Integrated stratigraphy of the Guşteriţa clay pit: a key section for the early Pannonian (late Miocene) of the Transylvanian Basin (Romania)

Abstract

Abstract The Neogene Transylvanian Basin (TB), enclosed between the eastern and southern Carpathians and the Apuseni Mountains in Romania, is a significant natural gas province with a long production history. In order to improve the (bio) stratigraphic resolution, correlations and dating in the several 100-m-thick upper Miocene (Pannonian) succession of the basin, the largest and most fossiliferous outcrop at Guşteriţa (northeastern part of Sibiu) was investigated and set as a reference section for the Congeria banatica zone in the entire TB. Grey, laminated and massive silty marl, deposited in the deep-water environment of Lake Pannon, was exposed in the ~55-m-high outcrop. The uppermost 25 m of the section was sampled in high resolution (sampling per metres) for macro- and microfossils, including palynology; for authigenic 10Be/9Be dating and for magnetostratigraphy; in addition, macrofossils and samples for authigenic 10Be/9Be isotopic measurements were collected from the lower part of the section as well. The studied sedimentary record belongs to the profundal C. banatica mollusc assemblage zone. The upper 25 m can be correlated to the Hemicytheria tenuistriata and Propontoniella candeo ostracod biozones, the uppermost part of the Spiniferites oblongus, the entire Pontiadinium pecsvaradense and the lowermost part of the Spiniferites hennersdorfensis organic-walled microplankton zones. All samples contained endemic Pannonian calcareous nannofossils, representing the Noelaerhabdus bozinovicae zone. Nine samples were analysed for authigenic 10Be/9Be isotopic measurements. The calculated age data of six samples provided a weighted mean value of 10.42 ± 0.39 Ma. However, three samples within the section exhibited higher isotopic ratios and yielded younger apparent ages. A nearly twofold change in the initial 10Be/9Be ratio is a possible reason for the higher measured isotopic ratios of these samples. Magnetostratigraphic samples showed normal polarity for the entire upper part of the outcrop and can be correlated with the C5n.2n polarity chron (11.056–9.984 Ma, ATNTS2012), which is in agreement with the biostratigraphic data. Based on these newly obtained data and correlation of the biozones with other parts of the Pannonian Basin System, the Guşteriţa section represents the ~ 11.0–10.5 Ma interval, and it is a key section for correlation of mollusc, ostracod, dinoflagellate and calcareous nannoplankton biostratigraphic records within this time interval.