ABSTRACT: Facies and Petrography of Upper Ordovician Mixed Carbonate-Siliciclastic Shelf Storm Deposits

BYERLEY, MICHAEL C., and MARIO CONIGLIO, University of Waterloo, Waterloo, Ontario, Canada

The Georgian Bay Formation is an Upper Ordovician 130 m thick shelf succession consisting largely of dolostones and shales with lesser amounts of limestones. Major lithologies include shale, packstone, and wackestone. The lower part of the succession is shale-dominated, the top portion becomes upwardly and progressively purer carbonate. Shale composition is 20 to 40% disseminated dolomite and/or ferroan calcite and 60 to 80% detrital silt-sized quartz with minor feldspar. Skeletal packstone and wackestone exhibit good allochem preservation. Packstone interparticle matrix is typically carbonate. Wackestones contain terrigenous or carbonate matrix and are commonly interbedded with shales.

The spectrum of sedimentary structures and ichnofabrics present indicate deposition in a shallowing-upward subtidal setting. As water depth decreased, storms became the dominant influence upon sedimentation. Storms were the primary cause of carbonate-siliciclastic mixing. Physical, biological, and diagenetic processes created three types and scales of mixed carbonate-siliciclastics, as illustrated with core, polished slabs, and thin section photomicrographs. Physical mixing occurred through storm processes such as scour and tempestite deposition. Biologically mixed sediments reflect the episodic impact of storms on benthic communities. Diagenetic mixing is seen where concentration of siliciclastics are enriched adjacent to hardgrounds and within dolomitized shales. Background sediment tion of fine-grained siliciclastics increased relative to carbonate during periods of carbonate sediment starvation and hardground formation. Decreased carbonate to siliciclastic ratios in dolomitized shale resulted from pressure solution removal of carbonate.

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