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 Volume 84 Issue 1 Article 5 1-6-2012 Extent and Mechanism of Footwall Shear Adjacent to the Ruby's Inn Thrust Fault, Southern Utah Skyler B. May Southern Utah University, [email protected] Roger E. Leavitt Southern Utah University, [email protected] John S. MacLean Southern Utah University, [email protected] Follow this and additional works at: https://digitalcommons.csbsju.edu/compass 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: 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 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 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 orogenies, 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 The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 30 during the Mesozoic and early Cenozoic attributed the thrust to the gravitational Sevier Orogeny and the subsequent collapse of the Marysvale Volcanic Laramide Orogeny. 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 Cretaceous 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). The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 31 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 Jurassic 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 late Cretaceous, 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 foreland basin 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 Paleozoic, Utah was at the compression from the west during the western edge of North America. The Eocene led to thick-skinned deformation of The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 32 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 Miocene (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). The Compass: Earth Science Journal of sigma Gamma Epsilon, v. 84(1), 2012 Page 33 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

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