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Aptian planktonic foraminiferal biostratigraphy and microfacies analyses of Mudurnu – Nallihan sequence (Ankara, Turkey)

Kayhan, Deniz Yağmur
The Mudurnu – Nallıhan Sequence that comprises the Upper Jurassic – Lower Cretaceous carbonate succession was cut by diabase dykes and intercalated with volcano-sedimentary rocks in the Nallıhan region, Ankara, Turkey. The sequence consists of the Yosunlukbayırı Formation and the Soğukçam Limestone. This study focuses on the high-resolution biostratigraphy and microfacies analyses of the Soğukçam Limestone widely exposed in the north of the Nallıhan town. For this purpose, the stratigraphic section was measured (94.73 m thick) and a total of 44 samples were collected from limestone, shale and diabase dyke. By using planktonic foraminiferal bioevents, the following biozones were established in ascending order: Globigerinelloides blowi Zone (Early Aptian), Leupoldina cabri Zone (Early Late Aptian) and Globigerinelloides ferreolensis Zone (Late Aptian). Eight species of Hedbergella, three species of Globigerinelloides, and Leupoldina cabri were identified in the study. Only the presence of these r-mode opportunistic planktonic foraminifera, Hedbergella and Globigerinelloides, in the studied samples indicates that eutrophic environmental conditions prevailed in the Aptian time in the studied region. The elongated chamber forms, such as Hedbergella roblesae and Leupoldina cabri, low-oxygen planktonic foraminifers suggests that the Oceanic Anoxic Event 1a (the Selli Event) has possibly been recorded in the Leupoldina cabri Zone of the studied sequence. In addition, small benthic foraminifera, radiolaria and Cadosina sp. were recorded through the measured section. Based on both lithofacies analysis (determination of the depositional texture), and biofacies analysis (fossil assemblages), microfacies types were defined as MF-1: Radiolarian mudstone / wackestone, MF-2: Planktonic foraminiferal and radiolarian wackestone / packestone and MF-3: Planktonic foraminiferal mudstone / wackestone. In the light of the microfacies analyses, the depositional environments of the Aptian part of the Soğukçam Limestone were determined as toe of slope to deep water basin.