Autoclastic Breccias Research Articles

Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.

Characterizing lithological, weathering, and hydrothermal alteration influences on volcanic rock properties via spectroscopy and laboratory testing: a case study of Mount Ruapehu volcano, New Zealand

The geomechanical characterization of volcanic material has important implications for geothermal and mineral exploration, engineering design, geophysical signals of volcano unrest, and models of instability and mass flows. Chemical weathering and hydrothermal systems can alter the host rock, leading to changes in mechanical behavior and failure mode. Here, we compare the physical and mechanical properties of lava, autoclastic breccia, and pyroclastic (scoria) rocks from Mount Ruapehu volcano (Ruapehu) in New Zealand to mineralogical composition determined via infrared spectroscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). We use correlation matrices, principal component analysis, and parametric analysis to determine which parameters can be used to predict physical and mechanical properties and form the basis for transfer functions. Laboratory-based spectroscopy shows that the samples contain absorption features indicative of Al- and Mg-rich hydrous phyllosilicates (e.g., kaolinite, halloysite, montmorillonite), Fe- oxides (e.g., goethite), and sulfates attributed to surface weathering, supergene, and steam-heated alteration. We find that porosity and primary lithology are the predominant control on physical and mechanical properties, followed by the pervasiveness of weathering/alteration, and then mineralogical composition. Several properties, such as porosity, uniaxial compressive strength, P-wave seismic velocity, density, and Young’s modulus, show strong correlations with other properties, indicating the potential for transfer functions between these properties. Hydrothermally altered rocks near the vent complex (up to ~ 400 m depth beneath the crater lake) with high-intensity hydrothermal alteration do not follow typical physical and mechanical property trends due to high clay content, low permeability, and low strength. The presence of these rocks within the edifice at Ruapehu implies local barriers to fluid flow and subsequent pore pressure variations. Additionally, they may have less than half the strength than would be dictated by typical porosity-strength trends for surface rocks, increasing the likelihood of structural failure. Trends in the pervasiveness of weathering with physical and mechanical properties, along with shifts in the position of spectral absorption peaks as hydrothermal/weathering alteration increases, suggest that it may be possible to extrapolate properties from imaging spectroscopy.

Preliminary Study of Accretion Complexes of Mangilu Area, Pangkep Ragency South Sulawesi

The exposure of Mesozoic rocks in the Mangilu area, both as a result of tectonic deformation in the form of mélange, low-high degree metamorphism and deep-sea sediments (flysch, radiolarian cherts, and sediment flow in the form of olistostrome), which formed in the subduction zone indicates complex accretionary rocks. The purpose of this study is to identify and study the accretionary complex trace rocks of the Mesozoic era. The method used in this study was field observation, including collection of geological data, tectonic deformation, rock groups and laboratory analysis (petrography). In general, the accretion complex is divided into 2 categories, namely a). oceanic volcanic groups and sediments of the oceanic crust; b). terigenous sediments from continents with sialic beds. The oceanic volcanic group consists of gabbro, diabase, dolerite, pillow and hyaloclastic lava, and oceanic sediments consist of radiolarian chert and mali limestone. The terigen sedimentary group consists of flysch type sandstones and olistostrome flow sediments with components in the form of granite, granodiorite, dacite and diabase. Other components found in autoclastic breccia rocks in the Mangilu area consist of metaperidotite, serpentinite, quartzite and metachert. Based on the Oceanic Plate Stratigraphy (OPS) arrangement, the autoclastic breccia rocks in the Mangilu area consist of bottom up, namely ultramafic/serpentinite, gabbro, diabase, pillow basaltic lava, hyaloclastic, radiolarian chert, mali limestone and flysch sediments. The mixing of rocks originating from oceanic plates with continental plates in autoclastic breccias indicates that the Mesozoic accretion complex in the Mangilu area has experienced strong deformation in the form of thurst in the Tertiary era.

Evidence for Upper Cretaceous seismites in the Abu Tartur area, Western Desert, Egypt

Upper Cretaceous tidal-flat sediments of the Abu Tartur region, Western Desert of Egypt, contain a variety of soft-sediment deformation structures (SSDSs) in the Mut Member of the Quseir Formation and the overlying Duwi Formation. Here, we document recognition of a set of SSDSs characterized by complex morphology and large size. The SSDS include: 1) water-escape structures, 2) minor recumbent, chevron, and slump folds, 3) syn-sedimentary minor faults, 4) neptunian dykes, 5) regular ball-and-pillow structures, flattened pseudo-nodules, and load casts, 6) syn-sedimentary undulations, 7) fractures and joints, 8) sand injectites, and 9) autoclastic breccia and brecciated siderite clasts. The dimensions of the SSDSs range from a few centimeters to several decimeters. Both extensional and contractional forces appear to have been involved in development of these structures. A detailed analysis of facies relationships, combined with consistent orientations of SSDSs and the regional tectonics of the target area suggests earthquakes as the triggering mechanism for sediment deformation and large-scale mass failure. The deformed strata are consistent with most of the well-known criteria of typical seismites. Thus, syn-sedimentary liquefaction was identified as the primary driving force in most of the observed deformation horizons for these Upper Cretaceous SSDSs in the Abu Tartur area.

Soft-sediment deformation structures in a lacustrine depositional context: An example from the Eocene Dongying Depression in the Bohai Bay Basin, East China

The study documents soft-sediment deformation structures (SSDSs) in the lacustrine Dongying Depression, a Paleogene synrift basin located in the southeastern corner of the Bohai Bay Basin, East China. The focus of this study is the lower part of the upper 4th member of the Eocene Shahejie Formation (Es4U), characterized by littoral to sublittoral deposits in the southwestern Dongying Depression. Nine lithofacies are grouped into the upper shoreface facies associations (FA1-1), lower shoreface facies associations (FA1-2), and sublittoral facies associations (FA2). Fine-grained calcareous sandstone was mainly deposited in FA1-1, with thinly alternating calcareous mudstone and siltstone or sandstone in FA1-2, while deposition in FA2 was dominated by mudstone, containing sediment gravity flow deposits. The SSDSs widely encountered in the lower Es4U can be classified into seven categories: (1) load structures, (2) water-escape structures, (3) folds and convolute lamination, (4) sediment-injection structures, (5) microfaults, (6) autoclastic breccias, and (7) slumps. Detailed observations of SSDSs from cores indicate liquefaction and fluidization as the main deformation mechanisms in the sands and muds. Moreover, rupturing of cohesive deposits also contributes to the soft-sediment deformation. The types and morphologies of the SSDSs are closely related to the rheologies of the sediments (e.g., the sand content), influenced by the facies that determine the lithologies and stratigraphic stacking patterns. Except for some deformation structures related to subaqueous sediment gravity flows (e.g., muddy debrite flows and slumps), most SSDSs are ascribed to rift-related seismicity due to the activity of the adjacent synsedimentary faults when the Dongying Depression underwent quick subsidence during the Eocene.

Soft-sediment deformation in the Sergipe-Alagoas Basin, Brazil: Implications for paleoseismicity in intraplate areas

Several studies have cited soft-sediment deformation as evidence of fault reactivation and paleoseismicity in intraplate coastal settings. However, a consensus has not been reached on the extent of paleoseismic activity in several regions and the link with continental-scale faults. This research aims to determine the origin of the deformation in soft-sediments, both in the Neogene Barreiras Formation and in Pleistocene sediments of the Sergipe-Alagoas Basin, on the continental margin of northeastern Brazil. The region shows a historical record since 1808 of earthquakes with maximum modified Mercalli intensity of VII and a 40-year instrumental record of events with maximum body-wave magnitude (mb) of 5.2. The brevity of these records leads to large uncertainties in estimates of fault slip rate, recurrence, and seismic hazard. Geologic studies of seismically induced soft-sediment deformation structures and their relationship to seismically active faults offer the opportunity to reduce these uncertainties. We present detailed descriptions of the deformation structures with emphasis on morphology, ductile or brittle character, size, and distribution of the features. In addition, we integrate geologic data with seismic reflection data, thus allowing for correlation between the normal faults at the seismic scale and the soft-sediment deformation structures at the outcrop scale. Faults associated with seismites at the outcrop scale exhibit the same strike/dip and kinematics and are spatially related to the major faults identified at the seismic-section scale. Regarding ductile features, the most remarkable are convoluted folds associated with a deformed muddy layer, giant sinking structures, and complex folds. Concerning brittle structures, the most striking are clastic dikes, mixed layers, autoclastic breccias, and fault breccias. The alternating records in the Neogene sediments indicate clustered seismic activity, combined with tectonic quiescence periods. In Pleistocene sediments, the size, frequency, lateral continuity, and spatial distribution of tens of kilometers of the deformation structures indicate a period of frequent earthquakes with body-wave magnitudes equal to or greater than 6. The recognition of seismic activity and brittle reactivation of preexisting faults during the Neogene-Quaternary has far-reaching implications for seismic hazards in intraplate settings.

Lithogeochemical and Hyperspectral Halos to Ag-Zn-Au Mineralization at Nimbus in the Eastern Goldfields Superterrane, Western Australia

With new advances in rapid-acquisition geochemical and hyperspectral techniques, exploration companies are now able to detect subtle halos surrounding orebodies at minimal expense. The Nimbus Ag-Zn-(Au) deposit is unique in the Archean Yilgarn Craton of Western Australia. Due to its mineralogy, alteration assemblages, geochemical affinity, and tectonic setting, it is interpreted to represent a shallow water (~650 mbsl) and low-temperature (&lt;250 °C) volcanogenic massive sulfide (VMS) deposit with epithermal characteristics (i.e., a hybrid bimodal felsic deposit). We present a detailed paragenetic account of the Nimbus deposit, and establish lithogeochemical and hyperspectral halos to mineralization to aid exploration. Mineralization at Nimbus is characterized by early units of barren massive pyrite that replace glassy dacitic lavas, and underlying zones of polymetallic sulfides that replace autoclastic monomict dacite breccias. The latter are dominated by pyrite-sphalerite-galena, a diverse suite of Ag-Sb ± Pb ± As ± (Cu)-bearing sulfosalts, minor pyrrhotite, arsenopyrite, and rare chalcopyrite. The main sulfosalt suite is characterized by pyrargyrite, and Ag-rich varieties of boulangerite, tetrahedrite, and bournonite. Zones of sulfide mineralization in quartz-sericite(±carbonate)-altered dacite are marked by significant mass gains in Fe, S, Zn, Pb, Sb, Ag, As, Cd, Ni, Cu, Ba, Co, Cr, Tl, Bi, and Au. Basaltic rocks show reduced mass gains in most elements, with zones of intense quartz-chlorite-carbonate±fuchsite alteration restricted to thick sequences of hyaloclastite, and near contacts with dacitic rocks. Broad zones of intense silica-sericite alteration surround mineralization in dacite, and are marked by high Alteration Index and Chlorite-Carbonate-Pyrite Index (CCPI) values, strong Na-Ca depletion, and an absence of feldspar (albite) in thermal infrared (TIR) data. White mica compositions are predominantly muscovitic in weakly altered sections of the dacitic footwall sequence. More paragonitic compositions are associated with zones of increased sericitization and high-grade polymetallic sulfide mineralization. Chlorite in dacitic rocks often occurs adjacent to zones of sulfide mineralization and is restricted to narrow intervals. Carbonate abundance is sporadic in dacite, but is most abundant outside the main zones of Na-Ca depletion. Basaltic rocks are characterized by strongly paragonitic white mica compositions, and abundant chlorite and carbonate. Shifts from Ca carbonates and Fe-rich chlorites to more Mg-rich compositions of both minerals occur in more intensely hydrothermally altered basaltic hyaloclastite, and near contacts with dacitic rocks. Hanging-wall polymict conglomerates are characterized by minor amounts of muscovitic to phengitic white mica (2205–2220 nm), and an absence of chlorite and carbonate alteration.

Mid-ocean-ridge rhyolite (MORR) eruptions on the East Pacific Rise lack the fizz to pop

Abstract Eruptions on the Alarcon Rise segment of the northern East Pacific Rise (23.55°N, 108.42°W) at 2500–2200 m below sea level (mbsl) produced the most compositionally diverse volcanic suite found along the submarine mid-ocean-ridge (MOR) system, offering an opportunity to compare mafic through silicic eruption styles at the same abyssal depth. Eruption styles that formed evolved volcanic rocks on the submarine MOR have not been studied in detail. The prevalence of lava flows along the MOR indicates that most eruptions are nonexplosive, but some volcaniclastic characteristics suggest that explosive styles also occur. Higher viscosities in intermediate (103–5 Pa·s) versus mafic (101 Pa·s) lavas on Alarcon Rise correspond with larger, more brecciated pillows, while highly viscous rhyolite lavas (106–7 Pa·s) formed rugged domes mostly composed of autoclastic breccia. Although high H2O contents (1.5–2.1 wt%), abundant volcaniclasts, and vesicularities up to 53% in rhyolite might imply eruption explosivity, limited fine-grained ash production and dispersal indicate an effusive origin. Higher viscosities of MOR rhyolite (MORR) magma and small eruption volumes, compared to MOR basalt (MORB), limit bubble coalescence and rapid magma ascent, two likely prerequisites for deep-marine eruption explosivity. This idea is supported by widespread dispersal of basaltic ash, but very limited production and dispersal of silicic ash on Alarcon Rise.

Geology and Petrogenesis of Igneous Rocks from Batur Paleovolcano, Gunungkidul, Yogyakarta: Evidence from their Textures, Mineralogy, and Major Elements Geochemistry

Batur paleovolcano is located in Wediombo Beach area, Gunungkidul Regency, Yogyakarta and is being part of Wuni Formation. Several volcanic products including lava flow, autoclastic breccia and volcanic breccia can be found associated with diorite intrusions. This research is aimed to characterize geological, mineralogical andgeochemical variations of igneous rocks from Batur paleovolcano to understand its petrogenesis. Detailed geological mapping with scale of 1:12,500 is conducted to identify geological aspects and delineate igneous rocks distributions. Igneous rocks and selected wall rocks samples were prepared for laboratory analysis including 8 samples for petrography and 5 samples for ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) analysis. Several geochemical data from previous study are also added to investigate the geochemical variations. Geological condition of the research area consists of four rock units including colluvial deposit, limestone, andesite lava and diorite intrusion. Geological structures found are normal fault and shear joint where the main stress direction is north–south. Petrography analysis showed that igneous rocks in this research area consist of diorite intrusion and andesite lava with phorphyritic texture. Plagioclase become the most abundant minerals found both as phenocryst phase and groundmass. Hornblende only occur as phenocryst phase in minor amounts as accesory mineral. Major elementsgeochemistry analysis showed the rocks are characterized by intermediate silica with low alkali content. They are can be categorized as calc-alkaline series. However, some samples are fall into tholeiitic series. Major elements variation and textural study also indicate the magma is experienced differentiation process by fractional crystallization mechanism. This study suggests that igneous rocks from Batur paleovolcano is formed by two phases of formation. Earlier phase is the formation of andesite lava in island arc tholeiitic tectonic setting then at the later phase is formation of diorite intrusion in the calc-alkaline basalts tectonic setting.

Phenomenon of schistic breccia, autoclastic breccia and radiolarian chert, a Mesozoic tectonic trace in Bantimala Tectonic Complex Area, Pangkep Regency South Sulawesi Province

The presence of the schistic breccia and the autoclastic breccias and the radiolarian chert on the Bantimala tectonic complex is one of the interesting geological phenomena to study. In addition to knowing the position and stratigraphic relationships between radiolarian chert with autoclastic breccia as basement rock, the secular breccia may reveal Bantimala’s tectonic environment in the Mesozoic time. This research uses secondary data assessment method from previous researcher and field investigation in the form of observation and rock sampling for petrographic and chemical analysis (X-Ray Fluorescence) to the chemical elements of rock. The appearance of a field indicating the presence of an alternating of the radiolarian chert with a schistic breccias and a sandstone insertion at the bottom of which is in the same composition as the schistic breccia and the autoclastic breccia as the source of some of its components, indicating the stratigraphy relation between radiolarian chert with schistic breccias are conformably, while the radiolarian chert with an autoclastic breccia is unconformably. The metamorphic rock components present in the schistic breccia and the autoclastic breccia are varied in the schistic breccias of about 42.94 - 91.44% SiO2, while the autoclastic breccia 33 - 92.02% SiO2 showing of protolith a mixing of sedimentary rocks, ultramafic, basalt and intermediates rocks. The result of plotting on the spider diagram of trace elements and rare earth elements indicates that the autoclastic breccias originate from the Mid-oceanic ridge basalt (MORB), while the schistic breccias are derived from MORB, OIB and active continental margin.

Seismically induced soft-sediment deformation structures in the Palaeogene deposits of the Liaodong Bay Depression in the Bohai Bay basin and their spatial stratigraphic distribution

Soft-sediment deformation structures (SSDS) have been identified from well cores in the Palaeogene deposits of the Liaodong Bay Depression in the Bohai Bay basin, China. These deposits formed as interbedded sand and mud at a delta front or on the slope toe of the prodelta. According to criteria proposed by previous research, we established that these SSDS were induced by earthquakes and that they can be divided into two groups: ductile deformation structures (plastic intrusions, ball-and-pillow structures, flame structures, boudinage structures, irregular convolute stratifications, and synsedimentary faults and folds) and brittle deformation structures (sand dykes and autoclastic breccias). Based on their level of deformation, size, and complexity, the SSDS were divided into three Groups, from weak to strong, to reflect the intensity of palaeo-earthquakes. With consideration of the palaeo-sedimentary environment, we proposed a model to account for the production and preservation of these SSDS. According to the classification adopted in this study and the spatial stratigraphic distribution of the SSDS, the tectonic activities of the Tan–Lu faults in the Bohai Bay basin were investigated. The A and B oilfields (assumed names) are located in the tectonically active zones of the west and east branches of these faults, respectively. The extension tectonic activities in the A oilfield region exhibit a sharply decreasing trend from E2s3 to E2s1, and increase again in E3d2; whereas the strike–slip tectonic activities in the B oilfield region exhibit an increasing trend from E2s3 to E2s1, and finally, reach a maximum to E3d3. The results of this study show that the method of analysis of the spatial stratigraphic distribution of SSDS is suitable for determining the evolution of tectonic activity and thus, it can provide a new perspective for basin analysis.

Soft-sediment deformation structures in the Cambrian (Series 2) tidal deposits (NW Estonia): Implications for identifying endogenic triggering mechanisms in ancient sedimentary record

Soft-sediment deformation structures (SSDS) are documented in several horizons within silt- and sandstones of the Cambrian Series 2 (Dominopolian Stage) Tiskre Formation, and some in the underlying argillaceous deposits of the Lükati Formation in NW Estonia, northern part of the Baltoscandian Palaeobasin. The aim of this study was to map, describe, and analyze these deformation features, and discuss their deformation mechanism and possible triggers. Load structures (simple load casts, pillows, flame structures, and convoluted lamination) with varying shapes and sizes occur in the Tiskre Formation in sedimentary interfaces within medium-bedded peritidal rhythmites (siltstone-argillaceous material) as well as within up to 3m thick slightly seaward inclined stacked sandstone sequences. Homogenized beds, dish-and-pillar structures, and severely deformed bedding are also found within these stacked silt- and sandstone units and within a large tidal runoff channel infill. Autoclastic breccias and water-escape channels are rare and occur only in small-scale — always related to thin, horizontal tidal laminae. Profound sedimentary dykes, sand volcanoes, and thrust faults, which are often related to earthquake-triggered soft sediment deformation, were not observed within the studied intervals. Deformation horizon or horizons with large flat-topped pillows often with elongated morphologies occur at or near the boundary between the Tiskre and Lükati formations. Deformation mechanisms identified in this study for the various deformation types are gravitationally unstable reversed density gradient (especially in case of load features that are related to profound sedimentary interfaces) and lateral shear stress due to sediment current drag (in case of deformation structures that are not related to loading at any apparent sedimentary interface). Synsedimentary liquefaction was identified as the primary driving force in most of the observed deformation horizons. Clay thixotropy may have contributed to the formation of large sandstone pillows within the Tiskre–Lükati boundary interval at some localities. It is discussed here that the formation of the observed SSDS is genetically related to the restless dynamics of the storm-influenced open marine tidal depositional environment. The most obvious causes of deformation were storm-wave loading, rapid-deposition and shear and slumping caused by tidal surges.

Growth after collapse: the volcanic and magmatic history of the Neostromboli lava cone (island of Stromboli, Italy)

This study focuses on the Holocene Neostromboli lava cone which is part of the Stromboli volcano (Aeolian Islands, Italy). It represents an important example of the structure and behaviour of lava cone re-growth shortly after a large-scale volcano sector collapse. Neostromboli’s eruptive style is dominated by extrusion of sheet-like a’a lava flows and associated autoclastic breccias. The lava cone developed during five phases of activity (phases A to E) representing discrete episodes of lava effusion characterised by rapid changes in magma composition, spatial development of the feeder system and physical properties of erupted lavas through time. Although they all belong to the potassic series (KS) with SiO2 49–55 wt% and MgO 2.50–6.25 wt%, the Neostromboli lavas have distinct petrogenetic characteristics ranging from leucite-free and low-Sr shoshonitic basalts (phases A and B) to leucite-bearing shoshonitic basalts with high (phase C) and intermediate (phase D) Sr contents, and to leucite-free, biotite-bearing shoshonites (phase E). Three different batches of mantle-derived, relatively primitive basic magmas fed the activity of phases A, C and D. The evolution phases A to B is inferred to have occurred in a continuously replenished, tapped and crystallizing (RTF) magma chamber whereas phase E products were dominantly generated by fractional crystallization from phase D magmas. The Neostromboli cone shows summit, flank and satellite vents and sheet intrusions with different geometry and location through time, reflecting the interplay of regional tectonics, precursory instability processes and volcano load.

Extensive soft-sediment deformation structures in the early Darriwilian (Middle Ordovician) shallow marine siliciclastic sediments formed on the Baltoscandian carbonate ramp, northwestern Estonia

Abstract The nearshore Middle Ordovician (early Darriwilian) deposits of the Baltoscandian Basin, formed on the flank of tectonically stable Fennoscandian Shield, comprise numerous soft-sediment deformation structures. The areal extent of the deformed interval extends up to 9000 km 2 from NW Estonia to SE Sweden. The deformation structures occur within a meter-scale sandstone horizon that includes load casts, flame structures, ball-and-pillow morphologies, up to 2 m wide sedimentary dykes, autoclastic breccias, sand volcanoes, and other related soft-sediment deformation structures. Evidently, these deformation structures formed during a single seismic event that caused liquefaction and fluidization of the water-saturated unconsolidated shallow marine sediments within almost the entire sandstone unit (Pakri Formation). Probably a unique catastrophic earthquake of magnitude 7 or higher occurred in this region penecontemporaneously with the deposition of these sediments. The ultimate cause of seismic event of such magnitude in the tectonically stable Baltoscandian Basin is unclear. However, deposition and deformation of the Pakri Formation fall within the Middle Ordovician meteoritic bombardment period which resulted from the disruption of a large parent body in the asteroid belt approximately 470 Ma ago. This in turn suggests a potential impact-origin for the single catastrophic earthquake in the tectonically stable study region. Although impact crater of suitable age is currently not known from the region, then previous studies have documented some impact metamorphic shock lamellae (i.e. PDFs) and abundant extraterrestrial chromite grains from the deformed sediments.

Progradational Holocene carbonate tidal flats of Crooked Island, south‐east Bahamas: An alternative to the humid channelled belt model

AbstractThe stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits. Representing accumulation at or near sea‐level, these deposits have provided insights into past palaeoenvironments, sea‐level and climate change. To expand understanding of carbonate peritidal systems, this study describes the geomorphology, sedimentology and stratigraphy of the tidal flats on the Crooked‐Acklins Platform, south‐east Bahamas. The Crooked Island tidal flats extend continuously for ca 18 km on the platformward flank of Crooked Island, reaching up to 2 km across. Tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics: (i) supratidal (continuous supratidal crust and pavement); (ii) upper intertidal, with the mangrove Avicennia germinans and the cyanobacteria Scytonema; (iii) lower intertidal (with the mangrove Rhizophora mangal) and (iv) non‐vegetated, heavily burrowed subtidal (submarine). These zones have gradational boundaries but follow shore‐parallel belts. Coring reveals that the thickness of this mud‐dominated sediment package generally is &lt;2 m, with depth to Pleistocene bedrock gradually shallowing landward. The facies succession under much of the tidal flat includes a basal compacted, organic‐rich skeletal‐lithoclast lag above the bedrock contact (suggesting initial flooding). This unit grades upward into rhizoturbated skeletal sandy mud (subtidal) overlain by coarsening‐upward peloid‐foraminifera‐gastropod muddy sand (reflecting shallowing to intertidal elevations). Cores from landward positions include stacked thin indurated layers with autoclastic breccia, root tubules and fenestrae (interpreted as supratidal conditions). Collectively, the data reveal an offlapping pattern on this prograding low‐energy shoreline, and these Holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats. Nonetheless, their vertical facies succession is akin to that present beneath channelled belt examples, suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology, lateral facies changes and heterogeneity in stratigraphic analogues.

Analysis of deformation structures in Pliocene and Quaternary deposits of the Hyblean Plateaux (south-eastern Sicily)

A singular association of deformation structures that have disturbed terrains up to Quaternary age, has been detected along the rocky coast of Vendicari (south-eastern Sicily). These structures are soft sediment deformations consisting of autoclastic breccias, diapyr-like injections and thixotropic wedges, as well as fractures, generally opened and filled by sediments. Both soft and brittle deformations can be due either to trigger mechanisms internal to the sedimentary environment or to external causes. In order to define the deformation mechanisms affecting the deposits, the local paleo-environmental reconstruction and detailed deformation structure investigations have been performed. Moreover, fractures have been analysed by a mesostructural study and the examination of the microscopic features of the filling materials, to highlight possible relationships with the stress field induced by regional tectonics. The multi-theme analysis allowed us to discard internal trigger mechanisms as responsible of the deformations and to reasonably infer these deformation structures as being seismically induced. This interpretation is also supported by similarities with structures studied in other areas worldwide and commonly interpreted as seismites. The link between these records of seismic activity and the eastern Sicily seismicity is set into evidence by the analysis of the historical accounts which report the occurrence of structures similar to those detected at Vendicari, in several sites close to the study area. A major result of the present study is to improve the catalogue of deformation features, recorded in sedimentary units, attributed to earthquakes.

Syndepositional tectonics recorded by soft-sediment deformation and liquefaction structures (continental Lower Permian sediments, Southern Alps, Northern Italy): Stratigraphic significance

The Lower Permian succession of the Central Southern Alps (Lombardy, Northern Italy) was deposited in fault-controlled continental basins, probably related to transtensional tectonics. We focussed our study on the stratigraphic record of the Lower Permian Orobic Basin, which consists of a 1000 m thick succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo Formation, PDV) resting on the underlying prevailing pyroclastic flows of the Cabianca Volcanite. The PDV consists of a lower part (composed of conglomerates passing laterally to sandstones and distally to silt and shales), a middle part (pelitic, with carbonates) and an upper part (alternating sandstone, silt and volcanic flows). Syndepositional tectonics during the deposition of the PDV is recorded by facies distribution, thickness changes and by the presence of deformation and liquefaction structures interpreted as seismites. Deformation is recorded by both ductile structures (ball-and-pillow, plastic intrusion, disturbed lamination, convolute stratification and slumps) and brittle structures (sand dykes and autoclastic breccias). Both the sedimentological features and the geodynamic setting of the depositional basin confidently support the interpretation of the described deformation features as related to seismic shocks. The most significant seismically-induced deformation is represented by a slumped horizon (about 4 m thick on average) which can be followed laterally for more than 5 km. The slumped bed consists of playa-lake deposits (alternating pelites and microbial carbonates, associated with mud cracks and vertebrate tracks). The lateral continuity and the evidence of deposition on a very low-angle surface along with the deformation/liquefaction of the sediments suggest that the slump was triggered by a high-magnitude earthquake. The stratigraphic distribution of the seismites allows us to identify time intervals of intense seismic activity, which correspond to rapid and basin-wide changes in the stratigraphical architecture of the depositional basin and/or to the reprise of the volcanic activity. The nature of the structures and their distribution suggest that the magnitude of the earthquakes responsible for the observed structures was likely higher than 5 (in order to produce sediment liquefaction) and probably reached intensity as high as 7 or more. The basin architecture suggests that the foci of these earthquakes were located close to the fault-controlled borders of the basin or within the basin itself.

Vulkanisme kompleks Gunung Patiayam di Kecamatan Jekulo, Kabupaten Kudus, Provinsi Jawa Tengah

http://dx.doi.org/10.17014/ijog.vol3no2.20082The Mount Patiayam Complex was interpreted by previous researchers as “Patiayam Dome”. That was reasoned by dips following its slope directions. Field data record that lithology of the complex of Mount Patiayam is dominated by volcanic rocks. The summit of the complex is composed of igneous rocks of pyroxene basalt rich in leucite minerals, associated with autoclastic breccia and beds of volcanic breccia rich in pyroxene basalt and pumice, pumiceous breccia, and tuff. Its flanks are composed of epiclastic rocks of lahar and fluvial deposits. Some river valleys, such as Pontang River, locally consist of pyroclastic breccia, autoclastic breccia and pumiceous breccia, that are overlain by marly limestone and black clay of swampy deposits. Based on the rock composition, the volcanism had more dominated geological processes compared with sedimentary. Mineral composition of volcanic deposits of the Mount Patiayam is closer to Mount Lasem volcanic rocks than Mount Muria, i.e. absarockites, shoshonites and trachyandesite. But, based on the long distance between Patiayam and Lasem, about 60 km, those volcanic rocks could not be produced by Mount Lasem. The Patiayam volcanic deposits were produced by its own volcanic activities. Therefore, the complex of Mount Patiayam is a paleo-volcano. The interpretation is also supported by the landsat imagery, showing depression-shapes in a caldera ring-like. There are four caldera features called as Rim 1,Rim 2,Rim 3, and Rim 4, which crosses each others.

Geology of the Side Crater of the Erebus volcano, Antarctica

The summit cone of the Erebus volcano contains two craters. The Main crater is roughly circular (∼ 500 m diameter) and contains an active persistent phonolite lava lake ∼ 200 m below the summit rim. The Side Crater is adjacent to the southwestern rim of the Main Crater. It is a smaller spoon-shaped Crater (250–350 m diameter, 50–100 m deep) and is inactive. The floor of the Side Crater is covered by snow/ice, volcanic colluvium or weakly developed volcanic soil in geothermal areas (a.k.a. warm ground). But in several places the walls of the Side Crater provide extensive vertical exposure of rock which offers an insight into the recent eruptive history of Erebus. The deposits consist of lava flows with subordinate volcanoclastic lithologies. Four lithostratigraphic units are described: SC 1 is a compound lava with complex internal flow fabrics; SC 2 consists of interbedded vitric lavas, autoclastic and pyroclastic breccias; SC 3 is a thick sequence of thin lavas with minor autoclastic breccias; SC 4 is a pyroclastic fall deposit containing large scoriaceous lava bombs in a matrix composed primarily of juvenile lapilli-sized pyroclasts. Ash-sized pyroclasts from SC 4 consist of two morphologic types, spongy and blocky, indicating a mixed strombolian-phreatomagmatic origin. All of the deposits are phonolitic and contain anorthoclase feldspar. The stratigraphy and morphology of the Side Crater provides a record of recent volcanic activity at the Erebus volcano and is divided into four stages. Stage I is the building of the main summit cone and eruption of lavas (SC 1 and SC 3) from Main Crater vent(s). A secondary cone was built during Stage II by effusive and explosive activity (SC 2) from the Side Crater vent. A mixed strombolian and phreatomagmatic eruption (SC 4) delimits Stage III. The final stage (IV) represents a period of erosion and enlargement of the Side Crater.

Geotechnical classification and characterisation of materials for stability analyses of large volcanic slopes

The main challenge faced by scientist studying large volcanic landslides is appropriate geomechanical characterisation of volcanic materials. Regardless of its significance, published data is scarce and does not present standard classifications or procedures. Herein a new global unifying geotechnical classification of volcanic materials encompassing geotechnical units potentially found in volcanic areas is proposed. Volcanic materials are divided into four main geotechnical units: lavas, autoclastic breccias, pyroclastic rocks and volcanic soils, which are further subdivided on the basis of hydrothermal alteration, welding and interlocking. A combination of new data from the slopes of Teide stratovolcano in Tenerife, Spain, and normalised reworked data from a very extensive literature review are presented for each geotechnical unit. Most rock matrix strength measurements in Tenerife were carried out with indirect methods (Schmidt hammer and point load tests) due to the remoteness of most outcrops and the abruptness of volcanic terrain. Rock mass quality was assessed in terms of the geological strength index (GSI). Lavas and strongly welded pyroclastic rock units have the highest uniaxial compression strengths ( σ ci = 107 and 82 MPa, respectively) although these values are largely affected by hydrothermal alteration. Lavas have a worse rock mass quality (GSI = 55) than strongly welded pyroclastic rocks (GSI = 80). Special emphasis has been placed on studying autoclastic breccia and weakly welded and/or interlocked pyroclastic rock units, which despite having been classified as disintegrated granular units in the past, are here classified as rock masses due to the high degree of interlocking between clasts. They present relatively weak rock matrices ( σ ci = 29 and 13 MPa, respectively) but good rock mass qualities (GSI = 60 and 80). Volcanic soils are the weakest materials found within edifices and therefore appear to be significant in the generation of large slope instabilities. The effects of high temperatures and pressures are discussed, as are the problems which arise when calculating equivalent linear Mohr–Coulomb geotechnical parameters for those rock masses which are characterised as Hoek and Brown materials.