40-2 Age Constraints for Relict Rotational Landslides, Southeastern Ohio

Session: Geomorphology (Posters)

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Landslides are common a geologic hazard of the Allegheny Plateau region of southeastern Ohio. Here these geomorphic features are often associated with claystone “redbeds” of the Upper Pennsylvanian Conemaugh and Monongahela Groups and Permian Dunkard Group. Field investigations, hillshade images, and close-interval contour maps have revealed an extensive swarm of 80+ landslides situated high on hillslopes in northwestern Athens County, Ohio. These landslides stretch for more than 16 kilometers along upper backslopes and have well-defined arcuate crowns, some of which are laterally contiguous over distances of 1200+ meters. Field observations suggest they are deep, rotational slides formed by failure within a zone of prominent claystone of the Upper Pennsylvanian, Mahoning “redbeds member” of the Glenshaw Formation. The qualitative surface roughness of the slopes disturbed by these landslides and the extensive colluvial deposits accumulated on their heads, along with the steepness of the main scarps, suggest they are ancient landforms. Two types have been recognized based on apparent relative age; Type 1) older slides with subtle features – covered scarps with diminished slopes (21-28^o), eroded blocks, internal drainage lines, and little evidence of distal reactivation, and; Type 2) younger slides with moderately defined features – partially exposed scarps with steeper slopes (34-44^o), no internal drainage lines, and distal reactivation obvious. Excavations through the accumulated colluvial deposits that bury the main heads of four of these relict landsides were conducted to access preserved remnants of pre-movement interfluve soils now buried beneath the colluvial deposits. In three of the younger landslides (Type 2), a buried soil horizon was encountered beneath the colluvial rubble at depths of about 2.1 meters. Bulk samples of this buried soil material were collected and radiocarbon-dated to provide a rough estimate of the minimum age of the initial landslide events. Dating both humin and humate fractions yielded minimum ages of 5601, 6191, and 7605 years BP.  Existence of the Type 1 slides remains speculative as no buried soil material was encountered at depths reaching 2.1 meters. However, given the presumed greater age of these slides, a thicker stack of colluvium and deeper burial is expected.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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