Sedimentary Rock Processes


Sedimentary rocks are those that have formed as a result of lithification of previously unconsolidated sediments. This is easy to conceptualize if you look at sandstone and at a handful of beach sand. At one point in its history the sandstone was simply a mass of unconsolidated beach sand. These eroded constituents of pre-existing igneous, metamorphic, or other sedimentary rocks are termed detrital sediments, and thus form detrital or clastic sedimentary rocks. A number of processes occur to sediment before it eventually becomes indurated. Weathering, transport, deposition and burial all take place in order for sediment to become sedimentary rock.

Sedimentary rocks can be divided into two general categories: clastic and carbonate. Clastic sedimentary rocks are discussed above. Carbonate sedimentary rocks are broadly defined broadly consist of limestone and dolostone. Limestone, unlike clastic rocks, are comprised of approximately 95% biogenic sediment. This is because carbonate sediment forms primarily from the shells of marine plants and animals which settle to the seafloor. These sediments will become cemented in situ, and thus, do not need to be buried before lithification can occur. Dolostone is usually a result of diagenetic alteration of the mineral calcite or aragonite, which make up limestone, to the mineral dolomite. It is common for carbonate rocks to contain both calcite and dolomite in a single unit. When there is more calcite present the rock is generally considered a limestone; when there is more dolomite present, it is usually referred to as dolomite.