Sequence Stratigraphy of the Angostura Formation (Ecuador) and Caleta Herradura Formation (Northern Chile): Evidence for Middle to Late Miocene Orbitally Paced Eustatic Sea-Level Changes

Di Celma, Claudio¹, Gino Cantalamessa², Luca Ragaini³, Gigliola Valleri⁴, Walter Landini³ (1) University of Liverpool, Liverpool, United Kingdom (2) Università di Camerino, Camerino (MC), Italy (3) Università di Pisa, Pisa, Italy (4) Università di Firenze, Firenze, Italy

It is now widely accepted that during much of the Cenozoic the expansions and contractions of large ice-sheets occurred in concert with eustatic sea-level fluctuations. However, whereas a plethora of outcrop studies made on Plio-Pleistocene deposits have emphasized the stratigraphic response to high-frequency orbital forcing, comparatively less attention has been focused on the attempt to relate Miocene, high-frequency, astronomically paced sea-level changes directly to marine sequences. A detailed sequence stratigraphic and palaeontological analysis 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 high-frequency, meter- to decameter-scale 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. Sequence bounding unconformities and intervening stratal units were identified on the basis of physical evidence, which includes abrupt basinward facies shifts, irregular contacts, bioturbation, palaeontological features of associated shell beds, and vertical facies stacking patterns. Biostratigraphic constraints allowed a reasonable correlation between sequences and the contemporaneous high-frequency glacio-eustatic changes derived from recent oxygen-isotope studies. This correlation has far-reaching implications and 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.