Study of water-level fluctuations throughout the deposition of the Gurpi Formation based on palynological data in the west of Dehrolan, south of Ilam

Abstract

Abstract To analyze the sedimentary palaeoenvironment of the Gurpi Formation in Bisheh–Deraz section in south of Ilam, various factors including diversity and abundance of dinocysts, a ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, palynofacies parameters and "selective preservation of organic matter" have been used. In lower parts of the Gurpi Formation of Campanian age is very argillaceous and then shallows up in the upper part of Lopha Limestone that contains the small size of bivalve shell debris. This is followed by a stagnant condition due to sea-level rise that favored the preservation of organic matter in early Maastrichtian. The Emam-Hassan Limestone with high percentages of non-fluorescent amorphous organic matter represents a basin environment. The Latter member is overlain by the marls of Late Maastrichtian. Microbiostratigraphy, sedimentology, and palynofacies observation indicate sea-level fall and a 3.5 Ma hiatus at the K/P boundary. This is followed by deepening and the deposition of the basal purple shale of the Pabdeh Formation. The relative sea-level changes curve drawn based on palynomorphs in this study is well in accord with the existing eustasy curves that indicate the great effect of eustasy factor relative to other factors. Key Words: Sea level change, Palynology, Gurpi Formation, South of Ilam, Zagros Introduction The reconstruction of water depth is an important factor to explain the palaeoecology/palaeoenvironment. Qualitative and Quantitative palynological analysis provides a high-resolution database for biostratigraphy and determines the main palaeoenvironment parameters, these parameters include diversity and abundance of dinocysts, a ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, palynofacies parameters and "selective the preservation of organic matter" factors. The Gurpi Formation in Bisheh–Deraz section in south of Ilam with a thickness of 201 meters consisting of an alternation of blueish-gray shales and marls, with two main members of Seymareh (Lopha) and Emam-Hassan limestones. The lower boundary with the Ilam Formation is continuous and without any erosion and its upper boundary with the Pabdeh Formation is gradual. The Bisheh–Deraz section, with grid coordinates of N 32° 49´ 52˝ and E 46° 59´ 01˝, is located in the southwest flank of Anaran Anticline in the west of Dehloran. The main aim of this paper is the study of palynofacies analysis and paleontological data (foraminifera and index dinocyst) to elucidate sedimentary environments and sea-level changes of the Gurpi Formation. Material & Methods The present study is based on seventy-four samples that were collected from the Gurpi Formation. The samples were processed by standard (HCl-HF) palynological preparation technique, without oxidation or ultrasonic treatments. The palynological residue was sieved through 20 μm nylon sieves. Three to five permanent slides were prepared using glycerin jelly as a mounting medium. The palynofacies analysis is based on the percentage frequency of different POM constituents categorized by Tyson (1995). Discussion of Results & Conclusions The changes in the palynofacies composition in the Bishederaz section and the AOM-palynomorph-phytoclast (APP) ternary plot reveal seven (II, IV, IX, V, VI, VII, VIII) palynofacies types in the Gurpi Formation. The percentage of each palynomaceral component is derived from the total POM frequencies. However, abundance (the number of all dinocyst taxa counted from two slides) and a simple diversity index (the number of species of dinocyst counted from two slides) and the ratio of Peridinioid to Gonyaulacoid (P/G) dinoflagellate cysts, were obtained from the total dinoflagellate cysts frequencies. The difference in the organic matter found in these regressive and transgressive offshore marine sediments have been documented and assessed within a sea-level change. The lower part of the Gurpi Formation of middle Campanian age is very argillaceous and is characterized by an abundance of fluorescence AOM and marine palynomorphs. The minimal size of the black equant phytoclasts and dinoflagellate cyst show that this interval was deposited in a quiet water environment under anoxic conditions in a pelagic environment. This formation shallows upward to boundstones of the Lopha Member in uppermost Campanian with very low amounts of organic matter. The Lopha Limestone consists of macrofossil assemblages (bivalves, oysters (Exogyra type) and echinoids) in different directions and recessive, bioturbated marly layers with scattered large Lopha in life position showing that this member was probably reworked and deposited on the foreland bulge depocenter. This is followed by an abrupt deepening and the deposition of pelagic marl. The high contents of transparent AOM, as well as the presence of pyrite and phosphates in these layers, suggest a dysoxic to the anoxic condition due to sea-level rise that favored accumulation and preservation of organic matter in early Maastrichtian. The Emam-Hassan Limestone represents a more pelagic fauna in a carbonate-rich deepwater setting. The high percentages of non-fluorescent AOM and black phytoclast indicate oxic conditions that were unsuitable for the preservation of marine organic matter. This member is overlain by the marls of Late Maastrichtian. Microbiostratigraphy, sedimentology, and palynofacies observation indicate sea-level fall and a hiatus at the K/P boundary. This is followed by deepening and the deposition of the purple shale of the base of the Pabdeh Formation. This interpretation is supported by sedimentological observation. The Gurpi Formation is entirely influenced by the compressional regime of the foreland basin.

Keywords

• sea level change
• palynology
• gurpi formation
• south of ilam
• zagros