Facies, microfacies, and diagenesis of late Callovian-early Oxfordian carbonates (La Manga Formation) in the west central Argentinean high andes

Abstract

At 32° 40′ south latitude the Andean orogen shows a typical thin-skinned fold and thrust belt that exposes the Mesozoic back-arc basin fill. The late Callovian - early Oxfordian limestones of the La Manga Formation were studied based on lithofacies, microfacies, and diagenetic processes in order to interpret the paleoenvironment depositional setting. Five lithofacies can be recognized within the La Manga Formation: (A) stromatolite mudstone, (B) massive and laminated carbonate mudstone, (C) massive wackestone, (D) coralline floatstone, and (E) oolitic-skeletal packstone-grainstone. The vertical succession reveals six closely related microfacies grouped as follows: (1) stromatolite mudstone, (2) peloidal wackestone, (3) bioclastic peloidal wackestone, (4) oolitic bioclastic packstone, (5) coral skeletal floatstone, and (6) oolitic-intraclastic grainstone. Diagenetic events, listed in sequence, include: 1) dissolution of high-Mg calcite and aragonite shells; 2) compaction; 3) pore filling; and 4) pressure-solution. Three types of cement, isopachous, granular calcite and syntaxial overgrowth, were identified. Based on CL petrography the cement sequence changes from non-luminescent to brightly luminescent, suggesting variations within the pore fluid system. Paleotemperature during deposition is estimated at 26°C according to stable isotope analyses on oyster shells. Lithofacies and microfacies data suggest that an intertidal-lower supratidal environment (lithofacies A and microfacies 1) was followed by a deepening succession (lithofacies B, C, and microfacies 2, 3), followed by a shallowing upward succession (lithofacies D, E, and microfacies 2,3), followed by a shallowing upward succession (lithofacies D, E, and microfacies 4, 5, 6). This facies assemblage is interpreted as part of a transgressive system tract and a high-stand succession. The sequence represents an upward-coarsening deposit on a shallow carbonate platform.