Pyroclastic Debris Flows Research Articles

Coarse calcite grains in hydrocarbon-rich tuffaceous mudstone in a Permian volcanic lake—An indicator for subaqueous volcanic deposition and related hydrocarbon generation

Sediments originated from volcanic-hydrothermal activities show clear relationship with hydrocarbon generation and accumulation in fine-grained sedimentary rocks at a regional scale. However, the fundamental processes and mechanisms of this relationship are still poorly understood at a microscopic scale to hamper efficient hydrocarbon exploration. In addition, coarse grains common in volcanic lacustrine fine-grained sediments have been traditionally interpreted as diagenetic products during early and late burial. Our study, however, suggests that coarse calcite grains are not diagenetic, but formed by subaqueous explosion. The calcite grains occur in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, NW China. They are sand sized, angular, mono- and polycrystalline, and discretely scatter in silt to clay-sized non-calcite pyroclastic matrix. The calcite grains and the mud matrix are interpreted as a pyroclastic debris flow deposit. In-situ 87Sr/86Sr ratios (0.704932–0.705840, and 0.705137 on average) of the calcite grains are similar to those of mantle rocks, indicating an origin closely related to mantle-derived fluids. The in-situ δ13C values (4.24‰ to 5.06‰, and 4.57‰ on average) and extremly negative δ18O values (−21.70‰ to −21.18‰, and − 21.49‰ on average) indicate that extensive hydrothermal alteration and microbial activities influenced the formation of the calcite. Element composition and homogenizaiton temperature of fluid incusions of calcite and biomarkers of the mudstone all suggest a relatively low-temperature hydrothermal condition in a saline and reducing environment. Furthermore, hydrocarbon-rich inclusions are rich in the calcite grains and have two modes of occurrence and maturity. They indicate that the organic matter was generated from algae bloom, which was caused by volcanic-hydrothermal activities. Alternatively, the hydrocarbon may have an inorganic origin originated from a mantle source. Our findings provide an alternative interpretation on the origin of coarse calcite grains in fine-grained sedimentary rocks and demonstrate a microscopic relationship between volcanic-hydrothermal activities and hydrocarbon generation and accumulation in a tectonic active rift basin.

Volcanic reconstruction of the Paleoproterozoic Stroud Breccia: Understanding the transition from a primitive to mature arc and its impact on VMS ore-formation, Snow Lake, Manitoba, Canada

The Stroud Breccia is a ≤400 m thick package of dominantly felsic volcaniclastic rocks within the Paleoproterozoic Snow Lake arc assemblage. It stratigraphically overlies the ore-interval for the Anderson, Stall and Rod volcanogenic massive sulfide (VMS) deposits at Snow Lake, Manitoba, Canada and occupies the stratigraphic interval between the underlying primitive arc Anderson sequence and the overlying mature arc Chisel sequence.The Stroud Breccia is interpreted to be a product of subaqueous pyroclastic and effusive, dominantly rhyolitic, volcanism, and concomitant subsidence. It consists of felsic tuff to lapilli-tuff, crystal-rich felsic tuff, felsic lapillistone, felsic tuff breccia, mafic tuff to lapilli-tuff, heterolithic tuff breccia to breccia and rhyolite. The finer felsic volcaniclastic units are characterised by angular or broken quartz crystals, angular lithic rhyolite lapilli, and pumice. Fine ash, broken crystals and pumice suggest they were deposited by eruption-fed density currents associated with a phreatomagmatic eruption. The coarser felsic volcaniclastic units are characterized by blocks of strongly quartz-feldspar porphyritic rhyolite and pumice and are interpreted to have been deposited during the reworking of pyroclastic debris flow deposits on the seafloor during mass wasting events at fault scarps. Planar and cross laminations, graded beds and bomb sag structures suggest the mafic tuff to lapilli-tuff unit was deposited by a combination of subaqueous eruption-fed density currents and eruption driven ballistically emplaced ejecta, likely through subaqueous tephra jetting.Lithofacies analyses, and geochemistry has allowed comparison between the felsic volcanic rocks of the Stroud Breccia and those that host the Anderson, Stall and Rod VMS deposits. Detailed mapping has led to the identification of synvolcanic faults, defining basins and volcanic vents, that indicate that the process of incipient rifting of the Anderson sequence primitive arc continued during emplacement of the Stroud Breccia. These structures also define magma and fluid pathways critical in controlling the formation of the Anderson VMS deposits, meaning they represent deep-seated and long-lived structures that can be used as vectors to mineralization at depth.

ESTIMATION OF WILLINGNESS TO PAY TOWARDS RIVERSIDE LANDSCAPE DESIGN

Riverside landscape design is developed not only to enhance amenity but also to address disaster risks. This study aims to estimate the value of the design of riverside landscape, in terms of amenity and disaster mitigation. Contingent valuation method (CVM) is used for valuation of the proposed design of riverside landscape. The case study was taken place in Yogyakarta City, Indonesia, that has three main rivers flowing through the city; Winongo River, Code River and Gajahwong River. As the rivers are headed from Merapi volcano, the community that occupied the riverside areas is threatened by the risks of cold pyroclastic debris flow, beside the regular riverine flood. Respondents are family representatives that chosen randomly from the total population of Yogyakarta City. There are 580 samples, consists of 333 samples of Type A and 247 samples of Type B. Visual image with explanation of amenity obtained higher WTP rather than with explanation of disaster mitigation indicates that the explanation of amenity features of the design is valued higher rather than disaster mitigation features. In addition, the value of extrapolated WTP results considerable amount in monetary term that represents the cost of realization of the design and also serves as feasibility of the project. However, visual image contain mixed value of the design to some extends, and the explanation increases the value of the design, due to the respondents may grasp other aspects than explained by viewing the visual image of the design. Reasons of willing to pay are all positively related to WTP, implies that people behaviour towards the riverside area is influencing the value, specifically related to their financial state. CVM as valuation technique for public good can be used to understand the perception and also to measure the acceptance of the public towards the proposed design.

The osteology and taphonomy of a Psittacosaurus bonebed assemblage of the Yixian Formation (Lower Cretaceous), Liaoning, China

We present the first detailed description of the osteology and mineralogy of an assemblage of Psittacosaurus juveniles (DMNH D2156) associated with a larger specimen from the Lujiatun beds of the Yixian Formation in Liaoning, China. We fully describe all of the material as well as discuss intraspecific variation in the sample set of twenty-four juveniles with associated postcranial material and compare with previously described ceratopsian juvenile material. Based on the development of the ends of long bones, it is suggested that the juveniles are not embryonic, but are post-hatchlings. In comparison with previous histologic analyses on P. lujiatunensis, it is shown that the large specimen associated with the juveniles is likely not yet an adult and that this assemblage is not an exemplar of parental care, but may be an example of post-hatching cooperation. The large specimen is however, shown to be positively associated with the assemblage and was not added on after excavation. Additionally, an allometric analysis of Psittacosaurus bone lengths is performed in order to determine ontogenetic trajectories and allometry using femur length as a proxy. This demonstrates that the vast majority of long bones and girdle elements in Psittacosaurus are isometric with body size, but supports previous analyses that the forelimb grew slower than the hindlimb. Finally, a mineralogical analysis using X-ray diffraction and petrographic thin sections of the block where DMNH D2156 is preserved shows that the animals were preserved in a volcanic-lithic arenite. The statistically significant alignment of specimens and preservation in a volcanic rock suggests that the taphonomic setting and reasoning for the exceptional preservation of the specimen is due to burial by a volcaniclastic debris flow. The vast majority of specimens in the Yixian Formation are found in lacustrine strata, recently suggested to be carried into lakes by pyroclastic debris flows. The preservation of DMNH D2156 in a clastic flow further supports the volcaniclastic flow preservation model for the fauna of the Yixian Formation, but we prefer a lahar flow interpretation for DMNH D2156 since there is no evidence of the bone microstructure being affected by intense heat associated with pyroclastic debris flows.

Rheological behaviour of reconstituted pyroclastic debris flow

An experimental study of the rheological behaviour of three natural pyroclastic soils with different depositional processes remixed with water was carried out with the help of a rotational rheometer and inclined plane tests. A homogeneous fluid-like behaviour is obtained only within a very narrow range of concentrations, typically not more than 10%. Below this range the material sedimentates rapidly; above this range it behaves like a solid. In the fluid-like range the typical rheological behaviour of these suspensions is that of a yield stress fluid exhibiting a static yield stress larger than its dynamic yield stress. This effect probably finds its origin in a ‘local' sedimentation effect: that is, the particles sedimentate just as necessary to form a structure more jammed than the structure during flow. As a result the flow of such materials is usually unstable: they will start to flow beyond a critical stress, but just beyond this value will reach a high shear rate associated with a high flowing velocity. The static and dynamic yield stresses of these materials increase widely from very low to very large values (several orders of magnitude). Inclined-plane tests were shown to provide reasonable although still approximate values for the static and dynamic yield stresses. These results suggest that in the field a small change in solid fraction will cause a slight decrease of the static yield stress, readily inducing a rapid flow that will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of pyroclastic debris flows, which are able to flow over very long distances, even on smooth slopes.

Theoretical analysis of tsunami generation by pyroclastic flows

Pyroclastic flows are a common product of explosive volcanism and have the potential to initiate tsunamis whenever thick, dense flows encounter bodies of water. We evaluate the process of tsunami generation by pyroclastic flow by decomposing the pyroclastic flow into two components, the dense underflow portion, which we term the pyroclastic debris flow, and the plume, which includes the surge and coignimbrite ash cloud parts of the flow. We consider five possible wave generation mechanisms. These mechanisms consist of steam explosion, pyroclastic debris flow, plume pressure, plume shear, and pressure impulse wave generation. Our theoretical analysis of tsunami generation by these mechanisms provides an estimate of tsunami features such as a characteristic wave amplitude and wavelength. We find that in most situations, tsunami generation is dominated by the pyroclastic debris flow component of a pyroclastic flow. This work presents information sufficient to construct tsunami sources for an arbitrary pyroclastic flow interacting with most bodies of water.

Origin of freshwater-diatom-rich pyroclastic-debris-flow deposit in a shallow-marine Tertiary forearc basin, NW Oregon

ABSTRACT An unusual freshwater-diatom-bearing pyroclastic-debris-flow deposit is present within the shallow-marine upper Eocene to Oligocene Pittsburg Bluff Formation of northwestern Oregon. The subaerially generated pyroclastic-debris flow rapidly debouched into the shallow-marine environment. The flow formed a wedge-shaped deposit, up to 3.5 m thick, that is mappable over several square kilometers. Thickness and maximum clast size decrease offshore; near the distal margins the debris flow mixed with seawater and became a high-density shelf turbidity current. Where emplaced above storm wave base, the deposit was locally reworked to form a thin lag conglomerate. Thalassinoides burrows are present in the upper 30 cm of the deposit. The rhyodacite chemical and mineral composition, regional geologic setting, and thickness and clast-size variations indicate a nearby highly explosive calcalkaline volcanic source to the northeast (i.e., Western Cascade are). This deposit documents the earliest known Cascade arc explosive event that directly affected sedimentation in the Tertiary forearc basin of western Oregon. A sanidine- and biotite-bearing ash fall tuff 3 m above the debris-flow deposit was derived from a backarc eruptive source (e.g., Oligocene John Day Formation). This tuff yielded an 40Ar/39Ar date of 29.83 Ma. The well-preserved diatom flora within the debris-flow deposit matrix extends the geologic range of some genera (e.g., Gomphonema) and widens the geographic distribution of others (e.g., Gomphopleura). Possible scenarios to explain the presence of exclusively freshwater diatoms in the debris-flow deposit in this shallow-marine section include: (1) a primary pyroclastic debris flow may have passed through a diatom-rich lake between the site of eruption and its entrance into the sea; (2) freshly erupted pyroclastics temporarily blocked a river drainage, creating an alpine lake that produced the debris flow upon dam failure; (3) primary pyroclastic debris may have been vented directly through a caldera or crater lake rich in diatomaceous sediment.

Application of multiple satellite data to the study of natural disasters

The ability observing a volcanic activity with a new airborne multispectral scanner (VAM-90A) of 9 bands is confirmed. Composite color images consisting visible, near IR, thermal IR are effective for clarifying lava dome condition. IR band image detects increasing temperature trend of craters, the surface temperature of lava domes and that of pyroclastic flow. Satellite data (Landsat/TM, MOS-1/MESSR and SPOT/HRV images) are used to detect land changes caused by pyroclastic debris flow, the debris accumulation and ash deposition.

Paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine Shirahama Group (Upper Miocene‐Pliocene), Japan

The cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations. Six to 13 samples were cored and measured from six essential andesite blocks 1–5 m in diameter. Thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps, each representing different blocking temperatures (Tb). Each sample has three components of remanent magnetization (C1, C2, and C3). C1 is characterized by a Tb of 250°C and an orientation coincident with the present geomagnetic field. C2 has a Tb of 250°–500°C and shows a consistent direction with reversed polarity. C3 has a Tb of 500°–600°C, and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts. On the basis of the remanence direction of each component, the differing Tb, and the petrography of the samples, C1 is interpreted as secondary viscous remanence, whereas C2 and C3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement. C3 was probably acquired while the blocks were rolling or sliding downslope, and C2 formed during continued cooling of the blocks after they came to rest. Heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted. If the eruptive vent was only 2–3 km distant, as suggested by field relations, a downslope transport rate of less than 36 km/h is implied.

Subaerial to submarine transitions in early Miocene pyroclastic flow deposits, southern San Joaquin basin, California

Dacitic pyroclastic flow deposits within the Tecuya Formation of the San Emigdio Mountains, California, exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the San Joaquin basin. The dacitic pyroclastic deposits are part of an informal volcanic member within the Tecuya Formation, consisting of a lower dacitic unit and an upper basaltic unit. Volcanogenic facies transitions within each of these units can be traced westward, obliquely across paleoshoreline, for 15 km. Depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits. The lowermost dacitic unit consists of pyroclastic surge and fallout deposits, which overlie nonmarine alluvial-fan deposits and were produced by subaerial, phreatomagmatic eruptions. A second, magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits, and within marine, nearshore to outer-shelf settings. These pyroclastic flows, generated from subaerial vents, were high-concentration, poorly fluidized flows which traveled down paleoslope into the marine part of the basin. Upon reaching water, the hot pyroclastic flows continued to flow subaqueously, partially mixed with water, and were quenched. Transitions to cool submarine pyroclastic debris flows may have occurred. Surface transformations of the submarine pyroclastic flows/pyroclastic debris flows generated dilute pyroclastic turbidity flows, which at times became detached and continued to flow independently downslope. Progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east. A second, probably submarine, vent also produced submarine pyroclastic flow deposits in the western part of the basin. Deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge, fallout, and flow deposition. The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits within the Tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin.

Stratigraphy of upper Paleozoic volcanic rocks and regional unconformities in part of the northern Sierra terrane, California

Upper Paleozoic volcanic rocks in the southern part of the Taylorsville strike belt are unconformably truncated at the North Fork of the American River by a lens of Upper Triassic conodont-bearing limestone and Lower and Middle Jurassic graywacke-slate flysch deposits of the Sailor Canyon Formation. The volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the Late Devonian Sierra Buttes Formation, andesitic turbidites of the Taylor Formation, a lower tuff-siltstone member and an upper chert member of the Peale Formation, and fine-grained tuff of the Reeve Formation. Newly discovered radiolarians date the chert member of the Peale as latest Late Mississippian and(or) Early Pennsylvanian. Shelly fossil fragments found 50 m above the chert member date the lower part of the Reeve in this area as late Early Permian. These rocks formed in the distal part of the submarine volcanic apron, presumably in an island-arc setting, and were deposited unconformably on quartz-rich clastic rocks and tectonically interlayered chert of the pre–Late Devonian Shoo Fly Complex. The Peale marks a period of quiescence in the late Paleozoic volcanism that culminated in deep-water chert deposition during latest Mississippian to the Middle Pennsylvanian. The Peale-Reeve contact is a regional hiatus, probably nondepositional in the south and becoming erosional to the north, that spans most or all of the Late Pennsylvanian and Early Permian. Post-late Early Permian to pre-Late Triassic deformation produced northeast-trending folds and a southeast-trending fault in the late Paleozoic volcanic rocks that are truncated nearly orthogonally by the Late Triassic unconformity and northwest-trending folds in the Jurassic Sailor Canyon Formation.

Volcanogenic Sedimentation in the Lesser Antilles Arc

Combination of deep-sea coring and land-based studies has provided a quantitative view of volcano genic sediments in and around the Lesser Antilles arc in the late-Quaternary. Of the total of $$527 km^{3}$$ of volcanics produced in the arc in the last $$10^{5}$$ years, over 80% has been deposited as volcanogenic sediments in the adjacent marine basins as ash-fall layers, pyroclastic debris flow deposits and volcanic sands. Over 70% of total volcanogenic sedimentation from the arc is received by the back-arc Grenada Basin west of the arc in the form of sediment gravity flows. The volumetric role of ash-fall layers is thus relatively small. Approximately 40% of the ash-fall in the Atlantic is dispersed in the sediment. The asymmetric distribution of volcanogenic sediments around the arc, with pyroclastic debris flow deposits predominating west of the arc and ash-fall layers in the Atlantic east of the arc, is attributed to the effects of prevailing high-altitude wind direction, different submarine arc slope...

The roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, lesser antilles arc

Abstract Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash. Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.