Sedimentary Facies

A sedimentary facies is a body of sediment with distinctive physical, chemical and biological attributes deposited in a specific depositional environment. The process of recognizing these bodies of sediment is termed facies analysis. Geoscientists have studied present-day depositional environments and determined the characteristics of the sediment deposits found in each environment. These observations have been used to assemble idealized facies models that describe the ideal, or typical sequences or associations of sediments that occur in a particular depositional environment. An observed sedimentary facies seen in the historical record can then be compared against idealized facies models, and the depositional environment responsible for the ancient sediments can be inferred.

Returning to the example in the Introduction of the well-sorted, well-rounded, high-sphericity medium sands with ripple cross-beds, the geoscientist may have also noted the sand has fossils that are known to be from marine fish and that the ripples in the sand are found to be symmetrical, indicating bi-directional waves rather than unidirectional river flows. They observe that the sands in a given bed generally get finer as they go upwards (FUS), then there is an erosion plane, and a new package of sands is deposited on top. Each successive package of sands once again starts coarse and become finer upwards. All these elements of the sequence are pulled together to describe a package of sediments that occurs together in a recognizable sequence. The exact sequence may not be identical in every outcrop, but the overall character is similar.

The geoscientist then examines modern depositional environments for sediments with similar characteristics. The fish indicates a marine environment. Clastic sediments are carried to the sea by rivers. Sand carried in a fast-moving river will not be held in suspension in lower energy ocean waters, and will settle out of the water column close to shore. The highest energy in the ocean occurs during storms, where near-shore sediments can be picked up and moved during the storm by wave energy. Some sand may be eroded at the start of the storm and then re-deposited as the storm abates with the coarsest material depositing first and finer material later. This leads to repeating sequences of fining upwards sand beds. Each part of the character can be explained by physical processes observable in the modern day. Comparison of the modern beach sediments to those observed from the past allows us to interpret the ancient sands as a beach facies.

Facies analysis uses the principle of uniformitarianism which holds that the forces and processes acting on Earth today are working in the same way and with the same intensity today as they did in the past. Facies analysis is part of the day-to-day job for geoscientists working in the oil industry. They use rocks recovered from deep boreholes to reconstruct past depositional environments to help them search for where oil is most likely to occur. Certain mineral deposits are also associated with sediments, and mineral exploration geologists will use facies models to help guide their searches.