Sr ages for the Chenque Formation in the Comodoro Rivadavia region (Golfo San Jorge Basin, Argentina): Stratigraphic implications

CUITIÑO, JOSÉ IGNACIO; ROBERTO ADRIÁN SCASSO; VENTURA SANTOS, ROBERTO; MANCINI, LUIS

Latin American Journal of Sedimentology and Basin Analysis

Asociación Argentina de Sedimentología

Año: 2015 vol. 22 p. 3 - 12

The age and stratigraphic subdivisions for the late Oligocene-Miocene, marinedeposits of Patagonia (Patagoniense succession) have been largely debated. Moststudies for the Comodoro Rivadavia region were focused on its biostratigraphyand stratigraphic relations with the underlying and overlying mammal-bearingterrestrial strata. We report here thirteen new 86Sr/87Sr ages obtained from wellpreservedoyster valves of the Chenque Formation in the classical Patagonienseoutcrops around the city of Comodoro Rivadavia (Golfo San Jorge Basin,Argentina). According to these ages, the deposition of the unit in the regionoccurred between 19.69 and 15.37 Ma (Burdigalian-early Langhian). A lowerinterval of normal sedimentation rate (Sequences 1 and 2) and an upper intervalof high sedimentation rate (Sequence 3) are defined according to these new agesof the Chenque Formation. Subsequently, the depositional age of the ChenqueFormation is compared with other well-dated marine successions of southernPatagonia located within the Austral Basin. This analysis suggests that the Australand Golfo San Jorge basins were flooded by the Atlantic sea at about the same time(~21-20 Ma), inundating what was before a vast continental region of southernPatagonia. A renewed phase of continentalization occurred at about 18 Ma in theAustral Basin and at about 15 Ma in the study region of the Golfo San Jorge Basin.This 3 my difference combined with the elevated sedimentation rate estimatedfor the upper part of the study interval, suggest that the Golfo San Jorge Basinmay have locally experienced relatively high subsidence rates during the earlyMiocene, allowing prolonged marine sedimentation in the Comodoro Rivadaviaarea. Consequently, the accumulation of the terrestrial deposits assigned to theSanta Cruz Formation started at about 15 Ma in the study region, later than inother localities of southern Patagonia. This results clearly indicate a remarkableinterdigitation between the Santa Cruz and Chenque formations in a west-easttransect across central Patagonia.