WATER-ESCAPE STRUCTURES IN SILICICLASTIC MUDSTONES

Water-escape structures are well known from coarse-grained sediments. Their formation is thought to be the result of local liquefaction triggered by an event, often an earthquake. Water-escape structures in fine-grained sediments, however, have not been recognized. The following contribution will use the Mississippian ‘False’ Bakken in North Dakota and Montana to outline water-escape structures in mudstones to help recognize similar features worldwide.

Water-escape structures in fine-grained siliciclastic rocks occur preferentially in slumped mudstones and consist of up to 0.5 mm wide dark mudstone-filled linear features (dikes) that are well-defined but irregular in width and bounded by silt grains on the bottom. These linear features cross-cut bedding, and commonly penetrate the rock at low angles to bedding; the material in the dikes is darker in color than that of the surrounding rock. When these structures show a vertical orientation, they are typically wider and less well defined and are strongly irregular to curved in cross-section. Water-escape structures may split in an upwards direction but generally consist of a single tube-like linear feature.

The water-escape structures in the ‘False’ Bakken are interpreted as conduits of fluid and sediment that escaped from water-rich material during synsedimentary folding. The synsedimentary deformation produced additional weight on unconsolidated muds, and forced a water-sediment mixture into the direction of the slump. This water-sediment mixture preferentially moved fine-grained sediment. Coarse-grained silt was pushed to the bottom. Water-escape conduits below a synsedimentary fold commonly maintain a low angle to bedding. However, when slumping ended, the linear sediment-water conduits were less constrained, which resulted in upward penetration through overlying sediment in a more vertical direction. These linear mudstone-filled dikes are darker than the matrix, vary in thickness, and the siltstone accumulations on only one side characterize them as fluid conduits. These features observed in the ‘False’ Bakken as well as the Alum Shale (Cambrian, Scandinavia), are likely to have formed in many mudstone successions worldwide especially when they were subjected to high energy events.