Logging Sedimentary Outcrops and Cores New 2

7/28/2019 Logging Sedimentary Outcrops and Cores New 2

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GEOL*4090 LOGGING SEDIMENTARY OUTCROPS AND CORES

Graphic logs are visual representations of the information you collect about the outcrop (see

attached example). It should summarize the following:

1. Thickness of units (vertical axis)2. Texture (average grain size-horizontal axis)

3. Lithology

4. Sedimentary structures5. Fossils

6. Diagenetic features

7. Contacts between units8. They may also contain additional descriptions, notes, measurements and so on if required

Guidelines for logging a section

1. Examine the outcrop from a distance to establish where the major changes and breaks insediment types are. Sketch a general view of the outcrop showing the major breaks and geometry

of the units you see (Fig. 1)

2. Establish and describe accurately the most typical sediment types (facies). Usually you will

have 5-6 fundamental types and a lot of variations. Describe the most characteristics types(facies). Name these facies with pre-established codes (like those of Miall shown in Figure 2) or

make up your own mnemonic ones. Keep the description short and to the point. You can do this

in paragraph form illustrated with sketches and photographs and/or you can set up a summarytable (see example below) (Fig. 3)

In the description make sure you note the following.

Lithology. Composition of grains and cement/matrix. Mean grain size, sorting, shape, fabric.

Degree of cementation or weathering.

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.


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3. Measure (from the bottom up) and describe the most characteristic exposed vertical sections.

Such sections will be composed of stacked up beds (units, facies associations) containing thesequences of facies you have selected, but some will show variations from the types you have

selected and described in detail. So as you measure and describe the sequence, note the facies

you encounter pointing out any differences from the type you selected (for example average

grain size, slightly different sedimentary structures, paleocurrents directions, and so on). Notethat facies associations (units) may repeat several times in the section and they may be separated

by erosional or other major breaks. Carefully note these facies associations and the breaks. They

represent respectively the environments and changes that occurred during deposition.

Try to describe things as you see them without bias. However in the field you may start

formulating an idea of what type of environment and hydrodynamic conditions produced thesediments.

4. Once you have collected your field data, draw up your logs following the examples provided

(Figs. 4,5,6,7), indicating diagrammatically the sediment unit you have identified, thickness,

grain size, sedimentary structures and other major features, using standard symbols or makingyour own if needed. Make sure that your log has a legend describing the meaning of the symbols

used.Add any extra measurement you made such as paleocurrents directions fossils if any, and

brief explanatory additional notes. You may add very simple sketches of unusual features that

are difficult to describe in words.Use a shorthand facies symbols in a column.

Make sure that your log is to scale.

Figure 1. (From Nichols,1999)


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Figure 2. Suggested Facies Codes (After Miall, 1978)


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Figure. 3. Example of Facies table


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Figure 4. Example of a form for stratigraphic/sedimentologic log (from Nichols, 1999)


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Figure 5. Example of sedimentologic log


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Figure 6. Example of interpreted log


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Figure 7. Standard symbols used in straigraphic/sedimentologic logs


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Figure 8. Examples of sand and gravel fluvial sequences (after Miall, 1978). These are simplified versions forcomparison of different fluvial systems only. As a result they are missing some information.


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Figure 9. Example of a panoramic picture of a sand and gravel pit face [Lesley Pit (Right side of upper picture t


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REFERENCES

Miall, A.D. 1978. Lithofacies types and vertical profile models in braided river deposits: a summary, In: Miall,

A.D. (Ed.), Fluvial Sedimentology. Canadian Society of Petroleum Geologists, Calgary, Memoir 5, pp. 578-

586.

Nichols, G., 1999. Sedimentology & Stratigraphy. Blackwell Science, Osney Mead, Oxford. pp 62-75

(Environments and facies)

Logging Sedimentary Outcrops and Cores New 2

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