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SEM examination of clay fabric in siltstones as old as Pliocene has proved useful in distinguishing rapidly deposited muds from hemipelagic clays: turbiditic siltstones are characterized by random orientation of clay minerals and hemipelagic units show preferred (parallel) orientation of clay minerals. Random orientation of clay minerals has been recognized in Paleozoic shales, indicating that postdepositional compaction of shales does not always result in parallel alignment of clay minerals. Therefore, the possibility exists that original depositional fabric of clay minerals is retained in rocks of Paleozoic age, and that clay-fabric analysis would provide an independent tool for interpreting the depositional environment of fine-grained clastic rocks.
This study shows that original depositional fabric of clay minerals is preserved in texturally homogeneous, nonbioturbated mudstones as old as Middle Cambrian. Clay fabric may be used in determining whether the mud deposition was rapid or slow (e.g., pelagic). Rapid mud deposition is characterized by a random clay fabric; slow sedimentation is characterized by preferred alignment of clay minerals. Bioturbation and bimodal grain size within a sample (e.g., a clayey siltstone) also are associated with a disordered clay fabric, so this analysis is most confidently used with mudstones that have a homogeneous texture and show no evidence of bioturbation. Therefore, environmental interpretations based on SEM analysis of clay fabric can be used with confidence provided that the caveats of bi odal grain size, bioturbation, and diagenesis are anticipated.
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