of the Pioche Shale Formation - A Burgess Shale Type Lagerstätte
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”,
a large trilobite
(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,
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
Shale Formation Location and Stratigraphy
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.
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.
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
(Paedeumias) chiefensis, Olenellus
(Paedeumias) terminatus, Bolbolenellus brevispinus, Nephrolenellus
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
of Yunnan Province, China and just older than the Burgess
Shale Fauna of British Columbia, Canada.