The Austral (or Magallanes) Basin, is located on the southwestern end of the South American Plate and it is bordered to the south by the Scotia Plate covering an area of approximately . In the studied area, the Austral Basin underwent three main tectonic stages: (i) a rift stage; (ii) a thermal subsidence stage; and (iii) a foreland stage. The rifting stage is related to the break-up of
Gondwana,
grabens and
half-grabens were formed and filled with volcaniclastic and volcanic rocks intercalated with epiclastic sediments of the
El Quemado and
Tobífera Formations. Subsequently, the
thermal subsidence stage resulted in the deposition of the
transgressive quartzose
sandstone of the
Springhill Formation, and the black
mudstone and
marl of the
Río Mayer Formation. Towards the end of this stage, the
Piedra Clavada Formation was deposited, representing a large passive-margin delta system. The foreland stage, in response to the regional change from extensive to compressive regime, resulted in the deposition of the continental Mata Amarilla Formation. This unit is mainly composed of grey and blackish
siltstone and
claystone, alternating with whitish and yellowish-grey fine to medium grained
sandstone. Varela (2014) recognized three informal sections (lower, middle, and upper) on the bases of sedimentological and sequence stratigraphic analysis. The lower section consists of fine-grained intervals with paleosols interbedded with laminated shale and coquina, representing coastal plain and lagoon paleoenvironments. The middle section comprises
sandstone and
siltstone representing meandering
fluvial channels and
crevasse splay deposits, intercalated with fine-grained
floodplains and subordinate
lacustrine deposits. The upper section is dominated by fine-grained deposits, related to distal fluvial channels.
Paleosol features and paleosol-derived climatic proxies suggest a subtropical temperate-warm, at and humid, with /yr, climate with marked rainfall seasonality during the deposition of this unit (Varela et al. 2012b; 2018), in accordance with previous paleobotanical interpretations. == Fossil content ==