Application of fluvial to marine transition zone concepts to ancient deltaic deposits, Lajas Fm., Neuquén basin, Argentina

Abstract

The seaward end of modern rivers is marked by the fluvial to marine transition zone (FMTZ), which is characterized by the interaction of marine and fluvial processes. In the study of ancient deposits, facies are only rarely interpreted as the result of process interactions, often leading to over-emphasis of a single process and potentially misleading depositional models. Because deltas show a range of different morphologies, architectures, facies and grain size distributions that are linked to the relative strength of fluvial, tidal and wave processes, understanding process interactions in ancient deposits is crucial to improve prediction of 3D depositional architecture and reservoir connectivity.This study provides an example where the FMTZ concept has been applied to an ancient deltaic succession. The Middle Jurassic Lajas Formation has been investigated in large-scale outcrops, 40 km south of Zapala (Figure 1). These deposits accumulated in different fluvial and shallow marine settings during the back-arc, post-rift phase of the Neuquén Basin. Sedimentological logging, correlation panel construction, and architectural element and facies analysis were used to constrain the depositional features and to evaluate the degree of tidal influence during deposition.