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Home , Aggradation, Coyote Gulch, Navajo Sandstone, Slot canyon

PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD IN THE , SOUTH-CENTRAL by David B. Loope1, Ronald J. Goble1, and Joel P. L. Johnson2

ABSTRACT

Within a four square kilometer portion of -Escalante National Monument, seven distinct slot canyons cut the . Four of the slots developed along separate reaches of a trunk (Dry Fork of Coyote ), and three (including canyons locally known as “Peekaboo” and “Spooky”) are at the distal ends of south-flowing drainages. All these slot canyons are examples of epigenetic gorges—bedrock reaches shifted laterally from previous reach locations. The previous channels became filled with , 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 ago, and that filling continued until at least 48,000 years ago. start to fill their channels when 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 ; (2) windier conditions increased the amount of eolian sand transport, perhaps destabilizing 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 cutting. This work illustrates a specific mechanism by which climate change can induce 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 and by flash These slot canyons are among the most-visited back- , slot canyons of the 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 . of walking, it is possible to see six different, broad canyons The drainage area of Dry Fork above the 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 - others, 1999). The Straight Cliffs escarpment, the north- ing. On maps and aerial photos, it becomes clear that each east margin of the (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 of Utah’s Far South: Utah Geological Association Publication 43, p. 11–24. 12 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. Figure 1. HikersinPeekabooSlot.Photo byJimElder. UGA Publication 43(2014) —Geology of Utah'sFar South 3203, 04

A PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – 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 CliŒs 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

5a Trunk #2 9a Brimstone Parking 11’07” W 111 11’07” 3096* *3097 “orphan” slot

Trunk #4 9b sandy wash subject to Loope, D.B., Goble, R.J., and Johnson, J.P.L 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 14 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. Figure 4. the seaofnakedNavajoSandstonebedrock(Jna). and sand, wind-blown of mantles alluvium, remaining of extent and canyons, sand-filled abandoned, and canyons slot of pairings Note 3. Figure ObliqueGoogle Earthimagelookingnorth. “P”markslocationofparking lot. Air photo looking downstream (eastward) above Dry Fork, with south-flowing : Peekaboo, Spooky, and Brimstone. Spooky, and Peekaboo, tributaries: south-flowing with Fork, Dry above (eastward) downstream looking photo Air UGA Publication 43(2014) —Geology of Utah'sFar South PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH –

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 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 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 16 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. at the southeast corner of our study area (sites 3338 and 3338 (sites area study our of corner southeast the at alcove an In developed. poorly very is bedding and sand, medium to fine by dominated is alluvium exposures, the of two but all 7B). At 3, (figures alluvium the covers sand localities (figure 2). At many sites, a mantle of wind-blown exposures. For OSL dating, we sampled alluvium at eleven unmapped small, many also are there but alluvium, Fork further fieldandlaboratorystudy. requires possibilities these Evaluating bedrock. weathered from sand the swept have may streams the Alternatively, streams. by dunes Pleistocene from reworked or streams into directly blown been have may It Formation). Entrada the possibly, and, Sandstone Navajo (the sandstones lian eo Jurassic from derived ultimately was sand this think Fork. Dry Weof headwaters the entered it before transport wind during sorted was that sediment of composed likely and JodiNorris. Figure 6. Williams (1984) mapped the largest expanses of Dry of expanses largest the mapped Williams(1984) Sooner Slide, a Holocene landslide at the base of the Straight Cliffs escarpment. See figure 2 for location. Photo by Jon Mason ALLUVIAL FILL - Dry Forkfrom thenorth. enter that 9C) (figure Brimstone and Spooky, Peekaboo, reworked by streams in the study area, especially those like actively being are dunes Modern alluvium. the within tion deposi eolian ancient indicating grainflows) lamination, (wind-ripple structures sedimentary no found we 8C), 2, wide (figures canyons within and uplands is both on today spread sand wind-blown Although lags. surface form top surface of the alluvium, and rhizolith fragments locally 1 centimeter in diameter are present within a paleosol at the to up Rhizoliths alluvium. the of portions upper the within present are (rhizoliths) cement calcite by preserved Ancient roots accumulating. was alluvium the as bedding nal tact withthemuchfiner, better-sorted alluvium. conglomerate with clasts up to 15 centimeters in diameter we found in erosive con 8C), (figure 3204 site sampling site. At this at alluvium the in present is cliff overhanging the from debris B). rockfall 8A, Angular (figures sand and silt of beds alternating of composed is alluvium the 3340), Rooted plants may have destroyed much of the origi the of much destroyed have may plants Rooted UGA Publication 43(2014) —Geology of Utah'sFar South - - - - PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L . Figure 7. Upper entrance to Spooky Slot. A. View looking east. Flow in wash is left to right; people are about to enter slot canyon. Slope on right is alluvium with thin mantle of windblown sand. B. View looking west. Flow in wash (and slot) is right to left.

MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 17 18 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. an OSL ageof~42.1ka;sampleincoarsermaterialat“O”(3204)returnedan ~39.8ka.Noteerosivecontact. of sequence. OSL sample 3340 (46.8 ka) was taken at “X.” Figure 8.

A . Thick sequence of interbedded silt and sand in an alcove near the mouth of trunk slot #4. C. Alluvium at sample site 3099-3204. Well-sorted sand (3099) at “X” returned UGA Publication 43(2014) B. Close-up of silty beds near base —Geology of Utah'sFar South PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L

Figure 9. Brimstone Canyon and its “orphan” slot. A. sand near the mouth of Brimstone Canyon. Person near dune crest for scale. . B. Mouth of the slot at its confluence with Dry Fork.C. Head of the slot canyon is at tip of upper arrow, 19 meters above the floor of Brim- stone Wash. Stream incised some of its alluvial fill and then cut the slot before abandoning the slot when it overflowed its (west) alluvial and reoccupied the broad canyon. It then rapidly excavated the remaining 19 meters of fill in that canyon, leaving the orphan slot high, dry, and open.

MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 19 20 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. Model (Galbraith andothers,1999)was moreappropriate. Minimum the and that Bailey indicated Age 2006) Arnold, of table decision distribution; (asymmetric distribution De Central the unless 1999) others, and (Galbraith Model Age D sample of Calculation criteria were discarded from the data set prior to averaging. these upon based unacceptable deemed feldspar.Aliquots detectable and 2011),Duller, and (Durcan component fast D dose, lent equiva the in error large (i.e. fits quality poor count-rate, insufficient for monitored were aliquots Individual 2008). (Rodnight, aliquots 50 of minimum a upon based are ages low saturation (D/D be were samples the whether determine to examined were were conducted (Murray and Wintle, 2003). Growth curves and Wintle, 2003). Dose-recovery and thermal transfer tests at the end of each SAR cycle for these old samples (Murray used was shinedown diode blue 280°C/40s a samples; est old the to doses regenerative the apply to used was 2005) (Bailey,irradiation Pulsed 280ºC. others, and Bailey 2004; selected based upon preheat plateau tests between 180º and were samples) (old 220°C/0s and 240°C/10s and samples) (young 180°C/0s and 200°C/10s of temperatures cutheat and Preheat used. was 2010) Wallinga, and Cunningham 2007; others, and (Ballarini subtraction background Early Wintle,2000). and (Murray technique (SAR) Dose erative Regen Aliquot Single the using diodes, infrared and blue with equipped TL/OSL-DA-20, and TL/OSL-DA-15B/C Models System Dating OSL Automated Riso on out ried techniques ofPrescottandHutton(1994). the using determined was dose-rate the to contribution mic cos The (1998). and of Aitken Adamiec method the using calculated were Dose-rates spectrometer. gamma olution using Silkospray. disks aluminum 1-centimeter-diameter of millimeters 5 or quartz grains were mounted on the innermost 2 millimeters etched The grains. feldspar residual remove to discarded fraction 90-micron the and resieved then was sample The 47%HCl. in 30minutes by followed acid, hyrdrofluoric 48% in minutes 75 for treated then solution, polytungstate sodium centimeter cubic per grams 2.7 a using flotation by remove organics. Quartz and feldspar grains were extracted (HCl) to remove carbonates and acid with hydrogen peroxide to hydrochloric with treated then and fraction, micron conditions. Samples were wet sieved to extract the 90-150- OSL SAMPLEPREPARATION/DOSE- Optically stimulated luminescence analyses were car high-res a using out carried were analyses Chemical Sample preparation was carried out under amber-light OPTICAL MEASUREMENTS RATE DETERMINATION e ), poor recycling ratio, strong medium versus medium strong ratio, recycling poor ), o < 2; Wintle and Murray, 2006). Optical e values was carried out using the using out carried was values ------ka), lie well below the elevation of the top of the alluvial the of top the of elevation the below well lie ka), samples, as well as samples 2896 (~44.4 ka) and 3096 (35.0 These UNL#3338). top, the (near ka 41.6 and UNL#3340) luvium at the alcove site are approximately 46.8 ka (lowest; al bedded superimposed from ka. Ages 39.8 about of age rived from the Straight Cliffs escarpment, and it returned an de clearly alluvium only the is 3099-3204 sites at surface erosional the above just lies that conglomerate The 2896. and 2895 sites to ka) 42.1 of age OSL an (with 3099 site of relationship the assess to difficult is it position, westerly cycle of alluviation was complete. Because of its upstream, earliest the when of estimate reliable most the provide 2) figure respectively; 3098, & ka 2895 UNL#’s (~49.6 ka; ~48.3 terrace and alluvial prominent a of surface upper the nearest samples two The time. this before well began 15 meters above the bed of Dry Fork, so alluviation clearly about lies 2) figure UNL#3097; ka; (~54.8 age oldest the ages of Holocene less than 3500 years (table returned 1). The sample returning samples three 3 remaining the ka; 30 were likely much more numerous there. Today,relatively there. numerous more much likely were cycles freeze-thaw cap, ice an develop to enough high not were CliffsStraight and Plateau Kaiparowits the Although plateaus. and mountains highest on formed Glaciers ing. heat summer diminished packs snow heavier and tinent, midcon the over pressure high of development stream blocked jet the of position southerly more a because teau Pla Colorado the reach to north enough far penetrate not did masses air tropical that suggest reconstructions mate Paleocli 2000). others, and (Anderson wetter and cooler 12,000 years ago), the climate of the was to (prior Pleistocene Late the During canyons. the through sediment of volumes huge move that discharges cre flood ate events precipitation These Plateau. Colorado the to rainfall heavy bring Monsoon the with sociated as storms Convective fall. and summer late the in region Plateau Colorado the into northward move masses air cal tropi maritime today’s climate, In channels. their fill then streams to undergo cycles in which they repeatedly cut and sion (figure 10). Earth’s constantly changing climate causes ero in results stream in increase an sediment); (deposit aggrade to stream a causes supply sediment the in supply with the water available to them. A relative increase cut andfill. of generations multiple of evidence field obvious no found we although cycle, alluvial earliest the of incision partial after renewed reflect possibly could and terrace RESULTS: TIMING OF ALLUVIATION Climate Change, Aggradation, andErosion Eleven samples returned Pleistocene ages greater than Streams adjust their profiles to carry the sediment carry to profiles their adjust Streams UGA Publication 43(2014) DISCUSSION —Geology of Utah'sFar South ------Table 1. PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Field and laboratory data, OSL results. Loope, D.B., Goble, R.J., and Johnson, J.P.L .

MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 21 22 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L.

the time were too cool for monsoonal storm activity, the activity, storm monsoonal for cool too were time the at temperatures Because supply. sediment their increasing streams, into ultimately and dunes into sand moved winds Strong bedrock. the from grains sand the swept then wind and wash Sheet sandstones. cemented weakly from sand more of release to led weathering enhanced that pothesize and cooler wetter, but also windier (Ellwein only and others, 2011). We not hy been have may climate ice-age The sand. of composed dominantly units from input more and units Cliffs Straight gravel-rich and clay- from input less suggesting sand, medium to fine of composed is alluvium the of bulk The beds. clay and cobbles, gravels, lack cally typi deposits alluvial the that indicate observations field mineralogy,our and abundances clay and sand of analysis Although we have not yet conducted a detailed stratigraphic CliffsStraight escarpment. the along exposed deposits vial eolian Entrada Formation) and not from the marine and flu from derived outcrops of the eolian Navajo Sandstone (and, possibly, dominantly the was canyons broad the buried that alluvium the that hypothesize 3). We (figure bedrock the from eroded sand wind-blown by buried partially and sandblasted are that domes and ridges “slickrock” naked forms Sandstone Navajo the Today, however. luviation, were dominantpriortoabout12,000yearsago. under the relatively wet sediment and non-monsoonal conditions that this of redistribution progressive from resulted reaches downstream in alluviation that possible escarp is It ment. Cliffs Straight the of slopes steep the from Fork Dry of headwaters the to delivery sediment more in sulted re have may rates weathering increased ago, years 60,000 and 50,000 between that, possible is It 2A). (figure spread wide more considerably are but prominent, less are flows features along this escarpment. Older landslides and debris prominent are flows debris and 6) (figure landslides fresh Blum from and Tornquist Modified (2000). true. is reverse the when incise and gime, when aggrade re discharge the of capacity transport Channels exceeds supply sediment supply. sediment and power stream between relationship the in changes emphasizing channels, vial 10. Figure S e d i We propose an alternate hypothesis for Dry Fork al Fork Dry for hypothesis alternate an Wepropose m e n Balance model for aggradation and incision of allu of incision and aggradation for model Balance t S u p p l y aggradation S t r ea m Pow e r ------ent field settings and described multiple gorges cutinto Moun along Henry TrailSandstone the Navajo in Canyon gorges multiple described and settings field ent differ from examples with formation gorge epigenetic of overview an provided (2008) others and .Ouimet or canyon previous the re-inhabiting and deposits solidated uncon only excavating simply than rather canyon former their of side-walls or spurs bedrock into down cut and tion alluvia of episodes during displaced laterally get channels of the cuttingofslotcanyonsinnewlocations. and fill sediment the of incision initiated and episode ing fill the terminated have basins. would drainage This small to power stream time-averaged in increase an and flooding increased brought have may change climate Subsequent sand. with filled being canyons to ultimately and time over aggradation gradual to leading today), do they (like Escalante River the to tributaries its and Fork Dry of channels the in deposited sand the of all move to able been have not small headwater streams (Etheredge and others, 2004) may important aspects of their geologic history: all three were three all history: geologic their of aspects important mechanism for streamaggradation. alluvial aggradation, rather than eolian deposition, was that the primary conclusively indicate here present we tions observa field subsequent the However, streams. trunk the blocking and migrating dunes sand to response in formed Slot Spooky and oth Slot Peekaboo and that suggested (2010) Johnson ers that note we Finally, channels. rower nar much of incision rapid and surface a slickrock over sloping path flow local steeper a to led then which walls, valley bedrock the in lows local through flow diverted tion aggrada valley that Slot, suggested (2010) Spooky others and and Johnson Slot Peekaboo of cases the In gorge). epigenetic an (forming bedrock or channel) an stream (maintaining older alluvium in occur can incision that and may occur wherever a channel reach happens to be located, that surprising be not should it Thus, month. single a in centimeters 40 by floor inner-channel bedrock a deepened runoff in snowmelt sustained Sandstone, reach Navajo channel steep particular a in that, showed tains flooding by Johnson and others (2010) in the Henry Moun and erosion of Monitoring events. flow during efficiently eroded be to scale grain the at enough weak nonetheless is it but landscape, dramatic a and cliffs high maintain to Mountains regions, the Navajo Sandstone is strong enough Henry and Escalante the In sediment. coarse of transport sion of bedrock may be more efficient than entrainment and ero stream situation, this In boulders. and cobbles coarser the of composed bed win a generating sediment, preferentially fine the now may action stream debris, landslide coarse from aggradation earlier by influenced is formation gorgeUtah. epigenetic south-central Where of region tains epigenetic gorges epigenetic h Dy ok lt ayn ae xeln examples excellent are canyons slot Fork Dry The ekbo Sok, n Bisoe ayn share Canyons Brimstone and Spooky, Peekaboo, UGA Publication 43(2014) Epigenetic Gorges (figure 11). These form when stream when form These 11). (figure —Geology of Utah'sFar South ------A B PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH –

1

C 2

3

4

Figure 11. Epigenetic gorges. See figure 2 for locations of photos. A. Stages of evolution of an epigenetic gorge (from Ouimet and others, 2008). B. Upstream entrance to trunk slot #3; C. Upstream entrance to trunk slot #4. In both B and C, the sediment-filled, broader canyon lies to the right (south) of the slot canyon. buried by alluvium, and, when removal of that alluvium windy. Cooler temperatures shut down the Arizona mon- was underway, all three developed slot canyons near their soon. Cooler, wetter conditions generally increase vegeta- junctures with Dry Fork. Today, Peekaboo Slot and Spooky tive cover, thereby stabilizing dune deposits (Hugenholtz Slot continue to carry floodwaters; the lowermost portions and Wolfe, 2005), but because a large portion of the Dry of their buried canyons remain sand-filled. Brimstone’s Fork drainage is a sea of bedrock, cooler and wetter condi- buried course, however, has been fully excavated (figure tions may not have greatly increased vegetative cover. We 9). The Brimstone slot canyon is an orphan—it is without hypothesize that more frequent freeze-dry cycles enhanced a large catchment basin, and its head is atop a high cliff, weathering of the Navajo and Entrada Sandstone and overlooking Brimstone’s broad canyon floor (figure 9). The stronger winds delivered more sand to headwater streams. stream that cut this slot escaped from its bedrock walls by Low-discharge streams laden with this sand buried canyons overflowing its alluvial bank. It later excavated a course that had previously been cut by more powerful streams. through the still-deeply-buried canyon, clearing out its fill When stream power increased relative to sediment supply, and leaving the slot high and dry. the stored sediment in Dry Fork was flushed downstream, Loope, D.B., Goble, R.J., and Johnson, J.P.L and seven new slot canyons were cut. CONCLUSION ACKNOWLEDGMENTS The slot canyons of Dry Fork are epigenetic gorges that developed during incision of thick alluvium that had We thank Joe Mason, Walter Loope, and Jackie Hun- buried older, broader canyons. We present new OSL dat- toon for their helpful reviews of the paper. Derek Burgess, ing of alluvium associated with slot-canyon incision, sug- Jim Elder, Cindy Loope, and Shirley Yik assisted with gesting alluviation occurred around 50,000 years ago. We fieldwork; Jon Mason, Jodi Norris, and Geoff Debenedetto suggest hypothetical linkages between climate, flood inten- helped with aerial photography. Kevin Phillips and Carolyn . sity, sediment supply, and channel incision. Previous work Shelton of Grand Staircase-Escalante National Monument on Colorado Plateau paleoclimate suggests that roughly provided encouragement and helpful suggestions through- 48,000 to 55,000 years ago, the climate was cool, wet, and out the duration of the study.

MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors 23 24 PRELUDE TO SEVEN SLOTS: FILLING AND SUBSEQUENT MODIFICATION OF SEVEN BROAD CANYONS IN THE NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH – Loope, D.B., Goble, R.J., and Johnson, J.P.L. Bailey, S.J., R.M., and Stokes, Armitage, S., 2005, investiga An in absorption dose of 1—simulation Paper 2004, R.M., Bailey, and R.H., Hevly,J.I., Mead, J.L., Betancourt, R.S., Anderson, fac conversion Dose-rate 1998, M., Aitken, and G., Adamiec, teeg, . 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