The spectacular, near- vertical roadcuts in the High Bridge Group (Middle Ordovician) in central Kentucky (Fig. 1) along U.S. Highway 27 just south of the Kentucky River (Fig. 2) afford an excellent opportunity to examine a major fault zone and study its complexity (Figs, 3, 4). These roadcuts are located in Garrard County approximately 12 miles south of Lexington and contain the oldest rocks exposed in Kentucky

Gilreath, J. A.

Losonsky, George

Potter, Paul E.

English

University of Kentucky

University of KentuckyUKnowledgeKentucky Geological Survey Map and Chart Kentucky Geological Survey1989Guide to Interpretation of Structural FeaturesAssociated with the Kentucky River Fault SystemAlong U.S. Highway 27 Near Camp Nelson,KentuckyJ. A. GilreathSchlumberger Well ServicesPaul E. PotterUniversity of CincinnatiGeorge LosonskyUniversity of CincinnatiRight click to open a feedback form in a new tab to let us know how this document benefits you.Follow this and additional works at: https://uknowledge.uky.edu/kgs_mcPart of the Geology CommonsThis Map and Chart is brought to you for free and open access by the Kentucky Geological Survey at UKnowledge. It has been accepted for inclusion inKentucky Geological Survey Map and Chart by an authorized administrator of UKnowledge. For more information, please contactUKnowledge@lsv.uky.edu.Repository CitationGilreath, J. A.; Potter, Paul E.; and Losonsky, George, "Guide to Interpretation of Structural Features Associated with the KentuckyRiver Fault System Along U.S. Highway 27 Near Camp Nelson, Kentucky" (1989). Kentucky Geological Survey Map and Chart. 212.https://uknowledge.uky.edu/kgs_mc/212KENTUCKY GEOLOGICAL SURVEY UNIVERSITY OF KENTUCKY, LEXINGTON Donald C. Haney, State Geologist and Director MAP AND CHART SERIES 1 Series XI, 1989 GUIDE TO INTERPRETATION OF STRUCTURAL FEATURES ASSOCIATED WITH THE KENTUCKY RIVER FAULT SYSTEM ALONG U.S. IDGHW AY 27 NEAR CAMP NELSON, KENTUCKY NORTH J. A. Gilreath, Paul E. Potter, and George Losonsky Artist's sketch of portion of Kentucky River Fault System on east side of U.S. Highway 27 near Camp Nelson, Kentucky. GUIDE TO INTERPRETATION OF STRUCTURAL FEATURES IN THE KENTUCKY RIVER FAULT SYSTEM ALONG U.S. HIGHWAY 27 NEAR CAMP NELSON, KENTUCKY By J. A. Gilreathl, Paul E. Potter2, and George Losonsky2 The spectacular, near- vertical roadcuts in the High Bridge Group (Middle Ordovician) in central Ken-tucky (Fig. 1) along U.S. Highway 27 just south of the Kentucky River (Fig. 2) afford an excellent opportunity to examine a major fault zone and study its complexity (Figs, 3, 4). These roadcuts are located in Garrard County approximately 12 m iles south of Lexington and contain the oldest rocks exposed in Kentucky. The Kentucky River Fault System (Fig. 1), a major structural feature in central Kentucky, consists of a com-plex mosaic of closely spaced grabens and horsts (Figs. 4, 5, 6). The s,ystem trends northeast-southwest: it is downdropped to the southeast and has as much as 700 feet (213m) of structural relief. The east side of the roadcut is downdropped about 300 feet (100m) so that the Camp Nelson Limestone and the younger Lexington Limestone both occur at road level (Figs. 3, 4). The Kentucky River Fault System was probably formed con-currently with the Appalachian orogeny and most likely reflects movement of basement rocks . For more de-tails about the stratigraphy and structure of the fault system, see Black and Haney (1975), Kuhnhenn and Haney (1986), and the map by Black and others (1977). Structural features to study include: a sharp kink fold (Figs. 7, 8, 9); breccias (Figs . 5, 6) ; both large and small normal faults; a small, unusual overthrust with puzzling geometry (Fig. 10) ; several small reverse and thrust faults ; conspicuous fractures that parallel the ma-jor fault trend ; small mineralized fractures; and slickensides . Sed imentary features of the limestones and dolomites indicate that they were deposited in a car-bonate tidal flat (Cressman and Nager, 1976). The kink fold northwest of the boundary fault (Figs. 7, 8, 9) is perhaps the most striking feature of the expo-sures . Th is sha,rp fold is easily m istaken for a fault , but careful tracing of the shale bed along both sides of the road proves otherwise (Fig. 6). We consider the kink to be a drag fold and wonder if perhaps its sharp flexure represents a very shallow level of deformation? 1Schlumberger Well Services, Gulf Coast Unit, New Orleans , LA 70130. 2H. N. Fisk Laboratory, University of Cincinnati, Cincinnati , OH 45221 - 0013. '--. FAULT BRECCIA Questions that deserve attention at this spectacular cut include: the relative ages of the structural features, the origin of the thrust fault in the M bed on the west side of the cut, and the relative roles of tension verses com-pression in the formation of the fault zone. The following tools will be useful in studying the structural features exposed in these outcrops: compass, notebook, camera, and the Little Hickman 7.5-minute Geologic Quadrangle Map (Wolcott, 1969). Park on e ither side of U.S . Highway 27, but be careful crossing it. It is instructive to consider the types of dipmeter patterns that would result if the structures exposed in this roadcut were in the subsurface and logs were run in boreholes at the locations shown on Figure 3 . The dip-meter is a four-armed logging tool that records eight (two from each pad) microconductivity curves (Fig. 11). Detailed correlation of these curves can be used to rec-ognize displacement across the borehole, which can be used to compute dips of sedimentary features. Dip· meter logs are used to inler structural dip, identify faults and unconformities, def ine stratigraphy, and identify depositional environments. Read about the dipmeter (Gilreath, 1977 , 1987a, 1987b: Gilreath and others, 1985) before you come. In order for a fault to be identified from a dip plot, either some type of distortion or a structural dip change must be present. Although most faults have one or both of these attributes, a lew types of faults have neither and cannot be identified from dipmeter results alone. Fortunately, two types of distortion, rollover and drag, are commonly associated with normal faults. Contemporaneous normal faults, those that de· velop during sedimentation , generally exhibit rollover, or d ip into the fault plane, on their downthrown sides. Rol-lover creates a pattern of downward-increasing dips in the downthrown block of a normal fault, with the maxi-mum dip located immediately adjacent to the fault plane. The downdip direction of such a dip pattern is toward the upthrown lault block and normal to the fault strike. Post- depositional normal faults, which occur after some format ion compaction has taken place, commonly exhibit drag in the beds immediately downthrown from the fault plane. The drag zone also produces a pattern of downward-increasing dips, with the maximum dip of the pattern located immediately adjacent to the fault plane. However, in the case of drag, the pattern of dip direction is toward the downthrown fault block, or in the same direction as the dip of the fault plane , and normal to the fault strike. Dip-sl ip reverse and thrust faults also develop drag on both sides of the fault plane, with the greatest amount of drag general ly in the overthrust block. The overthrust drag zone creates a downward increasing dip pattern, with the greatest dips being located very near the fault plane . The azimuth of drag-created dips in the SOUTH Figure S. Sketch of features in roadcut along the east side of U.S. Highway 27 south of the Kentucky River near Camp Nelson. Major features to look for are: shale bed in the Camp Nelson Limestone; kink fold; massive white micstone bed (M bed); boundary fault; and displaced ball-and-pillow bed in the Lexington Limestone. Offset across the fault zone is about 300 feet, downdropped to the southeast. Letters in boxes refer to faults shown on the geologic map (Fig . 2) and cross section (Fig. 3). SOUTH NORTH v v v • ' . v ~ ~-v v Shale -~ -v v FAULT BRECCIA WEDGE '---eoUNDARY FAULT Figure 6. Sketch of features in roadcut along the west side of U.S. Highway 27 south of the Kentucky River near Camp Nelson. :Major foatures to look for are: fault breccia and complex fault zone at the south end of the cut; boundary fault; and magnitude of displacements of the massive white micstone bed (M bed). The kink fold is in the ravine, and the shale bed is continuous across it. Rocks exposed in the west cut are mostly Camp Nelson Limestone. Letters in boxes refer to faults shown on the geologic map (Fig. 2) and cross section (Fig. 3) . overthrust block is in the direction of thrusting and nor-mal to the strike of the fault. Dip patterns in the downthrown block, which generally decrease downward, may be in either the same direction, or the opposite di-rection, of the overthrust. A hypothetical dipmeter at location 1 (Fig . 12) would cross the axis of the kink fold at 220 (numbers represent arbitrary units), and at location 2 the axis of the kink would be crossed at 30 (Fig. 13). A downward dip increase of about 20 degrees occurs across fault E, whi le a 5-degree decrease occurs across fault F (Fig. 13) . Such dip changes indicate a tilted block between fault E and fault F (Fig. 3). The slight increase of dip into faults E and F results from kink-created dip rather than from drag-zone dip. At dipmeter location 3 (Fig. 14), the breccia zone, located between faults A and B, produces only widely scattered dips, or no correlations. The block between Faults B and C is ti lted 28 degrees to the south-southwest. On the upthrown side of fault C structural dip is 1 0 degrees southwest (Fig, 14) . At dipmeter location 4 (Fig . 15), the downthrown drag zone of fau lt A is clearly seen. The pattern of downward-increasing southeast dips, which terminates at the fault plane, indicates a normal fault with its downdropped side dipping to the southeast. The kink fold is one of the most noticeable features exposed in the road cut. Northward along the cut (Figs. 3, 5, 6), all of the dips associated with the kink are ex-actly the same as those that would be expected in association with a down-to-the-southeast growth fault. Northwesterly dip increases downward to a maximum of 65 degrees near the axis of the kink (Figs. 12, 13) . From that point dip magnitudes decrease abruptly to 1 O degrees northwest (Figs. 12, 13). If this structure were Figure 1. Major tectonic featur es of Kentucky. Figure 7. Kink fold and shale bed in the Can1p N£1son Limestone a long the east side of U.S. Highway 27. View is to the north. F igure 8. Kink fold and boundary fa ult in roadcut a long the west side of U.S. Highway 27. Figure 9. Kink fold, sha le bed , and boundary fault in roadcut along the west side of U.S. Highway 27. V ie\.v is to the south. a growth fault rather than a kink fold, the fau lt would be located immediately below the point of maximum (65-degree) dip. Acknowledgments The drafting assistance of Lyla Messick , University of Cincinnati, and the financia l support of Schlumberger Well Services are greatly appreciated . References Cited Black, D. F. B., and Haney , D. C. , 1975 , Selected structural features and assoc iated dolostone occurrences in the vicinity of the Kentucky River Fault System: Guidebook, Geological Society of Kentucky Annual Field Conference (Published by the Kentucky Geological Survey). 27 p . Black, D. F. B., Keller , G. R., and Johnson , R. W., Jr., 1977, Structural geology , Bouguer gravity, and aeromagnettc inten-sity for a portion of central Kentucky, southeast sheet: Kentucky Geological Survey scale 1 :48 ,000. Cressman , E . R., andNoger , M. C ., 1976, Tidal-flat carbonate environments in the High Bridge Group (Middle Ordovician) of central Kentucky : Kentucky Geological Survey, A~pt. Inv. 18, ser . 10, 15 p . Gilreath , J. A .. 1977. The dipmeter, in Leroy, L. W . , Leroy , D. 0. , and Raese , J . W., eds .. Subsurface geology: petro-leum. mining. construction: Colorado School of Mines, Golden , Colo ., 4th ed. , p. 389-396. Gilreath, J. A., 1987a, Dlpmeter interpretation rules : Schlum-berger Technical Review, v . 35, no . 3 , supplement, 6 p . Gilreath, J. A., 1987b. Strategies for dipmeter interpretation : Schlumberger Technlcal Review , v. 35, no . . 3, p. 28-39. Giireath, J . A., Cox, J . W., Grace, M. L. , and Fett, T ., 1985, Practical dlpmeter interpretation: Schlumberger Educational Services. Kuhnhenn, G. L ., and Haney , D. C. , 1986, Middle Ordovician High Bridge Group and Kentucky River Fault System in central Kentucky ; Geological Society of America Centennial Field Guide, Southeastern Section , p. 25- 29. Wolcott, D. E., 1969, Geologic map of the Litt le Hickman Quad-rangle, central Kentucky: U .S. Geological Survey Map G0-792. Figu r e 10. Close-up view of unusual geometry or sma ll thrust fault involving the massive micstone bed (M) a long the west side of U.S. Highway 27. - - ----- -..._ ..._ -- ..._ ..._ -..._ ..._ -..._ -..._ ..._ -..._ ..._ -------------- ------- ---..._ -----..._ -..._ ----- --..._ --- --- -..._ -----Figure 11. Schematic diagram of dipmeter tool in borehole. As the tool is retrieved up the hole, it simu l-taneously records multiple resi s tivity curves ,vhich can be used in determining t he direction and 1nagnitude of d ip of the strata. 1000 0 2000 ft. 0 .5 1 m. Figure 2. Map of study area showing locations of faults associated with the Kentucky River Fau lt System near Camp Nelson. Area sho,vn is the Little Hickman 7 .5-minute quadrangle. SE 0 ,o ,00 " 4 3 BOU NDARY FAULT KEY ~ Camp Nelson ~ Lexington Limestone -- Fault '' '' Kink ,.(~5 Dip and Slrlke ~Talus " Figure 3. Detailed geologic map of roadcut a long U.S. Highway 27 just south of the Ken-tucky River. Note lin e of cross section and locations of hypothetical dipmeter plots indi-cated by numbers on line of section. ' ' NW ,shale Oc, M''°:==d T~1ck, White cs tone 2 OIPMET ER LOCATIONS 1 Figure 4. Cross section sh0\\1 in g structure on both sides of U.S. l·li gh,vay 27 at roadcut south of the Kentucky River. Vie,v is to the south,vest. Letters in boxes indicate faults, and c ircled nurnbers sho,v ·locations of hypothetical dipmeter plots. 300 200 100 0 -100 o• 3 0 ° 60° 90° I I ~ •••••• . • ... ~ .... .... ... .... .... ~ 'e .. .... .... .... ~ .... ~ I I~ I I ~ -, .... •··· . -. I I I I AXIS OF KINK METHOD OF PLOTTING ~-0° 1n° 20° N I w-r-E I s Figure 12. Hypothetical d ipm eter plot at location 1 . Scale is in arbitrary units. 300 .. • .. ._ .... .... 200 • • ~'II 100 .. ... .... - .... ••••• "'fl. .. .._. ... ~ • ... 0 ... ~ .. ... -100 ... ---••• -•• FAULT E FAULT F AXIS OF KINK F igu re 13. Hypothetical dipmeter plot at locat ion 2. Scale is in arbitrary units. o• ~ 30 0 FAULT A .... ~ ~ -BRECCIA • ~ 20 0 FAULT B '- -• .,. FAULT C 100 -• ~ ~,. -~ .,. 0 • .,. • • Figure 14. Hypothetical dipmeter plot at location 3 . Scale is in arbitrary units. o• 30° 60° 90° 100 'l FAULT A BRECCIA .. FAULT B '--~~--'-~~-'--___j .. F igure 15. Hypothetical dipmeter plot at location 4. Scale is in arbitrary units. https://doi.org/10.13023/kgs.mc01.11

Guide to Interpretation of Structural Features Associated with the Kentucky River Fault System Along U.S. Highway 27 Near Camp Nelson, Kentucky

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Guide to Interpretation of Structural Features Associated with the Kentucky River Fault System Along U.S. Highway 27 Near Camp Nelson, Kentucky

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