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

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 redefined 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 similar to1.5-2 km. (1) The lower clastic unit (similar to 30-19 Ma) contains 200-300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin. (2) The middle breccia and lava flow 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 with minor andesite lava flows. (3) The upper lava flow and breccia unit (similar to 15-12 Ma) consists almost entirely of up to similar to 600 meters of andesite lava flows 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 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, and 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 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. During deposition 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 position of the Gulf of California at about 25 Ma. At similar to 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 are once again stepped west forming eruptive centers along the coast near Loreto until similar to 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. (C) 2001 Elsevier Science B.V. All rights reserved.