STRATIGRAPHY OF THE SAHUARIPA BASIN AND PRELIMINARY COMPARISON TO THE RIO YAQUI BASIN, EAST-CENTRAL SONORA, MEXICO

The basins of east-central Sonora, Mexico, have been included in the Gulf extensional province (i.e.; Stock and Hodges, 1989; Henry and Aranda-Gomez, 1992), which experienced extension from ~12 to 6 Ma. Recent studies have indicated that significant extension in portions of the region occurred prior to this period (Gans, 1997; McDowell et al., 1997).

New field and geochronologic data from the Sahuaripa basin suggest the area experienced extension during latest Oligocene and early Miocene. A sequence of clastic and volcanic deposits is now well exposed due to incision of the Sahuaripa River. Samples of the igneous units have been dated by the ⁴⁰Ar/³⁹Ar method. At least two ignimbrites, dated at ~33.9 Ma and ~28.1 Ma, underlie the section. These tuffs shortly precede or post-date the initial faulting of the basin and are consistent with ages for the felsic components of the Sierra Madre Occidental immediately east of the basin. Basalt samples collected on the eastern side of the basin at the base of the sequence give ages of 28.3, 27.8 and 27.5 Ma. These are overlain by thin to thick bedded conglomerate and sandstone. Samples from a large mafic center at the northern end of the basin, east and north of the town of Sahuaripa, give ages of 25.3 and 24.87 Ma. This thick sequence of flows and the clastic beds overlying it dip ~35° east. The two youngest samples from the basin are dated at 15.9 and 15.2 Ma from gently east-dipping andesite flows found on the western side of the basin. These younger flows overly a sequence of coarse clastic material and are overlain by a sequence of generally finer-grained sediments. The difference in dip of the ~25 Ma and younger basalts indicates a significant share of the basin’s deformation happened before the younger flows were deposited.

The Sahuaripa basin can be compared to adjacent basins to learn more about the regional extension and basin formation. In the Rio Yaqui basin, to the west, most of the conglomeratic material is intercalated in a sequence with a 12.5 Ma ignimbrite as opposed to the 27.5 Ma basalts (McDowell et al., 1997). Near Santa Rosa, south of Sahuaripa basin, Gans (1997) found 17.5 Ma andesites overlying more steeply dipping basalts that are 25-26 Ma. This might suggest that basin development began earlier (20-15 Ma) in the eastern basins and progressed westward (see Gans et al., this volume).