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Fossils and sediments

Fossils only exist in sedimentary rocks, in some exceptional cases also in sediments which slightly metamorphosed like quartzites. Further metamorphosis destroys any fossil contained in a rock. Another exemption are deposited volcanic materials such as volcanic ash layers, which can also contain fossils. E.g. fossil human footprints on volcanic ash from the Pleistocene period are known from several places all over the world.

When hunting fossils it is key to gain proper knowledge of the types of rocks and how to recognize them. First, you keep yourself from searching for fossils in rocks which can't contain any fossils. Second, when prospecting this knowledge will help you recognize fossil bearing rocks or layers a lot easier. The information page covering rock types provides an overview of several types of sedimentary rocks with features and how to recognize them in the field.

 

Depositional environment

The fossils found in these sedimentary rocks often teach us about the depositional environment of that time, and because of that, fossils are often used for environmental reconstructions. Most research on fossils to determine the depositional environment is done on so called micro-fossils (e.g. foraminifera). The advantage of micro fossil research is that some rocks contain vast quantities of these small fossils. For instance 1 cm3 chalk/limestone  can contain hundreds of foraminifera. Also specific types of micro-fossils can be found globally and can be linked to 1 specific environment. At the same time macro fossils can also teach us about the depositional environment. To do so it is important to know if fossils are found in their original position and environment (in situ) or if they are washed up. When you find the individual valves of let’s say a clam/oyster, it is very well possible that they were transported by the current. On the other hand if you find a layer of clams/oysters that have continued to grow on top of each other and piled up over the years you probably found am actual oyster bed.  This example shows that the quality of preservation tells a lot about processes that have taken place before and during the fossilization of fossil. These processes are called taphonomic processes.

The rock itself can also provide information about the depositional environment. Here, the principle of uniformitarism, whereby we can assume that the processes responsible for erosion and sedimentation were not different from today's, is of importance. One of the most important characteristics of sedimentary rock is the grain size. Grain sizes of sediments can vary between coarse (boulders and gravel) and very fine (clay). Grain size within sedimentary rock is largely depending on the energetics/kinetics of the depositional environment.  For instance in a highly kinetic environment like a fast flowing stream, most of the fine particles are flushed away and gravel and boulders remain. How further away the sediment travels the calmer the environment eventually becomes, reflecting in the sediment where the presence of finer particles increases while coarse materials decrease. In a very calm environment like the deep-sea only small particles like clay remain. Sometimes one can find grains of different sizes in 1 bed of sediment, indicating changing kinetics in one depositional environment.

Another phenomenon that teaches us about depositional environment are sedimentary structures. Structures like wave or current patterns, shrinkage cracks etc. help us to recreate the original environment when they were formed. Certain types of structures are characteristic for deep-sea, river, coastal or even wind deposition. They all translate to a typical form sediment transportation. A wave patern for instance is quite symmetrical, caused by the back and forward movement of the ocean. A current pattern is relatively asymmetrical due to movement of the sediment in 1 direction. Highly layered and very fine grained sediments are indicative of deep-sea depositions, where hardly any movement takes place. 

Fossil ripple marks “in situ”

Mud cracks are formed when fine grained sediments are deposited and subsequently dry. They are quite rare and point towards a very specific depositional environment. The presence of mud cracks is a very clear indicator for that specific environment. 

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Fossil mud cracks (picture E Vercammen)

Apart from all the characteristics easily recognizable with the naked eye mentioned above, there are more subtle indicators for depositional environment. The chemical consistency of rocks is an aspect often used in geology to reconstruct the environment. A very obvious indicator is the presence or absence of chalk (calcium carbonate), which is usually formed by organisms like plankton and shellfish. Another important element for organisms  is oxygen. The rate of decay is largely determined by the presence of oxygen, most depositional environments show aerobic conditions, this means that oxygen is present. Sometimes a depositional environment is characterized by the absence or just limited presence of oxygen, this is called anaerobic (the sediment is without oxygen). Aerobic environments are often recognized by their light coloration, brown, red, purple or yellow rocks. These colors are formed when oxygen reacts with metals (e.g. Iron oxide, brown rust). Anaerobic environments are recognizable by dark colored, black and grey rocks. Because organic material slowly decays in an environment without oxygen, the organic material will build up/pile up over the years which in terms gives of a dark coloration. A good example of a rock formed in an anaerobic environment is coal. Anaerobic conditions sometimes result in exceptionally well preserved fossils.

 

Trace fossils

Activity of living organisms will continuously leave a permanent mark on the sediments which they choose to live on or live in. Sometimes these traces remain visible when the sediments turn into rocks. When this happens we speak of trace fossils or ichnofossils. Often we are dealing with burrowing traces or crawling tracks. Since specific burrowing organisms  live in specific conditions, the presence or absence of trace fossils can be an indication to estimate the water depth. We speak of bioturbation when a soils or sediment is reworked by organisms before it is fossilized.

 


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