TUNNEL VALLEY MORPHOLOGY AND SEDIMENTARY CHARACTERISTICS: CONSTRAINTS ON DEVELOPMENT OF HYPOTHESES CONCERNING SUBGLACIAL GEOMORPHIC PROCESSES

Tunnel valleys (TVs) are present in many glacial geologic records, but the origins of TVs are not well understood. However, fresh approaches to geologic mapping of glaciated landscapes, lakes, and sea floors provide data for development and testing of relevant hypotheses on subglacial processes. Though there is wide variability to the forms and stratigraphy of deglaciated beds of late Pleistocene continental ice sheets, suites of morphologic associations include: variably anastamozed patterns of TV networks; their overlapping, incised, and successive relationships to drumlin fields; variably overdeepened areas within TVs along longitudinal profiles; and relations of TVs to kettle patterns, to overdeepened basins upflow, and to hypothesized ice sheet margins and proglacial depositional systems downflow. The sedimentary characteristics of TVs include: basal erosional relations to underlying substrates, whether composed of pre-Quaternary bedrock or older Quaternary sediments; the thickness and nature of sedimentary fills within TVs; and relations of bedding to basal and internal disconformities. Sedimentary fills include: bedded diamicts intercalated with better sorted sand and gravel aquifers (including common friable and reactive shale clasts of local provenance); as well as sedimentary dikes, soft sediment deformation, and evidence for a wide-ranging portion of glacial ice that was transiently included in fills and variably segregated from sediment (e.g. from interstitial ice to kettle-forming blocks) prior to meltout. Flights of inset terraces at the tops of TVs include both diamicton and better sorted sand and gravel. Within TV troughs, postglacial fluvial, lacustrine, marine, and organic sediments range from minor to extensive. The large areas of consistent morphologic patterns, and relations of associated stratigraphy to geochronologic data indicate that extensive TV networks and associated stratigraphy were formed during latest Pleistocene ice sheet readvances (e.g. <12 ka BP ¹⁴C years). The size range and other characteristics of TVs are not consistent with the relatively steady conditions assumed in prevailing subglacial process models, thus motivating consideration of models of ice sheet advance and TV formation linked to transient large-magnitude meltwater routing.