REGIONAL GEOMETRY, PETROGRAPHIC VARIATION, AND ORIGINS OF UPPER ORDOVICIAN DOLOMITES IN HADELAND, NORWAY

Dolomites are widespread in the Upper Ordovician Kalvsjoen and Klinken berg Formations in Hadeland, Norway. Their distribution reflects gener ation by the repeated passage of fluids passing from a shallow shelf i n the east towards a deeper basin in the west. The Kalvesjoen Formatio n consists of thin-bedded limestone turbidites and shales deposited on a relatively deep slope. As sea level fell, these were incised to for m a series of shallow channels that were subsequently filled by chaoti c debris-flow deposits. The upper parts of this assembly were ultimate ly emergent. The Klinkenberg Formation marks a new transgression and i s a mixed carbonate-siliciclastic assemblage deposited on a storm-rewo rked shallow shelf or slope. Dolomite occurs throughout this interval. in the Kalvsjoen Formation it is generally dispersed, but is most com mon in eastern areas. Idiotopic and xenotopic textures show five disti nct cathodoluminescent zonal sequences and three non-luminescent forms . By contrast, the Klinkenberg Formation also includes a significant l aterally extensive dolostone unit. This is thickest in the east, altho ugh thickness has been reduced there by erosion. Three distinct dolomi te morphologies and five cathodoluminescent zone types are identified. The greatest concentrations of dolomite, the brightest zones, and the greatest variety of zones, are found in eastern regions, suggesting t hat the bulk of the waters responsible for dolomitization came from th at direction, driven down into the basin by shell drainage as sea leve l fell. The diversity of zones illustrates the chemically heterogeneou s and temporally variable nature of this Bow. In parts of the successi on, dolomite is associated with dissolution seams and results from Row of reactate solutions along these seams. Such Bow was sometimes confi ned within the seam but was more often pervasive. Evidence of dilation suggests that some seams were also paths For the upwards migration of overpressured fluids, a process which may have been driven by sea lev el change or by compaction.