REGIONAL STRATIGRAPHY, SEDIMENTOLOGY, AND TECTONIC SIGNIFICANCE OF OLIGOCENE-MIOCENE SEDIMENTARY AND VOLCANIC-ROCKS, NORTHERN BAJA-CALIFORNIA, MEXICO

Upper Oligocene (?) to middle Miocene sedimentary and volcanic rocks i n northern Baja California were deposited along the western margin of North America during subduction of the Guadalupe plate and southward m igration of the Rivera Triple Junction. Regional mapping and compilati on of stratigraphic data reveal a sequence of three regionally traceab le stratigraphic units. (1) Oligocene (?) to lower Miocene Mesa Format ion: basal quartz-rich fluvial sandstone, grus, conglomerate, and acce ssory facies, whose detrital compositions reflect the composition of l ocal pre-Tertiary basement rock. (2) Lower to middle Miocene Comondu F ormation: laterally variable sequence of volcaniclastic conglomerate, breccia, sandstone, tuff and minor volcanic flow units. (3) Widespread mesa-capping rhyolite tuff, typically welded and crystal-rich, probab ly upper Miocene in age. The Mesa Formation overlies a highly irregula r and deeply dissected erosional surface developed on pre-Tertiary bas ement rock. The shift from pre-Mesa erosion to widespread (though loca lized) deposition and valley-filling records the final phase of late C retaceous to middle Tertiary regional subsidence and eastward transgre ssion that resulted from slow cooling and thermal contraction of Creta ceous are crust during a temporal gap in magmatic activity along the w estern Cordilleran margin. Nonmarine sediments of the Mesa Formation w ere deposited in small, steep-walled paleovalleys and basins that grad ually filled and evolved to form through-going, low-energy ephemeral s tream systems. The gradational upward transition from the Mesa to Como ndu Formation records the early to middle Miocene onset of subduction- related are magmatism in eastern Baja California and related westward progradation of alluvial volcaniclastic aprons shed from high-standing eruptive volcanic centers. Pre-existing streams were choked with the new influx of volcanic detritus, causing the onset of rapid sediment d eposition by stream flows and dilute to viscous sediment gravity flows . Deposits of the Comondu Formation thin and fine systematically westw ard, from proximal volcanic conglomerate and breccia with thin basalt and andesite flows in the east, to distal volcaniclastic fluvial sands tone in the west. These proximal-distal relationships help to define t he location and paleogeography of active are-flanking volcaniclastic a lluvial aprons of the Miocene magmatic are in northern Baja California . A substantial late Miocene drop in regional base level (relative sea level) is best attributed to regional uplift caused by the renewal of magmatic and thermal activity in northern Baja California, which has continued to the present day.