Zur Herkunft des Quarzzements. Abschätzung der Quarzauflösung in Silt- und Sandsteinen

The origin of quartz cement in sandstones can be attributed to supplies (1) from the surrounding shales, and (2) to a lower degree from dissolution of quartz on stylolites within the sandstones. A supply from the surrounding shales, which has been shown by the porosity decrease near the upper and lower surfaces of different sandstones (Füchtbauer, 1974), can be explained by the following observations in Upper Triassic and Middle Jurassic sandstones and siltstones of Northern Germany as well as in concretions of Devonian to Upper Cretaceous age from different localities:

1. Quartz grains in silt layers are flattened by dissolution compared with quartz grains of the same size in the adjacent sandstones, the amount of shrinking being about 35 percent (fig. 1).
2. Concretions prevent the enclosed insoluble residues from diagenesis. The main difference between the concretions and the adjacent shale of 31 occurrences examined is the quartz content, which is by 10–50 percent lower in the adjacent shale, due to diagenetical dissolution (fig. 2).

It is suggested that the dissolved silica was brought to the sandstones by the compaction stream of interstitial water percolating through the rock sequence, and that the sandstones acted as sinks triggering the dissolution. Only a small amount of silica, about 10 percent of the silica from dissolved quartz, is provided by the transition montmorillonite — illite. Both sources together would be able to explain the precipitation of 20 percent quartz cement in a sequence composed of 1/3 sandstones and 2/3 shales. In the sandstones mentioned above stylolites can be observed (fig. 3), the amplitudes of which increase from 0,5–1 mm to 2–5 mm with increasing depth, between 1300 and 2600 metres. The real amount of dissolution on each stylolite — about 4 mm — has been calculated using large mica which were collected by the stylolites from the adjacent sandstone. Using this figure, the decrease of porosity in the sandstones shown in fig. 4 can be quantitatively explained by the frequency of stylolite intercalations. It is suggested that this process, which was due to local diffusion, occurred late in diagenesis, when the compaction stream was already insufficient to move large quantities of silica.