A new perspective on the zonation and correlation of barren strata: An integrated heavy mineral and palaeomagnetic study of the Sherwood Sandstone Group, East Irish Sea Basin and surrounding areas

The integration of data from high-resolution heavy mineral and palaeomagnet ic analyses of samples from nine East Irish Sea wells, and from onshore sec tions at St Bees Head and the North Cheshire Basin has generated a temporal ly-constrained heavy mineral stratigraphy and regional correlation within t he Triassic Sherwood Sandstone Group. These results have shown that the com bined application of these two techniques offers an effective tool for exte nding the zonation and correlation of barren strata into uncored wells. The lateral persistence of the heavy mineral zones, and the significant differ ences in the distributions of the heavy mineral varieties between individua l zones, may reflect tectonic events and, to a lesser extent, climatic vari ations. Palaeomagnetic results indicate that the original magnetisation of the anal ysed samples was acquired as a detrital remanent magnetisation and that the characteristic remanent magnetisation in the red beds was acquired during and shortly after deposition. Magnetopolarities have been interpolated usin g the available data from which a composite reference section has been cons tructed. All well sequences exhibit a regionally persistent three-fold zonation (Zon es A, B and C, locally with subzones), defined by systematic changes in cha racteristic heavy mineral suites. Using a common sample set, the independen tly erected heavy mineral stratigraphy was tied to specific magnetostratigr aphic Chrons. Heavy mineral Zone A coincides with a period of dominantly no rmal polarity, fixed by a 244 Ma Chron of Middle Scythian age. A change in polarity from normal to reversed at >243 Ma coincides with the boundary of Zones A/B. Zone B is Middle to Upper Scythian and shows a dominantly revers ed polarity. A well-defined Chron of 242.7 Ma has been detected close to th e boundary of Zones B/C. Zone C is Anisian in age and is characterised by a dominantly normal polarity.