6.3 SEDIMENTARY MODELS
223
9
~
Boundary of Pleistocene "basement"
. . . . . .
Landward limit of delta front sands
(now buried beneath present deltaic plain )
.... 9 .............. Landward limit of marine sands
(reworked delta front sands)
Fig. 6.33. Map of the modern Mississippi delta showing distribution of major sand facies. The present active
delta lobe is prograding to the southeast, causing a local regression. In the northeast, the sea transgresses over
the subsiding earlier Chandeleur delta lobe. Marine processes rework the subsiding delta lobe depositing the
arcuate sands of the Chandeleur barrier islands. In sequence stratigraphic terminology, "upper low stand"
and "transgressive" system tracts form synchronously, but without any change in sea level.
more, the Mississippi builds into a sheltered marine embayment of low tidal range. Thus
the marine processes that redistribute the alluvial sediments of most deltas are largely
absent from the Mississippi. Only about 25 % of the modern Mississippi's load is sand,
the rest is silt and clay. This means that a very small amount of each delta lobe is sand.
Almost all the sand load is deposited at the mouths of the distributary channels in what
are called "bar-finger sands." As the distributary extends seaward, the bar-finger sands
take on a linear geometry. Switching of the delta means that these bar-finger sands generate an overall pattern of radiating shoe-strings, analogous to the fingers of a hand.
In the older lobes of the Mississippi the main locus of sand deposition was the seaward
edge of the delta platform, where it is possible to define an arcuate belt of delta-front
sheet sands which were deposited at the mouths of distributary channels (Fig. 6.35). The
delta-front sheet sands pass shoreward into, and are overlain by, silts, clays, and peats
deposited in lev6e, interdistributary, and swamp environments.
Seaward progradation of the delta has thus generated an upward-coarsening sequence ranging from marine clays of the prodelta up into the delta-front sheet sands.
223
9
~
Boundary of Pleistocene "basement"
. . . . . .
Landward limit of delta front sands
(now buried beneath present deltaic plain )
.... 9 .............. Landward limit of marine sands
(reworked delta front sands)
Fig. 6.33. Map of the modern Mississippi delta showing distribution of major sand facies. The present active
delta lobe is prograding to the southeast, causing a local regression. In the northeast, the sea transgresses over
the subsiding earlier Chandeleur delta lobe. Marine processes rework the subsiding delta lobe depositing the
arcuate sands of the Chandeleur barrier islands. In sequence stratigraphic terminology, "upper low stand"
and "transgressive" system tracts form synchronously, but without any change in sea level.
more, the Mississippi builds into a sheltered marine embayment of low tidal range. Thus
the marine processes that redistribute the alluvial sediments of most deltas are largely
absent from the Mississippi. Only about 25 % of the modern Mississippi's load is sand,
the rest is silt and clay. This means that a very small amount of each delta lobe is sand.
Almost all the sand load is deposited at the mouths of the distributary channels in what
are called "bar-finger sands." As the distributary extends seaward, the bar-finger sands
take on a linear geometry. Switching of the delta means that these bar-finger sands generate an overall pattern of radiating shoe-strings, analogous to the fingers of a hand.
In the older lobes of the Mississippi the main locus of sand deposition was the seaward
edge of the delta platform, where it is possible to define an arcuate belt of delta-front
sheet sands which were deposited at the mouths of distributary channels (Fig. 6.35). The
delta-front sheet sands pass shoreward into, and are overlain by, silts, clays, and peats
deposited in lev6e, interdistributary, and swamp environments.
Seaward progradation of the delta has thus generated an upward-coarsening sequence ranging from marine clays of the prodelta up into the delta-front sheet sands.