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

Abstract

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 δ 18 O 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.