Intertidal Sand-Mud Transitions: Depositional Mechanisms, Mass Physical Properties, and Stratigraphic Interpretation

Burg Flemming, Monique Delafontaine, and Weidong Xu
Senckenberg Institute, Wilhelmshaven, Germany

The traditional paradigm by which intertidal sand-mud transitions are explained is progressive size sorting along energy gradients controlled by residual onshore currents in conjunction with the settling lag/scour lag mechanism. The poor sorting of such sediments is interpreted as reflecting inefficient separation of discrete size classes. This model has recently been modified by demonstrating that the deposition of fine-grained intertidal sediments is dominated by particle aggregates composed of grains <8 μm in size, their co-deposition with very fine sands and coarser silts suggesting hydraulic equivalence between these particle groups. Such sediments must therefore be viewed as being hydraulically well sorted, the proportional increase in mud content along sand-mud transitions being associated with a corresponding increase in the proportion of aggregated particles. The loose grain fabric of the aggregates results in a progressive increase in porosity and a decrease in bulk density with increasing mud content. Pure intertidal muds (<5% sand content) have dry bulk densities of 0.6 to 0.3 g/cm³, depending on the clay and fine silt content, whereas intertidal sands (<5% mud content) have bulk densities between 1.4 and 1.6 g/cm3. A consequence of the lower bulk densities (higher water contents) of muddy sediments is that the mass concentration of pure muds (mass per unit volume) is lower by factors between 2.5 and 5 than those of pure sand. This results in complex differential compaction in the course of consolidation, a factor which must be taken into account when reconstructing the depositional environment of muds in the rock record.

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