The Rise of Black Shale Giants
Prof Paul Wignall (SEE), Simon PoultonContact email: P.B.Wignall@leeds.ac.uk
Carbonate platform formation is often abruptly terminated by the onset of thick black shale deposition. The reason(s) for this fundamental change in depositional style is unclear and contentious and yet it is of great geoloigcal and economic significance because some of most economically-important hydrocarbon source rocks are formed in this way. The most spectacular example of this transition in the geological history of the British Isles occurred in the middle of the Carboniferous when shallow-water carbonates, that had been accumulating for tens of millions of years, were replaced by hundreds of metres of black shales - one of the thickest successions of this rock type ever to accumulate. Thus, the Dinantian Limestones of northern England are succeeded by up to 300 m of black shale belong to the Bowland Shale and in Ireland a similar thickness of Clare Shales occurs at the same level following the regional cessation of limestone formation. Equally intriguing the limestone and shale are often separated by a thin bed of phosphatic pebbles whose origin is poorly understood. This project aims to evaluate what happened at this transition and what depositional conditions were like during black shale deposition. The study will focus on the well-exposed Carboniferous outcrops in western Ireland where Lower Carboniferous (Dinantian) Limestones are overlain by hundreds of metres of black shale belonging to the Clare Shale Formation (Fig. 1). Comparison will also be made with the equivalent strata in the Pennines of northern England where similar limestones are succeeded by the thick Bowland Shales. The tectono-sedimentary situation in both regions was very different - the western Irish area probably saw a change from transtensional to foreland basin style subsidence, whilst subsidence style changes in the Pennines were governed by the transition from a syn-rift to post-rift transition in an extensional basin. Despite these different underlying tectonic controls, both regions experienced the same change in sedimentation style pointing to an overriding control by water column conditions, that was perhaps climatically driven. The onset of black shale deposition is the start of a prolonged period of clastic deposition that presumably heralds the onset of rainfall in the hinterlands. However, a humidity increase alone is not sufficient to shut down carbonate formation. For example, the late Dinantian sedimentation in north-eastern England consists of regular alternations of clastic and carbonate lithologies (Yoredale Cyclothems) showing that heterolithic sedimentation was occurring to the north of the black shale basins. Objectives This project will focus on the environemntal changes responsible for the carbonate-to-black shale transition using state-of-the-art approaches to determine water column oxygenation, including sedimentary logging, sedimentary petrography and the latest geochemical techniques to assess both redox and primary productivity. The student will specifically focus on pyrite framboid size distributions, Fe speciation and trace metal techniques to evaluate local and regional water column redox conditions. Many of these techniques were developed by the supervisory team (Wignall & Newton, 1998; Poulton & Canfield 2005) and have recently been calibrated for application to carbonate as well as black shale successions (Bond & Wignall, 2010; Clarkson et al.., 2014). This detailed evaluation of marine oxygenation levels will set the premise for understanding controls on the supply and recycling of the key nutrient phosphorus across the carbonate-black shale transitions. This will be one of the first times such an analysis has been possible thanks to the availability of novel P speciation techniques that have recently been developed by the supervisory team at Leeds for application to ancient marine rocks. Training i) Sedimentary logging in order to collect and measure the Mid Carboniferous strata spanning the carbonate-to-black shale transition in western Ireland and the Pennines. ii) Pyrite petrographic analysis on a scanning electron microscope combined with sulphur isotope analysis. These data can help evaluate water column conditions and determine the presence of sulfidic (euxinic) water column. iii) analyse water column redox controls on P cycling using Fe and P speciation, organic carbon contents and trace metal ratios. iv) Construction of regional depositional history using these datasets and a model to explain the mechanisms of carbonate platform shut down and the subsequent prolonged deposition of highly organic rich mudrocks.
A more detailed project description will follow shortly...
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