CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT Hydrothermal karst phenomena in Danian carbonates, North Patagonia, Argentina. SERGIO MATHEOS, AILEN BORYA, DANIELA CUCCARO & NICOLAS SCIVETTI, Centro de Investigaciones Geológicas, CONICET-UNLP, La Plata, Argentina. Hydrotermal karst caves produced by the action of hot ascending waters were formed in a 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 a Tembrao Quarry, which has a total thickness of 6-7 meters. The host sedimentary succession is mainly composed by limestone (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: neomorphics development 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 limestone are laminated and characterized by tabular stratofabric, low angle sigmoidal cross bedding, and hummocky cross stratification (HCS). This sequence, based on lilology, sedimentary structures and vertical facies arrangements shows an open marine beach-nearshore and partially restricted shallow marine associations. Numerous veins of milky-white of calcite crystalline are encountered in the quarry or intercepted by others smaller caves as consequence of karts action. Caves and cavities were formed by carbonate dissolution and consisting of numerous coalesced spherical forms creating characteristics as pop-corn mamelons mainly composed by 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 typify concentric bands botryiods, with long axes of the crystals 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 a thin, dark. The mamelons resulting from the fibrous growth show a fan structure ranging from acute angles to more than 180º, sometimes attaining an almost complete spherulite. Petrographic observation shows that the botryoids had undulose extinction assigned to this radial subcrystal fabric. Closer examination reveals the presence of rhombic terminations, 24 to 96 μm across in wide and 500 to 1125 μm in large. Under cathodoluminescence, the bands show moderately bright to dull. The rohombic terminations of the botryoids crystallites suggest that the Lower Tertiary botryoids were originally calcite, as replacement calcite tipically has square-ended terminations. There are some speleothems composed by grained deposits made of random needle-fibbers of low magnesian calcite.