LATE PLEISTOCENE CLAYS, AVON, MAINE: GLACIOMARINE OR GLACIOLACUSTRINE?

A small exposure of grey-brown silty clay can be found at the bottom of a shallow abandoned meander of the Sandy River in Avon, Maine. This clay is not shown on the 1:24000 surficial geologic map of the Phillips Quadrangle where the exposure is located. The oldest sediment found on the surface in this quadrangle is considered Late Pleistocene coarse glaciofluvial or glaciolacustrine sediment (Syverson and Greve, 2003). Two potential interpretations for the origin of the clay are: 1) glaciolacustrine, likely from ice damming, or 2) glaciomarine from a shallow estuary environment, a finger of the post-glacial De Geer Sea.

This investigation will compare samples from several representative deposits: a glaciomarine sample from an exposure of Presumpscot Formation in Norridgewock, a glaciolacustrine sample near Rangeley, a modern marine sample from mudflats in Wiscasset (as a reference for identifying foraminifera), and a sample from Avon as the unknown. Samples are being analyzed for the presence of foraminifera, and for salinity via XRF analysis of major salt water ions, primarily Ca, Mg, Na, K, and possibly Cl, SO₄^2-, and B. The samples will also be analyzed for grain size to help define the type of sediment each sample represents. This will allow for depositional environment comparison of the unknown Avon clay to the known samples.

If the Avon clay was glaciomarine, tests would find high concentrations of common marine salts and forams would be present in a sample. This would expand the maximum marine limit roughly 10 km upstream in the Sandy River valley from where it is currently mapped in the Strong quadrangle, to the southeast of Phillips. Alternatively, the Avon sample could be glaciolacustrine, with a low ion concentration, no presence of forams, and defined stratification. There are several glaciolacustrine deposits within the area.