Dissolution during burial diagenesis, and its Influence on further Cementation-Examples from the Siri Canyon, Danish North Sea
The Siri Canyon is a submarine canyon system eroded into the uppermost Chalk deposits and filled in with Palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstones deposited from sandy mass-flows and turbidites. Core samples from 7 wells were studied. The reservoir sands in these wells all contain authigenic silica/quartz of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy. Late Paleocene–early Eocene volcanic ash layers are recognized throughout most of the North Sea and also recorded from the Siri Canyon wells. Volcanic lithoclasts are strongly altered and associated with diagenetic opal/ microquartz coatings and zeolite. Zeolite crystals formed simultaneously with opal and prior to microquartz but dissolved with increased burial depth. The dissolution of zeolite followed two steps; the core of the zeolite crystals appears to have been more unstable than the rim and dissolved first. Later the entire crystal dissolved and left an impression of the euhedral zeolite crystal in the microquartz coating. Such openings in the microquartz coating are nucleation points for macroquartz. Thus, the precipitation of zeolite may later facilitate further quartz cementation, which might otherwise be retarded by the presence of disordered microquartz. The silica activity of pore fluids can influence zeolite precipitation. Although zeolite formation is clearly related to volcanic ash, zeolite has also formed in samples where no volcanic ash is demonstrated; it seems that a rapid supply of dissolved silica from dissolution of siliceous fossils was the main reason for the early co-precipitation of opal and zeolite. There are two important sources for Si: 1) Biogenic opal from diatoms or radiolarians, which are abundant in some of associated shales; and 2) volcanic ash. The dissolution of biogenic silica may result in a rapid release of silica thereby promoting the formation of diagenetic opal/microquartz, but there may be a limited release of Al. A limited release of Al may result in precipitation of Si-rich clinoptolite, or zeolite may not precipitate at all. The dissolution of volcanic lithoclasts may also release a high rate of Al, resulting in abundant formation of Al-rich clinoptilolite. If both sources interact, a compositional variation may occur with time. The compositional variation may result in variable solubility and give rise to dissolution of the internal core prior to its rim. This dissolution behavior may have impact on the further quartz cementation. The formation of various quartz morphologies influences the continued cementation by macroquartz. This effect may be counteracted by precipitation/dissolution of zeolite.