The Chichinales Formation, broadly speaking, is made up of grayish
sandstones, whitish
tuffs,
siltstones and greenish
claystones with paleo soil levels. It is divided into 3 members: lower, middle and upper. The lower one is composed of grayish brown sandstones with low-angle
cross bedding, alternating with light brown tuffs, tuffaceous sandstones, sandstones with carbonate cement and
shales. In this member, the presence of "
opalized" fossil logs is common. The middle member is formed by
paleosols and sequences of siltstones, shales and fine sands that in some sectors form
lenses in the form of fluvial channels. Finally, the upper member is represented by more homogeneous levels of tuffs, whitish gray
tuffites, siltstones, green claystones and tuffaceous sandstones This member, in some sectors, is laterally interdigitated with marginal marine levels of the
Gran Bajo del Gualicho Formation, which dates from the Late Oligocene to the Early Pliocene.[3][4] The Chichinales Formation is correlated with the contemporaneous fluvial and
lacustrine tuffaceous
Collón Curá Formation of the central Neuquén Basin.[5]
The sedimentological and paleontological evidence provided by the Chichinales Formation suggests the existence of open environments with low relief, restricted wooded areas with temporary water bodies and warm temperate climate, in agreement with other Colhuehuapian localities of Chubut Province. In this context, it is possible to interpret, at least tentatively, that the differences between the set of taxa represented in the
Cerro Bandera Formation and those of the Chichinales Formation are due to contrasts between the local environmental conditions of each unit rather than to differences in temporal range. It is likely that such differences are the result of the particular conditions that prevailed in the Neuquén region during the Early Miocene, which would be a consequence of its proximity to the Andean zone rather than its latitudinal location.[6]