KAOLINIZED, POTASSIUM-LEACHED ZONES AT THE CONTACTS OF THE GARN FORMATION, HALTENBANKEN, MID-NORWEGIAN CONTINENTAL-SHELF

The Middle Jurassic Garn Formation consists mainly of subarkosic arenite in which potassium feldspar is several times as abundant as plagioclase. In cores from eight wells, the top of the formation (upper 6-12 m) appears to have undergone relatively intense alteration of potassium feldspar to kaolinite, with the resulting loss of bulk potassium content. This could be explained as a result of subaerial weathering of the top of the sandstone body, except for the fact that in two of the wells the same pattern of alteration is observed at the base of the formation. An alternative explanation is that alteration resulted from the introduction of acid released by the maturation of organic matter in adjacent shales. In all instances the altered zones display a gradual increase in kaolinite content and a decrease in the ratio of bulk-rock potassium: aluminium over a distance of several meters approaching the formation boundary. This pattern indicates that the active species responsible for the alteration entered vertically from the formation contacts rather than being introduced by lateral flow along the sandstone bed. The cores studied are tentatively interpreted as recording different stages in the following depth-related sequence of diagenetic events: (1) early kaolinization throughout the formation by meteoric water flushing; (2) formation of additional kaolinite near the formation contacts by the influx of acid from surrounding shales at approximately 2.7 km depth below the sea floor; and (3) extensive alteration of kaolinite to illite at approximately 3.7 km depth. Illitization has been impeded in leached zones near the formation contacts due to a local deficiency in reactive potassium; here kaolinite survives unaltered, whereas kaolinite throughout the rest of the formation has been extensively replaced by illite. This implies that the illitization of sandstone can occur by short-range diffusion in a chemically closed system, solely in response to increasing thermal exposure at greater depth.