VOLCANIC ROCKS AND MICROFOSSILS CONFIRM A LATE MIOCENE AGE FOR MARINE STRATA ON ISLA TIBURON, GULF OF CALIFORNIA

The timing of earliest marine incursion in the Gulf of California is critical for models of paleogeography and 4-D plate boundary strain. Marine strata exposed on Isla Tiburón have been cited as evidence for a middle Miocene marine incursion ~7 Myr prior to localization of the Pacific-North American plate boundary. We synthesize new geochronologic data from this southwest Isla Tiburón marine basin, the corresponding (up-dip) non-marine Sauzal basin, and both underlying and overlying volcanic units with existing paleontology and geochronology. Beneath the marine section, ≥15 Ma andesitic breccias and flows are unconformably overlain by a 6.7 ± 0.8 Ma (Ar/Ar) white, rhyolitic, poorly-welded ash-flow tuff. The tuff is overlain by a landslide breccia of welded rhyolite tuff clasts that provides a key marker at the base of the marine rocks. We relocated the outcrop of the 12.9 ± 0.4 Ma (K-Ar) andesitic breccia that Smith et al. (1985) interpreted to be interbedded with marine strata. In this arroyo exposure, the landslide breccia directly overlies the andesitic breccia in an erosional window through the gently-dipping marine conglomerate. Thus the andesitic breccia is unconformably beneath, rather than within, the marine strata. We obtained an age of 18.70 ± 0.19 Ma (U/Pb) from the same breccia clast sampled by Smith et al. (1985). Interbedded within the lowermost marine sediments we mapped and dated a 6.01 ± 0.20 Ma (U/Pb) ash-flow tuff. Within stratigraphically equivalent basal non-marine deposits we document a 6.87 ± 0.07 Ma (U/Pb) air-fall ash bed. The ages of basin deposits broadly agree with planktonic and benthic foraminifera found in the marine strata, reported to be late Miocene to early Pliocene, ca. 6.4 - 4 Ma (Gastil et al., 1999). We also mapped and dated a 4.13 ± 0.09 Ma (U/Pb) dike-fed rhyodacite flow that overlies the marine section. This age is consistent with reported K-Ar feldspar ages of 4.16 ± 1.81 Ma from the flow top and 3.7 ± 0.9 Ma from the feeder dike. Detailed mapping shows that no dikes cut through the flow. The 11.2 ± 1.3 Ma (K-Ar) date for this flow by Gastil and Krummenacher (1977) is erroneous. In summary, age data from microfossils and volcanic rocks constrain the earliest marine strata on southwest Isla Tiburón to ca. 6.0 - 6.8 Ma, consistent with a latest Miocene marine incursion in the northern Gulf of California.