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Faunal and facies changes at the Early-Middle Frasnian boundary in the north-western East European platform

100%

Zhuravlev A. V., Sokiran E. V., Evdokimova I. O., Dorofeeva L. A., Rusetskaya G. A., Malkowski K.

Acta Palaeontologica Polonica

2006

tom 51

nr 4

747-758

Multidisciplinary study of the Early–Middle Frasnian boundary in the north−western East European Platform (Main Devonian Field) allows evaluation of changes in facies, brachiopod, ostracod, and conodont associations, as well as carbon isotope composition. Brachiopod and ostracod faunas, characterized by predominance of rhynchonellids, spiriferids, podocopids, and platycopids in the Early Frasnian, and by predominance of spiriferids, palaeocopids, and kloedenellocopids in the Middle Frasinan, demonstrate significant changes in dominance and diversity likely caused by regressiontransgression couplets in the extremely shallow−water environment. Changes in diversity of the conodont associations, represented mainly by shallow−water polygnathids and spathognathids, are controlled by sea−level fluctuations as well; however significant evolutionary turnover is absent. Deepening pulses lead to an increasing in diversity of both the benthic and nectic groups, whilst regressions cause diversity fall due to progressive habitat reduction. Most prominent decreasing in the fauna diversity is observed in the late Early Frasnian (Dubnik time) coinciding with the regressive phase of the Late Givetian to Early Frasnian eustatic cycle. Positive−negative δ¹³C excursion, detected in brachiopod calcite from the early interval of the Middle Frasnian, can be correlated with global isotopic perturbations near the Frasnian substage boundary: the positive 2.6‰ δ¹³C excursion is probably linked with enhanced primary production in high−nutrient regimes in the epeiric sea.

Benthic ostracods from the Early-Middle Frasnian transition in the north-western East European Platform, Russia

100%

Evdokimova I. O.

Acta Palaeontologica Polonica

2006

tom 51

nr 4

773-778

Pronounced changes in benthic ostracod associations in the north−west part of the East European Platform, across the Early–Middle Frasnian (Devonian) transition, track a marine transgression event. More than 80 ostracod species belonging to the Eifelian Mega−Assemblage were recorded. Cavellinidae and Acratiidae generally dominate the Early Frasnian Sargaevo Horizon. Middle Frasnian ostracod associations of the Semiluki Horizon are more diverse and are characterised by different dominant both geographically and stratigraphically, resulting from significant palaeo−basin bottom relief and benthic biotope differentiation. The ostracod associations are indicative of very shallow, well oxygenated semi−restricted epeiric environment, with occasional marine influence, alternating with short periods of open shelf conditions. The relative abundance of ostracod species in the different associations, and faunal diversity indices, show considerable variations throughout the Early–Middle Frasnian, and suggest a less restricted position within an intra−platform setting in the Middle Frasnian, relative to a more restricted ostracod habitat in the Early Frasnian. The succession of Early–Middle Frasnian micro−benthic associations within the Main Devonian Field seems to be of a regional scale, and resulted from a marine regression−transgression couplet, corresponding to eustatic cycles IIb–IIc. This was paired with synsedimentary tectonic subsidence in adjacent areas of the north−east East European Platform, progressively improving marine water circulation in the extremely shallow−water shelf seas.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

2 Evdokimova I. O.

1 Dorofeeva L. A.

1 Malkowski K.

1 Rusetskaya G. A.

1 Sokiran E. V.

1 Zhuravlev A. V.

2 2006

Wyniki wyszukiwania

Faunal and facies changes at the Early-Middle Frasnian boundary in the north-western East European platform

Benthic ostracods from the Early-Middle Frasnian transition in the north-western East European Platform, Russia