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Continental blocks of the earth's crust are mainly deep basin sediments, generally lithified and commonly metamorphosed. Observed rock transformations required input of heat in an environment of pressure ranging from hydrostatic to geostatic, and most occurred in the presence of water.
The role of hydrology in diagenesis and lithification of deep basin sediments, and in the leaching, transport, and reprecipitation of mineral constituents is now in a period of intensive reevaluation. New techniques of study, vastly improved methods of data collection and processing, and an enormous store of information on widespread conditions through a great depth range provide effective means for such reevaluation.
The hydrologic evolution of deep basin sediments prior to metamorphism occurs in two distinct phases. Discharge of connate water upward and toward the basin margin is the first phase; intake and throughflow of meteoric water comprise the second. The first phase may be considered near completion only when clay-mineral dehydration has entered its final stage. Each phase may span scores to hundreds of millions of years, and different parts of a basin may be in different phases at a specified time.
The hydrologic evolution of a sedimentary basin is related to its configuration and dimensions, its depositional and structural history, the relative thickness and areal distribution of sediments (by type) within it, and changes in its regional geomorphic setting. Evolutionary progress is evidenced by changes in formation-water composition and salinity as functions of depth and areal distribution, changes in the geothermal and interstitial fluid pressure regimes, and reduction of the water content of the rocks. Conditions in basin deposits ranging in age from early Paleozoic to Neogene illustrate these evolutionary processes.
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