ABSTRACT

Thinly interbedded limestones and dolostones are common in Paleozoic and Proterozoic carbonate sequences. Such rocks, called ribbon rocks, are characteristic of the 750-m-thick Upper Cambrian Conococheague limestone, a platform deposit exposed in western Maryland. Limestones of Conococheague ribbon rocks occur as continuous wavy thin beds to isolated centimeter-scale pods; commonly have flat bottoms and ripple-form tops; typically are internally current- or wave-ripple cross-laminated; and are composed of peloidal grainstones. Dolostones interbedded with these limestones are typically continuous on the outcrop scale; drape underlying limestone ripple-forms; are commonly mudcracked; and are composed of 10-40 B. dolomite rhombohedrons in a carbonate-clay matrix. Some ribbon rocks are di rupted by filled tubes, soft sediment deformation features, and abrupt lateral changes in rock type. Ribbon rocks within meter-scale cycles of the Conococheague have gradational contacts with cross-stratified grainstones below and grade into mud-cracked laminites above.

These observations suggest that the limestone-dolostone alternations of ribbon rocks reflect original carbonate sand-mud alternations. This interpretation is strongly amplified by comparing Conococheague ribbon rocks to examples of lenticular and wavy bedding from mixed siliciclastic sand and mud flats from the modern North Sea. Conococheague ribbon rocks are interpreted as ancient carbonate examples of lenticular and wavy bedding deposited on mixed carbonate sand and mudflats in a shallow subtidal to intertidal setting. This is consistent with an overall interpretation of Conococheague cycles as the regressive deposits of carbonate tidal flats.