The Balcarce Formation: stratigraphy and ichnology in a Gondwanian framework

PAZOS P.J.; GUTIERREZ C.; FERNÁNDEZ D. E.; SOLDATI, D.; COMERIO M.

Congreso; XX Congreso Geológico Argentino; 2017

1.- The Balcarce Formation is characterized by quarzites and undisputable tidal features.2.- Some ichnogenera like Arthrophycus, Cruziana and Herradurichnus remain valid.3.- At the Cabo Corrientes locality tidal bars and tidal channels migrating to the W with lateral accretion to the N and S suggest a clear supply from the E-NE.4.- The paleocurrent pattern conflicts with a supply from the Precordillera or Pampean Ranges as traditionally suggested.5.- The ichnological content favours an early Silurian age and post Hirnantian Glaciation.The Balcarce Formation has been subject of different correlation proposals and stratigraphic position. In a local frame the correlation with the Napostá Formation in the Ventana System is accepted. However, the relationship with the glacial diamictites of the El Volcán Formation remains debatable but the postglacial (Hirnantian) age of the unit is undisputable. A more precise correlation with other units of western Gondwana is suggested in this contribution. Firstly, quartzitic composition, tidal features and ichnological content are astonishingly similar to the Silurian part of the Port Stephens Formation in particular the Albemarle and Mount Alice (lower section) members in the Malvinas/Falkland islands. However, the polarity of the paleocurrent is opposite.The correlation with the Karoo Supergroup in Africa is straightforward in composition, paleocurrents and ichnological content. It is similar to the Skurweberg Formation of the Nardouw Group. This unit overlies the Hirnantian Glaciation recorded in the Cedarberg Formation. Secondly, the presence of K-feldspar in the Ventana Group and Devonian member (South Harbour Mb) in the Malvinas/Falkland Islands contrasts with the Balcarce Formation. All these evidences suggest a climatic change between Silurian and Devonian, and based on the absence of late Silurian ages a nondeposition is regionally suggested favouring a early Silurian age for the correlatable units.