(Uploaded 2/26/09)

The southwestern margin of the Cretaceous Western Interior Seaway laid extensive deposits in Arizona from the westernmost exposures near Pinedale to the Arizona-New Mexico border along the Mogollon Rim. Consisting of the intertongueing Dakota and Mancos Formations, it ranges in age from Cenomanian to Turnonian. The transgressive-regressive sediments consist of a sequence of sandstones, shales and limestones and represent fluvial, coastal and marine shelf environments.

The stratigraphic units in Arizona have been primarily extended from the New Mexico terminology by Wolfe and others to the following units: A basal unit, consisting of primarily a conglomerate of erosional clasts from the pre cretaceous underlying units of the Triassic Chinle formation, and meta sedimentary deposits from pre-Paleozoic volcanics. This unit has been named the Cliff Dwellers Sandstone of the Dakota formation and is up to 15 meters thick towards the eastern extensions of this study area.

Overlying the Cliff Dwellers Member of the Dakota is an intertongueing series of shaley beds that are three separate formations in New Mexico that pinch out around the Show Low area. They are in ascending sequence, the Clay Mesa Shale, Paguate Sandstone, and the Whitewater Arroyo members of the Mancos Shale. However, around the study area, the three are very thin, and have been combined to one 10 to 20m thick bed called the Clay Mesa/Whitewater Arroyo tongue of the Mancos Shale.

Conformably overlaying the above group is the Twowells Tongue of the Dakota Formation. This is the commonly seen yellow sandstone beds in and around the Show Low area, forming sometimes highly crossbedded massive cliffs and escarpments. It ranges in thickness from 10 to 20 meters in the study area and usually contains several limestone shell lag beds near the top ranging in thickness up to 10 meters and containing dominantly the mollusks Exogyra Levis and Pinna Petrina.

A tertiary basalt cap overlies the previous formation in much of the area especially in the eastern end of the section.

The Cretaceous sediments in Show Low - Pinedale area are locally fossiliferous, containing a dominantly molluscan macrofauna including oysters, ammonites, nautiloids and Pinnas, with an occasional shark or crocodile tooth. Terrestrial deposits include large amounts of permineralized wood, impressions of leaves and plant parts, and freshwater clams.


From the westernmost extension near Pinedale, to east of Show Low, the Cretaceous deposits in Arizona lie with either planar or angular unconformity over Triassic sediments. This can be seen clearly along Highway 260 past Heber, where the Moenkopi redbeds outcrop along numerous roadcuts, followed by an onlapping of sandy Cretaceous sediments. West of Show Low, the colorful Chinle bentonites form badlands topography in many canyons, with the overlying yellowish strata assignable to either the Twowells Sandstone, or the Cliff Dwellers member of the Dakota.

Large truck sized blocks of the basal Cliff Dwellers Member of the Dakota at Mesa Redonda overlay the bluish Triassic badlands of the Chinle formation.The basal is a bit lighter in color than the distant yellow Twowells Sandstone cliffs due to large amounts of included cherts derived from the underlying Chile formation.

Standing on a localized pile of basal conglomerate, Mesa Redonda looms in the distance. The massive yellow cliff is primarily Twowells Sandstone overlain by a deposit of black tertiary basalt as a cap. The slope leading up to the cliff is a mixture of the Cliff Dwellers member overlain by a scree slope of eroded material from the Twowells above.

A closer look at the Mesa Redonda revealing the very thick 15 meter basalt cap eroding onto the lower slopes below the Twowells sandstone. The Clay Mesa/Whitewater Arroyo member is almost completely concealed by this rubble and only outcrops in thinly exposed units.

One of my favorite scenes, the rounded multicolor hills of the Triassic Chinle Formation directly overlain by the blocky ledge of the Cliff Dwellers member of the Dakota. Note the large boulders that have rolled down from the ledge over the the dominantly gray and lavender bentonite (clay) beds of the Chinle.

Trying to drive into an area at the base of the Cliff Dwellers Member of the Dakota formation (basal) can be a challenge sometimes after a rain! Our Jeep and other four wheel drive vehicles are essential at times to be able to fully explore the Mesozoic stratigraphy.

Cliff Dwellers Member (Basal Conglomerate)

The basal Dakota Formation is called the Cliff Dwellers member, as extended from stratigraphically equivalent deposits from Cliff Dwellers Canyon, seven miles northeast of Gallup, New Mexico by Wolfe in 1989. Attributed to deposits of gravelly fluvial stream deposits over wide areas, this non marine unit is too coarse to preserve finely detailed fossils, however occasional pieces of abraded Triassic wood, and bark and limb impressions in some siltstone facies can be found. The clasts contained in the conglomerates are primarily angular to well rounded black and white cherts and silicified wood derived from erosional processes from the underlying Chinle Formation. Occasional Paleozoic fossils in cherts are also mixed in rarely from apparently more distant sources such as rugose corals from the permian Kaibab formation.

The lithology near the center of Show Low is more variable, Wolfe correlated the Cliff Dwellers with a ruddy coarse grained sandstone, with volcanic derived clasts only a meter or so thick. Near Pinedale, the basal members appear to be represented by coarse yellow sandstones over large areas.

East of Show Low a very typical scene of the yellow Cliff Dwellers Member overlain by a black, sometimes columnar basalt that is Tertiary in age. The ground is an erosional alluvium from the cliff, and consists of yellow sand, black and white chert pebbles, and some bentonite deposits.

The basal conglomerates of the Cliff Dweller member can be recognized easily at a distance by the rounded boulder strewn appearance, and lack of fine layering in the stratigraphy. A coarse gravelly texture can often be easily seen in a pair of binoculars as well from a distance. This is east of Show Low.

Sandstones interbedded with coarse conglomerates mark the base of Mesa Redonda east of Show Low. The boulders always weather into pillowy rounded masses, sometimes splitting along the finer grained sandstone bedding planes.

Truck sized block of basal conglomerate with strong crossbedding of low angle types indicating a river or braided stream deposit depositional environment. We found this in the basal Cliff Dwellers member at Mesa Redonda.

Interbedded siltstone deposit at the basal at a locality about 20 miles east of Show Low indicates a much slower flowing water deposition during an apparent hiatus of deposition of the conglomerate. Thin layers in the shale indicated this was probably not a single event, but occurred over an extended period of time.

Close up of cemented basal block of conglomerate. The clasts consisted of cherts and pieces of petrified wood that were very rounded from abrasion from transport. Also, much to our surprise we found Paleozoic fossils in some of the cherts, many of which appeared to be permian Kaibab corals. These clasts were very rounded and well worn as well.

Geologist Doug Wolfe in which we are indebted to for his help in understanding the stratigraphy of the Dakota formation, points out a common erosional feature found in the areas where the Twowells sandstone outcrops - deep beds of sand, relinquished from the sandstone forming stratigraphic features in their own right. These recent deposits are starting to lithify, and can form impressive canyons next to the exposed cliffs of the Twowells.

Block of Cliff Dweller basal with very distinct impressions of plant material. Such fossils found were stems, limbs and bark of trees that were buried rapidly in storm events followed by a quieter period of shale deposition to preserve the finer details. This differs from the many pieces of Triassic wood we find in the basal, which were actually fossils at the time swept into the gravels during deposition.


The Mancos Shale is a thin grey shaley unit near Show Low, and is a deeper water marine unit consisting of three thin intertongueing formations, the Clay Mesa Shale, Paguate Sandstone, and the Whitewater Arroyo shale units. The three units are not easily separable in the field, and will be treated as one here. The unit consists of shales, limestones and sandstones, with baseball to 1m sized concretions surrounding a core of a bit of shell or plant material. In many localities, we often found large barite/calcite accicular nodules near the top of the unit, just under the base of the Twowells Sandstone. The Clay Mesa/Whitewater Arroyo unit can contain a locally rich fossil assemblage of oysters, cephalopods, gastropods, sharks teeth and formainifera.

Paleontologist Steve Fowers examines a triple coal seam on Mesa Redonda. We found this bed in the Clay Mesa/Whitewater Arroyo member of the Mancos and shales above it contained excellent impressions of leaves and other cretaceous plant parts. The coal was a poor lignitic grade.

Near the top of Mesa Redonda, this small 2m exposure of very light colored shales corresponds to an easternmost extension of the Clay Mesa, and Whitewater Arroyo shales of the Mancos.


The main body of the Dakota Formation in this part of Arizona is the Twowells Sandstone. In the Show Low area it is the massive to bedded yellow sandstone units that predominate the Cretaceous deposits in this area. The unit contains both terrestrial and marine deposition with the general trend that in the westernmost extension of the unit such as near Pinedale, the deposits are primarily terrestrial in origin, and contain much petrified wood, ironstones with freshwater clams, and coal and clay beds. As you travel toward the New Mexico border, the unit becomes dominantly marine and deeper water, and contains more limestones and shales. Fossils include in these areas innumerable oysters, bivalves, sharks teeth, and occasional cephalopods.

The Twowells also contains numerous crossbeds, both planar and trough types in many areas. Other sedimentary structures include Thalassinoid burrows, desiccated fossil mud cracks, tooling marks, and bivalves preserved in life position, with both valves intact in the bottom of their burrows. We have never found as of yet any dinosaur or crocodile trackways in the Twowells, but we suspect on the west end of the depositional unit they must be there.

A key marker bed or series of beds to take note of near to top of the Twowells is the oyster lag bed. This deposit, typically 2 to 5m thick is a coquina of oyster shells of mostly Exogyra, but includes Lopha, Inoceramus and many unidentified fragments.

Inside Pinedale Clay mine. Thick deposits of yellow, pink and lavender clays contained within the thick sandstone sequences offer clues to the paleo environment of the cretaceous. A large pond or small lake was apparently present here, funneling local ash from volcanic sources into thin laminar beds of clays and black plant hash.

Just on the rim of the huge clay mine the lavender bentonites can be seen to be overlain by a marine sequence of sandstones that contained a non diversified assemblage of mollusks, and other marine fossils including the trace fossil Thallassinoides - ghost shrimp burrows. Over the top of this layer was a terrestrial sandstone that contained petrified wood and freshwater clams in an ironstone.

Twowells Sandstone east of Show Low and Pinedale. The cretaceous "beach". Much of what I am standing on is sand weathered from nearby exposures of the Twowells, forming new sedimentary layers.

Two part composite of a long Twowells cliff about 20 miles East of Show Low. Closely examine the foreground, and you'll see that the sand that has been liberated from the eroding sandstone now forms a thick ground cover, cut by a deep channel in the foreground.

View from the top of the ridge in the proceeding shot, showing the deeply incised channel in soft sand. This is a classic scene of the typical Twowells outcrop for the area near Show Low. Look carefully, and you can see someone in a white shirt standing on the cliff across the way.

The author stands by his trusty Jeep Wrangler at a scenic locality near Mesa Redonda. The yellow rocks in the background are Cliff Dwellers basal conglomerate overlain by the Twowells Sandstone forming the butte.

Oyster lag bed near the top of the Twowells East of Show Low. The bed is mostly thick limestone made of countless shell bits of Exogyra, Pinna, Lopha and other marine mollusks.

Parked in the Triassic Chinle formation en route to explore Mesa Redonda. Hiking the mesa is an extremely strenuous affair, and is much larger than it looks!

Top of the Twowells Sandstone cliff near Mesa Redonda. Very important - look at the thin 2m wide gray band near the top. This is a oyster lag bed which can be traced out for miles in this area.

Climbing Mesa Redonda, this view shows us in the basal Cliff Dwellers Sandstone of the Dakota. The entire ridge top seen here, which is NOT the top of the mesa is also basal.

A common sedimentary feature in the Twowells are these sandstone concretions, that usually weather out to perfect baseball sized spheres. Most of them contain nothing inside, except circular growth rings or an ironstone center.

A small isolated coal bed is dug out to search for plant fossils in the overlying shales.The main fossil here was not leaves or stems, but numerous root molds and carbonized roots.



Thallassinoides burrows at the clay mine in Pinedale. This is a classic marine fossil from the Paleozoic to present and today marks the presence of marine ghost shrimp, with their "T" and "Y" shaped branches. Some of the burrows had pelleted walls, and are called Ophiomorpha.

Mardy Zimmerman shows off the Cunningtoniceras Arizonaense ammonite she found East of Show Low. Some of the ammonites can reach large sizes in the cretaceous rocks.

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