Other Features of Facies

You will learn more in Lab 3 about what controls which facies appear where and how sedimentary environments change over time. You have learned that there are two major types of environments: clastic dominated and carbonate dominated. What determines whether a particular environment will be depositing clastic sediments or carbonates?


Clastic-dominated sequences usually indicate a tectonically active or unstable (uplifted) sourcelands.   Grain size will generally be coarsest near to shore (proximal) and finest furthest (distal) from shore. The thickness of a clastic rock unit can be used as a general indication of proximity to the sourceland and rate of deposition in the sedimentary depositional basin. Thick immature clastic sediments imply rapid uplift of a nearby sourceland, closer proximity to the sourceland, or a rapidly deepening sedimentary basin. Thin mature clastics imply tectonic stability and a distal sourceland.


Carbonates are generally formed where the input of clastic material is low. Shallow water carbonate-dominated sequences usually indicate a warm, shallow water environment that has little clastic input. Thick shallow water carbonate sequences typically imply stability of the sedimentary depositional system such as slowly rising sea levels or slow subsidence (sinking) of the sea-floor. Deep water carbonate sequences are developed where the location is far from shore beyond the range of clastic input.


You may also have noticed that some of the sediments within facies may exhibit a degree of cyclicity due to repeated sedimentary events. For example, repeated turbidity flows lead to repeated fining upwards sequences in a continental slope/submarine facies; meandering river channels have repeating channel centre and point bar deposits leading to repeated coarse and fine grained layers in a meandering river facies; floodplain deposits have repeating layers of silts and sands from seasonal flood events.


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