GLACIAL GEOFRAMEWORK AND GEOCHRONOLOGY: LESSONS FROM THE EASTERN U.S. APPLICABLE FROM MAINE TO MONTANA

This report reviews progress toward compilation of a 1:500k-scale surficial geologic map of northeastern U.S. in the USGS FEDMAP Glacial Regions project in cooperation with STATEMAP partners to build a National Cooperative Geologic Mapping Program national geoframework. Map examples, notably from Massachusetts, showcase USGS geodatabase methods, a regional lithostratigraphic framework, and ways to portray geochronology of ice-margin retreat positions of the last two glacial diachronic events. The glaciated bedrock surface is analyzed in three settings. 1) Uplands/hilltops contain widespread areas of shallow bedrock containing many outcrops and lidar-enhanced lineaments related to rock lithology where till is <2 m thick. 2) In hillslope settings, rock is mostly hidden by till 3-6 m thick, but rock-controlled stoss-and-lee slopes can be discerned in lidar data. In both till settings wide-spread thick till in drumlins forms the remnants of Illinoian drift. 3) Beneath meltwater deposits in lowlands, rock surfaces with closed depressions outline buried reaches of palimpsest valleys, including possible ice-stream beds. Moraines contain constructional elements of till, other diamictons, and sorted sediments in ice-hole deposits. Mapped moraines, newly extended by lidar data, guide regional correlations of ice-margin recessional deposits. Mappable meltwater deposits, coarse- and fine-grained, are differentiated by morphology, physical contact relationships (age), and distribution of 3D sedimentary facies that compose glacial channel, glaciofluvial, glaciodeltaic, ice-hole (ice-walled plain), lake/marine-bottom units. Each local morphosequence of ice-proximal to -distal deposits is recognized as a synchronous unit, deposited from a single ice-marginal source, graded to local base level in a local depositional basin. Most map deposits accumulated in informally named glacial-lake units that also are described in a genetic classification. Map overprint patterns show the extent of unit depositional plains. Line symbols specify sourcing ice-channel deposits and the positions of glacial ice margins used to correlate large deposits among valley basins. Postglacial deposits in the top map layer can be hidden to reveal continuity of underlying glacial deposits in our 2.5D layered model.