Bay sedimentation as controlled by regional crustal behaviour, local tectonics and eustatic sea-level changes: Coquimbo Formation (Miocene–Pliocene), Bay of Tongoy, central Chile

The north-facing Bay of Tongoy in central Chile is flanked by topographic highs in the west and east. During the Miocene and Pliocene, the bay extended inland at least 30 km farther south than a present. It was filled with muds, sands, coquinas and gravel during a series of transgressions and regressions related to regional and local tectonic movements combined with global sea-level variations. 87Sr/86Sr and microfossil dating indicates transgressions between 11.9–11.2 Ma, 10.1–9.5 Ma, 9.0–7.3 Ma, 6.3–5.3 Ma, 4.3–2.2 Ma and 1.7–1.4 Ma. The regional tectonic behaviour of the crust shows general uplifting from 10.5 Ma to 6.9 Ma, associated with subduction of the Juan Fernández Ridge (JFR) beneath this part of the continent. Subsidence followed between 6.9 and 2.1 Ma, in the wake of the southeastward-migrating JFR. The subsequent subduction of an oceanic plateau similar to the JFR caused rapid uplift that led to the final emergence of the bay above sea level. The Puerto Aldea normal fault along the western limit of the study area was reactivated during the regional uplift and subsidence events, with reverse faulting occurring during the latter phase. Sporadic fault reactivation probably triggered the rapid changes in water depth reflected in the recorded vertical succession of facies.