Heavy mineral analysis of the Pecan Island complex, Louisiana Chenier Plain, southwest Louisiana

Abstract

This study reports the results of a heavy mineral analysis of samples from Pecan Island and associated ridges on the Louisiana Chenier Plain. The goal of the study is to provide additional insight on the origins of the sand comprising the ridges. Chenier plains are multi-membered, progradational ridge-interridge mudflat sequences (Augustinus, 1989). The various suggested sources of the sand grains include: the Mississippi River, especially the Teche sub-delta; the Mermentau and Calcasieu Rivers; and the reworking of older mudflat and chenier ridges. Several liters of sand were collected from a depth of 0.5 to 2.0 meters at locations near the crest of the ridges, on Pecan Island, Sweet Bay Ridge, and Cane Ridge. Flank samples were collected at three locations on Pecan Island. The samples were separated using sieves (very fine sand, 3.5-4.0ö, and coarse silt, 4.0-4.5ö) and the heavy minerals concentrated with sodium polytungstate. Qualitative analyses by energy dispersive X-ray analysis with grain mount on the SEM (Scanning Electron Microscope) revealed the presence of epidote, garnet, ilmenite, pyrobole, sphene, spinel, zircon, staurolite, kyanite, apatite, biotite, iron oxides, chromite, corundum, dolomite, rutile, and barite. Approximately 100 grains per sample were used to estimate percent abundance of the heavy minerals. MANOVA, an ANOVA that allows for testing with multiple dependent variables was used to identify statistically significant differences in the abundance of the seven most common minerals (epidote, garnet, ilmenite, pyrobole, sphene, spinel, and zircon) with respect to sample site, location on the ridge crest, crest versus flank position (only for Pecan Island), and among the ridges. At each level, the null hypothesis was that samples were homogeneous. Most samples were homogeneous at the first three levels of analysis. The three ridges contained significant differences in epidote, garnet, pyrobole, sphene, spinel and zircon. They have distinct assemblages but the values overlap considerably making it difficult to identify a particular ridge by its heavy minerals. The differences in mineralogy confirm the general theory of ridge origin from a variety of source, including past and present mudflats, cheniers and beach ridges. Specific sources for the heavy minerals could not be identified.