A speleogenetic history of Novoafonskaya Cave in the Western Caucasus

Abstract

Speleogenesis in hypogene karst settings may be closely tied to regional tectonic dynamics and concomitant hydrochemical evolution of karst waters. However, placing temporal constraints on these processes can require a wider array of field observations and techniques than for typical karst systems. Herein, we present a comprehensive study of Novoafonskaya Cave (Western Caucasus, Abkhazia). The updated speleogenetic history of the cave comprises four stages: (1) the most ancient, a low-T hydrothermal (ca. 40–50°C) priming stage; (2) the main stage enabled by mixing of upwelling thermomineral and locally recharged common karst waters; (3) the late sulfuric-acid speleogenesis (SAS) stage, which left significant mineralogical overprint (gypsum and a suite of minerals resulting from the alteration of silicate sediments) but did not alter the morphology of the cave appreciably; and (4) the contemporary stage, occurring predominantly in the phreatic and epiphreatic zones. Ages of speleogenetic stages were constrained by radiometric dating (230Th-U and 40Ar/39Ar) and paleomagnetic data, alongside the Quaternary geological history of the region and paleodynamics of the Black Sea level. Conditions for the low-T hydrothermal karstification occurred from Miocene – Middle Pliocene. The main stage, which created the main volumes of the cave, took place from Late Pliocene – Middle Pleistocene. The cave emerged from the phreatic into the vadose zone after ca. 400 ka ago, due to the combined effect of the Black Sea regression and intensification of tectonic uplift in the Caucasus. The presence of sulfidic waters during dewatering led to the development of transient SAS processes during Middle to Late Pleistocene. Dating of calcite underlying SAS-associated overgrowths constrains the cessation of this SAS activity to within the last 147 ka. In its present state, the cave has no connection with thermomineral waters; however, such waters are found deeper in the southern parts of the karst massif (based on the hydrochemistry of Psyrtskha spring), implying that hypogene karstification, primarily driven by mixing corrosion, may still be active deep in the phreatic zone.