GEOLOGIC MAPPING IN GLACIATED AREAS: USING LANDFORM SEDIMENT ASSEMBLAGES TO MAP THE DES MOINES LOBE IN IOWA

Recent STATEMAP mapping efforts in Iowa have focused on mapping the Des Moines Lobe (DML). The DML encompasses a complex suite of depositional landforms and sediment sequences related to supraglacial, subglacial and proglacial sedimentation. To map a relatively large area with such diverse glacial terrains we selected a comprehensive mapping strategy-- landform sediment assemblage (LSA) mapping at 1:100,000 scale. To date, LSA mapping concepts have been mostly applied to large valleys at the 1:24,000 scale. There are distinct advantages to moving away from a morphostratigrahic mapping scheme and adopting an LSA mapping strategy. LSA mapping units allow the mapper to generalize and map complex glacial terrains by recognizing the genetic relationship among landforms and their underlying sediment sequence. Three broad terrestrial glacial environments are recognized on the DML: subglacial, supraglacial and proglacial. Each of these environments is characterized by a unique relief pattern, stratigraphy and sedimentology. In the map area, subglacially-controlled environments are characterized by a relief pattern of lineated ridges oriented transverse to glacier flow with a thin increment of supraglacial sediments (<3 m) mantling the till plain. The supraglacial environment is characterized by hummocky terrain that forms arcuate belts of moraine complexes and undulating plains with thick increments of supraglacial sediment (>3 m). The proglacial environment is characterized by a host of relief patterns that encompass coarse-grained glaciofluvial ice-contact sediments associated with glacial plains, fans and channel deposits, and fine-grained glaciolacustrine deposits associated with lake plains. Fourteen landform sediment assemblage units have been identified in the map area utilizing digital elevation models (DEMs), high altitude aerial photography, orthophotos, topographic expression, digitized soils data, and existing and new subsurface boring information. All mapping is accomplished on screen in ArcView by geologists. LSA maps produced in a digital form allow for quick generation of derivative maps for land-use planning and hazard assessment tools.