Penholoway Formation

The Penholoway Formation (also known as the Penholoway terrace or the Penholoway Coastal Complex is a geological formation located along the United States Atlan

Penholoway Formation
Penholoway Formation
Stratigraphic range: ~770 ka[1]
The Penholoway formation in the lower Savannah River area of Southeastern Georgia, USA
TypeGeological formation
Unit ofCoastal Plain Sediment deposits
OverliesWaccamaw Formation
Area25 square kilometers (15.5 square miles)
Thicknessup to 15 feet (4.6 m)
Lithology
PrimarySand, Clay
Location
LocationNortheastern Florida, Coastal Georgia, Coastal South Carolina, Southeastern Coastal North Carolina
RegionCoastal Plain
CountryUnited States

A map of the United States east coast coastal plain. The Penholoway Formation hugs the coastline from Jacksonville, Florida to Wilmington, North Carolina
Penholoway formation in northeastern Georgia

The Penholoway Formation (also known as the Penholoway terrace or the Penholoway Coastal Complex is a geological formation located along the United States Atlantic coastline. It is one of many coastal formations that formed during the Pleistocene. It can be found along the coast from northern Florida to southeastern North Carolina. It averages 15-21 meters (50-69 feet) above sea level and averages 10 to 25 kilometers (32 to 82 miles) wide.[2]

Geology

During the Pleistocene, the Atlantic Ocean shoreline was 15 to 40 kilometers (9 to 25 miles) further to the west than its current position. Due to erosion and time, the current boundary of the formation is almost straight compared to the current Atlantic coastline. In South Carolina, there is strong evidence for ancient sandbars, barrier islands, and spits.[3]

The formation consists of sand, clayey sand, and clay deposited in shallow-marine to marginal-marine environments. It's notable for a barrier and back-barrier complex (land behind a barrier island) near Summerville, South Carolina. The Woodstock fault bisects the formation as well.[4]

Terrace uplift and South Carolina earthquakes

See also: 1886 Charleston earthquake

The Penholoway Formation cuts through The Charleston Seismic Zone; the most active seismic zone on the United States east coast.[5] During the great Charleston earthquake in 1886, which had a magnitude estimated to be between 7.1 and 7.3, the Penholoway Formation measured uplift of ~1 meter (3.2 feet). Since the formation of the Peholoway formation ~770 ka, it's estimated that the formation has been uplifted by between 6-7 meters (19-23 feet) and 12-16 meters (40-50 feet) of dextral offset offset. There is also evidence for liquification of sediments in the formation during the earthquake.[1][6]

Measuring uplift between 770 ka to present, research can estimate the average interval between large earthquakes (to an extent). Data shows that the average interval between large earthquakes is ~100 ka. However this number is inaccurate. Geodetic studies show anomalously high strain rates to the southwest of the Penholoway Terrace, which suggests renewal intervals of hundreds of years. Present-day seismicity (1:5 < Mw < 4:5) tends to occurs to the southeast of the southern termination of the formation, which is also inferred mainshock rupture.[1]

References

  1. ^ a b c Bilham, Roger; Hough, Susan E. (1 October 2023). "The 1886 Charleston, South Carolina, Earthquake: Relic Railroad Offset Reveals Rupture". The Seismic Record. 3 (4): 278–288. Bibcode:2023SeisR...3..278B. doi:10.1785/0320230022. Retrieved 4 January 2026.
  2. ^ Soller, D.R.; Mills, H.H. (1991). "NGMDB Product Description Page". ngmdb.usgs.gov. Winston-Salem, North Carolina: Carolina Geological Society. pp. 290–308. Retrieved 4 January 2026.
  3. ^ Markewich, Helaine; Pavich, Milan; Mahan, Shannon; Bierman, Paul; Alemán‑González, Wilma; Schultz, Arthur (2021). "Open-File Report" (PDF). USGS. United States Geological Survey. doi:10.3133/ofr20211015. Retrieved 4 January 2026.
  4. ^ Marple, Ronald T.; Hurd, Jr., James D. (22 May 2020). "Interpretation of lineaments and faults near Summerville, South Carolina, USA, using LiDAR data: implications for the cause of the 1886 Charleston, South Carolina, earthquake". Atlantic Geology (in French and English). 56: 073–095. Bibcode:2020AtlG...56...73M. doi:10.4138/atlgeol.2020.003. Retrieved 4 January 2026.
  5. ^ Wulfeck, Andrew (23 August 2025). "South Carolina rattled by earthquake swarm". FOX Weather. Charleston, South Carolina. Retrieved 4 January 2026.
  6. ^ Tuttle, Martitia P.; Hartleb, Ross; Wolf, Lorraine; Mayne, Paul W. (13 July 2019). "Paleoliquefaction Studies and the Evaluation of Seismic Hazard". Geosciences. 9 (7): 311. Bibcode:2019Geosc...9..311T. doi:10.3390/geosciences9070311. Retrieved 4 January 2026.

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