Clay fabric of fluid-mud deposits from laboratory and field observations: Potential application to the stratigraphic record

The clay fabric of fluid-mud deposits was investigated with the aim of identifying characteristic features that potentially apply to the recognition of fluid-mud deposits in the stratigraphic record. We examined both experimentally formed and natural fluid-mud deposits. The clay fabric of experimentally formed fluid-mud deposits is characterized by aggregates of clay particles in face-to-face contact with each other (herein termed ‘FF-aggregates’), with long-axis lengths of up to 20 μm. Some flocs that formed in fluid with a high initial suspended sediment concentration (ISSC), such as fluid mud, would have a high preservation potential as FF-aggregates; once they have settled to the sea floor, to distinguish them from flocs that form in fluids with a lower ISSC. Aggregates that are similar to FF-aggregates are also observed in natural fluid-mud deposits that formed in a modern tide-dominated estuary at the mouth of the Rokkaku River in Kyushu, Japan. Thus, in conjunction with formally proposed lithofacies and ichnofacies features of fluid-mud deposits, the observation of FF-aggregates is potentially useful in identifying fluid-mud deposits when examining limited volumes of muddy samples or thin- to very thin-bedded muddy deposits in the stratigraphic record.