Lithostratigraphical subdivision of the Triassic Sherwood Sandstone Group in west Cumbria

Abstract

SUMMARY The subdivisions of the Sherwood Sandstone Group, previously recognized on geophysical and lithological criteria in boreholes offshore in the East Irish Sea Basin, can be identified in the onshore exposures and boreholes of west Cumbria. The term St Bees Sandstone Formation is here restricted to the lower, fluvial, fine-grained sandstone, with claystone and siltstone partings, exposed around St Bees. This formation is sharply overlain by softer and coarser grained, aeolian and subordinate fluvial sandstone, which contains abundant, well-rounded aeolian sand grains, referred to the newly defined Calder Sandstone Formation. Higher sandstone has been separated from the Calder Sandstone Formation by its finer grain size and by geophysical log signatures. This is referred to the Ormskirk Sandstone and is dominantly aeolian in origin. Similar divisions of the Sherwood Sandstone Group can be recognized in the Carlisle Basin, and may also be present in other onshore areas, but await formal definition.

Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the northeastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK

“…Essentially the same lithostratigraphical units were recognised onshore in west Cumbria by Barnes et al (1994) (see also Akhurst et al 1997; Survey 1999) from the study of outcrops and by the examination of the more complete sections provided by the several kilometres of core from boreholes drilled around Sellafield (Michie & Bowden 1994). However, some different names and a different hierarchy were proposed, principally because the Calder Sandstone Formation in this area was noticeably coarser grained than either the St Bees or the Ormskirk Sandstone formations, or its offshore equivalent, the Calder Sandstone Member (Tables 2, 3).…”

“…Barnes et al (1994) (also Jackson et al 1995;Jackson & Johnson 1996) suggested that these boundaries are disconformities, the uppermost corresponding to the Hardegsen Disconformity. These observations contradicted the previous investigations of Eastwood et al (1931) and Trotter et al (1937) who concluded that there were no major breaks in the sequence and that the aeolian beds are a local lithofacies of the fluvial rocks and cannot be separately mapped at surface.…”

“…Onshore, the boundary between the Calder and Ormskirk Sandstones is taken at the top of a prominent, persistent fluvial unit (Barnes et al 1994). …”

“…38;Akhurst et al 1997, fig. 28; Holliday et al 2004) (Table 1). Barnes et al (1994) (Table 3), and several alternative proposals have been made. Barnes et al (1994) broadly correlated the Ormskirk Sandstone and Kirklinton Sandstone, and compared the 'transitional' interbedding of St Bees Sandstone and Kirklinton Sandstone-type strata, described by Dixon et al (1926), with the alternating fluvial and aeolian facies of the Calder Sandstone Formation around Sellafield.…”

“…Barnes et al (1994) (Table 3), and several alternative proposals have been made. Barnes et al (1994) broadly correlated the Ormskirk Sandstone and Kirklinton Sandstone, and compared the 'transitional' interbedding of St Bees Sandstone and Kirklinton Sandstone-type strata, described by Dixon et al (1926), with the alternating fluvial and aeolian facies of the Calder Sandstone Formation around Sellafield. Jackson et al (1995) and Jackson & Johnson (1996, p. 70) • Is the Kirklinton Sandstone Formation a valid lithostratigraphical division and can it be adequately defined and distinguished from the St Bees Sandstone?…”