Shallow-Water Evaporite Cycles in the Middle Devonian of Western Canada
KENDALL, ALAN C., and GILL M. HARWOOD, University of East Anglia, Norwich, U.K.
The basal unit of the Prairie Evaporite, the Ratner Member, has been interpreted as a deep water (50 m +) deposit formed in subbasins between earlier carbonate buildups. This was suggested by lateral persistence of carbonate/evaporite laminae and the presence of turbidites.
Absence of Ratner laminites from buildup tops and flanks is, however, inconsistent with this interpretation. Subbasins contain different sequences that require continuous barriers in a deepwater basin. Detailed seismic coverage, however, indicates barriers are a series of carbonate buildups with intervening passages, which would have been ineffective in separating deep-water subbasins.
The Ratner Member consists of 2-3 cycles of mm-laminated sediments. Each cycle is composed of (a) even-laminated micritic carbonates, (b) crinkly-laminated carbonates that grade up into (c) interlaminated carbonate/anhydrite and then to (d) anhydrite. Crinkly-laminated carbonates are interpreted as syndepositional replacements of bottom-grown gypsum crusts and indicate maximum depositional depths of only a few tens of meters. Ratner cycles reflect upward salinity increases together with shallowing. The latter is consistent with brine-pan halite appearing immediately above the uppermost Ratner cycle.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)