Internal Structures of "Homogeneous" Sandstones

Abstract

The internal structure of 316 samples of seemingly homogeneous sandstones collected from 61 formations was analyzed by x-ray radiography. These analyses show that in 97 percent of the samples the individual grains are not packed together in a random fashion, but are arranged into distinctive structural units. The basic structural framework of the "homogeneous" sand stones studied consists of various types of cross-strata and rhythmic laminations. These structures may occur singly or in various combinations throughout a given bed and may be modified to some degree by burrowing organisms, secondary mineral growth, or penecontemporaneous deformation. Micro-cross-lamination is the most common structure and indicates that many seemingly homogeneous sandstones accumulated by deposition on the lee slope of various types of ripple marks. Large scale cross-strata are also abundant but details of their size and shape could not be determined by radiography. Apparently the various types of cross-strata found in deposits where sedimentary structures are well expressed also occur in seemingly homogeneous beds. Varve-like laminations were found in many samples but are not considered to be the product of repetitive sedimentation involving a definite cycle of time. Small, irregular lenses are typical of "structureless" mudstones and blocky shales.

It is concluded that most seemingly homogeneous sandstones do not represent a special sedimentary environment nor do they result from special rates of sedimntation. They develop by vertical and lateral accretion in much the same manner as sediments in which stratification is well expressed. The visual expression of stratification in elastic rocks is to a large degree fortuitous and may be controlled by many unrelated factors, such as nature of the source material, weathering, and diagenesis. Thus, there exists a complete gradation from deposits in which structural detail is well expressed to those n which structure appears to be completely lacking.

One of the most significant aspects of stratification is the size, shape, and orientation of the smallest layer within the rock. Such a layer is referred to as a "unit stratum" and is important in that it represents a depositional interface and reflects the processes operating within the environment during the time of deposition. Thickness of bedding has little genetic significance except as an indication of the amount of continuous deposition under a given set of conditions.