Silurian Beds Research Articles

The soft-sediment deformation structures and the siliceous concretion presence as indicators of the depositional processes during Middle Paleocene to Middle Eocene in the Apulian Platform Margins in Kefalonia Island, Greece

Kefalonia Island is situated in western Greece and geologically corresponds to the Apulian Platform Margins (APM), with the Apulian Platform lying to the west and the Ionian Basin to the east. The studied deposits, with a thickness up to 164.5, extend along the 650 m long Agia Efimia bay, ranging from the Middle Paleocene (Selandian) to the uppermost Eocene (Priabonian), during the latest part of the rift stage. Fieldwork measurements of soft-sediment deformation structures (SSDS), paleocurrent directions, the size and abundance of siliceous concretions (SC) and siliceous beds (SB), and shale horizons were correlated with the age of selected samples, and thus, the relation of the tectonic regime with the depositional conditions was presented. As SSDS could represent an indicator of tectonic activity and instability of the basin floor instability, the size and the abundance of the SSDS whereas used to identify the parameters that produced the basin floor instability like the intensity of tectonic activity. The studied section, with a general NE-SW orientation, was subdivided into five (5) different units. The changes of these units in paleocurrent directions, the size, the abundance, and the thickness of SSDS, SC, and SB and the presence of shale horizons in the upper unit, characterize a gradual change of the depositional conditions in the APM, from the Upper Paleocene (Selandian) to the Upper Eocene (Priabonian). The factor that influenced the abovementioned changes was related with the presence and the activity of normal faults that produced instability of the basin floor. Therefore, a reduction in tectonic activity from the Paleocene to the Eocene is suggested.

Zonation of Lithuanian Silurian graptolites and other faunal groups

The Baltic Silurian Basin, Lithuanian Depression and other structures are shown in the map, with marked by isopachs (contour lines of equal thickness) of the Silurian beds with graptolites and fauna of other groups. The Silurian facies vary greatly in the Depression – from clayey open-sea deep shelf to carbonaceous ones of shallow shelf, and low-energy lagoon facies. The history of investigations on East Baltic area graptolites begins from 1953–1958, when 15 graptolite zones were singled out, and proceeds to 35 zones defined now. Peculiarities in the graptolite scale from C. cyphus to N. lochkovensis inclusive are discussed. Transgressions and regressions of the Silurian marine basin, as well as shorter transgressions with wedges and graptolites of clayey facies shifted towards basin shores and regressions with partial extinction of graptolites are elucidated. During these investigations the graptolite scale has been detailed and added with new zones. Graptolite evolution in the zones has been analysed. Stages of graptolite evolution are analysed in relation to the following bioevents: Stačiūnai, Likėnai, Valgu, Ireviken, Mulde, Linde, Lau, Klev and Šilalė. Finally, two tables present graptolite zone correlation with conodont, vertebrate and ostracod zones revealing a highly detailed stratigraphy of the Lithuanian Silurian.

The Schandelah Scientific Drilling Project: A 25-million year record of Early Jurassic palaeo-environmental change from northern Germany

With the aim to understand prolonged and repeated marine anoxia after the Triassic-Jurassic mass- extinction event, a continuously cored, 338 metre thick succession of Rhaetian to Toarcian sediments was retrieved close to the village of Schandelah near Braunschweig (Lower Saxony, northern Germany). Here, preliminary biostratigraphical, lithological, sedimentological, geochemical, and geophysical borehole data are presented and discussed. Based on the presence of ammonites, ostracods, benthic foraminifers, calcareous nannofossils, and palynomorphs, all major Late Triassic and Early Jurassic stage boundaries and many of the standard Lower Jurassic ammonite zones could be defined. The deltaic Rhaetian sand- and siltstone succession (Exter Fm) contains evidence for seismic activity probably related to large-scale geodynamic processes. The Hettangian (Lias Alpha) is represented by a thick heterolithic succession composed of shallow marine sandy and silty beds with intercalated organic-rich shale, representing deposition on a shoreface with frequent storm activity. Progressive deepening during the Sinemurian to Toarcian resulted in repeated deposition of laminated organic-rich facies. Periods of relative sea-level fall likely occurred during the Late Hettangian, the Late Sinemurian, and Early Pliensbachian, where a series of hardgrounds occur indicating erosion on the sea floor followed by sea level rise leading to omission, and increased reworking. One of the most conspicuous features of the Lower Jurassic in the Schandelah-1 core is the presence of abundant authigenic carbonates (glendonites,concretions, beef-calcite) within the Upper Pliensbachian with partly very negative C-isotope values (down to –37‰ V-PDB) suggesting the anaerobic oxidation of methane. A high-resolution organic carbon isotope record based on 485 analyses shows two major negative carbon isotope excursions (CIEs) within the lowermost Hettangian and Lower Toarcian, respectively. Both excursions coincide with the onset of black shale deposition and are well-known features of these time-intervals in other regions. Despite the fact that black shale deposition also characterizes parts of the Sinemurian and Pliensbachian in similar facies, no large negative CIEs are apparent. The Schandelah-1 core thus provides an unique archive of sedimentary, biotic and geochemical records of long-term Triassic-Jurassic palaeo-environmental change in the European Epicontinental Seaway.

Mineralogy of siliceous concretions, cretaceous of ionian zone, western Greece: Implication for diagenesis and porosity

Siliceous (chert) concretions are a prominent component of Cretaceous limestones in the Ionian zone of western Greece. This study investigates the diagenetic evolution of such cherts from two localities: largely undeformed strata on Kastos Island and highly faulted strata at Araxos. In both areas, lower Cretaceous limestones contain abundant small chert nodules, whereas in the upper Cretaceous nodules are larger but less abundant. Seventy seven powdered samples, from 67 representative field samples, were analyzed by X-ray Powder diffraction (XRPD). The analyzed samples came from both nodular and siliceous beds and showed that quartz is the dominant mineral, with rare to common residual calcite. The presence of moganite and opal-A minerals suggests that quartz recrystallized from original amorphous silica. A few samples from Araxos contain traces of halite and anhydrite. The variability in the calcite content and the presence/absence of halite and anhydrite in the different studied areas are related with the different tectonic activity in these areas, with more intense tectonic activity in NW Peloponnesus than in Kastos Island. Size of nodules is related to variation in primary porosity of the host rocks. Finally, the presence of dolomite was related with the pre-existing underlying dolomites of Early Cretaceous age in Ionian basin, whereas the presence of maghemite is due to the presence of iron minerals in water solutions. • Zoned chert nodules and beds in Cretaceous calcturbidites and landslide deposits. • Moganite and opal-A minerals recrystallized from original amorphous silica. • Size of chert concretions related to porosity of host limestones. • Dolomite in cherts only where underlying dolostone. • Halite and anhydrite in cherts due to Paleogene deformation of Triassic evaporites .

Pore characterization and shale facies analysis of the Ordovician-Silurian transition of northern Guizhou, South China: The controls of shale facies on pore distribution

The Ordovician-Silurian Wufeng-Longmaxi Formations in the Upper Yangtze Platform have been considered as the most promising shale play in China. However, few studies of detailed shale facies and their influence on pore distribution have been conducted. Therefore, it is important to describe the heterogeneity of shale facies in detail and elucidate their relationship to pore networks and development. Seven distinct facies were identified on the basis of their characteristic physical, biological, and chemical attributes. Ion-milled samples from all facies were examined with SEM for identification of pore types. Point-counting was used to quantify porosity. Four major pore types appear dominant: framework pores, phyllosilicate framework (PF) pores, intrapores within inorganic grains, and organic matter (OM) pores. Due to variability in composition and depositional setting of the different facies, pore networks are not uniform. Framework pores between detrital grains and PF pores are widespread in bioturbated claystones, Pores in black siliceous shales are dominated by framework pores supported by authigenic silica. Most of these are filled with kerogen/bitumen that is host to “foam” and “bubble” type OM pores. Intrapores within calcite or dolomite grains are common in fossiliferous mudstones and black dolomitic siltstones, but are isolated and do not contribute to productive porosity. In muddy siltstones, PF pores are well developed within silty bands. Initial framework pores within silty beds or laminae, however, were subsequently cemented by calcite or destroyed by compaction. As most of primary PF pores and primary frameworks pores are lost during compaction, porosity is highly dependent on the population of framework pores between authigenic quartz grains. Secondary cracking of amorphous organic matter added porosity after initial infilling of porosity by oil and bitumen migration. Therefore, porosity is mainly controlled by abundance of authigenic silica and TOC, in spite of differences of burial diagenesis among various facies. Understanding these main controls on porosity should be helpful to predict and find and more porous facies and have a significant impact on future gas exploration in the Wufeng-Longmaxi Formations.

Geological significance of the discovery of Middle Triassic (Ladinian) radiolarians from the Hong Hoi Formation of the Lampang Group, Sukhothai Zone, northern Thailand

This paper presents a newly found fauna of Ladinian (Middle Triassic) radiolarians from the Hong Hoi Formation of the Lampang Group, Sukhothai Zone, northern Thailand. The Ladinian age determined by the radiolarian fauna concords with the age previously determined by mollusks. The study section consists of intercalated sandstone and shale in the lower part and a thick conglomerate in the upper part. The radiolarian-bearing siliceous beds are intercalated within the lower unit. The lithic sandstone of the lower unit consists mainly of volcanic rock fragments, quartz, and feldspar, whereas the thickly bedded conglomerate of the upper unit is characterized by abundant gravel-sized clasts of volcanic rock and limestone. These lithic features of the study section suggest that during deposition the Hong Hoi Formation was located near a supply of volcanic materials. A forearc basin close to the Sukhothai Arc would be the most suitable environment for the deposition. The Ladinian age determined by the radiolarian fauna supports the occurrence of intensive volcanic activity within the Sukhothai Arc during the Middle Triassic.

Minerals in the Triassic carbonaceous silicites of Sikhote Alin

Over 60 minerals, including native elements, intermetallic compounds, haloids, sulfides, sulfates, arsenides, oxides and hydroxides, silicates, borosilicates, wolframates, phosphates and REE phosphates, were established in Triassic siliceous rocks of Sikhote Alin. Allothigenic and authigenic minerals in the carbonaceous silicites were formed over a long period through several stages. Judging from morphology, chemical composition, and structural position, K-feldspar (K-Fsp), illite, kaolinite, metahalloysite, monazite, xenotime, zircon, rutile, or its polymorphs are the disintegration products of sialic rocks of continental crust. Authigenic sulfides are dominated by diagenetic pyrite (fine-crystalline, microglobular, framboidal, as well as those developed after biogenic siliceous and carbonate fragments), which has been formed prior to precipitation of siliceous cement and lithification of siliceous rocks. Most of other sulfides (sphalerite, galena, chalcopyrite, pyrrhotite, argentite, pentlandite, antimonite, ulmanite, and bravoite), arsenides and sulfoarsenides (arsenopyrite, nickeline, skutterudite, cobaltite, glaucodot, and gersdorffite), wolframates (scheelite and wolframite), intermetallides (Cu2Zn, Cu3Zn2, Cu3Zn, Cu4Zn, CuSn, Cu4Sn, Cu8Sn, Cu4Zn2Ni, Ni2Cu2Zn, Ni4Cd), and native elements (Au, Pd, Ag, Cu, Fe, W, Ni, Se) were crystallized later (during catagenesis after the lithification and brecciation of siliceous beds) from metals involved in the easily mobile fractions of bitumens. Supergene mineral formation was mainly expressed in the sulfide oxidation and replacement of diagenetic pyrite by jarosite and iron hydroxides.

A record of astronomically forced climate change in a late Ordovician (Sandbian) deep marine sequence, Ordos Basin, North China

The late Ordovician Pingliang Formation on the southwestern margin of the Ordos Basin, North China, consists of rhythmic alternations of shale, limestone, and siliceous beds. To explore the possible astronomical forcing preserved in this lithological record, continuous lithological rank and magnetic susceptibility (MS) stratigraphic series were obtained from a ~34m thick section of the Pingliang Formation at Guanzhuang. Power spectral analysis of the MS and rank series reveal 85.5cm to 124cm, 23cm to 38cm, and 15cm to 27cm thick sedimentary cycles that in ratio match that of late Ordovician short eccentricity, obliquity and precession astronomical cycles. The power spectrum of the MS time series, calibrated to interpreted short orbital eccentricity cycles, aligns with spectral peaks to astronomical parameters, including 95kyr short orbital eccentricity, 35.3kyr and 30.6kyr obliquity, and 19.6kyr and 16.3kyr precession cycles. The 15cm to 27cm thick limestone–shale couplets mainly represent precession cycles, and siliceous bed deposition may be related to both precession and obliquity forcing. We propose that precession-forced sea-level fluctuations mainly controlled production of lime mud in a shallow marine environment, and transport to the basin. Precession and obliquity controlled biogenic silica productivity, and temperature-dependent preservation of silica may have been influenced by obliquity forcing.

New data on the stratigraphy of the Ordovician and Silurian of the central region of Kotelnyi Island (New Siberian Islands) and correlation with the synchronous successions of the Eastern Arctic

Much of the material of Paleozoic successions of Kotelnyi Island (New Siberian Islands) is composed of various carbonates, which accumulated in relatively shallow conditions in the Ordovician–Middle Devonian. Such deposits are widespread in synchronous sections throughout the Eastern Arctic and therefore attract attention as a potential tool for interregional correlations. In paleotectonic reconstructions, Kotelnyi Island either is included in terrain which was disconnected from the Siberian Platform in the Paleozoic or is joined to it. These different interpretations result from lack of primary evidence. This paper presents new data on stratigraphy and macroand microfauna of the Upper Ordovician and Silurian of the central regions of Kotelnyi Island. For this interval, we propose a correlation of the studied sections with those of adjacent regions. A conclusion is drawn on the basis of the similarity of lithology and fossil assemblages and also shared trends in the evolution of the sedimentary environment that the Upper Ordovician–Silurian beds of Kotelnyi Island, Taimyr, and the Siberian Platform accumulated in a single shelf basin, which apparently also extends to northeastern Chukotka.

Detecting fold structures at the southern flank of the Neogene Vienna Basin in eastern Austria using near-subsurface geophysical methods

At the southern flank of the Vienna Basin, spectacular fold structures were exposed in the former Steinbrunn sand pit. The succession of Upper Pannonian age consists of decimeter- to meter-thick sandy, silty, and clayey beds, which are overlain by sandstone beds. Previously, these fold structures have been interpreted differently as of tectonic but also gravitational origin. To gain a more detailed insight into the 3D distribution and orientation of the folds, high-resolution geophysics such as electrical resistivity tomography (ERT), ground-penetrating radar (GPR), and electromagnetics (EM) were applied for mapping the subsurface in the surroundings of the sand pit. The EM results found that the uppermost layer was more clayey northwest and sandier southeast of the sand pit. To directly compare ERT and GPR results with the lithology of the fold structures observed in the sand pit, reference profiles behind the wall of the sand pit were performed. Both methods clearly revealed fold structures parallel to the folded Pannonian strata of the outcrop. Although the GPR data displayed boundaries between sandy and clayey sediments, and thus clearly imaged the fold geometry of the succession, the resistivities in the ERT profiles portrayed a smoother image. Almost all GPR profiles revealed wavelike structures with axes in the southeastern direction and dome-shaped structures with axes in the southwestern direction, deepening toward the west. This pattern was comparable with the sections of rounded buckle folds, which underlie almost the entire investigation area of approximately [Formula: see text] in size. Compared with fold structures as documented from a neighboring abandoned coal mine, the geophysical investigations around the Steinbrunn sand pit supported the hypothesis of a tectonic origin, formed by simple buckling of a planar surface and its confining volume along the southeastern flank of the Vienna Basin in Plio/Pleistocene times.

Sedimentary processes and depositional environments of a continuous marine succession across the Lower–Middle Pleistocene boundary: Kokumoto Formation, Kazusa Group, central Japan

The Chiba composite section of the Kokumoto Formation in central Japan is a remarkable, thick marine succession and is a candidate for formal recognition as the Global Boundary Stratotype Section and Point (GSSP) for the base of the Middle Pleistocene stage. Although there have been numerous studies of the Kokumoto Formation, the sedimentology of the formation, particularly in the Chiba composite section, has been unclear. We use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation. The Chiba composite section includes the Matuyama–Brunhes paleomagnetic reversal, which is the primary guide for the Lower–Middle Pleistocene boundary, along with multiple tephra deposits that provide isochronous correlational tie points. One bed, Byk-E (also known as Ontake-Byakubi Tephra), immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary. The predominant silty beds of the Chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites. However, minor sandy beds intercalated within the silty section, particularly in its lower part, are either (1) fining upward sequences with faint planar or ripple laminations and an erosional base, or (2) sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface. The silty beds are interpreted to be hemipelagite formed by deposition of fine-grained suspended material under stable and calm bottom-water conditions. The sandy deposits are interpreted to be sediment gravity-flow deposits (i.e., turbidites and debrites). These sedimentary processes and the trace fossil assemblage indicate that the Chiba composite section was formed in a continental slope environment. The depositional environments of the Chiba composite section, together with tephrostratigraphy, make it suitable for high-resolution studies on the paleoenvironment, paleoclimate, and paleoceanography of the northwest Pacific Ocean across the Lower–Middle Pleistocene boundary; it is therefore a suitable candidate to become the GSSP for the Middle Pleistocene.

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10–265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14C-dated peat deposition rates, and unpublished analyses of the island’s history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180–270 yr for at least the past 3500 yr.

Conodont biostratigraphy across the Permian-Triassic boundary at the Xinmin Section, Guizhou, South China

A continuous sedimentation at the Xinmin Section, Anshun, Guihzou Province, with the Upper Permian Talung Formation dominated by bedded siliceous rocks, and the Lower Triassic Luolou Formation consisting mudstones and marls as well as siliceous mudstones at its basal part indicates that it represents a deeper-water basinal facies across the Permian-Triassic boundary. Based on a systematic conodont biostratigraphic work, nine conodont species belong to two genera have been identified in this study. It enable us to establish five conodont zones at this section, in ascending order, they are: Clarkina changxingensis Zone (beds 1-3-4-2), Clarkina yini Zone (beds 4-3-5-1-1), Clarkina meishanensis Zone (beds 5-1-2-5-2), Hindeodus changxingensis Zone (beds 5-3-1-5-3-2) and Hindeodusparvus Zone (beds 5-3-3-5-3-4), respectively. According to the first occurrence of Hindeodus parvus in bed 5-3-3, the Permian-Triassic boundary is placed at the base of bed 5-3-3. This conodont zonation of the Xinmin Section provides precise biostratigraphic framework for further investigations on the geological events across the Permian-Triassic boundary at this section. In addition, the new conodont data also reveals that several siliceous beds occurred at the basal Triassic. It provides an exception of Early Triassic Chert Gap.

Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous Chimu Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au ± Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimu Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite–rutile ± digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The δ34S values for pyrite (1.7–2.2 ‰) and digenite (2.1 ‰) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimu Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimu Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz–alunite and a quartz–alunite–kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has δ34S values of 24.8–29.4 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of 6.8–13.9 ‰, consistent with H2S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0–2) and logfO2 of −28 to −30. Alunite–pyrite sulfur isotope thermometry records temperatures of 190–260 °C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by 40Ar/39Ar at 17.0 ± 0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite–pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ values of stage IV alunite vary between 11.5 and 11.7 ‰ and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (δ34S = 27.1 to 33.8 ‰ and $$ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $$ = 8.1 to 12.7 ‰). The Δ34Spy–alu isotope thermometry records temperatures of 210 to 280 °C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.

Measurements of riverbed hydraulic conductivity in a semi-arid lowland river system (Murray–Darling Basin, Australia)

Riverbed hydraulic conductivity (Kr) was measured along one river reach in four tributaries of the Murray–Darling Basin (MDB) in south-eastern Australia. Two techniques were trialled: in-river falling-head tests in high Kr sediments, and laboratory evaporation tests on intact riverbed cores for low Kr sediments. In-river falling-head tests were conducted using two types of permeameter: a steel-base permeameter or a stand-pipe permeameter. Kr was found to range from 10–10 to 10–3 m s–1, corresponding to a range in riverbed sediment textures from clay to silty gravels, respectively. Although the within-reach variability in Kr was also large, in general the river reaches could be divided in two groups, those with a low Kr (<10–8 m s–1) or a high Kr (>10–5 m s–1). The low Kr reach (Billabong Creek) was a clay-lined bed, whereas the others had silty sand or silty gravel beds. Thus, regional-scale assessments of Kr in the MDB could be made using a stratified sampling process in which reaches would be first classified into low or high Kr classes, and then Kr measurements made in a subsample of low and high Kr reaches. This would be an improvement over the current practice whereby riverbed Kr is estimated either from regional soil maps or through the calibration of groundwater models.

Experimental Investigation on Scour around Marine Pipelines in Silt Beds by Jet Trenching

Jetting power has been applied more and more frequently to create trench in the seabed to bury cables or pipelines under a protective layer of sand in offshore engineering. However, there are still several key problems to be understood further and solved both in their design and operating. This study investigates the topographic deformation due to the scour of silt beds which lie below marine pipelines and are impinged by moving two jets in a model experimental system. The response of silty sea beds was also examined during the experiment. The important parameter that influences the trenching performance, the jet velocity, was studied. The speed of the jet horizontal motion was varied to examine its effect on the scour profile. By presenting the variation of a scour profile with time after impingement, the change process was analyzed. The asymptotic scour profiles were found to be very similar when made non-dimensional, and a function was employed to describe the non-dimensional profile. The results of the above work will permit a better understanding of the mechanisms controlling the performances of underwater trenching by jet trenching machines.

Were Ediacaran siliciclastics of South Australia coastal or deep marine?

AbstractThe Late Neoproterozoic Ediacara Member of the Rawnsley Quartzite in South Australia has been considered aeolian, fluvial, intertidal and deep marine by various authors. Palaeosols would not be expected for the deep marine interpretation, but some palaeosols should be evident for the aeolian–fluvial–intertidal interpretations, and this is the first study to examine the Ediacara Member at a petrographic and geochemical scale appropriate to recognize potential palaeosols. Recognition of palaeosols and floodplain facies in Neoproterozoic rocks is a challenge because such rocks are too ancient for diagnostic non‐marine fossils such as root traces. The varied thickness of Ediacara Member red siltstones and white sandstones is distinct from laterally persistent overlying and underlying grey shales and limestones with acritarchs, stromatolites and other marine fossils. The sandstones are trough cross‐bedded and fill palaeovalleys. The red siltstones have poorly sorted, highly angular, silt‐size grains characteristic of loess. Particular sandy and silty beds were sampled for detailed petrographic and geochemical studies, because they include desiccation cracks, sand crystals, ice cracks, carbonate nodules and soft‐sediment deformation like those of palaeosols. Chemical and grain‐size variations within these beds reveal surficial clay formation and oxidation from feldspar as in soils. Petrographic studies also revealed surficial disruption of these palaeosols by filamentous structures comparable with microbial ropes of biological soil crusts. This array of palaeosol features may be of use for recognizing palaeosols in other Neoproterozoic siliciclastic sequences.

Siliceous and organic-rich sedimentation during the Cenomanian–Turonian Oceanic Anoxic Event (OAE2) on the northern margin of Africa: an evidence from the Bargou area, Tunisia

The late Cenomanian–early Turonian deposits in Tunisia recording the OAE2 event are commonly attributed to the Bahloul Formation. These deposits are composed of dark clayey limestone and organic-rich black shales with abundant planktic foraminifera. At the Bargou area, Oued Kharroub section, these organic-rich deposits include siliceous beds with abundant radiolarians recalling the “Livello Bonarelli”. A total of 25 radiolarian species are identified with a maximum of 18 species per sample. Moreover, the species skeleton preservation is usually poor to moderate in the most studied samples [estimated preservation index (PI) values ranging between 4 and 6]. Two successive assemblages are distinguished. The oldest one (RI) is dominated by nassellarians, which includes mainly Stichomitra, Pseudodictyomitra and Rhopalosyringium genera. It is included within the upper part of foraminiferal Rotalipora cushmani Zone and lower part of Whiteinella archaeocretacea zone. In contrast, the youngest assemblage (RII), which belongs to the upper part of the W. archaeocretacea zone and the earliest Turonian Watinoceras ammonite assemblage, is dominated by spumellarians and includes mainly Archaeocenosphaera, Cavaspongia, Pseudoeucyrtis and Pyramispongia genera. In addition, major and trace elements are analysed to test whether the geochemical record was synchronous to the biotic event or not. Thus, selected crossplots, Al2O3 vs. SiO2 and Al2O3 vs. TiO2, are used to evaluate the detrital input, and V/Cr vs. V/(V + Ni) and U/Th vs. V/(V + Ni) are used to evaluate the bottom redox conditions and the primary productivity within the Bahloul Formation. A strong Si/Al increase is marked in the lower part of the siliceous and organic-rich Bahloul Formation that could be explained by a local increase in upwelling-related biogenic SiO2 (silica-secreting organisms). Relative low abundance of terrigenous-related Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba (organic matter related trace elements) suggests that the Bahloul was of relatively elevated primary productivity and minimal detrital input. Enrichments in redox-sensitive trace metals U and V in the Bahloul Formation deposits and redox indices, such as V/(V + Ni), U/Th and V/Cr, indicate that oxygen-restricted conditions prevailed during the late Cenomanian–earliest Turonian times and correlate well with relative abundances of some radiolarian and foraminiferal paleo-environmental indicators. In addition, indicators of detrital flux variations are used. Al2O3 shows a slight positive correlation with TiO2 and a less distinct correlation with SiO2.

Seismites in the Kathmandu basin and seismic hazard in central Himalaya

Soft-sediment deformation structures have been analyzed at six sites of the Kathmandu valley. Microgranulometric study reveals that silty levels (60 to 80% silt) favor the development of soft-sediment deformation structures, while sandy levels (60 to 80% sand) are passively deformed. Nonetheless well sorted sand levels (more than 80% sand) generate over-fluid pressure during compaction if located beneath a silty cap, leading to fluidization and dike development. 3-D geometry of seismites indicates a very strong horizontal shearing during their development. Using a physical approach based on soil liquefaction during horizontal acceleration, we show that the fluidization zone progressively grows down-section during the shaking, but does not exactly begin at the surface. The comparison of bed-thickness and strength/depth evolution indicates three cases: i) no soft-sediment deformation occurs for thin (few centimeters) silty beds; ii) the thickness of soft-sediment deformation above sandy beds is controlled by the lithological contrast; iii) the thickness of soft-sediment deformation depends on the shaking intensity for very thick silty beds. These 3 cases are evidenced in the Kathmandu basin. We use the 30 cm-thick soft-sediment deformation level formed during the 1833 earthquake as a reference: the 1833 earthquake rupture zone extended very close to Kathmandu, inducing there MMI IX–X damages. A 90 cm-thick sediment deformation has therefore to be induced by an event greater than MMI X. From a compilation of paleo and historic seismology studies, it is found that the great (M ~ 8.1) historical earthquakes are not characteristic of the greatest earthquakes of Himalaya; hence earthquakes greater than M ~ 8.6 occurred. Kathmandu is located above one of the asperities that laterally limits the extent of mega-earthquake ruptures and two successive catastrophic events already affected Kathmandu, in 1255 located to the west of this asperity and in ~ 1100 to the east.

Petroleum Migration Direction of the Silurian Paleo‐pools in the Tarim Basin, Northwest China

Abstract: The results obtained in this paper indicate that carbazole‐type compounds have high thermal stability and also show stability in oxidation and bio‐degradation. This kind of compounds still has a high concentration and complete distribution in the analyzed dry asphalt samples, showing that they are particularly useful in studying petroleum migration of paleo‐pool. During the basin's first‐stage of oil‐gas pool formation in the Silurian in Tazhong and Tabei areas of Tarim Basin (at the end of Silurian period) and the second‐stage in the Awati area (in Permian), the petroleum experienced a long‐distance migration. During the formation of the Silurian paleo‐pools in Tazhong Uplift at the end of Silurian, the petroleum mainly came from the lower and middle Cambrian source rocks in the Manjiaer sag. The petroleum migrated towards the southwest‐south entering the Silurian reservoir beds in Tazhong first. Then, it further migrated within Silurian from northwest to southeast along the highs of the Structural Belts to the region of the Silurian pinchout boundary in Tazhong. In Tabei Uplift, during the first‐stage of pool formation, the petroleum was also from the lower and middle Cambrian source rocks in the Manjiaer sag. It migrated northwest entering the Silurian reservoir beds in the Tabei Uplift firstly, and then the migration continued in the same direction within the Silurian reservoirs and finally the petroleum was trapped in higher positions. During the second‐stage pool formation in the Silurian beds in the areas around Awati sag, the petroleum mainly came from the lower‐middle Cambrian source rocks in the Awati sag. The petroleum migrated from the generation center to Silurian reservoirs in all directions around the sag through major paths, and the petroleum was finally trapped in higher locations.