Hydrothermal karst phenomena in Danian carbonates, North Patagonia, Argentina

Abstract

Hydrotermal karst caves produced by the action of hot ascending waters were formed in marine carbonate sediments from the Lower Tertiary (Danian) in the Northeast of Nordpatagonian Massif (Río Negro Province, Argentina). The area profoundly altered of this sequence appears to be the top of the Danian carbonate sequence (Roca Formation) in the Tembrao Quarry, which has a total thickness of 6-7 meters. The host sedimentary succession is mainly composed of limestones (wackestone and packstone) dominated by skeletal grains (bivalves, ostracods, foraminifers, echinoderms, bryozoans and gastropods), associated with variable contents of carbonate cements, micrite and siliciclastics grains. The features of the different types of cements are: neomorphic developments into the grains during marine cementation; fine-grained calcite crystals (Iow-Mg ferroan composition) partially or totally filling the pore space; botryoidal aragonite and microdolomite rhombs euhedral to subhedral fine crystalline texture. The limestones are laminated and characterized by tabular stratofabric, low- angle sigmoidal cross-bedding, and hummocky cross-stratification (HCS). This sequence, based on lithology, sedimentary structures and vertical facies arrangements shows an open marine beach-nearshore and partially restricted shallow marine associations. Numerous veins of milky-white, cristaline calcite are encountered in the quarry or intercepted by other smaller caves as consequence of karts action. There are numerous coalescing spherical shapes (“pop-corn”) forming swellings mainly composed of botryoidal aragonite. The botryoids are well-developed generally between 2-5 cm in diameter, locally with overlying micrite and spectacular growth-form occurring as coalescent mamelons within cavities. Millimeter concentric bands are developed of botryoids whose long axes of the crystals are oriented perpendicular to the walls of the cavities. Their fabrics are characterized by elongated crystals which have a radial-fibrous habit and have different bands (4-6 cm in thickness) separated by thin dark bands. The mamelons resulting from the fibrous growths show a fan structure ranging from acute angles to more than 180º, sometimes attaining an almost complete spherulite. Petrographic observation shows that the botryoids have undulose extinction assigned to this radial subcrystal fabric. Closer examination reveals the presence of rhombic terminations, 24 to 96 μm across and 500 to 1125 μm in large. Under cathodoluminescence, the bands show moderately bright to dull. The rhombic terminations of the botryoids crystallites suggest that the Lower Tertiary botryoids were originally calcite, as replacement calcite tipically has square-ended termination.