The Offshore Transport of Mud: Why It Doesn’t Happen and the Stratigraphic Implications

Robert W. Dalrymple and Donald I. Cummings
Queen's University, Kingston, ON

It is commonly assumed that mud disperses widely in the ocean, with suspension fall-out occurring over large areas. However, recent observations in a number of modern deltaic systems 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 moderately 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 wind stress and/or the Coriolis Force. As a result, most mud accumulates in a belt that is only 40-50 km wide, in water typically less than 50 m deep (turbidity-current muds excepted).

This observation has 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) Mudstones in outer shelf and/or “basinal” settings may require larger excusions of the shoreline than might be expected. C) Most mud deposits should show clinoforms that downlap onto a condensed horizon rather than parallel (“railroad-track”) reflections or correlation surfaces. The clinoforms may not be visible, but should be present between parallel reflections. Slump deposits and/or mass-flow channels may attest to their presence. D) Mudrocks deposited “rapidly” near the coast should display different facies characteristics than muds deposited very slowly in distal, deeper-water locations.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005