PROVENANCE STUDY OF BARRIER SANDS USING 40AR/39AR LASER-PROBE DATING OF SINGLE CRYSTALS DETRITAL MUSCOVITES, MERRIMACK EMBAYMENT BARRIERS, NEW ENGLAND

For the first time, 40Ar/39Ar geochronology has been used to determine the origin of the sand comprising the largest barrier system in New England. The 34-km long progradational barrier chain, located in the Merrimack Embayment north of Boston Massachusetts, has been studied for over sixty years in an attempt to answer this provenance question. Recent investigations using sedimentological and bedform data argue that large amounts of sand are entering the coastal system directly from the Merrimack River and then reworked along shore. The Merrimack River drainage basin contains several large plutons which were deeply weathered during the Tertiary. The saprolite from the plutons was stripped away during multiple episodes of Pleistocene glaciation and redeposited down flow of their origin in the form of eskers, deltas, ice-contact heads, and outwash plains. The emplacement ages of plutons follows a distinctive temporal and spatial pattern within the drainage basin, allowing it to be partitioned into four segments: upper (115-170 Ma), upper-middle(255-310 Ma), lower-middle (355-390 Ma), and lower (430-450 Ma).

Twenty kilometers of Ground-penetrating radar (GPR) and thirty-nine cores collected from Castle Neck have revealed a complex evolution involving initial progradation, erosion, channel migration, swale filling, bar welding, shoreface accretion and dune formation. A subset of seven cores from the northern portion of the barrier were used to pick a total of one-hundred muscovite grains from four prominent facies. The ages of these single crystals were determined using a CO2 laser-probe. Preliminary results yielded dates between 230-315 +/-3.0 Ma. These data indicate that the barrier sand was sourced from the upper-middle segment of the Merrimack drainage basin. The dominance of sediment supplied from this location is consistent with the upper-middle portion of the trunk river running though a large glacial deposit. This extensive glacial deposit is located southeast of the 255-310 Ma pluton complex, which is directly down gradient of Wisconsinan ice flow in this region.