The Pimenteira Formation, also known as Pimenteiras Formation, is an Mid to Late Devonian geological formation of the Parnaíba Basin, northeastern Brazil. It mainly outcrops in the states of Piauí and Tocantins. It was deposited in a shallow sea, under the influence of currents and storm waves, as well as fluvio-deltaic influences.
Being part of the Malvinokaffric Realm, it haboured a diverse fauna of invertebrates such as trilobites, brachiopods, bivalves, gastropods, and tentaculids, as well as elamobranches and plant remains.
History The name 'Pimenteira' was first proposed by Small (1914) to ascribe a shale sequence 20 m in thickness close to the municipality of Pimenteiras, Piauí. Caster (1948), reported on the occurrence of several fossils in the area, like trilobites and shark fin bones, and Kegel (1953) later visited the region between Picos and Pimenteiras, collecting specimens and later describing them.
Initially, Kegel (1952) divided the formation into the Itaim and Picos members. Blankennagel (1952) later considered that only the Picos Member is part of the formation, with the Itaim Member being the upper part of the Serra Grande Formation.
Description
Geology The Pimenteiro Formation is made up of dense dark-grey shale sections interlayered with thick to thin sandstone, commonly found are also hummocky cross-stratification. The formation is interpreted as a transgressive system, deposited under a shallow marine environment with the influence of storm waves. It possesses two sub-units, the lower Picos Member and the upper Passagem Member.
The Picos Member is composed by sandstone and intercalated siltstone and shale, with common hummocky cross-stratification, representing the first great Devonian transgression into the Parnaíba Basin. The sandstone is well defined in the old strata, of Middle Devonian origin, but it gradually fizzles out as it approaches the upper layers, of Late Devonian origin. The Passagem Member is composed by thin and micaceous sandstone, with asymptotic and hummocky stratification. This unit represents a system of fluvio-deltaic floods, in which turbulent, episodic flows went beyond the coastal limit.
The formation has outcrops on the states of Piauí and Tocantins. Sediments on the eastern border of the formation are dated be from the Late Eifelian to the Early Givetian, while the central and western portions were dated to be from the Late Eifelian to the Early Famennian.
Paleoenvironment
When it was deposited, the Pimenteiro Formation was a shallow marine environment under the influence of currents and storm waves, as well as fluvio-deltaic influences. The Passagem Member has been proposed to be influenced by episodic floods. During the Mid to Late Devonian, the formation was situated below the antarctic circle, its fauna being part of the endemic Malvinokaffric Realm.
It haboured a diverse fauna of invertebrates such as trilobites, brachiopods, ostracods, bivalves, gastropods, hyolithids, and tentaculids, as well as fish, like elamobranches and acanthodians. Plant remains are also found, as well as ichnofossils.
Fossil Content
Flora
Genus Species Presence Description ImagesArchaeosigillariaA. picosensisPicos Member.A lepidodendrale lycophyte.HaplostigmaH. furqueiPassagem Member.A lycophyte.PaleostigmaP. sewardii Passagem Member.A lepidodendrale lycophyte.ProtolepidodendronP. kegeliPicos Member.A protolepidodendrale lycophyte.SigillariaS. sp. Picos Member.A lepidodendrale lycophyte.SpongiophytonS. sp.Picos Member. A spongiophytace.
Arthropods
Genus SpeciesPresence Description ImagesBurmeisteriaB. noticaPicos Member.A homalonotid trilobite.Eldredgeiacf. E. venustaPicos Member.A calmoniid trilobite.MetacryphaeusM. cf. australis M. kegeli
M. meloi
M. turbeculatusPicos Member. Passagem Member.A calmoniid trilobite.
Echinoderms
Genus Species Presence Description ImagesExaesiodiscusE. dimerocrinosus E. sp. Picos Member, mid-upper Eifelian.A crinoid. Possibly related to Dimerocrinites.LaudonomphalusL. aff. L. tuberosus L. sp. A
L. sp. B
L. sp. C
L. sp. DPicos Member.A flucticharacid crinoid.MarettocrinusM. sp.Picos Member.MonstrocrinusM. incognitusPicos Member, mid-upper Eifelian.A rhodocrinitid crinoid.
Brachipod
Genus Species Presence Description ImagesAmphigeniaA. cf. A. elongataPicos Member.A brachiopod.Australocoelia A. palmataPicos Member.A brachiopod.AustralospiriferA. iheringiPicos Member.A brachiopod.Chonetoidea indet.Passagem Member.A brachiopod.LingulaL. sp. Lingula cf. L. manniPicos Member. Passagem Member.A lingulid brachipod.MonsenetesM. cf. M. boliviensisPicos Member. Passagem Member.A brachiopod.MontsenetesM. carolinaePicos Member.MucrospiriferM. cf. M. pedroanusPicos Member. Passagem Member.A mucrospiriferid brachiopodOrbiculoideaO. sp.Picos Member.A discinid brachiopod.Pleurochonetes P. comstockiPassagem Member.A brachiopod.PustulatiaP. ? curupiraPassagem Member.A brachiopod.Spiriferida indet.Passagem Member.A spiriferida brachipod.Terebratulida indet.Picos Member. Passagem Member.A terebratulida brachiopod.TropidoleptusT. carinatusPicos Member. Passagem Member.A tropidoleptid brachiopod.
Conulariids
Genus Species Presence Description ImagesConulariaC. sp. ?C. cf. C. africanaPicos Member. Passagem Member.A conulariid.CtenoconulariaC. cf. C. undulataPicos Member.A conulariid.
Tentaculites
Genus Species Presence Description ImagesStyolinaS. clavulusPassagem Member.A tentaculite.TentaculitesT. cf. T. eldregianus T. oseryiPassagem Member.A tentaculitid tentaculite.
Annelids
Genus Species Presence Description ImagesAnnulitubusA. fernandesiUppermost Pimenteira Formation.A polychaete annelid.
Molluscs
Genus Species Presence Description ImagesBucanellaB. sp. B aff. B. deiremsi
B. laticarinata
B. quadrilobataPicos Member. Passagem Member.A bucanellid bivalve.CucullellaC. triquetraPicos Member. Passagem Member.A cucullellid bivalve.GrammysioideaG. lundiPassagem Member.A grammysiid bivalve.PalaeoneiloP. sp. P. sp. A
P. sp. BPicos Member. Passagem Member.A malletiid bivalve.PlatycerasP. (Platystoma) bainiPassagem Member.A platyceratid gastropod.PlectonotusP. (Plectonotus) derbyiPicos Member. Passagem Member.A plectonotine gastropod.NuculitesN. sp. N. aff. N. oblongatusPicos Member. Passagem Member.A cucullellid bivalve.SpathellaS. pimentanaPassagem Member.A modiomorphid bivalve.TropidodiscusT. sp.Picos Member.A tropidodiscid gastropod.
Fishes
Genus Species Presence Description ImagesAcanthodii indet.Picos Member.A fin spine. The lack of distinctive features prevents identification.Antarctilamnacf. A. sp.Picos Member.A dorsal spine, resembling A. ultima and A. seriponensis.Chondrichthyan indet.Picos Member.A fin spine, resembling that of Ctenacanthus.Climatiiformes indet. 1Picos Member.A fin spine, similar to that of Ptomacanthus anglicus.Climatiiformes indet. 2Picos Member.Fin spines, similar to that of Climatius enodicosta.CtenacanthusC. kegeli Aff. C. sp.Picos Member.A ctenacanth shark.Groenlandaspididae indet.Picos Member.Dermal plates of an unidentified groenlandaspidid placoderm.MachaeracanthusM. sp.Picos Member.An acanthodian fish.Xenacanth indet.Picos Member.A bicuspid tooth, resembling those of xenacanths.
Ichnofossils
Genus Species Presence Description ImagesArenicolitesPicos Member.Burrow made by some kind of worm.AsteriacitesA. isp. A. stelliformePicos Member.Burrow of sea stars on the sea floor.BifungitesB. munizi B. piauiensisPicos Member. Passagem Member.Possible Annulitubus burrow.ConichnusPicos Member.CruzianaPicos Member. Passagem Member.Trilobite burrows.DiplichnitesPicos Member.Arthropod trackways.GyrophylitesPassagem Member.MerostomichnitesM. piauiensisPicos Member.Eurypterid traces.NeoskolithosN. picosensisPicos Member.PalaeophycusP. tubularisPicos Member.Open burrows, probably made by polychaetes.PhicosiphonP. incertumPicos Member.PlanolitesP. beverleyensisPicos Member.Traces left by the feeding process of worm-like animals.RusophycusPicos Member. Passagem Member.A trilobite resting trace.Scolicia Picos Member.Traces assigned to various groups.SkolithosPicos Member.A burrow.ZoophycusPicos MemberTrace produced by moving and feeding polychaete worms.
References