TY - JOUR

T1 - Microfacies and C/O-isotopes in lacustrine dolomites reflect variable environmental conditions in the Germanic Basin (Arnstadt Formation, Upper Triassic)

AU - Hofbauer, Barbara

AU - Viehmann, Sebastian

AU - Gier, Susanne

AU - Bernasconi, Stefano M.

AU - Meister, Patrick

N1 - Funding information: We thank Gernot Arp for guiding us to the most relevant outcrops and providing essential information on the Arn-stadt Formation. He also provided valuable comments to the manuscript. Caroline Haberhauer and Stefanie Klackl assisted during fieldwork. Claudia Beybel, Leopold Slawek, and Ilka Wünsche are thankfully acknowledged for preparing high-quality thin sections. Beatrix Bethke supported us during laboratory work. The study was partially financed by the European Commission through IEF Project TRIADOL (no. 626025) and Horizon2020 Project ELEMIN (grant no. 746033), and by the Department of Geology at the University of Vienna. We thank Gerhard H. Bachmann for detailed comments to the regional geology and an anonymous reviewer for constructive feedback to our study.

PY - 2021/1

Y1 - 2021/1

N2 - The conditions in ancient evaporative environments conducive to authigenic carbonate (especially dolomite [CaMg(CO3)2]) formation are still insufficiently understood. Insights from microfacies analysis can help to constrain the conditions in these environments. We provide a brief overview of the microfacies association and carbon and oxygen isotope compositions of dolomite beds intercalated in a claystone-rich succession from the Norian Arnstadt Formation in Thuringia and Lower Saxony (Germany) in order to gain further insight into the depositional conditions and processes leading to the formation of authigenic Mg/Ca-carbonates in the Germanic Basin. The studied intervals are ascribed to lacustrine, partially evaporitic conditions, while the sedimentary structures were not obliterated by recrystallization. The microfacies of the dolomites is diverse, showing homogeneous micrite, mudclasts, lamination, and peloidal structures, and reflects a shallow to deeper water (below wave base) and episodically evaporative environment. The dolomites exhibit oxygen isotope values (δ18O) in the range from-5.21 to-0.36‰ VPDB and, hence, only represent a weak meteoric influence, suggesting that the authigenic carbonate generally formed under evaporative conditions. Carbon isotope values (δ13C) in the range of-4.28 to 1.39‰ VPDB indicate a small contribution of remineralized organic carbon, mainly in sediments that were presumably deposited in deeper water or under brackish conditions. Sedimentary structures, such as lamination with graded silt layers, reworked mudclasts embedded in a fine dolomicrite matrix, and peloids showing plastic deformation, indicate that the sediment was still unlithified. These observations would be consistent with an authigenic formation of Mg/Ca-carbonates directly from the lake water, and their deposition under variable conditions in a large playa-lake/per-ennial lake system.

AB - The conditions in ancient evaporative environments conducive to authigenic carbonate (especially dolomite [CaMg(CO3)2]) formation are still insufficiently understood. Insights from microfacies analysis can help to constrain the conditions in these environments. We provide a brief overview of the microfacies association and carbon and oxygen isotope compositions of dolomite beds intercalated in a claystone-rich succession from the Norian Arnstadt Formation in Thuringia and Lower Saxony (Germany) in order to gain further insight into the depositional conditions and processes leading to the formation of authigenic Mg/Ca-carbonates in the Germanic Basin. The studied intervals are ascribed to lacustrine, partially evaporitic conditions, while the sedimentary structures were not obliterated by recrystallization. The microfacies of the dolomites is diverse, showing homogeneous micrite, mudclasts, lamination, and peloidal structures, and reflects a shallow to deeper water (below wave base) and episodically evaporative environment. The dolomites exhibit oxygen isotope values (δ18O) in the range from-5.21 to-0.36‰ VPDB and, hence, only represent a weak meteoric influence, suggesting that the authigenic carbonate generally formed under evaporative conditions. Carbon isotope values (δ13C) in the range of-4.28 to 1.39‰ VPDB indicate a small contribution of remineralized organic carbon, mainly in sediments that were presumably deposited in deeper water or under brackish conditions. Sedimentary structures, such as lamination with graded silt layers, reworked mudclasts embedded in a fine dolomicrite matrix, and peloids showing plastic deformation, indicate that the sediment was still unlithified. These observations would be consistent with an authigenic formation of Mg/Ca-carbonates directly from the lake water, and their deposition under variable conditions in a large playa-lake/per-ennial lake system.

KW - Arnstadt Formation

KW - Authigenic carbonates

KW - Germanic Basin

KW - Microfacies

KW - Playa lake

KW - Primary dolomite

UR - http://www.scopus.com/inward/record.url?scp=85123935939&partnerID=8YFLogxK

U2 - 10.17738/ajes.2021.0004

DO - 10.17738/ajes.2021.0004

M3 - Article

AN - SCOPUS:85123935939

VL - 114

SP - 66

EP - 87

JO - Austrian Journal of Earth Sciences

JF - Austrian Journal of Earth Sciences

SN - 0251-7493

IS - 1

ER -