Depositional mechanisms of deep-sea coarse clastic sediments, Cap Enragé Formation, Quebec

Abstract

Cambro-Ordovician Cap Enragé deep-sea sediments consist of six main facies: (1) coarse conglomerates; (2) graded–stratified/cross-bedded fine conglomerates; (3) dispersed pebbly sandstones; (4) liquefied pebbly sandstones; (5) ungraded cross-bedded fine conglomerate and pebbly sandstones; (6) massive sandstones.Facies 1 and coarser parts of facies 2 beds have unimodal grain orientations in bedding, with unimodal a-axis imbrications. Finer facies 2 beds have unimodal or bimodal grain orientations in bedding, with bimodal (up- and downstream) a-axis imbrications. Facies 3, 4, and 6 beds have unimodal, bimodal, or random grain orientations in bedding. The a-axis imbrications vary as follows: mainly bimodal for facies 3, bimodal or unimodal for facies 4, and dominantly unimodal for facies 6.Facies 5 beds result from fractional reworking of previous deposits by strong bottom currents (velocities 70–125 cm/s) (? dilute spillover flows). All other facies were deposited mostly from concentrated clast dispersions at bases of large turbidity currents. Calculations show basal dispersions range from 15 to 27 cm thick beneath turbidity currents 100 m thick for facies 3 and facies 1 beds. Finer facies 2 beds had fractional phases, with maximum velocities 140–146 or 71–78 cm/s (depending on friction factors) for graded trough cross-bedded types. Facies 3, 4, and 6 basal dispersions had high apparent viscosities; for facies 1 and 2, inertial effects dominated. Syn- or early post-depositional deformation or liquefaction produced bimodal imbrications for facies 3, 4, and 6, and liquefaction structures in facies 4.