Cumulative palaeosols: A particular geological conditions for "accumulation" of sauropods eggs (Upper Cretaceous, Tama, La Rioja)

GIORGIO BASILICI; LUCAS FIORELLI; MARTÍN HECHENLEITNER; GERALD GRELLET-TINNER

Puerto Madryn

Congreso; XIV Reunión Argentina de Sedimentología; 2014

Asociación Argentina de Sedimentología

The Los Llanos Formation is a stratigraphic unit that crops out in La Rioja Province. Whilst the age of this formation is still debated, fossil remains of sauropods (bones and eggs) and ostracods justify a Cretaceous age. In this communication, the palaeoenvironmental aspects that allowed the "accumulation" (burial) of sauropod eggs in geological record will be discussed. In other words, we will consider the aspects of construction, accumulation, and preservation of the eggs, considering them as "clasts". At the discovery site of the eggs (Tama, La Rioja), Los Llanos Formation is 160 m thick. It consists of poorly sorted, on average fine- to coarse-grained sandstone, and few beds of sandy conglomerate. The sandstone beds represent a succession of paleosol profiles, whereas the sandy conglomerate beds appear to have been yielded by no-channelized subaquatic flows. The lack of clastic fraction less than coarse silt coupled with abundant rounded and frosted grains of medium sand and rare ventifact pebbles, suggest that the parent material was transported and deposited by wind. The palaeosols succession is characterised by a prevalent pale reddish orange colour, infrequent Bk horizons, and rare silica pseudomorphs after gypsum that suggests oxidising and arid palaeoenvironmental conditions. However, light gray horizons, ubiquitous root traces, and platy structures, separated by thin sparitic laminae, indicate localised stagnant-water in the soil profile and root activity. Thus, the palaeosols suggest semiarid conditions, with enough precipitation to sustain an adequate vegetation cover. The paleosol profiles are immature because pedogenic features, which form in time greater than 1 ka, are uncommon, and the Bk horizons are limited to isolated nodules. Moreover, the horizons show an exaggerated thickness, on average more than 1 m. These aspects suggest that the palaeosols were formed in environments subjected to high sedimentation rate by wind action. The pedogenesis kept pace with the sedimentation, and the deposits as soon as formed were pedogenised. However, the soils did not acquire well-developed structures because the continuous input of sediments increased the topographic surfaces and continuously rejuvenated the soils. The palaeosol profiles formed in this way are called cumulative. In conclusion, (1) the "construction" (ovideposition) of the sauropod eggs was promoted by a vegetated palaeoenvironment with shallow groundwater level, which was adequate for sauropods nesting; (2) the "accumulation" (burial) of the eggs was facilitated by the high rate of sedimentation; (3) the "preservation" is due to the space accommodation creation, which origin is in debate.