THE OFFSHORE TRANSPORT OF MUD: WHY IT DOESN'T HAPPEN AND THE STRATIGRAPHIC IMPLICATIONS

It is commonly assumed that mud disperses widely in the ocean, with suspension fall-out occurring over large areas. However, recent observations in modern deltaic systems, coupled with theoretical considerations, suggest that this is not the case for three reasons: 1) flocculation, even in areas landward of the salinity front, increases the particle settling rate; 2) high-concentration fluid mud, which is common in such settings, forms dense bodies that hug the bottom, promoting mud deposition in nearshore areas; and 3) sediment-laden river plumes do not move long distances offshore, but instead are deflected parallel to the coast by the Coriolis Force and/or wind stress. Thus, most shelf mud accumulates in a nearshore belt that is only 40-50 km wide, in water typically less than 50 m deep.

These observations have important implications for the interpretation of stratigraphic successions. A) Most mud is deposited relatively close to shore and accumulates during fair-weather, above storm wave base. B) Most mud deposits should contain clinoforms that downlap onto a condensed horizon or flooding surface; these surfaces typically show parallel “railroad-track” geometry that is mistakenly taken to indicate a draping morphology and suspension deposition. C) Mudstones located in outer shelf and/or “basinal” settings may require larger shoreline excursions than might be expected. D) Mudstone “blankets” may not indicate major sea-level rise and transgression; they might instead indicate a change to a mud-rich source. E) Many mudrocks were deposited “rapidly” near the coast and should display different facies characteristics than mudrocks deposited very slowly in distal, deeper-water locations.