Volume 2
R.C. Selley, in Encyclopedia of Geology (Second Edition), 2005
The Boundaries of Diagenesis
Diagenesis begins as soon as sediment is deposited. Fossilized beer bottles and other anthropogenic detritus found in modern ‘beach rock’ limestone picturesquely illustrate this. Ancient evidence of early diagenesis is further confirmed by the occurrence of intraformational conglomerates containing clasts, not only of contemporaneous limestone, but also of siderite-cemented sandstones and shales. As sediment is buried more deeply, temperature and pressure increase and, ultimately, diagenesis merges into metamorphism, with shale becoming slate, sandstone becoming quartzite, and limestone becoming marble. Field observation and laboratory experiments demonstrate that the boundary between these rock types, and hence diagenesis and metamorphism, is gradational. The sequence, deposition → diagenesis → metamorphism, is not a ‘one-way street’, however. At any time while sediment is on its way to metamorphism, it may be uplifted to the surface again. Rocks returned to the surface show a reversal of the trend of porosity decreasing with burial, and its enhancement by both physical and chemical processes. The term ‘epidiagenesis’ was applied by Fairbridge in 1967 to the diagenesis resulting from uplift and weathering. Epidiagenesis is of little significance in shales. It is, however, of great importance in sandstones and carbonates, because of the way in which it restores porosity and permeability to rocks that had previously lost these features. When buried beneath an unconformity, these epidiagenetically enhanced zones may provide excellent petroleum reservoirs. Epidiagenesis is also well known to mining geologists, being responsible for the ‘gossan’ sulphide ore bodies, such as those of Rio Tinto, Spain. Figure 1 delineates the boundaries of diagenesis.