The Poleta Formation is a geological unit known for the exceptional fossil preservation in the Indian Springs Lagerstätte, located in eastern California and Nevada.[2]
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.[1]
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 1,000 ft (300 m) thick. It is primarily composed of medium to light-gray oolitic limestone, containing an abundance of archeocyathid reefs.[3] The limestone 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 100 ft (30 m) thick.[1]
Middle Member: This member in some areas gets up to 600 ft (180 m) 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.[1]
Upper Member: This member is the thinnest, only getting up to 100 ft (30 m) thick. It is mostly composed of medium-gray limestone, which is very thin to thin-bedded. It also contains a few archaeocyathid fossils.[1]
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.[2]
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.[2]
Most of the fauna is biomineralized, including brachiopods, hyolithids, trilobites,[4] archeocyathids from the Gold Point Reef locality,[5][3] and helicoplacoids, which are often articulated.[2] Non-mineralized components of these fossils are also preserved, as are sponges, anomalocaridid parts, and a range of algae and cyanobacteria.[2]
Trace fossils, mainly Planolites, are also common; ichnofossils generally lie on the bedding plane and very few penetrate more than 1 millimetre (0.039 in) into the sediment. The biota of this formation has been noted to be similar to the Utah Lagerstätten, more so to the Spence Shale.[2]
Platysolenites-like tubular test, may be a taphonomic variant of Platysolenites. Also shows a resemblance to Spirosolenites spiralis, Although it differs to much in other ways when compared to Platysolenites and Spirosolenites.
Sessile frondose organism, although assignment has been questioned based on the possibility that known specimens could be referred to other known Cambrian frondose organisms.[17]
Valve-shaped fossil. Noted to be a mold of the shell, and poor preservation hinders taxonomic placement, although general morphology suggests it to be a mollusc.
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