Hierarchy of bounding surfaces in aeolian sandstones of the Tordillo Formation (Jurassic). Neuquén Basin, Argentina

Abstract

The Tordillo Formation is a continental clastic unit deposited in the Neuquén Basin during the Late Jurassic. This paper discusses the stratigraphy of the succession outcropping at the Quebrada del Sapo, with emphasis on the origin, dimensions and hierarchy of bounding surfaces of aeolian deposits. Field survey, supported by the measurement of three detailed stratigraphic sections and line drawings of photographic panels allow the identi- fication of four unconformity bounded units within the succession, informally named as T1, T2, T3 and T4. Units T1 and T3 are composed of conglomerates and pebbly sandstones deposited by density flows in a lacus- trine environment. Paleocurrents indicate a source area located in the northeast while the presence of angular sandstone blocks suggests resedimentation processes. T2 and T4 units are composed of fine to medium grained sandstones of aeolian origin, characterized by large scale dunes and minor dry interdunes. Both units have sharp bases, and overlie a deflation surface characterized by the presence of ventifacts. Paleocurrents suggest a pale- owind direction from the southwest. Internal bounding surfaces show a hierarchy of at least four discrete sur- faces which were numbered according to their crescent extension. Type 1 surfaces are related to the normal advance of the dune front. Type 2 are reactivation surfaces within a single dune set. Type 3 surfaces relate to set superposition. Type 4 surfaces are related to extensive deflation of the dune complex, and define at least nine elementary aeolian sequences in the T4 unit.