Zechstein 2 carbonate platform subfacies and grain-type distribution (upper Permian, northwest Germany)

The Upper Permian Zechstein 2 Carbonate (Stassfurt Carbonate, or Ca2) platform facies of Northwest Germany can be subdivided into twelve subfacies types using slabbed cores from fifteen representative wells. Thin section and scanning microscopic analysis further provide subfacies-specific characteristics, based on distribution, size, shape, and spatial arrangement of the grains contained in the different subfacies types. Thirteen grain types can be distinguished within the different subfacies types on the Ca2-platform: 1) one type of oncoid, 2) one type of grapestone, 3) three types of peloids, 4) four types of ooids and 5) four types of aggregate grains. Both presence and composition of grains are indicative of the different subfacies types. There is also a relation between grain composition and porosity of the Ca2-subfacies types. The size and quantity of ooids correlate positively with increasing porosity, whereas an increasing amount of algal structures (algal-lamination) correlates negatively with porosity. The Ca2-platform carbonates almost exclusively represent highstand systems tract and lowstand systems tract deposits. The presence or absence of type-3 aggregate grains within the grainy shoal and algal-laminated shoal subfacies allows the assignment of these subfacies to highstand (grains absent) or lowstand (grains present) systems tracts deposits. The Ca2-highstand deposits can be subdivided into four shallowing-upward parasequences (PS3 to PS7) bounded by parasequence boundaries (PSB3 to PSB6) and Zechstein sequence boundary ZSB4. In contrast to macroscopic core studies, microscopic studies to identify Ca2-subfacies types can utilize cutting material. This allows reconstruction of the subfacies distribution on the Caa-platform, and delineation of potentially porous zones in uncored Ca2 intervals.