The Cretaceous of the Rocky Mountain region contains sandstones that were deposited in marine, transitional, and nonmarine environments. Spatial dimensions of sandstones deposited in shallow neritic and transitional environments are regular in character and are easily defined. Only this type of sandstone is here considered, and examples illustrating minimum and maximum geographic distribution are treated.
Minimum size sand bodies are well shown by the Fox Hills sandstone where it is exposed on the northeast flank of the Rock Springs uplift, Wyoming. This formation consists of a series of barrier bar sandstones that change northwestward to lagoonal shales (Lance formation), and southeastward to marine shale (Lewis shale). Detailed surface analysis of one barrier bar shows a thickness of 30 feet and a width of 6–7 miles from the lagoonal shale and sandstone facies to the marine shale and siltstone facies. Each bar is believed to have extended along much of the western margin of the Cretaceous seaway.
The upper part of the Judith River formation of central and eastern Montana exemplifies a transitional and marine sandstone unit having a maximum width. The unit is 140 miles wide and was deposited between lagoonal shale facies to the west and marine shale facies (Pierre shale) to the east. Thickness of the unit ranges from a wedge edge to 100 feet.
The geometric pattern of most of the sand bodies that accumulated along the Cretaceous shoreline is similar in character to the above examples and ranges in size between these extremes.
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.