Facies analysis and sedimentary environments of deep marine deposits from the Sargelu Formation (middle Jurassic) in NW Zagros basin

Document Type : Original Article


1 Department of Geology, Faculty of Sciences, University of Hormozgan, Bandar Abbas, Iran

2 Faculty of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran

3 National Iranian Oil Company Exploration Directorate, Tehran, Iran

4 Department of Petroleum Geology, RIPI, Tehran, Iran


Introduction: The Zagros sedimentary Basin has long been considered by geologists because of the presence of the hydrocarbon reservoirs and source rocks. Variable sedimentology and facies studies in this basin especially in Jurassic and Cretaceous successions have displayed their high potential as hydrocarbon source rocks. The Sargelu Formation is one of these successions that consist of abundant thin-shelled bivalves. In this study diversity and abundance of the Posidonia-dominated faceis have applied to differentiation of the sub-environments and reconstruction of the palaeoecological conditions of the Sargelu Formation. Because of the importance of the Sargelu sediments as source rocks, the achieved results are useful for reconstruction of the paleogeographical condition and hydrocarbon exploration in the Zagros Basin.  
Materials and methods: The studied outcrops of the Sargelu Formation throughout Zagros fold-thrust belt including the Lorestan Zone consists of Tange-Havandeh, Kezi, Dowdan, Bizel, Tange-Mastan and Homajgah. To evaluation and determination of the sedimentary facies and sedimentary environment of the Sargelu sediments, a detailed sedimentary parameters include sediment/package/layers thickness, bedding boundaries (gradational, sharp, erosional), sedimentary structures, variable grain size, type and preservation mode of macrofossils (fragmented, whole, disarticulated, articulated) were recorded in the field. Based on microscopic asses on 500 thin-section, eight carbonate facies were identified in the Sargelu sediments in the studied area. Carbonate facies were classified/named according to Dunham (1962) classification. Identification of the facies/facies associations and their sedimentary environments were based on Wilson (1975) and Flugel (2010) scheme. 
Results and discussion: Based on the microscopic and field analysis, eight facies have been identified in the Sargelu sediments that were related to two facies associations including basin to outer ramp of a distally steepened ramp system. Mud-dominated, fine-grained facies were deposited in the basin environment below SWB, characterized by long time of the low sedimentation rate, low water circulation and low oxygen level, displayed by high abundance of the organic material in this sediments. Outer ramp association consists of thin- to medium-bedded dark limestone. These special features display periodically activity of the turbidity currents during deposition of the shell concentration and higher sedimentation rate and relatively favorable environmental conditions for the benthic communities, displayed by abundance bioturbation in these sediments.
Conclusion: Facies analysis of Sargelu Formation exposed in Lorestan Zone led to identification of two facies associations, representing basin to outer ramp of a distally steepened ramp system. Basin deposits in central Lorestan Zone include pelagic and hemi-pelagic deposits, displayed by mud-dominated, fine-grained facies. In contrast, Posidonia pelagic bivalves are interpreted as the result of the proliferation of this bivalve during periods of favorable environmental conditions on the sea floor. Disarticulation, fragmentation, and reorientation of Posidonia pelagic bivalves can be indicative of reworking, by turbidity currents which periodically interrupted periods of lower-energy suspension sedimentation.


Main Subjects

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