Aphanitic buildup from the onset of the Mulde Event (Homerian, middle Silurian) at Whitman’s Hill, Herefordshire, UK: ultrastructural insights into proposed microbial fabrics; pp. 287–292

Abstract

A microbial origin has been proposed for matrix-supported, low-diversity buildups reported from different palaeocontinents during the onset of the Mulde positive carbon isotope excursion. We have investigated a small aphanitic buildup from the Lower Quarried Limestone Member of the Much Wenlock Limestone Formation, exposed at Whitman's Hill (Herefordshire), corresponding to the central part of the Midland Platform (UK). Up to 50% of the rock volume in this buildup consists of mottled micrite. The SEM studies revealed that the micrite is largely detrital and does not show features characteristic of calcareous cyanobacteria or leiolites. The aphanitic character of the buildup is suggested to be controlled by the depositional rate, and the widespread occurrence of matrix-supported reefs in this interval to be driven by a mid-Homerian rapid eustatic transgression.

High-resolution correlation of the Homerian carbon isotope excursion (Silurian) across the interior of the Midland Platform (Avalonia), UK

“…The matrix is clotted and peloidal, suggestive of microbially precipitated micrite. This interval also contains reefs, not examined here, but characterized previously by Penn (1971) and Päßler et al (2014). The second and stratigraphically higher facies is mud-to wackestones with rare complete rugose corals in life positions.…”

“…Interpretations of facies, depositional environments and relative sea-level change have been previously published for the Storridge, Suckley (Hereford-Worcestershire) and Dudley (West Midlands) areas (e.g. Butler, 1939;Phipps & Reeve, 1967;Oliver, 1981;Ratcliffe, 1988;Ratcliffe & Thomas, 1999;Ray & Thomas, 2007;Ray et al 2010Ray et al , 2013Päßler et al 2014;Jarochowska et al 2018). Therefore, here we provide an overview with a focus upon those parts of the succession that show considerable regional variability and may consequently impact upon the local carbon isotopic signature (Figs 2, 3, 6).…”

“…The presence of green algae within much of the overlying Lower Quarried Limestone Member indicates deposition within the euphotic zone (Päßler et al 2014). All oncoids observed are small, smooth-surfaced, and spherical to subspherical (Type A oncoids of Ratcliffe, 1988) and indicate deposition within fair-weather wave base.…”

“…All oncoids observed are small, smooth-surfaced, and spherical to subspherical (Type A oncoids of Ratcliffe, 1988) and indicate deposition within fair-weather wave base. Only the topmost part of the member was deposited below the fair-weather wave base and the euphotic zone (Päßler et al 2014), with the amalgamation through burrowing organisms suggestive of low depositional rates at this time.…”

“…In combining samples from both Whitman's Hill Quarry and Bruff Business Park, a local composite carbon isotope curve has been made and, based upon the correlation of the marker bentonite and parasequences, there is an approximate sampling overlap of 6 m between sections. In terms of sedimentology, the results of hand-specimen analysis and a limited number of thin-sections have previously been reported for the Hereford-Worcestershire succession (Penn, 1971;Ray et al 2013;Päßler et al 2014), alongside more regional studies (Phipps & Reeve, 1967). However, as a means of further improving our understanding of the succession, a number of thin-sections were created to confirm and improve upon the existing sedimentological descriptions and to investigate intervals that show considerable regional variability and may consequently impact upon the local carbon isotopic signature.…”