THE ROLE OF QUARTZ-LINED MICROFRACTURES IN SANDSTONE DIAGENESIS

Microfractures exert a fundamental influence on the diagenetic history of quartz sandstones by increasing sites for quartz cement precipitation and providing fluid-flow pathways. SEM-based cathodoluminescence systems (scanned CL) allow improved detection of these microfractures, which are typically quartz lined and obscure to invisible using light microscopy. We have identified these microfractures in more than 80 different units. Sample ages range from Archean to Tertiary; sample locations thus far include 12 countries on 5 continents. Quartz-lined microfractures have been identified in sandstones buried to a wide range of depths and formed in diverse depositional environments. Microfracture densities measured range from 32 to 202 microfractures per square millimeter. Measurements of microfracture quartz cement per area (former microfracture porosity) are in the range of 0.5 to 2 percent.

Using scanned CL, most sandstones that have undergone moderate to deep burial can be shown to contain some quartz cement, even where none is evident in sandstone pore volume. In sandstones having low quartz-cement volume, most of the cement is found in microfractures. This is because microfractures provide fresh surfaces, in contrast to grain surfaces, which commonly have coatings or damage that inhibit precipitation. In some sandstones, early quartz cement nucleates in microfractures that penetrate grain boundaries and then spreads out along grain boundaries. Microfractures also increase surface area for precipitation, allowing greater overall cement volumes. In sandstones having little or no final porosity, microfractures provided fluid pathways that led to the final stages of porosity reduction.

Sandstones buried to any appreciable depth contain at least two sets of quartz-lined microfractures. These microfractures span the diagenetic history of the rock, with one set typically forming during compaction. Microfracture histories tend to increase in complexity with age of the sandstone and with depth of burial. Microfractures are common in flat-lying rocks from sedimentary basins that are not near faults or associated with tight folds. However, sandstones from structurally complex areas show increased numbers of microfractures. In some deformed samples, microfractures define a penetrative fabric.