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Book Chapter

Reefs

Published:
January 01, 2000

Abstract

The Boot Inlet Formation (Reynolds Point Group, Shaler Supergroup) is an early Neoproterozoic (< 1077 MA, >723 Ma) succession that crops out within the Minto Inlier on northern Victoria Island in the Canadian Arctic archipelago, and consists of strata that accumulated on a carbonate ramp. Inner-ramp facies comprise molar-tooth lime mudstone and current-bedded ooid grainstone (locally herringbone cross-laminated) with scalloped erosional surfaces. Ooid shoals (3–4 m thick) and sheets (0.5–1.0 m thick) are interbedded with 10–15 m thick stromatolite bioherms and biostromes forming complexes 0.5 to 5.0 km wide. The most common mid-ramp facies is parted to ribbon-bedded limestone with conspicuous ripples, gutter casts, hummocky cross-stratification, and intraformational breccias readily interprétable as storm deposits; these finegrained rocks form shallowing-upward, meter-scale cycles capped by oolitic limestone and small reefs. Outer-ramp facies comprise shale with large carbonate concretions.

Reefs are most common in the lower half of the succession, where overall sea-level rise combined with higher-order transgressions to produce maximum accommodation space. A pronounced zonation of reef types occurs across the ramp. A current-oriented biostrome of Baicalia? is the only reef type on the inner ramp. Patch reefs and table reefs characterize the inner- to mid-ramp transition, and consist of stacked meter-scale bushes of Tungussia that pass upward into broad domal sheets of parallel, columnar stromatolites (Baicalia) oriented at a high angle to the sheets. Overall upward decrease in diversity of growth form is accompanied by evidence for increasing wave and current energy. Concentric-sheet bioherms up to 60 m in diameter and 15 m high, composed of sheets of closely spaced “pencil stromatolites” (Jurusania), grew in outer-ramp facies during rapid transgression.

The Boot Inlet reefs are similar to other Prolerozoic reefs in being composed entirely of stromatolites, including some of the same forms as characterize other early Neoproterozoic patch reefs. Calcimicrobes are conspicuously absent, despite their abundance in coeval deeper-water reefs in the Mackenzie Mountains. The presence of kalyptra-like stromatolitic structures in the Boot Inlet reefs is similar to that of Early Cambrian calcimicrobe-archaeocyathan reefs, and lends support for the view that the Phanerozoic reef archetype originated during the Neoproterozoic.

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SEPM Special Publication

Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World

John P. Grotzinger
John P. Grotzinger
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Noel P. James
Noel P. James
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SEPM Society for Sedimentary Geology
Volume
67
ISBN electronic:
9781565761896
Publication date:
January 01, 2000

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