Acritarch Zones Research Articles

Acritarch biostratigraphy of the Lower-Middle Cambrian boundary in the Iberian Chains, province of Soria, northeastern Spain

The Lower and Middle Cambrian age acritarch assemblages are reported from a continuous succession of the siliciclastic-carbonate Ribota, Huermeda and Daroca Formations in the Iberian Chains, northeastern Spain. One new genus and four new species are described, one of them in open nomenclature. The microfossil assemblages allow to recognize the Lower Cambrian Heliosphaeridium dissimilare-Skiagia ciliosa and Volkovia dentifera-­Liepaina plana acritarch zones, and to discuss an informal lowermost Middle Cambrian acritarch zone. These zones correspond respectively to the Holmia kjerulfi, Prorolenus and Acadoparadoxides oelandicus trilobite zones, identified in the Acado-Baltic faunal province. The first appearance of Eliasum llaniscum, Celtiberium dedalinum and Retisphaeridium dichamerum, which are age-diagnostic for the Middle Cambrian, in the upper Daroca Formation is interpreted to point the Lower-Middle Cambrian boundary. This stratigraphic level underlies the previously established trilobite-based boundary. The depositional environments and biostratigraphy of the concerned strata are discussed.

A Walcottella-like bradoriid (Arthropoda) from the lower Cambrian of Sweden

A probable new bradoriid genus and species, represented by a single specimen, is described tentatively as Walcottella? aff. apicalis Ulrich & Bassler from lower Cambrian strata of Sweden. The specimen comes from the Bergmyrhobben section west of Storuman in Västerbotten County, northern Sweden. It was found in mudstone in the lower part of the Grammajukku Formation, co-occurring with a rich acritarch assemblage of the Skiagia ornata-Fimbriaglomerella membranacea acritarch Zone that corresponds to the Schmidtiellus trilobite Zone. The specimen represents one of the oldest bradoriids known from the paleocontinent Baltica. Characteristics of the specimen are a dorsomedian to mid-dorsal acuminate nod and a flattened shell border along the free margin, the latter feature unknown from other bradoriid taxa. The specimen occurs at the same level where the first trilobites were found in the investigated section. The biogeographic affinities of the Baltic bradoriid genera to the bradoriid fauna of Avalonia, western Gondwana and Laurentia are reviewed and discussed.

First biostratigraphic (palynological) dating of Middle and Late Cambrian strata in the subsurface of northwestern Algeria, North Africa: Implications for regional stratigraphy

The first biostratigraphic (palynological) evidence of Middle to Late Cambrian sedimentation in the Algerian subsurface is presented and discussed in this paper, resulting from the palynological analysis of pre-Ordovician clastic sequences penetrated by borehole AMG-1 in northwestern Algeria. Well preserved and moderately diversified acritarchs occur in a transgressive sequence comprising the upper third of the Sebkhet el Mellah Formation and most part of the Aïn en Nechea Formation. This acritarch suite corresponds to the Cristallinium cambriense — Eliasum/ Timofeevia acritarch Superzone established in Morocco as well as to the Adara alea acritarch Zone of eastern Newfondland, both directly correlated to the Paradoxides paradoxissimus Trilobite Zone of Middle Cambrian age. This stratigraphic interval correlates clearly with the mixed carbonate–siliciclastic successions outcropping in the Tacheddirt Valley of the Anti Atlas, Morocco. The uppermost part of the Aïn en Nechea Formation is possibly attributed to the Late Cambrian, based on the absence of the typical Middle Cambrian species, and the presence of taxa of Timofeevia and Cristallinium, associated to an increase in abundance and diversity of diacromorphs acritarchs, suggesting that the Late Cambrian stratal sequences must be considerably reduced and/or partly eroded in the study area. These results provide the basis for improved correlation with the acritarch-bearing Middle Cambrian sediments of the Anti Atlas in Morocco, and with the outcrops of the Ougarta Ranges further south in Algeria, facilitating future sequence stratigraphic studies of the pre-Ordovician clastic successions of the North Saharan Platform, with a higher resolution than previuosly available.

Early Cambrian acritarch assemblage zones in South Australia and global correlation

Lower Cambrian sediments in the Arrowie and Stansbury Basins were deposited on a rifting continental platform and contain abundant acritarchs, trace fossils, small shelly fossils, archaeocyaths, trilobites and brachiopods. Generally, faunas and acritarchs are abundant in transgressive and highstand deposits, but less common in lowstand fluvial deposits, responding both to sedimentary environments and preservational conditions. Seven acritarch Assemblage Zones are recognised in the lowest Cambrian to Toyonian sediments of South Australia. Assemblage Zone 1 embraces the Precambrian-Cambrian boundary, but a dramatic increase of spinose acritarchs (Assemblage Zone 2) is observed around 1 m above the first appearance of the trace fossil Rusophycus. Assemblage Zone 3 occurs in transgressive siltstones during deposition of the uppermost Parachilna Formation and is marked by the first appearance of several taxa, which are widespread and well correlated in South Australia. Assemblage Zones 4, 5 and 6 are dominated by Skiagia; the appearance of S. ornata followed by S. ciliosa in Zones 4 and 5, respectively, indicates correlation with the nominal acritarch zones on the East European Platform and concurrent Schmidtiellus mickwitzi and Holmia kjerulfi trilobite zones, and the Qiongzhusi Formation in southern China. Assemblage Zone 7 encompasses a major acritarch decrease in South Australia during the late early Cambrian regression and 'red-beds' deposition. Continuous occurrences of acritarch genera Skiagia, Ceratophyton and Corollasphaeridium during the early Cambrian provide valuable evidence of evolutionary development, and therefore are reliable for biostratigraphic zonation. The integration of biostratigraphy and unconformity-bounded sequence stratigraphy provides more precise intercontinental correlation.

Early Cambrian phytoplankton diversification and appearance of trilobites in the Swedish Caledonides with implications for coupled evolutionary events between primary producers and consumers

The newly examined Lower Cambrian strata in the Laisvall-Storuman area, central Swedish Caledonides, yield diverse and stratigraphically significant phytoplanktic organic-walled microfossils (acritarchs) associated with the olenellid trilobites of Holmia sp. that appear to be at the lowermost horizon ever recorded in the Caledonides. The acritarchs, recovered throughout the Grammajukku Formation, are taxonomically reviewed in the context of regional and global trends of phytoplankton diversity. The assemblages are assigned to acritarch zones Skiagia ornata-Fimbriaglomerella membranacea and Heliosphaeridium dissimilare-Skiagia ciliosa, corresponding to the Schmidtiellus mickwitzi and Holmia kjerulfi trilobite zones. The records of acritarch radiations and appearances of trilobites and other faunas in Baltica, and on a more interregional scale, are biochronologically correlated, revealing tightly coupled evolutionary events among primary producers and consumers. The timing of early diversifications of trilobites in various faunal provinces is discussed and the relative age of the oldest known non-mineralized arthropods from the Zawiszyn Formation in Poland is estimated.

Biochronology of the autochthonous Lower Cambrian in the Laisvall–Storuman area, Swedish Caledonides

New records of phytoplankton (acritarchs), ichnofossils and olenellid trilobites have been studied from the autochthonous upper Neoproterozoic–Lower Cambrian successions along the Caledonian Thrust Front in the Laisvall–Storuman region of northern Sweden. The fossils are from a newly examined natural outcrop at Bergmyrhobben near Lake Storuman, and from previously described fossiliferous outcrops at Delliknäs and Mt. Assjatj, the Laisvall mine and the Maiva borehole successions in the Laisvall area. Acritarch assemblages are recorded throughout the Grammajukku Formation. They are age-diagnostic for the Skiagia–Fimbriaglomerella acritarch Zone, time-equivalent to the Schmidtiellus mickwitzi trilobite Zone (the lower part of the formation), and the Heliosphaeridium–Skiagia acritarch Zone corresponding to the Holmia kjerulfi trilobite Zone (the upper part of the formation). The acritarch record from the Storuman area documents the presence of strata contemporaneous to the Schmidtiellus mickwitzi Zone for the first time in the Scandinavian Caledonides. This zone was previously only recognized in the platform regions of the Baltica palaeocontinent. The ichnofossils from the upper Såvvovare Formation, including ?Harlaniella, Phycodes, Gyrolithes and Palaeophycus ichnogenera, allowed the base of the Cambrian System to be determined within the Maiva Member and the coeval Kautsky Ore Member in the subsurface successions, and to attribute this part of the formation to the Lower Cambrian Platysolenites antiquissimus faunal Zone of Baltica. The trilobite fauna from the Storuman area, attributed tentatively to Holmia sp., occurs at the lowermost stratigraphic level among olenellids in the Caledonides. The range of this species, estimated from the concurrent acritarch biostratigraphy, is within the Schmidtiellus mickwitzi Zone. The stratigraphic significance of the acritarch assemblages and ichnofossils is analysed and the biochronology of the Grammajukku Formation and the upper Såvvovare Formation is discussed in detail in the context of Lower Cambrian zonation in Baltica.

Numerical ages of volcanic rocks and the earliest fauna1 zone within the Late Precambrian of east Poland

U–Pb age determinations by ion microprobe on zircons from two tuff samples within the Neoproterozoic–early Cambrian successions in Poland are presented. One sample, from the Kaplonosy borehole is within, or conformably below, rocks that contain the Sabellidites–Vendotaenia fossil assemblage of the Upper Vendian. The second sample is from the Ksi¹¿ Wielki IG-l borehole, from rocks that were referred alternatively to the early Cambrian or to the Vendian on lithostratigraphic evidence. The Kaplonosy zircons are euhedral and free of visible zircon cores, both optically and as back-scattered electron images, but they exhibit a range in 206 Pb/ 238 U ages that exceeds analytical error. The combined data-set can be resolved into three age-components in different proportions, which overlap in apparent age due to measurement errors. There is a well-defined principal age component at 551 ± 4 Ma (95% limits) and two minor detrital or inherited components at 588 ±8 Ma and 635 ± 10 Ma. The age of the Kaplonosy tuff is interpreted as equal to that of the youngest and principal component, 551 Ma. This age allows a maximum time difference of 17 ± 4 Ma between the top of the Stawatycze Formation and Lower Cambrian strata of the Heliosphaeridium dissimilare-Skiagia ciliosa acritarch Zone, the latter being time-equivalent to Middle Tommotian strata in northeastern Siberia, recently dated as 534.6 ± 0.5 Ma. The tuff from the Ksi¹¿ Wielki IG-l borehole has a similar spectrum of zircon ages, but also contains several detrital grains that have concordant and separate early Proterozoic and Archaean ages. The age of the major component is 549 ± 3 Ma (72% of the totak), and there are two older components at 578 ± 9 Ma (13%) and 619 ±s8 Ma (15%). The deposition of the tuff from the Ksig Wielki Formation therefore occurred at 549 Ma, indicating that the Ksi¹¿ Wielki Formation is not early Cambrian but Upper Vendian, unless all zircons in the tuff are detrital.

Constraints on the early Cambrian radiation and correlation of the Tommotian and Nemakit-Daldynian regional stages of eastern Siberia

Acritarchs of early Cambrian age occur in strata attributed to the Middle Tommotian in northeastern Siberia. A diagnostic assemblage from a part of the Tyusersk Formation referred to the Dokidocyathus regularis Zone (Middle Tommotian) at Chekurovka, and a much poorer assemblage from Ulakhan-Aldyarkhay, indicate time-equivalence with the Heliosphaeridium dissimilare–Skiagia ciliosa acritarch Zone and the correlative Holmia kjerulfi trilobite Zone in Baltica. The bulk of the species turnover which is part of the Cambrian explosion, including archaeocyathans, small shelly fossils, trilobites and acritarchs, is inferred to occur in the Skiagia ornate–Fimbriaglomerella membra-nacea (= Schmidtiellus/Rusophycus faunal zone) and H. dissimilare–S. ciliosa (= Holmia kjerulfi trilobite Zone) acritarch zones.

A palynological study of the Lower Carboniferous Lydebrook Sandstone and adjacent late Wenlock and Langsettian strata, Shropshire, England

The Lower Carboniferous Lydebrook Sandstone contains spores attributable to the NM and VF miospore Zones indicative of late Dinantian (Asbian-Brigantian) age. The overlying Brigantian limestones are succeeded by the Little Wenlock Basalt. Coal Measures, which yield spore assemblages assignable to the upper part of the SS Zone or the lower part of the RA Zone, Langsettian Stage of the Westphalian, unconformably overlie the Brigantian strata. The upper part of the late Wenlock Coalbrookdale Formation below the Lydebrook Sandstone can be assigned to the Cymatiosphaera pavimenta acritarch Zone. The data presented herein demonstrate the limitations of earlier lithostratigraphic correlations of Lower Carboniferous sand bodies in the region of the emergent Welsh London Brabant Massif. In the context of eustatic/tectonic controlled regional patterns of sedimentation, palynologically time constrained deposits of the platforms margins can be interpreted in respect of an evolving sequence of major eustatictectonic events.

Lower Cambrian acritarchs from northern Spain: the Precambrian-Cambrian boundary and biostratigraphic implications

AbstractAcritarchs are reported from basal Cambrian rock units inthe Cantabrian region of northern Spain that are known to contain archaeocyathan and trilobite faunas. Biostratigraphic correlation of the Iberian sequences with other regions has been hampered by the strong provincialism of these faunas. However, this report of evidently cosmopolitan acritarch taxaestablishes the time equivalence of early Cambrian trilobite faunas from Iberia, Baltoscandia and the East European Platform (EEP). Our data suggest that the detrital deposition of the Lower Cambrian Herreria Formation embraces at least three (and possibly four) Lower Cambrian acritarch zones previously identified in the EEP, eastern Siberia, Baltoscandia, Scotland, Greenland, Svalbard and western North America. The early Cambrian transgression in northern Spain was probably initiated in Talsy times (Schmidtiellus mickwitzi trilobite Zone in Baltoscandia and the EEP), in part corresponding to the Dokidocyathus regularis archaeocyathian Zone of the Middle Tommotian in Siberia.

Silurian acritarchs from the type Wenlock and Ludlow of Shropshire, England

Acritarch taxa are recorded and listed from the Wenlock and Ludlow from the type areas in Shropshire, England, and other localities in the Welsh Basin of South Wales and the Welsh Borderland of England. The ranges of 141 taxa of stratigraphic value are plotted. Five new genera are described: Hogklintia, Lepto brachion, Rhaco brachion, Salopidium and Wrensnestia. 36 new species are described, and 32 new combinations proposed. Seven acritarch zones are recognised in the Wenlock and Ludlow.

Espèces zonales d'Acritarches du Cambro-trémadocien de belgique et de l'Ardenne française

Nine acritarch zones are recognized in the Cambro-Tremadocian of Belgium and the French Ardennes. The author gives the main characteristics of these zones as well as an estimation of their position relative to the international chronostratigraphic scale. The zonation is based on the stratigraphic ranges of easily identifiable species, eight of which are new: Micrhystridium, lanceolatum, Priscogalea chevronensis, Leiosphaeridia fumiana, Lophosphaeridium bacilliferum, L. ? kryptoradiatum, Revinotesta microspinosa, Dictyotidium hasletianum and Polyedryxium ? pentagonale. Their description necessitated the establishment of a new genus: Revinotesta. The author proposes two new specific combinations and defines his concept of the genera Priscogalea, Cymatiogalea and Lophosphaeridium.