Characteristics of Geologic Materials and Formations - Geotechnical Engineering Investigation - page 358

PLATE 5.19 ( Color Plate 5.19 follows
page 396.)
Chert interbedded in the Edwards
Limestone (Round Rock, Texas).

TABLE 5.9
Characteristics of Detrital Sedimentary Rocks

Rock Type Characteristics Plate

Coarse-Grained (Rudites) (2 mm)

Conglomerates
General Rounded fragments of any rock type, but quartz predominates. Cementing 5.11

agent chiefly silica, but iron oxide, clay, and calcareous material also common

Pudding stone Gap-graded mixture of large particles in a fine matrix

Basal First member of a series; deposited unconformably

Breccia Angular fragments of any rock type. Resulting from glaciation, rock falls, cave

collapse, fault movements
Medium-Grained (Arenites) (

50% Sizes between 0.02 and 2.0 mm)

Sandstones Predominantly quartz grains cemented by silica, iron oxide, clay, or a 5.12

carbonate such as calcite.Color depends on cementing agent: yellow, brown,
or red — iron oxides predominate: lighter sandstones — silica or calcareous
material predominates. Porous and previous with porosity ranging from
5 to 30% or greater. Material hard, and thick beds are common

Arkose Similar to sandstone but with at least 25% feldspar 5.13

Graywacke Angular particles of a variety of minerals in addition to quartz and feldspar,

in a clay matrix. Gray in color: a strongly indurated, impure sandstone
Fine-Grained (Lutites)

Siltstone Composition similar to sandstone but at least 50% of grains are between

0.002 and 0.02 mm. Seldom forms thick beds, but is often hard

Shale 5.14

General Predominant particle size 0.002 mm (colloidal); a well-defined fissile fabric.

Red shales are colored by iron oxides and gray to black shales are often colored
by carbonaceous material. Commonly interbedded with sandstones and
relatively soft. Many varieties exist

Argillites Hard, indurated shales devoid of fissilily; similar to slates but without slaty cleavage

Calcareous Contain carbonates, especially calcite. With increase in calcareous

shales content becomes shaly limestone

Carbonaceous Black shales containing much organic matter, primarily carbon, often grading

shales to coal formations

Oil shales Contain carbonaceous matter that yields oil upon destructive distillation

Marine shales Commonly contain montmorillonite clays that are subject to very large

volume changes upon wetting or drying (see Section 6.7.3 and Figure 6.91)

Clay shales Moderately indurated shales 5.15

Claystones and Clay-sized particles compacted into rock without taking a fissile structure

mudstones (the geologist's term for clay soils in the stiff to hard consistency)

Note : 1. Sandstones and siltstones are frequently interbedded and grade into one another unless an unconformity

exists.
2. Flysch: A term used in Europe referring to a very thick series of sandstone, shales, and marls (impure
limestones) well developed in the western Alps.