Stratigraphy and provenance of late Pleistocene glacial sediments in the Pontiac South quadrangle, southeastern Michigan.

摘要

The stratigraphy and provenance of late Pleistocene sediments in the Pontiac South quadrangle were studied to determine the geologic history of the Defiance moraine, and make paleogeographic reconstructions. A Quaternary geologic map of the Pontiac South quadrangle was produced at a scale of 1:24,000. The map area is located 30 km northwest of Detroit, Michigan, and lies between 42°15' and 42°37'30'' latitude; and between 83° and 83°37'30" longitude. The results show that the map area is a relict late Pleistocene glacial landscape on which is superimposed a Holocene system of streams, lakes and wetlands. The glacial landscape consists of a morainal upland, characterized by knob-and-kettle topography and a series of outwash channels, bounded on the southeast by part of an extensive lacustrine lowland extending from Lake Huron to Lake Erie. The morainal upland includes an interlobate terrain of the Huron and Saginaw lobes, and parts of the Ft. Wayne, Defiance and Birmingham Moraines. The lacustrine lowland consists of the Detroit moraine, and a series of lacustrine terraces that descend in elevation progressively eastward related to proglacial paleolakes Maumee, Arkona, Whittlesey, Warren, Wayne, Grassmere, Lundy and Rouge. The map area is underlain by Paleozoic sedimentary rocks overlain by a sequence of Quaternary sediments 10-120 m thick. Stratigraphically, the late Pleistocene section underlying the morainal upland is comprised of three till units (River Rouge, Novi and Birmingham tills) composed of massive clayey diamicton; two lithologically variable units of ice contact affinity (Farmington Hills and Walled Lake formations) composed of diamicton, alluvial sand and gravel, and rhythmite; and deposits of alluvial sand and gravel underlying outwash channels and river terraces. The late Pleistocene Lacustrine Terrace Deposits unit underlying the lacustrine lowland includes a basal lithosome of bedded clayey diamicton, overlain by sand and gravel associated with relict lakebed plain, beach ridge and deltaic landforms, and rarely rhythmite. The Holocene section includes Recent Lake and Wetland deposits comprised in part of marl, peat and muck, and Recent Alluvium associated with the River Rouge, and Huron and Clinton Rivers. The gross structure of the morainal upland is a homocline dipping gently southeast. Folding and normal faulting, found mainly in the shallow subsurface, is related to the development of knob-and-kettle topography along the crest of the morainal upland. The map area is south of the region being affected significantly by post-glacial crustal rebound and although is not thought to be seismically active, seismites have been identified at several locations. Although some subsurface sediments may be Nissourian or older in age, the interlobate part of the morainal upland was probably formed during the early Port Bruce phase of late Wisconsinan time about 14,800 yr BP by the Saginaw and Huron lobes of the Laurentide Ice Sheet. The Ft. Wayne-Defiance and Birmingham moraines were deposited by the Huron lobe later during the Port Bruce phase between about 14,300 and 13,800 yr BP. The lacustrine lowland was formed by a series of proglacial lakes during the Port Bruce and Port Huron phases of late Wisconsinan time between about 14,300 and 11,900 yr BP. The Recent Lake and Wetland unit began forming during Port Bruce time, based on vertebrate fossils and 14C dates as old as 12,800 yr BP, and probably spans all of Holocene time. The Recent Alluvium unit, associated mainly with the modern drainage of the River Rouge may, in large part, be no older than about 4,500 yr.