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Diagenesis |
Geologisches Institut, Universität Bern, Baltzerstrasse-1, 3012 Bern, Switzerland
Koninklijke/Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, Rijswijk ZH, The Netherlands
AGAT Consultants Inc., 1215 18th Street, 260 Cable Building, Denver, Co 80202, USA
Diagenesis is the sum of those processes by which originally sedimentary clastic assemblages attempt to reach equilibrium with their environments. The subject has rapidly evolved, over the last 20 years, after several decades of routine petrographical analysis, to become a discipline of sophisticated analytical geochemistry. This progression may be traced through the evolution of depositional facies models, the growth of theoretical geochemistry, and the development and application of quantitative analytical techniques. Consequently, the study of diagenesis today involves the integration of data gathered from a range of interrelated disciplines.
The publication in recent years of such integrated studies, particularly from the Texas Gulf Coast of the U.S.A., has led to a number of wide ranging models being established. These models allow an interpretation of the sequence of authigenic minerals in terms of their relationship to the depositional environment or surface chemistry (eogenesis), burial or subsurface conditions (mesogenesis) and weathering or reexposure to surface conditions (telogenesis).
More specifically, once the pre-depositional controls on diagenesis have been established, it is possible to relate the inferred eogenetic mineral assemblages to exact geochemical sedimentary environments. Sedimentary mineral assemblages chemically are characterized by relative instability and so tend to interact with interstitial pore waters. Thus, in non-marine environments, mineral authigenesis may reflect arid-oxidizing, or wet-reducing pore-water conditions, and in the marine environment, either oxidizing or reducing pore waters.
During mesogenesis different processes become important. Elevated temperatures add energy to the reacting system, lowering reaction barriers and increasing reaction rates. Furthermore, widespread pore fluid migration at depth, transporting large quantities of solute, is likely to impart major, regional diagenetic changes to sediments. That there is a remarkable degree of consistency in deep burial settings suggests that depth-related processes conform to a predictable pattern.
Additional geochemical instability is introduced into the diagenetic system during uplift and exposure to the telogenetic realm. Minerals formed during burial, at elevated temperatures and pressures, and from concentrated formation waters, may become unstable in oxidizing, meteoric waters.
Diagenetic research requires the complete dissection of sedimentary rocks and, subsequently, the quantitative chemical and mineralogical analysis of their individual components. Such an approach to diagenetic studies, when related to an assessment of their paragenesis, may eventually lead to a predictive, integrated model for the evolution of sedimentary clastic rocks.
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  J. Parnell Diagenesis and fluid flow in response to uplift and exhumation Geological Society, London, Special Publications, 2002; 196: 433 - 446. [Abstract] [PDF]
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K. Bjorlykke Fluid-flow processes and diagenesis in sedimentary basins Geological Society, London, Special Publications, 1994; 78: 127 - 140. [Abstract] [PDF]
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S. N. Palmer and M. E. Barton Porosity reduction, microfabric and resultant lithification in UK uncemented sands Geological Society, London, Special Publications, 1987; 36: 29 - 40. [Abstract] [PDF]
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