STRATIGRAPHY AND GEOCHRONOLOGY OF THE COMONDÚ GROUP BETWEEN LA PAZ AND THE SAN JOSÉ ISLAND REGION, BAJA CALIFORNIA SUR, MEXICO

Volcanic and volcaniclastic rocks of the Comondú Group exposed along the Main Rift Escarpment between La Paz and the San José Island region of Baja California Sur were formed in an Oligocene-Miocene forearc basin and arc.  New ⁴⁰Ar/³⁹Ar ages and field observations give a framework stratigraphy for the study area between latitudes 24°30′ and 25°05′ N.  The oldest rocks are marine volcanic sandstones of the upper El Cien Formation, dated at ~25.5 Ma by previous workers.  The formation is not exposed north of 24°35′ and is conformably overlain by the terrestrial Comondú Group, from which ten samples were dated with ⁴⁰Ar/³⁹Ar techniques.  In the southern portion of the study area, the Comondú Group consists of a lower volcanic fluvial sandstone overlain by alternating rhyolitic tuffs and pebbly sandstone and conglomerate.  The lowest ash-flow tuff in the section is 23.34±0.21 Ma and the highest, the San Juan tuff, is newly dated at 19.37±0.06 Ma.  This unit is overlain by a widespread silty sandstone to sandy conglomerate.  The uppermost unit is an 18.23±0.20 Ma andesite flow that is overlain by volcanic breccia (?).  In the northern portion of the study area, the Comondú Group is a relatively young section underlain by an 18.64±0.13 Ma dacitic flow breccia.  Overlying the dacite are interfingering volcanic conglomerates and breccias.  The area is capped by pebbly sandstone and conglomerate that correlates laterally with the uppermost sandy conglomerate to the south.  A rhyolitic ash-flow tuff near the bottom of the Comondú section in the northernmost study area (25°05′N) is 23.02±0.11 Ma and likely present as far north as Timbabichi.  A depositional rate of ~100 m/my in the southern study area suggests that a change from marine to terrestrial conditions took place at ~24 Ma.  Between 23.34±0.21 and 19.37±0.06 Ma the fluvial facies became more conglomeratic.  At ~19 Ma there was a major change to dominantly proximal and core volcanic facies; the widespread sandstone and sandy conglomerate unit demonstrates a break within the proximal facies.  Major vertical facies changes are interpreted to represent a westward jump of the volcanic arc between 19.37 and 18.64 Ma, similar to what has been observed in the Loreto and Bahía Agua Verde area.   Rhyolitic tuffs from 23.34 to 19.37 Ma may correlate with those in the state of Nayarit as previous workers have suggested.