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Ordovician and Silurian, volcanic and sedimentary rocks, which are folded and intensely faulted, underlie an area three miles by one mile in the vicinity of Cobbs Arm, northeastern New World Island, Notre Dame Bay, Newfoundland. The Ordovician rocks are intermediate and basic volcanic flows, massive crystalline limestone, and graphitic, pyritic, black argillite. These rocks occur toward the top of a thick succession of Ordovician eugeosynclinal deposits that underlie a large part of the Notre Dame Bay region. The volcanic rocks are the oldest in the map area. Minor pyroclastics and limestone interbedded in the lava flows contain Middle Ordovician fossils. The black argillite contains Middle Ordovician graptolites. Grey-green argillite and silty argillite with Early Silurian fossils disconformably overlie the black argillite. Interlayered, terrigenous conglomerate, greywacke, sandstone, siltstone, and argillite are the other Silurian rocks of the map area. These Silurian rocks and terrestrial, volcanic and sedimentary rocks that occur stratigraphically above them represent the Silurian succession of northeastern Newfoundland.

Structural relationships are complex. A recumbent fold-system was overthrust into the map area from the southeast. Normal faulting of the folded and thrust-faulted rocks produced a graben which now forms a structural belt in the central part of the map area. Transverse faults subsequently modified this structure. High-angle, transcurrent, transverse faults are very numerous, particularly in the central structural belt. Rotational and normal, transverse faults are also present.

Ordovician limestone is quarried commercially at Cobbs Arm. The estimated reserve of recoverable, high-calcium limestone is one million tons.

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Supervisors: G. C. Milligan / Paul E. Schenk