Scalloped and Planar Erosion Surfaces, Middle Ordovician Limestones, Virginia: Analogues of Holocene Exposed Karst or Tidal Rock Platforms

Abstract

ABSTRACT Scalloped and planar erosion surfaces are developed at numerous stratigraphic levels above prograded tidal flat fenestral limestones and beneath more open marine skeletal limestones in the Middle Ordovician limestone sequence, Virginia. Scalloped surfaces consist of smooth curved basins (10 cm to over 1 m wide) with intervening steep, locally overhanging walls and sharp ridges. Relief is up to 30 cm. The scalloped surfaces pass laterally into planar erosional contacts. Erosional surfaces truncate grains and cements and rare incipiently brecciated (vadose) fabrics. Fossiliferous breccias occur rarely above contacts. Some contacts are bored locally and encrusted by calcareous algae and bryozoans. Erosional contacts between fenestral limestones and overlying skeletal beds are extremely s arp and there is no burrowing across the contacts. Locally, submarine hardgrounds converge onto scalloped/planar erosional contacts. The erosion surfaces are believed to have formed initially as exposed karst on supratidal, prograded, early lithified tidal flat limestones, as indicated by vadose features beneath some contacts. Further erosion in tidal rock platform environments is indicated by position of erosion surfaces beneath open marine carbonates, organic encrustation of contacts, local marine cements beneath some contacts, local bored contacts, and by reported higher rates of tidal erosion compared to subaerial erosion in the Recent. These surfaces do not appear to have been documented to any extent in ancient carbonate sequences. However, the abundance of such surfaces along modern coasts and in karst terrains, and in the Middle Ordovician limestones of Virginia, suggests that they might be more abundant elsewhere. Finally, these surfaces might be useful in defining paleoshorelines in the absence of tidal flat deposits.