Stratigraphy and Sedimentology
GLY 471


Guide to Petrologic Description of Sedimentary Rocks


Description

Hand Specimen

This should be brief, and hand lens must be used. Determine color, mean grain size (using comparative size scale), sorting, roundness, mineral composition of grains and cement, induration and weathering. Note any sedimentary structures, fossils, or tectonic features. Give a field name to the specimen, e.g. fine grained sandstone, oolitic limestone, black shale.

Thin Section

A thin section description should be accompanied by a sketch of the main features of the slide. It is often useful to look at a thin section under the hand lens before using the microscope.

The following properties should be described where possible or relevant, and you may need to describe other properties that are not listed below.

1. Estimate mean grain size and sorting (using comparative chart) and check with your hand specimen estimates. Find percentages of gravel, sand, silt, and clay sized particles.

2. Estimate roundness (using comparative chart) and elongation of the grains, and note how related to size and composition. Note that three-dimensional shape measures can not be determined in thin section.

3. Nature of grain contacts (e.g. long, point, sutured), relation of grains to detrital matrix (matrix versus grain supported), grain orientation, geopedal fabrics, etc...

4. Percentages of terrigenous and chemical/biogenic constituents. Describe terrigenous minerals and varieties of minerals in the following order: quartz, chert, feldspar, rock fragments, mica, detrital clay, heavy minerals. Describe chemical/biogenic grains separately, i.e. fossils (reference manuals can be used). ooliths, intraclasts, pellets, glauconite, etc...

For each grain type record its percentage, distribution and orientation in the slide, and any correlation between composition, size, shape, and alteration. Note also overgrowths, coatings, nuclei, inclusions, special optical properties, etc...

5. Composition and type of cement, or detrital matrix, or both. Percentage of whole rock.

6. For terrigenous rocks estimate mineralogical and textural maturity.

7. Classify rock using scheme provided. Note that, when using triangular diagrams, the percentages of the three end members must sum to 100.

Interpretation

From your observations you should be able to draw some conclusions about the origin of the rock. It must be realized, however, that full interpretation of the origin of sedimentary rocks must result from a compilation of all types of sedimentary information. In particular, knowledge of fossils, sedimentary structures, laterally equivalent and vertically adjacent lithofacies is essential. However, the following comments may be useful:

A. Source area


A mineralogical immature sediment may give an idea of rock types in the source area, and also indicate uplift and rapid erosion from high relief (minimal weathering and abrasion). A mature mineralogy commonly reflects recycling, although intense chemical weathering in hot humid lowland areas may be responsible in a single cycle.

B. Transportation and depositional environment

Terrigenous sediments:

A single grain size measure is not diagnostic of nature of transport except to indicate minimum competence of transporting fluid. Vertical and lateral variations in sedimentary structures and grain size are more useful here. However, thick conglomeratic/breccia sections indicate large gradients and proximity to source.
Abrasion (hence rounding and removal of mechanically unstable grains) is minimal for sand sizes (except when wind blown) but appreciable for gravel sizes. Hence good rounding in sands is mostly inherited. Unweathered and rounded unstable grains probably indicate substantial abrasion during a single sedimentary cycle, whereas weathered rounded unstable grains probably indicate weathering was an important factor in the rounding.
The clay content and sorting reflect final depositional setting.

Chemical/biogenic sediments:

The texture of chemical/biogenic grains can only give an idea of the nature of transportation if an in situ origin can be ruled out, e.g. ooliths and intraclasts with a sparry calcite cement. Carbonate sediments may be produced in situ with angular or rounded grains, with or without matrix of micrite. However presence of micrite normally implies an absence of current reworking, but the converse is not true.
The nature of grains themselves can give a good idea of depositional environment (e.g. fossils, glauconite).

 

C. Diagenetic History


Timing of cementation, dissolution, recrystallization. Some diagenetic textures and early diagenetic minerals can give an idea of depositional environment (e.g. calcite cements, dolomite, iron minerals, etc...). Some detrital matrix (if any) may be post depositionally filtered in or result from breakdown of volcanic or sedimentary rock fragments.



Return to Sedimentology Index