Sedimentology and authigenesis of the lower Devonian Torbrook Formation ironstone, Torbrook, Nova Scotia, Canada
LE3 .A278 2011
Pufahl, Peir K.
Bachelor of Science
Earth & Environmental Sciences
Ironstone in the lower Devonian Torbrook Formation of Nova Scotia accumulated in an array of storm-dominated, middle and distal shelf environments. Lithofacies stacking patterns indicate deposition occurred through the development of seven parasequences. Paraseqeunces range in thickness from 40 to 250 m thick and become progressively thicker through the Torbrook Formation. Individual parasequences coarsen from parallel laminated mudstones and ripple cross-laminated sandstones to iron cemented hummocky cross-stratified sandstones that are capped by marine flooding surfaces. Ironstone firmgrounds that mark flooding surfaces contain in situ and transported brachiopods and crinoids, suggesting mesotrophic nutrient levels predominated across the shelf. Ironstone development is interpreted to have been restricted to flooding surfaces because low or net negative rates of sedimentation stabilized the iron redox interface within the sediment, which allowed pore waters to become saturated with iron. Saturation and precipitation of iron minerals across this redox interface is interpreted to have occurred through iron redox pumping. Iron redox pumping is a cyclic mechanism that concentrates iron in pore water through the dissolution of iron (oxyhydr)oxide below the iron redox interface. Precipitation occurs when ferrous iron diffuses upward through the sediment to combine with oxygen in suboxic pore water. This process links the cycling of iron to phosphorus and is likely responsible for the high levels of bioavailable phosphorus necessary to sustain firmground populations of brachiopods and crinoids. Important authigenic phases in these ironstone firmgrounds include hematite, chamosite, brethierine, and francolite
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