Stratigraphic Evolution of Chandeleur Islands, Louisiana

John R. Suter, Shea Penland, S. J. Williams, J. Kindinger

Analyses of over 3,000 km of high-resolution seismic profiles, supplemented by vibracores and soil borings, illustrate the evolution of the Chandeleur Islands through transgressive processes associated with the abandonment of the St. Bernard complex of the Mississippi delta some 1,500 years ago. Historical maps show that the system has been eroding, migrating landward, and losing area for the last 100 years. At current rates, the subaerial integrity of the islands will be terminated in about 200 years. Hurricane impacts accelerate erosion and segment the islands, followed by limited recovery during fair weather periods. Relative sea level rise from both subsidence and possible eustatic factors contributes to the loss of island area.

From south to north, the geomorphology of the system varies from small, ephemeral islands and shoals to wide beaches, hummocky dune fields, flood-tide deltas, and washover fans. The last two deposits overlie and interfinger with the lagoonal muds of Chandeleur-Breton Sound, indicating landward migration--a fact confirmed by landward-dipping clinoformal reflectors in seismic sections. The numerous tidal inlets and recurved spits of the Chandeleur Islands have ubiquitous offshore equivalents in the form of buried tidal inlet scars and truncated spit platform deposits. Two major distributary zones, probably relating to different delta lobes, occur both landward and offshore of the islands in their southern and central portions. Individual channels within these zones reach 400 m in width nd 15 m in depth. Landward of the islands, distributary associated deposits in the upper few meters of sediment show more complex reflection patterns than their offshore equivalents, resulting from truncation of the latter by shoreface erosion. Adjoining the central distributary system, a zone of high-angle, seaward-dipping clinoformal reflectors reaches 6 m in thickness and extends several kilometers offshore. This deposit, evidence of former progradation, is interpreted as a truncated beach-ridge plain similar to the modern-day Cheniere Caminada. Erosional and depositional trends and the distribution of facies within the system clearly demonstrate the ongoing transgressive submergence of the Chandeleur Islands.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.