A multidisciplinary study of the Lower Cretaceous marine to continental transition in the northern Austral-Magallanes basin and its geodynamic significance

Sedimentologic studies together with preliminary anisotropy of magnetic susceptibility (AMS) analysis were carried out in a very particular and intriguing Lower Cretaceous depocenter in the northern Austral-Magallanes basin, in order to elucidate its geodynamic setting. Aptian-Albian regressive sequences from the coastal Río Belgrano Formation and the fluvial conglomerates of Río Tarde Formation prograded into the regionally distributed marine sequence from Río Mayer Formation. This "early" progradation of continental environments is a characteristic of the northern sector of the basin, since marine conditions prevailed up to Campanian times towards the south. The tectonic context indicates a sudden passage from a marine to a deltaic and continental river system occurred during the uplift of the Northern Patagonian Andes and extra-Andean Patagonia. Our sedimentologic observations suggest a Barremian to Aptian regression, probably driven by the generalized uplift of surrounding blocks. AMS data indicates that a tectonic overprint of the original magnetic fabric was likely the product of E-W shortening applied soon after deposition, representing one of the earliest evidences of Andean deformation. Although the sedimentary setting was broadly placed in the retro-arc, our data helps to place the specific tectonic environment of the depocenter that is still little known and open to debate.