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Date of Award

1979

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Professor Roger G. Walker

Abstract

The Cap Enragé Formation, Cambro-Ordovician in age, is approximately 250 m thick and consists of coarse clastic sediments in association with classical turbidites. Sediments are divided into seven major facies: (1) coarse-grained conglomerate; (2) graded-stratified/cross-stratified fine conglomerate and pebbly sandstone; (3) graded-dispersed fine conglomerate and pebbly sandstone; (4) graded-liquefied fine conglomerate, pebbly sandstone and sandstone; (5) ungraded-cross-stratified fine conglomerate, pebbly sandstone and sandstone; (6) structureless sandstone; and, (7) classical turbidites, mainly sandstone. Rare shale beds also occur.

Facies (7) beds follow the Bouma model and are interpreted as deposits from turbidity currents. Facies (5) beds are due to reworking of previous deposits by dilute flows associated with turbidity currents. Facies (1) to (4) and (6) record a two-stage process: i) grading patterns reflect segregation of clast sizes during main transport phases by turbity currents; and, ii) sedimentary structures and fabrics record transport and deposition from basal dispersions, which developed at the base of turbidity currents.

Facies (1) beds have an a-axis flow-parallel bedding fabric, with a-axis upstream imbrication. Beds were deposited en masse from dispersions, in which there was strong grain interaction and high dispersive pressures. Grading patterns are interpreted in terms of decreasing applied shear stress and concentrations of sediment, as follows:

upgraded → inverse → inverse-to-normal → normal grading.

Coarse-grained Facies (2) beds have the same fabric as the Facies (1) beds and are interpreted as en masse deposits from dispersions, with dominant inertial effects. Stratification in both the coarse-grained Facies (2) and Facies (1) beds reflects pulsating depostition from the basal dispersions.

Finer-grained Facies (2) beds have flow-parallel or bimodal a-axis bedding fabrics, with a-axis upstream or bimodal imbrications. Beds were deposited from waning turbidity currents under high velocity tractional flow conditions. Bimodal bedding fabrics reflect clast reorientation after initial deposition on the bed. Bimodal imbrications record deposition of clasts onto low-relief wave bedforms.

Facies (3), (4) and (6) beds all have a-axis flow-parallel, bimodal or random bedding fabrics. Differences arise in imbrication patterns: Facies (3) have bimodal imbrications; Facies (4) have bimodal or unimodal imbrications; and, Facies (6) have unimodal imbrications. All beds are interpreted as being deposited from basal dispersion, in which viscous effects dominated. Apparent viscous effects were due to high concentrations of sand, such that the coarser clasts responded to the flow as if it were very viscous. Facies (3) and (6) beds may have experienced some syn- or post-depositional deformation into low-relief wave-forms, yielding bimodal imbrications. Facies (4) beds experienced some syn- or early post-depositional liquefaction, producing fluid escape features. Some Facies (4) beds also underwent syn-depositional or early post-depositional deformation similar to Facies (3) and (6) beds.

The overall facies model suggests deposition within three main topographic levels: (i) main channels; (ii) bars within main channel networks; and, (iii) terraces above the main channel networks. Facies (1) beds are main channel deposits. Facies (2),(3) and (4) beds are bar-top deposits or marginal terrace deposits. Facies (6) are high terrace sediments. Facies (7) and shale were deposited on high 'distal' terraces, farthest removed from the main channel networks.

Small scale ( < 1 m - 5 m ) fining/thinning-up sequences record the abandonment of channels; coarsening/thickening-up sequences represent the occupation of channel sites. Large scale (10's - 100 m) sequences reflect interactions between main channel networks and high terrace sites. Transitions from channels to marginal terraces are fining/thinning sequences; sequences from marginal terrace to high terrace are fining/thickening; and, transitions from high terrace to 'distal' high terrace are fining/thinning.

Regional paleocurrent and facies trends within main channel networks suggest that sediments accumulated in a submarine braided valley system. Main channel networks were forced to swing parallel to the continental base-of-slope, perhaps by a small scale, uplifted continental block.

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