Isotope wallrock alteration associated with hypogene karst of the Crimean Piedmont, Ukraine

Abstract

Although it is commonly perceived that isotopic alteration of carbonate rock is suppressed at low temperatures (below ca. 80–100°C), this study demonstrates that alteration halos do develop in permeable limestones of relatively low textural maturity. The study of Paleogene carbonate bedrock constituting the walls of hypogene karst conduits of the Crimean Piedmont (Ukraine) revealed several events of water–rock interaction, each resulting in isotopic alteration of the bedrock.Early development of hypogene karst conduits shifted δ18O and δ13C toward lower values by 1–2‰ and 1–3‰, respectively. The width of the alteration zone locally exceeds several meters. The case hardening associated with cavernous weathering shifted δ18O and δ13C of the bedrock toward slightly more positive values (by up to 3‰ and 1‰, respectively). The thickness of isotopic halos ranges from a few centimeters to tens of centimeters. During late-stage hypogene karstification the δ18O values of the bedrock was shifted toward lower values (by 5–6‰), forming a thin (5–15mm) alteration halo. In most cases δ13C was also shifted toward lower values (by 6–9‰), but in one case a shift toward higher values (by ca. 2‰) was observed.Water involved in early hypogene karst and in the origin of cavernous weathering structures was enriched in 18О. Such waters are known in Crimea in association with mud volcanoes on the Kerch Peninsula. We suggest that similar deep-seated water affected the Crimean Piedmont during the Middle Miocene. Water involved in late hypogene karstification had δD values of −75 to −68‰ and δ18O values of −10 to −9‰ (fluid inclusion data), similar to present-day spring and well water in Crimea. Isotopic calculations and fluid-inclusion data indicate that the late hypogene karst processes occurred at low temperatures (<ca. 50°C). These processes ceased before 130ka, when the Crimean Piedmont was uplifted, dissected by erosion, and acquired its modern morphology.