Organic Matter‐Rich Shale Depositional Environments

Abstract

2.1 INTRODUCTIONShale is the most abundant rock type available at the surface of our planet and makes up about two-thirds of the stratigraphic record (Garrels and Mackenzie, 1969). The term "shale" 1 refers to all sedimentary rocks composed predominantly of mud 2 (>4ϕ or <0.0625 mm) particles (cf. Tourtelot, 1960, p. 342). Mud particles may be terrigenous, biogenous, or hydrogenous. Terrigenous-or siliciclastic-mud is always detrital, that is, produced by weathering and erosion of preexisting rocks, and comes into the depositional environment as individual particles and/or as aggregates. Biogenous mud is made by organisms, and includes a skeletal and an organic component. Skeletal mud may be calcareous or siliceous. Some benthic protozoans (agglutinated forams) build their tests by cementing terrigenous and other particles with an organic ligand. Some quartz silt aggregates in shales represent the collapsed or compacted tests of these organisms (Pike and Kemp, 1996;Schieber, 2009). When a shale is characterized by an organic matter content higher than the average marine shale (ca. 0.5%, Arthur, 1979), it is referred to as an organic matter-rich shale or, more commonly, a black shale. 3 Hydrogenous mud precipitates out of solution directly, either from seawater or from interstitial water during diagenesis, and it includes oxides and hydroxides, silicates, for example, zeolites and clay minerals, heavy metal sulfides, sulfates, carbonates, and phosphates. Clay minerals are therefore not only terrigenous, but they may also be hydrogenous, that is, formed in situ. Biogenous and hydrogenous mud may be detrital, that is, recycled from older deposits, in which case their origin is biochemical, but their texture will be clastic. The composition of the detrital fraction of a shale depends on the petrology of its source areas and on the intensity and effectiveness of chemical weathering.

Middle Ordovician Upwelling-Related Ironstone of North Wales: Coated Grains, Ocean Chemistry, and Biological Evolution

“…The reproducibility of TOC concentrations using LECO combustion is ±1%. Organic-matter-rich lithofacies contain >0.5 wt% TOC (Trabucho-Alexandre, 2015). The geostandards SY-4 (diorite gneiss), 692 (iron ore), GBM998-10 (multi-metal nickel ore), and Till-2 (till) were used as matrix matching standards for various facies.…”

Sedimentation of the Kimmeridge Clay Formation in the Cleveland Basin (Yorkshire, UK)

“…Geoscientists continue to study the KCF using techniques ranging from sedimentology [8][9][10], geochemistry [6,[11][12][13][14][15], to palaeontology [16][17][18][19], and climate modelling [20][21][22][23], in an effort to correlate deposits, elucidate depositional controls and reconstruct global climate conditions. Characterisation of the sediment facilitates the interpretation of ancient depositional environments, which provide a unique and important insight into global climate and carbon cycle dynamics [24]. However, lateral sediment transport and potentially multiple episodes of sediment remobilisation prior to deposition means that mudstones may record palaeoenvironment conditions from other areas of the ocean with little or no evidence of long-range transport; therefore, [36] reproduced with permission after Atar et al (2019b) [15].…”