MINOR FAULTS EXPOSED AT A TN-109 ROAD CUT NEAR GALLATIN, TN 1 Table of Contents Section Page Number Executive Summary 2 Introduction 2 Geologic Setting 3 Lithologies 7 Location and Orientation of Fractures 8 Structures and Regional Geology 8 Summary and Conclusions 9 References Cited 10 2 Summary The post-depositional features observed in the Sequatchie formation at the TN-109 road cut in Gallatin, TN, indicate that the region was subjected to uplifting and regional stress after its deposition. There are at least 6 faults which all share a common trend of 250-280 degrees. Three of them have greater than 1m of dip separation. One fault is a strike-slip trending 272 degrees with only 1.6cm of dip separation. However, it contains a significant amount of cataclasite. The only feature to which can be drawn similarities from the strikes alone, is the FHRFZ. Faults at this site 27km southwest of the Gallatin road cut have a similar trend of 250270, and they could be an example of en-echelon faulting (Abolins et al., 2018). These fault zones are likely similar in origin, but the origin is not clear. The author believes that they were formed during uplift of the Nashville Dome and reactivated on multiple occasions. Introduction The Central Basin of Tennessee is in an area surrounding Davidson, Rutherford, Wilson, and Sumner Counties. It is roughly centered over the Nashville Dome, which is an elliptical cratonic uplift, trending NNE with a land area of 12,000km2. Dome uplift and erosion in the basin has exposed strata varying in age from the middle Ordovician to the later-Tertiary period. Postdepositional features (faults, fissures, sinkholes, folds) can be observed around this area, drawing a connection between a diverse set of geological units and offering evidence for the e events of the past. 3 This paper will investigate a specific outcrop north of Gallatin, TN, where late Ordovician to Devonian strata can be observed. Lithological and structural features were studied to relate this specific location to the characteristics of the greater Nashville Dome. These faults are related to, and possibly an extension of the Freeman Hollow Road Fault Zone (FHRFZ). The FHRFZ is exposed 27km southwest of the TN-109. The faults at both locations have a similar trend of 250-270° (Abolins et al., 2018). It is possible that the faults visible in the roadcut are related to the Appalachian Mountains or the Nashville Rift. The evidence, however does not strongly support either hypotehsis, and were reactivated by uplift of the Nashville Dome. The slip we see on them today reflects the amount of uplift that was subjected to the Cambrian through Mississippian strata. (Abolins, et al., 2018). Geologic Setting This road cut is located on the northwest side of the Nashville dome on the boundary with the Highland rim. It is on highway TN-109 approximately 7 miles north of the TN-31E junction. There is outcropping of strata from Ordovician Sequatchie fm., Silurian Wayne group., and Devonian Fort Payne and Chattanooga Shale formations. These formations are quite diverse in lithology due to changes in local environmental conditions caused by uplifting of the dome and sea level cycles. 4 Figure 1: Map of Middle Tennessee showing the geographic location of multiple fault zones, Nashville Dome, and Nashville rift. From Abolins et al., 2018. 5 Structures are observed in the Sequatchie formation at road level. These rocks were deposited in a shallow sea with a moderate amount of silt input. The area was subsequently uplifted around 800ft on different occasions from the middle Ordovician to Early Cretaceous. This uplift of the Nashville Dome reactivated numerous pre-cambrian aged faults in several fault zones, including this zone being studied. (Wilson and Stearns, 1963). 6 (Wilson, 1961) Figure 2 from Wilson and Stearns 1963 Figure 3 from Wilson 1961. The black Chattanooga Shale unconformably lies overtop of the late Ordovician and Silurian sediments which have been thinned by about 200 feet at the crest of the dome relative to the flanking areas. This is mostly due to uplift and truncation, where the rocks were exposed at the crest of the dome due to uplift, causing them to weather away before burial by the Chattanooga Shale. (Wilson and Stearns, 1963). Additional 400ft of uplift happened during the 7 late Mississippian during the Alleghanian orogeny, attributed to flexure of the lithosphere during this time. (Abolins et al., 2018). Lithologies Forte Payne formation contains platy siltstone with shale, brownish black to olive-gray in color. At base is greenish-gray glauconitic shale or mudstone near 1 foot thick. Certain localities contain chert and crinoidal limestone. Chattanooga Shale is carbonaceous, grayish black with thinly laminated fissile bedding. Osgood formation is thin to medium beds of olive gray to yellow-green. It contains shale and claystone with calcareous nodules and thin beds. Sequatchie formation at this road cut contains thin to medium beds of gray argillaceous limestone. Some medium beds are laminated, containing thin nodules of chert or vugs filled with gypsum. There are some beds of grainstone which show soft sediment deformation including cross bedding, stylolites and seismite. (Ferguson et al., 1987). Location and Orientation of Fractures There are several faults in the TN-109 outcrop, visible on both the east and west side of the road. The strike can be determined by placing the transit in the fault plane visible across either 8 side of road cut. There is a large number of normal faults visible on both the east and west sides of the highway. They are mostly trending 250-270° with dips ranging from 55° to vertical. Table #1, strike, dip, and dip slip values for faults observed in Sequatchie Fm. Fault #, type Strike Dip, Dip slip 1 – Normal 251° 68° S 2 – Normal 264° 55° S 3 – Normal 272° 62° N , 90cm 4- Normal 285° 70 S, 20cm 5 – Reverse 093° 84° N, 1cm 6 – Normal 082° 72° S, 1.7m 7 – Normal 100° 90°, 1.4m 9 Relationship between Structures and Regional Geology The topography, faults, and fractures observed in the TN-109 road cut indicate that it is located in the northern highland rim fault zone. The strike of the fractures is at a high angle to the trend of both the Appalachian Mountains and the Nashville Rift. There is no evidence in this paper that the faults are directly related to either. However, the strike of the faults at TN-109 is very similar to faults exposed 27km to the southwest at the Freeman Hollow Road Fault Zone (FHRFZ). Alleghenian orogeny and have been reactivated due to isostatic factors more recently. An ancient rift has been interpreted to the south of this area due to gravity and magnetic anomalies of sub-surface rocks. It is the southernmost member of an inferred large- 10 scale mid-continental rift(MCR) spanning from Alabama to Ohio. (Stein et al., 2014). There is not much evidence linking the faults at Gallatin to any activity of the Nashville Rift. Instead the high angle of the faults relative to trend of the rift instead shows that they are not likely related. Figure 4. Gravity map showing the anomalies which indicate the position of an ancient rift buried beneath strata in the midcontinent of United States. From Stein et al., 2014. Summary and Conclusions The formations visible in the TN-109 outcrop north of Gallatin, Tennessee have clearly been affected by post-depositional forces. They contain multiple fractures and faults which have significant dip-slip. There has been close to 800ft of uplift since the deposition of the basement rocks in this area. However, this uplift happened in different stages and was influenced by many 11 factors including uplift of the Nashville Dome, regional stress from formation of the Appalachian Mountains, as well as isostatic uplift from erosion in the millions of years since. The faults overall have a trend close to 270° which is roughly perpendicular to the trends of both the Appalachian mountains and a hypothetical Nashville rift which had likely formed during the breakup of Rodinia. It is probable that all of these factors had an influence in the development and reactivation of these faults. 12 References Cited Abolins, M., Bush, R., Han, A., and Ogden, A., 2018, Mesoscale faults, macroscale folds, and inferred basement structures, Nashville dome, central Tennessee, in Engel, A.S., and Hatcher, R.D., Jr., eds., Geology at Every Scale: Field Excursions for the 2018 GSA Southeastern Section Meeting in Knoxville, Tennessee: Geological Society of America Field Guide 50, p. 95–119, https://doi.org/10.1130/2018.0050(07). Ferguson, C., Wilson, C., Jr., and Barnes, R., 1987, Geologic Map and Mineral Resources Summary of the Gallatin Quadrangle: Tennessee Division of Geology Geologic Quadrangle Map 313-NW, scale 1:24,000 Pope, M. C., Holland, S. M., Patzkowsky, M. E., The Cincinnati Arch: A Stationary Peripheral Bulge During the Late Ordovician, School of Earth and Environmental Sciences, Washington State University, Pullman, WA Stein, C.A., Stein, S., Elling, R., Keller, G.R., and Kley, J., Is the “Grenville Front” in the central United States really the Midcontinent Rift?, GSA Today, vol. 28, doi:10.1130/GSATG357A.1. Stein, C.A., Stein, S., Merino, M., Keller, G.R., Flesch, L.M., and Jurdy, D.M., 2014, Was the Midcontinent Rift part of a successful seafloor spreading Geophysical Research Letters, v. 41, p. 1465–1470, https://doi.org/ 10.1002/2013GL059176. Wilson, C., Jr., and Stearns, R., 1963, Quantitative Analysis of Ordovician and Younger Strata in the Nashville Dome Tennessee, Bulletin of the American Association of Petroleum Geologists, Vol. 46, pp823-832, May 1963. Wilson, C. W., 1961, Stratigraphy and Geologic history of Ordovician rocks of Central Tennessee, Geological Society of America Bulletin, v. 73, p. 481-504, April 1962.
