Polymodal Grain-size Modes in Long Island Sands, Silts, and Weathered Bedrock

The purpose of this study is to determine if grain size modes already present in weathered bedrock sediments can be retained and inherited in derived, or processed sediments. To test this theory, weathered bedrock sediment, as well as Cretaceous aged sands and silts, Glacial sands and silts, Loess-like silt sediment, and glacially transported granitic gneisses were collected and analyzed for grain size.;This study is based on the implicit assumption that grain size modes are indicative only of process, and are not reflective of modes present in the parent material of the sediment. A distinct pattern of grain size modes at about 20im, and 400im in most if not all pebbly-loess samples from various sites in Suffolk County and modes of 20im, 80 im, and 400im from till and outwash sands on the Stony Brook campus (Zhong, 2002, Kundic, 2005, A. Olaofe, personal communication, 2012 and T. Clare, personal communication 2013). This lead to investigating if these modes are found in other sediment samples. Glacial samples were collected from Centereach, Long Island, New York, and also were found to display the same pattern of modes seen at Stony Brook. Investigations of Cretaceous sediments from Caumsett State Park also display the same pattern of modes at about 20im, 80 im, and 400im. Analysis of weathered bedrock from Brookhaven, and mechanically crushed rocks, typical of those believed to be the parent rock of the glacial and loessic sediments was also conducted, showing the same mode pattern. Additionally, petrographic analysis of a sample of bedrock, similar to the weathered bedrock in our study, showed silt sized grains already present in the bedrock.;Analysis of possible machine bias towards certain modes was also conducted. All of the samples were analyzed with a Malvern Mastersizer 2000. It appears that the modes seen in sedimentary samples of loess, glacial sands, glacial till, cretaceous sands, weathered bedrock, and manufactured bedrock all show polymodial pattern of modes at about 10.00im-20.00im, 100.00im-200.00im, and 400.00im-500.00im may be inherent in the parent bedrock of the sedimentary material. Furthermore, analysis with a Sediment Grain Size Analyzer program shows that the high rate of 80 micron modes seen in the Mastersizer results are possibly due to a combination of modes seen between 90 microns and 100 microns, regardless of the type sediment.;In conclusion, we can state that grain size modes found in weathered bedrock on Long Island, New York, can be retained as common modes in derived sediments, such as Cretaceous materials, and glacial materials, such that process is not the only determination of the sediment mode characteristics. In addition, we can also state that silt sized particles can in fact be created without the aid of glacial grinding, as seen in the thin section analysis.