EPISODIC DEFORMATION AND DEPOSITION IN NW MEXICO FROM THE LATE CRETACEOUS TO MIDDLE TERTIARY AS RECORDED IN SAHUARIPA, SONORA

The structure and stratigraphy found in the area of Sahuaripa, Sonora, indicate multiple periods of deformation and volcanism between the Late Cretaceous and the middle Tertiary. Previously this area was thought to have experienced a more simple deformation history including middle Cretaceous thrusting and middle to late Tertiary extension. New mapping and ⁴⁰Ar/³⁹Ar geochronology coupled with the known tectonic framework presents a rather detailed geologic history for this region. The generally north-east dipping units are separated by gentle to moderate angular unconformities indicating a long period of episodic, similar style deformation. A 5 km thick volcaniclastic and clastic section likely represents syntectonic deposition into an alluvial and lacustrine basin. The section fines upward from a coarse limestone conglomerate and andesitic lavas and breccias into a much more fine-grained lacustrine unit punctuated by many tuffs and reworked tuffs. A tuff from the upper was dated by ⁴⁰Ar/³⁹Ar to be 73.7 ± 0.3 Ma. This unit is partly repeated by a sequence of NNW trending, SW dipping normal faults which do not cut the overlying Paleogene (?) gravels suggesting that extension in the region began in the latest Cretaceous or the earliest Tertiary. A 10-15° angular unconformity separates the two sequences, the gravels dip NE 30-35°. Ignimbrites and basaltic andesite dated at 37-33 Ma and 28.6-25 Ma, respectively, unconformably overly the fluvial gravels. The Oligo-Miocene section dips 20-30° reflecting an amount of tilting before volcanism. Magmatism ceased in the area until 16.15 Ma when another sequence of mafic flows were erupted until 15.2 Ma. This last sequence of flows were overlain by an alluvial sequence bounded by the W dipping normal faults which define the present geometry of the basin. 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.