Original paper

Ignimbrites of the Pliocene Bagaces Formation near Canas (Guanacaste, Costa Rica)

Semm, Wolf; Alvarado, Guillermo E.


By this study, four units within the so far mostly undivided Bagaces Formation - a major ignimbrite unit of the Plio- to Pleistocene magmatic succession in northern Costa Rica - were identified and their ignimbrite sequences stratigraphically distinguished. The basal welded Montenegro ignimbrite of mid-Pliocene age forms the lowermost stratigraphic member. It is followed by three non-welded ignimbrite units of Upper Pliocene age, Sandillal (4.1-4.15 ± 0.02 Ma) and Cañas (2.06 ± 0.02 Ma) and the locally exposed Río Tenorio ignimbrite (estimated age 2 Ma). The ignimbrite units of the Bagaces Formation show decreasing thicknesses from NE to SW. For the non-welded Sandillal and Cañas units, distal lobes could be defined. Lateral pumice and lithics size distribution, as well as isopach data indicate a paleotopographical control for the deposition of the ignimbrites and allowed defining the source area of the ignimbrites. Ar-Ar-ages and compositions analysis of the pyroclastic units refer to the eruptive evolution of the Neogene volcanic arc.