Recognition of an inferred Miocene marine incursion affecting areas from Colombia through Peru and Bolivia and into Argentina is essential to delineate the South American Seaway. In Bolivia, corresponding strata of inferred marine origin have been assigned to the late Miocene Yecua Formation. We carried out high-resolution {delta}13C and {delta}18O isotopic studies on 135 in situ carbonates from 3 outcrops, combined with detailed sedimentologic, paleontologic, and ichnologic analysis. Four less negative {delta}13C excursion levels were recorded that coincide well with beds containing marine body (barnacle) and trace (Ophiomorpha) fossils. These strata are interbedded with red-green beds containing mudcracks, plant roots, gypsum, and trace fossils of the continental Scoyenia ichnofacies. Our data are significant in that they show for the first time four possible short-lived marine incursions in the Bolivian central Andes during the late Miocene. The result is constrained by a new U-Pb date of 7.17 ± 0.34 Ma at the top of Yecua strata.

---

Uba, C. E.; Hasler, C.-A.; Buatois, L. A.; Schmitt, A. K.; Plessen, B. (2009): Isotopic, paleontologic, and ichnologic evidence for late Miocene pulses of marine incursions in the central Andes. Geology, 37, 9, 827-830.