Niveles yesíferos de la formación Sarmiento en el Valle Inferior del Río Chubut.

Abstract

The Sarmiento Formation (middle Eocene–lower Miocene) is a continental unit with a broadly geographical distribution in Patagonia central. It is worldwide renowned for their fossil content and by recording climatic and ecological changes linked to regional and global events. In the area of the Chubut river lower valley, the Sarmiento Formation characterize by displaying a succession bearing abundant gypsum, which lacks in detailed sedimentological studies. The presence of gypsum levels in this formation is an unusual feature within their known constituents members cropping out in other parts of the Chubut Province, thus representing an enigma the paleoenvironmental significance of such successions. This study was focused on the characterization of the sedimentological framework of the gypsum-bearing successions and determining the paleoenvironmental factors and potential controls that have influenced the formation of these gypsum deposits. The mapping of the beds bearing gypsum, by the use of ASTER satellite image and spectral analysis techniques restrict to the outcrop area to vicinity of La Angostura locality. In outcrops, distinct gypsum habits and textures were identified. The sedimentological study allowed the definition of four facies associations closely linked to an inland sabkha continental system, with deposits containing gypsum, either immersed in or intercalated with volcaniclastic and clastic deposits. These associations represent the record of the following sedimentary subenvironments: dry mudflat, saline mudflat, shallow saline lake, and perennial saline lake. Additionally, spring mounds were recognized in marginal areas of the system. Variations in gypsum abundance were determined, and various modes of occurrence within the studied sequences were evaluated. The gypsum found in these successions was related to two origins: primary (bottom growth, displacive, and brecciated) and secondary (enterolithic, nodular, cavernous, intergranular replacement). In stratigraphic sense, the studied Sarmiento Formation successions, bearing abundant gypsum, possess a distinctive lithological composition, distinct from those recognized in the various defined members of the unit, that allow suggest the denomination of a new member.