Mesozoic Black Shales Research Articles

Evolution of Sedimentary Basins as Recorded in Silica Concretions: An Example from the Ionian Zone, Western Greece

Chert concretions in thick limestone successions preserve a more complete paragenetic sequence of diagenetic minerals than their host limestone and interbedded shale. The goal of this study was to test the possible presence of a high-temperature mineralising system in the Ionian basin of western Greece. Upper Cretaceous chert nodules were sampled at Araxos, where rocks are highly faulted and uplifted by salt diapirism, and on Kastos Island, on the flanks of a regional anticline. Chert concretions have microporosity produced by recrystallisation of opal to quartz and fractures produced in the brittle chert during basin inversion. Diagenetic mineral textures were interpreted from backscattered electron images, and minerals were identified from their chemistry. Diagenetic minerals in pores and veins include sedimentary apatite (francolite), dolomite, Fe-chlorite, Fe oxide-hydroxide mixtures, sphalerite, barite and calcite. Sphalerite is restricted to Araxos, suggesting that inferred basinal fluids were hotter and more saline than at Kastos. At Araxos, the Fe oxide-hydroxide also includes minor Cu, Zn, and Ni. Whether the transported metals were derived from sub-salt clastic rocks and basement, or from enriched Mesozoic black shales, is unclear. The effectiveness of this novel approach to understanding fluid flow history in thick limestone successions is validated.

Mesozoic biochronostratigraphy and paleoenvironment of the South Atlantic: A revised framework based on 20 DSDP and ODP deep-water sites

The South American and West African margins of the South Atlantic and the Southern Ocean are characterized by Mesozoic black shales that serve as source rocks of known and unknown petroleum systems. The knowledge of regional chronostratigraphy and paleoenvironment enables the prediction of the stratigraphic and geographic distribution of black shales with precision, as well as the understanding of local heterogeneity in regional- or global-scale oceanographic changes, such as oceanic anoxic events. Through sample re-analyses, this study provides a high-resolution Mesozoic chronostratigraphic and paleoenvironmental framework with the help of new micropaleontological data generated from 17 DSDP and ODP sites (out of 20 study sites), ranging from the Equatorial South Atlantic to the Southern South Atlantic. A series of eight regional Mesozoic unconformities are proposed and correlated to previously known unconformities or seismic reflectors. The regional paleoenvironmental interpretation, which utilizes the standardized chronostratigraphic framework, is herein accomplished for four time-horizons, namely, Middle Jurassic–Valanginian, Hauterivian–Albian, Cenomanian–Turonian, and Coniacian–Maastrichtian. Chronostratigraphic cross-sections and reconstruction maps for the time-horizons show that significant changes in paleoenvironmental parameters, such as bottom-water oxygenation and allochthonous sedimentation, were governed by the tectonic evolution of the South Atlantic margins.

A unique ore-placer area of the Amur region with high-Hg gold

This work presents the geological structure and a description of the gold-ore occurrences and gold placers of the Un’ya-Bom ore-placer cluster of the Amur gold-bearing province. The host rocks are Late Paleozoic and Mesozoic black shales. Intrusive formations occur rarely. The sublatitudinal Un’ya Thrust is the principal ore-controlling structure. Paleozoic sandstones are thrust over Mesozoic flysch deposits along the Un’ya Thrust. The gold-ore occurrences are represented by quartz-vein zones. The ores are gold–quartz, low-sulfide. Ore minerals are arsenopyrite, scheelite, ferberite, galena, and native gold. High-Hg native gold was revealed in the ore occurrences and placers. The high Hg content in native gold is explained by the presence of the frontal part of the gold-bearing column located within the cluster; the rich placers were formed due to crushing of this column.

Tethyan–Mediterranean organic carbon‐rich sediments from Mesozoic black shales to sapropels

AbstractThe Jurassic to Holocene record of black shale deposition in the Tethys‐Mediterranean region is unrivalled by that of any other ocean basin, either in land sections or drill cores. The term ‘black shale’ is used here broadly for sediments with elevated organic carbon concentrations (> 1%), including the Pliocene to Recent sapropels. Most of the black shales are devoid of benthonic organisms, are laminated, and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic. The Tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales. These records have been central in understanding climate control on ocean dynamics and biogeochemical cycles. The Mesozoic black shales were deposited within well‐defined time envelopes of around 0·5 to 1 Myr. These black shales, which were confined to certain chronostratigraphic intervals in the Jurassic and Cretaceous, were recognized as expressions of global Oceanic Anoxic Events in the mid‐1970s and subsequently named after prominent researchers (Bonarelli, Selli, Goguel). The black shale episodes were dated by biostratigraphic methods and by high‐resolution chemostratigraphy and cyclostratigraphy. Mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate. Research into younger (Pliocene to Pleistocene) sedimentary cycles (including black shales, termed sapropels) exposed in land sections, or found in pelagic and hemipelagic marine sediment cores of Late Quaternary age, started in the 1950s. Main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes, palaeoclimate evolution of the circum‐Mediterranean area and biogeochemical dynamics in anoxic basins. After 50 years of intensive research, the Tethyan and Mediterranean black shales remain subjects of fascination in Earth Science. Tracing the origins and building upon recent progress, the current hypotheses on their formation are reviewed here. It is a panorama of complex interplays between global and regional tectonics, climate dynamics during both Ice House and Greenhouse states of global and regional climate, oceanographic responses to these climate changes, and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity.

Organic-carbon burial, climate change and ocean chemistry (Mesozoic–Palaeogene)

This thematic set brings together a selection of papers that were presented at a conference entitled ‘Organic-Carbon Burial, Climate Change and Ocean Chemistry (Mesozoic–Palaeogene)’ at the Geological Society of London, 9–11 December 2002. The conference, informally known as ‘the Black Shales Meeting’, was convened by Hugh Jenkyns, Juergen Thurow and Tom Wagner. It attracted over 100 scientists from 15 different countries, proving that current research into Black Shales is highly active. Twenty-seven talks were given during the meeting, and some 40 posters were also presented. Although the nine papers that are published here represent a relatively small part of the large number of topics that were discussed, we nevertheless hope that they reflect the diversity and far-reaching nature of the research that is currently being carried out on ‘Black Shales’. Whilst Mesozoic Black Shales have enormous economic significance as hydrocarbon source rocks, the emphasis of the December 2002 meeting was on the importance of Black Shales and associated lithologies as palaeoenvironmental indicators in the broadest sense. This importance comes about because individual Black Shale samples can contain a wide variety of records of prevailing ocean chemistry, while Black Shale successions—which are often stratigraphically complete, undisturbed, and available for study at a relatively high resolution—can reflect longer-term temporal changes in palaeoenvironmental conditions. It is over 20 years since the first international conference on Black Shales took place, on 5 May 1979, at the Geological Society of London (Hallam 1980). That conference was held at a time when …

Introduction to ‘Paleoclimatic and Paleoceanographic Records in Mediterranean Sapropels and Mesozoic Black Shales’

Introduction to ‘Paleoclimatic and Paleoceanographic Records in Mediterranean Sapropels and Mesozoic Black Shales’

The mid-Cenomanian Event: prelude to OAE 2

A detailed synthesis of litho- and biostratigraphic investigations of the Cenomanian pelagic limestones of the Scaglia Bianca Formation in the Umbria–Marche Basin shows that the Bonarelli Level, which is the regional sedimentary expression of the OAE 2, represents the climax of a ∼2 Myr-long cycle of black shale deposition that began in the mid-Cenomanian planktonic foraminiferal Rotalipora cushmani Zone. This long-term cycle began with a prominent short-term event, named mid-Cenomanian Event (MCE), which is associated with several changes in the biotic and abiotic records. In particular, a comparison of the available litho-, bio-, and chemostratigraphic data from the Umbria–Marche Basin allows recognition of the MCE based on: (a) a significant change in the sedimentary and tectonic regimes; (b) a reorganisation of planktonic foraminiferal assemblages; (c) a major change in benthic foraminifera; (d) a major turnover in the radiolarian assemblages; (e) a 0.7‰ positive shift in the carbon isotope values; and (f) a positive shift of carbonate Sr/Ca ratio. Moreover, the MCE is preceded (∼400 kyr) by a short-term 0.5‰ negative excursion in the oxygen isotope record. The above mentioned changes in carbon and oxygen isotope values can be confidently correlated over different basins and oceans and seem to occur in correspondence with a major sea-level fall. These lines of evidence define the MCE as an outstanding event associated with large-scale changes in the oceanic structure and palaeoclimate. The MCE and the subsequent cyclical deposition of organic-rich beds serve as a case study to better understand differences and similarities between Mesozoic black shales and Cenozoic sapropels and related palaeoceanographic changes controlled by orbital climate cycles.

Mineralogy and geochemistry of Mesozoic black shales and interbedded carbonates, southeastern Sicily: evaluation of diagenetic processes

AbstractUpper Triassic/Lower Jurassic organic-rich shales and interbedded carbonates (Rhaetian → Sinemurian) are widespread in the subsurface of southeastern Sicily where important oil fields have been found hosted in Triassic reservoirs. Core samples from wells drilled offshore and onshore were studied from petrographie and geochemical viewpoints.In the Hettangian/Sinemurian shale-carbonate sequences, which accumulated in a rapidly subsiding basin, the micritic aragonitic mud is still largely preserved. Mixed-layer I/S has remained randomly interstratified to a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finely crystalline, pore-filling dolomite and/or calcite. The carbonate component exhibits a high Sr content and fair amounts of Fe and Mn. Carbon and oxygen isotopic values suggest a subsurface interstitial formation for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters were retained in the sediment pores for a long time after deposition, thus enhancing the preservation of significant amounts of the original organic matter.In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I/S and a carbonate mineralogy dominated by low-Mg calcite in the uppermost beds and by near-stoichiometric dolomite in the lowermost ones. Petrographie, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence pervasive dolomitization in a sabkha-type environment.

The Denali fault system and Alaska Range of Alaska: Evidence for underplated Mesozoic flysch from magnetotelluric surveys

Regional magnetotelluric surveys recently completed across the central and eastern Alaska Range of Alaska provide evidence for large volumes of conductive rocks beneath the core of the range. These conductive rocks may represent a formerly extensive, but now collapsed, Mesozoic flysch basin formed on the leading edge of the Talkeetna superterrane (amalgamated Wrangellia, Peninsular, and Alexander terranes). The docking of the Talkeetna superterrane caused large-scale oblique thrusting, folding, and metamorphism in the flysch basin, and formation of a megasuture along which the Cenozoic strike-slip Denali fault system developed. The deep magnetotelluric soundings and seismic reflection data suggest the possibility that the highly conductive rocks were tectonically emplaced beneath the thin crystalline sheet constituting the southern Yukon-Tanana terrane over a broad region of the Alaska Range. The conductive rocks are locally correlated with surface outcrops of Mesozoic black shales that are part of Upper Jurassic and Cretaceous flysch but may be composed of Paleozoic carbonaceous shales as well. In either case, their extremely low resistivities make them a valuable marker horizon for tectonic studies. The conductive rocks are interpreted to extend to depths of greater than 20 km and were mapped north and northeast of the Denali fault for more than 50 km. The magnetotelluric surveys represent the first large-scale surveys done in Alaska, but the structures mapped are similar to those observed in large, compressed flysch basins in the eastern Alps and Carpathian Mountains of Europe. The results of these surveys bear on several key tectonic questions, including development of the ancestral Denali fault, and collapse and possible underplating of an extensive Mesozoic flysch system and associated igneous arc.

Irradiation of organic matter by uranium decay in the Alum Shale, Sweden

The Alum Shale of Sweden contains black shales with anomalously high uranium concentrations in excess of 100 ppm. Syngenetic or early diagenetic origin of this uranium indicates that organic matter within these shales has been irradiated by decaying uranium for approximately 500 Ma. Radiation-induced polymerization of alkanes through a free-radical cross-linking mechanism appears to be responsible for major alterations within the irradiated organic matter. Specific radiation-induced alterations include generation of condensate-like oils at reduced yields from hydrous pyrolysis experiments, decrease in atomic H/C ratios of kerogens, decrease in bitumen/organic-carbon ratios, and a relative increase in low-molecular weight triaromatic steroid hydrocarbons. Conversely, stable carbon isotopes of kerogens, reflectance of vitrinite-like macerais, oil-generation kinetics, and isomerization of 20R to 20S αα C 29-steranes were not affected by radiation. The radiation dosage needed to cause the alterations observed in the Alum Shale has been estimated to be in excess of 10 5 Mrads with respect to organic carbon. This value is used to estimate the potential for radiation damage to thermally immature organic matter in black shales through the geological rock record. High potential for radiation damage is not likely in Cenozoic and Mesozoic black shales but becomes more likely in lower Paleozoic and Precambrian black shales.

Deposition of Organic Carbon-Rich Sediments in Narrow Marine Basins and Open-Marine Upwelling Environments--New Results from the Ocean Drilling Program: ABSTRACT

Detailed sedimentological and organic geochemical investigations have been performed on Neogene sediments from ODP site 645 (Baffin Bay), ODP site 658 (upwelling area of northwest Africa), and ODP site 679 (upwelling area off Peru). The study is mainly based on (1) data derived from total organic carbon and nitrogen analyses, Rock-Eval pyrolysis, and kerogen microscopy (2) sedimentation rates, and (3) x-ray diffraction analyses. The main objective of this study was to point out the most important factors controlling the accumulation of organic carbon in the different sedimentary environments, such as supply of terrigenous organic matter, productivity of marine organic matter, and preservation of organic matter. These new results from the investigation of ODP sediments are compared with DSDP data from the Mesozoic Atlantic Ocean to characterize the depositional environments of Mesozoic black shales.