Stratigraphy and geochronology of the Comondu Group near Loreto, Baja California sur, Mexico

Upper Oligocene to Middle Miocene volcanic and sedimentary rocks in the Lor eto region, Baja California Sur, are widely exposed and make up the Comondu Group as redefined here following McFall. The Comondu Group is part of a v olcanic arc and forearc basin that formed along the northwestern margin of Mexico. Regional to detailed scale mapping, stratigraphic analysis, and geo chronology 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 similar to1.5-2 km. (1) The lower clastic unit (sim ilar to 30-19 Ma) contains 200-300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt fl ows that accumulated in a forearc basin. (2) The middle breccia and lava fl ow unit (similar to 19-15 Ma) is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded wi th minor andesite lava flows. (3) The upper lava flow and breccia unit (sim ilar to 15-12 Ma) consists almost entirely of up to similar to 600 meters o f andesite lava flows a few kilometers west of Loreto that are part of a co mposite volcano. North and south of Loreto for up to 25 km, the unit is com posed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers. The three units of the Comondu Group in the Loreto area correlated with the Salto, Pelones, a nd Ricason formations of the Bahia Concepcion area. The lower clastic unit of this study is time equivalent to the marine San Gregorio, El Cien, and I sidro formations on the western and southern Baja California peninsula. The middle and upper units of the Loreto area are the proximal equivalent to t he type section of the Comondu Group near the village of Comondu. During de position of the lower part of the Comondu Group, the arc lay to the cast on mainland Mexico in the Sierra Madre Occidental, and then migrated to the p osition of the Gulf of California at about 25 Ma. At similar to 19 Ma the p roximal part of the arc migrated abruptly > 50 km westward to approximately the position of Carmen Island and at 15 Ma the are once again stepped west forming eruptive centers along the coast near Loreto until similar to 12 M a. 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. (C) 2001 Elsevier Science B.V. All rights reserved.