تفسير تاريخچه رسوبگذاري سازند جهرم در ناحيه بوشهر بر مبناي روزن داران كف زي و ايزوتوپ استرانسيم

Depositional History of Jahrum Formation based on Distribution of Benthic Foraminifera and Strontium Isotope Stratigraphy on the Bushehr area

In order to determine the depositional environment and age of the earbonaeeous Jahrum Formation, bcnlhic foraminiferal assemblage of upper Eoeene in the Jahrum Formation carbonates, loeated in Gisakan Mountain, 10 km east of the Borazjan city and adjaeent fields, were studied. Based on the foraminiferal distribution and abundance, the paleoenvironment were reconstructed. Due to the presence of Nummiriiies fabianni and based on strontium isotopic analyses, the age between 33.7-35Ma (Priabonian) was determined, The mean sedimentation rate of ~ 2 m/10000 yr. was calculated. This study indicated Asmari Formation deposited afier a hiatus, more than four million years (Rupehan stage). Gradual decreases of the sea level and hence the distribution of large benthic foraminifera resulted in the deposition of six sedimentary faeies along the inner-middle to outer carbonate ramp system. These facies, from deep to shall ow parts ofthe basin, inelude argillaceous carbonates containing plank tonic foraminifera that were deposited in a hemipelagic to pelagie environment (whieh is the transition zone from the Pabdeh to Jahrum Formation), carbonate facies of operculina waekestone to packstone related to the outer carbonalc ramp, discocyclina /nummuliles wackeslone to packstone facies deposited in the middle to outer ramp, nummulites waekestone facies occurred in the middle ramp, nummulites orbitolites waekestone to packstone belong to the middle to inner ramp, and finally the orbitolites/bioclast packstone deposited in the inner ramp.