MARINE DIAGENESIS OF MESOPROTEROZOIC DEEP-WATER SEEP MOUNDS
Benthic clotted dolostone consists of millimetric clots of medium-crystalline dolomite and is interpreted to have precipitated on the seafloor as a direct result of fluid seepage, probably in the presence of microbes. The clots form a framework in which marine isopachous cement lines voids. Several forms of marine isopachous cement are present, including bladed length-slow dolomite, fibrous length-fast dolomite, and blunt-ended crystals replaced by clear, blocky, medium-crystalline dolomite. Bladed length-slow dolomite displays sweeping extinction through individual crystals, is well preserved, and contains growth zones defined by inclusions; it may be dolomite that precipitated from seawater. Fibrous length-fast dolomite is probably a replacement of calcite cement. Blunt-tipped cement crystals were possibly originally aragonite or another mineral that was diagenetically unstable. The blunt-tipped cement is present only in contact with clot surfaces, whereas the other cements have diverse micro-environmental and microstratigraphic relationships. Stable C isotope values fall in the normal range for Mesoproterozoic carbonate rocks and REE+Y patterns exhibit marine signatures, but the mounds’ REE+Y patterns also show that the water column was redox-stratified, and that mounds formed in anoxic conditions. The range of marine cement composition (aragonite, calcite, dolomite?) present in depositional void space among clots indicates a highly variable seafloor diagenetic environment. The complicated cement history was influenced by variable proportions of seawater and vent fluid, as well as the activity of the presumed clot-forming microbes. The variable chemical conditions and the anoxic environment may have promoted marine dolomite cement precipitation, similar to primary dolomite cement described in Neoproterozoic reefs, but is the first of its kind in Mesoproterozoic carbonate rocks.