Depth of Burial

Abstract

The clay mineral composition of slightly to moderately buried sediments commonly displays a wide diversity and variability, which reflects the various paleoenvironmental controls typically acting under conditions of the Earth’s surface. At depths of burial usually exceeding two kilometers, clay assemblages tend to become simpler, and to be more or less progressively enriched in illitic and chloride minerals. Smectite tends to disappear downwards, irregular mixed-layers tend to be less expandable and to include subregular to regular types. Kaolinite is preserved deeper than smectite, but also tends to diminish in abundance. This general evolution expresses the increase of temperature and pressure in deeply buried series, and corresponds to the transition from early to late diagenetic conditions. The precise depth of the downward passage to thermodynamically affected sediments primarily depends on the geothermal gradient, secondarily on pressure, lithology, permeability, tectonics, or hydrothermal circulation (see Chap. 16). Common geothermal gradients average 30°C/km, and significantly affect the clay mineralogy beyond 2.5 to 3.0 kilometers of burial, at temperatures exceeding 80°C. At greater temperature and pressure, anchimetamorphic conditions replace the diagenetic ones. Anchimetamorphism is characterized by an increase in illite crystallinity, the appearance of transitional minerals like pyrophyllite or paragonite, the development of a slaty cleavage and the passage from clay-sized to silt- and sand-sized mica and chlorite.