Interglacial and interstadial valley environments and adjustments of floodplain levels; Geological Society of America, 1998 annual meeting

Continental environments of the last interglacial are often defined by investigations of upland (i.e. paleosols) or closed basin records. Valley environments are accorded somewhat less attention, which is not surprising given the more fragmented and obscured character of valley deposits. In the upper Mississippi River basin, deposits and paleosols of the last interglacial, as well as the interglacial preceding it and the interstadial following it, are sporadically preserved and rarely exposed in valleys not completely scoured or widened by deglacial and post-glacial erosion. Alluvial paleosol evidence from a range of occurrences suggests last interglacial fluvial depositional environments not unlike the present. In contrast, last interstadial floodplains are often characterized by organic-rich soils formed under cooler and/or more moist conditions. During the last interglacial and subsequent interstadial, the floodplain of the Mississippi Valley stabilized at or close to modern floodplain levels. Despite differences among intervening glaciations, this general correspondence of interglacial and interstadial floodplain levels suggests a common underlying factor or set of factors leading to the similar level of floodplain stabilization. Furthermore, the alluvial record of the late Wisconsin to Holocene transition in the upper Mississippi Valley is well-dated and indicates the post-glacial adjustment to interglacial floodplain levels was relatively rapid. Coupled with the occurrence of similar floodplain levels during the last, relatively brief, interstadial, these data suggest post-glacial and post-stadial adjustments to new floodplain levels occur rapidly. With the onset of renewed glaciation, interglacial and interstadial floodplains and soils in tributary valleys are buried in a time transgressive fashion in an upstream direction in response to aggradation in principal outwash valleys.