The Naco Formation of the Mogollon Rim represents a Pennsylvanian marine sequence in the transitional zone on the southern tip of the Colorado Plateau. It is locally highly fossiliferous, and contains a host of vertebrate and invertebrate fossils including brachiopods, bryozoans, tabulate and rugose corals, sponges, crinoids, forams, gastropods, edrioasteroids, and such vertebrate remains as fish bone and sharks teeth.
In most locations, it is underlain by the marine Mississippian Redwall Limestone, and overlain by the red beds of the Hermit Shale and Schnebly Hill formation. The Naco Formation is subdivided into three informal members: A thin basal member consisting chiefly of reddish insoluble residues that accumulated on an ancient karst topography developed on the underlying Redwall limestone, a middle member consisting of alternating limestones and grayish shales or mud stones, a upper member consisting of marine shales and limestones that inter-tongue with reddish clastics characteristic of the partly non marine overlying Supai red beds.
A great deal of correlation work has been done in recent years, correlating the Naco Formation on the Rim, with the Naco Group in Southern and Southeastern Arizona with the included Earp and Horquilla formations. The western end of the Rim, extending off into Central Arizona west of Sedona has been correlated with the Grand Canyon Lower Supai Groups Pennsylvanian formations of the upper Manakacha and Wescogame formations. These will be covered under a separate section.
The Three Members of the Naco Formation:
Alpha Member - The lower member of the Naco is up to 90 feet thick, and is composed of reddish brown insoluble residues that developed on the karst topography of the upper surface of the Redwall. The Naco alpha member was deposited during early Desmoinesian time, which consolidated the insoluble Redwall cherts and corals into the basal unit.
Beta Member - The middle member of the Naco is the most fossiliferous member of the formation, and includes such famous localities as the Kohls Ranch site, and along the Control Road 64. Up to 900 feet thick, it is the primary member found in on the Rim. It was deposited during mid Desmoinesian, and contains two sub facies, a near shore limestone/shale facies characterized by alternating of ledge forming limestones and slope forming grayish shales and mud stones. The second subfacies is a cliff forming limestone formed further off shore.
Gamma Member - The upper member, up to 300 feet thick, consists of reddish brown clastics inter bedded with limestones that marks the transition between the carbonate of the beta member, and the red beds of the Hermit Shale and Schnebly Hill formations. Desmoinesian in age, and sparsely fossiliferous, it marks the Pennsylvanian Permian boundary on the Mogollon Rim.
North of Payson, the rolling hills and ridges yield an excellent opportunity to examine the Beta member of the Naco. This image, I shot from an adjacent ridge about a mile north of the East Fork of the Verde River. Most of the lower slopes are Mississippian Redwall Limestone, capped unconformably by the Pennsylvanian Naco formation. The pair of limestone beds near the upper part of the ridge is the Naco, and about halfway down the slope it becomes Mississippian. The top of the mesa is capped by tertiary basalts dated in some places at 12 million years old. The lower slopes contain cherts in the Redwall with marine fossils, and the upper slopes we found contained red chertized burrows, probably from a shrimp like arthropod.
This wash just off the Highway 260 East of Payson contains a rich crinoidal fauna preserved in red and orange cherts in the gray limestones. This bed of red chert, extends over much of the middle member of the Naco in the area, and can be traced to the entrance of the Control Road 64 off the Beeline Highway, and in the other direction, toward Diamond Point. While most fossils are fragmentary, a large percentage are fully articulated stems, and calyxes also preserved in red chert.
Here is a close-up of the ground at the above site, showing a typical softball sized boulder riddled with red chertized crinoidal material. Note the ground is also speckled with the red chertized remains, which also include tubular burrows and loose basal cups.
The Naco formation at the bottom of Colcord Canyon, off of Highway 260 East of Payson. This site, also the middle member of the Naco contains a rich assemblage of brachiopods in the limestones, and much crinoidal material. The predominant brachiopod, Composita Subtilita, is about half an inch in size, and usually preserved in huge numbers in the limestones like plums in a pudding.
Also taken at Colcord Canyon, this shows the lumpy cottage cheese constancy of the limestones typical of the Naco on the Mogollon Rim. This curdled appearance is diagnostic as well as the huge numbers of Composita brachiopods found.
Numerous thin bedded red chert beds define the Beta member of the Naco Formation on the Mogollon Rim. Many are obviously in fillings of low areas and cavities under the sea bottom. Here, at this site just off of the Control Road 64 near Payson, a burrow formed by an animal the size of a small shrimp made his tunnel in the sediment, which later filled with the silica gel to form the cherts. The source of the silica varies, however, while much is the remains of radiolarians, in the case of the Naco, it appears more likely that hexactanellid sponges (glass sponges) were the culprit. We do find occasional whole pancake shaped sponges, and always in the red cherts on the Mogollon Rim.
There are many spots to pull off Forest Road 200 in search of Naco fossils. At one locality along the roadway, we found huge numbers of red chertized sea urchin spines (Echinoids) about the size of a toothpick literally covering the gray limestone boulders. Although the process of coating the fossils with red chert tends to blur fine details, we have found many fine specimens here. The area appears devoid of all other fossils, just a gray micrite matrix. Such low diversity but abundant assemblages suggest stressful living conditions (i.e., hypersalinity) to many organisms or very limited food sources.
Close up of one of the limestone boulders at the above site, with large numbers of red chertized urchin spines. We also found the basal plates, and some few crinoid stems, however, nearly all fossils were chertized, and details were lacking for generic identification.
Searching for crinoids in the red chertized beds off of Control Road 64, near Diamond Point. This Beta member of the Naco site had large articulated crinoid stems, and complete crinoids with fully articulated arms and calyxes. Strangely enough, while the crinoids were preserved in red cherts, the brachiopods were simply calcified in the limestones.
This is the type of material we found in the red cherts at the Diamond Point site above. Giant garden hose sized crinoid stems were common. the red cherts in this part of the Naco forms thin beds and lenses in the gray limestones and is often extremely fossiliferous, weathering out fully by solution to concentrate the cherts - and fossils.
The "Famous Naco Site" 1 mile west of Kohls ranch. This is the Beta member of the Naco, and is one of the most popular fossil collecting sites in the state of Arizona! Just ahead of my Jeep, the old road bed winds south, and reconnects with the main Highway 260. The fossils are located in both the slope forming shales and in the limestone cliffs themselves. Here, you can find hordes of brachiopods including productids, fenestrate and tubular bryozoans, crinoid stems, calyxes, conularids and fish teeth. This site was destroyed and burried over with granite a few years back, and is now unavailable.
A group of fossil hunters explore the slopes at the Famous Naco Site near Kohls ranch. Many of the large boulders in the foreground and on the flats of the old road contain small pieces of bone and sharks teeth, and the slopes are filthy rich with brachiopods.
Fossil hunter Mardy Zimermann collects invertebrates at the Famous Naco Site east of Payson. The old roadbed which she walks on was once a road metal quarry used to grind up and fill in low spots on the road for construction in the 50s.