Problems in Interpretation of Clay Fabrics

Suzanne Reynolds

Several models have been developed to explain the origins of different clay fabrics as seen with the scanning electron microscope, but some of these models may be oversimplified.

One microfabric model suggests that bioturbation leads to a randomization of fabric; nonbioturbated fabrics should exhibit a preferred orientation (PO) of clay particles in the horizontal direction. However, in samples from the Los Angeles basin, California, it was discovered that bioturbated, hemipelagic mudstones had essentially the same clay fabric as nonbioturbated, turbiditic mudstones; both were highly random. The effect of bioturbation was also studied in anoxic-laminated, nonbioturbated muds which exhibited isolated burrows (Pico Formation, Rosario Group, California; Niobrara Formation, Colorado). The clay fabric inside and outside the burrows was similar; diagenesis appeared to be the controlling factor of these microfabrics.

Another common conception is that PO of clays is developed during consolidation. The only PO seen in the samples from the Los Angeles basin is of silt-sized detrital micas and diagenetic chlorite. Much of the PO which has been measured in recent sediments may be due to the PO of silt-sized micas, not clays; and PO in shales may be due to diagenetic growth of phyllosilicates under uniaxial pressure.

Another model states that pelagic settling of clays will lead to the development of PO. The nonbioturbated mudstones of the Pico Formation display random clay fabrics in both pelagic and turbiditic sediments.

These results are not meant to disprove previous clay fabric studies but instead are intended as a warning against oversimplification of the origin and significance of clay fabrics.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.