Short-Term Surface Elevation Variations at Matagorda Peninsula, Texas: A Response Produced by Shrink-and-Swell Clays?

Coastal elevation fluctuations play a critical role in modeling relative sea level change, and marsh landscape resilience and sustainability. Large, annual time scale elevation changes with maximum vertical displacement of 0.42 m were recorded in a survey period from 2007 to 2011, at East Matagorda Peninsula, Texas. These decimeter-scale movements are not reflected in the long term stratigraphic record here. This study considers the effects of shrink-and-swell clay minerals on vertical elevation changes in the region. To understand the effect of shrink-and-swell clays, sediment samples were collected from two trenches at 1 cm intervals (up to 6 cm in depth). This study employs 1) particle size distribution data to determine clay percentage, 2) X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) data to characterize clay mineralogy, and 3) the free swell index test (FSI) to illustrate the swelling potential of the sediment. Sediment clay fraction percentages range from 0.96 - 6.26. Mineral composition of the clay size fraction consists of more than 90% smectite, with trace amounts of mica, kaolinite, and quartz. FSI values range from 12 (low) to 35 (medium), with decreasing FSI from the surface to depth. Statistical analyses through the Pearson product-moment correlation coefficient show mean FSI values vary as a function of depth, due to variations in clay content. The presence and behavior of shrink-and-swell clays in the region indicate that elevation fluctuations could occur with variation in sediment/soil moisture content.