Diagenetic Modification of Knox Evaporative-Dolomite Geochemistry by Middle Ordovician Paleoaquifer/Burial Fluids

Isabel P. Montanez, J. Fred Read

Early Ordovician upper Knox cyclic carbonates (100,000-year periods) were deposited on a ramp in a semiarid setting. Inner ramp facies are completely dolomitized, whereas only cycle caps on the outer ramp are dolomite. Laminite caps on leading edges of prograding flats were dolomitized by evaporative, storm-recharged marine waters that precipitated CaCO₃ and minor amounts of CaSO₄. Subtidal facies beneath interiors of supratidal flats (up to 200 km wide) were dolomitized by shallow sub-surface brines as the flats prograded. Inner ramp areas existed as supratidal flats for as long as 100,000 years, hence only there are sequences completely dolomitized.

Tidal-flat dolomites consist of euhedral to subhedral fabric preservative mosaics (crystals 5-50 µm, and up to 300 µm in replaced pelletal layers). Dolomitized subtidal sediments are composed of 20-200 µm dolomite crystals. Most of the early dolomites are fabric retentive initially, becoming more fabric destructive with increasing exposure to dolomitizing fluids. Fe values are up to 2,000 ppm, and Mn values are up to 500 ppm. ^dgr¹⁸O values show strong geographic control, with samples depleted (up to -7^pmil) relative to pristine evaporative dolomites derived from concentrated Ordovician seawater. Burial dolomites are enriched in Fe (up to 14,000 ppm) and depleted in ^dgr¹⁸O, suggesting precipitation from basinal brines at elevated temperatures.

Geochemical data suggest that evaporative dolomites have undergone modification by isotopically lighter fluids. Alteration most likely occurred within a Middle Ordovician meteoric aquifer system or less likely by later Paleozoic basinal brines. Regional geologic data for Knox Group carbonates throughout the eastern United States are vital in constraining dolomitization models.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.