Stratigraphic architecture of a tide-influenced shelf-edge delta, Upper Cretaceous Dorotea Formation, Magallanes-Austral Basin, Patagonia

Abstract

The Magallanes-Austral Basin of Patagonian Chile and Argentina is a retroforeland basin associated with Late Cretaceous-Neogene uplift of the southern Andes. The Upper Cretaceous Dorotea Formation records the final phase of deposition in the Late Cretaceous foredeep, marked by southward progradation of a shelf-edge delta and slope. In the Ultima Esperanza district of Chile, laterally extensive, depositional dip-oriented exposures of the Dorotea Formation contain upper slope, delta-front and delta plain facies. Marginal and shallow marine deposits include abundant indicators of tidal activity including inclined heterolithic stratification, heterolithic to sandy tidal bundles, bidirectional palaeocurrent indicators, flaser/wavy/lenticular bedding, heterolithic tidal flat deposits and a relatively low-diversity Skolithos ichnofacies assemblage in delta plain facies. This work documents the stratigraphic architecture and evolution of the shelf-edge delta that was significantly influenced by strong tidal activity. Sediment was delivered to a large slump scar on the shelf-edge by a basin-axial fluvial system, where it was significantly reworked and redistributed by tides. A network of tidally modified mouth bars and tidal channels comprised the outermost reaches of the delta complex, which constituted the staging area and initiation point for gravity flows that dominated the slope and deeper basin. The extent of tidal influence on the Dorotea delta also has important implications for Magallanes-Austral Basin palaeogeography. Prior studies establish axial foreland palaeodrainage, long-term southward palaeotransport directions and large-scale topographic confinement within the foredeep throughout Late Cretaceous time. Abundant tidal features in Dorotea Formation strata further suggest that the Magallanes-Austral Basin was significantly embayed. This 'Magallanes embayment' was formed by an impinging fold-thrust belt to the west and a broad forebulge region to the east.