New findings of invertebrate trace fossils in a Late Miocene loess-paleosol succession, Cerro Azul Formation, La Pampa, Argentina

RENATA SOSTILLO; MARÍA CRISTINA CARDONATTO; CLAUDIA MONTALVO; GRACIELA VISCONTI; RICARDO NESTOR MELCHOR

Santa Rosa

Simposio; Segundo Simposio Latinoamericano de Icnología (SLIC2013); 2013

INCITAP - Facultad de Ciencias Exactas y Naturales (UNLPam)

The Cerro Azul Formation is a monotonous succession of reddish-brown massive siltstones and fine-grained sandstones interpreted as loess deposits with development of calcareous paleosols (mostly comparable with mollisols). Sedimentological and paleontological evidence suggest deposition under a semiarid temperate and seasonal climate and savanna-like vegetation. In this contribution new findings of invertebrate trace fossils belonging to eight localities of the unit in La Pampa province are described. These localities are distributed in an area with a maximum latitudinal range of 200 km and more than 300 km in the E-W direction. The new material can be assigned to Coprinisphaera, Rebuffoichnus, Celliforma, Rosellichnus and Taenidium. The description of the trace fossils from each locality is arranged according to the dominant ichnofossils. Coprinisphaera ispp. are only reported for Salinas Grandes de Hidalgo (SG; 37º 12? 55?S, 63º 35? 25?W) and Telén (TE; 36º 15? 15?S, 65º 30?48?W) localities. Coprinisphaera isp. are spherical or hemispherical trace fossils with constructed walls with an average diameter of 29 mm and occasional occurrence of an emergence hole. Specimens of Taenidium barretti with arcuate menisci were also recorded from SG. Previous records of ichnofossils from these localities include Attaichnus kuenzelii; Coprinisphaera isp., Celliforma isp., vertebrate burrows and invertebrate meniscate burrows. The material from the remaining localities is assigned to Rebuffoichnus, Celliforma and Rosellichnus. Specimens of Rebuffoichnus are recorded from Estancia El Recado (ER; 35º 41? 37??S, 64º 40? 08??W), Las Torrecitas (LT; 36º 24? 51?S, 67º 14? 37?W), Estancia La Malvina (LM; 36º 37? 04? S, 64º 19? 52?W), Punta de la Barda (PB; 37º 09? 02?S, 67º 14? 52?W), and Bajo Giuliani (BG; 36º 42? 55?S, 64º 17? 55?W). Rebuffoichnus cf. sciuttoi (LT and LM) is an ovoid structure (mean diameter 18 mm) with a slightly oval cross section, a broken end and a surface texture in the form of fine ridges or rounded mounds. R. cf. casamiquelai and Rebuffoichnus isp. (PB, ER, BG) are 20-36 mm in diameter empty chambers with a thick wall showing millimetre-sized holes and an internal smooth ovoid cavity. Celliforma germanica (ER) is a small cell (17 mm long, 7 mm wide) with a marked neck. Celliforma isp. was recorded from El Guanaco (EG; 36º 18? 32?S, 64º 16? 43?W) and Road 14 (R14; 36º 42? 50?S, 64º 26? 52?W) and consist of isolated cell casts (11 mm wide) with a rounded end and the other broken. Rosellichnus isp. (from EG) is a cluster containing at least 5 and probably 7 cells (about 32 mm long and 10 mm wide) arranged in a single layer and three rows with the ends of cells reaching the same level and showing a very thin wall. In addition, a 22 mm wide burrow fill with a pseudomeniscate structure and a surface texture consisting of small rounded mounds and millimetre root traces was recovered from EG. The trace fossil association from SG and TE is compared with the Coprinisphaera ichnofacies, typical of paleosols associated with herbaceous communities. Although scarce, the dominant trace fossil in the remaining localities are Celliforma and Rebuffoichnus in calcareous paleosols. The later are compared with the Celliforma ichnofacies, which is indicative of a drier climate and lower vegetation coverage than those represented by the Coprinisphaera ichnofacies, particularly scrubs and woodlands. The occurrence of both trace fossil associations probably reflects geographic and/or stratigraphic differences between the analyzed localities.