Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK)

Abstract

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.

Quartz cementation in Late Cretaceous mudstones, northern North Sea: Changes in rock properties due to dissolution of smectite and precipitation of micro-quartz crystals

“…On the other hand, the origin, authigenic or detrital, of silt-sized quartz (Blatt and Schultz, 1976;Milliken, 1994;Schieber et al, 2000;Williams et al, 2001) not identifiable as biogenically derived has been an issue of long-standing controversy (e.g., Milliken, 1994). Consistent with the findings of this study, silt-sized quartz verified by SEM/CL as authigenic, has been interpreted as re-crystallized siliceous planktonic organisms (Milliken, 1994;Schieber et al, 2000;Peltonen et al, 2009).…”

Rock-physics templates for clay-rich source rocks

“…Shale mineralogy may include kaolinite, montmorillonite-smectite, illite, and chlorite, so the term smectite-illite as used in this study may be representative for a mixture of clay minerals (Mondol et al, 2008). In the Kimmeridge Clay, there is also some percentage of silty quartz (Williams et al, 2001). Moreover, the smectite-illite layers represent an assembly of platelets, subject to internal hydration, so its mechanical properties such as the stiffnesses can vary depending on the source rock.…”

Black shale deposition during Toarcian super-greenhouse driven by sea level

“…The relative proportion of coarse versus fine detrital fractions of the sediment (represented by quartz and clay minerals, respectively) deposited at a given location is driven by accommodation space, and, consequently by the shoreline position (Williams et al, 2001;Coe, 2003). This sequence stratigraphic feature is typically used to recognise patterns in relative sea level change (de Graciansky et al, 1998;Williams et al, 2001;Coe, 2003;Hesselbo, 2008).…”

“…This sequence stratigraphic feature is typically used to recognise patterns in relative sea level change (de Graciansky et al, 1998;Williams et al, 2001;Coe, 2003;Hesselbo, 2008). Indeed, changes in the distance between the studied location and the coastline where sediments are supplied may be attributable to sea level assuming no significant change in the source of detrital supply, nor variation of the subsidence rate (Guillocheau et al, 2000).…”