section Oceanic anoxic events and the formation of Frasnian conden Alborz

General:
     In central Alborz, a condensed section of carboniferous black mudstones, glauconitic quartz sandstones, open marine limestones and phosphorite interheds has been deposited in the lower part of the Jirrud Formation (Upper Devonian). The characteristics of this group of facies arc quite similar to other sedimentary phosphate deposits of the world. The Late Frasnian condensed section, with a thickness of less than 100 meters, has been sedimented due to maximum sea transgression (during the highstand systems tracts). It is recommended that the facis have been accumulated on a continetal shelf and slope, along the margins of an extensive deep oceanic basin. Low latitudes, depth (ranging from 50 to 200 meters), warm waters, low elastic influx, and very slow rate of sedimentation are the controling factors. The studies indicate that some conditions such as oceanic upwelling, great numbers of living phytoplanktons and fishes, oceanic anoxic events, dominant oxygen minimum zones, low variations in sea water chemistry, and long time stability of the sedimentary basin in a tropical warm and wet climate, have been responsible for the formation of the Late Frasnian phosphate — bearing condensed section. These kinds of processes have occurred in northern Gondwana supercontinent. due to the rifling and the farmation of spreading passive margin of the Paleotethian oceanic basin Lasemi. 1379). As a result of thermal subsidence, after a post—rift unconformity, the Late Frasnian condensed section of the Jirrud
Formation has been formed progressively (onlap) on the older rocks.