AGE OF THE CARACOL FORMATION, SIERRA DE PARRAS, COAHUILA, MEXICO

The Caracol Formation crops out in a well exposed, overturned stratal succession in Cañon La Casita, Sierra de Parras, Coahuila. It overlies deepwater calciturbidites of the Indidura Formation and is overlain by gray shale of the Parras Shale. Microfossils indicate that the Indidura is late Cenomanian-Santonian, the Caracol is late Santonian-early Campanian, and the Parras is late Campanian (Longoria et al., 1999). The Caracol consists of graded volcaniclastic beds a few cm to 50 cm thick interbedded with gray shale that resembles shale beds of the Parras. The volcaniclastic beds are Bouma AB turbidites, normally graded from structureless medium-grained sandstone to laminated fine-grained sandstone. The sandstones are neovolcanic litharenites and contain approximately 40% plagioclase, 40% microcrystalline tuffaceous lapilli or siliceous volcanic rock fragments, 5% fresh biotite to 1 mm long, and 1% angular unstrained quartz grains. The sandstones thus lack demonstrable extrabasinal grains and appear to be reworked tuffs.

A single sample from a 50 cm thick tuff bed in the lower part of the formation yielded a weighted mean U-Pb zircon age of 85.9 ± 0.4 Ma (latest Coniacian; LA-ICP-MS; n = 22 zircon grains; MSWD = 0.88). Zircon grains (n = 41) in the sandstone range in age from 92 to 78 Ma, evidently demonstrating incorporation of a range of grain ages during reworking or systematic errors in the grain ages. The new age indicates that voluminous magmatism associated with subduction of the Farallon slab migrated into the region west of Sierra de Parras in the Coniacian. Foundering of the older carbonate platform system to deepwater deposits recorded by the Indidura Formation likely resulted from dynamic subsidence driven by Farallon subduction prior to major magmatism in the region, whereas the Caracol Formation signals the arrival of magmatism, crustal shortening and attendant flexural subsidence of the Sierra Madre foreland that became pronounced during Parras Shale deposition.