Preliminary Map Showing Quaternary Geology of the Franklin 7.5-Minute Quadrangle, Indiana

Abstract

The Franklin 7.5-minute quadrangle is situated near the maximum limit of Wisconsin Episode glacial deposits in southeastern Johnson County. Till of Wisconsin Episode age (Trafalgar Formation) dominates the surficial geology of the quadrangle. Areas of Wisconsin Episode outwash (Atherton Formation) are found in the Sugar Creek, Youngs Creek, and Hurricane Creek valleys and within former meltwater pathways not occupied by modern streams. Kame deposits (sand and gravel) are found in the northern half of the quadrangle and are part of a larger kame complex present in central Johnson County. Holocene (post-glacial) alluvium is found in the valleys of Sugar, Youngs, Nineveh, Buckhart, and Ray Creeks and tributaries. Unconsolidated sediment thickness generally increases from southwest to northeast across the quadrangle, from less than 15 ft along the southwestern margin of the quadrangle to over 200 ft thick in the far northeastern corner of the quadrangle. This transition of bedrock topography represents the buried northern end of the Knobstone Escarpment. Two significant bedrock paleovalleys exist in the quadrangle, one in the south-central part of the quadrangle which parallels the buried Knobstone Escarpment and one in the northeast corner of the quadrangle which is part of a large bedrock valley extending north into Marion County. The Quaternary sediments in the western half of the quadrangle are underlain by Mississippian Borden Group siliciclastic rocks. In the eastern half of the quadrangle, the Devonian New Albany Shale and Muscatatuck Group carbonate rocks subcrop under thick Quaternary sediment cover. The Wisconsin Episode maximum limit is located within one mile south of the southern boundary of the quadrangle. Radiocarbon ages from the adjacent Trafalgar 7.5-minute quadrangle indicate the Laurentide Ice Sheet reached its maximum extent just after 23,700 years ago. A second glacial limit, marked by the Crawfordsville Moraine in the adjacent Trafalgar 7.5-minute quadrangle, crosses the southern end of the Franklin 7.5-minute quadrangle, albeit diffuse. The Crawfordsville Moraine represents the limit of a readvance which occurred 21,700 years ago. This preliminary geologic map is an interim geologic map product that documents progress in mapping the Quaternary geology of Johnson County, Indiana. The lithologic classification of the Quaternary units of Indiana was used in assigning map units.