Cover image:

In northern Sierra Plomosa, south of Placer de Guadalupe –a classical locality of paleozoic exposures 100 km northeast of Chihuahua City– a siliciclastic Lower to Middle Jurassic marine-marginal succession, rest unconformable on carboniferous limestones of the Pastor Formation. The siliciclastic unit, considered previously as Early Permian in age was recently dated as Early to Middle Jurassic according to U-Pb (zr) ages from ignimbrite and rhyolitic flows that occur concordant in the succession. Photo by: José Rafael Barboza Gudiño, 2012.