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. It is overlain by the Harkless Formation, whilst it is underlain by the Campito Formation.
Members The formation contains three members, which are as follows, in ascending stratigraphic order (lowest to highest):
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
GenusSpeciesNotesImagesMesonacis M. fremontiOlenellid trilobite, originally described as Fremonita.Laudonia Laudonia sp.Olenellid trilobite.Olenellus O. clarkiOlenellid trilobite.Elliptocephala E. praenuntiusOlenellid trilobite.Teresellus Teresellus sp. T. goldfieldensisOlenellid rilobite.Nevadella N. eucharisNevadid trilobite.Nevadia N. parvoconicaNevadid trilobite.Judomia (?) Judomia (?) sp.Nevadid trilobite.Holmiella H. falxHolmiid trilobite.Holmia Holmia sp.Holmiid trilobite.Palmettaspis P. consorta P. lidensisHolmiid trilobite.Grandinasus Grandinasus sp. G. argentus G. auricampusHolmiid trilobite.Holmidd (?) ???Undetermined holmiid trilobite.Keeleaspis (?) K. (?) terhaariCorynexochid trilobite.Polliaxis P. hanseniCorynexochid trilobite.Geraldinella Geraldinella sp.Fallotaspidoid trilobite.Anomalocarididae ???Anomalocaridid sclerites and fragments.
Lophotrochozoa
GenusSpeciesNotesImagesMicrocornus Microcornus sp.Lophotrochozoan hyolith.Hyolithellus (?) Hyolithellus (?) sp.Lophotrochozoan hyolith.Obolella Obolellasp.Obelellid brachiopod.Kutorgina K. perugataKutorginid brachiopod.Lingulella Lingulella spp.Obolid brachiopod.Spinulothele S. dubiaAcrotheloid brachiopod.Pompeckium (?) P. argenteum (?)Eoorthid brachiopod.Mickwitzia M. occidensMickwitziid brachiopod.Swantonia S. weeksiBrachiopod of uncertain affinities.Lingulid ???Lingulid brachiopod.Obollelid ???Obollelid brachiopod, muscle scars preserved.Hyolithes H. princepsLophotrochozoan hyolith.Hyolithellus Hyolithellus (?) sp. H. insolitusLophotrochozoan hyolith.Hyolithid ???Lophotrochozoan hyolithids, too poorly preserved for taxonomic placement.Orthothecida ???Orthothecid hyoliths, too poorly preserved for taxonomic placement.Lapworthella (?) Lapworthella (?) sp.Lophotrochozoan tommotid.
Chancelloriidae
GenusSpeciesNotesImagesAllonia Allonia sp.Chancelloriid organism.Chancelloria Chancelloria sp.Chancelloriid organism.Chancelloriid ???Chancelloriid spicules.
Cnidaria
GenusSpeciesNotesImagesPaiutitubulites P. variabilis P. durhamiAnthozoan cnidarian.Cambrotubulites Cambrotubulites sp. C. triseptaAnthozoan cnidarian.Sphenothallus Sphenothallus sp.Cnidarian conulariid.Genus A Genus A sp.Anthozoan cnidarian, similar interior to Cambrotubulites and Paiutitubulites, though differs in having five septa.
Porifera (Sponges)
GenusSpeciesNotesImagesArchaeocyathus A. arborensisArcheocyathid sponge.Claruscoscinus Claruscoscinus sp.Archeocyathid sponge.Protopharetra P. junensisArcheocyathid sponge.Gatagacyathus Gatagacyathus sp.Archeocyathid sponge.Ethmophyllum E. whitneyiArcheocyathid sponge.Williamicyathus W. colvillensisArcheocyathid sponge.Robustocyathellus Robustocyathellus sp.Archeocyathid sponge.Paranacyathus Paranacyathus sp.Archeocyathid sponge.Protopharetra Protopharetra sp.Archeocyathid sponge.Metacyathellus (?) Metacyathellus (?) sp.Archeocyathid sponge.Cordilleracyathus (?) Cordilleracyathus (?) sp.Archeocyathid sponge.Wapkia (?) Wapkia (?) sp.Wapkiid demosponge.
Echniodermata
GenusSpeciesNotesImagesHelicoplacus Helicoplacus sp. H. gilbertiOblong echinoderm.Eocystites Eocystites sp.Echinoderm.
Foraminifera
GenusSpeciesNotesImagesPlatysolenitesP. antiquissimusAgglutinated hyperamminid foraminiferan.Problematicum A ???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.
Petalonamae
GenusSpeciesNotesImagesSwartpuntiaSwartpuntia sp.Sessile frondose organism, although assignment has been questioned based on the possibility that known specimens could be referred to other known Cambrian frondose organisms.
incertae sedis
GenusSpeciesNotesImagesOnuphionella O. claytonensisTubular fossil.Salterella Salterella sp.Tubular salterellid fossil.Westgardia W. giganteaValve-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.
Flora
GenusSpeciesNotesImagesCyanobacteria (?) ???Calcareous cyanobacteria.Renalcis Renalcis sp.Cyanophyte cyanobacteria.
Ichnogenera
GenusSpeciesNotesImagesCruzianaCruziana sp.Trails.PlanolitesPlanolites sp.Burrows.ScoliciaScolicia sp.Burrows.RusophycusRusophycus sp.Resting trace of trilobites.BergaueriaBergaueria sp.Resting trace of cnidarians.LaevicyclusLaevicyclus sp.Traces.Psammichnites (?)Psammichnites (?) sp.Burrows.MonocraterionMonocraterion sp.Burrows.SkolithosSkolithos sp.Burrows.
See also
List of fossiliferous stratigraphic units in Nevada Paleontology in Nevada
References