Abstract

Although stylolites in quartz-rich sandstones have been described and modeled, more feldspathic sandstones have not been similarly investigated. The Upper Jurassic Ula sandstone, Norwegian North Sea, a subarkose with 15 to 25% feldspar, displays stylolites with prominent low-porosity haloes up to 10 cm wide. Detailed mineralogical and chemical profiles across several stylolites and haloes are used to investigate the role of feldspar in mineral reactions at stylolites and to constrain the transport of chemical components.

Quartz and feldspar are intensely dissolved at the margins of stylolites. The surrounding haloes are characterized by: dissolution of quartz and feldspar, intense compaction, anomalously high feldspar content, high concentrations of clay, and oxide cement volumes that increase away from the stylolite. Chemical mass-balance calculations demonstrate that silica and elements associated with feldspar (Al2O3, Na2O, K2O) were exported from the haloes. Grain-size data and evidence for aluminum loss from the halo indicate that primary concentrations of clay and mica were not higher near the stylolites and that the petrographic and geochemical patterns reflect the diagenetic concentration of clay. The analysis suggests that K-feldspar dissolution at and near the stylolite resulted in precipitation of authigenic illite near the site of K-feldspar dissolution, due to the low solubility of Al3+. This in turn catalyzed chemical dissolution of quartz in the low-porosity halo.

Transport of CaO, MgO, and MnO into the haloes suggests that the haloes were zones of higher pH. The increased pH, perhaps due to the presence of clays, would have also increased quartz solubility, consistent with evidence for silica export from the halo. The dissolution of K-feldspar in the absence of kaolinite would also have raised pH around the site of K-feldspar dissolution and illite precipitation, favoring carbonate precipitation.

This interpretation of stylolite diagenesis differs from conventional models based on quartz-rich sandstones, in which quartz cement volumes decrease away from stylolites. Models for compaction in arkosic sandstones need to account for the enhanced dissolution of quartz that is driven by K-feldspar dissolution and for catalysis by the resulting illite.

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