High resolution δ 13C stratigraphy of the Homerian (Wenlock) of the English Midlands and Wenlock Edge

Research output: Contribution to journalArticlepeer-review

Authors

  • Carly Marshall
  • Alan T. Thomas
  • Ian Boomer
  • David C. Ray

External organisations

  • University of Birmingham
  • Neftex Petroleum Consultants Ltd.

Abstract

High resolution δ 13C calb 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 upper-most Coalbrookdale, Much Wenlock Limestone and basal Lower Elton formations. Deposition there occurred in a mid-shelf setting. A similar 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 δ 13C calb values rising to +5.5%o VPDB. Values fall to +0.8%c VPDB higher in the section before rising again to +4.1%o VPDB (Nodular Beds Member). Analysis of lithofacies varia-tion 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 δ 13C carb values are consistently 2%c higher, and accord more closely with values published for sec-tions elsewhere. The lower of the two peaks found in the West Midlands cannot be identified on Wenlock Edge, where 813Ccarb values fluctuate somewhat around +2%c VPDB. The upper peak, though less distinct, can be identified on Wenlock Edge with values rising to +3.8%c VPDB. Correlations based on biostratigraphy, sequence stratigraphy and bentonite geochemistry suggest that not all changes in δ 13C calb occurred synchronously in the two areas studied, de-spite their close proximity.

Details

Original languageEnglish
Pages (from-to)669-679
Number of pages11
JournalBulletin of Geosciences
Volume87
Issue number4
Publication statusPublished - 2012

Keywords

  • δ C stratigraphy, English Midlands, Homerian, Mulde Excursion, Silurian, Wenlock