عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In this research for the first time, conglomerate layers of the Triassic (Elika Formation) and Jurassic (Dalichai Formation) successions has been studied in regards to sedimentary environment and origin in Zanjan area. The Triassic conglomerates are only composed of intraclast with floatstone-rudstone texture. These intraformational conglomerates with an erosional surface and graded-bedding structure (fining upward), indicate a high-energy condition and effect of storm process in shallow marine environments. While in middle Jurassic strata, the presence of thin shell bivalves, radiolarian, ammonite, belemnite, planktonic foraminifera and microbialites show relatively deeper marine environments. Thick-bedded conglomerates along with these fossil contents are oligomictic, which contains microbialites, bryozoans, ammonite and belemnite. These layers are poorly sorted with grain size from 3 to 30 cm and clearly show a high-energy condition during deposition possibly related to falling stage of sea level across middle-upper Jurassic?. Angular to rounded grains, irregular fabric, no specific sedimentary structure such as bouma sequence show sedimentation with debris flow. Finally, the results show that the Zanjan area as a small part of northern Tethys realm, has been changed from near-shore marine environment to relatively off-shore area as e result of eustatic sea level rise during Triassic-middle Jurassic and then followed by a sea level fall at the middle/upper Jurassic boundary?. In this large-scale sedimentary package, the Triassic carbonate conglomerate has been considered as tempestite facies in shallow marine environment. Whereas conglomerate sequences in upper part of Dalichai Formation shows effect of ocean currents on microbialites in relatively deeper marine environments.