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Journal of the Geological Society; 2007; v. 164; issue.3; p. 653-665;
DOI: 10.1144/0016-76492006-001
© 2007 Geological Society of London

Original Article

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

Gino Cantalamessa1, Claudio Di Celma1,2, Luca Ragaini3, Gigliola Valleri4 & Walter Landini3

1 1Dipartimento di Scienze della Terra, Università di Camerino, I-62032 Camerino (MC), Italy
2 2Present address: STRAT Group, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK (e-mail: c.di-celma@liverpool.ac.uk)
3 3Dipartimento di Scienze della Terra, Università di Pisa, I-56126 Pisa, Italy
4 4Dipartimento di Scienze della Terra, Università di Firenze, I-50121 Firenze, Italy

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 {delta}18O 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.