رویدادهای اقیانوسی فاقد اکسیژن و تشکیل توالی فشرده‌ی فرازنین (Frasnian condensed section) در البرز مرکزی

         In central Alborz, a condensed section of carboniferous black mudstones, glauconitic quartz sandstones, open marine limestones and phosphorite interbeds has been deposited in the lower part of the Jirrud Formation (Upper Devonian). The characteristics of this group of facies are quite similar to other sedimentary phosphate deposits of  the world. The Late Frasnian condensed section, with a thickness of  less than ۱۰۰ meters, has been sedimented due to maximum sea transgression (during the highstand systems tracts).  It is recommended that the facies have been accumulated on a continetal shelf and slope, along the margins of an extensive deep oceanic basin. Low latitudes, depth (ranging from ۵۰ to ۲۰۰ meters), warm waters, low clastic 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 rifting and the formation of spreading passive margin of the Paleotethian oceanic basin (Lasemi, ۱۳۷۹). 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.