Grand Isle is a barrier island composed entirely of sediments, most of which are terrigenous. Surface sediments of the island are primarily fine- to very fine-grained sand, some silt and with some clay. The sand fraction is basically quartz but contains 18.7 percent feldspar. Size characteristics of the sediments parallel geomorphic features such as the beach, dune, back-island ridges, and inter-ridge areas. Silt and clay fractions of the sediment generally increase behind the dune complex of the island. This finer fraction is interpreted as being secondarily introduced from the bay-sound environment by influxes of high water and by aeolian transport.
Sorting coefficients closely parallel geomorphic features of the island. Median grain-size diameters for the island range downward from 0.166 mm. Two factors indicate that beach material moves into the Grand Isle area via littoral currents from the southwest: (1) all grain-size characteristics of beach material increase to the southwest; and (2) sediment buildup occurs on the southwest sides of the jetty and groins.
Organic content of the sediment is lowest in environments of high physical energy and high oxidation, and these generally parallel geomorphic features.
Approximately 25 sets of relict beach and dune ridges are present behind the active beach and dunes. The orientation of these low relief back-island features indicates that the island has grown at its distal ends and seaward by accretion.
The Holocene-Pleistocene contact is at approximately 400 ft below the surface at Grand Isle. Four sands are present in the Holocene column, these are interlayered with clay and silty clay. The upper two sands have a linear configuration and are as much as 43 ft thick. The deepest sand is more than 120 ft thick.
Subsurface strata studied at Grand Isle are related to the Holocene marine transgression and the Mississippi River deltaic progradation. Holocene sediments of Grand Isle are 20,000 yrs old or less. The “D” Sand (at about 200 ft below sea level) was deposited during the Holocene marine transgression. The “C” Sand was deposited as a nearshore Gulf deposit. The “A” and “B” Sands were deposited as barrier islands or barrier island spits. The two uppermost sand bodies are less than 1000 yrs old. The “A” and “B” Sands were deposited by northeasterly moving littoral currents which flow contrary to the prevailing longshore currents. The oldest, continuously subaerial, segment of the present island is interpreted to be less than 700 yrs old. The island probably originated as a barrier spit which extended northeastward from the Lafourche delta front. The barrier spit waspossibly separated from the mainland by a tidal current breakthrough or surge of storm waters. Since separation from the mainland, the island hasgrown to each end, and seaward, by ridge accretion.