Megaflora of the Laguna Flecha Negra locality, Chon Aike Formation (Middle-Late Jurassic), Santa Cruz Province, Argentina.

SAGASTI, A.J.; CHANNING, A.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

The Laguna Flecha Negra locality is located in the central-western region of Santa Cruz Province, Argentina. The outcrops have been assigned to the ignimbrite-dominated Chon Aike Formation based on their sedimentological characteristics and stratigraphic position. The Chon Aike Formation is part of the volcanic and volcaniclastic Bahía Laura Group, which also includes the tuff-dominated, and very fossiliferous, La Matilde Formation. The major outcrops of the Laguna Flecha Negra locality were investigated during two field trips, and a sedimentological profile was logged. This field work allowed us to identify various outcrops with distinctive fossil content and differing styles of preservation. These are: an in situ fossil forest with conifer-like stumps, a chert bed and two fossiliferous levels from which we obtained a large palaeobotanical collection. In the southwestern area of the Laguna Flecha Negra locality there are coarse sandstones with poorly preserved fossils, and massive lacustrine siltstones with an abundant, well preserved, compression-impression taphoflora. Conifers are the dominant component with two types of leafy branches, dispersed ovuliferous scales of Araucarites sp. and ovulate cones assignable to Pararaucaria sp. The genus Brachyphyllum is the most abundant of the leafy branches; it has helicoidally arranged adpressed leaves and two or three orders of shoots in organic connection are commonly observed. Branches assignable to Pagiophyllum sp. are a minor element of this taphoflora. Second in abundance are bennettitalean monopinnate leaves of Otozamites sp. that appear at the silt horizon in association with Brachyphyllum and Araucarites and often have cuticular preservation. Elatocladus leafy branches, Sphenopteris and Gleichenites fronds, and small oval-circular conifer strobili with helically arranged sporophylls are minor elements of the assemblage. The elements of this paleofloristic association suggest that the conifers were the dominant group, with an abundance of Araucariaceae and a minor component of Cheirolepidiaceae. Middle to Late Jurassic floras of the Bahía Laura Group have been mostly described from the La Matilde Formation, with only one record from the Chon Aike Formation in the Bajo Pellegrini locality that is composed only of Dictyozamites fronds. The study of the Laguna Flecha Negra locality allows an improved knowledge of the Bahía Laura Group paleofloras.