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[Display omitted] • Organic-rich shales mainly formed from late Katian to early-middle Rhuddanian. • Occurrence pattern of organic-rich shales within graptolite biozones was delineated. • Graptolite biozones are crucial in evaluating gas-enriched, productive intervals. Typical outcrops and shale gas wells from the upper Ordovician (Katian and Hirnantian) Wufeng Formation to the lower Silurian (Rhuddanian) Longmaxi Formation in the Sichuan Basin of southern China were selected to investigate the shale quality and shale gas potential. The chronostratigraphy was studied based on the graptolite biozones and well logging. The sedimentary environment, depositional facies, biostratigraphic distribution, depositional model and lithological characteristics were characterized. Organic-rich shale primarily developed in the late Katian to the early–middle Rhuddanian. Two organic-rich shale depocentres were identified in the southern and eastern-north-eastern Sichuan Basin. The spatial distribution of organic-rich shale of different graptolite biozones was determined and showed gradual migration northwest. The depocentre of the Dicellograptus complexus Biozone to the Cystograptus vesiculosus Biozone was controlled by paleo-uplift and a submarine high. The black shale typically lacks some graptolite biozones, and even if it is fully developed, the thickness of organic-rich shale is thin and has a poor shale gas source potential. The control effect of the graptolite biozone on shale-gas-enrichment helps to evaluate the shale-gas-enriched and productive intervals accurately. This study characterises the organic-rich shale distribution in the Sichuan Basin and helps in assessing organic-rich shale and identifying sweet spots of shale gas potential. [ABSTRACT FROM AUTHOR]

Trace fossils are important biogenic signatures in sedimentary environments because they indicate the paleoecologic controls during deposition. In some cases, trace fossils are the unique preserved biologic evidence, allowing to infer paleoenvironments in controversial stratigraphic units. This is the case of Furnas Formation from Paraná Basin, Brazil. This unit was widely interpreted as fluvial-generated up to 1990's, but facies association analysis and the ichnologic data have been used as tools to suggest a marine origin for the unit. However, some recent studies have been ignored that information and continuous to interpret Furnas Formation as a fluvial-generated unit. In this study we analyzed some outcrops at northeast of the Ponta Grossa Arch aiming to infer depositional processes and paleoenvironments for basal and intermediated units of Furnas Formation (Lower Silurian). Nine ichnotaxa (Cruziana, Didymaulichnus lyelli, Didymauliponomos rowei, Heimdallia chatwini, Palaeophycus tubularis, Rusophycus, Skolithos, Taenidium dieslingi, and Thalassinoides) expressing proximal and depauperate Cruziana ichnofacies were reported for the studied outcrops, corroborating the shallow marine paleoenvironment inferred for the unit. • The Furnas Formation presents Cruziana ichnofacies. • Lower and middle units of Furnas Formation were deposited in Lower Silurian. • Trace fossils from Furnas Formation indicate a shallow marine depositional setting. [ABSTRACT FROM AUTHOR]

The Baolun gold deposit is a mesothermal orogenic gold deposit located in the southwestern part of Hainan Island, South China. The deposit comprises a series of NNW-trending quartz-sulfide lodes situated within a parallel array of fault zones traversing a sequence of variably foliated flysch siliciclastic rocks of the Lower Silurian. Detailed field mapping documented at least five phases of deformation in the deposit including NNW-trending folding of the Lower Silurian rocks (D1), development of NNW-trending, steeply dipping ductile shear zones with an oblique dextral sense corresponding to NNE-SSW shortening (D2), WNW-ESE shortening and extension associated with an early oblique sinistral ductile shearing along the NNW-trending fault zones (D3), ENE-WSW shortening (D4), and near N-S extension (D5). The gold-bearing quartz lodes cut the strata folded in the D1, show some laminar layering related to ductile shear in the D2 and are overprinted by brittle structures formed in the D3 to D5. 40 Ar– 39 Ar dating on muscovite from an auriferous quartz lode yielded an age of 242 ± 2.5 Ma, which, together with the age of 232 ± 2.5 Ma for an aplite vein in the deposit, suggests that the mineralization may be related to a tectono-thermal event in the Triassic. In the context of the southern South China plate tectonics, the formation of the Baolun gold deposit is interpreted to be related to the oblique dextral ductile shearing (D2) along the NNW-trending fault zones during the Indosinian orogeny, in relation to the convergence between the Indochina and South China plates. [ABSTRACT FROM AUTHOR]

In northern Guizhou Province (Upper Yangtze Platform, South China Block) two types of reef communities developed in the Lower Silurian (upper Aeronian, Llandovery) Shihniulan Formation; they are calcimicrobial- and metazoan-dominated structures, and existed because of northward deepening of the shallow-marine ramp setting in which they grew. Stromatolitic communities are the focus of the present study and dominated the shallowest reef structures, while metazoan-dominated reefs, previously described in other papers, grew in the outer shelf portion of the ramp. Stromatolitic reefs occur in several sections (Daijiagou, Baishanxi, Jianba and Lianghekou), palaeogeographically close to Qianzhong Land. Within the stromatolite units, laminar sheets of microbial mats and columns are pronounced, with individual stromatolite thicknesses generally less than 1 m. Some very small stromatolites are only centimeters in diameter and thickness. Stromatolitic units are cyanobacterial bindstones mostly associated with shales, siltstones and thin-bedded bioclastic limestones. Their growth was frequently punctuated by siliciclastic sediments, and their shallow-water nature is demonstrated by association with birds-eye structures, cross-stratified sediments, and Lingulella -bearing silts in intertidal or/and lagoonal environments. The stromatolites formed during a regression and erosion surfaces are common at the top of the Shihniulan Formation. The Tongzi Uplift, a short-duration expansion of Qianzhong Land, ended the deposition of the late Aeronian limestones. [ABSTRACT FROM AUTHOR]

Abstract: In the Valongo Anticline, northern Portugal, the Upper Ordovician-age Sobrido Formation records glacioeustatic sea-level changes and glacially-influenced depositional environments on the north Gondwana platform associated with the Hirnantian glaciation. New stratigraphic data from the Sobrido Formation, taking into account lateral facies variations not exposed in the type section, allow for a better understanding of Hirnantian glacial sedimentary processes, sea-level changes and palaeogeography. The lower member of the Sobrido Formation is composed of quartzites alternating with siltstones and mudstones. The transition to the upper member is most commonly an erosional contact marked by a ferruginous horizon. The lowermost part of the upper member contains a thinly laminated and rhythmically-bedded mudstone that is capped by a thin (mm-scale) shelly bed and overlain by massive diamictites. Laterally, conglomerate beds that have erosion bases are rhythmically interbedded with slates and diamictites that sometimes contain outsized mudstone clasts. The uppermost part of the upper member is very variable in thickness laterally and contains massive to laminated diamictite beds intercalated with quartzites, conglomerates and slates. Massive diamictites commonly exhibit thin horizons with Fe-oxides and occasionally manganese, phosphate and oolitic chamosite. The lower member of the Sobrido Formation is interpreted as ice-proximal outwash that is transitional laterally to subaqueous ice-contact fans formed by debris and mass flows. Erosion followed by chemical weathering of the lower member and variably within the lowermost part of the upper member is interpreted as a response to sea-level fluctuations and development of spatially-variable anoxic conditions and groundwater migration within isolated marine basins. Facies within the upper member of the Sobrido Formation are interpreted as ice-distal (laminated diamictites) and ice-proximal deposits (massive diamictites, quartzites and conglomerates). Groove casts and hummocky cross-stratification within quartzite beds suggest deposition in a shallow shelf setting. The Upper Ordovician–Lower Silurian transition is locally marked by a ferruginous horizon. Black basal quartzites are interpreted to record development of an erosional unconformity during Silurian sea-level rise. In other areas a chloritoid layer indicates proximity to dry land. These palaeoenvironmental interpretations suggest dynamic changes in sedimentary and geochemical processes in response to sea level and related palaeogeographic changes during the Upper Ordovician and transitional to the Lower Silurian on the north Gondwana platform. [Copyright &y& Elsevier]

Arthrophycus brongniartii (Harlan, 1832) is common in marginal-marine deposits in the Silurian Red Mountain Formation of Alabama. The ichnospecies, the second to be named in North America, is revived and emended after long disuse. Transitional forms to Rusophycus isp. and other morphologic evidence indicate that the maker of Arthrophycus was an arthropod, perhaps a trinucleine (raphiophorid?) trilobite. Interconnection of Arthrophycus and Nereites biserialis, as well as intergradation of Arthrophycus with Cruziana aff. quadrata, Phycodes flabellum, and Asterosoma ludwigae, indicate that these Red Mountain trace fossils were made by the same species of arthropod. Possible relationships with Arthrophycus alleghaniensis (Harlan, 1831) in the Silurian belt from Ontario to Tennessee are also explored. Ichnofamily Arthrophycidae Schimper, 1879 is emended. The ichnofamily is interpreted as chiefly the work of arthropods. Arthrophycus and other trace fossils from the Silurian of Alabama constitute a test case to build criteria for recognizing the members of complexes of trace fossils. In general, criteria such as interconnection of different forms, intergradation among unconnected forms, similarity of size, similarity of morphologic elements, and co-occurrence should be examined in order to determine the biologic and ethologic interrelationships of trace fossils. [Copyright &y& Elsevier]

More than 100 m-long core sections from Western Lithuania (Baltic Basin), presenting an almost continuous lower Silurian fine-grained siliciclastic organic matter-rich sedimentary record from Aeronian to Homerian, were investigated to evaluate the main forces governing the organic matter enrichment. The methods applied include organic and inorganic geochemistry, organic petrology, sedimentological and petrographic studies along with framboidal pyrite analysis, as well as analysis of stable nitrogen and organic carbon isotopes. The petrographic observations showed distinct mudstone diversity reflecting a variety of paleoenvironmental depositional conditions evolving against a backdrop of post-Ordovician climate, thus, sea level variations, and progressing development of foreland basin throughout the early Silurian. Reflectance measurements on vitrinite-like particles, supported by molecular organic geochemical and Rock-Eval data prove that kerogen from the studied section reached the oil window maturity stage. Rock-Eval pyrolysis, organic geochemical and petrographical as well as δ13C org data clearly confirm the marine origin of the organic matter. Various geochemical proxies and sedimentary features indicate that in Aeronian a period of more humid climate favored enhanced nutrient input, which in turn intensified organic matter productivity resulting in large amounts of organic matter accumulation. Redox proxies (TOC/TS ratio, trace elements enrichment, molecular organic geochemical indices, framboidal pyrite analysis, TOC/P ratio) demonstrate a rather broad spectrum of bottom water oxygenation levels in the lower Silurian sediments accumulated under oxic - dysoxic – anoxic conditions interrupted by several episodes of euxinia. A highly condensed section of Aeronian black-grey mudstones manifests a very good organic matter preservation under sediment-starved and oxygen-depleted settings. Here, the oxygen-depleted water bottom conditions are well evidenced by low values of δ15N and δ13C org pointing to the presence of diazotrophic N-fixing and chemoautotrophic organisms. The Sheinwoodian-Homerian section of monotonous grey calcareous mudstones evenly and moderately enriched in organic matter reflects the most steady environmental settings developed under more arid climate in comparison to Llandovery. Throughout the early Silurian sedimentation rates were overall low, although probably enhanced in Wenlock, and played a role in organic matter preservation. It was a critical factor during sedimentation of grey-greenish mudstone of the Telychian characterized by comparatively the lowest organic matter preservation efficiency. Variations in the intensity of the pycnocline, thus organic matter preservation, were probably controlled by climate-triggered sea level changes, although tectonically-driven paleoenvironmantal effects, bathymetry, and, at some point, intensified paleoproductivity inducing oxygen-restricted conditions, were likely contributing. Although Mo EF /U EF ratios mainly indicate fully open marine settings during the entire early Silurian, Mo–TOC covariation clearly displays strict basinal restriction in the Aeronian. The profile of δ13C org demonstrates six carbon isotope excursions along the lower Silurian section. The identified Late Aeronian and Early Sheinwoodian Carbon Isotope Excursions are linked to global sea-level changes, thus, changes in oceanic water circulation pattern. A high percentage of tiny pyrite framboids within the interval of Early Sheinwoodian CIE could reflect an increased pyrite burial under highly reducing conditions and might be associated with an Oceanic Anoxic Event. The three positive δ13C org excursions within the Telychian seem to be related to an early diagenetic partial oxidation of organic matter. • Lower Silurian facies, hence TOC distribution, were partly tectonically controlled. • Rates of sediment accumulation influence organic matter preservation efficiency. • Humid-climate driven siliciclastic weathering enhanced higher marine productivity. • Sea-level changes ruled redox bottom water conditions and watermass restriction. • Carbon isotope curve throughout the lower Silurian recorded several excursions. [ABSTRACT FROM AUTHOR]

Early Silurian black shales are widely deposited over the entire Yangtze platform and their origin are highly controversial. In order to identify the controlling factors of these black shales formation, the mineralogy and geochemistry of the Early Silurian organic-rich shales from the Yangtze Platform, South China, were investigated by using an optical microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and field emission scanning electron microscopy. These black shales are characterized by high total organic carbon (0.37%–10.47%, avg. 3.86%) and total sulphur (0.02%–1.68%, avg. 0.67%) content as well as enrichment of some trace elements, including V (70.66 ppm–1740.52 ppm, avg. 312.75 ppm), Cr (68.72 ppm–301.94 ppm, avg. 117.97 ppm), U (3.90 ppm–52.49 ppm, avg. 16.58 ppm), Ba (316.69 ppm–2985.82 ppm, avg. 1523.38 ppm) and rare earth elements (94.26 ppm–340.50 ppm, avg. 202.46 ppm). The minerals mainly consist of quartz and clay mineral with average content of 54.8% and 29.3%, respectively. K-feldspar, dolomite, calcite and pyrite are next in abundance, followed by trace amounts of apatite, barite and Cr-bearing minerals. The dissolution of biogenic silica and recrystallization during burial is an important source of quartz cement. In addition to biological sources, terrestrial, volcanic and authigenic sources are responsible for these mineralogical and geochemical compositions. The Al 2 O 3 /TiO 2 , Zr/Sc, Th/Sc, LREE/HREE and Eu anomalies indicate that the sediment-source region is derived from a mixture of felsic and intermediate rocks of the old upper continental crust. The Th/U and V/(V + Ni) ratios, Ce anomalies and authigenic U suggest that these black shales are deposited under an anoxic bottom water environment. The elevated concentrations of U-V-Ni-Cu-Zn-Cr-Ba and REE in these shales are predominated by the marine environment during deposition and influenced by biologic production in water columns and volcanic ash fall. High marine productivity resulting from volcanic nutrients promote the anoxic/sulfidic water column conditions of the restricted Yangtze basin, further resulting in organic matter enrichment. Elemental enrichment processes usually result in a valence change under anoxic environment, which is associated either with the clay minerals or direct precipitation from sea water. Understanding the coenrichments of organic carbon and trace elements in these black shales is important for studying the formation mechanism and resource potential of hydrocarbon and critical elements. • Longmaxi black shales are characterized by the enrichment of trace elements. • Longmaxi black shales mainly consist of quartz and clay minerals. • Longmaxi black shales deposited under an anoxic bottom water environment. • Terrestrial, biological and volcanic sources controlled geochemical compositions. [ABSTRACT FROM AUTHOR]

Lithuania is situated in the central and eastern parts of the Baltic Sedimentary Basin. Several prospective Lower Palaeozoic gas shale formations are identified in the basin. Most of the published studies concern the organic chemistry and evaluation of gas potential, whereas only scarce data are provided on the petrophysical and mechanical properties of oil/gas shales. It is typical also for other basins, essentially concerning parameters like cation exchange capacity, swelling index, erodibility etc. Mineralogical, petrophysical and mechanical properties of the Lower Silurian organic rich shales of west Lithuania (within petroleum window area), considered as a most prospective body for unconventional oil/gas exploitation, are discussed in the present study. The most distinct feature due to anomalous organic matter (OM) content is the Middle Llandovery black shales that are compatible to the Llandovery "hot" shales defined in sedimentary basins of Africa and Arabian Peninsula. Brittleness index (BI) of shales was calculated from mineral composition, shales are classified as moderately brittle (predominate) to moderately ductile. It was stated that clay content and quartz content directly correlate with the BI, despite that they have a reverse effect on shale brittleness. The unconfined compressive strength and tensile strength also indicate low brittleness of shales that is accounted to high natural fracturing of the Lower Silurian shales. Low porosity, varying from 12 to 16% to less than 3%, have no discernible effect on shale strength properties. Cation exchange capacity measurements show low values that is related to high illite and detrital minerals content in shales. Organic matter (OM) content have noticeable statistically positive correlation with gamma ray intensity in organic most rich "hot" shales. Roller oven technique was applied to measure the shale erodibility. Experiments show that application of the tap water leads to high erodibility of shales, they are classified as the moderate to high erodibility rocks. Reversely, diesel considerably reduces erodibility index causing low erodibility of shales. As regards the acid solutions, the best effect in decreasing shale erodibility is measured when K 2 Cl (7%) solution is applied (moderate erodibility). Generally low erosion susceptibility is explained in terms of high content of detrital silt and predominance of illite and chlorite in the clay fraction. Cation suction time (CST) characterises the shale swelling capacity. It was noticed that CST negatively correlates with the GR intensity logs as they mainly reflect the OM content in shales. The most organic rich black Middle Llandovery "hot" shales show the lowest swelling capacity compared to other less OM enriched shales. The measured parameters are therefore evaluated as positive for oil/gas exploitation (low swelling, low erodibility, and low stiffness) or of moderate quality (brittleness), some parameters decrease the quality of shales (e.g. low CEC, low porosity). • The Lower Silurian shales contain a high amount of detrital silt grains. • A high content of the silt fraction causes a rather high brittleness. • The measured CEC values are very low and is a negative parameter. • The erodibility of shales is also low and positively correlates with the CST. • Porosity is typically low for prospective shales. [ABSTRACT FROM AUTHOR]