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The Compass: Earth Science Journal of Sigma Gamma Epsilon

Volume 84 Issue 1 Article 5

1-6-2012

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

Skyler B. May Southern Utah University, [email protected]

Roger E. Leavitt Southern Utah University, [email protected]

John S. MacLean Southern Utah University, [email protected]

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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 , Southern Utah," The Compass: Earth Science Journal of Sigma Gamma Epsilon: Vol. 84: Iss. 1, Article 5. Available at: https://digitalcommons.csbsju.edu/compass/vol84/iss1/5

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 editor of DigitalCommons@CSB/SJU. For more information, please contact [email protected].

EXTENT AND MECHANISM OF FOOTWALL SHEAR ADJACENT TO THE RUBY'S INN THRUST FAULT, SOUTHERN UTAH

Skyler B. May, Roger E. Leavitt, and John S. MacLean Department of Physical Science Southern Utah University 351 W. University Blvd. Cedar City, UT 84720 [email protected]

ABSTRACT

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 east- directed contractional structures of the Sevier . The Paleocene to 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 though these mountain building episodes demonstrate different modes of folding and Utah’s complex geologic history has faulting, both typically show significant included eperic seas, extensional basins, and east-directed contraction, and evidence of significant orogenic events. The most this thrusting can be observed throughout recognized of these , the Sevier the central portion of the state. Southern and Laramide, were part of the building of Utah, in particular, has many outcrops that the North American Cordillera. Even demonstrate the east-directed thrust faulting

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during the and early Cenozoic attributed the thrust to the gravitational Sevier Orogeny and the subsequent collapse of the Marysvale Volcanic . Complex based on age relationships The Ruby’s Inn Thrust Fault, located (Lundin, 1989; Merle et al., 1993). Davis just north of Bryce Canyon National Park, is (1997) also related conjugate shear a rare south-directed structure found on the structures in Bryce Canyon’s hoodoos to the Paunsaugunt Plateau that thrusts thrusting and suggested a possible Straight Cliffs Formation over Paleocene- relationship between these structures and the Eocene Claron Formation (fig.1). In the late formation of the hoodoos (fig. 2). 1980s and early 1990s, researchers

Figure 1. Geologic map of Bryce Canyon/Marysvale (modified from Hintze et al, 2000).

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

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the Laramide Orogeny (Reading et al., continues today in the western part of the 1998). Several phases of volcanics and state (Reading et al., 1998; Wright, 1987; Basin and Range extension began in the Sahagian et al., 2002). Oligocene and continued through the (Reading et al., 1998). Extension

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).

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

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early stages of Laramide deformation volcanic field (30-20 Ma; Steven et al., temporally overlapped the late stages of 1984), which is first exposed less than 20 Sevier deformation (Elliot, 2011; Yonkee & kilometers to the north. This interpretation Weil, 2011). Figure 5 shows the affected 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 south- directed 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 area of the Sevier Orogeny in Utah. structures are apparent throughout the extent Figure 5. Generalized map of Sevier Orogeny of the thrust (Fig. 6), cross-cutting deformation extent (simplified from DeCelles lithologies and often showing offset. The and Coogan, 2004). structural planes of the conjugate shear structures display very well developed The Ruby's Inn Thrust demonstrates slickensides and slickenlines. These significant south-directed displacement and conjugate structures range from a few many associated footwall structures that centimeters to 10s of meters across. There have not been interpreted as a product of the are also pervasive conjugate-thrust Sevier or Laramide Orogenies. Researchers deformation bands exposed in the middle in the late 1980s and 1990s (Davis and Cretaceous Straight Cliffs Formation within Krantz, 1986; Lundin, 1987, 1989; Lundin the hanging wall (Davis, 1997). Davis and Davis, 1987; Merle et al., 1993; Davis, observed the presence of these vertical fault 1997) interpreted the timing of the south- surfaces and conjugate shear structures directed thrusts and fault-propagation folds within the hoodoos and spires of the Claron associated with the Ruby’s Inn Thrust Formation of Bryce Canyon (Fig. 2) and system as the result of gravitational postulated a correlation between the spreading and collapse of the Marysvale

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structures and the development of the METHODS landforms. 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 Figure 6. Representative conjugate shear shear planes, as well as plunges and bearings structure in the Claron Formation. of slickenlines. Sterographs and rose diagrams were produced from each station

Figure 7. Map of field area showing stations.

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collected by OpenStereo software (GNU, complicated conjugate shear structures and 2011). Using Trimble’s Pathfinder Office beautifully exposed slickensides with deep software (Trimble, 2011) we differentially slickenlines that show three distinct corrected the GPS points and exported them orientations of slip; approx. 000°, approx. into the ArcGIS Desktop software. Using 045°, and approx. 315°. To the south of GIS, we produced a map that geographically station 5, evidence of shearing gradually displays our stations and graphically shows diminishes until it ends approximately 40 the measured orientations using the km from the main thrust fault. Interestingly, stereographs and rose diagrams. some southern stations initially showed little evidence of conjugate shear structures, but RESULTS gentle hammering on the outcrop caused the seemingly undeformed, homogenous The northern extent of the shear zone limestone to fracture into perfect, large associated with the Rubyʼs Inn Thrust conjugate sets. These beautiful conjugate begins just north of the main thrusts on the shear structures are either underdeveloped hanging wall and continues to the south. surfaces, or they were hidden beneath The thrust system spans a 29 kilometer N-S deposition of a thin layer of mud. The width near the northern boundary of Bryce former appears to generally decrease in Canyon National Park. Shearing intensifies intensity and prevalence of structures to the slowly from the main thrust at the northern south. end of our traverse to a maximum intensity The hoodoos of the region appear to have at 13 kilometers south of the thrust, where it some form of structural fabric indicative of then slowly diminishes until an abrupt end shearing within them. On our north-south approximately 29 kilometers south of the transect, very few hoodoos did not show thrust (Fig.7). some sort of conjugate shear set. The The main thrust exhibits large, well hoodoos to the north are tall, slender, and defined fault scarps and large slickenlines well defined, while the hoodoos to the south and grooves on the hanging wall. To the are less well developed (Fig. 9). south of the main fault, increased conjugate shear structures become evident. Although DISCUSSION there are many hoodoos that had substantial shearing and conjugate sets, access to the The Rubyʼs Inn Thrust Fault and the sheared surfaces varied, so measurements footwall shear structures appear to be related are of the most easily accessed outcrops in because of the similarities in strike and the study area. Figure 8 shows orientations slickenline orientation. The dominant north- of structures from several stations. As seen south orientation of slickenlines, with the in figure 8, the dominant strike orientation exception of station 5, is generally on the conjugate shear surfaces is east-west. perpendicular to the strike of the shear Intensity increases from the north to the planes. Considering the orientation of south until station 5, Badger Creek, where slickenlines and the bisection of the acute several outcrops display extremely

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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 are tall, slender, and isolated, while the interpret a similar north-south orientation of hoodoos to the south become more massive

σ1. The complicated structural geometry at and consolidated. Figure 10 shows a station 5 could be explained by reversing the representative outcrop from the central orientations of the intermediate and region of the transect showing defined minimum compressive stresses, σ2 and σ3, hoodoos and conjugate features compared to while maintaining a constant north-south a representative southern outcrop that lacks orientation of σ1 (Fig. 9). This scenario distinct structural features. could occur if σ2 and σ3 forces were nearly equivalent. As discussed earlier, our north- CONCLUSIONS south transect revealed very few hoodoos that did not express some sort of conjugate Using the structural relationships, we shear structures. Just as the conjugate shear determined an approximately 40-km north- structures diminish in intensity from the south extent of shearing in the footwall of north to the south, the hoodoos in the north the Ruby’s Inn Thrust Fault directly to the

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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 σ2 and σ3 while maintaining a constant σ1. 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 Figure 10. (Upper Photo) A well-developed and hoodoo development. conjugate shear structure on a well-developed hoodoo. (Lower Photo) A poorly-developed shear structure on a poorly-developed hoodoo.

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