The Larne No. 2 Borehole: discovery of a new Permian volcanic centre

Abstract

SYNOPSIS The Larne No. 2 Borehole is situated in an area where Mesozoic and Tertiary rocks cover the assumed westerly extension of the Midland Valley of Scotland. It penetrated 2880-36 m of New Red Sandstone sediments and volcanic rocks and is one of the deepest boreholes drilled in Ireland. Beneath thin Quaternary clays, a Triassic succession consisting of saliferous Mercia Mudstone Group (958-3 m), overlying Sherwood Sandstone Group (648-3 m) was encountered. Permian Upper Marls (185-4 m), including a thick (113-1 m) basal halite, and Magnesian Limestone (21-6 m) successively underlie the Triassic rocks and with the latter may be correlated with similar sequences in Northern Ireland, England and the Irish Sea. The Permian succession beneath the Magnesian Limestone resembles that of south-west Scotland and comprises 440-7 m of sandstone and pebble conglomerate overlying a hitherto unsuspected 616-7 m proven thickness of intermediate to basic volcanic rocks. These altered lavas have yielded K/Ar ages of c. 245 ± 1 3 Ma and the balance of evidence suggests that they are of early Permian age.

The petrogenesis of the Tertiary basaltic and intermediate lavas of northeast Ireland

“…They suggest that the major and trace element chemistry of the mafic alkalic Permian dyke swarm in western Scotland has the requisite characteristics to have caused this depletion. A 1am-prophyre dyke swarm of Permian age occurs in NE Ireland (Reynolds 1931) and recent geothermal drilling operations in the Larne area have revealed igneous bodies of assumed Permian age underlying the Antrim lavas (Penn et al 1983). The lamprophyres of the Ards Swarm (Reynolds 1931) comprise a suite of mainly vogesites and minettes whose major element chemistry differs slightly from the Skye example quoted by Morrison et al (1980).…”

The origin of Upper Palaeozoic sedimentary basins in Northern Ireland and relationships with the Canadian Maritime Provinces

“…rocks recorded in Northern Ireland occurs in the Larne No 2 Borehole and consists of three main units comprising 62.5 m of unbottomed, very coarse breccio-conglomerates and sandstones at the base, overlain by 554 m of heavily altered intermediate and basic volcanic rocks, tufts and tuffaceous sediments with 440 m of purple to red-brown sandstones and pebble conglomerates at the top (Penn et al 1983). Deposition may have been from braided streams or on alluvial fans in a location proximal to the sediment source.…”

The East Irish Sea Basin — from caterpillar to butterfly, a thirty-year metamorphosis

“…The Lower Permian correlations in these wells implied the presence at the centre of the EISB of an area of shale and mudstone, equivalent to the coarser Collyhurst sandstone (Colter & Barr 1975, fig. 5A (Penn et al 1993), proving its presence in three basins. The long recognized unconformity at the base of the old Keuper Sandstone (Helsby and Ormskirk Sandstone formations of the Cheshire and East Irish Sea basins respectively) was interpreted in 1975 to be marked by an abrupt decrease in gamma-ray activity near the top of the Sherwood Sandstone in the onshore and offshore wells studied.…”