Glacial stagnation-zone retreat in New England: Bedrock control

Abstract

Glaciofluvial and glaciolacustrine sediments in New England have been grouped and mapped in time-stratigraphic units called out wash sequences (also morphologic sequences and morphosequences). This sequence concept is, in part, a response model complementary to a process model known as stagnation-zone retreat, both originally proposed in 1941. It is suggested that the operating mechanism for this apparently discontinuous type of marginal retreat, whereby the active continental glacier periodically sheds a fringe of stagnant ice, is by downwasting to a critical thickness below which valley ice tongues can no longer support ice movement. At this point the marginal shear zone shifts up glacier, and a new outwash gradient is developed, creating a new outwash sequence. This process is dependent on suitable bedrock topography, which accounts for its prevalence in New England. This mechanism also helps to account for the general absence of ice-contact stratified drift on higher topography and for the observation that outwash sequences are generally not clearly correctable between valleys. For this paper, morphosequences are termed “suites,” so that the term “sequence” can be reserved for a series of suites along a drainage pathway. Parallel suites occupy adjacent valleys and may be time-equivalent. Recognition of serial suites (sequences) and parallel suites will encourage the study of depositional systems over broader regions. A system of tributary sequences occupying a single drainage basin is a sequence net. Because of the tributary arrangement of sequences, the suites within them may be either simple or bifurcating. There are two mechanisms for developing serial suites. Stagnation-zone retreat alone generates a shingled sequence, wherein suites share a common base level but originate from relocated heads of outwash. An outlet sequence, on the other hand, is generated primarily by sequential relocations of base-level thresholds. Outlet sequences also commonly have displaced heads of outwash.