Neogene marine strata crop out in the present Coastal Cordillera and Longitudinal Depression of south-central Chile between 38°30′S and 41°45′S, indicating the onset of a major marine transgression that covered most of the forearc in this area. In order to determine the sedimentary environment, paleobathymetry, and age of these deposits, we carried out integrated sedimentologic, ichnologic, and micropaleontologic studies on samples from oil wells and outcrops in the region. Our results indicate that these successions were deposited at lower-bathyal depths (>2000 m) during the middle to late Miocene. Shallow-marine deposition followed in the southwestern part of the study area during the Pliocene(?). We attribute deep-marine sedimentation in this area to a major event of subsidence in the Miocene that affected the entire forearc and that was caused by basal subduction erosion. We suggest that the anomalously thin crust that characterizes this area may have facilitated forearc subsidence and allowed the Miocene transgression to advance much farther inland here than in other regions of Chile. Subsequent uplift of the forearc is ascribed to basal accretion or underplating of sediments. Our conclusions contradict previous studies that favor a stable margin at these latitudes since the Jurassic. Deep-marine sedimentation in this area during the Miocene implies that the present Coastal Cordillera and Longitudinal Depression were probably submerged during that epoch.