Identifying organic-rich layers and reconstructing trophic levels in the Dalichai Formation in north Damghan
Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epif...
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| Format: | Article |
| Language: | fas |
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University of Isfahan
2023-06-01
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| Series: | Journal of Stratigraphy and Sedimentology Researches |
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| Online Access: | https://jssr.ui.ac.ir/article_28066_2d1c5748ea65b7e0b39665c281804b60.pdf |
| Summary: | Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epifaunal to infaunal benthic foraminifera, and the ratio of surface to deep infaunal, were utilized to identify organic-rich layers and reconstruct trophic levels. These factors confirm a mesotrophic environment with average oxygen and nutrient levels for the Dalichai Formation (Bajocian to Lower Callovian) in the studied section. The primary reasons for low organic matter in the Dalichai Formation are attributed to low production and poor preservation of organic matter. However, dark-colored deposits with high organic matter at the Bajocian–Bathonian boundary and the lower and upper Callovian boundaries suggest increased production rates in anaerobic and eutrophic conditions. Macrofossil fragments (ammonites and echinoids) within limestone deposits at the uppermost part of the Dalichai Formation, exhibiting low organic matter, and the presence of Zeophycus trace fossils, suggest an oligotrophic environment with dysoxic conditions in the Upper Callovian.Keywords: Trophic level, Organic facies, Dalichai Formation, Foraminifera, Palynomorphs Introduction The stratigraphic section of Ahvano, studied near the village in the north of Damghan, has coordinates E 54° 10΄ 42˝ longitude and N 36° 12΄44˝ latitude. The Dalichai Formation, 78 meters thick, exhibits shale-marl lithology with limestone interlayers, characteristic of the structural range of eastern Alborz. The lower boundary adjoins the Shemshak Formation with a parallel discontinuity, while the upper boundary is overlaid by the limestones of the Lar Formation. The presence of facies rich in organic matter depends not only on suitable conditions for their production but also on two other fundamental factors: the rate of sedimentation and the presence of oxygen-poor waters to protect them (Bombardiere and Gorin 2000). To investigate and determine the trophic level and its role in the formation of the organic facies of the Delichai Formation in the studied section, various tools, including palynomorphs, foraminifers, and microfacies studies, were employed. Material & MethodsFor palynological studies, 18 samples were taken at intervals of one to five meters. The samples were prepared using the standard Travers method (Travers 2007). Each sample (50 grams) underwent treatment with hydrochloric acid and 30% hydrofluoric acid, followed by neutralization with a heavy solution of hydrogen chloride (ZnCl2). The treatment has been done at the biostratigraphy laboratory of the Damghan University. Thin sections were prepared and employed for the foraminifera study. Statistical analyses involved selecting 20 fields for each sample, and graphs were created using Excel. Discussion of Results & ConclusionsMicrofacies studies, palynological analyses, and fossil evidence, encompassing foraminifera diversity and abundance, the ratio of epifaunal to infaunal forms, and the ratio of surface infaunal to deep infaunal forms, were employed to reconstruct trophic levels during the Dalichai Formation deposition in the Ahvanu section in eastern Alborz. The presence of Nannoceratopsis spp. from dinoflagellate proximate forms, low percentages of amorphous organic materials (AOM), and benthic foraminifer presence, especially epifaunal species like Ophthalmidium sp., Triloculina sp., Quinquluculina sp., and Glomospira sp., with terrestrial debris influx, indicates a gradual rise in sea level in the late Bajocian deposits, creating a mesotrophic environment. However, at the Bajocian–Bathonian boundary, a significant decrease in the diversity and abundance of epifaunal benthic foraminifers is observed, with an increase in forms such as Nodosaria sp., Textularia sp., and Lenticulina sp. Palynological studies support these conditions, revealing the appearance and abundance of planktonic foraminifers Globigerina spp. and radiolarians, indicating a eutrophic environment (Baumgartner 2013).From sample number 10 to sample number 13 in microfacies A2 (peloid foraminifera wackestone) and microfacies A3 (bioclast Wackestone), there is an increase in the ratio of epifaunal benthic forms to infaunal, with the presence of radiolarian and sponge spicules, indicative of a mesotrophic environment. However, from sample number 13 to 15, a gradual decrease in the ratio of epifaunal benthic forms to infaunal is observed, with an increase in the abundance of deep infaunal forms such as Nodosaria sp. and Dentalina sp., leading to a decline in benthic foraminiferal diversity. In palynological slides, better preservation of marine palynomorphs is noted. These conditions indicate the persistence of a mesotrophic environment until the lower Callovian. Gradually, in the limestone deposits at the uppermost part of the Dalichai Formation in the section under study, there is an increase in the ratio of epifaunal to infaunal forms, fragments of macrofossils (ammonites, echinoids, and bryozoans) with a micrite matrix. Together with the influence of Zoophycus fossils, these observations signify a shift from a mesotrophic to an oligotrophic environment during the Callovian.Microfacies studies, palynological analysis, and fossil evidence, including foraminiferal diversity, epifaunal to infaunal ratios, and surface-to-deep infaunal ratios, were utilized to reconstruct the trophic levels during the Dalichai Formation deposition in the Ahano section. These indicators confirm a mesotrophic environment for the Dalichai Formation deposits in the studied area. Towards the end of the Dalichai Formation, there is a gradual rise in the epifaunal to infaunal ratio, and the presence of macrofossils (ammonites, echinoids, and bryozoans) in micrite background, suggesting a shift from mesotrophic to oligotrophic conditions during the Callovian. However, at the Bajocene–Batonian and early–late Callovian boundaries, there's a notable decrease in the diversity and abundance of epifaunal benthic foraminifers. Planktonic foraminifers (Globigerina sp.), along with increased bioturbation, Posidonia, and radiolarian abundance, indicate elevated production and organic matter preservation, suggesting a transition to a eutrophic environment. |
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| ISSN: | 2008-7888 2423-8007 |