Palaeogeographical and eodiagenetic settings of host‐replacing phreatic calcrete intervals developed in mud deposits of the Famennian Kinnesswood Formation in the Pennyseorach Subbasin of south‐west Scotland

Abstract

ABSTRACTIn the Pennyseorach Subbasin of south‐west Scotland, the Famennian Kinnesswood Formation of the Inverclyde Group includes several, ca 1 to 3 m thick erosional remnants of host‐replacing phreatic calcrete hardpans developed in mud deposits. The latter are most unusual hosts for such calcretes due to their relative impermeability, which would normally prevent them from sufficiently accommodating the circulation of an aquifer. In previously documented phreatic calcretes, mud deposits clearly acted as aquicludes that sharply constrained calcrete development. In the Kinnesswood Formation at Pennyseorach, tall desiccation fissures allowed groundwater to circulate in thick, semi‐consolidated mud deposits. Because of the inferred development of an adjacent evaporitic basin, the mixing of fresh and evaporitic groundwaters raised the pH enough for the replacement of phyllosilicates by calcite to occur along the fissure walls, forming ‘fissure calcretes’. The latter gradually expanded and eventually coalesced into mature, ‘columnar host‐replacing phreatic calcrete hardpans’ in which 90 to 100% of the muddy host material was replaced by calcrete. At Pennyseorach, host‐replacing phreatic calcrete hardpan formation not only affected the transition zone between the Doughend Sandstone and Foul Port Members of the Kinnesswood Formation (the Dunagoil Calcrete interval), as in other localities of south‐west Scotland, but also affected the uppermost part of the formation, which is marked by several erosion surfaces downcutting into columnar host‐replacing phreatic calcrete hardpan intervals in association with the episodic rise and fall of base‐level. These newly recorded host‐replacing phreatic calcrete hardpan intervals from the upper part of the Kinnesswood Formation have a tightly constrained stable isotopic signature that is distinct from that of both the Dunagoil Calcrete and the successive host‐replacing phreatic calcrete hardpans of the Visean Clyde Sandstone Formation within the same group, underlining their potential as stratigraphic markers and palaeoenvironmental indicators.