INVESTIGATING FAULTED TERRACE DEPOSITS AT THE SMITHSONIAN NATIONAL ZOO, WASHINGTON, D.C

The damage to Washington, D.C. and surrounding areas by the 2011 Mineral, Va. earthquake exemplifies the potential damage that even moderate earthquakes in the eastern U.S. can cause, and highlights the potential importance of the paleoseismic record for hazard assessments. One such record is the earthquake history of the Adams Mill fault on the SE side of the Smithsonian National Zoo, where part of a 92-year-old roadcut exposure preserved in an enclosure shows a reverse fault that emplaced crystalline bedrock of the Cambrian Laurel Fm. over fluvial deposits of unconfirmed age.

We used ground penetrating radar to locate the fault ~10 m west of the original fault exposure and dug a pit to expose the fault in three dimensions. Multiple measurements of the fault have an average strike and dip of N24°W, 53°SW in the enclosure and N24°W, 52°SW in the pit. Foliation dips 46° to 79° NE in nearby outcrops of phyllitic metagraywacke and metapelite, and has an average dip of 59°SW in brecciated, saprolitized phyllite within 10 cm of the fault. This change in the dip direction of foliation within the hanging wall is consistent with rotation proximal to the fault. Slickenside striations on smooth, curviplanar surfaces were observed on brecciated phyllite near the fault. Bedding in the footwall gravel and sand dips 19°NW within the enclosure.

A second pit was dug in the terrace alluvium ~10 m north of the first pit to collect samples for ¹⁰Be depth-profile dating to bracket the timing of faulting. The pit exposed a poorly sorted, clast-supported sandy cobble gravel that consists of well-rounded quartzite and vein quartz cobbles, pebbles, and small boulders, with most cobbles being hard but some are weathered and rotten. These cobbles are interbedded with cross-bedded coarse sand and a silt layer. A sample was collected for optically stimulated luminescence (OSL) dating from the sand, and the silt was sampled for pollen analyses. The faulted gravel has been interpreted as a Cretaceous (Potomac Fm) or Pliocene to Pleistocene deposit. Cambrian-age Scolithos tubes present in some cobbles suggest a Miocene or younger age for the deposit, based on regional provenance studies. Forthcoming results from this study should provide a maximum age for faulting to determine whether the Adams Mill fault has been active within the Quaternary.