Abstract: Abstract: Diagenetic Processes in Sandstones
The range of physical and chemical conditions included in diagenesis is 0 to 200°C and 1 to 2,000 kg/cu cm. Water salinities range from fresh to brines twice as concentrated as the Dead Sea. Numerical values of these parameters vary not only with depth but areally at any single depth. In addition, gross differences in heat flow in different tectonic settings can cause the average temperature at a given depth in a eugeocline to be double that in a miogeocline.
Despite the large variation in physical and chemical conditions during diagenesis, calcite and quartz are the dominant chemical precipitates in pore spaces of sandstones. To a first approximation, this reflects two facts: (1) most preserved sands were deposited in marine environments where seawater is saturated with respect to calcium carbonate; and (2) the average sand contains about 65% quartz and rates of flow of subsurface waters are so slow that quartz is the main buffer for the silica content of these waters.
The slow rate of movement of subsurface waters places major restrictions on the time when cementation of a sand can occur. To lithify a sand, the circulating water must be supersaturated with respect to the solid to be precipitated in the pore space. Also, the number of pore volumes of water that must flow through the sand must be very large. If the sand is widespread areally, calculations for well-sorted pure quartz sand indicate that cementation of the unit by quartz by horizontal flow of subsurface water is impossible within geologically reasonable periods of time. Plugging of pore spaces by quartz must result from vertical circulation, probably when the depth of sand burial does not exceed a few hundred meters. This cementation can occur either soon after deposition of the sedimen or at any subsequent time when tectonic forces elevate the sediment to a shallow depth.
AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado