Ichnofabric analysis as a tool for characterization and differentiation between calcareous contourites and calciturbidites

ABSTRACT The Eocene–Miocene Cyprus paleoslope system records complex deep-marine sedimentation comprising background vertical settling of autochthonous pelagic–hemipelagic particles (chalks) which were punctuated by calcareous bottom currents (contourites) and gravity flows (calciturbidites). The Eocene Lefkara Formation at the Petra Tou Romiou beach section (Cyprus) shows the incidence of deep-marine bottom currents and distal turbiditic episodes in a context of pelagic–hemipelagic sedimentation. Trace-fossil analysis of this section, using an ichnofabric approach (i.e., ichnodiversity, Bioturbation Index, Bedding Plane Horizontal Index and crosscutting relationships), was conducted to precisely describe the paleoenvironmental conditions of this complex setting. Ichnofabric analysis allow the characterization and differentiation of sporadic turbiditic events that disrupted both pelagic–hemipelagic and contourite deposition. Calciturbidite intervals show ichnofabrics consisting of postdepositional U-shaped traces (i.e., Arenicolites isp., ?Diplocraterion isp.,) and vertical borings typical of consolidated substrates. High-energy sandy contourite deposits are dominated by horizontal deposit-feeder traces and the development of ichnofabrics with Planolites isp., and Thalassinoides isp. The record of ichnofabrics with slightly deformed Planolites in the interbeds of sandy contourites or in the transition between the facies reveals variations in sedimentation in the bi-gradational contourite succession, and can potentially act as an indicator of depositional hiatus.

Magmatic thickening of crust in non–plate tectonic settings initiated the subaerial rise of Earth’s first continents 3.3 to 3.2 billion years ago

When and how Earth's earliest continents—the cratons—first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure–temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary–magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth’s earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (∼50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision–driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric–oceanic oxygenation (O2-whiffs) and glaciation on Archean Earth.

Geological, Mineralogical and Geochemical Studies of Kolosh Formation, Dokan Area, Kurdistan Region, Iraq

The geology, mineralogy and geochemical of The Kolosh Formation in Dokan area, northern Iraq has been studied. The formation sequence includes gray and dark gray marl that alternate from clastic submarine sediments represented by turbid deposits resulted from the last stages of the collision movement between the continental plates. The geochemical study showed that the Kolosh Formation is mainly dominated by detrital sediments (Clay) with a dominance of kaolinite illite, and albite wıth low amounts of quartz. The analysis revealed that the Kolosh Formation is dominated by relatively marginal marine sedimentation where shelf bay facies was deposited with carbonate facies deposited as shallow marine. In contrast, SiO2 is strongly negatively correlated with CaO and MgO, this supports their derivation from terrigenous sources during the deposition of Kolosh sediments. Keywords: Kolosh Formation, sequences, mineralogy, geochemistry.

New fossil discoveries illustrate the diversity of past terrestrial ecosystems in New Caledonia

New Caledonia was, until recently, considered an old continental island harbouring a rich biota with outstanding Gondwanan relicts. However, deep marine sedimentation and tectonic evidence suggest complete submergence of the island during the latest Cretaceous to the Paleocene. Molecular phylogenies provide evidence for some deeply-diverging clades that may predate the Eocene and abundant post-Oligocene colonisation events. Extinction and colonization biases, as well as survival of some groups in refuges on neighbouring paleo-islands, may have obscured biogeographic trends over long time scales. Fossil data are therefore crucial for understanding the history of the New Caledonian biota, but occurrences are sparse and have received only limited attention. Here we describe five exceptional fossil assemblages that provide important new insights into New Caledonia’s terrestrial paleobiota from three key time intervals: prior to the submersion of the island, following re-emergence, and prior to Pleistocene climatic shifts. These will be of major importance for elucidating changes in New Caledonia’s floristic composition over time.

Studi Mekanisme Sedimentasi Formasi Dolokapa, Gorontalo

The Dolokapa Formation is a sedimentary rock formation formed in a deep-sea depositional environment with a fairly complex level of deformation and tectonic arrangement. Analysis of the sedimentation mechanism is carried out to determine how much tectonic influence on the mechanisms that occur in a depositional environment and the variations in the sedimentation mechanism formed. Research on the sedimentation mechanism needs to be carried out to determine the history of the formation of Gorontalo sedimentary rocks, especially in the Dolokapa Formation which was formed during the Miocene. The purpose of this study is to know the mechanisms of deep-marine sedimentation based on the identification of lithological characteristics, layer stacking patterns, and sedimentary structures. The method used was measuring sections using a range of ropes divided into four measurement paths. After that, a correlation was performed based on the genesis of deep marine formation. Based on the results of processing and analysis of the data, obtained units of lithology that insertion silty-clay, and the sandstone graining insertion of silt. In vertical succession, the layering pattern formed generally thickens upwards which describes the energy of the depositional currents. The sedimentary structure consists of rip up-clast, parallel lamination, graded bedding, convolute, slump, and trace fossils of nereites trace fossils of nereites that characterize the sedimentation of traction currents and turbidite currents in the deep-sea environment. The sedimentation mechanism formed is the traction current mechanism which is a further development of turbidite current and high-low concentration turbidity current mechanism that occurs slowly on a suspension-controlled grain. The stratigraphic relationship of the rock units in the research area is aligned based on the genesis formation that is located in the setting of the deep marine.

Geochemical Trends Reflecting Hydrocarbon Generation, Migration and Accumulation in Unconventional Reservoirs Based on Pyrolysis Data (on the Example of the Bazhenov Formation)

The current study is devoted to the determination and interpretation of geochemical trends reflecting hydrocarbon generation, migration and accumulation in unconventional reservoirs; the study is performed on the Bazhenov shale rock formation (Western Siberia, Russia). Results are based on more than 3000 Rock-Eval analyses of the samples from 34 wells drilled in the central part of the West Siberian petroleum basin, which is characterized by common marine sedimentation environments. Pyrolysis studies were carried out before and after the extraction of rocks by organic solvent. As a result, we have improved the accuracy of kerogen content and maturity determination and complemented the standard set of pyrolysis parameters with the content of heavy fraction of hydrocarbons. The data obtained for the wells from areas of different organic matter maturity was summarized in the form of cross-plots and diagrams reflecting geochemical evolution of the source rocks from the beginning to the end of the oil window. Interpretation of the obtained results revealed quantitative trends in the changes of generation potential, amount, and composition of generated hydrocarbons in rocks at different stages of oil generation process. The analysis of geochemical trends allowed us to improve approaches for the productivity evaluation of the formation and study the effect of organic matter maturity on distribution of productive intervals of different types.

Central- and North-East Greenland Rifted Margin Composite Tectono-Sedimentary Element, From Onshore East Greenland to the Greenland Sea

The Devonian to lower Eocene Central-East and NE Greenland Composite Tectono-Sedimentary Element CTSE) is a part of the North-East Atlantic rift system. East and NE Greenland geology is therefore analogues to that of the prolific basins on the conjugate Atlantic margin and in the North Sea in many respects. None the less, hydrocarbon discoveries remain. The presence of world-class source rocks, reservoirs and seals, together with large structures, may suggest an East and NE Greenland petroleum potential, however. The TSE was established through Devonian - Carboniferous, Permian - Triassic and Jurassic - Cretaceous rifting interspersed by periods of uplift and post-rift sagging. Subsequently, Paleocene - Eocene magma-rich rifting accompanied the North-East Atlantic break-up. Depositional environments through time varied in response to the changing tectonism and climate. None-marine deposition dominated until the end of the Triassic, only interrupted by marine sedimentation during Late Permian - Early Triassic times. Subsequently, marine conditions prevailed during the Jurassic and Cretaceous. Volumetric series of basalt erupted over most of the CTSE during the latest Paleocene - early Eocene following a significant latest Cretaceous - Paleocene regression, uplift and erosion event. Since the Eocene, denudation pulses have removed much of these basalts uniquely exposing the up to 17 km strata of the CTSE.