Microstructures in the Martinsburg Formation in east-central Pennsylvania provide strong evidence supporting Maxwell's hypothesis (1962) that the slaty cleavage formed during deformation of unconsolidated sediments by rotation of phyllosilicate grains in a loosely compacted sedimentary succession held open by abnormally high pore-fluid pressures.

The structures fall into two classes: those which indicate the unconsolidated condition of the strata at the time of slaty cleavage development, and those which indicate that pore-fluid pressures were high prior to dewatering.

The absence of dewatering phenomena across the strata where slaty cleavage parallels bedding provides a clue to the process of cleavage development. Because sedimentary intrusions and other dewatering structures occur only where phyllosilicate particles are at some angle to bedding, the inference is that dewatering across bedding occurred when clays rotated away from parallelism with the strata, providing avenues of escape for pore water. It is concluded, therefore, that rotation and alignment of phyllosilicates caused the dewatering rather than the converse, as is commonly thought.

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