Penecontemporaneous Dolomitization and Preservation of "Oncolitic" Stromatoporoids in Lower Middle Devonian of Ohio

Roger J. Bain

Lower Middle Devonian carbonates of north-central Ohio record environments that changed upward from hypersaline supratidal-tidal lagoon (Lucas) to barrier beach to shallow marine (Columbus-Delaware). Within the upper Lucas and lower Columbus, molds of stromatoporoids are common, occurring generally as scattered disoriented remnants in various stages of dissolution. In an areally restricted portion of the upper Lucas, stromatoporoids are much more abundant and are extremely well preserved. Growth forms vary from massive low-profile encrustations to "oncolitic" spherical to prolate masses. Laminae within the "oncolitic" forms mimic the overlapping semiconcentric laminar structures of algal oncolites and are interpreted as having formed by similar interrupted upward growth o organic masses that experience episodic movement and overturning. Transport and burial resulted in additional deformation and fracture of semirigid skeletons.

I suggest the encrusting and "oncolitic" stromatoporoids lived in a tidally influenced channel or flat adjacent to a restricted hypersaline tidal lagoon.

Exceptional preservation resulted from penecontemporaneous lithification of the Lucas by dolomitizing solutions, both refluxing brines and mixing water. Lithification reduced the sediment permeability sufficiently so that later diagenetic waters, either magnesium rich or fresh, were unable to migrate through the sediment and dissolve the aragonitic stromatoporoids. Slow calcite replacement preserved skeletal details of Lucas stromatoporoids, whereas in the porous permeable Columbus (barrier beach), stromatoporoids were dissolved during diagenesis and dolomitization.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.