Pioche Formation

Fossils of the Pioche Shale Formation - A Burgess Shale Type Lagerstätte

The Pioche Formation, also called the Pioche Shale, varies from early to middle Cambrian. Pioche Formation fits the category of a Burgess shale-type Lagerstätte. The Pioche Formation contains a diverse early Cambrian to middle Cambrian biota, including a “Terror Of The Cambrian”, Anomalocaris, a large trilobite fauna, brachiopods, early crinoids (eocrinoids), hyolithids, phyllocarid Crustaceans, porifera (sponges), and basal soft-bodied and shelled arthropods, some of which are placed in an unofficial subphylum called trilobitomorpha. Importantly, the formation also contains soft-bodied fauna such as occurs in the Burgess Shale of Canada and the Maotianshan Shales and Kaili Formation of China, including Anomalocaris, Canadaspis, Emeraldella, Crumillospongia and Tuzoia. The Burgess shale-type Lagerstätten provide a fossil record of the sequence and geographical dispersion of the ostensibly fast development of extant phyla, as well as phyla that may not have survived, in a process called the Cambrian Explosion.

Pioche Shale Formation Location and Stratigraphy

The Pioche Formation is named for exposures to the southeast of town of Pioche, Nevada near the southeast border with Utah. It is composed of both arenaceous and argillaceous shale layers along with limestone layers. Because of the heterogeneous lithology, the name Pioche Formation is more appropriate than Pioche Shale at all localities in eastern Nevada and west-central Utah. Major exposures occur in both Nevada and Utah. The formation is some 210 feet thick at Pioche, Nevada, 200 feet thick at Eureka, Nevada, 125 feet thick in House Range of Utah, and 250 feet thick at Big Cottonwood, Utah. The Pioche Formation overlies the Prospect Mountain Formation and underlies Howell formation (new). Early Cambrian exposures have abundant Olenellus trilobite fauna (Order Redlichiida), a genus restricted to 520 to 540 million years ago. In the House Range, lower part of revised Pioche is predominantly quartzite with interbedded siltstone and shale. Early–Middle Cambrian boundary; fossils from the Early Cambrian are preserved in botryoidal haematite, whereas those from the Middle Cambrian are preserved in carbon films.

Generally speaking, Burgess Shale–type faunas are characterized by a distinct early Paleozoic temporal range, and a distribution in geographic locality and concomitant paleoenvironments. They are found across almost the entire globe, and are concentrated from the lower to Middle Cambrian. Burgess Shale–type formations are characterized by a taxonomically and paleoecologically diverse fauna that has provided profound insights into the relationship between taxa that are well known and those that are enigmatic.

Interestingly, the extinction of trilobite Redliichia suborder Olenellina can be found near the top of the Combined Metals Limestone members of the Pioche Formation. Laurentia at the time was an independent continent, after breaking away from the supercontinent Pannotia in the Precambrian about 540 million years ago. The Olenellid species diversity is very rich below this Laurentian boundary and include: Olenellus fowleri, Olenellus (Paedeumias) chiefensis, Olenellus (Paedeumias) terminatus, Bolbolenellus brevispinus, Nephrolenellus geniculatus, Olenellus gilberti, and Olenellus howelli. The Comet Shale Member of the Pioche Formation in Nevada spans the transition of Early to Middle Cambrian which saw the extinction of the Olenellid trilobites. It is just younger than comparable taxa from the Chengjiana Biota of Yunnan Province, China and just older than the Burgess Shale Fauna of British Columbia, Canada.

Pioche Formation References