Carbonate botryoids in Lower Tertiary limestone in the northeast Patagonian Massif (Rio Negro Province, Argentina)

MATHEOS, S. D., BEILINSON, E., LOURENCO, F. J.

Belem, Brasil

Congreso; 3rd Latinamerican Congress of Sedimentology; 2003

Universidade Federal de Pará

The aim of this work is  to distinguish the original mineralogy of botryoids in paleokarst and relative diagenetic aspects of carbonate sediments from the Lower Tertiary (Danian), assigned to Roca Formation in 66º 17´ W and 41º 07´ S, Northeast Patagonian Massif (Río Negro Province, Argentina). The unit is mainly composed of limestones (wackstone, packstone and grainstone) dominated by skeletal grains  with normal marine fauna, associated with variable contents of carbonate cements, micirte, intraclastos and siliciclastioc grains. The limestones are laminated and characteised by tabular stratofabric, low-angle sigmoidal cross bedding and hummocky cross stratification. Matheos & Tunik (1998) defined in this unit the main diagenetic processes and features, as mentioned: 1- neomorphic grains; 2- fine-grained calcite crystals (LMC) partially or totally filling the pore space between detrital, larger carbonate and bioclastic grains, 3- silica replacement during later diagénesis bringing as result nodular chert; 4- first stage of dissolution with formation of interparticle porosity; 5- botryoidal structure; 6- radiaxial fibrous calcite with spherulitic growth; 7- second stage of dissolution, which generated vuggy and fenestral porosity, later infilled by calcite; 8-paleokarst features (spaleothems formed by needle-fibres of LMC and some type of globulites growing in degraded carbonate hosts), caves, vugs, and enlarged fractures; 9- partial and local dolomitization. Well-developed botryoids, generally between 2-5 mm in diameter, occur locally overlying micrite and spectacular growth-forms occurring as mamelons, mainly coalescing to form milimiter-thick fringes within cavities. Concentric sub-milimeter bands differing in inclusion content typify the botryoids. Their fabric is characterised by fans of elongated crystals that exhibit a radial.fibrous habit and show different bands (4 to 6 cm in thickness) separated by a fine dark cape. Under CL, the bands show moderately bright to dull luminescence. 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. The botryoids studied have not been observed to be bored, encrusted or interlayered with marine sediment. Under crossed polars, unulose extinction may be assigned to this radial subcrystal fabric. Closer examination reveals the presence of rhombic terminations, 24 to 96 mm wide and 500 to 1125 mm long. The rhombic terminations of the botryoids crystallites suggest that the Lower Tertiary botryoids described here were originally calcite, as replacement calcite typically has square-ended terminations or assumes a blocky distribution (Sandberg, 1985; Aissaoui, 1985). Undulose extinction is generally taken as a clear indicator of neomorphic replacement, but in this case it is due to the radial fibrous disposition, which gives the botryoids this apparent undulose characteristic. Botryoids appear to be of aragonitic origin only; somewhat similar structures in calcite do form in meteoric, spelean-vadose diagenetic environment (Tucker, 1991). Thus, adding the results of petrographical studies to the fact that the botryoids were found in paleokarstic caves, we assume that they were formed in a spelean diagenetic environment.