The Ediacaran / Lower Cambrian transition in Northwest Argentina: New paleontological evidence along the proto margin of Gondwana

ACEÑOLAZA, G.F., FEDONKIN, M., ACEÑOLAZA, F. AND VICKERS-RICH, P.,

Windoek, Namibia

Simposio; 2nd Symposium on Neoproterozoic-Early Paleozoic Events in southwestern Gondwana:; 2005

Geological Survey of Namibia, geological Society of Namibia

The Neoproterozoic / Early Cambrian  siliciclastic sediments of the Central Andean Basin of South American records a varied spectrum of environmental settings, where slates, sandstones, limestones and conglomerates display a complex arrangement of facies. The Puncoviscana Formation represents its most emblematic unit, with slightly metamorphosed and highly deformed slates and sandstones whose strata provides, in certain areas, distinctive fossils denoting an Early Cambrian age.             The early referred complexity of facies and the multiple-phase deformation suffered by the strata obscures interrelations that precludes a clear sight of the Neoproterozoic / Early Cambrian transition in the region. Over 30 years of systematic studies has provided a better understanding of certain aspects related to the basin, where trace fossils played an important role on the early interpretation of the “Puncoviscana sea”. On the basis of sedimentological features and the ichnological arrangement of associations, two areas have been distinguished: 1- An westwards flysch-like sector of a deeper environmental setting, referred as the “Oldhamia belt”; and 2- An eastwards shallower area affected by wave action, classically referred as the “Nereites belt”. Even though most of the data came out from the ichnological and sedimentological analysis, a noticeable lack of body fossils was recorded in the bibliography until few years ago. In this opportunity, recent field work on the eastward “Nereites belt” provided new and well preserved fossils that allow a reinterpretation of material early described as “medusoids” (e.g. “Beltanelliformis” and “Sekwia”), as the sea anemone-like gregarious organism Nemiana simplex. Originally described from the Vendian strata of Siberia, Nemiana is one of the simplest Neoproterozoic – Cambrian fossils preserved under different paleoenvironmental conditions in Siberia, Australia, Namibia and Newfoundland. Nemiana represents a sessile metazoan with a saclike body difficult to interpret (Fedonkin, 1990; 1992). Morphologically is distinguished by a round, convex shape body lacking tentacles and occasionally with a central mark that is understood as representing the mouth (Fedonkin, 1994).  It has been related to sea anemones, cnidarians and large protists (Palij, 1976; Seilacher, 1989; Fedonkin, 1992). The new material described in this contribution comes from the slates and sandstones of the Puncoviscana formation cropping out in the town of Purmamarca, in Jujuy Province (Cordillera Oriental of Northern Argentina). Abundant Nemiana were found associated in group, on surface of highly tectonized reddish and dark gray layers of sandstones and shales up to 20 cm thick. Fossils are mostly associated in-group (over 100 individuals) on upper surface of strata, with a three-dimensional development deeping up to 1.5 cm into the strata. Diameter varies from small forms (0.5 cm) to large rounded samples (2.3 cm), where larger forms usually display certain relief above the surface (0.5 mm maximum). Well-preserved material provides some distinguishable morphological characters on the surface of fossils, some preserving a central structure interpreted as mouth. The only paleontological data provided by the presence of Nemiana in the slates and sandstones at Purmamarca, do not allow a confident assignation of strata either to the Neoproterozoic or to the Early Cambrian, but gives a new remarkable element to the paleontology of an almost barren fossiliferous unit in the Andean margin of South America..  All new data on the paleontological content of the Puncoviscana Formation and related units provides important clues to better understand the Neoproterozoic / Early Cambrian transition of a little known area in the western margin of the Proto-Gondwana.