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Sedimentary Petrology
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
Areas of extreme aridity and local areas of internal drainage with a high rate of evaporation, as in continental or coastal sabkhas, are ideal for precipitation of evaporites. Another ideal situation for evaporite deposition is a barred (silled) basin formed as a bay or cut-off arm of a sea (see discussions on euxinic or silled basins). Circulation in such a closed basin is maintained by the influx of salt-laden sea-water flowing above the sill and return (reflux) of this water into the sea as a subsurface current. The amount of evaporite deposited into the closed basin is governed by the amount of salt flowing into the basin and the amount carried back to the sea during reflux. In this process the least soluble salts (anhydrite and gypsum) are deposited nearer to the sill and the highly soluble salts (halite and sylvite) are deposited away from the sill, in the distal part of the basin (see Chapter 6, Fig. 6.17).
Aluminium-normalised trace-element paleoredox proxies and their application to the study of the conditions of Burgess Shale-type preservation
Published in Australian Journal of Earth Sciences, 2023
K. C. Meehan, W. G. Powell, D. M. McKirdy, P. A. Hall, C. Nedin, P. A. Johnston, C. J. Collom
It is well established that Mo, Ag, Zn, Ni, Cu, V, U and the platinum-group elements (PGEs) tend to be significantly enriched in modern muds within anoxic basins and in ancient black shales (e.g. Calvert & Pedersen, 1993; Kimmig & Pratt, 2022; Piper & Calvert, 2009; Tribovillard et al., 2006; Vine & Tourtelot, 1970). In fact, black shales deposited under sustained euxinic conditions may result in extreme concentration of metals with redox-dependent solubility, such that they attract economic interest (Coveney & Martin, 1983; Coveney & Nansheng, 1991; Ripley et al., 1990). These redox-sensitive metals accumulate under reducing conditions, in part owing to the delivery of seawater-derived elements to the sediment as insoluble mineral precipitates, or as ions adsorbed onto the surfaces of settling pelagic sediment and organic debris (Algeo & Li, 2020; Calvert & Pedersen, 1993). However, additional sources may contribute to the total metal content of a sediment, including a lithogenous component in the form of detrital mineral grains (Calvert & Pedersen, 1993). Therefore, total metal concentrations must be normalised against the lithogenous component to produce an effective geochemical index of redox conditions. The terms ‘normalising’ and ‘normal’ shales are in reference to the post-Archean Australian Shale (Taylor & McLennan, 1985).
Authigenic calcium carbonate precipitation in the “bathtub ring” around the anoxic Alum Shale Basin during the Furongian SPICE event (Baltic Basin, northern Poland)
Published in GFF, 2021
Aleksander Majchrzyk, Wojciech Kozłowski, Anna Żylińska
One of the earliest pronounced Phanerozoic anoxic events occurred during the early Furongian Epoch (late Cambrian), and is postulated to be associated with the Steptoean Positive Carbon Isotope Excursion (SPICE), a 4–6 ‰ positive shift in δ13Ccarb (Saltzman et al. 1998, 2000; Gill et al. 2011; Gerhard & Gill 2016). The SPICE event is regarded as a synchronous, worldwide event, and records have been recovered from all palaeocontinents (e.g., Gill et al. 2011). The excursion is related to major sea-level changes − the onset of the excursion coincides with a regressive event, and its peak is concurrent with a lowstand (Saltzman et al. 2000, 2004; Gill et al. 2011; Schiffbauer 2017) − and a significant trilobite extinction in Laurentia (the end-Marjumiid biomere extinction; Saltzman et al. 2000; Gerhardt & Gill 2016; Schiffbauer et al. 2017). The coeval positive sulphur isotope excursion (recorded in carbonate-associated sulphates) and increase in total organic carbon (TOC) levels suggest enhanced organic carbon and pyrite burial in euxinic basins (Gill et al. 2011; Saltzman et al. 2011), although the global scale of such anoxia has been questioned (Egenhoff et al. 2015; Wotte & Strauss 2015). The event probably resulted in a strong, global-scale seawater sulphate depletion (Gill et al. 2011), possibly coinciding with a rise in carbonate saturation.
Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia
Published in Australian Journal of Earth Sciences, 2021
P. A. Hall, D. M. McKirdy, G. P. Halverson, J. B. Jago, A. S. Collins
The two sections of the Talisker Formation (Figure 3) show strong covariance of TOC, MoPAAS, V/Sc and Ni/Co (Figure 7g, h). Calcareous siltstone and shale at the base of the formation passes upward into pyritic shale, reflecting the transgressive nature of the formation (Gum, 1998; Turner, 1994). Accordingly, at both sites there is an overall trend towards more reducing conditions up section. At Frankton-1 this trend is both rapid and pronounced. Similar excursions are exhibited by MoPAAS at 43 and 72 m height in the Frankton-2 section. The consensus of the redox proxies is that the Talisker shales were deposited beneath dysoxic to oxic bottom waters; the high value of U/Th at 60 m in Frankton-1 is clearly anomalous. The high concentrations of pyrite in these shales might reasonably be regarded as implying euxinic conditions at the sea floor. However, high concentrations of dissolved sulfide in anoxic pore waters commonly occur at or within a few millimetres of the sediment–water interface in many shallow sea-floor environments, above which fully oxic conditions prevail (Piper & Calvert, 2009). Moreover, the hydrothermal fluids that sourced the local mineralisation (Gum, 1998) could have supplied sufficient hydrogen sulfide to create localised euxinic conditions, as is believed to have happened on the Yangtze Platform, South China (Och et al., 2013).