PALEOKARST-INFLUENCED DEPOSITIONAL AND DIAGENETIC PATTERNS IN UPPER-PERMIAN CARBONATES AND EVAPORITES, KARSTRYGGEN AREA, CENTRAL EAST GREENLAND

The Karstryggen area of eastern Greenland represents the western edge of sedimentation in the Jameson Land Basin, an arm of the northern Zec hstein seaway. Upper Permian strata of this area were deposited as two major sequences. The first marine incursion transgressed largely pene planed Lower Permian strata and deposited thin, paralic conglomerates, sandstones and shales (the Huledal Formation) followed by a thick pac kage of carbonates and evaporites (the Karstryggen Formation). Althoug h the Karstryggen Formation represents the transgressive maximum of th is sequence, it contains only marginal oi restricted marine strata, in cluding micritic, stromatolitic and peloidal carbonates and thick, but localized, bedded gypsum deposits. These lithofacies indicate that re latively arid climates prevailed in this basin, as in most of the Zech stein region. A major regression, associated with a change to a more h umid climate, terminated Karstryggen sedimentation. Pre-existing evapo rites and carbonates underwent diagenetic alteration, including widesp read calcitization and dissolution of gypsum. More importantly, topogr aphic relief in excess of 120 m was generated by fluvial drainage syst ems and karstic sinkholes. A second marine incursion, accompanied by a return to a semi-arid climate, drowned this high relief topography, p roducing a complex sequence of strata (the Wegener Halvo Formation) in which sedimentation was greatly influenced by the rugged underlying t errain. Marine cemented algal-molluscan grainstones draped pre-existin g palaeotopography during the initial stages of flooding. Continued dr owning led to differential sedimentation on 'highs' and in 'lows'. Ool itic and bryozoan-brachiopod grainstones formed as shoals on the crest s of most prominences, whereas shales, conglomeratic debris flows, eva porites. or oolitic turbidites were deposited in the lows. More restri cted sedimentation took place in the westernmost areas which lay close st to the mainland shoreline and were situated to the west of a palaeo topographic ridge. There, oolitic, stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions. Three subcycles mark smaller scale rel ative changes of sea level that occurred during deposition of the Wege ner Halvo Formation; they are delimited by regional surfaces with mode rate relief(5-20 m) developed during subaerial exposure. Widespread di agenetic changes, including leaching of aragonitic grains, dissolution /collapse brecciation of evaporites and meteoric calcite cementation, occurred in association with these smaller scale sequence boundaries, again reflecting climatic oscillations. Relative sea level fluctuation s, coupled with regional climate changes, played a dominant role in de termining both depositional and diagenetic relations in these strata. These features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern Zechstein Basin a nd Barents Shelf, and may have economic significance for the localizat ion of hydrocarbons.