Abstract

ZARATE, Marcelo A et al. Estructuras biogénicas en el Cenozoico tardío de Mar del Plata (Argentina) atribuibles a grandes mamíferos. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.2, pp.95-103. ISSN 1853-6360.

The occurrence of  mammals caves and burrows is a notable feature of the Late Cenozoic deposits of the Mar del Plata area (Fig. 1). Although these structures have been known for more than a hundred years, few studies have been devoted to their description and interpretation. This paper is focused on the morphological characteristics, the stratigraphic setting and the field criteria of recognition of the biogenic structures attributed to big mammals. We discuss the associated fauna and their probable builders and evaluate their importance as stratigraphic and sedimentological indicators. The structures possess discrete boundaries and subcircular to slightly elliptical cross-sections. Longitudinal sections are also very frequent, but not usually reported, possibly because they have been misinterpreted as features of  fluvial deposition.  Their maximum width (horizontal axis)  which usually exceeds the height slightly, varies between 0.75 m and 2 m (Fig. 2). Some structures show flat floors and arched roofs (Fig. 3f). Although few structures have been measured (n=42) they tend to fall into two sizes ranges. The maximum width of most ranges from 80 cm to 130 cm whereas many of the remaining structures are approximately 180 cm in width (Fig. 2). The major size variation might reflect different species of diggers; size variations between 15-30 cm are possibly caused by measurement of width along a plane oblique to the longitudinal axis of the structure (due to difficulties in recognizing the true axis); those between 5 cm and 10 cm are attributable to measurement error caused by boundaries that are difficult to distinguish. Recently, several tunnels, reaching up to 1.8 m in width and 40 m in length and devoid of sedimentary filling, have been discovered in the Mar del Plata area. The tunnels are interpreted as animal  burrows. The studied biogenic structures were formed in very fine sandstones or siltstones (loess-like sediments) and are located either in floodplain or interfluvial paleoenvironments. The tunnels are filled with thinly stratified fine sands and silts alternating with laminated clay layers. Scutes of Dasypodidae (Zárate, 1989) and disarticulated remains of rodents have occasionally been recovered from the sedimentary filling.  Temporally, the structures are found from the Pliocene to the late Pleistocene, and possibly extended into Late Glacial Maximum-Late Glacial times. The main criteria for field recognition of these structures are their stratigraphic relationships, sizes and closed boundaries. Sedimentary fillings and associated faunal remains are further criteria that may  be applied once the contextual framework has been established based on the main criteria. Frenguelli (1921) attributed these structures to Glyptodontidae (Glyptodon, Doedicurus, Neuryurus). More recently, Imbellone & Teruggi (1988), Imbellone et al. (1990) and Quintana (1992) attributed them to large armadillos such as Pampatherium (Pampatheriidae), Eutatus (Dasypodidae) and Propraopus (Dasypodidae). Anatomical, allometric, and biomechanical analysis recently performed on Pleistocene milodontids (Scelidotherium, Glossotherium, Lestodon)  indicate that their limbs were well prepared for digging (Bargo et al., submitted). Also, the diameters of the structures coincide with or slightly exceed the inferred diameters of  Scelidotherium and Glossotherium, and supports the interpretation that the structures were excavated by these mylodontids. Furthermore, the scratchs found on the walls and roof of a burrow devoid of sedimentary filling in Mar del Plata and originally attributed to large armadillos by Quintana (1992), are better explained as having been formed by the two more developed unguals digits of Scelidotherium and Glossotherium. The paleoenvironmental implications of these biogenic structures remain limited, as the ecological reasons for their construction have not been fully  elucidated. The structures may be useful in the interpretation of the diagenetic history of the deposits. The burrows and caves may have been at least 5-6 m deep based on the shallowest empty burrows found in Mar del Plata, and thus traverse different stratigraphic and age units. This transgression of litho- and chronostratigraphic units renders problematic the determination of their time of origin and relative abundance at any stratigraphic level. Based on the size and abundance of the structures, we infer that the Pleistocene fossorial fauna of large mammals must have played a significant role as potential agents of erosion, generating substantial volume of sediments.

Keywords : Argentina; Buenos Aires; Late Cenozoic; Biogenic structures; Big mammals.

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