Geological Society of America

T285 Examining Big Questions or Paradigms with Geochronology

Wednesday, 7 November 2018
1:30 PM - 5:30 PM
Room 136-137

USING OPTICALLY STIMULATED LUMINESCENCE DATING OF BEACH RIDGES TO MEASURE BARRIER PROGRADATION OF GRAND ISLE, LA

Paper number: T285-10
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Julie A. Torres Presenting Author
University of New Orleans, New Orleans, LA
Mark A. Kulp
University of New Orleans, New Orleans, LA
Ioannis Georgiou
Univ. of New Orleans, New Orleans, LA
Duncan M. FitzGerald
Boston University, Boston, MA
Kenneth Lepper
North Dakota State University, Fargo, ND

Abstract

The fundamental processes driving long-term (millennial) and short-term (seasonal to annual) morphological changes within barrier island systems are well developed and understood. However, the details necessary to understand barrier island evolution at decadal to centennial time scales are still lacking for most barrier island systems, including those of Louisiana. One method for resolving barrier island growth through time uses optically stimulated luminescence (OSL) dating of beach ridge sediments. In Louisiana, while this age-dating method has been used to document deltaic progradation, it has not been used to evaluate the regional barrier island systems. Here, we use OSL to document the timing of beach ridge formation, rates of sediment transport, and the fundamental geologic framework of Grand Isle, the only inhabited barrier island in Louisiana.

Grand Isle is composed of beach ridges organized in distinct, unconformable sets. Ridges increase in spacing from west to east, likely due to decreasing accommodation space that existed at the time of deposition. Grand Isle began forming by accretion of beach ridges approximately 720 years ago. Progradation took place to the northeast until approximately 505 years ago when deposition ceased and the seaward portion of the ridges were eroded, followed by renewed deposition, albeit in a slightly more eastward direction. The central ridges formed between 370±30 and 170±10 years ago at an average rate of 13 yr/ridge or progradation rate of 14 m/yr. The sediment source for Grand Isle is a rapidly eroding older beach ridge plain of the adjacent Caminada headland. We calculate the Grand Isle barrier lithosome volume to be ~9.26x10⁷ m³, which requires an average longshore transport rate of 128,625 m³/yr available for deposition during the period of barrier formation. This relatively large volume of sediment can be reconciled by the high rates of erosion and eastward transport of Caminada beach ridge sand.

It is likely that the thickness of the Grand Isle barrier lithosome (~10 m), location of deeper thick compacted sand, and steady supply of sediment explains the island stability relative to other Louisiana barrier systems and despite rates of relative sea-level rise on the order of 1 cm/yr.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318605

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