Sedimentology and local basin analysis of the Lower Conception Group (Hadrynian), Avalon Zone, Newfoundland

Gardiner, Scott (1984) Sedimentology and local basin analysis of the Lower Conception Group (Hadrynian), Avalon Zone, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Argillites and medium to very fine grained sandstones in the Mall Bay (approx. 800 m thick) and Drook (approx. 1500 m thick) Formations of the lower Conception Group (Hadrynian) are well exposed along the coast on the east side of St. Mary's Bay, Avalon Peninsula, Newfoundland. These volcaniclastic sediments contain graded beds, convolute and parallel laminae, ripples, and rare floating shale clasts. A few sedimentary slide deposits are also present. Inferred sediment transport mechanisms are turbidity currents, low-density, low velocity lutite flows, cohesive gravity flows, and modified grain flows. Superimposed depositional processes include grain interaction and fluid escape. -- Seven facies are recognized based on lithology, grain size, bed thickness, and sedimentary structures. Six of these have been grouped into facies associations I, II, and III, which are interpreted to represent sedimentation on basin plain/fan fringe, lower basin-slope, and lower mid-fan environments respectively, within a turbidite fan depositional system. The intensely faulted, chaotic nature of the seventh facies precluded interpretation. -- Field measurements of solemarks (grooves and prodmarks), linguoid and straight-crested ripples, and laboratory measurements of sandstone fabric indicate a complex paleoflow pattern dominated by a southwest-northeast trend. Paleocurrent directions obtained from sandstone fabric agree with those obtained from solemarks and ripples. Stratigraphic variation of paleoflow directions does occur. During deposition of the Mall Bay Formation, turbidity currents flowed toward the northwest and southwest, while marine currents flowing to the east reworked muds and silts on the adjacent lower basin-slope. During Drook deposition, turbidity current flow was initially toward the southwest, but later switched to the northeast from a new southerly source. -- The average sandstone composition of the lower Conception Group is 39.6% plagioclase (albite), 36.6% volcanic rock fragments, 14.2% matrix, 8.2% quartz, 1.4% carbonate, and trace amounts of epidote and accessory heavy minerals (e.g., zircon). Based on electron microprobe data, albitization of feldspar is pervasive. -- The dominant source terrane consisted of rhyolitic and basic volcanic rocks. There was also an older sedimentary source. A tectonic setting for these sandstones could not be determined using the QFL and QmFLt diagrams of Dickinson (1983) and coworkers. The sandstone composition is similar to that around island arcs beside both modern and ancient subduction zones, but the paucity of other features indicative of subduction, such as andesitic volcanism, melange, and ophiolites, precludes an unequivocal subduction-zone interpretation.

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