STRATIGRAPHY OF THE COMONDÚ GROUP NEAR TIMBABICHI, BAJA CALIFORNIA SUR, MEXICO

We propose an informal Comondú group for rocks exposed in the Sierra La Giganta near Timbabichi. The 700-750 m thick succession formed in a Late Oligocene to Miocene forearc basin. Unit A (~50 m) overlies conformably the marine Timbabichi member. It is composed of sandstone, siltstone, tuffaceous mudstone, and little conglomerate. A few tuffs occur and one of them we dated at 25.9±1.0 Ma. Unit A grades into unit B. Unit B (~60 m) is composed of sandstone, pebbly sandstone, and conglomerate. A few tuffs are also found. The contact with unit C is erosional. Unit C (~100-150 m) contains andesitic breccia, grading laterally to the west into conglomerate and pebbly sandstone. The contact with unit D is highly irregular. Unit D (~80-90 m) consists of sandstone, pebbly sandstone, siltstone, and conglomerate. A prominent tuff occurs in the upper part. The contact with unit E varies from planar to erosive. Unit E (46 m) is dominated by siltstone and slightly phosphatic sandstone, in addition to some coquina. The contact with unit F is planar or gradual. We estimate the age of unit E as Lower or early Middle Miocene. Unit F (~100 - >200 m, average ~200 m): The lower part is composed of sandstone, siltstone, pebbly sandstone, and conglomerate, grading towards the upper part into conglomerate and coarse sandstone. Unit G (80-200 m) contains volcanic breccia and displays large-scale synsedimentary deformation and sliding structures. Thickness varies due to the strongly erosive lower and an irregular upper contact. West of the divide, unit G interfingers laterally with conglomerate and sandstone. Unit H (<30 - >200 m) consists of sandstone, pebbly sandstone, and conglomerate. The Comondú group is covered by basaltic lava flows, which interfinger with unit H and thin to the west. Similar basalt at Punta Mechudo has been dated at 10.1 ± 0.3 Ma and an andesitic dike near San Carlos at 10.4 ± 0.4 Ma. In the Comondú group, we observe two sedimentary cycles of terrestrial volcaniclastic deposition (units A-D and F-H), separated by marine strata (unit E). Unit E, deposited during a marine ingression, represents the first record of marine deposits in the Comondú group. Their presence implies that for a certain time period proliferation of volcaniclastic material from the magmatic arc ceased. Deposition of the Comondú group terminated in the early Late Miocene.