Chertification in places along fractures and porous zones in the dolomite units has produced pod-like chert masses with entrapped "islands" of dolomite.
It is at localities where the Alibates is directly overlain by Ogallala that the most extensive chertification has taken place.
There, chertification has taken place most extensively and chert has completely replaced the upper dolomite member (Bowers 1975).
Some rocks are chertified dolomite (10 to 50% silica) and represent both intermediate stages of chertification and transitional contact zones between chert and dolomite rock.
An empty or partly filled cavity may be less than 5.0 mm away from a completely filled cavity, thus showing extreme localization or selectivity of the chertification. No evidence was observed, however, to suggest that silica precipitation was selective of certain cavities because of pre-existing chemical or mineralogical conditions.
The chertification process is revealed under the microscope.
The main conclusion, based on correlative black shale of the Dobele Formation and micritic limestone of the Saarde Formation containing chertification, barite occurrences, and increased [delta][sup.13]C, is that the Aeronian in the East Baltic was a period of high primary bioproductivity (Kiipli et al.
As stated above, black shale in the deep shelf, contemporaneous micritic limestone formation, widespread chertification, barite occurrences, and [delta][sup.13]C positive mild shift in the shallower shelf were interpreted as indicators of high primary bioproductivity (Kiipli et al.
High primary bioproductivity in the Aeronian was suggested considering organic-carbon-rich black shale in the deep shelf contemporaneous with micritic limestone, large-scale chertification, barite occurrences, and a mild positive excursion in [delta][sup.13]C in the shallow shelf carbonate facies.