Preferred formation of porosity preserving microquartz in shoreface sandstones of the Upper Jurassic Danish central grab

Some intervals of the deeply buried Upper Jurassic sandstones in the Danish Central Graben of the North Sea have higher than expected porosities of up to and above 20% even at depths of more than 4 km. These relatively high porosities correlate with the presence of microquartz coatings in the sandstones. The presence of microquartz coatings is generally delimited to sandstones deposited in the shoreface environment (Ravn Member and ‘Outer Rough Sand’) and offshore gravity flows, while microquartz is absent in sandstones deposited in fluvial, back-barrier and estuarine environments (Gert Member). Occasionally, impressions after sponge spicules occur in the microquartz-bearing sandstones, implying that the presence of microquartz is determined by the preferred habitats of silica sponges. Microquartz has been observed at burial depths of 2.8–4.8 km. Below ca. 3 km the individual crystals slowly begin to increase in size, eventually growing into incipient and interlocking quartz overgrowths. No microquartz is present below 4.8 km. This may be due to thermodynamic or chemical instability of microquartz at increased depth and temperature. Instead, unusual quartz-calcite interactions are present in calcite-cemented sandstones, perhaps indicating recrystallization of an initial grain-coating microquartz or opal phase.