Pleistocene? megamammal burrows in late Miocene calcareous paleosols from central Argentina

MELCHOR, RICARDO NÉSTOR; CARDONATTO, MARÍA CRISTINA; BEDATOU, EMILIO; MONTALVO, CLAUDIA

Santa Marta

Simposio; IV Simposio Latinoamericano de icnología; 2018

Universidad de Caldas

Meter-scale cylindrical structures occurring in the Cerro Azul Formation from central Argentina are herein interpreted as the burrows of megamammals. The formation is a monotonous succession of loess containing moderately developed paleosols that has been assigned to the late Miocene based on its mammal remains. The studied locality is a roadcut at national road 154 in the southeast of La Pampa province (37º 49? 28.5? S, 64º 4? 8.9? W). The local section is about 2 m thick and includes massive reddish-brown siltstone and fine-grained sandstone with two closely stacked paleosol profiles and a capping calcrete with a nodular to platy texture. At least four burrow fills are easily distinguished from the host rock by its laminated structure. Burrow fills appear in transverse or longitudinal (7.7 m long) sections. Cross-sections are subcircular to elliptical with two sizes: the smaller is 1.2 m wide by 0.89 m high, whereas the larger are 2.25 m wide and 1.8 to 2.25 m high. The burrow fills display a finning-upward tendency with a lower interval composed of massive siltstone to fine-grained sandstone and an upper laminated claystone and siltstone interval. In one of the burrow fills, a decimeter-scale plano-convex body of matrix-supported mudstone breccia was identified. Carbonate rhizoliths ranging in diameter from 1 to 45 mm and up to 0.5 m long are abundant in the burrow fills and oriented parallel to the burrow axis. Cylindrical to elliptical carbonate bodies (about 50 mm in diameter and up to 120 mm long) found in the bottom of one burrow fill are interpreted as possible coprolites. This origin is suggested by the external morphology, including overall cylindrical shape, tapering at one end, a flattened side, presence of rounded lumps, and shrinkage cracks. In thin section, they are composed by micrite and siliciclastic grains with no organic inclusions and no calcium phosphate was identified by XRD. In consequence, its organic origin cannot be confirmed. The burrow fills are herein considered to postdate significantly the deposition of the Cerro Azul Formation, probably of Pleistocene age. The reasons for this inference are: 1) burrow diameters are comparable with burrows attributed to the Pleistocene megafauna; 2) the burrow cut the carbonate nodules of the capping calcrete, and 3) regional evidence suggest that the calcretization occurred on sedimentary sequences ranging in age from late Miocene to Pleistocene. After abandonment, the burrows received sediment passively after heavy (massive interval) or light rains (laminated interval). The mound of brecciated mudstone suggest that some roof collapse occurred. Considering the horizontal diameter, cross sectional shape and Pleistocene fossil mammals recorded for the western Pampean region the most likely candidates are the Glyptodontidae (Glyptodontinae, Doedicurinae and Hoplophorinae) and Mylodontidae (Mylodontinae and Scelidotherinae).