Paleosol ichnofabrics of the Pinturas Formation (Early Miocene), Patagonia, Argentina

BEDATOU, E.; BELLOSI, E.; CANTIL, L.; GENISE, J.F.; MELCHOR, R.N.; SÁNCHEZ, M. V.

Colonia

Simposio; III Simposio Latinoamericano de Icnología 2016; 2015

Universidad de La Republica

The Pinturas Formation (Early Miocene) includes a sedimentary succession exposed at the Río Pinturas valley, Santa Cruz province, Argentina. It is separated in three members. The lower and the upper ones are dominated by ash fall episodes alternated with paleosol formation; while the middle member is represented by desert eolian deposits. Paleosols of the lower member bear a moderately diverse ichnofauna including Coprinisphaera murguiai, C. ispp., Loloichnus baqueroensis, Feoichnus challa, Taenidium barretti, vertical burrows and a three-dimensional system of interconnected burrows.Four ichnofabrics (IF) were defined: the Coprinisphaera, Loloichnus, Vertical Burrows and Boxwork IF. The Coprinisphaera IF is characterized by scattered Coprinisphaera ispp., and by a Bioturbation Index (BI) = 2 with 13% of Ichnofabric (IF), 65% of Pedofabric (PF) and 22% of Original Bedding (OB). F. challa, an accessory component, may be locally abundant. The Loloichnus IF shows L. baqueroensis following random tortuous paths and intertwining usually without touching with other. Coprinisphaera ispp., Taenidium barretti and other indeterminate burrows may accompany L. baqueroensis, although showing no cross-cuttings among them. BI of this ichnofabric is 1 (4% IF, 91% PF, 5%OB). The Vertical Burrows IF is formed by abundant burrows (5-7 mm in diameter), vertically aligned, but with some oblique to sub-horizontal portions. Filling is usually finer-grained and lithologically different to hosting paleosol. The BI is 3 (33% IF, 67% PF, 0%OB). The Boxwork IF is formed by an intricate tridimensional system of interconnected burrows, with irregular diameters (4-8 mm) and orientation. Joints usually show enlargements up to 40 mm in diameter. In fine lithologies (silt to very fine sand sizes), burrows show a thin clay lining, which is missing in coarser lithologies. Filling is mostly massive but, in some cases, there are little ovoid pellets. Its BI is 3 (51% IF, 49% PF, 0% OB). All IF are present in similar facies of massive very fine tuffaceous sandstones and siltstones. Granular and platy soil structures were recorded. Relationship between IF and paleosol/facies types are under study.Vertical Burrows IF and Boxwork IF are assigned to earthworms, and the high BI indicates good conditions for producer emplacement. Such conditions are mostly wet but not flooded soils and abundant, available organic matter. Different ecological groups of modern earthworms produce different types of burrow systems; probably those IF could be representing two of these groups. The Coprinisphaera IF and Loloichnus IF, occur individually or forming a composite IF. There are no cross-cutting relationships to evaluate the time of emplacement of both. Coprinisphaera IF indicates well drained and drier soils with low water table; while the Loloichnus IF suggests wetter soils with a higher water table. Its presence suggests changes in water availability conditions, probably reflecting some climatic seasonality.