Sedimentology and depositional history of the Chapel Island Formation (late Precambrian to early Cambrian), southeast Newfoundland

Myrow, Paul Michael (1987) Sedimentology and depositional history of the Chapel Island Formation (late Precambrian to early Cambrian), southeast Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Chapel Island Formation, a 1000 m thick shelf sequence of late Precambrian-Early Cambrian age, is receiving considerable attention as a leading candidate for the boundary stratotype for the Precambrian-Cambrian boundary. The formation, composed of five informally defined members, has been divided into six facies associations (1-6). -- Facies Association 1 consists of red and green sandstones and shales (Facies 1.1/1.2), and grey to black, thinly laminated shales and silty shales (Facies 1.3), deposited in tidally-influenced peritidal and semi-restricted shoreline environments, respectively. -- Facies Association 2 consists of very thin to medium bedded sandstones and siltstones (Facies 2.1/2.2) with subordinate green and red laminated siltstones (Facies 2.3/2/4), deposited within a muddy deltaic system. Storm-generated sandstones (tempestites), and other storm-related features (e.g., gutter and pot casts), dominate Facies 2.1 and 2.2. These facies are also characterized by abundant and diverse slide and gravity-flow features (debrites, turbidites, and liquefied flow deposits) formed under conditions of high sediment supply in a delta front/upper prodelta setting. Thin bedded and laminated siltstones of Facies 2.3/2.4 form excellent marker horizons and represent delta abandonment facies. -- Laminated green siltstones of Facies Association 3 are considered outer shelf deposits. Sandstone laminae are interpreted as distal tempestites deposited by waning flows below storm wave base. -- Facies Association 4 consists of red, green and gray mudstones of inner shelf origin and thing fossiliferous algal limestones deposited in peritidal environments. Deposition took place on a low energy, oxygen-stratified muddy shelf, with the limestone beds forming during low stands of sea level. -- Facies Associations 5 and 6 were deposited on a storm- and wave-dominated shelf below and above the influence of storm waves, respectively. The thin to medium bedded sandstones and siltstones of Facies Association 5 represent distal tempestites. The planar laminated and hummocky/swaly cross-stratified sandstones of Facies Association 6 represent inner shelf and shoreface environments. -- The depositional history of the Chapel Island Formation records a shift from tectonically-active fault-bounded basins (underlying Rencontre Formation) to a tectonically-stable platform (overlying Random formation). Three distinct phases of sedimentation have been delineated. Phase 1, corresponding to the lower part of the formation (members 1-3), is a period of relative sea level rise recording a shift from marginal marine environments to delta front;/prodelta settings to the outer shelf. Phase 2, represented by members 3 and 4, is interpreted as a time of dropping relative sea level and low sediment input. Small scale relative sea level changes are recorded by variations in color, sedimentary and biogenic structures, diagenetic features and the deposition of thin peritidal limestone beds. Phase 3 (member 5) is a period of rapid deepening followed by shallowing with the progradation of a storm- and wave-dominated shelf. This phase, recording high sediment influx and strong storm and wave influence, is associated with major uplift and re-orientation of the shoreline from the southwest to the northeast.

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