Paleosol Ichnofabrics of the Pinturas Formation (Middle Miocene), Patagonia, Argentina

EMILIO BEDATOU; EDUARDO SERGIO BELLOSI; LILIANA CANTIL; JORGE FERNANDO GENISE; RICARDO NESTOR MELCHOR; MARÍA VICTORIA SÁNCHEZ

Colonia del Sacramento

Simposio; Tercer Simposio Latinoamericano de Icnología (SLIC2015); 2015

Facultad de Ciencias, Universidad de la República

The Pinturas Formation includes a continental Neogene sedimentary succession exposed at Río Pinturas valley, NW Santa Cruz province, Argentina. According to its mammal fauna it is considered as middle Miocene in age. The unit is, mostly, aeolian in origin but with successive calm periods, where numerous palaeosols were developed. These palaeosols bears a moderately diverse, but abundant, ichnofauna. Discrete trace fossils presents are Coprinisphaera ispp., Loloichnus baqueroensis, Feoichnus challa and, more rarely, Taenidium barretti. In addition, there are two unclassified ichnofossils named here Vertical Burrows and Boxwork.Four ichnofabrics (IF) were defined: the Coprinisphaera, Loloichnus, Vertical Burrows and Boxwork IFs. The Coprinisphaera IF characterizes by scattered Coprinisphaera ispp., being most of times its only component, and by a low percentage of bioturbation (max. 13%). F. challa is an accessory component, which may be locally abundant. The Loloichnus IF shows L. baqueroensis without any regular pattern or disposition. Burrows follow random and tortuous paths, intertwining among them; usually without touching, or just contacting, with each other. In some cases, they crosscut to each other. L. baqueroensis shows, in soil tops, transversal sections suggesting a vertical access conduct. Coprinisphaera ispp., Beaconites coronus and other indeterminate burrows may accompany to L. baqueroensis, but always without cut relationships between them. The representation of this ichnofabric ranges from 3% to 6%. The Vertical Burrows IF is formed by abundant unlined and passively filled burrows, dominantly vertically aligned, but with some oblique to near-horizontal portions. Rarely, they may show branching or active filling. Filling is usually finer and lithologically different to paleosols material. This IF reach up to 33% of the bed. The Boxwork IF is formed by an intricate tridimensional system of interconnected burrows, very irregular in diameters and orientations, forming a boxwork. Joints usually shows enlargements. In finer lithologies, burrows show a thin clay lining, but it is not present in coarser lithologies. Filling is passive, quite loose, and easily erodes. In some cases, in the filling there are many little ovoid pellets. This IF ranges from 45% to 56% of bioturbation. Vertical Burrows IF and Boxwork IF with moderate to high values of bioturbation indicates good conditions for the emplacement of earthworms, its producers. Such conditions are wet but not flooded soils with abundant organic matter available. The Coprinisphaera IF and the Loloichnus IF, in some cases occurring in the same bed, are not coeval. They indicates different environmental conditions, Coprinisphaera IF indicates well drained and dry soils with low water table; while the Loloichnus IF suggests wetter soils and a water table closer to the surface. Its presence in the same bed suggests environmental changes, especially in water availability conditions.