Thrombolites (microbial reefs) of the form-genus Favosamaceria in the Upper Cambrian of the Eastern Precordillera, San Juan, Argentina. In the upper part of the La Flecha Formation (Furongian, Upper Cambrian) of the eastern Precordillera, Argentina, there are abundant, metre-sized, domical patch-reefs flanked by peloidal and intraclastic grainstone. These shallow-water reefs are thrombolites and they consist of a radial-columnar microbial framework composed mainly of clotted micrite with subordinate micrite, peloids and lumps. In between the framework elements are intraclastic and bioclastic peloidal grainstones. In longitudinal section the framework appears as upward-radiating, branching and anastomosing columns which in transverse section range from roughly circular, elongate, lobate, or labyrinthiform in outline. Morphologically they conform to the form-genus Favosamaceria Shapiro and Awramik. However, despite partial dolomitization, they do not exhibit the distinct secondary microstructure diagnostic of the type form-species, F. cooperi Shapiro and Awramik. Accordingly, the thrombolites are placed in a new taxon -F. precordillerana forma-sp. nov.- which is characterized by a primary clotted and peloidal micritic microstructure.