A linear, continuous deposit, composed primarily of fine and very fine sand, which occupies the channel and distributaries of the lower Mississippi River is termed a “bar-finger sand.” One explanation relates the origin to deposition of the thick sand units at the mouth of the river as distributary-mouth bars. This process is acknowledged as the initial phase; however, it is postulated that development of these sands continues after the river has prograded beyond the original site of bar formation. This continuing process is postulated to be related directly to the annual discharge pattern of the river.
The intrusion of salt water into the river channel during periods of less than maximum discharge reduces the current and carrying power of the river directly above its bed and causes deposition of the coarser fractions. This process is postulated to be the other major cause for the deposition of large amounts of sand along the channel. Inasmuch as this phenomenon occurs during much of the year, a net buildup of sediment occurs. Recognition of the importance of bar-finger sands may aid the exploration geologist in his quest for petroleum in areas where the deltaic process has been active.