Geology topics

Ron Counts

Publications and source records attributed to Ron Counts.

3 recordsLinked to original sources

USGS2025

Neotectonic origins for the Meadow Bank scarp, Wabash Valley seismic zone USA

The Meadow Bank scarp (MBS) in southeastern Illinois is a linear geomorphic expression, ∼10 km long and ∼8 m high above a relatively flat landscape. It parallels an underlying northeast‐oriented Late‐Precambrian–Early‐Cambrian structural fabric, called the Wabash Valley fault zone, and is within an area of modern, historic, and paleo seismicity, called the Wabash Valley seismic zone. In addition, the MBS acts as a boundary of the Wabash River floodplain, as well as Pleistocene glacial outwash channels, which show evidence of frequent outburst flood events. To better understand the MBS’s equivocal origin in this complex geologic environment, we acquired a 917‐m‐long seismic‐reflection survey across its axis to assess the subsurface geologic configuration. The resultant image indicates a complex set of faults that antiformally fold and displace the top of Paleozoic bedrock by ∼12 m across the survey. Moreover, fault and/or fold deformation extends into the shallowest imaged Quaternary strata at ∼6 m below the ground surface. This suggests the MBS origin is related to underlying Quaternary reactivated, positively inverted faults rather than exclusively to glacial outburst flood erosion. These results provide rare paleoearthquake spatial constraints for central U.S. regional seismic hazard consideration.

Illinois↗

USGS2021

River terrace evidence of tectonic processes in the eastern North American plate interior, South Anna River, Virginia

We show that long-recognized seismicity in the central Virginia seismic zone of the eastern North American intraplate setting arises primarily from tectonic processes predicted by new, fully coupled plate tectonic geodynamic models. The study leverages much new geophysical and geologic data following the 2011 Mineral, Virginia, earthquake that ruptured a steeply dipping, northwest-verging reverse fault traversed by the South Anna River. The data are primarily assembled from a flight of six fluvial terrace geomorphic markers identified and correlated on texture, relative weathering, and numeric ages including one terrestrial cosmogenic nuclide (TCN) profile and 30 luminescence dates. Terrace thickness, stratigraphic age models, and incision rates downstream and upstream of the 2011 rupture are different. Long-term river incision rates of ∼25–30 m/My are superimposed on regional TCN-determined erosion rates of ∼8.5 m/My; however, there are at least 10 m of tectonically driven incision in the epicentral region at rates of ∼30–94 m/My. The inferred deformation resembles a hanging wall anticline above a blind reverse fault with a diffuse overlying carapace of minor brittle faults, an interpretation supported by seismology as well as bedrock and saprolite mapped across the epicentral region. These results are further supported by channel metrics that show nonuniform channel steepness ( k sn ) and a predicted steady-state channel elevation different from the actual channel elevation across the epicentral region. If all of the observed deformation is a consequence of the fault that ruptured in 2011, the recurrence interval of Mineral-sized events would be ∼5.5 ky.

Virginia↗

USGS2019

Quaternary displacement on the Joiner Ridge Fault, eastern Arkansas

The New Madrid seismic zone of the central United States is an intraplate seismic zone with blind structures that are not seismically active but may pose seismic hazards. The Joiner Ridge fault is the 35 km long east-bounding fault of the Joiner Ridge blind horst located in eastern Arkansas approximately 50 km northwest of Memphis, Tennessee. Shallow S-wave (SH-mode) seismic reflection profiles, continuous cores, and radiometric dating of Quaternary alluvium across the Joiner Ridge fault reveal down-to-the-east reverse faulting and folding within of the top of the Eocene strata and overlying Quaternary Mississippi River alluvium. The base of the Quaternary alluvium has an age of 20.3 ka and is vertically displaced 12 m, resulting in an average slip rate of 0.6 + 0.1 mm/yr over the past 20.3 ka. The overlying late Wisconsinan and Holocene alluvial facies are also displaced by the Joiner Ridge fault. These facies increase in thickness across the Joiner Ridge fault and were used to calculate late Wisconsinan and Holocene slip rates. The JRF slipped 7 m between 20.3 ka and 17.5 ka (2.8 ka), reflecting a slip rate of 2.5 + 0.3 mm/yr. From 12.3 ka to 11.5 ka (0.8 ka) the JRF slipped 3 m at an average slip rate of 3.8 + 0.9 mm/yr. There were 2 m of slip on the JRF between 11.5 ka and 8.9 ka (2.6 ka), reflecting a slip rate of 0.8 + 0.3 mm/yr. No apparent slip has occurred on the JRF within the last 8.90 ka. This research illustrates that slip rates on the JRF have varied through the late Wisconsinan and early Holocene, but the Joiner Ridge fault has been inactive since the middle Holocene.

Arkansas↗