ANALYSIS OF HEAVY MINERAL DISTRIBUTION IN MODERN BARRIER ISLAND DEPOSITIONAL ENVIRONMENTS

Magnetic susceptibility (MS) is a method largely used for paleoenvironmental reconstruction and sedimentological interpretation. MS values vary based on the concentration of Fe/Mn-bearing minerals also known as heavy minerals (HM). The deposition of HM in coastal settings is controlled by processes operating on short as well as across longer-term time scales. Daily processes such as wave winnowing, longshore transport and tidal currents are often superseded and reworked by infrequent, but much larger-scale storm events. Moreover, over time, barrier island rollover and migration from sea level rise is also in play.

To better understand the processes impacting HM deposition in modern coastal environments, sediment samples were collected along transects perpendicular to the beach on Assateague and Wallops Islands (AI & WI), Virginia, USA. Samples were analyzed for MS and grain size (GS). Results show that sediments from AI were about 40% coarser and MS values were 67% lower than WI. The difference in MS and GS is likely due to the higher energy environment on AI and anthropogenic activities (beach nourishment) on WI that artificially impact the HM assemblage in that setting. To concentrate on natural depositional processes, only sediment samples from AI are reported here. Additional GS and MS analyses reveal an interesting correlation between. GS decreases from the swash zone towards back-barrier areas, whereas MS increases respectively. The lower foreshore beach environment is dominated by wave-winnowing, which suspends fine-grained HM and moves these sediments down current. Upper foreshore beach and dunes have higher MS as aeolian processes remove low-density minerals, allowing higher concentrations of fine HM particles. Washover fans had the highest MS values. These environments are impacted by storm events and transport large amounts of HM.

Understanding the dynamics of the processes that create different HM assemblages in coastal settings is critical for accurate sedimentological analysis when using the MS method. The results of this study contributed to ongoing research by providing quantitative evidence of environmental change that other traditional methods failed to identify. Therefore, this study helped to enhance the interpretation of AI geological history.