RECOGNITION OF FOSSIL NEBKHA DEPOSITS: CLUES FROM NEOICHNOLOGY AND SEDIMENTOLOGY

CARDONATTO, MARÍA CRISTINA; MELCHOR, RICARDO NÉSTOR

PALAIOS

SEPM-SOC SEDIMENTARY GEOLOGY

Lugar: Lawrence; Año: 2020 p. 1 - 39

0883-1351

This study includes the first neoichnologic characterization of the burrow systems of Tympanoctomys barrerae (Rodentia: Octodontidae) and sedimentologic features of the modern nebkhas where they occur. Tympanoctomys is a South American solitary and fossorial rodent that have ecomorphofunctional adaptations for living in saline environments and construct its burrow in nebkhas with halophyte shrubs. The purpose of this work is to identify the ichnologic signatures of T. barrerae burrow systems and to provide combined ichnologic-sedimentologic criteria for identification of Cenozoic nebkha deposits. Tympanoctomys barrerae burrow systems are subhorizontal, typically with ten or more entrances, two or three levels, closed circuits, average complexity of 48, average tortuosity of 3.25, and an average ratio of total chamber volume to total tunnel volume of 0.04. The size of the tunnels averages 85 mm in horizontal diameter and 64 mm in vertical diameter, and cross-section ranges from elliptical flattened to plane-convex with incipient bilobed floor. Surface ornamentation is typified by a coexistence of primary (sets of four claw traces forming an arcuate pattern produced during digging) and secondary (numerous arthropod burrows excavated from the burrow lumen) surface ornamentation. Nebkha deposits in upper Cenozoic sequences can be recognized by a combination of ichnologic and sedimentologic features: fossil burrows having the mentioned ichnologic features of T. barrerae burrow systems and presence of rhizoliths of shrubby plants occurring in well-sorted sandy deposits with low-angle cross-bedding. Extant rodents from other continents having similar adaptations and constructing comparable burrow systems in nebkhas extend the potential utility of these criteria.