New age, stratigraphic and paleoenvironmental interpretation of the Loma de los Piojos Formation (San Juan Province, Argentina) based on new palynologic and stratigraphic information

Abstract

A physical stratigraphic study of the Loma de los Piojos Formation (LPF) and related units at the homonymous locality (5 km north of Jachal city, San Juan Province, Argentina) was carried out. The first palynologic result is presented herein from the shale beds of the Loma de los Piojos Formation, just below the deposits bearing floral remains of the Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii (FNC) informal zone of Balseiro et al. (Serpukhovian sensu lato). The Loma de los Piojos does not have an exposed base, and the Guandacol Formation (late Serpukhovian–early Pennsylvanian) overlies it. Also, the field work analysis allowed the association of LPF shales with the last depositional sequence of the recently defined and underlying Don Buenaventura Formation, of late Mississippian–earliest Pennsylvanian age. The four fertile samples obtained from the shale beds of the LPF yielded 45 species of terrestrial plants. Monosaccate pollen grains (Cannanoropollis janakii, Cannanoropollis mehtae, Circumplicatipollis plicatus, Crucisaccites monoletus) represent the Cordaitalean and Coniferalean that varied in their frequency c. 25 % in the lower two samples to 34 % and 7 % in the other two. Spore species of lycophytes dominate in all samples (ca. 50–80 %) along with other spores that appeared in the late Serpukhovian–Bashkirian subzone A of the Raistrickia densa–Convolutispora muriornata Palynozone (DMZ). However, in the lowest sample the key spore Spelaeotriletes ybertii and a well-preserved specimen of the striate bisaccate pollen Illinites unicus allowed the correlation with the late Bashkirian–Moscovian Subzone B of DMZ. Therefore, the informal floral zone Frenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesarii is here reallocated to the Nothorhacopteris–Botrychiopsis–Ginkgophyllum (NBG) Zone, which is relevant in the global context of the evolution of the plant groups involved. This dark shale section would correspond to a warmer interval during which the maximum flooding zone was dated c. 320 Ma locally at the Huaco section (also close to Jachal city), also documented across the Paganzo Basin up to the Paraná Basin in Brazil. The three Late Paleozoic mentioned units were deposited within the largest incised valley complex of this region based on the amount of preserved depositional sequences (7), thickness (>1 km), and time span (from Late Mississippian to lower Pennsylvanian). This complex stratigraphic succession formed within a large paleovalley probably acted as a trunk drainage system connecting depositional areas from the continental interior with coastal areas of the Gondwanan continental western margin.