Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK

Abstract

For the purposes of a high-resolution multi-disciplinary study of the Upper Jurassic Kimmeridge Clay Formation, two boreholes were drilled at Swanworth Quarry and one at Metherhills, south Dorset, UK. Together, the cores represent the first complete section through the entire formation close to the type section. We present graphic logs that record the stratigraphy of the cores, and outline the complementary geophysical and analytical data sets (gamma ray, magnetic susceptibility, total organic carbon, carbonate, δ13Corg). Of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area. Detailed logs are available for download from the Kimmeridge Drilling Project web-site at http://kimmeridge.earth.ox.ac.uk/. Of further interest is a mid-eudoxus Zone positive shift in the δ13Corg record, a feature that is also registered in Tethyan carbonate successions, suggesting that it is a regional event and may therefore be useful for correlation. The lithostratigraphy of the cores has been precisely correlated with the nearby cliff section, which has also been examined and re-described. Magnetic-susceptibility and spectral gamma-ray measurements were made at a regular spacing through the succession, and facilitate core-to-exposure correlation. The strata of the exposure and core have been subdivided into four main mudrock lithological types: (a) medium-dark–dark-grey marl; (b) medium-dark–dark grey–greenish black shale; (c) dark-grey–olive-black laminated shale; (d) greyish-black–brownish-black mudstone. The sections also contain subordinate amounts of siltstone, limestone and dolostone. Comparison of the type section with the cores reveals slight lithological variation and notable thickness differences between the coeval strata. The proximity of the boreholes and different parts of the type section to the Purbeck–Isle of Wight Disturbance is proposed as a likely control on the thickness changes.

The age of the Tojeira Formation (Late Jurassic, Early Kimmeridgian), of Montejunto, west-central Portugal

“…MS magnetic susceptibility carbonate carbon in the Tethyan realm and organic carbon in the Boreal realm. Boreal realm organic carbon records from age equivalent sections in the UK (Morgans-Bell et al 2001), Scotland (Nunn et al 2009) and Russia (Riboulleau et al 1998;Price and Rogov, 2009) show a conspicuous positive excursion in the R. cymodoce Zone (Sub-boreal equivalent to the S. platynota/A. hypselocyclum zones) and are correlated here to the Tojeira-1 section C-isotope record (Fig.…”

“…4). The lowermost Kimmeridgian ammonite zone (Pictonia baylei) is represented by an interval of stratigraphic incompleteness in the type section of the Kimmeridge Clay Formation (KCF) in Dorset (Morgans-Bell et al 2001) and, therefore, this zone cannot be correlated to. However, although there is evidence for erosional surfaces in the overlying R. cymodoce-A.…”

The White Stone Band of the Kimmeridge Clay Formation, an integrated high-resolution approach to understanding environmental change

“…In addition to the subtle variations in organic matter content that define the Milankovitch-scale cycles, there are also intervals (Morgans-Bell et al 2001) where the Kimmeridge Clay Formation goes through a more rapid series of alternations (on a millimetre to metre scale) between the end-member lithologies of coccolith limestone and oil shale. If we can understand the mechanisms that caused the Kimmeridge environment to switch between these end-members then we have the potential to understand the controls of cyclic alternation within the longer formation-scale time-series.…”

Tethyan-to-boreal correlation in the Kimmeridgian using high-resolution sequence stratigraphy (Vocontian Basin, Swiss Jura, Boulonnais, Dorset)

“…As Weedon et al (2004) draw their cyclostratigraphic interpretation from only one (composite) type section (cf. Morgans-Bell et al 2001), it would be of interest to know if other sections in different (structural) positions furnish exactly the same results as the examined type section. In comparison to southern England, the Jura platform experienced a relatively homogenous subsidence during the Late Jurassic.…”

“…This major plate reorganisation was associated with variations in the oceanic spreading rates and intraplate stresses (Cloething 1986(Cloething , 1988aKarner 1986;Lambeck et al 1987) giving rise to doming, subsidence and break up in the adjacent North Sea (Ziegler and van Horn 1989), and reactivations of ancient fractures over the whole incipient North Atlantic margin. In southern England, the extensional tectonics led to the development of W-E trending half-grabens, resulting in block tilting and thus markedly different thicknesses of the Kimmeridge Clay depending on footwall or hanging wall position (Chadwick 1986;Newell 2000;Morgans-Bell et al 2001). Within such an environment defined by high tectonic activity, it has to be considered that processes such as erosion, sediment redistribution, and frequent changes in subsidence rate may be triggered which can distort the recording of an existing orbital signal (tectonic forcing).…”