NON-SYNCHRONOUS RETREAT OF LOBES OF THE SOUTHEASTERN MARGIN OF THE LAURENTIDE ICE SHEET REVEALED BY DIGITAL DERIVATIVE MAPPING IN RHODE ISLAND

Eleven USGS 1:24,000/1:31,680 Quaternary (surficial) maps (1955-1961) were digitized, compiled into a seamless 1:24k NAD83 projection and reinterpreted using new morphosequence and stratigraphic information. Depositional environments were inferred from stratigraphic facies models reconstructed from subsurface data obtained from well logs, test boreholes and vibracores. Ice margins were determined by identifying features such as end moraines, ice-contact topography and landforms, and the collapsed “heads of outwash”. New morphosequences were then designated by retracing meltwater paths from ice margin positions. Derivative maps were created that illustrated details of late-glacial drainage patterns, ice margin locations, particularly interlobate margins, glacial lake deposits and spillways, and resulting significant aquifers.

Laurentide deglaciation in Rhode Island (RI) is characterized by the systematic retreat of two glacial lobes: the Central Rhode Island/ Eastern Connecticut (CRI) lobe and the Narragansett Basin/Buzzards Bay (NB) lobe. After the onset of deglaciation, 1) the ice sheet in southern New England may have undergone fluctuations resulting in the advance of both lobes at different times, 2) CRI lobe retreated rapidly north-northwest over uplands, thinned over low relief bedrock controlled topography and formed up-valley ice lobes, and 3) the NB lobe retreated more slowly as it occupied the deep bedrock valleys within the Narragansett structural basin. Ice recession from the terminal position probably began shortly after the ice sheet reached its maximum extent around 26 kyr BP. The ice margin retreated to the Charlestown end moraine by 21.3 kyr BP (calendar years based on calibrated ¹⁰Be dates). Although significant warming did not begin until 15 kyr BP, rapid retreat of both lobes probably began around 19.5 kyr BP. Laurentide ice is inferred to have retreated from RI by 18.5 kyr BP.