Gnishik Formation
The Gnishik Formation is a geologic formation in Armenia, Azerbaijan and Iran. It preserves fossils dated to the Wordian age of the Permian period.
The thin-bedded limestones of the formation reach a thickness of in the Julfa section of northwestern Iran and in the Arpa River valley of Armenia. The sediments were deposited in an open marine setting at the northern edge of the Paleo-Tethys Ocean.
The coral Wentzellophyllum gnishikense was named after the formation.
Description
The Gnishik Formation was first formally described by Arakelyan in 1964, based on a section in the Gnishik River valley, after which the formation was named. The formation is mostly represented by thin-bedded, occasionally shaly, dark grey and black bituminous foraminiferal-algal biodetrital limestones. The occasional admixture of clayey and terrigenous material colors the limestones light grey and yellowish. The thin-bedded layers alternate with coarser-bedded compact varieties.The open marine limestones of the formation reach a thickness of in the Julfa section of northwestern Iran. In Iran, the formation overlies the Dorud Formation and is overlain by the Khachik Formation, while in Armenia the formation rests on top of the Asni Formation and is overlain by the Arpa Formation. The Gnishik Formation represents an increase in subsidence rate in the Permian. The sediments were deposited at the northern edge of the Paleo-Tethys Ocean.
The formation is dated to the late Murgabian, which belongs to the Wordian stage of the Middle Permian. The Gnishik Formation is correlated with the Ruteh and Nesen Formations, and the Kuffengian stage of Chinese chronostratigraphy.
Fossil content
Among many others, the following fossils have been reported from the formation:Invertebrates
;TrilobitesPseudophillipsia armenicaPseudophillipsia (Carniphillipsia) paffenholziAcropyge sp.;Cephalopods
;GastropodsPharkidonotus khairliensisNaticopsis cf. spectatusBellerophon sp.Naticopsis sp.Straparollus sp.?Euphemites sp.?Platyceras sp.
;CoralsIpciphyllum araxenseI. flexuosumI. originaleI. ex gr. restriseptatumI. simplexI. subtimoricumI. subelegansLonsdaleia aff. gracilisL. aff. jenningsiLophocarinophyllum lophophyllidumL. pulchrumL. aff. zaphrentoideumMichelinia vesiculosaParaipciphyllum transcaucasicumParawentzelella (Parawentzelella) canaliferaPentaphyllum leptoconicumPraetachylasma alternatumProtomichelinia microstomaSinopora asiaticaSzechuanophyllum szechuanenseUfimia elongataWentzelella armenicaWentzelella (Wentzelella) densicolumnataWentzellophyllum gnishikenseW. parvusW. volziYatsengia asiaticaLophophyllidium sp.Wentzelella sp.
;Crinoids?Poteriocrinites ardjichensis
;RhynchonellataCryptospirifer omeishanensisSeptospirigerella baissalensisTerebratuloidea davidsoniCrenispirifer sp.Septospirigerella sp.
;StrophomenataChonostegoides baissalensisC. ogbinensisEdriosteges poyangensisLeptodus sp.
;StenolaemataMackinneyella dzhulfensis Rhabdomeson floriferumSeptopora lineata
;FusulininaSichotenella cf. sutschanicaYangchienia cf. haydeniNankinella cf. ovataN. orbiculariaStaffella sphaericaS. suborientalisEoverbeekina cf. intermediaPisolina abichiP. subsphaericaLeella ex gr. bellulaSphaerulina crassispiraS. ogbinensisVerbeekina cf. heimiV. verbeekiEopolydiexodina darwasicaE. persicaPolydiexodina chekiangensisChusenella abichiC. doraschamensisDunbarula sp.
;Foraminifera
Flora
;DasycladophyceaeAtractyliopsis fecundusDiplopora americanaEndoina stellataEpimastopora nipponicaGyroporella tenuimarginataJohnsonia spinosaMacroporella spinosaMizzia velebitanaPseudogyroporella grandisPseudovermiporella serbicaStolleyella yabei;RhodophyceaeSuccodium ambiguumS. difficileGymnocodium bellerophonteG. novumPermocalculus forospinusP. fragilisP. piai