Geochemical discrimination of Silurian mudstones according to depositional process and provenance within the Southern Welsh Basin

Abstract

A preliminary geochemical investigation of Silurian (Llandovery) basinal mudstones (turbidites and hemipelagites) from the Southern Welsh Basin is described. Turbidite mudstones show higher concentrations of Fe2O3, MgO, TiO2, MnO, LOI, Zn and Zr than laminated hemipelagites. This is consistent with the observed higher concentrations of chlorite and Ti-bearing minerals in turbidite mudstones. Laminated hemipelagites show higher values of REEs (Ce and La), concentrated within authigenic monazites, and Ni, As, Cu and Pb within sulphide minerals (pyrite and galena) reflecting the influence of primary organic carbon levels and anoxic bottom waters on early diagenesis. Deposition of hemipelagites under oxidizing conditions is reflected in lower concentrations of authigenic sulphide mineral hosted elements compared with laminated hemipelagic lithologies. There is a distinct geochemical difference between mudstones of easterly and southerly provenance in the Southern Welsh Basin. This is shown for both turbidites and hemipelagites. The differences are due to the increased input of illite and the chemical elements associated with this mineral (K2O, A12O3, Rb and Ba). Turbidite mudstones sourced from the south show increased levels of heavy minerals, especially those associated with Ti-rich minerals. There is also an increase in elements associated with detrital monazites: Th and Y. The hemipelagites show higher values of REE and chalcophile elements consistent with their more reduced nature.Peer reviewe

Sequence stratigraphical analysis of late ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment

“…These units vary from conglomeratic debrites and channelized turbidite sandstones and conglomerates to thin-bedded turbidite-lobe facies. The unifying feature of the slope-apron systems is their proved or inferred supply from the east or southeast, directly across the edge of the Midland Platform (Ball et al 1992;Morton et al 1992).…”

The Caban Conglomerate and Ystrad Meurig Grits Formation — nested channels and lobe switching on a mud-dominated latest Ashgill to Llandovery slope-apron, Welsh Basin, UK

“…The slope-apron facies comprise thinly-interbedded, easterly-sourced mudstone turbidites and hemipelagites (Ball et al 1992). The mudstone turbidites comprise thin-to very thin-bedded, siltstone/mudstone couplets (Bouma T de divisions), thin to very thin, structureless or graded mudstones (Bouma Te division) and silt-laminated mudstones capped by graded and/or structureless mudstone.…”

“…Each pair of laminae is thought to represent an annual varve (Dimberline et al 1990). Very low levels of bioturbation and the preservation of organic debris demonstrates that such hemipelagites accumulated under anoxic bottarn conditions (Ball et al 1992) Fig. 26.2b and c).…”

“…26.2b and c). The presence of pervasive bioturbation reflects deposition under relatively oxic bottarn conditions, while the pale colour reflects the destruction of organic carbon in both the hemipelagite and the upper part of the underlying turbidite mudstone by downward-migrating oxidation fronts (Cave 1979;Smith 1987;Ball et al 1992), Three slope-apron facies are recognized: anoxic, oxic and mixed. Anoxie facies mainly comprise laminated hemipelagites and mudstone turbidites (Bouma Te) (Fig.…”

A new Early Silurian turbidite system in Central Wales: insights into eustatic and tectonic controls on deposition in the southern Welsh Basin

“…During deposition of the latest Ashgill to middle Llandovery series, between around 445 and 430 Ma (after Melchin, Cooper & Sadler, 2004), the southern Welsh Basin recorded the development of a broad NWfacing slope apron, constructed principally from turbiditic and hemipelagic mudstones supplied from the east (Ball et al 1992;Davies et al 1997), that extended west from the Tywi Lineament, a contemporaneous structural high separating basinal and shelfal domains ( Fig. 1; Woodcock, 1984).…”