Cambrian and Ordovician rocks of southeastern Arizona and southwestern New Mexico
— Philip T. Hayes

The Cambrian and Lower Ordovician rocks of southeastern Arizona were deposited near the eastern shore of a shallow shelf sea that encroached from the west or southwest across an area of generally low relief carved in diverse metamorphic, igneous and sedimentary rocks of Precambrian age. The sea, which apparently had a very broad intertidal zone, entered the region in late Early Cambrian or early Middle Cambrian time and extended eastward to about the position of the present Arizona-New Mexico boundary by about the end of Middle Cambrian time (fig. 1). The position of the eastern shoreline of the sea fluctuated during Late Cambrian time, but before the end of the Cambrian began a more rapid eastward advance across western New Mexico. The region was covered by this sea until latest Early Ordovician time.

 

The region was probably slightly emergent during most of Middle Ordovician time and was subjected to some erosion, more to the north than to the south. A sea again encroached, this time from the southeast, near the end of Middle Ordovician time and reached at least as far as the eastern edge of southern Arizona and possibly extended across much of it. No sedimentary rocks and thus no record of events exists for the Silurian in Arizona, but a Silurian sea did reach into south-western New Mexico; southeastern Arizona may have been inundated at that time or may have been slightly emergent. The entire region was emergent and tilted slightly southeast-ward during Early and much of Middle Devonian time, when Cambrian and Ordovician rocks were eroded to progressively lower stratigraphic levels towards the west and north. Consequently, late Middle Devonian and Late Devonian seas, which subsequently covered the region, left their deposits on Middle Cambrian strata in the western part of southeastern Arizona (fig. 2), on Upper Cambrian strata in most of south-eastern Arizona that are progressively younger eastward, on Lower Ordovician strata in most of easternmost Arizona and part of westernmost New Mexico, on Middle and Upper Ordovician strata in a very small part of the same area, and on Silurian strata farther east in southwestern New Mexico.

 

The Cambrian and Ordovician rocks of southeastern Arizona have been affected by many other tectonic and erosional events in post-Devonian time. In some areas these rocks were faulted, intruded, locally mineralized and locally eroded away during Triassic and Jurassic time. They were removed from large areas in the northern part of southeastern Arizona and in part of southwestern New Mexico by erosion during Cretaceous time (fig. 3). They were locally thrust-faulted, folded, intruded, thermally and hydrothermal!), altered and possibly wrench-faulted during Late Cretaceous and early Tertiary (Laramide) time. They were very locally intruded, metamorphosed and apparently faulted again in mid-Tertiary time. Finally, along with other rocks of the region, they were involved in Basin and Range faulting and subsequent local erosion in late Tertiary and Quaternary time.

 

What is now known of the Cambrian and Ordovician rocks of the region has been gleaned from study of the variously faulted and altered rocks in mountain-range outcrops that fortuitously make up about 0.5 percent of the present land surface; very few drill holes exist to provide information on these rocks under the great thicknesses of younger rocks in the intermontane basins.