Supergene sulphuric acid speleogenesis and the origin of hypogene caves: evidence from the Northern Pennines, UK

Abstract

The maze caves of the Northern Pennines are rectilinear joint‐controlled networks of predominantly tall vertical rifts developed on one level towards the top of a ~20 m thick limestone bed; they were all intersected by underground mines and have no relationship to the present landscape. Passage walls commonly have large, non‐directional scallops; speleothems are uncommon. The caves were previously identified as hypogene in origin, i.e. formed by groundwater ascending from depth, but reassessment of their origin using published data shows that they lack diagnostic hypogene features (rising wall channels, ceiling channels, ceiling cupolas and dome‐pits), and the low permeability strata above and below the limestone bed greatly restrict vertical groundwater flow through the caves. Instead the maze caves were dissolved by the sulphuric acid released by oxidation of iron sulphides (and perhaps chalcopyrite) in the mineralized veins adjacent to all these caves; passage sizes decrease away from the veins and gypsum encrusts the walls of one cave. The maze caves were not formed by vertical groundwater flow, and dissolution was focussed in a relatively small area of limestone beneath an impermeable confining layer. The caves began to form when river incision due to the probably Late Cenozoic uplift of northern England exposed the iron sulphides to weathering and oxidation. The process that formed the maze caves is here termed supergene sulphuric acid speleogenesis, because generation of the acidity was due to near‐surface supergene sulphide oxidation, and differentiated from hypogene sulphuric acid speleogenesis, where the source is at depth beneath the cave. To clarify usage of the term hypogene, it should be restricted to Palmer's geochemical definition (Speleogenesis: Evolution of Karst Aquifers, eds Klimchouk et al., National Speleological Society: Huntsville, AL, 2000; 77–90): dissolution by a deep‐seated source of acidity. Caves dissolved by ascending groundwater containing carbonic acid with a near‐surface origin, e.g. on the rising limb of a phreatic loop, are better identified as epigene. © 2020 John Wiley & Sons Ltd

Discussion on: Supergene sulphuric acid speleogenesis and the origin of hypogene caves: Evidence from the Northern Pennines, UK in Earth Surface Processes and Landforms, 46, 455–464 (2021)

“…The evidence for these suggestions is limited. Webb (2021, p. 461) notes ‘the deposits of limonite adjacent to several of the maze caves confirm … that oxidation of iron‐containing minerals … was the key factor in maze cave formation’. However only limited references for this statement are noted, namely that ‘limonite is present “in great quantity”’ near Silverband Caverns and at Knock Fell Caverns and as ‘platy goethite’ at Faggergill.…”

“…The author of Webb (2021) puts his finger on the nub of the 'hypogenic or epigenic?' problem when he refers to the slightly differing definitions of hypogenic speleogenesis promoted by Palmer and by Klimchouk, the former offering a geochemical definition and the latter a hydrogeological one.…”

“…In Webb (2021) the earlier hypothesis (Dale et al, 2015; Harrison, 2016) that the caves were formed by hypogenic activity is disputed using three arguments: (a) the apparent lack of some of the features attributed to hypogenic speleogenesis by Klimchouk (2007) and known as the morphological suite of rising flow (MSRF), (b) the fact that several wall features in the caves are consistent with formation by lateral water rather than vertical rising water, and (c) the geological setting with impermeable beds of shale and limestone underlying and overlying the Great Limestone Member which prevents upward moving groundwater.…”

“…The author of Webb (2021) puts his finger on the nub of the ‘hypogenic or epigenic?’ problem when he refers to the slightly differing definitions of hypogenic speleogenesis promoted by Palmer and by Klimchouk, the former offering a geochemical definition and the latter a hydrogeological one. Palmer's definition relies on the source of aggressiveness, stating that hypogenic caves ‘owe their origin to processes below the surface’ (Palmer, 2007, p. 209) but he notes that in the transverse hypogenic cave origin concept ‘meteoric groundwater following deep paths is assumed to be sufficient’ (Palmer, 2011, p. 10).…”

“…The second area of debate highlighted in Webb (2021) is the extent of the involvement of iron sulphides in the speleogenesis of these caves. It was previously hypothesized (Dale et al, 2015) that galena – and to a lesser extent, its zinc analogue, sphalerite – is converted in the presence of carbonic acid to cerussite so releasing sulphide anions subsequently oxidized to create acidity and sulphate ions that react with and dissolve calcite to form gypsum.…”