ABSTRACT
The Middle Ordovician Shoal Arm Formation, which is located in the central volcanic belt of north-central Newfoundland, is a tripartite assemblage of hematitic argillites, grey cherts, and black shales directly underlying a flysch sequence. The hematitic argillites are enriched in Fe, Mn, Ni, Pb, and Co. Factor analysis and principal component analysis indicate the presence of a hydrothermal component, presumably derived from hydrothermal activity in the coeval Lawrence Head volcanics. Unusual, (?) calcareous Mn-Fe-oxide nodules are present in the top parts of turbidites in the hematitic argillites. Electron microprobe analysis of a color transition from a red to a green argillite indicates fractionation of Mn from Fe by diagenetic mobilization of Mn and subsequent precipitation as Mn-carbonate in adjacent green, calcareous argillites. The detrital component of the Shoal Arm Formation is influenced by several, geochemically different, clastic sources. The top part of the Shoal Arm Formation is characterized by a Zr-, Nb-, and Y-rich clastic component that may reflect either erosion or volcanic activity of lateral equivalents of the Lawrence Head volcanics. The hydrothermal component disappeared with erosion of these volcanics. The overlying grey, mottled and laminated cherts reflect a biogenic bloom, which preceded euxinification of the depositional basin. Synchronous and diachronous depositional models are proposed to explain the tectonic history of the Shoal Arm Formation. The synchronous model emphasizes the high biological productivity and limited circulation in a restricted basin as the cause for the observed euxinification. The diachronous model explains the black shale facies with a prograding, deep-water anoxic layer that developed during rapid basin subsidence as the result of thrust-loading. In this model, the black shales were deposited in front of flysch sediments derived from a southeastward prograding thrust stack.
The Middle Ordovician Taconic sequence of New York (i.e., the upper part of the Poultney Formation, the Indian River, and the Mt. Merino Formations) exhibits hematitic argillites in a similar lithostratigraphic position relative to black shale and flysch as the Shoal Arm Formation. Comparison of the Shoal Arm Formation with this part of the Taconic sequence indicates that the two tectonic models are also applicable to this sequence. Both the Indian River Formation and the Mt. Merino Formation are slightly enriched in Fe, Mn, and the trace elements Pb and Ni. This modest metal enrichment is explained either by recycling of Fe and Mn into the seawater in expanded oxygen minimum zones and subsequent precipitation at oxic/anoxic interfaces, or by a distal hydrothermal component. A continental source of Fe is excluded. Minor enrichment of biogenically derived material in the Mt. Merino Formation suggests that biological productivity may not have been the determining factor for euxinification.
The comparison with Precambrian sequences that contain Superior-type banded iron-formations and black shales in comparable stratigraphic positions indicates little geochemical similarity with the two Ordovician sequences. Enrichments of Fe, Mn, Pb, Ni, Co, and Cr in both iron-formations and black shales are generally stronger than in the Ordovician cases. Interpretations of biological productivity are hampered by the insufficient knowledge of inorganic element associations with biological matter in Precambrian oceans. As a consequence, it is difficult to test the proposed tectonic models with the available geochemical data. Comparisons to that point have to rely upon field observations alone.

Brüchert, V., 1992. The origin of metalliferous argillites in the Shoal Arm Formation of north-central Newfoundland. Unpublished MSc. thesis, State University of New York at Albany. 262 pp., +xii
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1992 B79

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