Depositional Environments
Phosphates are known to be deposited in a wide range of depositional environments. Normally phosphates are deposited in very shallow, near shore marine or low energy environments. This includes environments such as supratidal zones, littoral or intertidal zones, and most importantly estuarine. Currently, areas of oceanic upwelling cause the formation of phosphates. This is because of the constant stream of phosphate brought from the large, deep ocean reservoir (see below). This cycle allows continuous growth of organisms.
Supratidal zones: Supratidal environments are part of the tidal flat system where the presence of strong wave activity is nonexistent. Tidal flat systems are created along open coasts and relatively low wave energy environments. They can also develop on high energy coasts behind barrier islands where they are blocked from the high energy wave action. Within the tidal flat system, the supratidal zone lies in a very high tide level. However, it can be flooded by extreme tides and cut across by tidal channels. This is also subaerially exposed, but is flooded twice a month by spring tides.
Littoral environments/ intertidal zones: Intertidal zones are also part of the tidal flat system. The intertidal zone is located within the mean high and low tide levels. It is subject to tidal shifts, which means that it is subaerially exposed once or twice a day. However, it is not exposed long enough to withhold vegetation. The zone contains both suspension sedimentation and bed load.
Estuarine environments: Estuarine environments, or estuaries, are located at the lower parts of rivers that stream into the open sea. Since they are in the seaward section of the downed valley system they receive sediment from both marine and fluvial sources. These contain facies that are affected by tide and wave fluvial processes. From the top, the estuary is considered to stretch from in the landward limit of tidal facies to the seaward limit of costal facies. Phosphorites are often deposited in fjords within estuarine environments. These are estuaries with shallow sill constrictions. During Holocene sea-level rise, these estuaries built a U shaped valley profile formed by drowning the glacially eroded valleys.
The most common occurrence of phosphorites are related to strong marine upwelling of sediments. Upwelling is caused by deep water currents that are brought up to coastal surfaces where a large deposition of phosphorites may occur. This type of environment is the main reason why phosphorites are commonly associated with silica and chert. Estuaries are also known as a phosphorus “trap”. This is because coastal estuaries contain a high productivity of phosphorus from marsh grass and benthic alge which allow an equilibrium exchange between living and dead organisms.
Read more about this topic: Phosphorite
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