Abstract

The Mississippian-Pennsylvanian Tesnus Formation in the Marathon Basin, Trans-Pecos Texas, was rapidly deposited as a large submarine fan complex in an unstable, migrating foredeep. Primary depositional fabric in siliciclastic mass-flow deposits of the Tesnus Formation was extensively modified during intense soft-sediment deformation. Liquefaction, fluidization, and clastic intrusion were the most common processes, and produced clastic injection structures possessing a remarkable array of shapes and orientation with respect to bedding. The most common intrusive bodies are dikes which were non-systematically injected into overlying host sediments at angles ranging from just greater than zero to 90 degrees. Clastic sills appear to be nearly as common as dikes and, based on their generally greater thickness, may be responsible for a greater proportion of post-depositional sediment remobilization than any other type of intrusion. Unusual clastic injection structures in the Tesnus include concordant and discordant cylindrical clastic plugs—irregularly shaped wave-like intrusions where one surface is concordant and the other discordant - and intrusions that are both concordant and discordant along different parts of the injection. With clastic dikes and sills as end members, heretofore undescribed clastic injection structures in the Tesnus Formation define a spectrum of features with geometries transitional between concordant and discordant structures.