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United Kingdom |
Shell Canada Ltd, Calgary Research Center, PO Box 2506, Calgary, Alberta T2P 3S6, Canada
The petrographic textures in the upper member of the Cadeby (Magnesian Limestone) Formation demonstrate the combined effects of original depositional setting and later diagenetic alteration of a carbonate rock.
Dolomitization was the most ubiquitous diagenetic process to affect these rocks. Dolomites formed (1) penecontemporaneously, (2) perhaps as primary precipitates, (3) as cements during early diagenesis, and (4) as locally controlled late diagenetic replacement and cement. These dolomite textures were subsequently altered by the advent of new diagenetic environments. A drop in Zechstein sea-level exposed much of the upper (Sprotbrough) member to the influence of meteoric waters. The mixing of fresh and marine waters produced solutions capable of dissolving unstable carbonate matter, dolomitizing more resistant grains and replacing earlier dolomite textures with new ones. Generally, later dolomitization events produced coarser crystal textures than the earlier.
Evaporite minerals were formed as early nodules within supratidal sediments or as late diagenetic pore-filling cements in the sub-surface. Their dissolution led to the development of considerable vuggy porosity and the addition of Ca2+ to ground waters. Ca2+-enriched fluids calcitized surrounding dolomites to produce dedolomite.
Porosity was controlled by the changes from one diagenetic environment to another. Primary pores were modified by cementation, recrystallization and pressure-solution. Secondary porosity was created by the dissolution of minerals that were unstable in pore fluids of the later diagenetic episodes.
