Poleta Formation

The Poleta Formation is a geological unit known for the exceptional fossil preservation in the Indian Springs Lagerstätte, located in eastern California and Nevada.

Occurrence

The formation dates to the Stage 3 of the yet-to-be-ratified Cambrian Series 2; the lower portion base of the formation and the youngest Lagerstätte beds date to the Stage 3 Nevadella trilobite zone (= Laurentian Montezuman stage), with higher beds dating to the Stage 4 Olenellus trilobite zone (= Laurentian Dyeran stage), making the formation the same age as the Sirius Passet and just younger than the Chengjiang. It outcrops in Esmeralda County in western Nevada.

Geology

The Poleta Formation is mainly composed of limestone rocks within its lower and upper sections, with its middle section containing siltstone, quartzite, and some limestone.

Members

The formation contains three members, which are as follows, in ascending age:

• Lower Member: This member is the thickest of the three, getting up to thick. It is primarily composed of medium to light-gray oolitic limestone, containing an abundance of archeocyathid reefs. The limestone is mostly well-bedded, although it is occasionally thin to very thin-bedded. There is also a single inter-stratified layer of greenish-gray or olive-gray siltstone, getting up to thick.

• Middle Member: This member in some areas gets up to thick. It is dominated by siltstones, limestones, and sandstones or quartzites. The siltstone layers are made up of grayish-olive, pale-olive, and locally pale-yellowish-brown or light-brown siltstone, shale, and phylitic silstone, which are composed of subangular silt-sized grains of quartz, embedded in a fine-textured matrix of muscovite or mica clay, and chlorite. The limestone layers are composed of medium-gray to grayish-orange or mottled-gray to light-brown aphanitic to medium crystalline limestone, which is inter-stratified with occasional siltstone. The sandstone or quartzite layers are primarily pale-brown, light-brown and very-pale-orange, very fine to fine grained and commonly evenly laminated. In some areas, these layers can grade into a coarse siltstone. They also contain quartz and feldspars.

• Upper Member: This member is the thinnest, only getting up to thick. It is mostly composed of medium-gray limestone, which is very thin to thin-bedded. It also contains a few archaeocyathid fossils.

Depositional setting

The formation was deposited on an offshore shelf between the wave base and storm base, and experienced storm-related pulses of siliciclastic sediment input. Like many other Burgess Shale-type Lagerstätten, this unit was deposited on the Cordilleran margin of the Laurentian continent; it is among the oldest of the Lagerstätten from this region.

Taphonomy

The modes of preservation of the non-biomineralised material found within the Indian Springs Lagerstätte has been noted to be either clay-mineral replacement, and pyrite or iron oxide staining. This is similar to the modes of preservation seen in the Chengjiang Lagerstätte. The Lagerstätte is also an example of a obrution-type deposition, which is a rapid burial of organisms, with some helicoplacoids being buried alive, still in their upright position. This form of deposition will have also obscured the buried specimens from scavengers.

Paleobiota

Most of the fauna is biomineralized, including brachiopods, hyolithids, trilobites, archeocyathids from the Gold Point Reef locality, and helicoplacoids, which are often articulated. Non-mineralized components of these fossils are also preserved, as are sponges, anomalocaridid parts, and a range of algae and cyanobacteria.

Trace fossils, mainly Planolites, are also common; ichnofossils generally lie on the bedding plane and very few penetrate more than into the sediment. The biota of this formation has been noted to be similar to the Utah Lagerstätten, more so to the Spence Shale.

Arthropoda

Lophotrochozoa

Chancelloriidae

Cnidaria

Porifera (Sponges)

Echniodermata

Foraminifera

Petalonamae

incertae sedis

Flora

Ichnogenera

See also

• List of fossiliferous stratigraphic units in Nevada
• Paleontology in Nevada

References