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

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