2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 2
Presentation Time: 8:15 AM


BLAIR, Karen, Dept of Geological Sciences, Univ of California, Santa Barbara, CA 93106 and GANS, Phillip, Dept. of Geological Sciences, Univ of California, Santa Barbara, CA 93106, kblair@umail.ucsb.edu

Extensional basins of northwestern Mexico expose thick sections of Tertiary non-marine sequences, which previous workers generally lumped together into the Oliocene(?) - Miocene(?) Báucarit Formation. Dumble (1900) and King (1939) defined the Báucarit as any slightly indurated well-bedded sandstone, conglomerate and some clays including basaltic flows at the base. Some of the individual basins have recently been dated using interbedded lava flows and tuffs revealing important differences in their ages, sizes, sedimentary environments, and associated magnitude of extension (e.g. Gans, 1997, McDowell et all, 1997).

The well-exposed Sahuaripa basin provides important constraints on the timing and magnitude of episodic extension in eastern Sonora. A 5 km thick Late Cretaceous to Early Tertiary(?) volcaniclastic and lacustrine sedimentary section is tilted 45-55° and repeated by several NNW trending, SW dipping normal faults. The unconformably overlying Paleocene(?) to Eocene(?) package of fluvial conglomerate dips somewhat less (30-35° NE), indicating a period of extensional faulting and tilting in the early Tertiary. The Paleocene gravels are well rounded, come from a distal source, and indicate a much different environment than the typical Basin and Range (Blair and Gans, 2005). A sequence of Late Eocene - Oligocene ignimbrite and basaltic andesite dated by 40Ar/39Ar to be 37-33 Ma and 28.6-25 Ma, respectively, unconformably overlies the fluvial gravels, dipping about 20-30° indicating a period of slight tilting before volcanism. Mafic magmatism probably accompanied faulting along the eastern bounding fault. The area experienced a hiatus of magmatism from 25 to 16 Ma followed by a sequence of andesite flows from 16.15 to 15.2 Ma. These mafic flows along with a thick sequence of overlying conglomerate, dip only 10-20° ENE with steeper tilts localized near fault zones. The basin's extensional history was episodic and included at least three periods of faulting: 1) Late Cretaceous (post 74.7 Ma) to Paleocene, also the highest magnitude of tilting (15-20°), 2) mid Eocene (before 37 Ma), and 3) post-15 Ma. Rather consistent dip direction with decreasing dips of the stratigraphy implies the bounding faults may have been reactivated, such that many of the basins and basin-fill sequences are composite features.