Using the preferred orientation of chlorite, we measured vertical compaction in 53 samples from the Devonian Catskill Delta in central New York State. The marine part of this delta contains three levels based on different amounts of vertical compaction. These levels correspond roughly to (1) black shales at the base, (2) prodelta turbidites, and (3) a cap of shallow-water sediments including abundant storm-washed shell hashes deposited within the wave base. The cap is normally compacted, whereas the lower two levels are undercompacted. Tectonic (cross-fold) joints that propagated during the late Paleozoic Alleghanian orogeny are restricted to the deeper, undercompacted levels of the Catskill Delta, whereas unloading (cross-fold) joints pervade the cap. The correlation between undercompaction and the distribution of tectonic joints indicates that abnormal fluid pressure was a key mechanism during the propagation of these joints.

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