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

DI PASQUO MERCEDES; MILANA JUAN PABLO

SEDIMENTARY GEOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2023

0037-0738

A physical stratigraphic study of the Loma de los Piojos Formation (LPF) and related units at thehomonymous locality (5 km north of Jachal city, San Juan Province, Argentina) was carried out. A firstpalynologic result is presented herein from the shale beds of the Loma de los Piojos Formation, just belowthe deposits bearing floral remains of theFrenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpuscesarii(FNC) informal zone of Balseiro et al. (Serpukhovian sensulato). The Loma de los Piojos does not havean exposed base, and the Guandacol Formation (late Serpukhovian-early Pennsylvanian) overlies it. Also, thefield work analysis allowed the association of LPF shales with the last depositional sequence of the recentlydefined and underlying Don Buenaventura Formation, of late Mississippian-earliest Pennsylvanian age. Thefour 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 % inthe 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 theRaistrickia densa-Convolutispora muriornataPalynozone (DMZ). However, in the lowest sample the key sporeSpelaeotriletes ybertiiand a well-preserved specimen of the striate bisaccate pollenIllinites unicusallowedthe correlation with the late Bashkirian-Moscovian Subzone B of DMZ. Therefore, the informal floral zoneFrenguellia eximia–Nothorhacopteris kellaybelenensis–Cordaicarpus cesariiis here reallocated to theNothorhacopteris–Botrychiopsis – Ginkgophyllum(NBG) Zone, which is relevant in the global context of theevolution of the plant groups involved. This dark shale section would correspond to a warmer intervalduring which the maximum flooding zone was dated c. 320 Ma locally at Huaco section (also close to Jachalcity), also documented across the Paganzo Basin up to Paraná Basin in Brazil. The three Late Paleozoicmentioned units were deposited within the largest incised valley complex of this region both in the amountof preserved depositional sequences (7), thickness (>1 km), and time span (from Late Mississippian to lowerPennsylvanian). This complex stratigraphic succession formed within a large paleovalley probably acted as atrunk drainage system connecting depositional areas from the continental interior with coastal areas of theGondwanan continental western margin