Early Cretaceous Black Shales Research Articles

Temporal Variation in the Chemical Index of Alteration in Early Cretaceous Black Shale as a Proxy for Paleoclimate

Temporal Variation in the Chemical Index of Alteration in Early Cretaceous Black Shale as a Proxy for Paleoclimate

Continental chemical weathering during the Early Cretaceous Oceanic Anoxic Event (OAE1b): a case study from the Fuxin fluvio-lacustrine basin, Liaoning Province, NE China

This study focuses on Early Cretaceous mudstones from the Shahai and Fuxin formations in the Fuxin continental basin. We analyse chemical weathering, land surface temperatures and palaeoclimates based on chemical weathering indices, and emphasize the implications of continental chemical weathering on nutrient fluxes into lakes and oceans. According to Cr and Ni abundance, Al2O3-TiO2, La/Sc-Th/Co and V-Ni-Th×10 plots, as well as rare earth element (REE) analysis, mudstone samples from the Shahai and Fuxin formations were derived from the same type of provenance comprising mainly felsic igneous rocks. Chemical weathering trends reflected by the Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP) and the Mafic Index of Alteration for Oxidative weathering environments (MIA(O)) are consistent with each other and allow the geological succession to be divided into four stages. Land surface temperatures of the Shahai and Fuxin formations are estimated based on the linear relationship of CIA to temperature, and also can be divided into four stages consistent with those determined from chemical weathering trends. During Stage A (early part of the late Aptian) chemical weathering and land surface temperatures were relatively low and showed characteristic high fluctuations, while Stage B (latest Aptian) represented a transitional period where weathering rates and temperatures increased, and high amplitude fluctuations continued. Conditions changed markedly in Stage C (early Albian) with very high and stable weathering, and warm, humid climates, while in Stage D (middle and late Albian) conditions returned to low chemical weathering and land surface temperatures. These stages of chemical weathering and land surface temperature fluctuations represent responses to global climate fluctuations during the Early Cretaceous, with the early Albian high weathering intensities and warm, humid climates combining to create high nutrient levels that would have flushed through rivers into lakes and ultimately oceans. This correlates stratigraphically with the development of Early Cretaceous black shales during Ocean Anoxic Event 1b, showing the importance of continental weathering regimes as a causal mechanism for lake and ocean anoxia.

Early Cretaceous black shale in the Fajiaying Formation (Lingshan Island, East China): Terrestrial record of hothouse climate

The significance of Cretaceous hothouse climate in terrestrial systems is currently unclear, and its records in terrestrial strata are rarely reported. This study investigated the sedimentary environment of the Fajiaying Formation, characterised by abundant black shale and soft-sediment deformation structures. Five lithofacies have been identified for the Fajiaying Formation, e.g. Facies A: black shale with siltstone inter-bedding, Facies B: black and gray-black shale and siltstone alternations with minor sandstone, Facies C: black and gray-black shale interbedded with gray-black sandstone, Facies D: pebbly sandstone and Facies E: gray-black shale and siltstone. The average TOC content of the shales decreases from facies A (av. 1.32%) to facies E (av. 0.46%) through facies B (av. 0.57%) and facies C (av. 0.40%). The paleosalinity gradually decreases from lithofacies A to lithofacies E, corresponding to the change of paleoclimate. In addition, Bivariate Scatter Plot of SiO2 and Al2O3 + K2O + Na2O and the clay mineral assemblages suggest an arid climate. Combined with distribution characteristics and paleontology of saliferous strata in eastern China, we determine that hothouse climate influenced the salification of lakes located in eastern China during the Early Cretaceous. Furthermore, the Fajiaying Formation in Lingshan Island was deposited in a terrestrial salified lake. Salinization of the sedimentary water bodies of the Fajiaying Formation might be related to an oceanic anoxic event. The black shale developed extensively in the Fajiaying Formation and represents the terrestrial record of hothouse climate.

Contrasting origins of Early Cretaceous black shales in the Vocontian basin: Evidence from palynological and calcareous nannofossil records

Detailed records of spore–pollen assemblages, particulate organic matter (OM), dinoflagellate cysts and calcareous nannofossils provide new insights into the palaeoclimatic and palaeoceanographic conditions during formation of Early Cretaceous black shales in the Vocontian basin (southeastern France). The early Aptian Niveau Goguel, which corresponds to the OAE1a, and the regionally distributed late Aptian Niveau Jacob have been studied with regard to changes in terrestrial vegetation patterns, terrigenous inputs and palaeofertility conditions. Palynological results from both black shale intervals exhibit a rich and stable floral pattern, dominated by various ferns, different types of cycads, bennettites as well as by several conifer families. Dinoflagellate cyst assemblages and the calcareous nannofossil-based nutrient index show no prominent changes in surface water productivity across the two studied intervals in the Vocontian basin. Significant variations are observed in terrestrial detrital input indicated by changes in absolute abundances of marine and terrestrial palynomorphs. According to our results, the laminated, OM-rich horizons of the Niveau Goguel interval reflect deposition during times of reduced siliciclastic input. Episodes of pronounced condensation were accompanied by anoxic conditions preventing degradation of the predominantly marine-derived OM. In contrast, the Niveau Jacob is characterised by a strong increase in terrestrial palynomorphs, most probably reflecting an abrupt increase in riverine runoff. The enhanced terrestrial OM input may have triggered oxygen-depletion in bottom waters, resulting in turn in increased OM preservation. Our results highlight the variety of processes that controlled the accumulation of OM in the Vocontian basin and they illustrate that enhanced surface water productivity is not an indispensable prerequisite for the formation of mid-Cretaceous black shales.

Late Jurassic to Early Cretaceous black shale formation and paleoenvironment in high northern latitudes: Examples from the Norwegian‐Greenland Seaway

The Late Jurassic to Early Cretaceous (Volgian‐Ryazanian) was a period of a second‐order sea‐level low stand, and it provided excellent conditions for the formation of shallow marine black shales in the Norwegian‐Greenland Seaway (NGS). IKU Petroleum Research drilling cores taken offshore along the Norwegian shelf were investigated with geochemical and microscopic approaches to (1) determine the composition of the organic matter, (2) characterize the depositional environments, and (3) discuss the mechanisms which may have controlled production, accumulation, and preservation of the organic matter. The black shale sequences show a wide range of organic carbon contents (0.5–7.0 wt %) and consist of thermally immature organic matter of type II to II/III kerogen. Rock‐Eval pyrolysis revealed fair to very good petroleum source rock potential, suggesting a deposition in restricted shallow marine basins. Well‐developed lamination and the formation of autochthonous pyrite framboids further indicate suboxic to anoxic bottom water conditions. In combination with very low sedimentation rates it seems likely that preservation was the principal control on organic matter accumulation. However, a decrease of organic carbon preservation and an increase of refractory organic matter from the Volgian to the Hauterivian are superimposed on short‐term variations (probably reflecting Milankovitch cycles). Various parameters indicate that black shale formation in the NGS was gradually terminated by increased oxidative conditions in the course of a sea‐level rise.

Structure and kinematics of a foothills transect, Lago Viedma, southern Andes (49°30′S)

The western edge of Patagonia, south of 47°S, experienced a major tectonic reorganization during the Tertiary. The Chile ridge, separating Nazca from Antarctica, collided obliquely with western Tierra del Fuego at about 14 Ma and the triple point migrated northwards to its present position at about 47°S. Consequently, the southern tip of South America has passed from a Miocene context of rapid oblique convergence (ENE–WSW at about 9 cm/yr) between Nazca and South America, to a Pliocene context of slow frontal convergence (EW at about 2 cm/yr) between Antarctica and South America. The Andean foreland fold-and-thrust belt lies on the eastern side of the Patagonian Cordillera and is well exposed along the northern shore of Lago Viedma (49°30′S). Structural observations, digital mapping, subsurface data, balancing of a cross-section and kinematic analysis of fault populations provide new information on the structure of the fold-and-thrust belt, the timing and style of deformation and their relationship with Tertiary plate tectonics. Along the studied transect, synsedimentary structures show that compressional deformation began at least during the Late Cretaceous, was ongoing during the syntectonic emplacement of the Lower Miocene granitic Monte Fitz Roy pluton and continued into the Pliocene. Folds and thrusts are thick-skinned in the west, and mostly thin-skinned above a décollement in Early Cretaceous black shales in the east. Analysis of fault populations, measured within Jurassic basement and its Cretaceous cover, provides subhorizontal principal directions of shortening, striking between E–W and ENE–WSW. Compressional deformation was associated with a major component of right-lateral wrenching parallel to the Cordillera.

A new, efficient, one-step method for the removal of organic matter from clay-containing sediments

AbstractFor the purpose of clay examination, a new one-step method for the removal of organic matter was developed using sodium peroxodisulphate combined with different buffers such as sodium hydrogen carbonate, disodium hydrogen phosphate and disodium tetraborate. From an early Cretaceous black shale from the Apennines with a high organic carbon content, the <2 µm clay fraction was separated and contained 10.9 wt% organic carbon. To prevent decomposition of the clay layers, the period of oxidation was short (25-60 min) and the pH was kept between 5 and 9.5. Up to 98% of carbon was removed by this method.

The genesis of Colombian emeralds: a restatement

A renewal of metallogenetical studies of Colombian emerald deposits produced new geological and geochemical data that favour a hydrothermal-sedimentary genetic model for these deposits. A comprehensive model is presented which integrates both chemical and structural aspects and invalidates some aspects of the model recently presented by Ottaway et al. The deposits result from a two-stage process in which shortening tectonics affect the two borders of the Eastern Cordillera of Colombia and provoke decollement planes, thrusting, and thrust-fault related folds in the Early Cretaceous black shale series. Three major descriptive and interpretative aspects of the deposits are presented: (1) time relationships between the two different sets of barren and mineralized extensional veins; (2) the association of graphite and emerald mineralization, and (3) the thermochemical sulfate reduction reaction acting at the site of the emerald deposits to reduce sulfate of evaporitic origin from the hydrothermal brines into hydrogen sulfide by interaction with organic-rich strata.