Upper Oligocene to Middle Miocene volcanic and sedimentary rocks in the Loreto region, Baja California Sur, are widely exposed and make up the Comondu  Group as rede®ned here following McFall. The Comondu  Group is part of a volcanic arc and forearc basin that formed along the northwestern margin of Mexico. Regional to detailed scale mapping, stratigraphic analysis, and geochronology in a 10±20-km-wide and 70-km-long belt from the gulf escarpment to the coast near Loreto reveal three main units in the Comondu  Group and a composite thickness of ,1.5±2 km. (1) The lower clastic unit (,30±19 Ma) contains 200±300 m of ¯uvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt ¯ows that accumulated in a forearc basin. (2) The middle breccia and lava ¯ow unit (,19±15 Ma) is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris ¯ows interbedded with minor andesite lava ¯ows. (3) The upper lava ¯ow and breccia unit (,15±12 Ma) consists almost entirely of up to ,600 meters of andesite lava ¯ows a few kilometers west of Loreto that are part of a composite volcano. North and south of Loreto for up to 25 km, the unit is composed of andesite lava ¯ows and massive andesite breccia that formed from thick debris ¯ows near that and other volcanic centers. The three units of the Comondu  Group in the Loreto area correlated with the Salto, Pelones, and Ricaso Ân formations of the Bahõ Âa Concepcio Ân area. The lower clastic unit of this study is time equivalent to the marine San Gregorio, El Cien, and Isidro formations on the western and southern Baja California peninsula. The middle and upper units of the Loreto area are the proximal equivalent to the type section of the Comondu  Group near the village of Comondu Â. During deposition of the lower part of the Comondu  Group, the arc lay to the east on mainland Mexico in the Sierra Madre Occidental, and then migrated to the position of the Gulf of California at about 25 Ma. At ,19 Ma the proximal part of the arc migrated abruptly .50 km westward to approximately the position of Carmen Island and at 15 Ma the arc once again stepped west forming eruptive centers along the coast near Loreto until ,12 Ma. Also at 15 Ma, the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment.