MAPPING AND PETROGRAPHIC ANALYSIS IN THE COMONDÚ ARC, CONCEPCIÓN PENINSULA, BAJA CALIFORNIA SUR, GULF OF CALIFORNIA (MEXICO): RESULTS FROM BAJA BASINS IRES

A 7 km thick section of Oligocene to mid-Miocene Comondú Group arc volcanic, sedimentary, and intrusive rocks is exposed on the Bahía Concepción Peninsula. We created an updated map of the northern peninsula using field observations, petrography, and Google Earth imagery to establish stratigraphic and intrusive relations, from forearc sedimentation to arc magmatism and the onset of extension. Formation names defined by McFall¹ are retained.

The Oligocene Salto Formation represents a 27 Ma forearc basin, with eolian and fluvial sandstones and distal ignimbrites. This is conformably overlain by the Miocene Pelones Formation, a >1 km thick section of block-and-ash-flow tuffs (plag+cpx) and minor volcanic debris flow deposits. These contain irregularly-shaped sills indicating vent proximity. This is conformably overlain by the Minitas and Pilares Formations. McFall¹ mapped these two ca. 1 km thick formations in fault contact, but we show that they interfinger and thicken in opposite directions. The Minitas Formation is a volcanic lithic conglomerate-sandstone unit with progradational alluvial fan cycles, formed along a contemporaneous fault scarp. Sandstones consist of glassy volcanic lithics with cpx+plag. The Pilares Formation is scoriaceous to amygdaloidal structurally ponded lavas with large (≤6 mm) plag (40%), small cpx (5%), and local manganese veins.

We map more intrusions than previously mapped by McFall¹. Cutting the Salto and Pelones formations are: Beatriz porphyritic and fine-grained holocrystalline intrusions (plag±hb); Mantita anorthosite (95% plag, 5% cpx); (3) Guadalupe intrusive complex (75% plag, 5% cpx in a fine-grained groundmass); and a sill complex with 8% cpx and trace plag in a fine-grained groundmass. The Pelones and Pilares/Minitas Formations are cut by a red jointed intrusion with altered plag, and a 3 m thick white sill with plag+hb in a glassy groundmass.

An updated map and detailed characterization of the petrography is important to reconstruct magmagenesis and sedimentation during subduction and extension in the Gulf of California.

plag-plagioclase, cpx=clinopyroxene, hb=hornblende

¹C. McFall, 1968 Geol. Sci. Stanford University Pubs 10:5, 25 pp