<p><span style="font-size:14px"><span style="font-family:Arial,Helvetica,sans-serif">The determination of paleodepth and even its changes can have many applications in </span></span><span style="font-size:14px"><span style="font-family:Arial,Helvetica,sans-serif">study</span></span><span style="font-size:14px"><span style="font-family:Arial,Helvetica,sans-serif"> the depositional environment and also the factors that depend on the water depth. Due to the abundance of foraminifers, especially planktonic foraminifera in the Gurpi Formation, one of the best ways to determine the changes in paleodepth is the investigation of planktonic foraminifera morphotypes abundance. By the investigation of the existing foraminifera morphotypes in the studied samples of the Gurpi Formation (Santonian - Maastrichtian) in the Bachun stratigraphic section (West of Firuzabad), an increase in the depth of water in Campanian and a decrease in depth of water in Maastrichtian is observed.&nbsp;</span></span></p>