THE GEOLOGICAL HISTORY OF THE 

 MARSHES OF COASTAL LOUISIANA 



Rufus J. LeBlanc 



Owner, Rufe LeBlanc School of Clastic Sediments 



3751 Underwood Street 



Houston, TX 77025 



ABSTRACT 



The marshes of coastal southeast Louisiana occur over an area of about 6,600 mi 2 , and they 

 constitute about 60% of the Mississippi River Deltaic Plain complex, the newest land added to 

 the Gulf Coastal Region during the past few thousand years. Any local, State, or Federal program 

 concerned with the management of these deltaic plain marshes must be based upon a firm 

 understanding of the natural geological processes which created them. 



Part I of this paper consists of a brief description of the deltaic plain complex. Part II is con- 

 cerned with the description of the coastal bays, sounds, transgressive barrier islands, and offshore 

 shoals which are related to the delta complex. Part III discusses 51 significant papers on the delta 

 complex which have been written during the past 58 years. Part IV is a brief summary of the 

 origin and development of the coastal region of southeast Louisiana based upon the research 

 outlined above. Illustrations show how the mighty Mississippi River created about 14,000 mi 2 of 

 new land in the Gulf of Mexico, in the form of a series of deltas, during the past 7,000 years. 

 Attention is also focused on the natural processes of river diversion, delta abandonment, and 

 compaction and subsidence of abandoned delta sediments, which permitted the gulf to move inland 

 and reclaim about 7,000 mi 2 of this new land. 



In spite of the massive research effort at Louisiana State University over a period of 58 years, 

 there are still many citizens of coastal Louisiana who do not understand the basic principles of 

 natural deltaic sedimentation and the concurrent loss of land that had been previously created by 

 the deltas. Over 80% of the shorelines of coastal Louisiana are and should be under natural 

 transgressive conditions today. Wherever humans choose to live upon the large deltas of the world 

 they must be prepared to suffer the inevitable consequences of natural river diversions, delta 

 abandonment, compaction and subsidence, and the great loss of land as the seas transgress over 

 large portions of the deltas. 



INTRODUCTION 



The Mississippi River, one of the largest rivers of the world, drains all or part of 32 States 

 stretching from Yellowstone National Park on the west to New York on the east, and from Min- 

 nesota on the north to the Gulf of Mexico on the south. This river also drains parts of southern 

 Alberta and Saskatchewan, Canada. This 1,300,000-mi 2 drainage basin is shown on Figure 1. 



During the past 7,000 years, the sediments transported southward by this great river, under 

 natural conditions, have created thousands of square miles of new land in the Gulf of Mexico which 

 comprise the southeast part of Louisiana. The Mississippi River has changed its course, also 

 under natural conditions, several times and as a result has constructed four deltas in the region 



