I am a senior at University of Tennessee in the Geology program. I love what I study! So much information is held within a rock. They have many clues as to the environment they formed in preserved within the rock in textures and mineral assemblages. Geologists have found that certain environments produce specific sequences which are visible within the structure of the rock. The sum of these characteristics is referred to as a facies. Metamorphic facies can only tell us about the temperature and pressure that the rock formed in. On the other hand, sedimentary facies preserve much of the original constituents and patterns due to the fact that they are a surface process and no melting occurs during their formation.
How are Sedimentary Facies described?
Firstly, in order to recognize and classify facies, scientists have drilled cores in present day environments making note of the patterns and sequences that are within them. We assume that the same processes that are at work today also formed similar sequences in the past. If this were not true most of science would go to hell in a hand basket.
When describing facies the following traits are used:
- Areal extent
- Types of boundaries between beds
- Bedding styles
- Sedimentary structures
- Grain size
- Grain type (mineralogy)
- Color
- Biogenetic materials
The sum of these characteristics of a sedimentary unit is referred to as the facies. By recognizing what facies are present and what sequence they occur in one can reconstruct the transport and deposition of the area. Basically, one can construct a general geologic history for the area based on the known facies that are found. It is like a puzzle, you have the pieces and know where they go but you cannot see the big picture until they are all together in the proper place!
Facies assemblages are then given names which typically are related to the environment in which they are formed. Sometimes they are named after people as in the case of the Bouma Sequence. This faces was recognized by Arnold H. Bouma as a common deposit sequence in the bottoms of lakes, oceans, and rivers.
The Bouma sequence (to the left) ideally has five characteristic beds which represent a marine environment with a sudden influx of material as turbidity currents move down the slope. As one moves up the beds, we can see the energy waning. In layer A, the current has just begun and has a lot of energy. Therefore, the largest clasts and particles are carried down and deposited as the current loses energy and can no longer keep them in suspension. This results in a graded bed, the larger, heavier sediment is deposited first and gets progressively finer as a result of the waning energy. In layer B, the energy has decreased and deposited much of the large sediment leaving sand sized particles behind. There is still enough energy in the flow to move these sand particles. This can result in fluting or groove casts preserved within laminations as sand grains slide and roll until they are deposited. In layer C, similar sized particles are present however the bedding typically has characteristic ripples. These ripples are formed by a process called saltation. Saltation bascially amounts to jumping grains of sand. The flow energy has decreased even more at this point in time and it no longer has the energy to roll and slide sand particles. The energy may only be enough to pull it away from the surface and before long it will be deposited in another location. In layer D, energy has decreased to the point that most sand size particles can no longer enter the water column. All of the sediment from the event has not been deposited yet. Very fine grains, mostly silt sized, are still held within suspension. These slowly settle out of suspension producing a finely laminated sandstone. Any alterations in flow energy during this time will be recognized by a change in grain size. Finally, layer E, is deposited once the current has ceased. This results in the very fine grained silt and clay sized particles left in suspension to finally settle to the floor. This results in a characteristic bed of shale.
Facies models will not always perfectly align with findings in the field. That is just part of geology. It is always at work weathering and eroding so some layers may be altered or missing. In the case of the Bouma sequence, if energy is not high enough some layers (A and B) may not even be present! Perhaps another even may occur before flow velocity decreases to deposit sediment and another conglomerate layer appears before energy dissipates and grading continues. Things are not so cut and dry in geology so you may have to test a few ideas and do a few thought experiments to determine what happened!
