HARWOOD, GILL M., and ALAN C. KENDALL, School of Environmental Sciences, University of East Anglia, Norwich, UK
ABSTRACT: Dolomitization, Carbonate Leaching and Sulfate Diagenesis Associated with Permian Evaporite Basins, USA and North Europe
Common to many evaporite basins, the Delaware and Zechstein basins were floored by basal aquitards formed from basin-floor, fine-grained and organic-rich carbonate and siliciclastic facies. After draw-down at the start of evaporite sedimentation, these aquicludes restricted downward brine reflux, aiding brine retention and evaporite deposition within the basins.
Basin-flanking slope carbonates were more permeable. Once brine levels and evaporite sediment deposition overstepped the basal aquiclude, brine migration out of the basin commenced. Migration was predominantly lateral, as vertical flow was restricted by low-permeability basinal and toe-of-slope carbonate facies, formed during earlier carbonate progradation episodes. Within basin-flanking limestones, brines caused dolomitization and sulfate replacement/cementation. Sulfates formed as calcium-poor, but sulfate-rich, brines dissolved the more soluble carbonates present and dolomitized the host limestones. These processes released calcium ions which reacted with the brine sulfate. Emplacement of sulfate markedly reduced basin-flank permeabilities, thus allowing brines to be retained withi the basin, so allowing the continuation of evaporite sedimentation.
Controls on dolomitization and sulfate replacement in the lower slope facies were essentially the relative permeabilities of the different sub-facies (in part controlled by their early cementation history), together with the rate of brine reflux and brine composition. In upper slope and, particularly, the shelf margin facies, the early diagenetic history was a more important factor. Facies there had more early cementation (both aragonite and calcite), exposure and earlier (syn-sedimentary) dolomitization. Reaction of dolomitized sediments with sulfate-rich brines caused further replacement by anhydrite and/or magnesite.
AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.