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Biostratigraphy of the Neotethys Ocean Early Cretaceous deposits in the Lut Block based on nannoplankton fauna
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Marziyeh Notghi Moghaddam    |
| PNU university , m.n.moghaddam@gmail.com |
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Abstract: (448 Views) |
Background and Objectives: Calcareous nannoplankton organisms evolved rapidly during the Mesoozoic especially in Cretaceous, and are of great value as stratigraphic indicators. The early Cretaceous rocks are well developed in the Lut Block basin. These sediments have attracted the attention of many stratigraphers and paleontologists because of their fossil content. In the present research, in order to studies of the calcareous nannoplankton biostratigraphy of the Lut Block Cretaceous deposits and age determination, stratigraphic section of the Charmeh in the northwest of Sarayan (east margin of Lut Block) was selected and sampled. Thickness of the studied section is 209 meters and lithology of this section contains grey to dark green silty marls. The lower boundary of these deposits is conglomerate and sandstones and its upper boundary is grey massive limestone.
Methods: In the present research 91 smear slides of the studied sediment prepared following the standard smear slide technique. These slides were examined at standard light microscope techniques under XPL and PPL in order to estimate relative calcareous nannoplankton abundance and preservation. All species were identified according to researchers' descriptions and morphological characteristics. The preservation of nannoplanktons was evaluated qualitatively by using visual criteria concerning degree of etching and overgrowth. For the age determination of the Cretaceous deposits, standard zonation was utilized.
Findings: In this study, a common and index calcareous nannoplankton assemblage (34 species and 19 genera) is documented for the first time in this account from the early Cretaceous deposits succession of the Lut Block Basin (Charmeh section). Calcareous nannoplanktons are medium to well-preserved in this section, which facilitates detailed identification of the floral assemblages. First occurrence of the Cretarhabdus loriei and Nannoconus abundans and last occurrence of the Speetonia colligate are bio-events that records in this research.
Conclusion: Based on index nannoplankton species and associated fossil assemblages, Cretarhabdus loriei Zone (CC4) and Lithraphidites bollii Zone (CC5) were determined. The identified zones are consistent with middle last part of NK3B and NC4A، NC4B and NC5. The nannoplankton species show the age of early Hauterivian to late Hauterivian for the studied successions. Furthermore investigation of the relationship between change in abundance, size and paleoenvironmental conditions in the Charmeh ection allows to reconstruction paleo-conditions and possibility of discussing the influencing factors in the form of paleoenvironmental conditions during the Hauterivian in the Lut Block sedimentary basin. The genus Nannoconus appears to be particularly sensitive to global fertility conditions and local continental input while change in abundance of Micrantholithus is clearly associated with salinity levels. In calcareous nannoplankton assemblages of the early Cretaceous deposits of the Charmeh section, the warming event with mesotrophic to oligotrophic conditions is marked by appearance of the low latitudinal species and an acme of tropical species such as Watznaueria barnesaiae, Lithraphidites spp., Eiffellithus spp., Nannoconus spp., and Micrantholithus spp. Therefore base on the abundance of index nannoplankton species, studied sequence was deposited in shallow marine environment in relatively low latitude and warm surface water condition with low productivity at the early Hautrevian - late Hautrevian were suggested.
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| Keywords: Biostratigraphy, Cretaceous, Lut Block, Calcareous nannoplankton, Charmeh |
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Full-Text [PDF 1494 kb]
(148 Downloads)
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Type of Study: Research/ Original/ Regular Article |
Subject:
Marine Geology
Received: 2024/02/4 | Revised: 2024/04/20 | Accepted: 2024/04/20 | ePublished: 2024/03/5
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