A Tour of Selected Marine and Lacustrine

Formations in Arizona

By Chris Schur

A former Arizona Governor once said that there are only two primary disciplines that amateurs can make significant contributions in science: Astronomy, and Paleontology. And our state offers a golden opportunity to meet that challenge. For Paleontology, the reason is simple: While most of this States geological exploration is driven by mining of ores and consumable resources, paleontological research is usually a secondary byproduct of this exploration. Because of this, much of the initial early exploration of the various geologic formations that has taken place in this state was done in the early 1900's.

Recently, very little has been accomplished, and a strict study of the paleofauna and flora of our state is long overdue. Because of this lack of work done here, the amateur can make a very significant contribution to the science of paleontology in Arizona in particular. Virtually anything you study or write about is often new and is eagerly consumed by the professional community. With that said, I'll give an overview of the states geology as it pertains to sedimentary rocks and the fossils they contain, and a few points for new explorers in the field.

The Three Geologic Provinces

Our state is divided by climate into three bands running roughly east-west called "provinces". The northernmost province is the Colorado Plateau Province and includes The Grand Canyon, Flagstaff and onward north to the border. It ranges from 4000 to 9000 feet in elevation, and contains numerous cliff like outcrops and hills which contain a mixture of both marine and terrestrial components in the sedimentary record. Ranging from mid-Paleozoic to Recent in age, the sedimentary outcrops cover an enormous area of land.

The Central Highlands Province is bounded on the north side by the area south of the Mogollon Rim, which is the southern end of the Colorado Plateau Province, and contains Camp Verde, Prescott, Globe, Payson, and its eastward extension in Safford. This is a transitional region between the southerly Basin and Range Province and the Colorado Plateau Province and contains characteristics of both. Low rounded hills give way to jagged volcanic peaks to the south. The northern portion of this province contains the sediments that contain the fossils.

The Basin and Range Province extends from just north of Phoenix to the southern border of the state. Primarily volcanic, this area is characterized by jagged mountains and peaks, with much mineralization. Because of this, most of the geologic study in our state has been in this area, where ore concentrations are high. But also much uplifting has occurred, and a wide range of Paleozoic to Mesozoic deposits occurs here. Most of the sedimentary rocks lay in the south eastern part of the province including areas such as Tucson, Bisbee, Tombstone and the southern border.

Exploring Ariozna

I won't waste your time on the basics of geological formations and facies and get right to it! Collection of fossils from the allowable areas (See Collecting in Arizona) can be a very rewarding and scientifically productive endeavor. While we don't often have the magnificent whole crinoid preservation such as in Indiana, or the complete articulated trilobite assemblages that are common in New York, we DO have a wide variety of marine and terrestrial fossil types, some of which are found NO WHERE ELSE in the world. The oldest sedimentary outcrops in the state reflect a Precambrian age with algal mats and stromatolites in the 1.2 Billion year old Mescal Limestone found in isolated outcrops in northern Arizona. Although fossil stromatolites doesn't sound like the most exciting thing in the world, you have to remember just how old these fossils are. Think of a time over a billion years ago, with nothing but algae and bacteria as the dominant life forms on the planet. You can visit the remains of this ancient ocean scene in our state.

The Cambrian is not very widespread either. You have the Tapeats Sandstone in the Central Highlands and in the northern part of the state which contains some trace fossils, such as Scolithos and Correphioides burrows, and in the Grand Canyon, some areas have prominent Caruziana - traces generally attributed to trilobites. The Bright Angel Shale, outcropping in the Grand Canyon and in isolated patches to the south contain abundant trace fossils as well, and locally abundant trilobite remains such as Zacanthoides, the enigmatic Hyalithids, and sponges in great numbers. In the Basin and Range Province, the Abrigo Limestone and Bolsa Quartzite contains locally abundant trilobite hash and parts, hyolithids and some Burgess shale type remains.

The Ordovician is only present in the Basin and Range Province, and is contained in a few small isolated outcrops of the Longfellow Limestone near Morenci. Gastropods are reported from these sediments. We have also found gastropods and orthoceras nautiloids in the isolated outcrops of the El Paso Limestone, near Dos Cabezos in southern Arizona.

The Devonian is widespread throughout Arizona, and is dominated by the Martin Formation. Huge numbers of silicified corals, mollusks, brachiopods bryozoans, and other reef type colonial organisms are found. The Martin exists in both the Basin and Range Province and Central Highlands, with a few very isolated outcrops that have been tectonically uplifted occurring in the Colorado Plateau Province. (Mount Elden in Flagstaff has a small Martin outcrop containing fish material) The Portal formation in south eastern Arizona contains very small crinoids, and some trilobite hash.

The Mississippian is widespread throughout Arizona. Two primary formations are present, Which in reality are the same paleo-ocean, but named differently because of the two parts of the state that they outcrop. The northern extension of the Mississippian sea is the Redwall limestone, occupying much of the slopes of the Mogollon Rim. It is extremely fossilliferous, and contains mainly molds of brachs, bryozoans, trilobite pygidia, corals - both tabulate and rugose, fish material, mollusks and large numbers of echinoderm parts including some calyxes unique to Arizona. This is the best formation to begin fossil hunting in the state because the Redwall can't be beat for its variety and abundance. The southern extension of the Mississippian sea is the Escabrosa Limestone. it is much less fossiliferous but contains exactly the same fauna as the Redwall, but far less in cherts. Despite this, we have found some spectacular fossils in the Escabrosa including giant stromatoporoids, complete crinoid calyxes with feathery arms, urchin material and even a complete edrioasteroid.

Pennsylvanian outcrops are dominated by the Naco Limestone. Consisting of both shales and limestones with some dolomite. It too can be richly fossiliferous in isolated areas. Crinoid calyxes, bryozoans, brachs, some mollusks, and hordes of crinoid stems are dominant. We have also found the extremely rare Edrioasteroids and plenty of fish material in this formation. Most of the bright red cherts are the remains of softball sized pancake shaped glass sponges which can be found in abundance on the Mogollon Rim. The edge of the Mogollon Rim is the prime place in northern Arizona to find the Naco, between the Redwall and the Permian redbeds. In southern Arizona, the Naco forms awesome cliffs and escarpments, but contains less fossils. Some great outcrops are found near Winkelman.

Permian is a mixed combination of both terrestrial and marine formations. the Mogollon Rim area is covered with the huge redbeds associated with the Hermit Shale, Supai Group, and Schnebly Hill Formations. They are terrestrial in origin and contain beds of shale and mudstones with some occasional excellent trace fossils and plant material. The Supai and Hermit Formation contains some of the most unique fossils in the world, including vertebrate trackways, insect and arthropod trace fossils, and much conifer and fern impressions. Even the footprints of Dimetrodon have been spotted near Sedona. A marine bed, the Fort Apache Limestone is a band, less than 100 feet thick that lies within the Schnebly Hill sandstone, and is primarily a sparsely fossiliferous dolomite unit that contains a few mollusks and urchin spines in the areas on the reservation near Fort Apache Arizona and off the Reservation on Highway 260.

The Permian Kaibab Limestone however is highly fossliferous, and outcrops in many areas on the Colorado Plateau. Containing two or three discreet zones of paleo-fauna, that includes giant productids, hordes of fossil glass sponges (Leucon Hexactanellids), chertized shrimp burrows, brachs, mollusks including giant nautiloids, and a true prize for any state - permian trilobites! (Anisopyge sp.) Although the preservation of the specimens in dolomite makes extraction difficult, this is surely offset by the rich faunal assemblage.

Above the Permian lies a number of marine and terrestrial formations that include the Cretaceous Dakota and Mancos Shales containing giant (how does 2 feet in diameter sound?!) ammonites and nautiloids, hordes of oysters and clams, and some great trace fossils. The Cretaceous also outcrops in south eastern Arizona as the Bisbee Group. We have collected ammoites, mollusks and even cretaceous sand dollars from these outcrops, which are extensive and locally very fossilliferous.

The next marine sequence lies also on the Colorado Plateau, and is a very isolated outcrop of Triassic marine material near Fredonia on the northern border of the state. We have found in this highly sandy Virgin limestone scaphopods, bellerophontid gastropods and red chertized clams. Nearly all of the fossils found were preserved as red chert casts in a limey matrix that contained huge amounts of pure white beach sand.

And to complete our tour of the marine sedimentary formations of Arizona, the Tertiary Bouse-Wash Formation near Yuma is a tertiary marine sequence, little studied. I have seen sand dollars and urchin parts come out of the formation which is limited in extent to very isolated outcrops.

The Verde formation is a Tertiary lacustrine deposit in Clarkdale and Camp Verde. Characterized by magnificent cliffs and arroyos, the sparsely fossiliferous deposit contains vertebrate trackways, ostracods, and turriculate form gastropods. It is time equivelent for the most part with the Payson Basin, Tonto Basin and San Carlos Basin formations which contian many of the same fossils.

Numerous other Tertiary lacustrine sequences can be found in Arizona. In the north, the extensive Bidahochi formation with its fish fossils, Owl Rock member of the Chinle, Blue Ridge Tertiary up on the Rim filled with fossil cattails and rhizomes, and the magnificent Chalk Canyon formation near Black Canyon City with its huge slabs of fossilized mud cracks.

A surprising number of Holocene and "Recentocene" fossils have been explored by us as well. These consist primarily of travertine deposits down in Fossil Creek which contain molds of leaves, stems and strangely enough - mosses. A few localities below the Martin formation on the Rim also contain travertineized fossils.

This completes our tour of the basic marine and lacustrine sedimentary sequences in Arizona. With this background knowledge, we can now put all of the formations in context, and get a real understanding of the sediments you will be collecting in.

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