Techniques: Sediment Description and Analysis

Introduction 

Describing sediments is a valauble geomorphological skills. Using such information it may be possible to deduce the origin(s) of the sediment(s). Such reconstruction can be achieved by making inferences about the erosional, transportational and depositional history using the sediment characteristics (such as particle size, shape, roundness, etc.). Inferences can also be made about the processes operating and, ultimately, the deduction of the environment of deposition of the sediment.

The main skills which are used are basic observation, the ability to concisely describe, lateral thinking, and deduction and inference.

Equipment 

You will require the following: 

• grain size card showing a measurement, roundness and sorting scales 
• large callipers 
• spade or trowel (to clean the section face) 
• plastic bags (to collect samples) 
• permanent marker pen (to label sample bags) 
• notebook or clipboard (covered by plastic sheet in case of rain) 

Procedure 

Look at the sediment section and identify different units (represented by different colour, particle size, etc.). For each unit systematically record what you see using the following criteria: 

Particle size and texture 

1. Measure the particle size range (i.e. the extremes in particle size) of the sediemnt sample. Using rulers or callipers, record the a, b and c axes of largest particles. Use simple tests to find out the approximate size of the finest sediment, i.e. whether fine sand, silt or clay.
   
2. Use the Wentworth classification of particle size to classify your measurements. Note that each category (clay, sand, gravel, cobbles, boulders) may be further separated into fine, medium and coarse. 
   
3. Assess the texture of the sample, i.e. the overall proportion (%) of each sediment fraction (i.e. % of silt + clay; % of sand; % of gravel + cobbles + boulders). For each sample, plot your rough estimates on a simple ternary diagram. 
   
4. How sorted is the sample? (Use Power's 1953 shape indices: extremely well sorted, well sorted, poorly sorted, extremely poorly sorted). 
   
5. Does one particle size fraction dominate (= unimodal), or are there extremes of particle size (= bimodal, i.e. 2 main particle size fractions, e.g. sand and cobbles), or a range of size with no single fraction dominant (= polymodal)? 
   
6. If the sample is bimodal, is the sample clast-supported ( = coarse particles volumetrically dominant, particles touching each other, but with an infill of fines) or matrix-supported ( = fine material is volumetrically dominant, coarse particles 'floating' in fines)? 

Sediment shape 

1. Examine the shape of the gravel, cobbles and boulders (if any) in each sample. From each sample, take a random sample of at least 10 clasts and record the roundness of each. Use rounded, sub-rounded, sub-angular, angular. Plot this data as a histogram for each sediment sample.
   
2. For the same random sample, also measure the a, b and c axes. Plot your results on a Zingg diagram. Are the sediments mostly oblate, equant, bladed or prolate? 

Lithology and organic content 

1. What is the colour of the sediment? Use a Munsell chart for soil samples. 
   
2. Can you recognise any different rock types or minerals? 
   
3. Is there any organic matter, e.g. rootlets or broken twigs, in the samples? If the sample is particularly brown and 'dirty'-looking, then it probably contains some organic matter. If the sediment looks 'clean', then it does not contain any organic matter. 

Inference of the environment of deposition (origin) of the sediment 

By integrating your results it is deduce the most likely mode of origin of each sediment sample. It is good practice to list the reasons (i.e. diagnostic characteristics) for your selection. Inference is based on interpretation of the characteristics. You need to make inferences about process (e.g. energy regime, distance of transport) in order to work out the environment(s) of deposition. Remember that some of the characteristics are the result of the processes which have occurred prior to deposition (e.g. transport). 

Hints and tips 

• The particle size distribution is the most important feature. Very fine, unimodal is typical of low energy environments such as flood plain alluvium or lake sediment. Beach gravels are typically coarse and unimodal. 
  
• Sorting: A well sorted sediment often represents selective separation of fractions due to either: 

(i) deposition of coarse material in rivers because of fall in flow velocity unable to transport large particles. Thus fine material is transported downstream and deposited in low-energy environments, such as a lake or on a floodplain.

(ii) preferential sorting by waves and longshore drift on a beach. 

Poorly sorted is usually indicative of glacial tills (e.g. lodgement till, terminal and lateral moraine tills). 

• Texture: Predominantly coarse material = beach or fluvial gravels (high energy environments). Predominatly fine material = floodplain alluvium or lake sediments (low energy environments). 
  
• The roundness of clasts may indicate their mode of origin. Beach gravels are commonly well rounded; river gravels are less well rounded; till is generally sub-angular to sub-rounded; scree material is typically angular. 
  
• Colour is strongly related to particle size distribution. Different colours may reflect different sources of material (i.e. from different lithologies). Colour may also be the result of weathering processes, the development of a soil, or the deposition of iron (red, brown) or manganese (black) in response to changing water table conditions. 
  
• Organic content: A high organic content is usually indicative of flood plain alluvium or lake sediments. No organic content is associated with beach material, glacial till or fluvioglacial deposits. 

Lithofacies analysis 

A sediment or sequence of sediments is called a lithofacies. Many environments have diagnostic characteristics. For instance, meandering river sediments tend to fine upwards, whilst deltaic sediments tend to coarsen upwards. The lithofacies of an environment is a reflection of the processes operating combined with the characteristics of the in-coming sediment.