Abstract
The Upper Triassic-Lower Jurassic Gassum Formation is a shallow marine or deltaic sandstone body with intercalated mudstones. The sandstones are primarily arkoses and subarkoses, but the composition is strongly influenced by diagenesis. Depths of burial vary between ea. 550 m and 3350 m, and the diagenetic modifications are strongly dependent on depth. The diagenetic pattern is also related to primary lithology. At shallow depth of burial, the most important diagenetic changes are compaction, calcite cementation, formation of kaolinite, and dissolution of feldspar. In deeply buried parts of the formation, the most important diagenetic changes are reduction of primary and secondary porosity by strong cementation by quartz, albitization of partly dissolved feldspar and ankerite cementation. In chlorite-cemented parts, quartz cementation was inhibited, and here pressure solution occurred in deep parts of the formation. The porosity in these deeply buried samples may be reduced to secondary intragranular porosity in partly dissolved feldspar and intercrystalline porosity in clay mineral cements. Other diagenetic minerals are siderite, pyrite, anhydrite, potassium feldspar and illite. The diagenesis evolved through the eodiagenesis and the immature and semimature stages of mesodiagenesis, whereas mature stages have not been reached.
Highlights
The Upper Triassic-Lower Jurassic Gassum Formation is a shallow marine or deltaic sandstone body with intercalated mudstones
Th e late precipitation of ankerite is probably not re lated to feldspar dissolution a lthou gh fe ldspar may be rep laced by a nkerite. This is seen from Plate 4, H, where the ankerite crystal grows into already open intragranular porosity of a framework feldspar and Plate 4,G where authigenic albite has been replaced by a nk erite
The zoned crystals may constitute up to 25% of the bulk volume, but generally makes up less than 2%
Summary
Number of thin sections from different sample locations Number of point - counted thin sections. The distinct growth of quartz into the intragranular porosity of partly dissolved feldspar (Plate l,A,B; Plate 2,G) demonstrates that a major phase of quartz cementation occurred after dissolution of the feldspar Both detrital grains and quartz overgrowths may be strongly replaced by ankerite (Plate 4,G,H), and there is no evidence that quartz growth took place after cementation and replacement by ankerite. The feldspar overgrowths occur in two major textural modes These are: a) thick rim cements up to 80 μm, which have grown in competition with quartz, as demonstrated by typical triplejunctions between quartzquartz-feldspar (Plate 2,A,B) and b) intragranular growth of small individual crystals on remnant skeletal fragments of detrital feldspar grains (Plate 2,C).
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