Mesozoic Geology and Paleontology of the Four Corners Area

About this guidebook

The essence of “The Land of Enchantment” extends throughout the Four Corners region and many of you will feel as though you never left New Mexico. Much of this essence derives from the flat-lying to gently dipping Mesozoic sedimentary rocks, which in turn define much of the topography and scenic quality of the Colorado Plateau physiographic province, we will be touring near the center of this province. These sedimentary rocks, their fossil record and correlation, and the depositional environments they represent, are the primary focus of this book. Given that most of the pre-Cretaceous Mesozoic stratigraphic record preserved in this region is that of nonmarine sedimentation, age-diagnostic fossils are sparse. We, accordingly, had to sharpen our I ithostratigraphic skills in anticipation of this challenge, and we consequently (perhaps predictably) found that the stratigraphic interpretations of some of the earlier workers differed from ours. Field conference participants will have ample opportunity to evaluate these contrasting interpretations on the outcrop (the real value of these events) and perhaps contribute some of their own. Upper Cretaceous rocks in the conference area are, by contrast, dominantly marine. Foremost amongst this marine epicontinental sequence is the Mancos Shale, the type area of which we will traverse on Day 2. The Mancos spans the first four stages of the Late Cretaceous and encompasses as much as 13 to 15 Ma; it attains thicknesses in excess of 2100 ft, consisting of fine grained clastics and lesser carbonate. An exhaustive treatment of the entire section is presented by Mark Leckie and coauthors, wherein they provide detailed bio- and lithostratigraphic data, and relate the section to the deeper or more basinal facies to the east. Before the thick peat deposits in the paralic coastal swamps (associated with the regressing Cretaceous seaway) had been transformed into the coals of the present Fruitland Formation, intrusive activity had begun in the Four Corners area. Age data supplied by the Laboratory of Geochronology at the New Mexico Bureau of Mines and Mineral Resources and presented herein by Steven Semken and William McIntosh confirms the pre-(classic) Laramide age of one of these intrusive centers– the Carrizo Mountains–and at least one other–the La Plata Mountains–is known from previous work to be of similar age. The doming over the sites of these early magmatic centers resulted in the deposition of distinctive proximal sediments such as those of the Animas Formation, which we will examine (south of Durango) on our last day. The tectonic history of the area has been summarized by Lee Woodward and others. Bill Chenoweth has provided a historic and technical account of the uranium mining industry in this region. Coal, petroleum, and other matters pertaining to economic geology are for the most part presented in the roadlogs, but petroleum-related topics are presented as contributed papers as well.