Upper Jurassic–Lower Cretaceous Source Rocks in the North of Western Siberia: Comprehensive Geochemical Characterization and Reconstruction of Paleo-Sedimentation Conditions

The present work is devoted to geochemical studies of the Bazhenov Formation in the north of the West Siberian Petroleum Basin. The object is the Upper Jurassic–Lower Cretaceous section, characterized by significant variations in total organic carbon content and petroleum generation potential of organic matter at the beginning of the oil window. The manuscript presents the integration of isotopic and geochemical analyses aimed at the evaluation of the genesis of the rocks in the peripheral part of the Bazhenov Sea and reconstruction of paleoenvironments that controlled the accumulation of organic matter in sediments, its composition and diagenetic alterations. According to the obtained data, the sediments were accumulated under marine conditions with a generally moderate and periodically increasing terrigenous influx. The variations in organic matter composition are determined by redox conditions and terrigenous input which correlate with the eustatic sea level changes during transgressive/regressive cycles and activation of currents. Transgression is associated with an intensive accumulation of organic matter under anoxic to euxinic conditions and insignificant influence of terrigenous sources, resulting in the formation of rocks with oil-generating properties. During the regression periods, the terrigenous sedimentation increased along with the dissolved oxygen concentration, and deposits with low organic matter content and gas-generating properties were formed.

Terrigenous sedimentation on the submarine Shirshov Ridge (Bering Sea) during the last deglaciation

The submarine Shirshov Ridge is an independent system of terrigenous sedimentation, which is geomorphologically isolated from bottom terrigenous influx into the deep-water basin of the Bering Sea. Using the ridge as example, we studied background hemipelagic sedimentation of the finely dispersed terrigenous suspended material from water column and deposition of the coarser grained ice-rafted material in the western part of the deep-water basin. The grain-size and mineral composition of postglacial sediments of the Shirshov Ridge was studied in cores SO201-2-85KL and SO201-2-77KL taken from local basins in the central and southern parts of the ridge, respectively. Statistic treatment of uninterrupted grain-size distributions (GD) of terrigenous component of the postglacial sediments by end-member (EM) modelling revealed that the grain-size distributions of terrigenous sediments from two cores are determined by the mixing of three EMs. EM-1 and EM-2 reflect the hemipelagic sedimentation with and without bottom currents influence respectively, while EM-3 with mode at fine-grained sand characterizes GD of the ice-rafted material. Reconstructed mechanisms of terrigenous influx on the Shirshov Ridge involve advection of the suspended matter with surface and intermediate water masses and ice-rafting. The relative role of both mechanisms of the terrigenous sedimentation under the influence of varying bottom current velocities for intervals of Last Glacial Maximum, early deglaciation, Heinrich event 1, Bølling–Allerød, Younger Dryas, and Early Holocene is estimated. It is ascertained that the grain-size distribution of terrigenous component is defined by climate variations, sea ice coverage, sea ice drift pathways, conditions of fast ice melting, and mobility of bottom waters. High concentrations of drifting ice or seasonal sea ice cover likely existed above the southern part of the ridge during the second half of the Heinrich 1 event. The low mobility of bottom waters facilitated only the subice hemipelagic sedimentation of fine fractions from the background reserve of suspended material. A sharp reduction of ice-rafted flux was reconstructed for the Bølling–Allerød warming interval. Bottom currents affected sedimentation in the central part of the ridge during the entire deglaciation (in addition to the second half of the Heinrich 1 event), and in the southern part during the Bølling–Allerød, Younger Dryas, and Early Holocene.

The Variable Influences of Sea Level, Sedimentation and Exposure on Holocene Reef Development over a Cross-Shelf Transect, Central Great Barrier Reef

Coral reefs globally are impacted by natural and anthropogenic stressors that are compounded by climate change. Understanding past reef responses to natural stressors (cyclones, sea-level change, freshwater inputs, and sedimentation) can provide important insights to further understand recent (within the past century) trends in coral cover and diversity. Here we use a compilation of recently published data to investigate the Holocene development of five fringing reefs that are located on a cross-shelf transect on the central Great Barrier Reef, and that are exposed to varying degrees of natural and anthropogenic sedimentation, storm exposure, and Holocene sea-level change. Forty-two reef cores collected using a combination of manual percussion coring and hydraulic drilling techniques, were analysed and dated using uranium-thorium methods. The chronostratigraphic records of reef development established using 105 recently published radiometric ages and seven new uranium-thorium ages from the reef cores and fossil microatolls preserved across the reef flats were compared to investigate cross-shelf variations in reef development. This is the first study to conduct an internal investigation of reef framework across an inshore–offshore gradient to examine the varying levels of influence of sedimentation, sea level and cyclones. Our observations from the central Great Barrier Reef show that reefs furthest offshore from the mainland coast were typically initiated earliest after the post-glacial marine transgression. Reef flat size, morphology, and growth style varied according to constraints placed on reef development by the composition, depth, shape, and relief of the underlying substrate. We establish that terrigenous sedimentation had a marked effect on the development of inshore reefs closest to the mainland (within 10 km of the mainland coast). Periods of relatively high terrigenous sedimentation correspond with enhanced reef accretion rates, and also resulted in a superior record of palaeo-ecological coral composition (i.e., better preservation) at inshore sites. In contrast, mid-Holocene cyclones played a seemingly more important role in the development of reefs >10 km from the mainland; although cyclones clearly affect reefs closer inshore, their geomorphology is affected by a range of controlling factors. Insights provided by these five Holocene reef chronostratigraphies provide useful baseline understanding of reef condition and growth along a cross-shelf transect where the reefs are exposed to variable stressors.

Caracterização Morfossedimentar da Plataforma Continental Brasileira (Morphosedimentary characterization of the Brazilian Continental Shelf )

Este trabalho discute as diferenciações existentes ao longo da plataforma continental brasileira, bem como sua gênese e evolução. Foi realizada uma pesquisa bibliográfica afim de evidenciar as particularidades da mesma, especialmente do setor pernambucano, reconhecido como último elo de ligação entre os continentes americano e africano. A morfologia e as características sedimentológicas da área em estudo são altamente influenciadas por diferentes condições climáticas, hidrológicas e aporte sedimentar continental ao longo do território brasileiro, além de diferentes estágios de formação e evolução da margem continental brasileira. Assim estes fatores promovem quatro grandes diferentes setores na plataforma continental brasileira. O setor norte, marcado pela largura ampla e predomínio de sedimentação terrígena, recebe grande influência continental pela presença de grandes rios e clima equatorial. O setor nordeste, mais estreito e com predomínio de sedimentação biogênica, recebe grande influência marinha, com território continental desprovido de grandes rios, de clima tropical e semiárido. A porção central da plataforma continental brasileira apresenta as mesmas características hidrológica, climática e sedimentar do setor anterior, contudo com larguras geralmente menores, como é o caso da plataforma adjacente ao município de Salvador. No setor sul volta a se apresentar mais progradante com predomínio de sedimentação terrígena, voltando a receber vasta influência continental, com ocorrência de rios mais relevantes e clima subtropical. A B S T R A C TThis paper discusses the existing differences along the Brazilian continental shelf, as well as its genesis and evolution. A bibliographical research was carried out in order to highlight its peculiarities, especially in Pernambuco’s sector recognized as the last link between the American and African continents. The morphology and sedimentological characteristics of the studied area are highly influenced by different climatic, hydrological and continental sedimentary contribution conditions along the Brazilian territory, besides different stages of formation and evolution of the Brazilian continental margin. Thus, these factors are responsible for four large sectors in the Brazilian continental shelf. The northern sector, marked by wide width and predominance of terrigenous sedimentation, receives great continental influence, due to the presence of large rivers and equatorial climate. The Northeast sector, which is narrower and has a predominance of biogenic sedimentation, receives great marine influence, the continental territory is devoided of large rivers, with tropical and semi-arid climate. The central portion of the Brazilian continental shelf presents the same hydrological, climatic and sedimentary characteristics of the previous sector, however usually with smaller widths; this is the case of the platform adjacent to the municipality of Salvador. The southern sector presents more prograde with a predominance of terrigenous sedimentation, receiving once again vast continental influence, with more relevant rivers and subtropical climate.Keywords: morphology, sedimentary dynamics, continental margin, Pernambuco.