Sedimentology and high-frequency sequence stratigraphy of a forearc extensional basin: The Miocene Caleta Herradura Formation, Mejillones Peninsula, northern Chile

Abstract

Facies and sequence stratigraphic interpretation for the 380-m-thick Caleta Herradura Formation (Miocene) are presented, based on detailed and comprehensive outcrop data from the Caleta Herradura half-graben, Mejillones Peninsula, northern Chile. The Caleta Herradura Formation contains an array of lithofacies comprising sandstones, sandy mudstones, diatomites and breccio-conglomerates that are interpreted as the products of inner-shelf to non-marine depositional settings. Complete exposure allows for recognition of a number of distinct and laterally persistent key stratal surfaces that permitted the identification of twenty-five high-frequency (meter to tens of meters-scale) sequences. Based on their internal organization, two main motifs of such unconformity-bounded depositional packages can be distinguished: deepening and deepening-to-shallowing upwards. Transgressive and highstand systems tracts were resolved based on such facies stacking patterns. The origin of sequences is difficult to decipher and remains somewhat uncertain. However, several lines of direct and indirect evidence constrain interpretations for both the sequence architecture and bounding unconformities, and help to discriminate between the various mechanisms that may have driven their development. In essence, these demonstrate that neither tectonically driven sea-level oscillations nor climatically induced changes in sediment supply can adequately explain the distinctive features of unconformities and the facies architecture of sediments they delimit. Instead, these high-frequency changes in relative sea level are best explained as a consequence of glacio-eustatic oscillations. The contemporaneous deep-ocean proxy records appear to support a causal link between the observed stratigraphic cyclicity and glacio-eustatic changes in sea level due to modulation of short-term Milankovitch-scale events by longer-period astronomical variations. However, the type of astronomical forcing remains elusive, hindered by lack of adequate age controls and a poor understanding of pacing mechanisms of high-frequency climate changes during the middle to late Miocene.