PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH by David B. Loope1, Ronald J. Goble1, and Joel P. L. Johnson2 ABSTRACT Within a four square kilometer portion of Grand Staircase-Escalante National Monument, seven distinct slot canyons cut the Jurassic Navajo Sandstone. Four of the slots developed along separate reaches of a trunk stream (Dry Fork of Coyote Gulch), and three (including canyons locally known as “Peekaboo” and “Spooky”) are at the distal ends of south-flowing tributary drainages. All these slot canyons are examples of epigenetic gorges—bedrock channel reaches shifted laterally from previous reach locations. The previous channels became filled with alluvium, allowing active channels to shift laterally in places and to subsequently re-incise through bedrock elsewhere. New evidence, based on optically stimulated luminescence (OSL) ages, indicates that this thick alluvium started to fill broad, pre-existing, bedrock canyons before 55,000 years ago, and that filling continued until at least 48,000 years ago. Streams start to fill their channels when sediment supply increases relative to stream power. The following conditions favored alluviation in the study area: (1) a cooler, wetter climate increased the rate of mass wasting along the Straight Cliffs (the headwaters of Dry Fork) and the rate of weathering of the broad outcrops of Navajo and Entrada Sandstone; (2) windier conditions increased the amount of eolian sand transport, perhaps destabilizing dunes and moving their stored sediment into stream channels; and (3) southward migration of the jet stream dimin- ished the frequency and severity of convective storms. We hypothesize that a subsequent increase in the frequency of intense runoff events after 48 ka, combined with the diversion of flow over steep but unchannelized bedrock surfaces, led to a brief and unusual episode of rapid canyon cutting. This work illustrates a specific mechanism by which climate change can induce river incision, and conversely how information on climate may be recorded in the morphology of erosional landscapes. INTRODUCTION a silt line left by floodwaters that was positioned 4.5 meters above the streambed. Carved into thick sandstones and limestones by flash These slot canyons are among the most-visited back- floods, slot canyons of the Colorado Plateau are beautiful, country sites in Grand Staircase-Escalante National Monu- dangerous, and popular. The slot canyons of our study area ment, and hikers sometimes become disoriented by this (figures 1, 2) are cut into the Jurassic Navajo Sandstone landscape with “too many canyons." With only a few hours and lie along Dry Fork—a tributary of the Escalante River. of walking, it is possible to see six different, broad canyons The drainage area of Dry Fork above the confluence with without a sizable, active stream as well as six short, narrow Spooky Slot (figure 2) is 65 square kilometers (Wohl and slots that show clear evidence of large-scale, recent flood- others, 1999). The Straight Cliffs escarpment, the north- ing. On maps and aerial photos, it becomes clear that each east margin of the Kaiparowits Plateau (figure 2A), is 2200 of these six slot canyons is paired with a broad, abandoned meters above sea level and forms the southwestern drain- canyon (figures 2C, 3, 4, 5). A seventh slot canyon (just age divide of Dry Fork, which lies 700 meters below that east of Brimstone Canyon, figures 2C, 4), is an “orphan.” escarpment. Peekaboo Slot and Spooky Slot (figure 2) have It is no longer connected to the stream that cut it, and its drainage areas of less than 10 square kilometers. Even in adjacent “parent” canyon has no thick alluvium and floods these small drainages, runoff from bedrock during heavy regularly. rainstorms generates violent floods. From Dry Fork slot Van Williams (1984) of the U.S. Geological Survey (trunk slot #1 in figure 2), Wohl and others (1999) reported was the first to study these canyons. He recognized two 1Department of Earth & Atmospheric Sciences, University of Nebraska, Loope, D.B., Goble, R.J., and Johnson, J.P.L., 2014, Prelude to seven slots—filling Lincoln, NE and subsequent modification of seven broad canyons in the Navajo Sandstone, 2The University of Texas at Austin, Department of Geological Sciences, south-central Utah, in MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors, Jackson School of Geosciences, Austin, TX Geology of Utah’s Far South: Utah Geological Association Publication 43, p. 11–24. Loope, D.B., Goble, R.J., and Johnson, J.P.L. Johnson, and R.J., Goble, D.B., Loope, Figure 1. Hikers in Peekaboo Slot. Photo by Jim Elder. PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – – UTAH SOUTH-CENTRAL SANDSTONE, NAVAJO THE IN CANYONS BROAD SEVEN OF MODIFICATION SUBSEQUENT AND FILLING SLOTS: SEVEN TO PRELUDE 12 UGA Publication 43 (2014)—Geology of Utah's Far South 3203, 04 A – UTAH SOUTH-CENTRAL SANDSTONE, NAVAJO THE IN CANYONS BROAD SEVEN OF MODIFICATION SUBSEQUENT AND FILLING SLOTS: SEVEN TO PRELUDE 3099 study area N 8c ** recent landslides 3 Dr y F older landslides ork debris ow colluvium Big Hollow Wash and alluvium Coyote OSL sample* 4 gure in text 0 5 10 H Gulch o l e- kilometers in - t h e- Ro ck B ID R WY o ad N Straight Clis NV 6 UT CO study area * pre-55,000 yrs AZ present 37 29’ 20” N 4 C 7a 7b 3098 Peekaboo * 2896 Spooky * N 2895 * 5b Trunk #1 11b Trunk #3 9c Trunk #2 5a 9a W 11’07” Brimstone Parking 111 3096* *3097 “orphan” slot Trunk #4 9b sandy wash subject to J.P.L Johnson, and R.J., Goble, D.B., Loope, ooding 11c slot canyon * 3415 abandoned no ooding water course 8ab abandoned (“orphan”) slot 0 0.5 1.0 ** ow direction of oods 3340 kilometers 3338 OSL sample* 3 gure in text . Figure 2. Maps of the study area, showing locations of sample sites and figures. A. Surficial geologic map of Williams (1985), showing setting of study area. B. Configuration of drainages prior to alluviation. C. Present configuration of slot canyons and abandoned water- courses. MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 13 Loope, D.B., Goble, R.J., and Johnson, J.P.L. Johnson, and R.J., Goble, D.B., Loope, Figure 3. Air photo looking downstream (eastward) above Dry Fork, with south-flowing tributaries: Peekaboo, Spooky, and Brimstone. Note pairings of slot canyons and abandoned, sand-filled canyons, and extent of remaining alluvium, mantles of wind-blown sand, and the sea of naked Navajo Sandstone bedrock (Jna). PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – – UTAH SOUTH-CENTRAL SANDSTONE, NAVAJO THE IN CANYONS BROAD SEVEN OF MODIFICATION SUBSEQUENT AND FILLING SLOTS: SEVEN TO PRELUDE Figure 4. Oblique Google Earth image looking north. “P” marks location of parking lot. 14 UGA Publication 43 (2014)—Geology of Utah's Far South PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – – UTAH SOUTH-CENTRAL SANDSTONE, NAVAJO THE IN CANYONS BROAD SEVEN OF MODIFICATION SUBSEQUENT AND FILLING SLOTS: SEVEN TO PRELUDE Figure 5. Alluvial fills and slot canyons. White arrows show people for scale. See figure 2 for locations. A. Northward view of thick alluvial fill of the trunk of Dry Fork #1 as seen from entry trail. People are about to enter trunk slot #1. B. Northward view of the alluvium-filled former course of Peekaboo and of Peekaboo Slot at its junction with Dry Fork. Loope, D.B., Goble, R.J., and Johnson, J.P.L Johnson, and R.J., Goble, D.B., Loope, important things: (1) the slot canyons formed during epi- Boison and Patton (1985) hypothesized that Dry Fork sodes of rapid down-cutting by Dry Fork into its own thick, received less of the alluvium because it, unlike Big Hollow sandy alluvium; and (2) filling of the broad canyons could (figure 2A), did not receive landslide debris in recent (late have resulted from a Pleistocene increase in landslides and Holocene) times. These geologists realized that Dry Fork, debris flows in the steep tributaries that drain the Straight during an earlier episode of alluviation (too old for radio- Cliffs escarpment (figure 6). His radiocarbon dates showed carbon dating), had filled to a much greater depth than 10 that the most recent episode of canyon filling (alluviation) meters. In this preliminary report of our work at Dry Fork, in Dry Fork and nearby Big Hollow is Holocene, and be- we present optically stimulated luminescence (OSL) ages gan about 2500 years ago and ended prior to 730 years ago from alluvium that now lies as much as 40 meters above . (Williams, 1984; Boison and Patton, 1985). Alluvium that the floor of Dry Fork, and we tentatively hypothesize that, now lies as much as 10 meters above the modern channel although some of alluvium may have been sourced from floor records that episode. Pleistocene landslides, much of the ancient alluvium is MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 15 Loope, D.B., Goble, R.J., and Johnson, J.P.L. Johnson, and R.J., Goble, D.B., Loope, Figure 6. Sooner Slide, a Holocene landslide at the base of the Straight Cliffs escarpment. See figure 2 for location. Photo by Jon Mason and Jodi Norris. likely composed of sediment that was sorted during wind 3340), the alluvium is composed of alternating beds of silt transport before it entered the headwaters of Dry Fork.