Fort Apache Limestone outcrop along the Highland Trail

Fossils of The Permian

Fort Apache Limestone

East of Payson

  Trilobite pygidium - One of the rarest fossils found

Updated  11/30/17
 

 Formation Description

Nestled between two red beds of the upper Schnebly Hill Formation is a marine sequence consisting of a colorless gray and massively bedded limestone. The unit varies in thickness from about 100 feet at Fort Apache Arizona, and pinches out to zero west of Sedona. The Fort Apache Limestone is a Permian sequence representing a marine transgression onto the Schnebly Hill dunes which deposited a thin bed of limestone. Fossils are very scarce in this formation, in Strawberry for example, the massive beds visible on the north west end of town while driving north on Highway 87 are completely unfossiliferous. On the Indian reservation however, the typical Permian fossils begin to appear, including large brachiopods, sea urchin material, gastropods, and occasional cephalopods. There are few outcrops of this formation OFF the reservation (which you can collect on) which contain fossils however, and we have collected at these sites some remarkable marine invertebrates.

Fossil collection and preservation

The Fort Apache Limestone is a light gray in color and grades from nearly pure carbonate rock with some sand and silt on the reservation, to a much more dolomitic limestone (doesn't do well in acid) as you go westward towards Sedona. The amount of terriginous material in the limestone also increases as you go westward. Rock is collected only if it shows a surface weathering of small silicified micro fossils such as larger clams pieces and urchin spines. You cannot be certain that any good fossils will be had from a given rock in the field, so many pieces are collected from a variety of locations to increase the odds of getting a richer sampling.

Back home, The rock is put in plastic bins and filled with water so they are just covered. Then the 30% muriatic acid available from ACE Hardware or a building supply (Dont get the odor free stuff, its no good) such that the dilution is roughly one part acid to three parts water. The acid will attack the limestone vigorously and free the enclosed silicified fossils. Any calcified or phosphatized fossils are gonners. You wont find conodonts, or sharks teeth! After 24 hours, the bubbling stops and the water is clear and neutralized. Dump out the clear top liquid and if required add more water and acid until the rock is all dissolved. Finally, we use three different size metal sieves to wash out the dirt and sand, but leave the tiny micro fossils and anything larger. The washed material is set out to dry in a Teflon coated baking pan in the Sun.

Sorting the fossils is done with an optivisor flip magnifier for the larger material, but many hours can be spent on going through the finer material under the stereo microscope - One pea sized scoop at a time. Fossils are picked out with fine tweezers or a wet toothpick for the most delicate such as some of the ostracod shells. Fossils are then sorted and placed in small snap lid pill bottles from Walgreens. Larger specimens are put in small cardboard boxes or directly on the fossil shelf. The smallest fossils are a fraction of a pinhead in size, and the largest one so far is a huge 10 inch spiral nautiloid.

Articles on Fort Apache Limestone Fossils:

 
 Ostracods Hundreds of these small crustaceans in several different types. The article can be found here.

 Trilobites Half a dozen pygidiums, free cheeks, glabellas, and numerous pieces have been found. The article can be found here.

 Pelecypods Most are very small, but this is the second most common complete fossil found in the formation. But large amounts of shell fragments dominate the rocks. The article can be found here.

 Echinoids (Urchins) Spines, plates of possibly three species were found here, all of small 1 - 2 inch sized urchins. Preservation was liminted by crystalization but some good spines were located in the silicified fines. The article can be found here.

 Microconchids These annelid tubes are a minor part of the fossil assemblage however at this new locality, we found scores of them in the acid residues. Although they look much like coiled gastropods, they are annelid worms homes. The article can be found here.

 Bryozoans Three types of small bryozoans have been found in our seived finds, and as are all invertebrates from this formation - They are very small and fragmented. But they help reveal the story of the conditions during this time in greater detail. The article can be found here.
 Porifera (Sponges) Sponges are very rare in our searches, only a few specimens were found and all are calcareous types. One specimen was a fragment of either a flat fan shaped sponge or a large vase shaped tube. The other specimens are flattened tubes that were orignally round and quite hollow. The article can be found here.
 Scaphopods (Tusk Shells) One of the larger macro fossils found in the limestones come in two varieties, smooth and ribbed. The article can be found here.
 Brachipods Dominated by productids, this minor faunal element shows once again that nearly every invertebrate found is diminuative in size compared to the overlying Permian Kaibab formation. A huge influx of terriginous sediment is most likely the cause. The article can be found here.
Permian Scaphopods II Juvenile Teleoconchs A not so rare and enigmatic fossil which turned out to be something very special! The article can be found here.
 Echinoderm Stem Ossicles Very few crinoid and blastoid stems were found in this formation. Here is the pictorial of our finds. The article can be found here.
 Cnidarians (Rugose Corals) We found ONE tiny rugose coral in our searches in the acid fines. That makes this one of the most precious finds in this formation. Its a beauty though, and had a secret friend inside... The article can be found here.
 Nautiloids Cephalopods are a minor but very important part of the Fort Apache Sea fauna. They are epifaunal and would have been floating above the muddy bottom which was a deterent to many invertebrates. The article can be found here.
 Straparollus Gastropods This is the only planispiral gastropod we found in this formation. Two different species were found in a huge range of sizes from pinhead sized up to 6 inches! The article can be found here.
Gastrpods and Monoplacophorans A massive project, since the dominant fossil by far were countless pin head sized gastropods. 22 types were found, many of which we were able to identify with Winters monograph. The article can be found here.
 Tiny Pectens, Branching Bryozoans, and Stellate Sponges A few unusual and really interesting finds from the Formation. The article can be found here.
 Quartz Microcrystals Over a hundred pinhead sized crystals were found in with the fossils! The article can be found here.
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