Abstract

The history of tectonism in southeastern Arizona during Late Cretaceous time is recorded by the shoreline movements and the composition of age-equivalent marine sandstone units of the San Juan Basin to the northeast. Three of these sandstone units, the Gallup, Point Lookout, and Pictured Cliffs Sandstones, were deposited during major regressions of the Cretaceous sea. Deposition of the Gallup and the Pictured Cliffs Sandstones corresponded to known tectonic events in southeastern Arizona. No major tectonism has been identified in southeastern Arizona during deposition of the Point Lookout Sandstone, but it is likely the regression during which the sandstone accumulated resulted from a large, relatively sudden increase in sediment supplied to the San Juan Basin area. Therefore, the Point Lookout regression is indirect evidence for a period of uplift.

The composition of the Gallup, Point Lookout, and the Pictured Cliffs Sandstones can be related to rock types which were uplifted in southeastern Arizona during Late Cretaceous time. Late Turonian tectonism uplifted a thick clastic sedimentary succession from which the quartz-rich Gallup Sandstone was derived. Much of this succession was subsequently eroded exposing a variety of older rock types. These older rocks contributed clasts to the younger Point Lookout Sandstone, which contains abundant varied rock fragments. The time of deposition of the Pictured Cliffs Sandstone coincides with a major episode of uplift and volcanic activity in southern Arizona. The abundance of volcanic rock fragments in the Pictured Cliffs indicate the sandstone was derived from this highland.