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Deep-Sea and Fluvial Braided Channel Conglomerates: A Comparison of Two Case Studies
Detailed comparisons of modern gravelly fluvial deposits from rivers in the Canadian Rocky Mountains and Cambro-Ordovician deep-sea valley-fill conglomerates (Cap Enrage Formation, Quebec) reveal some similarities and differences.
Braided channel deposits in both deep-sea and fluvial settings occur in laterally fining/thinning units within concave-up lenses bounded by major scour surfaces at the base. Vertical sequences are mainly fining-upward, less commonly coarsening, 5–10 m thick for deep-sea and 2–3 m thick for fluvial channel fills. Main channel deposits in both settings are dominantly structureless.
Braid bar deposits in deep-sea and fluvial sediments occur in laterally coarsening/thickening units with flat bases and convex-up top surfaces. Vertical sequences vary depending upon locations within the bar complex and the type of braid bar. Generally, vertical sequences are fining-upward, 1–2 m thick, horizontally stratified or cross-bedded units. Planar tabular cross-stratification is more common in fluvial bars than in deep-sea bar deposits. Graded trough cross-stratification, graded horizontal stratification and irregular inclined cross-stratification are very common in deep-sea bar deposits. These features were not recognized in the fluvial conglomerates. Horizontal stratification consists of layers with different clast sizes or layers with alternating matrix-filled and open-work texture in fluvial conglomerates. Horizontal stratification consists of layers with different clast sizes or layers with alternating clast-supported and clast-dispersed texture in deep-sea conglomerates. Open-work texture was not observed in the deep-sea conglomerates.
Grading types and gravel fabric patterns are perhaps the most useful criteria in the distinction of fluvial from deep-sea conglomerates. Fluvial conglomerates are mainly ungraded, with less common normal or inversely-graded beds. Deep-sea conglomerates are mainly normally graded, with less common ungraded conglomerates and rare inversely or complexly graded beds. Deep-sea channel conglomerates have a-axis flow-parallel, a-axis upstream imbricate fabrics. In fluvial channels the smaller clasts may also assume an a-axis flow-parallel, a-axis imbricate upstream pattern. However, the coarser clasts are generally aligned in fluvial channels with a-axis flow-transverse, b-axis imbricate upstream. Braid bar deposits are distinguished on the basis of imbrication: in fluvial deposits imbrications are b-axis upstream; in deep-sea deposits imbrications are either a-axis upstream or a-axis upstream and downstream (bimodal). A-axis orientations in bedding are quite variable in both fluvial and deep-sea bar deposits and are not very reliable.
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