The rise of black shale giants: case studies on the Exshaw Shale and Bowland Shale

The thickest developments of black shales are often found overlying shallow-water limestone, and mark fundamental changes in depositional environment. This is the case in the Carboniferous successions of the British Isles where thick black shale reserves belonging to the mid-Carboniferous Bowland Shale formations sit on Lower Carboniferous slope and platform carbonates. A similar transition is seen around the Devonian-Carboniferous boundary in western Canada where the Exshaw Shale overlies ramp carbonates. Intriguingly phosphate nodule beds often occur at this latter transition.
Comprehensive petrology and geochemical study of Jura Creek, a classic section in Western Canada Sedimentary Basin in western Laurentia shows that the onset of black shale deposition was preceded by intense pyritization of the topmost limestone suggesting deoxygenation began before shale deposition. The succeeding thin phosphate-rich bed was likely produced by internal wave action at a density interface within the water column. Following onset of black shale deposition in the lower Exshaw Shale Formation there was considerable redox variation. Elevation in FeHR/FeT, Fepy/FeHR, pronounced enrichment in trace metals (Mo, U, V and Re) combined with tiny framboid populations indicate euxinia. This was followed by a transition to a ferruginous water column as confirmed by moderate enrichment in trace metals, but low Fepy/FeHR and remarkably no framboidal pyrite. In addition, Mo-U covariation also suggests a preferential enrichment in Mo which could be associated with particulate shuttle (Fe cycle in ferruginous conditions). The Devonian- Carboniferous boundary saw the return of better ventilated conditions in the Exshaw Shale marked by a decrease in trace metal content and loss of syngenetic pyrite. The level of the Hangenberg Crisis at Jura Creek coincides with the improved ventilation, from euxinic to ferruginous conditions, a distinct contrast from European sections where anoxia intensifies at this level.
The Bowland Shale of the mid-Carboniferous Bowland Basin of northern England has been examined in 16 sections ranging from basin centre to basin margin sites. Based on calibrated redox threshold, it is found that the basin experienced a broad range of redox conditions from oxic, ferruginous, weakly euxinic ultimately to highly euxinic conditions. Initial deoxygenation in the Lower Bowland Shale (ferruginous and euxinic conditions) showed considerable local variation whilst carbonate productivity on adjacent platforms was maintained with the result that substantial volumes of carbonate detritus was supplied to the basin resulting in discrete calciturbidite fans composed of calcarenite and a substantial component of fine calcisilt in the interbedded shales. Much of this material was likely sourced from the Central Lancashire High (CLH), a small carbonate platform to the SE of the basin. The much more extensive platform on the Askrigg Block to the north of the Basin sourced little material probably because depositional dip was to the north. A major phase of onlap, associated with a transition from syn to post-rift conditions, saw black shales of the Upper Bowland Shale overlie the CLH carbonate platform. The extensive Upper Bowland Shales record intense euxinia indicated by presence of abundant tiny framboids, elevated enrichment in Mo (MoEF > 40), low ratio in U/Mo as well as Fepy/FeHR > 0.6. The petrography of this euxinic facies is dominated by clay lenses (up to 0.4 mm in length) with organic filaments and pelagic components (syngenetic framboids, planktonic larval shells) and further clay in the interstitial sediment. Most recent studies have interpreted such clay lenses to be intraclasts transported as bedload or in low concentration sediment gravity flows, but it is proposed that hemipelagic depositional processes dominate, as is likely the case for most organic-rich shales formed in euxinic conditions. The clay lenses are interpreted to be faecal pellets formed above the redoxcline before settling to the seabed.
By comparing the study of Exshaw Shale and Bowland Shale, it is found that the relationship between shallow-water carbonate demise and black shale deposition is complex. The basin morphology (whether it is a ramp or platform) is likely an important factor in controlling the timing of carbonate shutdown. Specifically, it is contended that a euxinic water body can more readily “poison” the lower reaches of a carbonate ramp than in a fault-bounded basin where water circulation is constrained. Only when the highly euxinic water body spills across a basin margin, perhaps due to basin margin collapse, can it shutdown carbonate production.
Insight from the Bowland Shale study enables a new model to be postulated that shows that input of warm saline bottom water provides an option to create an ideal environment for black shale deposition. Especially if the basins are surrounded by carbonate platform where potential reasons (e.g. elevated productivity, fresh water input) for black shale deposition cannot be adopted. However, it is ironic that the hydrogen sulphide produced by these strongly stratified and poorly circulated basinal waters may compromise the carbonate production on the platform in return. Furthermore the demise of carbonate production is destined in a basin-block setting.