STRATIGRAPHIC ANALYSIS OF LATE MIOCENE TO EARLY PLIOCENE (?) SEDIMENTARY ROCKS, SW ISLA TIBURóN, SONORA, MEXICO

Late Miocene to early Pliocene (?) sedimentary rocks exposed on southwest Isla Tiburón, Mexico, provide a record of basin evolution during initiation of focused plate-boundary strain in the northern Gulf of California. These rocks include the earliest marine deposits that accumulated during incursion of seawater into the northern Gulf. Structures active at this time, including the La Cruz fault, were likely involved in opening of the northern Gulf, so analysis of the sedimentology and basin architecture provides insight into structural controls on deposition of these earliest marine rocks. The stratigraphy of SW Isla Tiburón can be divided into two unconformity-bounded sequences that record two stages of basin formation and deformation. The lower sequence includes landslide breccia that overlies a tuff dated at 6.7 +/- 0.8 Ma (Oskin, 2002), a subaqueous (likely submarine) olistostrome, debris flow deposits, and fossiliferous sandstone and conglomerate. It is cut by a series of basin-bounding oblique normal faults and capped by a distinctive tuff named the "upper tuff" (as-yet undated). The upper sequence consists of marine Gilbert-delta bottomset turbidites, foreset conglomerate and sandstone that dip 15-30°, and non-marine topset deposits. The upper sequence records the growth of a broad monocline during progradation of the Gilbert-delta system to the northwest along the La Cruz fault. The mean transport direction based on orientation of imbricated conglomerate clasts is toward 342.6 +/- 5.4°, ~17° clockwise from the transport direction determined from average down-dip direction of untilted foreset deposits. This discrepancy may be due to clockwise rotation of deposits close to the La Cruz fault in response to dextral slip on the fault. Stratigraphic analysis shows that the marine section on SW Isla Tiburón is ~285 m thick, much thinner than previously thought. Combined with existing age data, this study confirms that marine incursion into the northern Gulf of California slightly post-dates 6.7 +/- 0.8 Ma. The association of fault-controlled basin formation with earliest marine deposition shortly after 6.7 Ma supports a hypothesis that latest Miocene marine incursion into the northern Gulf of California resulted from localization of regional transtension along the La Cruz and similar strike-slip faults.