In order to interpret the depositional environment of sedimentary
rocks it is necessary to describe them in detail in three-dimensions.
Only rarely are sedimentary rocks exposed extensively in
three-dimensions. In most cases three-dimensional description must be
pieced together from spatially correlatable one- or two-dimensional
exposures. Description normally follows a logical sequence as
follows:
1. Brief reconnaissance of the whole outcrop to gain a
preliminary impression of the main sedimentary features such as
sedimentary rock types present (as defined by color, composition, and
texture), geometry and orientation of beds and bedsets, sedimentary
structures, and associated paleocurrents, trace and body fossils,
vertical and lateral variation in these sedimentary properties. An
effort should be made to define preliminary lithofacies and
lithofacies associations (sequences). You will need a notebook, hand
lens, clinocompass, measuring tape or stick, grainsize comparator,
acid bottle, etc.
2. Detailed description. A large two- or three-dimensional
outcrop is difficult to describe completely in detail. The first step
is to describe the large-scale bedding geometry and distribution of
sedimentary facies. This is normally done using a field sketch or,
preferable, with photographs. The end product should be a scale
diagram of the exposure showing major bedding surfaces and
lithofacies. Clearly, a stream section does not lend itself to this
type of approach.
More detailed description of a two- or three-dimensional outcrop is
accomplished by measuring a series of vertical sedimentological logs
spaced judiciously across the outcrop. Such vertical logs should be
tied very closely to each other (i.e., correlated) and to the
large-scale outcrop diagram. Spacing of the logs will depend on
accessibility and lateral facies variations. If facies do not vary
much laterally, there is clearly no need for many logs.
When measuring sedimentological logs, start at the base of the
outcrop and, where possible, record the following information for
each bed or bed set:
Lithology. Composition of grains and cement/matrix. Mean grain size, sorting, shape, fabric. Degree of cementation or weathering. Color on fresh and weathered surfaces. Spatial variations of lithology in a bed or bed set. Name the sedimentary rock based on this information.
Bedding Geometry. Thickness, including lateral variation. Are beds sheet-like, lenticular, etc.? Nature of upper and lower boundaries (erosional, sharp, gradational, etc...)
Sedimentary Structures. Describe internal and external structures, including their dimensions, orientations, spatial variation, etc. If difficult to see, collect a sample for laboratory analysis.
Direction Indicators. Record direction azimuth separately for each structure. If regional dip exceeds approximately 15°, a correction is required to obtain the true paleocurrent azimuth.
Fossils. Identify body and trace fossils, and collect samples if possible. Record variation in size and shape, abundance and spatial distribution, orientation, sediment-fossil and fossil-fossil associations, nature of preservation.
Annotated scale sketches or photos should be used copiously to aid in
description. Be prepared to move to the left or right of the line of
section in order to allow complete description. As sedimentary rocks
tend to occur in repetitive sequences, you will find that, after the
initial stages, it is not necessary to determine all of the above
sedimentary properties for every bed or bed set. It is obviously an
advantage to recognize distinctive facies at an early stage. Regional
dip and strike should be recorded for each outcrop, also nature of
cleavage, folding, and faulting.
The best way of recording sedimentological information in the field
is to construct a 'rough' graphic log and write detailed observations
next to it, supplemented by field sketches. The graphic log is then
redrafted in the laboratory at an appropriate scale. There are
reasonably standard ways of compiling the descriptive information
graphically. Examples are given on attached sheets. Notice that it is
possible to display a large amount of information on graphic logs,
including mean grain size, composition, bed geometry, sedimentary
structures, paleocurrents, fossils, colors, etc.. It is critical to
think about the best scales to use.