Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK)
✍ Scribed by Carolyn J. Williams; Stephen P. Hesselbo; Hugh C. Jenkyns; Helen S. Morgans-Bell
- Book ID
- 104463334
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 359 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0954-4879
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✦ Synopsis
Sequence‐stratigraphic interpretation of mudrocks is often carried out using proxy indicators for grain size or by consideration of other aspects of lithology thought to relate to sea‐level change, such as organic‐matter content. An alternative stratigraphic analysis, based on direct estimation of quartz‐silt content, was carried out on a major Late Jurassic mudrock (and oil source rock), the Kimmeridge Clay Formation of the Wessex Basin, Dorset, UK. The new data, generated by backscatter SEM, X‐ray and image analysis, show decametre‐scale stratigraphic patterns that are incompatible with many previous sequence‐stratigraphic interpretations based on gamma‐ray logs or visual lithofacies and biofacies description. Correlation with a basin‐margin section in the Boulonnais, northern France, indicates that silt‐rich intervals in basinal facies are coeval, within the limits of biostratigraphic resolution, with shallow‐water sand‐rich packages on the margin. Variation in silt content in the Kimmeridge Clay therefore appears to be a record of relative sea‐level change of at least regional extent. It is suggested that analysis of silt content offers the most reliable basis for generation of a regional sequence stratigraphy in basinal mudrocks. A revised relative sea‐level curve for the Wessex Basin Kimmeridgian and early Tithonian is presented based on this premise.