Permo-Carboniferous Radiolarians from the Wupata＇erkan Group, Western South Tianshan, Xinjiang, China

The Wupata‘erkan Group, also called Wupata‘erkan Formation, distributed in the South Tianshan, Xinjiang,China, mainly consists of gray and dark gray fine-grained clastic rocks, interlayered with volcanic rocks, carbonates and cherts. Some ultra-basic rocks (blocks) punctuate the formation. The formation was variously assigned to Silurian-Middle Devonian, Silurian-Lower Devonian, and pre-Devonian, mainly based on Atrypa bodini Mansuy, Hypothyridina parallelepipedia (Brour.) and Prismatophyllum hexagonum Yoh collected from the limestone interlayers, respectively.However, radiolarian fossils obtained from 24 chert specimens of the Wupata‘erkan Group, mainly include Albaillella sp.cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon, Albaillella sp. cf. A. indensis Won, Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto, Albaillella sp. and Latentifistulidae gen. et. sp.indet., are earliest Carboniferous and Late Permian. The earliest Carboniferous assemblage is characterized by Albaillella sp. cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon and Albaillella sp. cf. A. indensis Won, and the Late Permian assemblage by Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto. This new stratigraphic evidence indicates that the Wupata‘erkan Group is possibly composed of rocks with different ages from Silurian to Permian, and therefore, it is probably an ophiolite mrlange. The discovery of Late Permian Albaillella sp. cf. A. excelsa provides more reliable evidence supporting the existence of a Permian relic ancient oceanic basin in the western part of Xinjiang South Tianshan.     

Fault geometry and mechanics of marly carbonate multilayers: An integrated field and laboratory study from the Northern Apennines,Italy

Sealing layers are often represented by sedimentary sequences characterized by alternating strong and weak lithologies. When involved in faulting processes, these mechanically heterogeneous multilayers develop complex fault geometries. Here we investigate fault initiation and evolution within a mechanical multilayer by integrating field observations and rock deformation experiments. Faults initiate with a staircase trajectory that partially reflects the mechanical properties of the involved lithologies, as suggested by our deformation experiments. However, some faults initiating at low angles in calcite-rich layers (θ_i = 5°–20°) and at high angles in clay-rich layers (θ_i = 45°–86°) indicate the important role of structural inheritance at the onset of faulting. With increasing displacement, faults develop well-organized fault cores characterized by a marly, foliated matrix embedding fragments of limestone. The angles of fault reactivation, which concentrate between 30° and 60°, are consistent with the low friction coefficient measured during our experiments on marls (μ_s = 0.39), indicating that clay minerals exert a main control on fault mechanics. Moreover, our integrated analysis suggests that fracturing and faulting are the main mechanisms allowing fluid circulation within the low-permeability multilayer, and that its sealing integrity can be compromised only by the activity of larger faults cutting across its entire thickness.     

Displacement velocity effects on rock fracture shear strengths

Triaxial shear tests are performed to assess the effects of displacement velocity and confining pressure on shear strengths and dilations of tension-induced fractures and smooth saw-cut surfaces prepared in granite, sandstone and marl specimens. A polyaxial load frame is used to apply confining pressures between 1 and 18 MPa with displacement velocities ranging from 1.15 × 10⁻⁵ to 1.15 × 10⁻² mm/s. The results indicate that the shearing resistances of smooth saw-cut surfaces tend to be independent of the displacement velocity and confining pressure. Under each confinement the peak and residual shear strengths and dilation rates of rough fractures increase with displacement velocities. The sheared-off areas increase when the confining pressure increases, and the displacement rate decreases. The velocity-dependent shear strengths tend to act more under high confining pressures for the rough fractures in strong rock (granite) than for the smoother fractures in weaker rocks (sandstone and marl). An empirical criterion that explicitly incorporates the effects of shear velocity is proposed to describe the peak and residual shear strengths. The criterion fits well to the test results for the three tested rocks.     

Recent contributions to the standard Albian/Cenomanian boundary chronology from Hokkaido,Japan: A review for data reintegration and numerical age recalibration

The numerical age of the Albian/Cenomanian (A/C) boundary (=Early/Late Cretaceous Epoch boundary) has recently been updated using new radiometric dates from Hokkaido, Japan. Yet, an element of uncertainty still remains in the age scaling due to inaccurate chronostratigraphic contexts surrounding the tuff beds that have been geochronologically studied in the Oyubari and Soeushinai areas. To ensure stability in the Cretaceous time scale, this paper rigorously reevaluates the current status of Hokkaido A/C chronology, clarifying the limitations and making refinements via proper reintegration of published chronostratigraphic data. In the case of the Oyubari area, its A/C biostratigraphic scheme is limited by near-absence of ammonites and planktonic foraminifera at the Albian–Cenomanian transition, resulting in a large discrepancy in the proposed boundary levels based on these macro- and microfossil taxa. These problems can be solved by solid data integration aided by event- and δ¹³C-sratigraphy from the adjacent Ashibetsu area, wherein the A/C boundary is well documented by the superior planktonic foraminiferal record. Based on the resultant A/C boundary redefinition for the Oyubari area in conjunction with a simple linear interpolation from two published UPb ages, the A/C boundary age can be independently calibrated to be 100.8 Ma [with ±1.4 Myr uncertainty (=analytical precision)], which is consistent with that currently in use in the Geologic Time Scale 2012 (100.5 ± 0.4 Ma). In the case of the Soeushinai area, its chronostratigraphic framework is limited by rather complex geological structure and vertically/laterally variable lithologies. Moreover, the accuracy of geochronological data from this area is undermined by the fact that the dated tuff levels do not bracket the inferred A/C boundary.     

Morphology and internal structure of two new abelisaurid remains (Theropoda,Dinosauria) from the Adamantina Formation (Turonian – Maastrichtian), Bauru Group,Paraná Basin,Brazil

The knowledge on the Brazilian theropod fauna is hampered by the limited number of specimens unearthed so far. The most potential deposits for the finding of those dinosaurs are the layers of Bauru Group, which comprises several different formations of Late Cretaceous age. Most of those remains are referred to Abelisauroidea, a clade that is particularly well represented in Gondwana. Here we report two new abelisaurid specimens comprising a left ilium (DGM 927-R) and the distal articulation of a right femur (MCT 1857-R) unearthed from the outcrops of Adamantina Formation (Turonian – Maastrichtian) in the locality known as Santo Anastácio, that comprises an abandoned quarry located in the outskirts of the homonymous city in São Paulo State. The ilium has a preacetabular process horizontally oriented and ventrally deflected, as well as an acute angle between this process and the pubic peduncle. The femur shows a well-developed anteromedial crest, but not to the same degree as in noasaurids, and ossified ridges between condyles and tibiofibular crest. Those features allow us to assign both specimens to Abelisauridae. CT-scans of those specimens shows that the ilium has pneumaticities similar to the camellate pattern previously recorded in neosauropods and reveals the diverticula invasion of abdominal air-sacs. The femur presents a developed anisotropic trabecular system, which suggests that the main muscular strain is located at the distal portion of this bone. These features have not been previously reported in Abelisauridae, indicating that the internal organization of the postcranial skeleton in the group is rather complex and quite variable.     

Modeling discontinuous rock mass based on smoothed finite element method

This paper aims at developing a method for modeling rock mass with preexisting multiple discontinuities within the framework of the smoothed finite element method (SFEM). The discontinuity is simulated by an interface element with zero thickness, the stiffness matrix of which are derived explicitly based on the SFEM. An elastic damage constitutive relation with residual strength is introduced in order to describe the nonlinear mechanical behavior of the discontinuities. The computation codes of the present method were developed. The present method has been verified to be a sound approach for modeling discontinuous rock mass, inheriting the advantages of the SFEM.     

Volcanological study of the middle Miocene Okiduse Volcanic Group,Woodlark Island (Muyuw), eastern Papua

Woodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km² of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition.     

青藏高原拉萨地块西部念青唐古拉岩群的地球化学特征及构造意义

拉萨地块西部呈断块状沿狮泉河-申扎-嘉黎蛇绿混杂岩带附近分布的念青唐古拉岩群被认为是前寒武纪变质基底。本文对念青唐古拉岩群进行了系统的岩石学、地球化学、同位素年代学及构造地质学研究。研究结果表明片岩-片麻岩-变粒岩含十字石、石榴子石等特征变质矿物,遵循粒度分异规律,其原岩可能为来自冈瓦纳古陆核北缘中新元古代弧盆体系的活动大陆边缘浊积岩。斜长角闪岩具低硅、高铁镁、富钙的基性岩特征,其原岩为岛弧型基性火山岩。念青唐古拉岩群中的花岗伟晶岩锆石LA-ICPMS U-Pb年龄为1150±13Ma,具过铝质S型花岗岩地球化学特征,可能为中元古代(1150±13 Ma)以前就开始沉积的念青唐古拉岩群基底岩石通过部分熔融形成。与花岗伟晶岩渐变过渡接触的二云斜长片麻岩第一组变质重结晶锆石U-Pb年龄为701±15 Ma,结合十字石特征变质矿物,暗示了该地区中温高压变质作用的峰期变质,变质程度达角闪岩相;第二组热液流体锆石UPb年龄为301±8.4 Ma,可能与冈瓦纳大陆北缘古特提斯洋演化过程中的岩浆热液作用有关。     

The Carrancas Formation,Bambuí Group: A record of pre-Marinoan sedimentation on the southern São Francisco craton,Brazil

The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuí Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitaí Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have δ¹³C values ranging from +7.1 to +9.6‰, which is a unique C isotopic composition for the lowermost Bambuí Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaí fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ¹³C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ¹³C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ¹³C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaí Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuí Group strata and has implications for the development of the Bambuí basin.