Top Of Member Research Articles

Lower Triassic conodont biostratigraphy of the Guryul Ravine section, Kashmir

The Guryul Ravine section in Kashmir, northern India is an important reference section for global Lower Triassic stratigraphy. Once a candidate Global Stratotype Section and Point (GSSP) for the Permian-Triassic Boundary (PTB), the section has attracted intense attention for the PTB beds, but few studies have concerned the entire Lower Triassic stratigraphy. As one of the most continuous marine Lower Triassic successions on the northern margins of Gondwana, the Guryul Ravine section provides an important conodont biostratigraphic yardstick for worldwide correlations. This study presents recent results of the Lower Triassic conodont zonation from the Guryul Ravine section. A total of 6500 elements including 1600 P1 elements were recovered. Ten conodont zones were recognized from the Griesbachian to Spathian: Hindeodus parvus, Isarcicella staeschei, Clarkina planata, Neoclarkina krystyni, Neospathodus dieneri, Ns. pakistanensis, Novispathodus waageni, Scythogondolella mosheri, Nv. pingdingshanensis, and Nv. abruptus – Nv. brevissimus zones. The PTB is placed within Bed 52 in Unit E2, 80 cm above the base of Unit E2, based on the first occurrence (FO) of H. parvus. The Induan–Olenekian boundary (IOB) is tentatively placed at the base of Member G according to the positive maximum δ13Ccarb values, but it could be slightly higher based on conodonts. The Smithian-Spathian boundary (SSB) is drawn at the top of Member H based on the FO of Nv. pingdingshanensis and the carbon mdpt(N3-P3). Several conodont taxa are newly recognized near the top of the study section where the conodont fauna is dominated by segminiplanate elements of the Neogondolellinae. Many of these age-diagnostic species have been identified in other sections around the world, and their occurrence in Guryul Ravine supports their potential for worldwide correlation. The conodont sequence erected from the study section corresponds well to those of South China (e.g. Meishan) and elsewhere worldwide. The newly established conodont zonation from Kashmir provides a high precision time-frame to consider biotic evolution and environmental change during the Early Triassic, a crucial period of Earth history.

Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada

The initiation of widespread penetrative bioturbation in the earliest Phanerozoic is regarded as such a significant geobiological event that the boundary between Ediacaran and Cambrian strata is defined by the appearance of diagnostic trace fossils. While ichnofabric analyses have yielded differing interpretations of the impact of Fortunian bioturbation, the disruption of sediments previously sealed by microbial mats is likely to have effected at least local changes in carbon and sulfur cycling. To assess the geochemical effects of penetrative bioturbation, we conducted a high resolution chemostratigraphic analysis of the siliciclastic-dominated basal Cambrian Global Stratotype Section and Point (GSSP; Chapel Island Formation, Newfoundland, Canada). A positive δ13C excursion in organic matter starts at the Ediacaran–Cambrian boundary and returns to stably depleted values near the top of member 2, while the δ13C of carbonate carbon increases from strongly depleted values toward seawater values beginning near the top of member 2. Pyrite sulfur coincidently undergoes significant 34S depletion at the Ediacaran–Cambrian boundary. These isotope anomalies most likely reflect progressive ventilation and oxygenation of shallow sediments as a consequence of bioturbation. In this interpretation, sediment ventilation in the earliest Cambrian may have spurred a temporary increase in microbial sulfate reduction and benthic sulfur cycling under low-oxygen conditions. In the late Fortunian, local carbon cycling appears to have stabilized as reductants were depleted and more oxygenated conditions predominated in the shallow substrate. Overall, these data attest to the geochemical significance of the initiation of sediment ventilation by animals at the dawn of the Phanerozoic.

Paleo-ocean redox environments of the Upper Ordovician Wufeng and the first member in lower Silurian Longmaxi formations in the Jiaoshiba area, Sichuan Basin

A study of paleo-ocean redox environments is important for understanding the deposition of black shale and has practical implications for shale gas exploration. Here, we selected a total of 52 shale samples from JY1 Well, the first shale gas well of commercial exploitation in China, to analyze the redox conditions of Upper Ordovician Wufeng (O3w) and the first member in lower Silurian Longmaxi shale (S1l1) in the Jiaoshiba area. Abundant pyrite framboids are observed in these units, with average framboid diameters ranging from 3.1 to 4.7 μm and maximum diameter about 10 μm. Analyses of redox-sensitive trace elements suggest the redox environment has evolved from an anoxic condition in the bottom of Member A to a dysoxic condition in the top of Member A, and to a dysoxic–oxic environment in Members B and C. Graptolite and radiolarian are discovered in these shale samples, indicating an oxygenated seafloor, which seems to be contradictory to the results from pyrite framboids and redox-sensitive trace elements. This contradiction is explained as follows: when the shale was deposited, the sedimentation was dominated by euxinic conditions; however, some oxygen may also occasionally migrate to the bottom water due to oxygen-deficient environment, deglaciation, and (or) strong upwelling of nutrient-enriched seawater, while the limited oxygen and anoxic environment is favorable for the preservation and accumulation of organic matter. Combined with the geological characteristics and redox conditions, Member A, especially the shales at the bottom of Member A, is expected to be the most favorable shale gas reservoir in the Jiaoshiba area.

New paleomagnetic results from the Ediacaran Doushantuo Formation in South China and their paleogeographic implications

A new paleomagnetic pole is obtained from the top of Member 3 of the Doushantuo Formation in the Jiulongwan section of the Yangtze Gorges area in South China. A total of 191 samples from 23 sites were collected and subjected to stepwise alternating field (AF) and thermal demagnetization. After the removal of a soft component (SC) of viscous magnetic remanence acquired in present geomagnetic field, a high temperature component (HC), likely carried by magnetite, was isolated. The HC includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great-circle analysis. Both the vectors only and the combined vectors and arc constraints of the HC passed a reversal test. This is the first Ediacaran paleomagnetic remanence from the South China Block (SCB) that passes a reversal test, and it is interpreted as a primary remanence. The HC vectors were thus used to calculate the paleomagnetic pole, and in combination with arc constraints, were used for polarity interpretation. The vectors give a mean direction of (D=75.2°, I=41.0°, α95=2.5°, N=147) after bedding correction, and a corresponding paleomagnetic pole at 23.9°N, 187.0°E (dm/dp=3.0/1.8). This pole differs significantly from previously published results of the Doushantuo Formation, which may have been subjected to Paleozoic remagnetization. Our results provide a 23.5±1.8°N paleolatitude for the Yangtze Gorges area of the SCB and suggest several alternative paleogeographic options for the Ediacaran–Cambrian amalgamation of East Gondwana, depending on chronostratigraphic correlations.

On the Lower Cambrian biotic and geochemical record of the Hetang Formation (Yangtze Platform, south China): evidence for biogenic silica and possible presence of Radiolaria

Abstract. Research on Lower Cambrian siliceous sedimentary rocks is important for understanding the origin and early involvement of polycystine Radiolaria in the silica cycle. During our study, thin sections and HF acid processing of black cherts and shales from the Hetang Formation that crops out in the Xintangwu section (west Zhejiang Province, south China) were made. We report on the presence of siliceous spherical microfossils (possibly Radiolaria) associated with sponge spicules and acritarchs. Their size and the presence of residual spines on some spherical siliceous microfossils observed in both residues and thin sections of cherts from the top of Member ‘a’ of the Hetang Formation argue for the possible presence of radiolarians. Based on the Small Shelly Fossil assemblages reported in previous studies, this interval should be considered as Qiongzhusian (Atdabanian–Early Botomian) in age. Finally, the values of the Ge/Si ratio measured on black cherts of the Hetang Formation point to a biogenic origin of the silica.

Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China

In South China, the Doushantuo Formation represents 90% of the time interval of the Neoproterozoic Ediacaran Period (635–542Ma). However, subdivision and correlation of strata in this formation is limited by sporadic occurrences of fossils of uncertain age and a scarcity of radiometric time constraints. In order to obtain Re–Os ages and initial 187Os/188Os (Osi) data, black shale samples from the Doushantuo Formation (Yangtze Platform, South China) were analyzed using the CrO3-H2SO4 digestion technique. A Re–Os date of 595±22Ma (Osi=0.85±0.18, MSWD=29, n=7) was obtained for black shale samples from the basal part of the fossil-bearing Doushantuo Member 4 (“Miaohe Member”) at Jiulongwan section, Three Gorges area, South China. A subset of these samples define a more precise isochron age of 591.1±5.3Ma (Osi=0.874±0.038, MSWD=1.3, n=4). We interpret the high MSWD of 29 and larger uncertainties for the 595±22Ma date to reflect heterogeneity of the Osi of samples related to temporal change of 187Os/188Os ratio of seawater when the sedimentary rocks were deposited. Taking 591.1±5.3Ma as the age for the base of the Doushantuo Member 4, together with the age of a tuff bed on the top of Member 4 (551.1±0.7Ma, zircon U–Pb dating, Condon et al., 2005), a minimum time difference of 35m.y. between the base and top of Member 4 is obtained. This estimate suggests very low sedimentation rates during black shale deposition (∼0.4mm/ka). The time difference of 35m.y. also represents a minimum duration of the Shuram–Wonoka δ13C excursion in South China. Black shale samples from Doushantuo Member 2 at Baiguoyuan section yield an imprecise date of 592±68Ma (Osi=0.534±0.057, MSWD=77) due to limited spread in 187Re/188Os ratio of the samples and a relatively large sampled stratigraphic interval (∼6m). If deposition occurred in water masses with access to the open ocean, Osi of the hydrogenous fraction from these organic matter rich sedimentary rocks may record the Os isotopic composition of coeval seawater. Thus, combined with previously published Re–Os data, the highly radiogenic initial 187Os/188Os ratio of 0.874±0.038 derived from Re–Os isochron for the base of Doushantuo Member 4, and even more radiogenic values near the top of Member 4 (1.08–1.17, calculated at 551Ma) indicate a dramatic increase of the flux of 187Os into the ocean during the late Ediacaran. The enhanced 187Os flux can only result from enhanced continental weathering and oxidation rates during that period.

Microstratigraphy of the Late Ediacaran to the Ordovician in NW Iran (Takab area)

Abstract Platform sediments of Iran span the Late Ediacaran and extend into the Palaeozoic. They are primarily clastic and carbonaceous in nature, deposited in coastal and lagoonal settings. They crop out in the Takab area, as the Soltanieh Formation (Late Proterozoic to Atdabanian), the Barut Formation (Early Bottomian), the Zaigun Formation (Middle Bottomian), the Lalun Formation (Late Bottomian) and the Mila Formation (Middle Cambrian to Early Ordovician). A Precambrian–Cambrian assemblage has been identified at the base of Member 3 and the top of Member 2 in the Soltanieh Formation. The Soltanieh Formation consists of shallow marine sediments. Vertical microfacies variation reflects two regressive sequences, the earliest Cambrian System being a regressive cycle. The presence of Chuaria and Vendotaenia in the Soltanieh Formation allows correlation of Iranian sequences to those elsewhere in the world, indicating a late Neoproterozoic age. Detailed facies analyses of the Soltanieh, Barut, Zaigun, Lalun, and Mila formations allow refined palaeoenvironmental reconstructions to be made for the Ediacaran–Cambrian/Ordovician in Iran.

Karst-controlled diagenesis and reservoir development: Example from the Ordovician main-reservoir carbonate rocks on the eastern margin of the Ordos basin, China

The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from 8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity. Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.

Mixed Siliciclastic-Carbonate Deposition in an Early Cambrian Oxygen-Stratified Basin, Chapel Island Formation, Southeastern Newfoundland

ABSTRACT The upper portion of member 3 and member 4 of the uppermost Precambrian (Vendian)-Lower Cambrian Chapel Island Formation represents a large-scale shoaling-up deposit dominated by shelf siliciclastic mudstone. Medium-scale, higher-order shoaling-up cycles at the top of member 3 and in member 4 are capped by peritidal limestones that contain a wide variety of stromatolitic buildups and algal mud mounds. Sedimentation took place in an oxygen-stratified basin with accumulation of gray dysaerobic mudstone offshore and green and red mudstone in shallower, more oxygenated waters. Biofacies models for dysaerobic basins can be applied to this sequence and help in understanding the distribution of trace fossils, skeletal fossils (shells and pyritic steinkerns), and diagenetic features. These da a are important for the understanding of the stratigraphic distribution of shelly fossils within this lowest Cambrian unit. A mixed carbonate-siliciclastic facies model is presented in which carbonate sediments formed in intertidal areas along the shoreline of a siliciclastic muddy shelf. The carbonate shoreline prograded during periods of increasing rate of relative sea-level fall or decreasing rate of relative sea-level rise. Restriction of carbonate sediments to the peritidal environment may have been due to a combination of higher rates of carbonate sediment production and accumulation in the peritidal zone, seaward dilution by siliciclastic mud, and a paleogeographic position near the latitudinal climatic extreme for carbonate production.

Magnetostratigraphy of the Lower Cretaceous Vectis Formation (Wealden Group) on the Isle of Wight, Southern England

Magnetostratigraphic results are described from two sections within the Lower Cretaceous Vectis Formation (formerly Wealden Shales) of the Isle of Wight, Southern England. The sections are located at Shepherd's Chine and Yaverland, respectively. In both of these sections a reliably-established reverse polarity magnetozone, herein defined as the Vectis magnetozone, occurs within the Shepherd's Chine Member. This magnetozone has a thickness of some 15 m and its upper boundary is well defined in the Yaverland section but is obscured in the Shepherd's Chine Section due to cliff slumping. The lower boundary is well established in both sections and lies some 15 m above the Barnes High Sandstone Member in the Shepherd's Chine section and 12 m above this unit at Yaverland. This boundary provides a useful chron-stratigraphic datum. A comparison with established geomagnetic polarity timescales for the Cretaceous indicates that the Vectis magnetozone may be correlated either with Lower Aptian reverse polarity Chron CM-0 or with Lower Barremian Chron CM-1, with the former being the more likely. An important implication of this interpretation is that the base of the Aptian stage, as defined from Tethyan foraminiferal zones, must lie within the Vectis Formation, at least 25-30 m beneath the top of the Shepherd's Chine Member, rather than at the top of this member as hitherto commonly accepted. Beds characterized by anomalously high intensities of magnetization, which occur at the base of the Vectis Formation and a few metres above the Barnes High Sandstone in the two sections, are tentatively interpreted as having a volcanogenic origin.

A candidate stratotype for the Precambrian–Cambrian boundary, Fortune Head, Burin Peninsula, southeastern Newfoundland

The Burin Peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the Precambrian–Cambrian transition. Fossils are abundant and include trace fossils, small shelly fossils, vendotaenid algae, soft-bodied megafossils, and microfossils. The Burin Peninsula is readily accessible and has long been considered a potential area for a Precambrian–Cambrian boundary stratotype.A continuous section through the upper part of member 1 and all of member 2 of the Chapel Island Formation is exposed at Fortune Head, and this section is herein proposed as a global stratotype for the Precambrian–Cambrian boundary. The boundary horizon is located 2.4 m above the base of member 2 of the Chapel Island Formation. This horizon marks the base of the basal Cambrian Phycodes pedum (ichnofossil) Zone and immediately overlies the top of the Late Precambrian Harlaniella podolica (ichnofossil) Zone. Shelly fossils (sabelliditids) first appear a few metres below the proposed boundary. Soft-bodied megafossils, carbonaceous impressions of vendotaenid algae, and organic-walled microfossils occur both below and above this boundary and enhance global correlation with this section.Fossils of the Rusophycus avalonensis (ichnofossil) Zone first appear midway through member 2 (approximately 135 m above the proposed boundary) and occur commonly throughout the upper part of the Chapel Island Formation and the overlying Random Formation. Calcareous small shelly fossils (?Circotheca sp.) appear near the top of member 2 (approximately 400 m above the proposed boundary), and a more diverse Aldanella attleborensis small shelly fossil assemblage characterizes the uppermost strata of member 3 and all of member 4 of the Chapel Island Formation (approximately 550–650 m above the proposed boundary). The lowest trilobites, representatives of the Callavia broeggeri Zone, first appear more than 1000 m above the proposed Precambrian–Cambrian boundary.

Paleosols and Alunite Silcretes in Continental Cenozoic of Western Portugal

Two different origins are proposed for fossil silcretes in continental Cenozoic sedimentary rocks of Western Portugal. These soils developed in more or less the same parent materials. The lower silcrete, 1 to 10 m thick, is a consistent indicator at the top of an upward-fining member and shows clear evidence of weathering processes: the decay of illite and its replacement by kaolinite, the redistribution and concentration of iron oxides, the impregnation of the sediment by silica minerals (opal-CT with the optical properties of quartzine under the petrographic microscope), and a late concentration of alunite rhombs along cracks in the upper part of the silcrete. This silcrete resulted from prolonged exposure of sediment to soil processes in a well-drained environment. The silcretes interbedded in a younger member are locally silicified layers at the top of upward-fining units. Silica minerals (opal-CT) precipitated where palygorskite also formed in abundance. Alkaline ponds are postulated by the numerous slickensides, due to shrinkage and swelling of clay minerals, indicate temporary subaerial exposure of the sediments and later silicification. Many rhombic boxworks in these silcretes could be the expression of a previous alunite precipitation, similar to alunite concretions at the top of this member. These silcretes in the younger member formed by repetitive processes acting in local swampy environments under a relatively dry climate. Alunite, which is abundant at the top of the lower silcrete, must not be considered as a final product of weathering. It is probably a later mineral phase introduced when the overlying beds were laid down.