SAYLOR, BEVERLY Z.¹, J. A. (TONI) SIMO², Y. S. (LUKE) CHOI², and CHARLES W. BYERS^2
1Case Western Reserve University, Cleveland, OH
²University of Wisconsin, Madison, WI

Abstract: Facies Control on Omission Surface Position and Paragenesis

Burrowed and mineralized omission surfaces are pervasive throughout Middle and Upper Ordovician carbonate and fine siliciclastic rocks of the Upper Midwest. They are particularly characteristic of the Galena Formation. Patterns of omission-surface stacking, relation to facies, and paragenesis reflect both basin geometry and detrital input.

Variations in clay and skeletal-grain abundances and bedding style define three facies belts. The central belt, which typifies the Galena Formation, is dominated by carbonate mudstone and punctuated by closely spaced omission surfaces. This widespread belt is bounded landward by clay-rich facies and seaward by bryozoan-crinoidal grainstone facies.

Omission surface morphologies are characterized by large burrow cavities and overhangs. Cavities are filled by material from the overlying bed or, in some cases, by a winnowed lag. Omission surfaces in the mudstone belt are impregnated by Fe-stained rinds. Borings and laminated phosphate coatings indicate more extensive seafloor cementation and mineralization, but are largely restricted to the clay-rich belt. Omission surfaces in the grainstone belt lack Fe-rinds, borings, and phosphate. Homogeneous mudstone facies show little or no change across omission surfaces. Landward, where mudstone passes into cleaning-upward cycles of the clay-rich belt, omission surfaces preferentially form boundaries between clay-rich and clay-poor facies. Omission surfaces in the grainstone belt separate stacked coquina event beds.

The grainstone and clay-rich belts represent end members of omission-surface genesis in this low-accumulation-rate basin. Event-bed deposition episodically interrupted seafloor starvation in the grainstone belt. Near land, clay accumulation was more continuous. Episodes of widespread nondeposition occurred during transitional periods when both carbonate and clay input were minimized. Land-derived nutrients enhanced phosphate and Fe- mineralization in the clay-rich belt. What mechanism controlled deposition and nondeposition in the mudstone belt? Minimal evidence for shallowing, winnowing, or marine carbonate cements suggest a limited role for abrasion in generating omission surfaces. Yet, bundles of omission surfaces seem to correlate for several tens of square kilometers, hinting at a process more widespread than localized event-bed deposition, perhaps some variation in the carbonate supply. The mudstone belt in particular and the basin as a whole appear to have been carbonate-sediment limited, possibly due to the epeiric sea's combination of low carbonate saturation state and large area for sediment redistribution. These conditions were especially suited to the formation of omission surfaces.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas