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Shallow‐water gravity‐flow deposits, Chapel Island Formation, southeast Newfoundland, Canada
Author(s) -
MYROW PAUL M.,
HISCOTT RICHARD N.
Publication year - 1991
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1991.tb01880.x
Subject(s) - geology , siltstone , turbidity current , slumping , turbidite , sedimentary depositional environment , sedimentary structures , sedimentary rock , geomorphology , graded bedding , lamination , debris flow , sediment , geochemistry , debris , structural basin , chemistry , oceanography , organic chemistry , layer (electronics) , facies
A remarkable suite of shallow‐water, gravity‐flow deposits are found within very thinly‐bedded siltstones and storm‐generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non‐graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine‐grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore‐fluid pressure dissipation support the feasibility of such transitions. Raft‐bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm ‐1 and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm ‐2 and a depositional slope estimate of 0.77°. The lithologies and sedimentary structures in member 2 indicate an overall grain‐size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure included in situ liquefaction (‘disturbed bedding’), sliding and slumping. Raft‐bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross‐stratified sandstone directly overlies slide deposits and raft‐bearing beds, linking sediment failure to the cyclical wave loading associated with large storms. The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud‐rich, marine deltas. Details of deltaic gravity‐flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies primarily carried out with side‐scan sonar.