Palaeomagnetism of the Cretaceous Pirgua Subgroup (Argentina) and the age of the opening of the South Atlantic

Abstract

Palaeomagnetic data for the Cretaceous Pirgua Subgroup from 14 different time units of basalts and red beds exposed in the north-western part of Argentina (25° 45′ S 65° 50′ W) are given. After cleaning all the units show normally polarized magnetic remanence and yield a palaeomagnetic pole at 222° E 85° S (dΦ= 7°, dχ= 10°). The palaeomagnetic poles for the Pirgua Subgroup (Early to Late Cretaceous, 114–77 Myr), for the Vulcanitas Cerro Rumipalla Formation (Early Cretaceous,<118 Myr, Valencio & Vilas) and for the Pocos de Caldas Alkaline Complex (Late Cretaceous, 75 Myr, Opdyke & McDonald) form a ‘time-group’ reflecting a quasi-static interval (mean pole position, 220° E 85° S, α95= 6°) and define a westward polar wander in Early Cretaceous time for South America. Comparison of the positions of the Cretaceous palaeomagnetic poles for South America with those for Africa suggests that the separation of South America and Africa occurred in late Early Cretaceous time, after the effusion of the Serra Geral basalts. The K-Ar ages of basalts of the Pirgua Subgroup (114 ± 5; 98 ± 1 and 77 ± 1 Myr) fix points of reference for three periods of normal polarity within the Cretaceous palaeomagnetic polarity column.