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Abstract

The Carboniferous Tesnus Formation in the Marathon basin of west Texas was deposited as a large submarine-fan complex in a tectonically active, migrating foredeep. Primary depositional fabrics in siliciclastic mass-flow deposits of the Tesnus Formation were extensively modified during intense soft-sediment deformation. Fluidization and clastic intrusion were common processes and produced clastic injection structures possessing a remarkable array of shapes and orientations with respect to bedding. The most commonly recognized intrusive bodies are dikes, which were nonsystematically injected into overlying host sediments at angles ranging from just greater than 0–90°. Dike orientations, corrected for tectonic deformation, show no correlation with paleoslope or later structural trends and, based on traditional decompaction methods, are interpreted to have been injected at relatively shallow burial depths (tens to hundreds of meters). Clastic sills are nearly as common as dikes and, based on their generally greater thickness, are interpreted to have been responsible for accommodating a greater proportion of postdepositional sediment remobilization than any other type of intrusion. Other, more unusual clastic injection structures in the Tesnus include concordant and discordant cylindrical clastic pipes; irregularly shaped, wavelike intrusions in which one surface is concordant and the other is discordant; and intrusions that are both concordant and discordant along different parts of the injection body. With clastic dikes and sills as end members, previously undescribed clastic injection structures in the Tesnus Formation define a spectrum of features with geometries transitional between concordance and discordance.

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