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 gulf ward. 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 relatively thin upper transition zone, with more silt and clay, which grades upward into natural-levee and delta-plain deposits; 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 of ancient bar-finger deposits.
Figures & Tables
Geometry of Sandstone Bodies
This volume contains the eight papers presented as a symposium of the Research Committee of The American Association of Petroleum Geologists at the 1960 annual meeting in Atlantic City, New Jersey. One paper presented in the General Session at that meeting, one reprinted paper, and three other solicited papers are also included.
The choice of “Geometry of Sandstone Bodies” as a timely and pertinent subject lor the 1960 symposium was made after an extensive canvass of Research Committee members and about fifty other geologists vitally interested in research in petroleum geology. From a group of about 15 proposed subjects, this one was selected as first choice by almost all those canvassed. Partly because of this high level of interest, the decision was made to attempt publication of the symposium as a special volume of The American Association of Petroleum Geologists. The word geometry in the title probably had several different meanings among the selectors, and for this reason an attempt was made to define the term adequately in order to establish uniformity of communication among symposium participants.
The dictionary definition of the word “geometry” is the science of magnitudes in space. In applying the term to the symposium theme, some modification and interpretation of its formal meaning were needed, and the following definition was therefore proposed for use in this volume—
Geometry of Sandstone Bodies—Spatial relationships of sandstone deposits within the sedimentary framework.
As used in this book, the subject is more than just a three-dimensional study in which thickness is added to areal distribution.