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GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 45 (1995), Pages 644-646

Abstract: The Eastern Chenier Plain: An Update on Downdrift Coastal Progradation Associated with the Building of a New Holocene Delta Lobe in the Mississippi Delta Complex

Harry H. Roberts, Oscar K. Huh


Howe et al. (1935) and Russell and Howe (1935) interpreted the origin of the Chenier Plain of southwestern Louisiana as resulting from alternations between deposition and wave erosion caused by shifting Mississippi River discharge (delta switching). It was not until H. N. Fisk and R. J. LeBlanc cored the Chenier Plain that sediment body geometries became known (R. J. LeBlanc, personal communication). From this work, the first maps of Chenier Plain sedimentary thickness were constructed (Fisk, 1955). Later, more complete work by Gould and McFarlan (1959) defined the sedimentary architecture and temporal history of the Chenier Plain. Their work verified earlier interpretations by Howe et al. (1935) and Russell and Howe (1935) that suggested Chenier Plain development was intricately linked to the availability of suspended sediment from a nearby Mississippi River source. Their work identified the chenier ridges as transgressive sediment bodies, whereas muds between ridges represented regressive deposits. Recently, Penland and Suter (1989) suggested a new chronology for Chenier Plain construction incorporating new data indicating sea level was 5-6 m lower as recently as 3,000 yrs BP. They suggest that the most landward cheniers developed after the rapid rise to the near-present sea level. However, the formative processes interpreted by earlier workers have remained unchallenged.

Prior to the middle of the century, the Chenier Plain coast had been in retreat for hundreds of years. However, the Atchafalaya diversion changed this picture, and the eastern Chenier Plain coast began receiving fine-grained sediments of sufficient quantities in the late 1940's so that mudflats episodically and temporarily appeared in front of previously retreating beaches (Fisk, 1952). Recent research by Kemp (1986), Roberts et al. (1989), and Huh et al. (1991) has monitored this new era of Chenier Plain growth as a downdrift product of westward sediment transport associated with rapid delta lobe development in Atchafalaya Bay. Figure 1 illustrates the dramatic changes that were just beginning to take place in the late 1940s-early 1950s. This change from erosion and coastal retreat to deposition along the eastern Chenier Plain shoreline was the result of filling of the Atchafalaya Basin to near capacity with lacustrine deltas and swamp deposits. Filling of the basin allowed sediment to be passed through to Atchafalaya Bay and downdrift coasts.

By the early 1950s, the basin-filling process was nearly complete so that significant suspended sediments reached the coast to start a new delta-building event in Atchafalaya Bay and initiate progradation of the eastern Chenier Plain where coastal erosion (3-7m/yr) had been active for many centuries. Fine-grained sediments flushed through Atchafalaya Bay as a mudstream, flowing mostly westward, bringing volumes of fine sediment to the inshore zone. Onshore transport of fluid mud deposited on the inner shelf has resulted in mudflats that have prograded the eastern 20 km of this coast. Progradation rates during the late 1980s and early 1990s averaged 50m/yr in the most actively accreting coastal sectors. Recent research (Kemp, 1986; Roberts et al., 1989; and Huh et al., 1991) has confirmed that both winter cold front passages (20-30/year) and occasional tropical storms result in water level set-up along the coast and shore-normal transport of fluid mud from the nearshore shelf onto the shoreface where it is stranded. In the winter cold front case, water level set-down plus dry air and cloud-free post-frontal conditions promote rapid water loss in stranded muds resulting in dessication. Sheets of fluid mud composed of clay-sized particles are mudcracked into resistant polygonal clasts (centimeters in diameter) which armor the coast against erosion. Cores through new accretion units along the coast reflect a vertical accretion related to repeated deposition of 2-10 cm thick units of fluid mud over a thin layer of silt to sand. Each one of these sedimentary couplets is interpreted as a depositional response to a cold front passage or tropical storm. Time-series remote sensing data suggest that mudflat progradation is shifting westward with time.

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FIGURE 1FIGURE 1. [Grey Scale] continued from page 11. A time-series of photographs of the eastern Chenier Plain coast illustrating rapid coastal progradation between 1987 to 1993. In this sector of the coast the average progradation rate has been about 50 m/yr during this time period. The "flood" of fine-grained sediments supplied by the Atchafalaya River coastal mudstream initiates and feeds these dramatic coastal changes. (a) NOAA-12 satellite AVHRR image of coastal turbidity. Turbid waters from the Atchafalaya are advected westward to the prograding Chenier Plain coast. (b) Aerial photograph from January 27, 1987 illustrates a sector of the coastline at the initial stages of rapid progradation. (c) Aerial photograph from January 22, 1988 illustrates that a mud arc is starting to form along the coast. (d) Aerial photograph from April 1993 reveals a 0.5 km zone of new land which is being rapidly colonized by coastal plants on the landward side and is fronted by semi-consolidated mudflats on the seaward side.

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Fisk, H. N., 1952, Geologic investigation of the Atchafalaya Basin and the problem of Mississippi River diversion: U. S. Army Corp of Engineers, Mississippi River Commission, Vicksburg, Ms., 145 p.

Fisk, H. N., 1955, Sand facies of recent Mississippi delta deposits: Proceedings 4th World Petroleum Congress, Rome, Italy, Section 1-C, p. 377-398. Howe, H. V, R. J. Russell, and J. H. McGuirt, 1935, Physiography of coastal southwest Louisiana: Department of Conservation, Louisiana Geological Survey, Bulletin 6, p. 1-68.

Huh, O. K., H. H. Roberts, L. J. Rouse, and D. A. Rickman, 1991, Fine grain sediment transport and deposition in the Atchafalaya and Chenier Plain sedimentary system: Proceedings Coastal Sediments '91, Seattle, Washington, p. 817-830.

Kemp, G. P., 1986, Mud deposition at the shoreface: Wave and sediment dynamics on the Chenier Plain of Louisiana: PhD dissertation, Marine Sciences Department, Louisiana State University, 146 p.

Penland, S., and J. R. Suter, 1989, The geomorphology of the Mississippi River Chenier Plain: Marine Geology, v. 90, p. 231-258.

Roberts, H. H., O. K. Huh, S. A. Hsu, L. J. Rouse, Jr., and D. A. Rickman, 1989, Winter storm impacts on the Chenier Plain coast of southwestern Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 39, p. 515-522.

Russell, R. J., and H. V. Howe, 1935, Cheniers of Southwestern Louisiana: Geographic Review, v. 25, p. 449-461.

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Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803

This research work on the Chenier Plain has been supported by a grant from NASA (NAG13-4) Land Processes Branch, Space Science and Applications Division. Remote Sensing, sedimentological, and physical process research on this new chapter of progradation of the western Louisiana coast continues under NASA support.

Copyright © 1999 by The Gulf Coast Association of Geological Societies