High resolution δ13Ccarb stratigraphy of the Homerian (Wenlock) of the English Midlands and Wenlock Edge


Authors: Marshall C, Thomas AT, Boomer IRay DC

Published in: Bulletin of Geosciences, volume 87, issue 4; pages: 669 - 679; Received 31 July 2011; Accepted in revised form 30 May 2012; Online 17 October 2012

Keywords: Silurian, Wenlock, Homerian, δ13C stratigraphy, Mulde Excursion, English Midlands,

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High resolution δ13Ccarb data are presented for two composite sections in England covering much of the Homerian Stage. Micrite samples collected at ˜0.5 m intervals from outcrop and core in the Dudley area, West Midlands, span the uppermost Coalbrookdale, Much Wenlock Limestone and basal Lower Elton formations. Deposition there occurred in a mid-shelf setting.Asimilar suite of samples from the Wenlock type area, Wenlock Edge, Shropshire, represents a coeval sequence deposited closer to the shelf-basin margin. The successions concerned extend from the upper lundgreni to nilssoni graptolite biozones and provide a detailed record of variation in stable carbon isotope ratios across the well-known double-peaked Homerian positive excursion (Mulde Excursion), a time of significant global biological and chemical perturbation. In the West Midlands, this excursion occurs in the Much Wenlock Limestone Formation. The lower peak (Lower Quarried Limestone Member) has δ13Ccarb values rising to +5.5‰ VPDB. Values fall to +0.8‰ VPDB higher in the section before rising again to +4.1‰ VPDB (Nodular Beds Member). Analysis of lithofacies variation in this interval indicates two transgressive-regressive cycles, the two positive peaks of the excursion correlating with relative sea-level lows and the intervening dip with a relative sea-level high, the local expression of Johnson’s (2006) Highstand 5A. The double-peaked nature of the excursion at Dudley resembles that previously recorded for the area; however, our δ13Ccarb values are consistently 2‰ higher, and accord more closely with values published for sections elsewhere. The lower of the two peaks found in the West Midlands cannot be identified on Wenlock Edge, where δ13Ccarb values fluctuate somewhat around +2‰ VPDB. The upper peak, though less distinct, can be identified on Wenlock Edge with values rising to +3.8‰ VPDB. Correlations based on biostratigraphy, sequence stratigraphy and bentonite geochemistry suggest that not all changes in δ13Ccarb occurred synchronously in the two areas studied, despite their close proximity.