil window and have an average potential for oil and gas generation. According to our data the Karaağaç Formation has a moderate TOC content, is mature and overmature. In the study area, there are potential stratigraphic traps (submarine fans and channels of the Karaağaç and Keşan Formations, fluvial channels of the Fıçıtepe Formation and reefs of the Soğucak Formation), potential fractured tuffs (the Gaziköy Formation) and, potential structural traps related to the folds, thrusts and the WPNAF. Although 17 exploration wells have been drilled in the study area and vicinity, no discovery has yet been made. The cause of this may be that the wells were terminated at depths shallower than potential targets or possibly drilling may have been terminated at the allochthonous block facies of the Gaziköy Formation, which is especially derived from ophiolite mélange and was misidentified as basement.

The Chimborazo sector collapse and debris avalanche: Deposit characteristics as evidence of emplacement mechanisms

Chimborazo is a Late Pleistocene to Holocene stratovolcano located at the southwest end of the main Ecuadorian volcanic arc. It experienced a large sector collapse and debris avalanche (DA) of the initial edifice (CH-I). This left a 4 km wide scar, removing 8.0 ± 0.5 km 3 of the edifice. The debris avalanche deposit (DAD) is abundantly exposed throughout the Riobamba Basin to the Río Chambo, more than 35 km southeast of the volcano. The DAD averages a thickness of 40 m, covers about 280 km 2, and has a volume of > 11 km 3. Two main DAD facies are recognized: block and mixed facies. The block facies is derived predominantly from edifice lava and forms > 80 vol.% of the DAD, with a probable volume increase of 15–25 vol.%. The mixed facies was essentially created by mixing brecciated edifice rock with substratum and is found mainly in distal and marginal areas. The DAD has clear surface ridges and hummocks, and internal structures such as jigsaw cracks, injections, and shear-zone features are widespread. Structures such as stretched blocks along the base contact indicate high basal shear. Substratum incorporation is directly observed at the base and is inferred from the presence of substratum-derived material in the DAD body. Based on the facies and structural interpretation, we propose an emplacement model of a lava-rich avalanche strongly cataclased before and/or during failure initiation. The flow mobilises and incorporates significant substrata (10–14 vol.%) while developing a fine lubricating basal layer. The substrata-dominated mixed facies is transported to the DAD interior and top in dykes invading previously-formed fractures.

Characterisation of a Pleistocene debris-avalanche deposit in the Tenteniguada Basin, Gran Canaria Island, Spain

We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies, hummocky topography and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present. The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials involved in the movement rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes, strength reduction of the rocks, effective stress decrease, active gully erosion and water table rise during Pleistocene humid episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic earthquake.

Reinterpretation of the Alika debris-avalanche deposits, West Hawaii: multiple slope failures

The Giant Wave hypothesis for the deposition of chaotic gravels high on the coastal slopes of the Hawaiian Islands remains controversial, in part due to the scarcity of detailed submarine information regarding the Alika debris-ava- lanche deposits. In order to interpret the Alika debris avalanches, a comprehensive submarine investigation using high-resolu- tion bathymetric maps, ocean-floor observations and samples from submersibles, piston cores, and improved GLORIA side- scan images, was carried out. The sequence of debris-avalanche events is based on bathymetric investigations, draped sedi- ments on megablocks in the South Kona slide area, and turbidites recovered by a piston core sampling 135 km southwest of the island of Hawaii (PC-13). The chemical compositions of the draped sediments and turbidite layers in the piston core are very similar, but differ from the matrix and block facies of the South Kona slide. These compositions suggest that the source for the turbidite layers was probably lava erupted subaerially from Mauna Loa. This geological evidence suggests that the Alika debris-avalanche deposits are unlikely to have formed in a single event or in rapid succession. They can be redefined as three geologically separated landslide events: the Kealakekua rotational slump, the Alika 1 debris avalanche, and the Alika 2 debris avalanche. The Kealakekua rotational slump probably occurred after the South Kona slide complex and before the Alika debris avalanches and was emplaced slowly. The Alika 1 and 2 debris-avalanche deposits can be divided into subfacies: (1) the main body of the debris-avalanche deposits (Alika 1d and 2d), (2) megaturbidites (Alika 1t and 2t) accompanying the debris ava- lanches, and (3) levee-overflow sediments (Alika 2s) derived from the suspension and reworking of Alika 2d. Thus, complicat- ed bathymetric characteristics in the Alika debris avalanche deposits can be interpreted as an overlapping of multiple slope failures. Therefore, the newly estimated volumes of the landslide deposits, their boundaries delineated by high-resolution digi- tal elevation models, are smaller than those of previous estimations.

Volcanic natural dams: identification, stability, and secondary effects

Volcanic activity can enhance several secondary effects, including the formation of one or more natural dams. A common example is from volcanic collapse, where huge mass volumes are rapidly emplaced, obstructing the drainage around a volcano. Their duration depends on the volume of the obstructing mass, inflow rate, and on its textural characteristics. A block facies of a debris avalanche produces durable and permeable dams that consist of decimeter to meter-sized blocks without matrix, whereas a mixed facies is easily eroded after overflowing. Analysis of the sedimentological characteristics of different volcaniclastic deposits that formed natural dams indicate that a mean grain size (Md) equal to −1 phi divides the field of debris avalanche dams (Md < −1 phi) from that formed from other types of volcanic deposits. In addition, the matrix proportion of dams formed by debris avalanches are less than the 50% and the percentage of mud fraction is highly variable, up to 30%. Combining the granulometric textures with duration time of the dam shows no clear relation. Dam durability is probably more dependent on the volume of the lake and the inflow rate. Only in some cases, as mud fraction increases is the blockage also less durable because the lower permeability favors rapid infilling. The texture of the dam also determines the types of secondary flows that originate by their breakdown. These vary from cohesive debris flow to hyperconcentrated flow, representing different hazards due to their magnitude and their different behavior downstream.

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

AbstractGeological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year−1 compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.

An Archaean submarine volcanic debris avalanche deposit, Yilgarn Craton, western Australia, with komatiite, basalt and dacite megablocks: The product of dome collapse

The Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, western Australia contains excellent exposure of Archaean felsic and ultramafic breccias characterised by facies associations interpreted to reflect a volcanic debris avalanche mode of deposition. Such Archaean volcanic deposits are typically difficult to identify due to poor preservation and exposure. However, primary volcanological and sedimentological features are preserved within the relatively low strain and low metamorphic grade (up to lower greenschist facies) Boorara Domain that allow accurate facies reconstruction. The breccia deposit is characterised by two clast populations. A ‘block facies’ comprised of metre- to decimetre-scale megablocks of dacite, basalt and komatiite is preserved within a ‘mixed’ matrix breccia facies of angular, coarse sand- to boulder-sized clasts. The megablocks preserve original stratigraphy and show fracturing and jigsaw-fit textures within the poorly sorted, unstratified, genetically related matrix. Overlying the volcanic debris avalanche deposit, are a series of stratified horizons. These deposits show evidence of hydraulic sorting within bedforms exhibiting normal grain-size grading and tractional scour and fill structures along their basal contacts. The stratified facies is interpreted to have been deposited by high concentration, high competency turbidity currents, triggered by slope stabilization slides in the source region. Primary contacts and volcanic textures preserved in decimetre-scale volcanic blocks allow reconstruction of the pre-collapse palaeovolcanological history of the source region. The volcanic debris avalanche deposit, together with the associated stratified sedimentary horizons, were produced by sector collapse of a submarine, dacitic volcanic dome. Contemporaneous komatiite intrusion into the dacite dome may have caused dome flank instability. However, the volcanic debris avalanche trigger is interpreted to be a post-lithification tectonic influence.

Emplacement mechanisms of the South Kona slide complex, Hawaii Island: sampling and observations by remotely operated vehicle Kaiko

Emplacement of a giant submarine slide complex, offshore of South Kona, Hawaii Island, was investigated in 2001 by visual observation and in-situ sampling on the bench scarp and a megablock, during two dives utilizing the Remotely Operated Vehicle (ROV) Kaiko and its mother ship R/V Kairei. Topography of the bench scarp and megablocks were defined in 3-D perspective, using high-resolution digital bathymetric data acquired during the cruise. Compositions of 34 rock samples provide constraints on the landslide source regions and emplacement mechanisms. The bench scarp consists mainly of highly fractured, vesiculated, and oxidized a’a lavas that slumped from the subaerial flank of ancestral Mauna Loa. The megablock contains three units: block facies, matrix facies, and draped sediment. The block facies contains hyaloclastite interbedded with massive lava, which slid from the shallow submarine flank of ancestral Mauna Loa, as indicated by glassy groundmass of the hyaloclastite, low oxidation state, and low sulfur content. The matrix facies, which directly overlies the block facies and is similar to a lahar deposit, is thought to have been deposited from the water column immediately after the South Kona slide event. The draped sediment is a thin high-density turbidite layer that may be a distal facies of the Alika-2 debris-avalanche deposit; its composition overlaps with rocks from subaerial Mauna Loa. The deposits generated by the South Kona slide vary from debris avalanche deposit to turbidite. Spatial distribution of the deposits is consistent with deposits related to large landslides adjacent to other Hawaiian volcanoes and the Canary Islands.

Flank collapse at Mount Wrangell, Alaska, recorded by volcanic mass-flow deposits in the Copper River lowland

An areally extensive volcanic mass-flow deposit of Pleistocene age, known as the Chetaslina volcanic mass-flow deposit, is a prominent and visually striking deposit in the southeastern Copper River lowland of south-central Alaska. The mass-flow deposit consists of a diverse mixture of colorful, variably altered volcanic rocks, lahar deposits, glaciolacustrine diamicton, and till that record a major flank collapse on the southwest flank of Mount Wrangell. The deposit is well exposed near its presumed source, and thick, continuous, stratigraphic exposures have permitted us to study its sedimentary characteristics as a means of better understanding the origin, significance, and evolution of the deposit. Deposits of the Chetaslina volcanic mass flow in the Chetaslina River drainage are primary debris-avalanche deposits and consist of two principal facies types, a near-source block facies and a distal mixed facies. The block facies is composed entirely of block-supported, shattered and fractured blocks with individual blocks up to 40 m in diameter. The mixed facies consists of block-sized particles in a matrix of poorly sorted rock rubble, sand, and silt generated by the comminution of larger blocks. Deposits of the Chetaslina volcanic mass flow exposed along the Copper, Tonsina, and Chitina rivers are debris-flow deposits that evolved from the debris-avalanche component of the flow and from erosion and entrainment of local glacial and glaciolacustrine diamicton in the Copper River lowland. The debris-flow deposits were probably generated through mixing of the distal debris avalanche with the ancestral Copper River, or through breaching of a debris-avalanche dam across the ancestral river. The distribution of facies types and major-element chemistry of clasts in the deposit indicate that its source was an ancestral volcanic edifice, informally known as the Chetaslina vent, on the southwest side of Mount Wrangell. A major sector collapse of the Chetaslina vent initiated the Chetaslina volcanic mass flow forming a debris avalanche of about 4 km3 that subsequently transformed to a debris flow of unknown volume.

Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone, rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal, and reefal environments. Deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.

Volcanic debris avalanche deposits of the upper Maronne valley (Cantal Volcano, France): evidence for contrasted formation and transport mechanisms

The deposits of two volcanic debris avalanches (VDA I and II) that occur in the upper Maronne valley, northwest sector of Cantal Volcano, France, were studied to establish their mechanisms of formation, transport and deposition. These two volcanic debris avalanches that clearly differ with regard to their structures, textures and extensions, exemplify the wide spectrum of events associated with large-scale sector collapse.VDA I is voluminous (∼1km3 in the upper Maronne valley) and widespread. The deposits comprise two distinct facies: the block facies that forms the intermediate and upper part of the unit and the mixed facies that crops out essentially at the base of the unit. The block facies consists of more or less brecciated lava, block-and-ash-flow breccia and pumice-flow tuff megablocks set in breccias resulting from block disaggregation. Mixing and differential movements are almost absent in this part of the VDA. The mixed facies consists of breccias rich in fine particles that originate from block disagregation, as well as being picked up from the substratum during movement. Mixing and differential movements are predominant in this zone. Analysis of fractures on lava megablocks suggests that shear stress during the initial sliding is the principal cause of fracture. These data strongly indicate that VDA I is purely gravitational and argue for a model in which the initial sliding mass transforms into a flow due to differential in situ fragmentation caused by the shear stress.VDA II is restricted to low-topography areas. Its volume, in the studied area, is about 0.3km3. The deposits consist of brecciated, rounded blocks and megablocks set in a fine-grained matrix composed essentially of volcanic glass. This unit is stratified, with a massive layer that contains all the megablocks at the base and in the intermediate part, and in the upper part a normally graded layer that contains only blocks <1m in size. The different lithologies present are totally mixed. These observations suggest that VDA II may be of the Bezymianny-type and that it underwent a flow transformation from a turbulent to a stratified flow consisting of a basal hyperconcentrated laminar body overlain by a dilute layer.

Intracaldera structure and megabreccias at Dorobu caldera, northeastern Honshu, Japan

In the Pliocene Dorobu caldera, 13 by 6 km in diameter, on northeastern Honshu, Japan, a number of occurrences of a distinctive megabreccia block facies is found within voluminous ash-flow tuffs, the Dorobu ash-flow tuffs A and B. Dorobu tuff A is believed to be the deposit of the initial caldera-forming eruption. An isopach map of tuff B and orientation of narrow dikes indicate that an internal large depression occurred inside the present caldera margin. This and locations of post-caldera conduits and near vent facies of Dorobu tuff C suggest that the caldera has a nested collapse structure, and possible conduits of the tuff B are within the internal depression. Annular collapses that sequentially moved towards the caldera center occurred during eruption of tuffs A and B. The overall structure of Dorobu caldera is nested, and it possibly has step-like floor, as described elsewhere from piecemeal calderas. Megabreccia block facies consists of polyhedral massive blocks composed of Miocene to Pliocene country rocks with large diameters (up to several hundreds meters). The block facies as a whole does not consist of sheets or piles of debris, as described in the San Juan Mountains and Stillwater Range, USA, but as isolated blocks set in the matrix of Dorobu tuffs A and B. In previous studies of caldera development, the mode of emplacement of caldera-collapse breccias has been considered to be by rock fall or rock slide during the caldera forming eruption. A paleo-magnetic study of the block facies shows two significant characteristics. Predominant variations of the magnetic directions of blocks indicate that they were emplaced with rotations, caused by rock slide, rock fall, or a toppling motion. In contrast, magnetized directions of one megabreccia block almost coincide with that of the caldera country rock, suggesting that this block must have kept its original magnetic direction. Our findings suggest that it is not a rock slide deposit derived from caldera walls, but rather a segment of subsided caldera floor. The mode of emplacement of the megabreccia block facies at Dorobu caldera is mostly similar to the other megabreccias, however, additionally, it is necessary to distinguish segments of caldera floor from rolled or toppled megabreccia blocks.

Multiple-pulsed debris avalanche emplacement at Mount St. Helens in 1980: Evidence from numerical continuum flow simulations

The complex 1980 Mount St. Helens debris avalanche is modeled numerically as a transient biviscous fluid flow. Several approaches are considered, including two-step rheologically-distinct models for avalanche I and combined avalanches II/III, and a composite flow model consisting of retrogressive slides of identical rheology successively accreted to the main avalanche flow. For the two-step situation, flow rheologies are evaluated separately for the initial avalanche, comprising the debris avalanche block facies, and an ensuing explosive-influenced flow. Strengths (normalized by density) as high as 250 m 2/s 2 and apparent Newtonian viscosities as much as 275 m 2/s were established for the block facies. These parameters for the explosively-influenced flow are an order of magnitude lower. The distribution of stratigraphic units within flowing model debris, compared with field distributions, suggests that the higher-strength emplacement models are appropriate for debris deposited on Johnston Ridge and in the upper parts and flanks of the North Fork Toutle River valley. In general, models for which constant rheology is assumed throughout the flow process provide lower-bound emplacement times, and excessive early velocities, as compared to the prototype event. Because model calibration is based on matching runout by trial and error, it is therefore biased toward the rheologic parameters essential to achieving that runout. These values characterize the flow in its latter stages, whereas the actual strength and viscosity may have substantially decreased as a function of displacement. Two-dimensional models predict debris accumulation about twice as thick as that observed at the foot of Mount St. Helens, where flow divergence was significant. This discrepancy is lessened with a quasi-three-dimensional modification of the flow model. Accretionary composite flow models with homogeneous rheology simulate the overriding of early avalanche debris by later debris pulses. The accretionary composite model also predicts a thick flow snout down-valley. Since this feature is not indicated in the debris deposit, the model lends support to the concept of flow separation by rheology. The two-step and accretionary composite flow models complement one another and provide model-based support for the Harry Glicken hypothesis for complex emplacement at Mount St. Helens.