The Cambro-Ordovician platform carbonates of the Great Valley, central Appalachians, are notable for the interlayering of limestones and dolostones with well-preserved primary sedimentary structures. Despite such primary structures there are several observations that demonstrate a secondary origin for much of the dolomite, the most significant being that dolostone-limestone boundaries commonly crosscut bedding planes. The nature of such dolomitization is clearly revealed by certain mottled dolostones in which mudcracks and burrows are haloed by limestone. Stratification can be traced through the 2 to 10-mm wide limestone halos into the dolostone without break. The limestone halos have a peloidal grainstone texture with high minus-cement porosity and unbroken shells wherea the dolostone is a sucrosic mudstone with fragmented shells. We interpret these observations to mean that early cementation around permeable mudcracks and burrows has protected these zones from later compaction and dolomitization. We extend this idea of selective early cementation of permeable sediments to explain why the limestones are invariably grainstones while the dolostones are mudrocks. The idea is supported by the observation that laminae of dolostones are bent around limestone mudcrack halos, ripple lenses, etc, showing that the limestones have acted competently and the dolostones incompetently during compaction.

The timing of this post-early cementation dolomitization is not unequivocal, but it is certainly pre-stylolitization, as demonstrated by dissolution of dolomite rhombs along bedding-plane stylolites. Our observations do not rule out the possibility of a syndepositional origin for at least some dolomite in these rocks (there are dolomites and dolomites!) but they do negate the pressure-solution origin of dolomite advanced by Wanless (1979).

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