Middle Late Silurian Research Articles

Middle–Late Silurian and Early Devonian Chitinozoans from the Chacoparaná Basin, Salta Province, Argentina

Middle–Late Silurian and Early Devonian Chitinozoans from the Chacoparaná Basin, Salta Province, Argentina

Seismic expression of shallow-water carbonate structures through geologic time

Shallow-water carbonate structures are characterized by different shapes, sizes, and identifying features, which depend, among other factors, on the age of deposition and on the carbonate factory associated with a specific geologic period. These variations have a significant impact on the imaging of these structures in reflection seismic data. This study aims at providing an overall, albeit incomplete, picture of how the seismic expression of shallow-water carbonate structures has evolved through deep time. Here, 297 shallow-water carbonate systems of different ages, spanning from Precambrian to present, with a worldwide distribution of 159 sedimentary basins, have been studied. For each epoch, representative seismic examples of shallow-water carbonate structures are described through the assessment of a selection of discriminating seismic criteria or parameters. The thinnest structures, commonly represented by ramp systems, usually occurred after mass extinction events and are mainly recognizable in seismic data through prograding clinoform reflectors. The main diagnostic seismic features of most of the thickest structures, which are found to be Precambrian, Late Devonian, Middle-Late Triassic, Middle-Late Jurassic, some Early Cretaceous presalt systems, “middle” and Late Cretaceous, Middle-Late Miocene, and Plio-Pleistocene, are steep slopes and reefal facies. Slope-basinal resedimented seismic facies are mostly observed in thick steep-slope platforms, and they are more common, except for megabreccias, in post-Triassic structures. Seismic-scale early karst-related dissolution features are mostly observed in icehouse platform deposits. Pinnacle structures and the thickest margin rims are concentrated in a few epochs, such as Middle-Late Silurian, Middle-Late Devonian, earliest Permian, Late Triassic, Late Jurassic, Late Paleocene, Middle-Upper Miocene, and Plio-Pleistocene, which are all characterized by high-efficiency reef builders.

Fracture, Dissolution, and Cementation Events in Ordovician Carbonate Reservoirs, Tarim Basin, NW China

Ordovician carbonate rocks of the Yijianfang Formation in the Tabei Uplift, Tarim Basin, contain deeply buried (>6000 m), highly productive oil and gas reservoirs associated with large cavities (>10 m). Previous workers inferred that large cavities are paleocaves (paleokarst) formed near the surface and subsequently buried. Alternately, caves may have formed by dissolution at depth along faults. Using 227 samples from 16 cores, we document textures and cement compositions bearing on cavity histories with petrographic, high-resolution scanning electron microscopy (SEM), isotopic, and fluid inclusion microthermometric observations. Results show that dissolution occurred at depth and was caused by (1) acidic fluids derived from Middle-Late Silurian and/or Devonian-Permian hydrocarbon generation and maturation, (2) high-temperature fluids, of which some were associated with Late Permian igneous activity, and (3) Mg-rich fluids that accompanied Jurassic-Cretaceous deformation and the formation of partially open fractures and stylobreccias (fault breccias). The relative paragenetic sequence of the structure-related diagenesis suggests seven stages of fracturing, dissolution, and cementation. Mottle fabrics in the Yijianfang Formation contain argillaceous carbonate-rich silt and are bioturbation features formed within the marine environment. Those mottled fabrics differ from clearly karstic features in the overlying Lianglitage Formation, which formed by near-surface dissolution and subsequent infilling of cavities by allochthonous sediment. Mottle fabrics are crosscut by compacted fractures filled with phreatic-vadose marine cements and followed by subsequent generations of cement-filled fractures and vugs indicating that some fractures and vugs became cement filled prior to later dissolution events. Calcite cements in fractures and vugs show progressively depleted values of δ18O documenting cement precipitation within the shallow (~220 m), intermediate (~625 m), and deep (~2000 m) diagenetic environments. Deep (mesogenetic) dissolution associated with fractures is therefore the principal source of the high porosity-permeability in the reservoir, consistent with other pieces of evidence for cavities localized near faults.

The Middle–Late Silurian granitoids in the Eastern Kunlun Orogenic Belt, NW China: petrogenesis and implications for tectonic evolution

The Eastern Kunlun Orogenic Belt lay on a significant tectonic boundary between Gondwana and Eurasia, produced widespread granitoids, and recorded at least two orogenic episodes since the Phanerozoic eon. In this paper, we provided new petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopic research on the Middle–Late Silurian granitoids from the eastern segment of The Eastern Kunlun Orogenic Belt. These granitoids comprised of Zhongzhigou granodiorites and Xintuo alkali-feldspar granites, formed in the Middle–Late Silurian (420–425 Ma). Geochemically, the granodiorites exhibit high Sr/Y (31.2–35.2) as adakitic affinities, and alkali-feldspar granites belong to highly fractionated I-type granites. Additionally, granodiorites have zircon eHf(t) values show limited positive variations from − 0.5 to + 3.2, with two-stage Hf model ages (TDM2) of 1105–1306 Ma, while alkali-feldspar granites show negative zircon eHf(t) values ranged from − 4.6 to − 0.1 with two-stage Hf model ages (TDM2) of 1289–1541 Ma, implying that their primary magma originated from different sources. Taken together, we conclude that granodiorites had been derived from partial melting of the juvenile newly underplated mafic lower crust, while alkali-feldspar granites were derived from partial melting of the shallower ancient crustal source. Combined with temporal and spatial distribution of magmatic activities, sedimentation, and metamorphism, it can be concluded that the whole EKOB underwent a transition from syn-collision compression to post-collision extension and extensive crustal reworking occurred during the Middle–Late Silurian.

Geochemistry and charge history of oils from the Yuqi area of Tarim Basin, NW China

The geochemistry, origin and charge history of oils from the Yuqi area of Tarim Basin have been investigated, through GC, GC-MS and fluid inclusion microthermometry analysis. The Yuqi oils accumulated mainly in three intervals: (1) in the Lower-Middle Ordovician Yingshan Formation (O1-2y) carbonate reservoirs; (2) in the overlying Upper Triassic Halahatang Formation (T3h); and (3) in the Lower Cretaceous Yageliemu Formation (K1y) sandstones. Oils from different reservoirs have distinct physical properties, varying from extra-heavy (O1-2y), heavy (T3h), to light oils (T3h and K1y). However, their geochemical compositions show a high degree of similarity, which indicates that they derive from the same source rock. Abundant tricyclic terpanes, gammacerane, dibenzothiophene and C21C22steranes, together with a low level of diasteranes, indicate an anoxic marine source rock for oils in the Yuqi area. Oil-oil correlation shows that Yuqi oils derive from the same source bed as Tahe oils. The co-occurrence of intact n-alkanes and 25-norhopanes in all the samples supports the proposition that there is a mixture of an early filled severely biodegraded oil and a late filled fresh oil.In this study, charge history is examined on the basis of integration of fluid inclusion homogenization temperature data with 1D burial-thermal history models. Two episodes of oil charging are identified in the O1-2y reservoir (well YQX1-1) at around 436-420 Ma (Middle-Late Silurian) and 10-3 Ma (Miocene to Pliocene), respectively. For the samples from the T3h and K1y intervals, only one episode of oil charge is indicated by the homogenization temperatures of coexisting aqueous inclusions with an inferred timing around 10-3 Ma. The T3h heavy oil reservoir is assumed to be a secondary hydrocarbon pool, which accumulated by re-migration and re-distribution of hydrocarbons from O1-2y hydrocarbon pools. The few early biodegraded oils in the K1y light oils were probably picked up along the migration pathway during the late fresh oil charging.

Geochemistry and zircon U–Pb geochronology of granitic rocks in the Buqingshan tectonic mélange belt, northern Tibet Plateau, China and its implications for Prototethyan evolution

The Yikehalaer intrusives are distributed in the Buqingshan tectonic mélange belt at the southern margin of East Kunlun, China. They tectonically intruded the conglomerate of Permian Gequ Formation to the south and the marble of Mesoproterozoic Kuhai Group to the north. These intrusives mainly consist of off-white coarse- and fine-grained granodiorite and granite. U–Pb dating of zircons from the intrusive samples yielded weighted mean 206Pb/238U ages of 436±7Ma and 437±6Ma, indicating Early Silurian crystallization age. The intrusives are high-silica, Na-rich, I-type calc-alkalic rocks. They are divisible into two subgroups on the basis of their trace element content: adakitic rocks in the northern part and normal calc-alkalic arc rocks in the southern part. Rocks in the northern part exhibit high Sr (280–493ppm), low Y (3.8–11.7ppm), high Sr/Y ratios (23.9–125.0), high light rare earth elements (LREE)/ heavy rare earth elements (HREE) (15.1–35.7), and invisible Eu/Eu∗. They resemble adakites formed by slab melting in subduction zones. However, the rocks in the southern part possess the attributes of normal arc rocks. In addition, the εHf(t) values of the intrusives are consistently positive (0.6–7.6), implying that they were generated by melting of an isotopically depleted mantle source, with insignificant contributions from crustal materials. The source residues likely contained 10–20% garnet and hornblende, and plagioclase was either absent or totally consumed during partial melting. Combining the regional tectonic data, we conclude that the Buqingshan paleo-ocean may have started to subduct in Middle Cambrian until Early Silurian (436Ma), generating an important adakitic magma event during the late stage of oceanic crust subduction. Moreover, the Proto-Tethyan Ocean closed, and the Bayan Har, East Kunlun, and Qaidam blocks collided during the Middle–Late Silurian.

Detrital zircon ages of the Jiageda and Woduhe formations: Constrains on the tectonic attribute of the Xing’ an terrane in the central Great Xing’ an Range, NE China

To ascertain the late Early Paleozoic tectonic attribute of the Xing’ an terrane, and the distribution and nature of the “Precambrian” basement in NE China, we studied the geochronology, petrology, and geochemistry of the metamorphic rocks of the Jiageda and Woduhe formations in the central Great Xing’ an Range. U–Pb ages of detrital zircons from the Jiageda and Woduhe formations vary from 963 to 374Ma, and can be divided into three groups (963–679, 570–468, and 457–374Ma). The youngest detrital zircons of the three samples from the Jiageda Formation yield weighted mean ages of 413±3, 414±9, and 416±1Ma, respectively, indicating that this formation was formed during the late Silurian–early Devonian. The youngest detrital zircons of the sample from the Woduhe Formation yield a weighted mean age of 429±4Ma, indicating that this formation was formed during the middle–late Silurian. The protoliths of the metamorphic rocks from the Jiageda and Woduhe formations are terrigenous clastic rocks, with some mafic–intermediate volcanic rocks, characterized mainly by volcanic–sedimentary formation. Considering the geochemical characteristics of the rocks and the regional tectonic setting, we suggest that both the Woduhe and Jiageda formations were formed in an island-arc active-continental-margin environment, and that the Xing’an terrane was located in an active continental margin related to the subduction of the Paleo-Asian Ocean during the middle Silurian-early Devonian. A comparison between the Jiageda and Woduhe formations with respect to their protoliths, geochemical properties, sedimentary environment, and geochronological characteristics shows that the Jiageda Formation does not constitute Precambrian crystalline basement, but instead consists of metamorphosed and deformed late Silurian–early Devonian sedimentary strata intercalated with volcanic rocks. By combining these results with the regional geology, we conclude that the Precambrian basement in NE China is not extensively distributed, being limited to Precambrian metamorphic rocks in the northern Erguna massif and Xing’ an terrane, eastern Songliao terrane, and southern Jiamusi massif, and the protolith ages are younger than Neoproterozoic.

Middle Paleozoic initial amalgamation and crustal growth in the West Junggar (NW China): Constraints from geochronology, geochemistry and Sr–Nd–Hf–Os isotopes of calc-alkaline and alkaline intrusions in the Xiemisitai-Saier Mountains

As one of the important parts of the Central Asian Orogenic Belt, the Early–Middle Paleozoic subduction–accretion history of the West Junggar is still needed to be further constrained although major progress has been made in the past years. In particular, the geodynamic setting for Late Silurian–Early Devonian A-type granites and associated mafic to felsic magmatism in the northern West Junggar is still controversial. In this study, we report new geochronological and geochemical results for Middle Paleozoic plutons in the Xiemisitai-Saier Mountains, northern West Junggar, including undeformed peralkaline granite, alkali feldspar granite, monzogranite, quartz syenite, syenite, monzodiorite, diorite, and hornblende gabbro. These plutons crop out in the eastern segment of the Early–Middle Paleozoic Boshchekul–Chingiz arc and invade into Early–Middle Paleozoic marine volcanic and pyroclastic sequences. New and published zircon U–Pb ages indicate that these intrusions were emplaced during the Late Silurian–Middle Devonian (424–392Ma). Late Silurian hornblende gabbro, diorite, and monzodiorite commonly contain abundant hornblendes and exhibit mantle-like Nb/Ta ratios (15.9–18.7), negligible Eu anomalies, with primitive Sr–Nd–Hf [I(Sr)=0.70370–0.70375, εNd(t)=+5.1 to +6.0 and εHf(t)=+14.3 to +16.2] and radiogenetic Os (I(Os)=0.2099–0.8795) isotopic signatures. They may be originated from a fluid metasomatized, H2O-rich and isotopically depleted mantle source and suffered some crustal contamination. Early–Middle Devonian calc-alkaline monzogranite and alkaline–peralkaline granitoids may have distinct origins. The monzogranite is I-type granite with high La/Yb ratios, depleted εNd(t) values (+5.1), and crust-like Nb/Ta ratios (8.5–8.9), which is suggested to be partial melt of juvenile lower crust containing massive hornblende±garnet. The peralkaline–alkaline granitoids include A-type and highly fractionated I-type granites, which contain mafic–intermediate enclaves and chemically exhibit variable element ratios, analogical εNd(t) values (+5.1 to +6.2), and significantly high I(Os) value (2.1247). They are interpreted to be produced by fractional crystallization of the hybrid magma contributed from mantle and crustal sources.Based on our new data, along with other geological evidence, it is speculated that these A-type granites and associated mafic to felsic intrusions may occur in a post-collisional extension setting since the end-Silurian, after the initial amalgamation of the West Junggar terrane during the Middle–Late Silurian. On-going lithosphere extension during the Early–Middle Devonian would result in the rifting of newly amalgamated West Junggar and thus induce asthenosphere upwelling, which gave rise to the mantle-derived magmatism in the northern and southern West Junggar and juvenile oceanic crust in central West Junggar. It is suggested that the Middle Paleozoic is an important episode of crustal growth in the West Junggar.

Distribution of Tuvaella brachiopod fauna and its tectonic significance

Tuvaella brachiopod fauna are distributed in an arc in three areas, including the Zeya-Hailar (海拉尔)-Shinejinst area, the Najramdal Uul-Kyzyl area, and the Barkol-Fuyun (富蕴) area. The Tuvaella fauna in the Zeya-Hailar-Shinejinst area are distributed in the northern margin of Jiamusi (佳木斯)-Mongolia block along the Mongolia-Okhotsk suture between the Siberia plate and the Jiamusi-Mongolia block but not appear in the corresponding Siberia plate. The Tuvaella fauna in the Najramdal Uul-Kyzyl area are distributed in the southwestern part of the Siberia plate as the accelerated fold belts during the Salairian-Caledonian periods, and those in the Barkol-Fuyun area are distributed in the northeastern margin of Kazakhstan plate. They are distributed along the Irtysh suture between the Siberia plate and the Kazakhstan plate. The Tuvaella fauna became domestic fauna during the Wenlock and Pridoli (Middle-Late Silurian) and form a unique biogeographic province, i.e., the Mongolia-Okhotsk Province. This biogeographic province is sustained to the Mississippian (Carboniferous), perhaps due to the semiclosed state of the Mongolia-Okhotsk Ocean. The northeastern margin (right now the southwestern margin) of Siberia plate, the northern margin of Jiamusi-Mongolia block, and the northeastern margin of Kazakhstan plate might be conjuncted in an arc region from Wenlock to Mississippian, along the southern margin of the Irtysh-Mid Mongolia-Okhotsk Ocean. This is coincident with the fact that the Siberia plate rotated 180° clockwise since the Mesozoic and located in the middle-high latitude during Devonian, and the coral reef limestone is widely developed in the Jiamusi-Mongolia block during the Early and Middle Devonian, which suggests that the block was positioning in middle-lower latitude at that time and that the Kazakhstan plate was also located in low latitude then.

Silurian atmospheric O 2 changes and the early radiation of gnathostomes

The number of known Silurian gnathostome fossils has increased significantly during the last two decades, and greatly improved our understanding of the early diversification of gnathostomes (jawed vertebrates). Primitive gnathostome remains from the Silurian of China are of special interest in bridging morphological gaps between osteichthyans and non-osteichthyan gnathostome groups. A review of these early fishes shows that gnathostomes had already obtained a wide distribution and experienced an early radiation in the Middle-Late Silurian. Environmental conditions in the Silurian are inferred from recent advances in geochemistry and paleoclimate models. Atmospheric oxygen concentration, an environmental factor critical in organismal evolution, rose gradually during the Silurian and reached modern levels for the first time. Compared to the Middle Ordovician when invertebrates and agnathans underwent a great radiation, the Middle-Late Silurian is distinctive for its high atmospheric oxygen level. We suggest that the rise of the atmospheric oxygen concentration would have triggered the early radiation of jawed vertebrates in the Silurian, which paved the way for the high generic diversity of vertebrates in the Devonian.

The Modernization of Landscapes during the Late Paleozoic-Early Mesozoic

Since their first appearance in the Middle-Late Silurian, land plants have played an increasingly important role in shaping terrestrial ecosystems and landscapes. It is difficult to overestimate their role because they form the framework for terrestrial ecosystems, provide habitats for terrestrial animals, form an important part of the food chain, affect weathering processes and have a direct impact on soil formation, and, last but not least, play a primary role in the oxygen/carbon cycles.

Evolution of the early Paleozoic deep-water basin of north Greenland—Aulacogen or narrow ocean?

A major lower Paleozoic deep-water sequence is exposed for more than 800 km along the coast of north Greenland. The maximum preserved width of the basin is about 200 km, and the thickness of the sedimentary sequence may reach 8 km. Nine evolutionary stages in basin evolution are recognized, each characterized by distinct sedimentary facies and structural style of the shelf-basin boundary: (1) the oldest sequence of unknown age is so deformed that inferences about the depositional environment are precluded; (2) late Precambrian(?)-Early Cambrian incipient basin; (3) Early Cambrian narrow turbidite basin; (4) Middle Cambrian-Ordovician basin expansion and initial starvation; (5) latest Ordovician-Early Silurian longitudinally filled turbidite basin; (6) Middle Silurian basin expansion and starvation; (7) Middle-Late Silurian wide turbidite basin; (8) Middle-Late Silurian Caledonian thrusting and conglomerate deposition; and (9) probably latest Silurian megasliding and advance of the Caledonian front. The basin evolution that came to an end during the mid-Paleozoic Ellesmerian orogeny was punctuated by events related to the closure of the Iapetus ocean and formation of the Caledonian mountain belt to the east. The basin may have been fully ensialic and formed during the early rifting stages preceding back-arc spreading, or it may have reached a narrow ocean stage. At present, however, we are inclined to favor an interpretation of the basin as being an aulacogen extending deeply into an old continent at a right angle to the Caledonian orogenic front to the east.