The previous chapters have traced the story of sediments, from weathering, erosion, and
transportation, until their deposition in diverse sedimentary environments. The third
and final part of the story is to tell how sediments are turned back into solid rock, a process termed lithifaction. Before doing this, it is essential to consider the subsurface environment in which these changes take place. First, it is necessary to consider the temperatures and pressures that prevail, then the chemistry of the various subsurface fluids
in which the many chemical reactions take place, and finally the motions of those fluids
through the sedimentary basin.
7.1 SUBSURFACE TEMPERATURES
It is abundantly clear to miners that temperature increases with depth beneath the surface of the earth. Detailed observations show, however, that the increase in temperature
with depth varies regionally and vertically. The internal heat of the earth is believed to
be derived from the radioactive breakdown of potassium, uranium, and thorium (Lillie,
1999). The heat flow of the earth's crust is the product of the geothermal gradient and
the thermal conductivity of the rocks. The global average heat flow is some 1.5 ixcal/
(cmZ)(s), but it varies from place to place. Old Pre-Cambrian shield areas have rates of
1.0 txcal/(cmZ)(s), active zones of subduction have rates of about 1.5/xcal/(cmZ)(s), and
mid-ocean ridges have rates in excess of 3 txcal/(cmZ)(s) (Press and Siever, 1982). These
variations are directly related to the thickness of the earth's crust and to convection cells
in the mantle, as discussed in Section 10.1.3. The geothermal gradient is normally expressed as the rate of temperature increase with increasing depth. The global average
geothermal gradient is taken as about 22~
but ranges from as low as 10 in old
shield areas, to as much as 50~
in active zones of sea floor spreading (North, 1985).
When measured in a borehole, however, there may be several zones with different geothermal gradients. These normally reflect variations in the thermal conductivity of the
strata penetrated.
The thermal conductivity of rocks ranges from as high as 5.5 W/m/~ for evaporites
such as halite and anhydrite, down to 0.3 W/m/~ for coal. Values of conductivity for
sandstones range from 2.6 to 4.0, and for limestones from 2.8 to 3.5 W/m/~ (data from
Evans, 1977). Variations in conductivity in sands, clays, and carbonates are largely due
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transportation, until their deposition in diverse sedimentary environments. The third
and final part of the story is to tell how sediments are turned back into solid rock, a process termed lithifaction. Before doing this, it is essential to consider the subsurface environment in which these changes take place. First, it is necessary to consider the temperatures and pressures that prevail, then the chemistry of the various subsurface fluids
in which the many chemical reactions take place, and finally the motions of those fluids
through the sedimentary basin.
7.1 SUBSURFACE TEMPERATURES
It is abundantly clear to miners that temperature increases with depth beneath the surface of the earth. Detailed observations show, however, that the increase in temperature
with depth varies regionally and vertically. The internal heat of the earth is believed to
be derived from the radioactive breakdown of potassium, uranium, and thorium (Lillie,
1999). The heat flow of the earth's crust is the product of the geothermal gradient and
the thermal conductivity of the rocks. The global average heat flow is some 1.5 ixcal/
(cmZ)(s), but it varies from place to place. Old Pre-Cambrian shield areas have rates of
1.0 txcal/(cmZ)(s), active zones of subduction have rates of about 1.5/xcal/(cmZ)(s), and
mid-ocean ridges have rates in excess of 3 txcal/(cmZ)(s) (Press and Siever, 1982). These
variations are directly related to the thickness of the earth's crust and to convection cells
in the mantle, as discussed in Section 10.1.3. The geothermal gradient is normally expressed as the rate of temperature increase with increasing depth. The global average
geothermal gradient is taken as about 22~
but ranges from as low as 10 in old
shield areas, to as much as 50~
in active zones of sea floor spreading (North, 1985).
When measured in a borehole, however, there may be several zones with different geothermal gradients. These normally reflect variations in the thermal conductivity of the
strata penetrated.
The thermal conductivity of rocks ranges from as high as 5.5 W/m/~ for evaporites
such as halite and anhydrite, down to 0.3 W/m/~ for coal. Values of conductivity for
sandstones range from 2.6 to 4.0, and for limestones from 2.8 to 3.5 W/m/~ (data from
Evans, 1977). Variations in conductivity in sands, clays, and carbonates are largely due
309