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,. including prodelta , delta front , and lower deltaic plain <br /> faces . <br /> The Lower Bowie Submember is basically <br /> nonmarine. However , it is overlain by a marine shale and <br /> sandstone sequence almost identical to the upper portion of <br /> the Iles Formation. The contact between the marine and <br /> nonmarine beds varies in vertical position, as would be <br /> expected if a sea overlapped an area of complex deltaic <br /> sedimentation . <br /> In some areas , continuous sedimentation is <br /> indicated, grading from nonmarine to marine rocks , while in <br /> other localities a subaqueous or subaerial diastem may <br /> exist . The overlap may be due to sudden subsidence below <br /> • sea level, with a corresponding lack of sediment influx, or <br /> to the normal constructional-destructional phases of a large <br /> deltaic system. <br /> There are three reasons for classifying the <br /> sequence as marine : a rapid increase in the density of <br /> trace fossils occurs above the boundary ; the fossil types <br /> present indicate a marine environment ; a marked change in <br /> bedding occurs with much more regular bedding in the marine <br /> section. <br /> The only significant difference between the <br /> upper Iles and this marine portion of the Bowie is the much <br /> finer development of burrow structures in the sandstones and <br /> • shales of the latter . Cvlindrichnus reptilis and Thalas- <br /> 22 <br />