Microfossils and microfacies of the Late Jurassic–Early Cretaceous Chia Gara Formation from NE Iraq

The Jurassic–Cretaceous period is marked by considerable geological and environmental changes, coupled with increased tectonic activity on both regional and global levels, along with significant paleoceanographic changes (e.g., Wimbledon et al., 2016).
The Chia Gara Formation (Late Jurassic–Early Cretaceous) is one of the common formations in northeastern Iraq. The formation is observed to be complete in certain sections, such as the studied sections at Surdash and Zeni Warte (Figure 1), situated in northeastern Iraq, at the northeastern boundary of the Arabian Plate. This site signifies the intersection where the continental regions of the Eurasian margin collide with the Arabian Plate (Stampfli & Borel, 2002). Tectonically, these sections are located in the High Folded and Imbricated Zone of Iraq (Figure 1), which form a part of the tripartite tectonic divisions of Iraq. They are characterized by intense folding and faulting of Mesozoic and Paleozoic successions (Jassim & Buday, 2006).
Analysed sections consist of mixed calcareous-clayey rocks (calcareous shales, marls, marly limestones). The content of clay is largest in the samples of the lower part of the sections and decreases towards the top of the sections. Fragments of organic matter are common and locally concentrated. Pyrite and small admixture of silt-sized quartz grains is present in some samples.
Several microfacies were distinguished in the studied samples: muddy micrite (MFT1A), mudstone with rare microfossils or muddy micrite with rare microfossils (MFT1B), radiolarian mudstone or muddy radiolarian micrite (MFT2A), radiolarian wackestone or muddy radiolarian micrite (MFT2B), radiolarian wackestone to packstone or muddy radiolarian micrite to muddy radiolarian limestone (MFT2C), Nannoconus wackestone to packstone or muddy Nannoconus micrite to muddy Nannoconus limestone (MFT3). Due to the mixed nature of the sediments, both Dunham (1962) and Mount (1985 with Flügel, 2004 modifications) classifications of rocks were applied.
Microfossils appear in the analysed sections, but their preservation is poor. Rare ostracods are observed in lower parts of the sections. In upper parts of the sections very rare specimens that may possibly represent poorly preserved calpionellids appear.
Rare calcareous dinocysts are present especially in the upper part of the sections but are represented mainly by relatively long ranged species. Occurrence of Colomisphaera tenuis (Nagy), Colomisphaera lucida Borza, and Colomisphaera heliosphaera (Vogler) confirms Late Tithonian and Berriasian age of the sections.
Foraminifera observed in the whole section are very rare, strongly recrystallized, locally pyritized, and very poorly preserved. They are represented by stratigraphically irrelevant calcareous benthic forms, predominated by Lenticulininae, Nodosariana and Involutina as well as planktic foraminifera preserved mostly as recrystallized fillings of Hedbergella-Lilliputianella and Globigerinelloides-type morphotype. Planktonic highspired foraminifera (?) Globuligerina bathoninana (Pazdrowa) was identified in the lower part of the section, indicating the Jurassic age of deposits, while in upper part the ?Lilliputianella eocretacea (Neagu) and Hedbergella ?handousi (Salaj) confirmed the Cretaceous age.
Radiolarians are common in the lower parts of the sections and absent in the upper parts. All together 10 samples were processed to obtain radiolarians, however only one sample was productive. The preservation and diversity of radiolarians in the sample was very poor. Based on the first occurrence of Pseudodictyomitra carpatica (Lozyniak) and of the frequently present Holocryptocanium barbui Dumitrica appearing in the latest Tithonian, together with Loopus primitivus (Matsuoka et Yao) and genera Zhamoidellum having its last occurrence in the Late Tithonian, the age of the sample is set as probably latest Tithonian (UA Zone 13; Baumgartner et al., 1995).
Fossils, lithology, limited input of siliciclastic silt and lack of coarser siliciclastic material, suggest deposition in deep shelf to basinal environment. Shift from more clay, radiolarian facies towards more calcareous nannoconus facies is observed. Small sizes of both foraminifera and calcareous dinocysts suggest restricted environmental conditions.
Acknowledgements: The research was carried out thanks to the financial support from the subsidy funds of AGH No. 16.16.140.315, by the Slovak Research and Development Agency (APVV-21-0281), and by the Agentúra Ministerstva Školstva, Vedy, Výskumu a Športu SR (VEGA 2/0012/24, VEGA 1/0021/25).