The Smithsonian National Zoo’s own geological feature
Deborah Painter (USA)
There is a fault zone that trends southwest to northeast from Stafford County, Virginia USA and joins a fault that extends through the capital of the United States. It is a high-angle reverse fault. Cambrian and Ordovician mylonites, schists and phyllites lie unconformably atop Cretaceous gravels or Quaternary gravels along this fault zone.
Sixty-four kilometres north in Washington, D. C. the fault, known as the Dumfries Fault Zone or Stafford Fault Zone, becomes a low profile overthrust fault. It has been obscured or damaged in most neighbourhoods in the capital, but can be seen in a stone and chain link enclosure at the intersection of Adams Mill Road and Clydesdale Place on National Zoo property (Fig. 1).
Geologist Nathaniel Horatio Darton of the United States Geological Survey (USGS) recognised the fault in1893. This, known now as the Adams Mill Fault, was the first of several relatively young faults that Darton discovered in the city. In the early twentieth century, the main entrance to the zoo was at this cross street in the Adams Morgan neighbourhood along the south-eastern border for the zoo (Fig. 2).
When “America’s Zoo” opened a larger entrance in the 1930s at 3001 Connecticut Avenue, the Federal government closed and demolished the one near the overthrust fault. It damaged the feature by doing so. Banded Palaeozoic mylonite, a recrystallised metamorphic rock, lies unconformably atop Pleistocene river gravels in the exposed intact fault that one can see off the zoo’s premises, in Rock Creek Park to the north. Elsewhere, this fault system is covered by fill or is damaged, as at the Adams Mill fault and several others that once could be seen (Fig. 3).
The Rock Creek shear zone (area of a concentration of stress within the crust) was visible at the Adams Mill Street and Clydesdale Place exposure prior to damage in the 1930s, and illustrated millions of years of missing geologic history in the fault at this location and in nearby neighbourhoods. The fault zone went through two distinct times of activity.
Geologist N. H. Darton asked the Smithsonian Institution to protect what was left at the street intersection. As a result, the Smithsonian constructed a stone and chain link fence enclosure with signage within the fence to explain the significance. It is located at coordinates 38.92515, -77.044605.
However, visitors can see better evidence of the shear zone for a related fault if they are willing to do plenty of hiking in Rock Creek Park to the north of the zoo. Piney Branch Trail, accessible from 17th Street NW after hiking for about 15 minutes from the Columbia Heights Metro stop, is a good place to see quartzite and other metamorphic rocks resulting from collision. Look for a large brown National Park sign. Soapstone Valley, which one has to cross busy Beach Drive to access, was named for the metamorphic rock soapstone (also known as steatite) here that was quarried by Native Americans thousands of years ago (Figs. 4 and 5).
Overthrust faults are reverse faults with a very low-dip angle of 45 degrees or less. Older rocks are thrust over younger ones, unlike normal or “gravity” faults (Fig. 6).
We encounter thrust faults in areas where rocks experience, or have experienced, compressional force, as opposed to tensional force that pulls rocks apart (Fig. 7). This compression is seen in the orogenic belts where either subduction zone accretion or collision of plates takes place.
Why didn’t the rock break initially rather than bend? The combination of very warm temperature, pressure within the earth’s crust, and time is the answer. Gradual motion bends rather than breaks rock. Mountain ranges result. The Alps are an example of a mountain range with often conspicuous overthrust faults.
Do the Stafford Fault Zone and the Adams Mill overthrust fault have any connection to the unusual 2011 Mineral, Virginia earthquake that made national news (Fig. 8)?
Do they share a fault? In the D.C. area, the Rock Creek shear zone and Stafford fault zone are apparently connected because of the ages of the relevant rock and the fact that both are varieties of reverse faults. The Lakeside Fault Zone, where the epicentre of the Mineral earthquake was located, is not considered connected to the others. There is currently no mountain building taking place in the region. Virginia, and the U.S. capital region are hundreds of kilometres from the nearest area of significant earthquake activity, the New Madrid Seismic Zone in the central Mississippi River valley.
The entire East Coast of the United States is likewise very far from any plate boundary. The last time there was volcanism to generate earthquakes was during the Eocene epoch, approximately 49 million years ago. The last time any compressional mountain building took place was during the Carboniferous and Permian periods, 365 to 200 million years ago. Thus, earthquakes are rare and none above approximately 3.0 on the Richter Scale have occurred since seismologists began recording seismic activity in Virginia… until 23 August 2011 when a Mw 5.8 (moment magnitude) earthquake occurred approximately 130km southwest of Washington near the Lakeside Fault Zone and Spotsylvania Fault.
Damage to structures in Mineral was moderate, with overturned and rearranged furniture, falling plaster and partial collapse of non-reinforced concrete structures. Seismic data gathered by the USGS revealed greater intensity was felt in Washington and in Alexandria, a densely urbanised Virginia suburb of Washington, D.C., than in more rural counties between Mineral and Alexandria.
Geologist Anthony Fleming hypothesised that thick fill soils in Alexandria, which tend to concentrate seismic disturbances (shaking), were responsible. In the summer of 2021, Greg Easson and a team of fellow geologists from the University of Mississippi conducted seismic refraction microtremor studies, ground penetrating radar surveys and resistivity surveys within the zoo property (Figs. 9 and 10), as well as the Washington Monument and Lincoln Memorial.
The objective was to find new faults that might have been missed. A 2022 thesis by Kristian Macias summarised findings from the field study. The Adams Mill fault is present in the zoo but under layers of fill, revealed by the instrumentation employed (Figs. 9 and 10).
Another fault may be present approximately 4.34km north. Ground penetrating radar data at the site of the Washington Monument (Fig. 11) showed a previously unknown fault here. However, thick fill made the findings difficult to obtain.
The team also identified a fault 80m west of the Adams Mill fault. However, this fault may not be connected to the Adams Mill fault. Due north of this fault lies a fault that was identified in 1976 by USGS drilling in Lafayette Square. If the faults at Lafayette Square and the Washington Monument are connected and continue further south, the fault trend would directly coincide with a very straight channel section of the Potomac River.
These results suggest the fault at the Washington Monument may be a previously unknown north-south trending fault that would seem to be the reason the Potomac River channel south of Washington, D.C. is straighter than elsewhere. The Adams Mill fault may connect this fault along the Potomac to the north-south trending Rock Creek Shear Zone. Macias deduced that these Quaternary faults have implications for seismic hazard analyses for the entire region.
About the author
Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.
References
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