Origin of Massive Dolostone: The Upper Knox Model

ABSTRACT Previous studies of Cambro-Ordovician dolostones from various sites in the U.S. midcontinent revealed similar textures and geochemistry, prompting similar explanations of their origin. Before these interpretations can be incorporated in a model to explain other massive dolostones they need verification in a contrasting setting. The DuPont Geohydrological Survey (DGHS) well, continuously cored through the Knox Group in west-central Tennessee, provides such a contrast. This core provides a complete sample of the Knox in the setting of a shallowly buried, flat-lying, structurally and stratigraphically simple platform sequence, which contrasts with the relatively deeply buried and structurally complex environments that dominate most prior studies. Light and luminescence petrography, XRD, SEM, EDS, XRF, ESR, and mass spectrometry were used to document the petrology and geochemistry of the Upper Knox Group (Lower Ordovician) from 456 m to 1126 m subsea (2018 ft to 4215 ft below the Kelly Bushing of the DGHS well). Of the 87 samples, 81 are dolostone. Early (E-1) dolomite forms a fine to medium crystalline replacive dolostone that lacks luminescence banding. Its geochemical signature is 13C = -2.83 ± 0.76o/oo; 18O = -5.85 ± 2.01o/oo; stoichiometry = 51.6 ± 1.2 mole % Ca; MnPR = 79 ± 27; Fe = 681 ± 237 ppm; Mn = 79 ± 32 ppm; and Sr = 110 ± 63 ppm. E-2 dolomite forms a medium to coarse crystalline replacive dolostone that luminescences with a relatively dull core and brighter red rim. Its geochemical signature is 13C = -2.07 ± 0.32o/oo; 18O = -6.96 ± 1.28o/oo; stoichiometry = 51.0 ± 1.3 mole % Ca; MnPR = 74 ± 20; Fe = 880 ± 843 ppm; Mn = 84 ± 52 ppm; and Sr = 79 ± 29 ppm. Late (L-1) dolomite lines vugs and fractures. Three limestone beds are unaltered remnants of the precursor to E-2 dolomite. Calcite is present both as relics in dolomite rhombs and as a vug and fracture fill postdating L-1 dolomite. Three quartz arenite beds and the presence of 1% to 5% well-rounded quartz sand grains in many dolostones denote an intermittent clastic source. E-1 dolomite began as syndepositional dolomicrite deposited in an upper intertidal to supratidal environment, and it was later modified during shallow burial. E-2 dolomite formed when subtidal grainstones and packstones were dolomitized during shallow burial. L-1 is a late void fill. Texture and geochemistry of dolostone in the DGHS well is similar to that from coeval Appalachian, Ozark, and Texas-Oklahoma units. Numerous vugs, fractures, and zones of rubble occur throughout the Upper Knox core, suggesting that multiple exposures occurred during its deposition. There is no pattern of change with depth for these features or for any other measured or observed parameter. We suggest that massive dolostone formation--involving dolomite grain growth, calcite replacement, and dolomite cement--took place in association with a succession of exposure-related events. The large volume of dolomite thus created required a large amount of magnesium, so large in fact that a volume of seawater equal to that of the entire modern ocean must have pumped through collective Upper Knox exposure surfaces.