STORM-REWORKED NODULES AND CONCRETIONS ON WALLOPS ISLAND, VIRGINIA: IMPLICATIONS FOR STORM-BED DEPOSITION AND TIME AVERAGING IN THE ROCK RECORD

As part of ongoing shoreline monitoring research at NASA’s Wallops Island Flight Facility, sediment and shell-bed samples have been collected during mapping efforts in this part of the Mid-Atlantic. Immediately following Hurricane Irene in September 2011, a few minor silty-concretions were noted on both the northern end of Wallops near the Chincoteague Inlet, as well as to the south near the former Assawoman Inlet. Overwash, barrier island rollback and associated scour were noted as a result of the storm. Apparently derived from buried marsh and bay sediments, these semi-indurated, irregular nodular concretions were incorporated into heavily winnowed shell beds deposited as storm swash deposits. Few concretions were noted in subsequent surveys. However, following Hurricane Sandy in October 2012, a number of new concretions were once again incorporated into the shell beds. These new occurrences included specimens showing evidence of exposure on the seafloor. In addition to being better indurated, some are iron-stained, and others have evidence for micro and macro borings and some evidence of encrustation. At least one is a partial infill of a bivalve shell that was cemented, reworked and possibly re-bored. Based on these observations, it appears that these concretions may have been exhumed from some depth by Irene and left exposed on the offshore sea-floor. These were then reworked intermittently and exposed to biological activity (encrustation/borings) until they were re-worked and deposited on the beach by Hurricane Sandy a little over a year later. This study is significant in that it provides insights into how complex storm bed deposits from the rock record might be generated. This is especially relevant to discussions concerned with development of intraformational conglomerates and “time-rich” and “condensed” intervals.