Sequence stratigraphy of the Angostura Formation (Ecuador) and Caleta Herradura Formation (Northern Chile): evidence for Miocene orbitally-driven eustatic sea-level changes

Earth’s climatic history during much of the Cenozoic has been controlled by Milankovitch variations in the planetary orbit comprising alternate periods of expansions and contractions of large ice-sheets. However, whereas a plethora of outcrop studies made on Plio-Pleistocene deposits have emphasized the stratigraphic response to high-frequency, orbitally-driven eustatic sea-level fluctuations, comparatively less attention has been focused on the attempt to relate Miocene eustatic sea-level changes directly to shallow-marine sequences. As a result, far less is known about these sediments that, having been deposited during a period of Southern Hemisphere-driven, glacio-eustatic sea-level changes, should feature a very similar record in term of sedimentary cyclicity. This bias is likely to arise from the general paucity of (i) adequate high-resolution sea-level histories from deep-sea oxygen isotope records; and (ii) high quality geological records documenting the associated transgressions and regressions across the world’s continental shelves as most continental margin remain flooded. A detailed sequence stratigraphic and palaeontological examination of the shallow-marine, middle to late Miocene Angostura Formation (Ecuador) and Caleta Herradura Formation (northern Chile), led to the identification of an extraordinary record of cyclical, high-frequency sea-level fluctuations. The Angostura and Caleta Herradura Formations comprise eight and twenty-five, meter to tens of meters-scale high-frequency sequences, respectively. Sequences are skewed towards preferential development of the TST, being HST deposits absent or poorly developed and deposits attributable to the FSST and LST not present at all. This distinctive architectural style is remarkably similar to that described for high-frequency sequences formed under severe icehouse regimes characterized by repeated glacio-eustatic oscillations in sea-level, such as during the Early Permian, the Oligo-Miocene, or the Plio-Pleistocene. Sequence bounding unconformities and intervening stratal units were identified on the basis of abrupt basinward facies shifts, irregular contacts, bioturbation, palaeontological features of associated shell beds, and vertical facies stacking patterns. A reasonable correlation between sequences and the contemporaneous high-frequency glacio-eustatic changes derived from recent δ18O studies is provided by biostratigraphic constraints. This correlation leads to the conclusion that (i) glacio-eustasy regulated by modulation of short-term Milankovitch-scale events by longer-period astronomical variations might has been the principal factor regulating stratigraphic packaging; and (ii) these sequences provide an excellent shallow-marine outcrop record of middle to late Miocene Antarctic glaciations.