Neogene-Quaternary coastal and offshore sedimentation in north central Chile: Record of sea-level changes and implications for Andean tectonism

Abstract

The stratigraphy, sedimentology, and paleontology of the Coquimbo Formation in a coastal section approximately 100km north of La Serena was Studied to establish relative low-order sea-level changes during the Neogene. During the Early-Middle Miocene, a wide coastal platform was bevelled onto Palaeozoic basement rocks, and marine sedimentation may have taken place within a deep coastal inlet later transformed into a submarine canyon. Transgression over this platform commenced during the late Burdigalian-early Langhian (16-15 Ma) and culminated in a sea-level highstand near the end of the Serravalian (11.8 Ma). This transgression was followed by regression during the rest of the Serravalian and Tortonian until approximately 9 Ma. Renewed transgression during the Messinian was Succeeded by regression lasting until 5.5 Ma, when another transgressive cycle bevelled a ravinement Surface into the underlying deposits. This cycle terminated in a marine highstand during the Zanclean at 4.9 Ma. Thereafter, the succession changed front enter platform to middle shoreface deposition. Rapid flooding at 3.9 Ma resulted in the formation of an extensively bored phosphatic hardground, which represents a condensed section. A lowstand during the Piacenzian is represented by inner-shelfdeposits, followed by a return to outer-shelf deposition with contemporaneous upper continental slope sedimentation below the ancient shell'break. The succession is capped by Gelasian upper shoreface deposits dated at 2.0-1.8 Ma. The reemergence of the shelf during the Pleistocene (1.0 Ma) formed an extensive coastal plain covered by fluvioestuarine, shelly gravel. Subsequently, four marine terraces were carved into the Succession in the last 0.5 Ma.

A comparison of the recorded sea-level changes with global sea-level Curves provides a reconstruction of local tectonic uplift and downwarp events during the Neogene-Quaternary. The migration of the Juan Fernandez Ridge underneath this part of the Chilean flat-slab sectorcaused a relative uplift of 60 m, followed by subsidence of 125 m in its wake. The generalized tectonic rate of change varied front 0.02 to 0.08 mm/yr.