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Bypass-zone Tempestite Facies Model and Proximality Trends for an Ancient Muddy Shoreline and Shelf
Paul M. Myrow
Member 2 of the late Precambrian to Lower Cambrian Chapel Island Formation of southeast Newfoundland was primarily deposited in storm-influenced, nearshore and shelf environments along a fine-grained coastline. Deposition took place within or adjacent to a deltaic system that supplied abundant silts and clays. Thin-bedded graded sandstone beds, hummocky cross-stratified beds, and a variety of very thin conglomerate units contain evidence for deposition by storm currents.
Sedimentological and stratigraphic evidence is used to reconstruct the relative paleobathymetric position of the three major facies in member 2. The Gutter Cast Facies is a shallow-subtidal deposit characterized by very thin sandstone laminae and abundant pot and gutter casts. The Siltstone-Dominated Facies contains more laterally extensive thin-bedded sandstones and fewer erosional features, and was deposited in the inner shelf. The Sandstone-Dominated Facies consists of thin to medium, graded and hummocky cross-stratified sandstone beds deposited in a more distal shelf setting, but above storm wave base.
The facies model and set of proximality trends developed for member 2 deposits may be applicable to other storm-influenced fine-grained shorelines. In this model, the shallow subtidal is a zone of throughput with high-velocity, sediment-laden flows eroding deep narrow scours (gutter casts) and depositing very little sand outside of these scours. As the storm-generated flows move into deeper water they decelerate, resulting in less erosion of the sea floor and depositing thicker and more continuous sand beds (Siltstone-Dominated Facies). Further from shore, bed thickness reaches a maximum (Sandstone-Dominated Facies) and hummocky crossstratification is abundant. Even more distally, bed thickness decreases again (represented by thinly laminated siltstones of the overlying member 3). Ana ysis of sedimentary structures and paleocurrent data suggests deposition by storm currents that transported sediment nearly perpendicular to shore.
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