ABSTRACT: Franciscan Ichnofossil Distribution: Does the Disturbance-Diversity Hypothesis Account for Assemblage-Level Patterns?

W. Miller, III

Disturbance regime is a neglected concept in studies of deep-water marine ichnofaunas. Disturbances are departures from normal environmental variations and differ in their origin, spatial coverage, intensity, frequency, and duration. These events range from environmental fluctuations with insignificant impact, disasters from which recovery is possible, to catastrophes completely eradicating local ecosystems. Nature of disturbances and associated process rates influencing the development of benthic communities (consisting of potential tracemakers) define a particular disturbance regime.

Surfaces of submarine fans consist of mosaics of varied disturbance regimes influencing distribution and diversity of benthic organisms. Large-scale disturbances such as slumps and turbidity currents can be recognized in sedimentary sequences by assessing bedding geometry, textures, and physical structures; small-scale disturbances (e.g., burrowing activity of motile infauna) could be preserved as more localized structures within bedding units. Thus near-continent deep-water deposits are expected to contain tangible evidence of both major and minor disturbance events, punctuating the longer intervals of normal background conditions.

Fine-grained turbidites in the late Mesozoic Franciscan Complex at Point Saint George (facies D) and Trinidad Harbor (facies C) contain rich ichnofaunas dominated by fodinichnia and graphoglyptids assignable to the Nereites ichnofacies. Coarse-grained turbidites (facies B) in the same areas contain few ichnofossils (mostly pascichnia). Similar interstratal variations in richness and ethologic groups have been noted in other flysch sequences. Although different preservation processes are at work here, impact of different disturbance regimes on the original deep-sea ecosystems also could be reflected in these patterns.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990