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Cambrian and Ordovician stratigraphy related to structure and seismic profiles in the western part of the English Midlands

Published online by Cambridge University Press:  01 May 2009

N. J. P. Smith  and
A. W. A. Rushton
N. J. P. Smith
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
A. W. A. Rushton
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
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Abstract

In England the Cambrian successions include relatively continuous, dominantly argillaceous sequences (Nuneaton area, > 900 m thick) and arenaceous successions punctuated by hiatuses (Malvern, Shropshire). The Tremadoc (lower Ordovician) succeeds paraconformably; it is widespread, dominantly argillaceous and locally very thick (> 2000 m). Deposition of the lower Tremadoc is thought to have been rapid. Uplift of regional extent occurred in early Ordovician (post-Tremadoc) times and inversion of the basins, such as the Tremadoc Worcester Graben, occurred during the Variscan Orogeny. The disposition of the Cambrian–Tremadoc beneath the Upper Llandovery unconformity is thought to reflect the arrangement of basins and highs at that time. Evidence from seismic profiles indicates deposition in half-grabens. In the Welsh Borderland, evidence from outcrops and boreholes suggests that the St David's Series (Middle Cambrian) is locally thickened against syn-depositional faults. The thinning and local absence of the St David's Series across such faults is attributed to the influence of tilt-block highs. Rifting and tilting allowed thick Tremadoc to accumulate (e.g. in a precursor Worcester Graben), but evidence for the direction of thickening is masked by lack of seismic markers and by the effect of pre-landovery erosion.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 5 , September 1993 , pp. 665 - 671
Copyright
Copyright © Cambridge University Press 1993

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