Learner resource 2–Using graphic logs
Introduction
Graphic logs are a powerful tool used by geologists to record the detail of a sequence of rocks in a graphical form. A graphic log can be constructed from a rock exposure (such as a cliff, road cutting or beach platform), cores from a borehole or using remote sensing images. Graphic logs are a pictorial representation that summarises lots of data in a way that allows geologists to recognise patterns, test hypotheses and make correlations between sequences. There are standard conventions for all graphic logs although there are a variety of styles depending on who drew it and why it was drawn (field summary, project report, detailed correlation, facies analysis).
A graphic log is a type of horizontal bar chart, although there may be additional columns to the left/right of the bar chart displaying additional data:
The y-axis of a graphic log represents the cumulative thickness of the sequence of beds or rock units. The height of each bar represents the thickness of an individual bed. The origin of the y-axis may be the base of a cliff (with values increasing upwards), the ground surface for boreholes (with vales increasing downward), or a marker bed
The x-axis of a graphic log represents the grain size of the rock unit/bed (for siliciclastic rocks); for limestones it represents the % mud in the rock. Fine grain size (or high mud %) is to the left and coarse grain size (or low % mud) is to the right.
The rock type (lithology) of each bed is shown by a stipple (pattern) either in the bar or in a separate column. Sands are shown by dots, fine sediment by horizontal lines and limestone by brickwork, as shown below.
Sedimentary structures may be shown as a symbol in the bed/bar or as a sketch (sometimes in a separate column)
Graphic logs may be free drawn in a field notebook to summarise observations, drawn to scale using logging sheets or produced using software such as SedLog. Oil company graphic logs can be very complex as they will include data from down hole logging tools.
Version 11© OCR 2017
Interpreting the past – Delivery gude
Example key for graphic log (from SedLog)
Graphic log frame of coastal transgression sequence
The most common misconceptions with graphic logs are all to do with beds. Many learners find it difficult to decide what a bed is when they are in the field; it can help to stand back from the exposure and decide what level of detail is appropriate. A common beginner’s error is to superimpose all the beds rather than stacking them sequentially; using a stack of books can be a useful visual model. Graded beds can also be confusing for beginners who expect the beds to be presented as rectangular bars. Introduce complexity gradually and allow learners to construct and manipulate graphic logs. Learners should take care drawing the lithology stipples, sandstones are represented by drawn dots of appropriate size rather than loud repetitious dotting with the pencil point.
A simple PowerPoint introduces graphic logs and two classroom simulations are provided. The first is an example of how to make use of an urban environment: the drinking fountain in the market square at Saffron Walden is constructed from at least eight different siliciclastic, carbonate and metamorphic rocks that can be treated as a stratigraphic sequence to be logged. The second example is a field exposure of Triassic sandstone in Morayshire: learners complete a graphic log by identifying sedimentary features and adding symbols to a partially completed graphic log.
Six partially completed graphic logs can be used by teachers to support learners in understanding fluvial (meandering and braided channel), hot dessert, littoral (siliciclastic and carbonate) and deep water carbonate environments. A second set of example graphic logs for use with SedLog (slg and csv files) are available in a zip file. These example logs include a sample of the sedimentary environments from Modules 4.1.2 and 5.1.1. You will need to install SedLog in order to use some of these resources
Activity
Saffron Walden drinking fountain: draw a graphic log of the lowest seven ‘beds’, use symbols to represent the rock types (lithology) and sedimentary features in each bed. The thickness of the sequence is around 2.2 metres.
Burghead Triassic Sandstone: using the partially completed logging sheet identify the beds in the photograph of the cliff and add the sedimentary features and comments to your graphic log. What sedimentary environment where these sandstones deposited in?
Example graphic logs: using the rock description on graphic log add any sedimentary features onto your log and complete the lithology column with the appropriate stipple.
Version 11© OCR 2017