Abstract

Detailed petrologic and stratigraphic principles can be combined to solve a river engineering problem of a bank failure and recession in fine grained deltaic sediments A study was made based on ten deep borings, some comprising more than 200 feet of oriented and undisturbed samples, drilled in the caved and noncaved bank of a bend of the Mississippi River west of Fort Jackson, Louisiana. Parameters examined include texture, composition of the sand residue, color, water content, plasticity, unconfined compression strength, microfauna, and primary sedimentary structures.

The resulting lithologic and microfaunal sequence establishes the detailed stratigraphic sequence and records the depositional processes once active in this part of the delta. Examination of the 200-plus feet of stratigraphic section in cores of the noncaved bank reveals several incursions of fluviomarine sediments prograding seaward over deltaic marine sediments. The vertical sequence from the base to the top of the noncaved borings records the progressive shoaling and increase in rate of deposition accompanying the change from the shelf to deltaic top-set sedimentation.

Borings in the caved and noncaved portions of the bank can be correlated precisely making it possible to interpret the causes of the bank recession. The bend has a tendency to grow by normal processes of river erosion active in the entire bend area; this erosion is supplemented by deep failure. Oversteepening of the slope eventually results in failure by shear. This shearing results in a wedge type of failure (nonrotational) involving lateral translation of sediment blocks. Correlations show that the lateral displacement during failure has been occurring above- a well-defined, horizontally bedded shell-hash layer at an elevation of approximately -195 to -205 feet.