PALEOLIQUEFACTION WITHIN MOUND F AT ANGEL MOUNDS: EVIDENCE FOR LATE HOLOCENE SEISMICITY IN THE MIDCONTINENTAL USA FROM AN ARCHAEOLOGICAL SITE, SOUTHWESTERN INDIANA

Angel Mounds, a prehistoric town occupied AD 1050-1450, occurs along the Ohio River near Evansville, Indiana and includes eleven earthen mounds. Two of the largest mounds (A, F) were studied in detail and, based on 14C ages, were constructed in 2-3 stages between AD 1050-1400. Mound F, excavated in 1939-42 and 1964-65, had two buried platforms. The first (“Inner Mound”) was ~1 m high and included a few buildings; thatch from one yielded a 14C age of 900 BP (~1050 AD). Other 14C ages show the Inner Mound was buried by a ~2 m high platform (“Primary Mound”) at ~750 BP (~1200 AD), and was later capped by ~3-4 m of fill (“Secondary Mound”) after 530 BP (~1400 AD).

A profile of Mound F left intact after the 1965 excavations was exhumed in 2013 and revealed two anomalous sand units. One unit, located at the mound base, is a horizontal, 2-4 cm thick bed of medium sand and is the source of a small (~0.5 cm wide by ~100 cm long) clastic dike that upwardly penetrated the overlying fine-grained mound fill by hydraulic fracturing. A second, larger sand body (~1m high by ~4m wide) is lenticular and contains many detached fragments of mound fill. This sand unit is clearly not part of the mound building fill because in many places it disrupts/truncates the mound fill, the mound surface, and other cultural features. In some places the sand appears to have vented onto the Inner Mound surface, but disturbances from the 1965 excavation confuse this relationship. Although feeder dikes were not exposed in the trenches, the two sand units are interpreted as seismogenic liquefaction features. The mound is too small to have produced such liquefaction via slumping, and staged mound construction was not rapid enough to induce a sudden loading to generate hydraulic fracturing.

These data suggest a large earthquake occurred at Angel Mounds after 1100 AD. Forthcoming OSL ages, however, will improve our understanding of the site history, including whether sand vented onto the Inner Mound surface. Large, mid-Holocene sand/gravel dikes and at least one Holocene fault, likely associated with the Wabash Valley Seismic Zone, occur within ~20-40 km of the site and demonstrate that large-scale, Holocene seismicity is common in the area. Seismicity at Mound F likely affected the occupation and mound construction at Angel Mounds and may have contributed to the post-AD 1450 site abandonment.