A Maastrichtian reef and associated limestones in the Roca Formation of Patagonia (Neuquén Province, Argentina)

KIESSLING, W; SCASSO, R.A; ABERHAN, M.; RUIZ, L; WEIDEMEYER, S

FOSSIL RECORD

WILEY-V C H VERLAG GMBH

Año: 2006 vol. 9 p. 183 - 197

1435-1943

We describe a small microbial reef and associated limestones occurring in a Maastrichtian transgressive succession of mixed carbonate-siliciclastic lithologies at Sierra Huantraico near Chos Malal (Neuque´n, Argentina). Strontium isotope data suggest that the reef is of earliest Maastrichtian age. The small reef (0.8 m thick, 2 m wide) is mostly composed of peloidal bindstone, dense stromatolite-cement crusts and thrombolite. Except for some ostracods, no metazoan fossils were found in the reef structure, although the majority of peloids are fecal pellets, probably of larger crustaceans. Small foraminifers with calcite tests and probable green algae have also been noted. Sedimentological data and fossils within and immediately above the reef suggest that the reef was formed in a transgressive systems tract under freshwater to brackish-water conditions. Limestones above the reef are serpulid-bryozoan packstones and intraclast-ooid grainstones. These limestones yield a mixture of typical non-tropical (common serpulids and bryozoans) and typical tropical aspects (common dasycladaceans and ooids). This mosaic is explained by salinity fluctuations, which in our case dominate over temperature in determining the grain associations.