Extent and Mechanism of Footwall Shear Adjacent to the Ruby's Inn Thrust Fault, Southern Utah

Transcription

1 The Compass: Earth Science Journal of Sigma Gamma Epsilon Volume 84 Issue 1 Article Extent and Mechanism of Footwall Shear Adjacent to the Ruby's Inn Thrust Fault, Southern Utah Skyler B. May Southern Utah University, sku_man@msn.com Roger E. Leavitt Southern Utah University, hikingllama@gmail.com John S. MacLean Southern Utah University, johnmaclean@suu.edu Follow this and additional works at: Part of the Earth Sciences Commons Recommended Citation May, Skyler B.; Leavitt, Roger E.; and MacLean, John S. (2012) "Extent and Mechanism of Footwall Shear Adjacent to the Ruby's Inn Thrust Fault, Southern Utah," The Compass: Earth Science Journal of Sigma Gamma Epsilon: Vol. 84: Iss. 1, Article 5. Available at: This Article is brought to you for free and open access by DigitalCommons@CSB/SJU. It has been accepted for inclusion in The Compass: Earth Science Journal of Sigma Gamma Epsilon by an authorized administrator of DigitalCommons@CSB/SJU. For more information, please contact digitalcommons@csbsju.edu.

2 EXTENT AND MECHANISM OF FOOTWALL SHEAR ADJACENT TO THE RUBY'S INN THRUST FAULT, SOUTHERN UTAH ABSTRACT Skyler B. May, Roger E. Leavitt, and John S. MacLean Department of Physical Science Southern Utah University 351 W. University Blvd. Cedar City, UT The Ruby s Inn Thrust, located in the Bryce Canyon region, is an uncharacteristic demonstration of a south-directed shortening episode located near the predominately eastdirected contractional structures of the Sevier Orogeny. The Paleocene to Eocene Claron Formation in the footwall of the Ruby s Inn Thrust contains conjugate shear structures and vertical fault planes with slickensides and slickenlines, indicating complex multidirectional shearing. We determined the north-south extent of a broad shear zone along a traverse immediately west of Bryce Canyon National Park, and shearing intensifies slowly from the main thrust at the northern end of our traverse to a maximum intensity at 13 kilometers south of the thrust, where it then gradually diminishes until an abrupt end approximately 29 kilometers south of the thrust. No evidence of conjugate shear structures in the hanging wall of the thrust was observed. The footwall outcrops adjacent to the thrust and at the southern portion of the traverse contained the structures, but they were more difficult to visually recognize, whereas the structures within the outcrops of the central region were obvious. The conjugate shear structures crosscut bedding and vary from small scale (a few centimeters) to large scale (tens of meters) throughout each outcrop, and are best observed parallel to their east-west strike. The conjugate shear structures contain distinct structural planar surfaces that include very well developed slickensides and slickenlines. This research supports the idea that the deformation structures were a significant contributor in the formation of hoodoos found in the Claron Formation. KEYWORDS: Ruby s Inn Thrust, Bryce Canyon National Park, Claron Formation INTRODUCTION Utah s complex geologic history has included eperic seas, extensional basins, and significant orogenic events. The most recognized of these orogenies, the Sevier and Laramide, were part of the building of the North American Cordillera. Even though these mountain building episodes demonstrate different modes of folding and faulting, both typically show significant east-directed contraction, and evidence of this thrusting can be observed throughout the central portion of the state. Southern Utah, in particular, has many outcrops that demonstrate the east-directed thrust faulting The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 30

3 during the Mesozoic and early Cenozoic Sevier Orogeny and the subsequent Laramide Orogeny. The Ruby s Inn Thrust Fault, located just north of Bryce Canyon National Park, is a rare south-directed structure found on the Paunsaugunt Plateau that thrusts Cretaceous Straight Cliffs Formation over Paleocene- Eocene Claron Formation (fig.1). In the late 1980s and early 1990s, researchers attributed the thrust to the gravitational collapse of the Marysvale Volcanic Complex based on age relationships (Lundin, 1989; Merle et al., 1993). Davis (1997) also related conjugate shear structures in Bryce Canyon s hoodoos to the thrusting and suggested a possible relationship between these structures and the formation of the hoodoos (fig. 2). Figure 1. Geologic map of Bryce Canyon/Marysvale (modified from Hintze et al, 2000). The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 31

4 Figure 2. A conjugate shear structure (highlighted with black box) in the hoodoos of the Claron Formation. Complex is based on the premise that the faults in the Paleocene-Eocene Claron Formation is likely younger than the Sevier Orogeny. However, the Wasatch Formation in northern Utah and the Flagstaff Formation in central Utah, which are age-equivalent to the Claron Formation of Bryce Canyon, were deformed by the last stages of the Sevier Orogeny (Yonkee & Weil, 2011; Elliot, 2011). This allows an alternative cause of the Ruby s Inn Thrust Fault. Therefore, the goal of our field investigation was to gain a better understanding of the structural relationships in the region. GEOLOGIC SETTING Stratigraphy Exposures in Utah provide an exceptional stratigraphic history (Fig. 3). During the Paleozoic, Utah was at the western edge of North America. The western portion was below sea level; the eastern portion of the state was a low plain near sea level (Reading et al., 1998). During the Triassic period, sea level fluctuated with a series of transgressions and regressions. In the Jurassic period the paleo-environment varied from vast deserts of windblown sand to hot, swampy lowlands with mountains and volcanoes. Rivers and lakes were plentiful during this time, and dinosaurs thrived. During the late Cretaceous, orogenic events from the west produced the Sevier foreland fold and thrust belt, which led to a foreland basin covered by the Western Interior Seaway in the eastern portion of the state (Reading et al., 1998). During the Paleocene to Eocene epochs, erosion from the mountains to the west led to deposition in major lakes in central and eastern Utah (Fig. 4). Continued compression from the west during the Eocene led to thick-skinned deformation of The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 32

5 the Laramide Orogeny (Reading et al., 1998). Several phases of volcanics and Basin and Range extension began in the Oligocene and continued through the Miocene (Reading et al., 1998). Extension continues today in the western part of the state (Reading et al., 1998; Wright, 1987; Sahagian et al., 2002). Figure 3. Stratigraphic column for the Bryce Canyon area, after Hintze (1988) and Pollock and Davis (2004). Figure 4. Generalized palogeography map of Utah s Paleocene-Eocene lakes (simplified from Davis et al., 2009). The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 33

6 The depositional environment of the Claron Formation is interpreted (e.g. Goldstrand, 1990) as a shallow marine to a freshwater environment including streams and lakes (Fig. 4). The Claron Formation is commonly divided into two members: the older Pink Member that often includes the towering hoodoos of Bryce Canyon National Park, and the younger White Member. The Claron Formation is predominantly composed of limestone interbedded with siltstone, sandstone and paleosols with a large variegated sandstone bed, sometimes referred to as an informal member (Lundin, 1989). In this region the relatively flat-lying Paleocene to middle Eocene Claron Formation disconformably overlies, youngest to oldest, the Kaiperowits, Wahweap, and Straight Cliffs Formations (Lundin, 1989). The Pink Member of the Claron Formation was commonly altered by pedogenic processes on the flood plains and in the shallow and seasonally ephemeral lake (Mullet, 1989). These pedogenic processes, most commonly caused by abundant plant growth, caused an oxidizing environment that ensured available iron would assume the form of hematite, contributing the pink and red hues (Davis & Pollock, 2010). Beginning in the late Paleocene, southwest Utah hosted the shallow Lake Claron in a slowly subsiding basin (Davis et al., 2009) forming the carbonates of the White Member. The lack of fossils in the White Member indicates poor conditions for life (Davis & Pollock, 2010). Goldstrand (1990) reported that the basal Claron Formation is time-transgressive, decreasing in age from late Paleocene in the west near the Pine Valley Mountains to middle Eocene in the east near the Table Cliff Plateau region, suggesting an east to northeast transgression of the lake. Structural Geology The Bryce Canyon region of the Paunsaugunt Plateau contains an assortment of intriguing contractional features and typical Basin and Range extensional faults and structures. Normal faults associated with Basin and Range extension include the Paunsaugunt normal fault to the east and Sevier normal fault to the west (Fig. 1). Both of these normal faults demonstrate hanging walls that drop down to the west at high angles and demonstrate significant displacement that can be seen easily from the ground and from the air (Davis, 2010). The large scale contractional structures found on the western Colorado Plateau, the restored eastern Basin and Range, and the transition zone between them are typically associated with the Sevier Orogeny and the subsequent Laramide Orogeny. The Sevier Orogeny is regarded as a region of classic thin-skinned fold and thrust belts (Armstrong, 1968), and the Laramide Orogeny represents a transition in structural style to thick-skinned deformation (Elliot, 2011; Yonkee & Weil, 2011). Through the mountain building episodes of the Sevier and Laramide Orogenies, an immense thickening of the crust occurred throughout Utah and surrounding states. Deformation due to the Sevier Orogeny in central/southern Utah began in the Cretaceous and continued into the Eocene (Elliot, 2011; Yonkee & Weil, 2011). The The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 34

7 early stages of Laramide deformation temporally overlapped the late stages of Sevier deformation (Elliot, 2011; Yonkee & Weil, 2011). Figure 5 shows the affected area of the Sevier Orogeny in Utah. Figure 5. Generalized map of Sevier Orogeny deformation extent (simplified from DeCelles and Coogan, 2004). The Ruby's Inn Thrust demonstrates significant south-directed displacement and many associated footwall structures that have not been interpreted as a product of the Sevier or Laramide Orogenies. Researchers in the late 1980s and 1990s (Davis and Krantz, 1986; Lundin, 1987, 1989; Lundin and Davis, 1987; Merle et al., 1993; Davis, 1997) interpreted the timing of the southdirected thrusts and fault-propagation folds associated with the Ruby s Inn Thrust system as the result of gravitational spreading and collapse of the Marysvale volcanic field (30-20 Ma; Steven et al., 1984), which is first exposed less than 20 kilometers to the north. This interpretation is supported by: 1) cross-cutting relationships and age constraints of the Claron Formation (presumably Eocene) and the overlying, undeformed 30 Ma Wah Wah Springs Formation (Best and Grant, 1987); 2) the arcuate nature of the fault trace that seems to curve around the southern exposures of the volcanic field; 3) the modern analog of Mt. Etna where such faulting and folding has been shown to result from volcanic activity (Borgia et al.,1992); and 4) the lack of another mechanism that would produce southdirected structures in a region that has experienced primarily east-west directed structures throughout geologic history. Throughout the extent of the Ruby s Inn thrust system there are a variety of different structures. Conjugate shear structures are apparent throughout the extent of the thrust (Fig. 6), cross-cutting lithologies and often showing offset. The structural planes of the conjugate shear structures display very well developed slickensides and slickenlines. These conjugate structures range from a few centimeters to 10s of meters across. There are also pervasive conjugate-thrust deformation bands exposed in the middle Cretaceous Straight Cliffs Formation within the hanging wall (Davis, 1997). Davis observed the presence of these vertical fault surfaces and conjugate shear structures within the hoodoos and spires of the Claron Formation of Bryce Canyon (Fig. 2) and postulated a correlation between the The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 35

8 structures and the development of the landforms. Figure 6. Representative conjugate shear structure in the Claron Formation. METHODS Using traditional mapping tools (topographic contour maps, aerial photographs, transit compasses, etc.), supplemented by global positioning system (GPS) devices and geographic information system (GIS) technology, data were collected from 10 stations displaying contractional deformation along a north to south traverse immediately to the west of Bryce Canyon National Park (Fig. 7). Measurements include strikes and dips of bedding planes, fault planes, and conjugate shear planes, as well as plunges and bearings of slickenlines. Sterographs and rose diagrams were produced from each station Figure 7. Map of field area showing stations. The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 36

9 collected by OpenStereo software (GNU, 2011). Using Trimble s Pathfinder Office software (Trimble, 2011) we differentially corrected the GPS points and exported them into the ArcGIS Desktop software. Using GIS, we produced a map that geographically displays our stations and graphically shows the measured orientations using the stereographs and rose diagrams. RESULTS The northern extent of the shear zone associated with the Rubyʼs Inn Thrust begins just north of the main thrusts on the hanging wall and continues to the south. The thrust system spans a 29 kilometer N-S width near the northern boundary of Bryce Canyon National Park. Shearing intensifies slowly from the main thrust at the northern end of our traverse to a maximum intensity at 13 kilometers south of the thrust, where it then slowly diminishes until an abrupt end approximately 29 kilometers south of the thrust (Fig.7). The main thrust exhibits large, well defined fault scarps and large slickenlines and grooves on the hanging wall. To the south of the main fault, increased conjugate shear structures become evident. Although there are many hoodoos that had substantial shearing and conjugate sets, access to the sheared surfaces varied, so measurements are of the most easily accessed outcrops in the study area. Figure 8 shows orientations of structures from several stations. As seen in figure 8, the dominant strike orientation on the conjugate shear surfaces is east-west. Intensity increases from the north to the south until station 5, Badger Creek, where several outcrops display extremely complicated conjugate shear structures and beautifully exposed slickensides with deep slickenlines that show three distinct orientations of slip; approx. 000, approx. 045, and approx To the south of station 5, evidence of shearing gradually diminishes until it ends approximately 40 km from the main thrust fault. Interestingly, some southern stations initially showed little evidence of conjugate shear structures, but gentle hammering on the outcrop caused the seemingly undeformed, homogenous limestone to fracture into perfect, large conjugate sets. These beautiful conjugate shear structures are either underdeveloped surfaces, or they were hidden beneath deposition of a thin layer of mud. The former appears to generally decrease in intensity and prevalence of structures to the south. The hoodoos of the region appear to have some form of structural fabric indicative of shearing within them. On our north-south transect, very few hoodoos did not show some sort of conjugate shear set. The hoodoos to the north are tall, slender, and well defined, while the hoodoos to the south are less well developed (Fig. 9). DISCUSSION The Rubyʼs Inn Thrust Fault and the footwall shear structures appear to be related because of the similarities in strike and slickenline orientation. The dominant northsouth orientation of slickenlines, with the exception of station 5, is generally perpendicular to the strike of the shear planes. Considering the orientation of slickenlines and the bisection of the acute The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 37

10 Figure 8. Stereographs and rose diagrams of measured structural planes and slickenlines, respectively. The "n" corresponds to the number of shear plane orientations in the stereograph. "Average strike" corresponds to the shear planes. "Average Bearing" corresponds to the slickenlines. angle between conjugate shear surfaces, we interpret a similar north-south orientation of σ 1. The complicated structural geometry at station 5 could be explained by reversing the orientations of the intermediate and minimum compressive stresses, σ 2 and σ 3, while maintaining a constant north-south orientation of σ 1 (Fig. 9). This scenario could occur if σ 2 and σ 3 forces were nearly equivalent. As discussed earlier, our northsouth transect revealed very few hoodoos that did not express some sort of conjugate shear structures. Just as the conjugate shear structures diminish in intensity from the north to the south, the hoodoos in the north are tall, slender, and isolated, while the hoodoos to the south become more massive and consolidated. Figure 10 shows a representative outcrop from the central region of the transect showing defined hoodoos and conjugate features compared to a representative southern outcrop that lacks distinct structural features. CONCLUSIONS Using the structural relationships, we determined an approximately 40-km northsouth extent of shearing in the footwall of the Ruby s Inn Thrust Fault directly to the The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 38

11 This preliminary study of the Ruby s Inn Thrust Fault, and its associated structures has revealed several questions. Future research could include continued inventory of conjugate shear structures to the west toward Red Canyon and Hillsdale Canyon, mapping of the conjugate shear structures within a nearby anticline (the Johns Valley Anticline on the northeast side of Bryce Canyon National Park) that could constrain relative timing, and mapping of structures to the north and east of the Marysvale Volcanic Complex to test the postulated radial distribution of contractional structures related to gravitational collapse. Figure 9. Illustrations of conjugate shear structures resulting from altering the orientations of σ 2 and σ 3 while maintaining a constant σ 1. The top illustration is exemplified at the majority of stations; the bottom illustration is exemplified at station 5. west of Bryce Canyon National Park. We interpret the orientation of the maximum compressive stress that caused these structures to be generally north-south, which corroborates their relationship to the generally south-directed Ruby s Inn Thrust Fault. We also interpret a correlation between the hoodoo formation and conjugate shear structure formation based on the relationship between shearing intensity and hoodoo development. Figure 10. (Upper Photo) A well-developed conjugate shear structure on a well-developed hoodoo. (Lower Photo) A poorly-developed shear structure on a poorly-developed hoodoo. The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 39

12 REFERENCES CITED Armstrong, R. L., Sevier orogenic belt in Nevada and Utah. Geological Society of America Bulletin, v. 79(4), p Best, M. G., & Grant, S. K., Stratigraphy of the volcanic Oligocene Needles Range Group in southwestern Utah and eastern Nevada. U.S. Geological Survey Professional Paper, p Borgia, A., Ferrari, L., & Pasquara, G. P., Importance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna. Nature, v. 357(6375), p Davis, G. H., Field Guide to Geologic Structures in the Bryce Canyon Region, Utah. American Association of Petroleum Geologists Hedberg Research Conference. Bryce Canyon: American Association of Petroleum Geologists, p Davis, G. H., & Krantz, R. W., Post- Laramide thrust faults in the Claron Formation, Bryce Canyon National Park, Utah. Geological Society of America Abstracts with Programs, v. 18(5), p. 98. Davis, G. H., & Pollock, G. L., Geology of Bryce Canyon Naional Park, Utah. In Geology of Utah's Parks and Monuments. Salt Lake City: Utah Geological Association and Bryce Canyon Natural History Association. p Davis, S. J., Mulch, A., Carroll, A. R., Horton, T. W., Chamberlain, C., & Chamberlain, C. P., Paleogene landscape evolution of the central North American Cordillera, Geological Society of America Bulletin, v. 121(1/2), p DeCelles, P. G., & Coogan, J. C., Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah. Geological Society of America Bulletin, v. 118(7/8), p Elliot, D. H., Judge, S. A., & Wilson, T. J., Road guide to the geology of the Sanpete Valley region, Sevier thrust belt, central Utah. Sevier thrust belt: northern and central Utah and adjacent areas. Utah Geological Association Publication, p GNU, OpenStereo General Public License version 3. Goldstrand, P. M., Stratigraphy and paleogeography of late Cretaceous and Paleogene rocks of southwest Utah. Utah Geological and Mineral Survey Miscellaneous Publications, v. 90(2). Hintze, L. F., Geologic history of Utah. Brigham Young University Geology Studies Special Publication, p Hintze, L. F., Willis, G. C., Laes, D. Y., Sprinkel, D. A., & Brown, K. D., Digital Geologic Map of Utah. Utah Geological Survey. Lundin, E. R., Thrusting of the Claron Formation, the Bryce Canyon region, Utah. The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 40

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