--> Abstract: Evolution of a Ramp into an Anoxic Basin: Middle Devonian of Saskatchewan, Canada, by G. Harwood, C. Latimer, and A. Kendall; #91012 (1992).

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ABSTRACT: Evolution of a Ramp into an Anoxic Basin: Middle Devonian of Saskatchewan, Canada

HARWOOD, GILL, CAMERON LATIMER, and ALAN KENDALL, University of East Anglia, Norwich, U.K.

Initial deposition of the Winnipegosis Formation occurred on a basin-wide (>1600 km) ramp, with deposition of bioturbated wackestones. Subsequently this ramp differentiated into a marginal bank

facies and an archipelago of isolated mounds separated by deeper water in which organic-rich laminites formed. Thickness variations (mounds a maximum of 90 m; anoxic basinal microlaminites maximum 3 m) indicate accumulation occurred within a stratified and starved basin. After growth of carbonate mounds ceased, the basin became hypersaline, generating mm-laminated carbonates and calcium sulfates. This sequence is confined to basin floors, and onlaps mound flanks.

Different areas of the basin have different stratigraphic sequences. In a northern "subbasin" there was a rapid change to anoxia. Here, the ramp top is an oncolitic horizon, abruptly overlain by thick (up to 3 m) organic laminites overlain by thick (15 m) cyclic hypersaline laminites. This contrasts with the succession into a southern "subbasin" where there is a rapid vertical change to hypersalinity. Here oncolites are commonly absent, the organic microlaminite is absent or is but feebly developed (<1 cm), and is followed by a thinned hypersaline laminite sequence (3 m). Close to the marginal bank, thick, laminated carbonates (with irregular and discontinuous organic films) above the ramp represent anoxic forebank deposits, time-equivalents of the starved basin deposit further to he north. Correlations between subbasins are contentious and have implications for the development of source rocks in different parts of the basin.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)