Continuity of sandstone beds in the Ordovician Cloridorme Formation, Gaspé Peninsula, Quebec

Ma, Cuiyan (1996) Continuity of sandstone beds in the Ordovician Cloridorme Formation, Gaspé Peninsula, Quebec. Masters thesis, Memorial University of Newfoundland.

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Abstract

The objectives of this thesis are to re-evaluate the continuity of sandstone beds and to investigate the factors that control sandstone bed continuity in the Cloridorme Formation. The study was carried out in a 5 km-long strike exposure of unit G of the β7 member of Enos (1965) between Grande-Vallée and Petite-Vallée. -- Thirteen facies and four facies associations were recognized in the field. The thirteen facies are: (1) coarse- to fine-grained, graded sandstones; (2) "slurry" sandstone beds; (3) sandstones containing abundant shale clasts; (4) sandstones that feed injections; (5) sandstones with shallow erosional bases; (6) cross laminated or/and parallel laminated sandstones or siltstones; (7) thinly bedded, laminated sandstones or siltstones; (8) sandstones containing large laminated slabs; (9) sandstones with bidirectional paleoflow indicators (Megaturbidite); (10) bi-partite sandstones; (11) thinly bedded sandstones; (12) concretions; and (13) laminated, silty mudstones. -- The four facies associations are: (1) erosively based sandstone packets; (2) sandstones interealated with mudstones or siltstone; (3) interealated sandstones, siltstones and silty mudstones; and (4) silty mudstones interealated with siltstones or sandstones. -- The interpretation of sandstone facies and facies associations described from the β7 member suggests that these turbidites accumulated in the distributary channels, the outer fan, and the fan fringe of a transverse fan near the margin of the foreland basin, and on the adjacent basin plain. The proximal parts of the marginal fans are believed to have been destroyed by later tectonic uplift and erosion. -- The turbidite succession, where bed-by-bed tracing is possible, contains outer fan and fan-fringe deposits of a transverse fan, and basin-plain deposits. The lateral correlation of individual beds proves that the continuity of sandstone beds in this succession is extremely low. Only 27 of the 94 traced beds extend beyond 3 km. The relationship between bed length (L) and percentage of beds longer than L (Y) is defined by the following equation: -- Y(%) = -49(log L) + 200; r² = 0.98 -- Extrapolation to Y=0% suggests that all beds are shorter than 12 km along this two-dimensional outcrop, although inclusion of partial beds in the regression analysis means that this is a minimum estimate of maximum bed length. -- Nine types of sandstone bed terminations were identified. They are; (1) beds feeding injections; (2) sills; (3) gradual taper; (4) scour-and-fill geometry; (5) erosional truncation; (6) depositional mounds; (7) shale-clast "nests"; (8) faulting; and (9) gradual passage into shale. The percentage of discontinuous beds accounted for by these type of terminations are follows: (a) scour and focussed deposition in scours (Types 4 and 5), 44.6%; (b) depositional thinning on unscoured surfaces as a result of patchy deposition (Types 3 and 6), 24.4%; (c) liquefaction and sand redistribution (Type 1), 10.8%; (d) masking of bed continuity by extensive "nests" of shale clasts (Type 7), 10.8%; (e) apparent lateral changes in the texture of watery debris flows (Type 9), 8.1%. -- The study of bed terminations suggests that the major factors controlling sandstone bed continuity are depositional and fault-controlled sea-bed morphology, and tectonics of the depositional setting. Sandstone bed continuity depends strongly on the rates and magnitudes of sea-bed deformation, the frequencies and magnitudes of the earthquakes that are inferred to have liquefied many of the Cloridorme Formation beds, and proximity to the termini of shifting feeder channels. Knowledge of the tectonic setting alone is not enough to permit a reliable prediction of sandstone bed continuity.

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