Sedimentology and high resolution sequence stratigraphy of the late middle to late Miocene Angostura Formation (western Borbón Basin, north-western Ecuador)

An integrated analysis of facies and palaeontological content of the middle to late Miocene (c. 12.4-9.1 Ma) Angostura Formation led to the identification of facies associations indicative of shoreface and inner-shelf settings. The Angostura Formation comprises eight high-frequency sequences that are stacked to form a tectonically driven lowstand sequence set. The most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision: (1) a basal suite of deepening-upward shoreface sediments (transgressive systems tract), including a base-of-cycle shell concentration; (2) a mid-cycle shell bed, consisting of molluscan shells dispersed in a matrix of inner-shelf muddy fine-grained sandstones; (3) a shallowing-upward unit of inner-shelf to lower shoreface sediments almost barren of mollusc fossils (highstand systems tract). Biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late Miocene high-frequency glacio-eustatic changes derived from recent δ18O studies. This correlation has far-reaching implications and leads to the following conclusions: (1) glacio-eustasy in tune with oxygen isotope changes at fourth-order frequency (200 ka-1 Ma duration) may have been the principal factor regulating stratigraphic packaging in the Angostura Formation; (2) these sequences provide an excellent shallow-marine outcrop record of late middle to late Miocene Antarctic glaciations.