Abstract: Timing of Carbonate and Clay Diagenesis in Cambrian-Ordovician Cow Head Breccia, Newfoundland

John F. Hubert, Robert K. Suchecki, Edward A. Perry, Jr.

The Cow Head Breccia accumulated from the Middle Cambrian through Middle Ordovician on the western slope of the proto-Atlantic. The 310-m sequence consists of carbonate megabreccia, green shale, lime mudstone, calcarenite, and radiolarian chert. As the "Taconic" klippe is oriented, the paleoslope dipped northeast, interrupted by a narrow carbonate platform that trended northwest-southeast. Gravity-mass flows from the platform deposited the breccia.

The original carbonate mud of the slope lime mudstone, and matrix of the breccia, is now entirely calcite microspar and pseudospar. Growth was very early because dense layers of pseudospar are deformed in erosionally truncated slump folds and soft-sediment folds in blocks of limestone and shale in breccias. Ubiquitous clay is concentrated around smooth, loaf-shaped crystals. Pervasive growth of pseudospar reflects lowering the Mg/Ca ratio in the pore water below the 5 to 1 molar ratio of sea water. Mg was depleted by (1) growth of 5 to 60-µ ferroan dolomite rhombs in the mud, (2) incorporation of pore water Mg in smectite, and (3) postdepositional exchange of Mg for Fe in the smectite. Thirty-two chemical analyses of the < 2-µ silicate fraction show Mg increases with the proportion of pseudospar. Burial diagenesis converted the Mg-rich smectite to corrensite in equilibrium with illite and occasionally chlorite.

Many boulders of platform algal limestone were lithified completely before incorporation into mass flows. For 70 m.y., the trilobites and conodonts in the boulders record biozones that match those of graptolites and conodonts in the interbedded limestone and shale. Continuous production of platform boulders dates their diagenetic fabrics as early, less than a graptolite biozone of a few million years. These include (1) nodules and crusts of radiaxial fibrous calcite, partly replaced by lutecite, in both collapse-solution karst breccia and bird's-eye openings in stromatolites, (2) calcite void-filling mosaics after vadose solution of aragonite shells and ooids, and (3) 50-µ "dusty" dolomite rhombs formed by sabkha and brine-reflux processes. Limpid dolomite rhombs averaging 60 to 00 µ are present along postlithification stylolites cut by Acadian ferroan calcite veins.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA