The Chalk Canyon Formation

in Central Arizona
Updated 9/3/09


The Chalk Canyon formation is a series of Early Miocene (15 - 22My) aged basaltic volcanic and tuffaceous limestone lacustrine sedimentary deposits in Central Arizona. It is subdivided into an upper and lower member by an unconformity, the lower member, 22 to 17 million years old consisting of alkaline basalts interbedded with crystal and lithic tuffs, and the upper member, which is 17 to 15 million years in age is primarily fluvial and lacustrine (lake deposit) sediments with a subordinate amount of basaltic rocks. Although the Chalk Canyon formation extends from southwest of Lake Pleasant to north of Black Canyon City, it is the upper lacustrine and fluvial member that is of primary interest to sedimentologists and paleontologists.

The upper member can most easily be recognized by the extremely white chalky sediments extending along I17, the main north - south freeway in the state, from about North of Cave Creek up to about a few miles north of Black Canyon City. This area represented a lower depression in Early Miocene times, and quickly filled in with the lake and river sediments we see today as limestones, coarse conglomerates and tuffaceous "shales". The white limy beds are interbedded with thin brown coarse conglomerate deposits, with clasts containing a variety of rocks with the exception of basalts.

Fossils and sedimentary structures

While most of the beds in the chalk canyon formation are a white amorphous and thickly bedded sequence of tuffaceous rocks, there are also many areas with finely bedded laminar beds of very finely grained sediments with sedimentary structures. Most common are mudcracks. We have found many thousands of pure white slabs with fossil mudcracks ranging from 1/16 inch cells up to many feet across. Most are in the range of 1 inch cell size. Along with mudcracks, crossbedding, channel fill deposits, and raindrop impressions were seen.

Fossils are rare. Most abundant are highly bioturbated worm burrow slabs, with small tubular burrows about 2 to 5mm across saturating the surfaces of localized areas. This is the only fossils mentioned in the literature. But we also found other types of trace fossils including Aulichinites - attributed to small gastropods grazing traces, and a few vertebrate toe and foot imprints similar in size to a small carnivore type animal might produce. Its key to note that any areas with mudcracks always indicates a period of subareal exposure, and the possibility of animals leaving traces is always present. One body fossil of a small annelid worm was also found, similar if not identical to Deros sp., a small tube building worm, that lived on rotting vegetation in lakes and streams as today.

Finally, we found the Chalk canyon formation difficult to photograph because of its extreme whiteness. It is even whiter than the tertiary marls in the Verde formation and challenging to get satisfactory images. We also found some interesting heavy manganese minerals in flat sheet-like beds in the formation, and a few areas with abundant cherts and psudomorphs.


Erosion of the soft limy sediments create low rounded hills and cliffs such as this butte in Black Canyon city, just east of the freeway I17. The slopes of the hills in this area contained a large amount of chert and calcedony nodules, along with psudomorphs of calcite after actinolite like mineral crystals.

This image best reveals the color variations in the upper Chalk Canyon formation beds, seen here in Black Canyon city. The topmost bed here is extremely white and consists of a limy facies composed of limestone, chert and subordinate conglomerates.

Massively bedded tuffaceous materials are also very common in the upper member. They are unfossiliferous, and have few sedimentary structures. However, this outcrop also had beds of re-vitrified tuff, seen as a very white porcelain like material, that was glassy in appearance and very hard. Black Canyon City.

Blinding white shaley ledge in a wash, north of New River along Highway I17. At this locality, which is on the southernmost end of the depositional basin of the lacustrine deposits we find sheet like beds of tuffaceous shales, and large numbers of fossil mudcracks.

Interbedded conglomerate bed in the upper member of the formation. This ledge is in a wash west of Black Canyon City, and was overlain by the sheetlike white tuffaceous shaley beds. It appears to be a fluvial deposit of river bed origin, that proceeded the deeper water lacustrine environment.

Close up of the very typical interbedded sandy conglomerate beds in the upper member of the formation. The keys are shown for scale. Interestingly, the clasts do not contain any basalt, and appear to predate the Hickey formation volcanics.

Dick and Mardy Zimmerman examine the hordes of mudcracked shales north of New River. Several of the slabs found here contained what looked like fossil salt crystal casts, in addition to extensive bioturbation from large worms. The rolling hills and ridges seen here are typical of the deposit border localities.

The Author searches for more annelid worm fossils in the dry wash. The thinly bedded shales contained beds of both smooth shales and those with extensive mudcracks. Certainly, some of the slabs made for very nice display pieces.


Close up of some of the extremely white "fossil" mudcracks found in Black Canyon City. Close examination of some of them revealed the toe prints of small carnivores, perhaps cat-like in nature. The material weathers to a very chalky consistency, and you soon realize you are covered with a white powder!

Spectacular fossil mudcracks found north of New River. This was at the bottom of a dry wash, and was over 50 feet long. This represents and ancient shore environment with at least temporary subareal exposure. (above water) No trace fossils were found here, indicating that the moist period was very short.

Primarily square shaped mudcracks from north of New River. Although these were about 1/2 inch in size, the smallest were only 1/16 inch across!

"Extreme bioturbation" best describes the worm burrows found in the Chalk Canyon Formation. This three inch slab was found on the top of one the rolling hills found so commonly in the formation near New River. This is the most common fossil in the formation.

Gastropod trackway Aulichnites found north of New River. Close examination will show the double ridged tracks of the snail approaching what appears to be the imprints of some plant material, perhaps grasses either in the water or just on the shoreline.

The two vertebrate toe impressions seen here (two dimples on right edge) are a sure sign that some 15 million years ago, a small carnivore type mammal crossed this mudflat. Although quite rare in the Chalk Canyon formation, these trace fossils provide a great deal of paleo environmental data for researchers.

Fossil foot print of a small carnivore in a huge slab bed in a dry wash in Black Canyon City. Although toe prints were more common, we did find several complete footprints in other areas. They are difficult as you see here, to photograph unless the sun angle is low and just the right angle.

A small tube building annelid worm, similar to Deros sp., found in the deposits north of New River. The "worm" itself is about 1/2 inch long, and shows remarkable detail. It is a carbonized film from the upper member of the Chalk Canyon formation - a simple imprint of a small creature that lived 15 million years ago.

This latest find, from a new outcrop south of Black Canyon City are fossil plant rhizomes, the root molds of land plants that were later infilled with siliclastic ash. These are the only plant fossils we have ever found in the Chalk Canyon formation at this point, so I decieded to add them in as an update.

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