Abstract

The Avalon tectonostratigraphic zone occupies about 25% of the area of insular Newfoundland. Late Precambrian development of the Avalon terrane is distinctly different than that of other parts of the Appalachian orogen and cratonic North America. Late Precambrian volcanism and tectonic instability gave way to deposition of about 2 km of Cambrian and Lower Ordovician dominantly epiclastic sediments on a slowly subsiding, relatively stable shelf platform.The Lower Cambrian Random Formation is part of the platformal sequence and is bounded by regional disconformities over much of the Newfoundland Avalon Zone, except around Fortune Bay where it is underlain by older terrestrial and shallow-marine sediments. The Random Formation was deposited during a time of global sea-level rise, and consists of up to 250 m of shoreline, nearshore, and open-shelf deposits that record macrotidal conditions and periodic storm activity. Cross-bedded quartzarenites occur in units to about 50 m thick (one exceptional unit is 110 m thick) that tend to have gradational bases and abrupt tops. Bimodal–bipolar paleocurrent data are best explained by reversing tidal currents. These sandstone units show no evidence of intertidal (beach) processes and are therefore interpreted as subtidal ridges or shoals. The intertidal environment is represented by a shaly unit, with thin sandstone beds, that is characterized by flaser and lenticular bedding, oscillation (wave) ripples, some flat-topped ripples, and abundant synaeresis (dehydration) cracks. A muddy shoreline and subtidal sands suggest a macrotidal setting.Storm deposits range from: (1) fine, micaceous, red sandstones with fiat, gently dipping lamination, low-angle truncation surfaces, rare hummocks, shale-clast horizons, granule lags, and steep-sided erosional gulleys believed to represent lower shoreface rip-current channels; to (2) mudstones with graded, sole-marked, glauconitic sandstone beds deposited on an open shelf by storm-generated density currents.

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