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

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. Pelagic disarticulated bivalves concentrations in the deep marine sediments have been introduced as filaments that have recorded in variable middle Jurassic sediments around the world. 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.  

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. 

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 (calcareous shale, mudstone, pelagic bivalve wackestone/floatstone) including pelagic and hemi-pelagic deposits 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 (Pomar et al, 2012; Bayet-Goll et al, 2018; Sharafi et al, 2016, 2019). Outer ramp association consists of thin- to medium-bedded dark limestone including abundant Posidonia-dominated shell concentration with sharp-based, chaotic to preferentially parallel bed orientation of the skeletal elements and fining up-ward fabrics. 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 (with respect to the oxygen and nutrients) for the benthic communities, displayed by abundance bioturbation in these sediments.

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. These sediments were deposited in an anoxic to dysoxic, relatively deep, sea-floor environment as revealed by the presence of: a) organic-rich shales and limestones, b) undisrupted lamination in most facies, c) absence benthic fauna. 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. Thin to medium beds of graded wackestone/packstone to rudstone/floatstone and accumulations of thin shells of Posidonia pelagic bivalves indicate sporadic reworking of the sea floor by turbidity currents. 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.