GEOLOGY AND STRATIGRAPHY OF THE SHEET 3246-II MIRAMAR, COSTA RICA

Abstract

The oldest rocks of the Miramar sheet are represented by sediments of the Punta Carballo Formation(Miocene), which are exposed at the SW corner. However, volcanic rocks of the older Aguacate Group (Miocene-Pliocene) and overlying Monteverde Formation (Lower Pleistocene) dominate the area. The Pliocene to Pleistocenesilicic ignimbrites and Quaternary sediments are subordinate. New K-Ar ages yielded 4,41±0,15 to 4,99±0,36 Ma forthe Aguacate Group and 1,71±0,22 to 1,77±0,11 Ma for the Monteverde Formation. Gabrodioritic stock was newlymapped in the Aguacate Group accompanying numerous other intrusive bodies of variable composition. New K-Arages were obtained for the rhyodacitic domes: 1,71±0,14 Ma for the cerro La Cruz Dome and 1,59±0,12 Ma for thecerro San Miguel Dome. Definition of volcanic units is based on detailed petrography, XRD, microprobe and 33 newbulk-rock chemical analyses.