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Many zones within Phanerozoic cratonic carbonates exhibit numerous features demonstrating synsedimentary submarine lithification at or near the sediment-water interface in normal-marine settings. Two such units, one from the Middle Ordovician Verulam Formation in Ontario, the other from the Upper Jurassic Sundance Formation in Wyoming, occur in sequences with multiple hardground zones, consist of coarse grainstones, and contain numerous borings of endolithic invertebrates which crosscut both allochems and early cement. The presence of borings in these grainstone units provides an unequivocal criterion for the discrimination and evaluation of hardground cement habits in that they constrain the timing of early cementation. Marine cement in both units consists of equant crys als of clear, inclusion-free, luminescent, low-magnesium calcite.
These cements are identical in habit to meteoric phreatic cement in calcitized Pleistocene sequences and are strikingly unlike Holocene marine cements which consist almost exclusively of acicular crystals of aragonite and/or high-magnesium calcite. Their presence in these two Phanerozoic normal marine carbonate sequences demonstrates that generalizations which relate cement morphologies to cement compositions, and which relate cement compositions to the chemistry of water in various cementation environments in modern systems, may not be valid when applied to ancient limestones.
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