Tag Query Results

Marine diagenesis embraces all processes that take place on the sea floor after sedimentation and before burial. It includes bioturbation, boring and micritisation, cementation, dolomitization and, less commonly, dissolution

Calcite and aragonite cementation takes place in the marine phreatic environment on platform margins from seawater that is supersaturated with respect to calcium carbonate. This occurs where there is a continual flux of seawater, at high flow rates, though primary macropore networks. Cementation is facilitated by the warming of rising ocean water.

In the platform interior, marine diagenesis includes bioturbation, boring and micritisation, cementaiton and dolomitization.

The mixing zone forms along coastlines, where the oceanward flux of groundwater interacts with saline water that migrates landwards. Density stratification results in a lower warm saline water body and a cooler, uppermost freshwater lens; circulation entrains seawater into the freshwater lens resulting in salinization and formation of a mixing zone (Hubert, 1940; Plummer, 1975). The position of the mixing zone fluctuates in response to changes in sea-level and groundwater flux.

Hardgrounds form in submarine settings on carbonate platforms when there is a break in sedimentation, which allows circulation of seawater and precipitation of calcite cements. They might be indicative of a local change in sedimentary conditions or a more platform-wide cessation of deposition.

When carbonate platforms become emergent, above sea level, then deposition is terminated and a series of diagentic processes begin. What type of diagenetic modification occurs, and its extent, is dependent upon a number of parameters - in particular palaeoclimate and length of emergence

Incipient surfaces are stratigraphic marker horizons that are indicative of a break in sedimentation, but which are hard to identify and may have a polygenetic origin (e.g Rameil et al., 2012). Despite their subtle features, they might record long periods of non-deposition on a carbonate platform

Calcretes, or caliche, are a highly complex suite of vertically-zoned features formed by inorganic and biologically-mediated calcite precipitation and physical modification of the sediment, particularly by plant roots. When sufficient time and rainfall allows soil formation, aluminosilicate clays become abundant, forming distinctive, red coloured lateritic soils (so-called terra rossa). Calcretes and palaeosols would not have formed in the Lower Palaeozoic, prior to the evolution of land plants.

Karst is a broad term to describe the large (metre-scale and above) solution modification of emergent carbonate platforms. Karstification largely involves dissolution and can occur either immediatly following deposition, or sometime later, following uplift of the platform.

Cementation by groundwater takes place in both the vadose zone (above the water table) and the phreatic zone (beneath the water table), but is most significant in the phreatic zone, where pores are completely saturated by water.

Burial diagenesis embraces all diagenetic events that take place within carbonate rocks once they are buried to a depth where they are no longer influenced by unmodified surface fluids

Pervasive carbonate cementation can take place within the burial realm, filling primary and secondary macropores and fractures.

Sulphates can occur as replacive or pore filling cements within carbonate sediments. Anhydrite is by far the most common sulphate, but celestite and barite might also commonly be observed. The presence of depositional and diagenetic anhydrite substantially increases the chance of hydrocarbon-related reactions

Burial diagenesis might be associated with a wide range of non-carbonate minerals, including quartz, fluorite, sulphides, sulphates and clay minerals (particularly kaolinite)

Compaction is a physical process by which porosity is reduced during burial. In carbonate rocks, there are two types of compaction:

• Mechanical compaction, where pores are closed and grains are pushed closer together. This can result in grain breakage and the formation of grain-grain contacts
• Chemical compaction, where carbonate dissolves at point contacts. Soluble carbonate moves away from the point contact and insoluble material remains to define a solution seam or stylolite

Although the burial realm is typically a zone of net porosity degradation, there is growing evidence that porosity can increase during burial as a result of dissolution.