The Shunbei area, located in northern-central Tarim (Northwest China), represents a new, important area for petroleum and gas exploration. This study presents high-resolution, conodont ...biostratigraphically controlled stable carbon isotope data from 540 samples of four deeply buried (∼7000–8000 m) wells in the Shunbei area and its nearby Tahe area spanning the Middle to Upper Ordovician for detailed chemostratigraphic correlations. Additionally, thin sections are obtained from the two studied areas for sedimentary facies analysis. The δ13C data from Shunbei area correlate well with the results from Tahe area and other outcrops in northwestern Tarim. The major global δ13C excursions of the mid-Darriwilian Isotopic Carbon Excursion (MDICE, ∼+1‰) and the Guttenberg Isotopic Carbon Excursion (GICE, ∼+1‰) are recognized from the upper Yijianfang and the Lianglitag formations respectively. Two minor (∼+0.5‰) δ13C positive peaks from the Yingshan Formation of the Shunbei area are comparable to lower Darriwilian strata of platform facies in Tarim, providing a potential tool for correlating lower Darriwilian hydrocarbon reservoirs, such as the Yingshan and Yijianfang formations in the Tarim Basin. A major sedimentary gap is detected in the lower Qiaerbak Formation of the Shunbei area and the upper Yijianfang Formation of the Tahe area, indicating an erosional event occurred in the early Sandbian which could be correlated with the globally tectonic-active interval during the Darriwilian–Sandbian transition. The completely preserved MDICE and GICE in the Shunbei area, as well as its relatively deeper-water sedimentary environment in comparison with the Tahe area, collectively suggest that the Shunbei area is of significance for further geochemical studies on the Middle and Late Ordovician environments and their resources.
The Silurian climate was volatile and punctuated by multiple short-lived climate events, each associated with biotic crisis and perturbations to the global carbon cycle. In total, seven positive ...carbon isotopic excursions are globally recognized in the Silurian with the large excursions of the Wenlock-Ludlow interval in the Baltic succession garnering the most research; these excursions are recorded in association with cyclic facies changes and the enigmatic resurgence of microbial carbonates. Climate models, hypothesizing cycles of alternating humid and arid climate states, have been developed to explain these paired lithologic-isotopic fluctuations; however, little work has been done to test these models on lower Silurian strata. To test for the presence of δ13Ccarb excursions and the stratigraphic motif of proposed climate events, we developed a high-resolution integrated stratigraphic framework for the upper Hirnantian to mid-Telychian (Upper Ordovician to Llandovery) carbonate-dominated succession of Anticosti Island, by examining ~450 m of strata from a recent drill core, supplemented by ~120 m of outcrop. Four facies assemblages and three time-specific facies are identified and organized into three orders of superimposed transgressive-regressive cycles. High-resolution isotopic curves reveal the presence of four global positive carbon isotope excursions all sequentially recorded in the study interval; the HICE (peak of +5‰), Early Aeronian (peak of +2‰), Late Aeronian (peak of +6‰), and Valgu (peak of +3.5‰) excursions. Common cyclic stratigraphic trends are recognized with these δ13Ccarb excursions, where each excursion is associated with a shift from shale-rich to shale-poor facies, an intermediate-order transgression, and a minor δ18O excursion. These repetitive lithologic-isotopic patterns in the Llandovery succession of Anticosti Island are linked to cyclic fluctuations between low-latitude humid and arid climate states. In addition to these patterns, microbial-rich carbonates are associated with the Late Aeronian and Valgu δ13Ccarb excursions on Anticosti Island; this suggests a genetic link between the elevated δ13C values and the proliferation of calcified cyanobacteria and associated microfossils. This integrated stratigraphic model linking carbon excursions with climate and carbonate factory shifts provides an excellent framework for future studies, using a wide variety of geochemical systems, to better understand the processes that drove environmental and biodiversity changes through the Llandovery.
•Integrated stratigraphic framework of an expanded Llandovery succession.•Sequential Llandovery isotopic carbon excursions.•Highest recorded values for the Late Aeronian δ13Ccarb excursion.•Paired lithological-isotopic signal linked to climatic fluctuations.•Local development of microbial-rich skeletal limestones.
As praias arenosas da Ilha do Maranhão localizam-se em ambiente macromaré, confinadas entre falésias e desenbocaduras fluviais. A morfodinâmica destas praias varia em função do regime de ...precipitações da região, com periodicidade semi-anual tendo a ZCIT como principal sistema meteorológico. O objetivo deste estudo foi caracterizar os sedimentos do mesolitoral das praias arenosas oceânicas (Ponta da Areia, São Marcos, Calhau, Olho D’Água, Meio, Araçagy e Mangue Seco e Carimã) e abrigadas (Guia e Panaquatira) da Ilha do Maranhão. Para tal, coletou-se 125 amostras que foram submetidas a análises granulométricas. As análises levaram em consideração o diâmetro médio do grão (Φ), o selecionamento (σI), a assimetria (SKI) e curtose (KG). Os sedimentos praiais foram predominantemente do tamanho areia fina e muito fina, variando de bem a moderadamente selecionados, com assimetrias e angulosidade variadas, refletindo as dinâmicas de interações inerentes de cada compartimento praial, na dependência da orientação da linha de costa, suprimento sedimentar e da geologia antecedente. O morfotipo de barreira costeira, na qual encontram-se as praias arenosas da Ilha do Maranhão, exerce controle morfodinâmico, apontando setores com características erosivas em barreiras de praias anexadas, enquanto e as barreiras de esporões arenosos ligadas a cabeços de promontórios indicam características deposicionais.
Internally generated, or autogenic, terrestrial and marine sediment-transport dynamics can produce depositional patterns similar to those associated with climatic, tectonic, or sea level changes. A ...central challenge in accurately interpreting the sedimentary archive is determining what scales and types of deposits reflect autogenic controls on sedimentation in different environments. Autogenic sediment-transport dynamics commonly result from intermittent sediment storage in transient landforms, which produces episodic, spatially discontinuous sedimentation across a basin. The transition from localized, variable sedimentation to even, basin-wide sedimentation marks the shift from stochastic landscape dynamics to deterministic deposition responding to the long-term balance between sediment supply and the creation of space to accommodate sediment. This threshold can be measured in a wide variety of stratigraphic successions and has important bearing on whether climatic, tectonic, or sea level signals can be recognized in physical sedimentary deposits.
