Transposition--A Somewhat Neglected Mechanism of Sedimentary Emplacement: ABSTRACT

Abstract

Transposition is the mechanism of postdepositional or syndepositional intrastratal sediment movement caused either by gravitationally unstable stratification or by liquefaction. Movement may be upward, downward, or lateral. Transposition structures include sand dikes, sand sills, and sand plugs (formed by injection), sand and mud volcanoes and water-expulsion pipes (formed by ejection), and some convolute stratification and load structures. Slump structures or structures formed by fluid-drag action on the sediment surface are not included. All of the above structures previously have been described as separate phenomena. They are, however, genetically interrelated; they commonly are found together and form a spectrum of secondary inorganic sedimentary structures. Rapidly deposited, alternate water-rich muds and fine sands in alluvial, lacustrine, deltaic, and turbidite sequences seem to be most suitable for their occurrence. The similarity of some transposition structures to common primary sedimentary structures is striking. However, they may easily be confused. For example, polygonal patterns of sand dikes may resemble sand-filled mudcracks; cross-stratification caused by ejection may resemble current-formed cross-stratification; some sand dikes may resemble vertically walled channels; downward collapse structures may resemble wave-excavated scoops. Well-exposed examples of transposition structures are present in the Mississippian Horton Group of Nova Scotia and in the late Precambrian Cabot Group of eastern Newfoundland. End_of_Article - Last_Page 783------------