Fe- and Mn-Enrichment in Middle Ordovician Hematitic Argillites Preceding Black Shale and Flysch Deposition: The Shoal Arm Formation, North-Central Newfoundland

The Middle Ordovician Shoal Arm Formation in the central volcanic belt of north-central Newfoundland consists of hematitic argillites overlain by grey cherts, then black shales directly underneath a Late Ordovician/Early Silurian flysch sequence. Using principal component analysis, geochemically definable components within related lithologie groups are: (1) biogenic, (2) mixed detrital, (3) hydrothermal, and (4) Mn, Ca-carbonate. Close sampling of the whole 350-m thick sequence provides reconstruction of variations among the sediment components through time. At the base of the hematitic section, a sharp increase in the hydrothermal component (enrichment in Mn, Fe, Ni, Pb, and Zn) decreases stratigraphically upward and disappears in the upper red Shoal Arm Formation. The Mn, Ca-carbonate component also decreases upward but persists into the grey cherts, indicating an additional source of Mn. The clastic component is largely mixed mafic/pelagic clay-like detritus with a strong pulse of Zr-, Nb-, and Y-rich detritus in the top of the hematitic unit. This latter component was derived from lateral equivalents of the alkaline/subalkaline Lawrence Head volcanics. The red hematitic argillites are not overall strongly Fe-enriched but represent the product of oxic bottom conditions under slower sedimentation rates than those of the earlier underlying sediments of the Wild Bight Group island-arc-derived volcaniclastics. The grey cherts mark a transitional stage between the hematitic sediments (oxic) and the black shales (anoxic). The change to increasingly$O_{2}$-deficient conditions is explained by either (a) an increase in the biological productivity in the overlying water column and basinwide synchronous development of anoxic porewaters (or seawater) by increased$C_{org}$-oxidation or (b) the diachronous subsidence of the basin floor into a deep-water anoxic layer as a result of the loading of the floor by an approaching accretionary thrust stack. In the second hypothesis, which we prefer, the anoxic water layer would permit significant lateral transport of recycled dissolved$Mn^{2+}$subsequently precipitated in more oxygenated parts of the basin. It would also explain the significant Mn-enrichment in the upper red Shoal Arm Formation and the grey cherts.