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

<p dir="ltr">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.</p><p dir="ltr">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.</p><p dir="ltr">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.</p>