Sedimentary Evolution Of The Late Cretaceous Strata In Lurestan-Fars Areas,Northern Margin Of Arabia

The Late Cretaceous is one of the important time intervals in the geological history of the Arabian plate when many major tectonic events affected this area happened.In Turonian-Maastrichtian.,the Neotethys Ocean began to close and Ophiolite complexes were developed in different areas of the present Zagros imbricate zone.Some tectonic activities represented by ophiolite obduction had brought important sedimentary environment changes to the northern margin of the Arabian Plate.The sedimentary strata had been gradually transferred from marine carbonate rocks of Gurpi Formation in the Late Cretaceous to the clastic rocks of Amiran Formation in the earliest Cenozoic.Previous studies on the age restriction of ophiolite tectonic activity through microfacies are few.Based on this,the Late Cretaceous marine strata developed in the Lurestan and Fars areas on the northern margin of the Arabian Plate were systematically studied in terms of stratigraphy,petrography and sedimentology in this paper.By analyzing carbonate microfacies and sedimentary environment of Savark Formation(Cenomanian-Early Coniacian),Ilam Formation(Early Coniacian-Early Campanian)/and Gurpi Formation(Early Campanian-Early Palaeocene)in the northern margin of the Arabian Plate during the Late Cretaceous,this paper provides a new understanding on the timing of the onset of the ophiolite obduction and its sedimentary response in the Zagros area.Upper Cretaceous marine carbonate rocks were widely distributed in Lurestan and Fars areas in the northern margin of Arabia.In this paper,the stratigraphic age of the upper Cretaceous in the study section is defined by the biostratigraphic study.Based on texture,sedimentary structure,gain composition,and fossil type and content,13 microfacies(MF1-MF13)and 3 microfacies assembles(A1-A3)overall were identified.They are Calcisphere Wackestone(MF1),Planktonic foraminifera Wackestone(MF2),Calcisphere Wackestone/ Packstone(MF3),Planktonic–benthic foraminifera Wackstone with glauconite(MF4),Intrabioclastic Packstone(MF5),Bioclastic wackestone/Floatstone(MF6),Bioclastic Floatstone /Rudstone(MF7),Bioclast-large benthic foraminifera Wakestone(MF8),Diverse bioclastic Wackestone/Packstone(MF9),Rudist debrisintraclasts Grainstone(MF10),Cortoids grainstone(MF11),Peloid Grainstone(MF12),Dasyclad-Large benthic foraminifera Wackstone(MF13).The sedimentary facies model of carbonate ramp in the study area was established and can be further divided into inner ramp(lagoon and bioclastic shoal),middle ramp(upper and lower parts)and outer ramp sedimentary environment.There were obvious differences in carbonate sedimentary characteristics between Kermanshan section and Khoramabad section in Lurestan area and Khormoj section in Fars area.There was a depositional gap between Savark and Ilam Formations in Khormoj section during the early Late Cretaceous,which changed from a deep-sea environment to a shallow marine environment.However,there was no depositional gap between Savark and Ilam Formation in Kermanshan section and Khoramabad section in Lurestan.Late Campanian Gurpi Formation in these two sections were interbedded with clastic limestone and gravel layers,which were different from the Khormoj section.Microfacies analysis showed the paleo-water depth of Kermanshan section and Khoramabad section became shallower during the late Campanian(80-72 Ma),changed from outer ramp environment to shallow marine inner ramp environment and developed shallow water biological assemblages.Combined with previous studies,it is considered that the depositional gap between Savark Formation and Ilam Formation in the Fars may be related to the emplacement of Neyriz ophiolite in the northeast,the Hormoz basement fault and paleo-uplift in the east.Also,the warm and humid erosional climate and the declining of global sea level contributed to the large-scale development of the depositional gap.The shallowing of paleo-water in Lurestan during the Late Cretaceous may be related to the Kermanshah Ophiolite obduction at about 85 Ma in the northeast of Lurestan.Therefore sedimentary responses existed in the Kermanshan section and Khoramabad section.Because Kermanshan section was close to the Kermanshan Ophiolite zone(about 85km)and may be located at a specific paleogeographic location,such as the foreland basin forebulge,thus the sedimentary response of the Kermanshah Ophiolite zone could occur earlier and for a longer time.However,the Khoramabad section was far away from the Ophiolite outcrop(about 150km),and briefly shallower sedimentary records appear in the Late Campanian.However,the Khormoj section in Fars was more than200 km away from the Ophiolite belt and was not affected by the Ophiolite obduction and did not show obvious sedimentary environment fluctuations.