Summary

The two bayhead deltas forming in Atchafalaya Bay along Louisiana's central coast represent an embryonic developmental stage of the next major delta lobe of the Mississippi River. Major lobes evolve through a series of stages lacustrine deltas to bayhead deltas to shelf deltas. The bayhead Atchafalaya-Wax Lake delta started in the early 1950s after centuries of filling the Atchafalaya Basin with lacustrine deltas and swamp deposits. The abnormally high flood of 1973 forced these bayhead deltas from a subaqueous to a subaerial phase of delta evolution. Flow down the Atchafalaya River is regulated to ~ 30% of the Mississippi River discharge at the Old River cutoff plus the contribution made by the Red River.

Since 1973 new land in both bayhead deltas has dramatically expanded. Both mapping of air photos/satellite images and output from terrain models based cross-bay survey data (bathymetric-topographic) indicate that the bayhead stage of development is rapidly progressing toward the final shelf delta stage. Using 1994 data, both deltas comprise about 153 km² (59 mi²) above a -0.6 m datum. The Atchafalaya delta accounts for 101.5 km² (39 mi²) while the Wax Lake delta has an area of 51.1 km² (19.7 mi). Predictions are that by the year 2000 the Atchafalaya delta will have expanded to 115.0 km² (44.4 mi²) and the Wax Lake delta will have grown to 84.2 km² (32.5 mi²), a total of over 199.2 km² (76.9 mi²) of new land in Atchafalaya Bay (nearly 43% of the entire bay). Extensive coring of both the Atchafalaya and Wax Lake deltas indicate that both deltas are sand-rich and have only thin prodelta-distal bar facies. Sand thicknesses, as percentages of total delta thicknesses, range from ~ 35% in the distal delta to over 86% in proximal areas. An average of about 65-70% is typical of both deltas.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah