About This Item
Share This Item
The Upper Permian Cadeby Formation was deposited on the western margin of the Zechstein evaporite basin. The formation comprises two carbonate members which were nearly entirely dolomitized during early diagenesis. Recent meteoric dissolution in the shallow subsurface precludes the presence of evaporites within the carbonates at outcrop, but both direct and indirect evidence indicates the existence of former displacive and replacive evaporites.
Direct evidence of former evaporites is present in the form of both pseudomorphs and casts. In the lower member, textural preservation of internal fabrics within some anhydrite nodules was produced by calcitization of both anhydrite cleavage flakes and possible small chevron halite crystals . Where no replacement preceded dissolution, cauliflower-shaped molds 2 to 3 cm (.78 to 1.18 in.) in diameter evidence the former presence of anhydrite nodules in lower member lagoon wackestones and mudstones. Anhydrite nodule development distorted bedding laminae, indicating displacive growth within soft sediments. Halite hopper casts surround some former nodules. Lack of evidence of subaerial exposure or intertidal sedimentation suggests both sediment deposition and anhydrite nodule growth were s baqueous; evaporites forming when lagoon waters became restricted and progressively hypersaline.
Further direct evidence of interbedded evaporites is shown by fabrics of vertically oriented calcite crystals with scattered pseudomorphs of gypsum crystal margins. Ghosts of foraminifera, only visible under cathodoluminescence, and numerous dolomite inclusions are present within the calcite. The vertical orientation suggests relict gypsum precipitates on the sediment surface. These calcitized gypsum horizons are underlain by skeletal lime wackestones and packstones and overlain by irregular dolomite mudstones with casts of displacive gypsum rosettes, indicating subaqueous evaporite growth within sediment during periodic hypersaline conditions.
In the upper member, indirect evidence of former bedded evaporites overlying the Cadeby Formation is provided by an irregular bed of dedolomite (calcitized dolomite). The dedolomite transects facies boundaries, being found in supratidal carbonates with incipient tepee structures, intertidal and subtidal cryptalgal laminate boundstones, and ooid grainstone shoals. Dedolomite thickness varies from 0.5 to 2.5 m (1.5 to 8 ft) within individual outcrops. Dedolomitization was due to the action of fluids with high Ca2+/Mg2+ ratios, created by dissolution of the overlying Hayton Evaporite Formation, with dedolomite thickness dependent on local differences in the permeability of the preexisting dolomites. Petrographically, these dedolomites contrast with ferroan dedolomit s elsewhere in the Cadeby Formation and not spatially associated with evaporites.
The presence of former replacive evaporites is shown by irregular vugs which crosscut sedimentary laminations. Such vugs are more numerous in the upper member. Petrography indicates this replacement occurred before final compaction of the carbonates and was possibly related to diagenetic changes within the overlying anhydrite formation.
End_of_Article - Last_Page 480------------