Bar-Finger Sands of Mississippi Delta1

Abstract

Recent core tests in the Mississippi bird-foot delta provide additional information on the geometry and facies characteristics of bar-finger sands. These elongate lenticular sand bodies underlie the 15- to 20-mile-long major distributaries of the river and are characterized by a branching pattern with interbranch areas widening gulfward. Originating as distributary-mouth bar deposits, the fingers reach maximum widths comparable to those of the present-day bars--approximately 5 miles. Their lenticular form and maximum thickness of more than 250 feet result largely from displacement of water-rich delta-platform clayey silts by the mass of accumulating bar sands. Each finger comprises three zones--a central zone of clean sand with minor amounts of silt and clay; a relative y thin upper transition zone, with more silt and clay, which grades upward into natural-levee and delta-plain deports; and a lithologically similar, but relatively thick, lower transition zone which grades downward and laterally into delta-front deposits. Typical internal features of the fingers include--thin layers with unidirectional cross-bedding in the upper transition and central zones, and thin festoon cross-beds throughout; laminae of plant fragments and scattered laminae of clayey silt; minor faults and contorted beds in the lower transition zone; and an absence of both microfauna and macrofauna. Locally, the bar fingers have been deformed by upward movement of mud lumps. The lower Pennsylvanian Booch sand of the greater Seminole district, Oklahoma, provides an excellent example f ancient bar-finger deposits.