SANDY SHALES (SILTSTONES) AND SANDY FIRECLAYS

Sandy shales (or siltstones) and sandy fireclays are distinguished from sandstones because they are darker in color, less gritty to the touch and do not have a distinctly grainy appearance. They can be distinguished from shales and other fine-grained rocks because they are distinctly more gritty to the touch. There are two major criteria for distinguishing different types of gritty, fine-grained rocks–bedding (fracture) and color. These properties are described below.

Bedding or Fracture

Three major types of bedding or fracture are found in sandy shales and sandy fireclays–(1) layered and streaked, (2) massive or uniform and (3) churned, which includes root penetrated or burrowed.

Sandy Shales with Layered or Streaked Structure consist of bands or layers of sandstone interlayered with shale lying in planes at, or nearly at, right angles to the side of the core. Where sandstone makes up between 80% and 20% of the total volume of the rock, the rock is said to be a Shale and Interbedded Sandstone (e.g. 50L). Where the sandstone makes up between 20% and 1% of the core, the rock is designated as a Shale Streaked with Sandstone (e.g. 55R). (Where sandstone layers exceed 80%, as in specimen 35R, the rock is designated as a Sandstone with Shale Streaks.)

Among shale and interbedded sandstones, it is desirable, in some cases, to indicate the character of the sandstone layers. Where the sandstone layers are of uniform thickness from one side of the core to the other (e.g. 54L), the rock can be designated Shale and Interbedded Sandstone, Flat. Where the sandstone occurs as lenticular layers, the rock is to be designated Shale and Interbedded Sandstone, Rippled (e.g. 50R). Fracture in all layered and streaked sandy shales is parallel to the plane of the layering.

Sandy Shales with Massive Structure lack distinct layering or any other kinds of obvious lines or spots (e.g. 58L). The term "massive", in this case implies a homogeneous or uniform appearance. The way the rock fractures does indicate, however, differing degrees of grain orientation which leads to the differentiation of two types of "massive" structure. In specimen 58L, for example, the grain orientation is parallel, or nearly so, and fracture is distinctly planar. Such rocks should be designated Massive Sandy Shale. In contrast, some cores like specimens 60L and 60R show slight traces of irregularly oriented material on the side of the core and show, like specimen lower 66L, a slightly irregular fracture of the broken butt end. Both features reflect minor grain disorientation. Such rocks that are outwardly massive but which show evidence of minor grain disorientation are designated Massive Churned Sandy Shales.

Sandy Shales with Churned Bedding can be recognized by irregularly distributed spots, bands, or wisps on the side of the core or by highly irregular fracture. The irregularly distributed spots or bands and grain disorientation in churned rocks are produced by the organic activity of animal burrowing or root penetration by plants and other processes of soil formation. An example of burrows produced by worms or possibly small shellfish is specimen 63L, whereas an example of root penetration is specimen 61L. Root penetration can be distinguished from burrowing by thin carbonaceous or coaly films that surround the root structure. Such materials are not found surrounding animal burrows. In addition, root penetrated rocks usually have a much more irregular fracture than burrowed rocks.

Color

There are three major colors of sandy shales–black, dark gray and light gray green–of which dark gray is, by far, the most common. Black (e.g. 51R) and dark gray (e.g. 51L) are generally recognized color terms. Light gray green (e.g. 62R) is used here for rocks that are light gray but commonly have varying tints of green or occasionally tan.

Among sandy shales with massive or churned structure, the colors are distributed with reasonable uniformity throughout the rock and the color terms apply to the entire rock. In sandy shales with layered or streaked structure, darker shaley material is interbedded with sandstone of a lighter shade. In these rocks, the color designation is applied to the shaley portion.

A key for identifying the commonly occurring types of sandy shale is given in Table 3.