FE-ENRICHMENT 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 overla in by grey cherts, then black shales directly underneath a Late Ordovi cian/Early Silurian flysch sequence. Using principal component analysi s, geochemically definable components within related lithologic groups are: (1) biogenic, (2) mixed detrital, (3) hydrothermal, and (4) Mn,C a-carbonate. Close sampling of the whole 350-m thick sequence provides reconstruction of variations among the sediment components through ti me. At the base of the hematitic section, a sharp increase in the hydr othermal component (enrichment in Mn, Fe, Ni, Pb, and Zn) decreases st ratigraphically upward and disappears in the upper red Shoal Arm Forma tion. The Mn,Ca-carbonate component also decreases upward but persists into the grey cherts, indicating an additional source of Mn. The clas tic 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 hemat itic 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 pro duct of oxic bottom conditions under slower sedimentation rates than t hose of the earlier underlying sediments of the Wild Bight Group islan d-arc-derived volcaniclastics. The grey cherts mark a transitional sta ge between the hematitic sediments (oxic) and the black shales (anoxic ). The change to increasingly O2-deficient conditions is explained by either (a) an increase in the biological productivity in the overlying water column and basinwide synchronous development of anoxic porewate rs (or seawater) by increased C(org)-oxidation or (b) the diachronous subsidence of the basin floor into a deep-water anoxic layer as a resu lt of the loading of the floor by an approaching accretionary thrust s tack. In the second hypothesis, which we prefer, the anoxic water laye r would permit significant lateral transport of recycled dissolved Mn2 + subsequently precipitated in more oxygenated parts of the basin. It would also explain the significant Mn-enrichment in the upper red Shoa l Arm Formation and the grey cherts.