John McKervey

Sedimentary and diagenetic environments of the Wildmoor Sandstone Formation (UK): implications for groundwater and contaminant transport, and sand production

Abstract The Wildmoor Sandstone Formation, proved in three boreholes drilled at Birmingham University, is dominated by fine- to medium-grained sandstones deposited in a braided river environment, within which channel lag, channel fill and abandoned channel facies are recognized. Minor proportions of aeolian sandsheet are present, as are dolocretes, not previously reported in the formation. The sandstones are feldspathic and lithic arenites, and typically are clay-poor. Early dolomite dominates the diagenetic overprint, and is preferentially developed in channellag deposits. Burial diagenetic effects are minor. Late calcite occurs as a pore-filling phase and within fractures. Minor fractures and granulation seams are oriented parallel to the NE-SW Birmingham Fault. ‘Conventional’ granulation seams, with comminution of detrital material, and more complex seams containing comminuted dolomite cement with a millimetre-wide halo of dolomite cement are present, the latter implying that the sandstone was dolomitecemented at the time of fracturing. Several scales of heterogeneity will affect groundwater solute transport. The palaeosols and abandoned channel mudstones may act as barriers to vertical flow at the decimetre scale. Dolomite-cemented channel-lag deposits may act similarly at smaller scales. Granulation seams have permeabilities of two-three orders of magnitude lower than their host sandstones, but their limited occurrence may limit their impact on larger scale flow. Matrix permeability is controlled by grain size and dolomite cement. The fines in the fine-grained, ripple cross-laminatied sandstones were extensively washed out during coring, and this lithology may be a source of sand yields in some sandstone boreholes. Although no enhancement of particle yields was seen during packer testing, the possibility remains that more comprehensive failure may occur at higher pumping rates.

Warwickshire Group (Pennsylvanian) red-beds of the Canonbie Coalfield, England–Scotland border, and their regional palaeogeographical implications

Late Carboniferous red-beds, < 700 m thick, at outcrop and in the subsurface of the Canonbie Coalfield can be assigned to the Warwickshire Group. They are preserved within the axial part of the Solway Syncline and are divisible into the Eskbank Wood, Canonbie Bridge Sandstone and Becklees Sandstone formations. Sedimentation largely took place on a well-drained alluvial plain, characterized mainly by early, primary oxidation of the strata. Large, northerly-flowing braided river systems were common, with overbank and floodplain fines deposited lateral to the channels; soils formed during intervals of low sediment aggradation. The Canonbie succession includes some of the youngest Carboniferous rocks preserved in the UK. Correlation of the Eskbank Wood Formation is equivocal, but using petrographical, heavy mineral, zircon age dating and palaeocurrent data, the Canonbie Bridge Sandstone Formation can be unambiguously correlated with the Halesowen Formation of Warwickshire, the Pennant Sandstone Formation of South Wales and the offshore Boulton Formation. This suggests that southerly-derived detritus travelled considerable distances from the Variscan highlands of Brittany and/or central Germany across the southern North Sea and UK areas, to a position some hundreds of kilometres north of that previously recognized. The Becklees Sandstone Formation has much in common with the Salop Formation of the English Midlands. It appears to have no preserved equivalent elsewhere in the UK or in the UK sector of the southern North Sea but resembles stratigraphically higher parts of the southern North Sea succession seen in the Dutch sector.

Chromite and PGE in the Logar Ophiolite Complex, Afghanistan

Abstract The Logar Ophiolite Complex (LOC) is located 30 km south of Kabul, Afghanistan, and extends over approximately 2000 km2. It comprises a lower lherzolitic-dunitic-harzburgitic-gabbro ultramafic-mafic unit that passes upwards into a dolerite dyke complex, basaltic pillow lavas and an uppermost sequence of volcaniclastic- and terrigenous-dominated sedimentary units. The ophiolite represents an obducted remnant of intra-Tethyan basin oceanic crust, thrust onto a platform-style cover component of the Kabul Terrane during the Himalayan orogeny. Platinum-group minerals have been detected for the first time in chromitites and ultramafic units from the LOC. Two distinct types of chromitites and ultramafic lithologies with different origins have also been identified in this study. The first type is a low Cr, PGE-poor chromitite interpreted to have been produced in a mid ocean ridge (MOR) environment. The second type is a high Cr, relatively PGE-rich dunite and peridotite from a boninitic magma produced in a supra-subduction zone (SSZ) setting. Platinum group element (PGE) abundances in these chromitites average 12 ppb and 26 ppb for Pt+Pd+Rh for the dunite and peridotite. Chondrite-normalised PGE patterns have two distinct trends: (a) the MOR rocks have a positive Ru anomaly with a negative Pt anomaly and a generally negative slope; and (b) the SSZ rocks show weak positive Ru and Pt anomalies and a positive slope. It is concluded that the negative sloping pattern is typical of PGE in most ophiolites elsewhere. In contrast, the positively sloping pattern is more unusual and may indicate PGE remobilisation and enrichment.