USE OF PALYNOLOGY TO COMPLEMENT THE ANALYSIS AND DATING OF MEGALANDSLIDES IN THE COLORADO PLATEAU Conor M. Watkins, J. David Rogers, & Francisca Oboh-Ikuenobe

University of Missouri - Rolla (e-mail: cwatkin@umr.edu) University of Missouri - Rolla (e-mail: rogersda@umr.edu) University of Missouri - Rolla (e-mail: ikuenobe@umr.edu)

Abstract: The Colorado Plateau (CP) region of the United States contains numerous examples of surface drainages that have been disrupted by megalandslides. Two types of drainage disruption due to landslippage have been recognized in the CP; the damming of surface streams and the formation of closed basins within headscarp grabens. These traps tend to accumulate lacustrine, aeolian, debris flows/fans, and colluvial materials, often eroded from the headscarps.

We recently employed palynology, the study of pollen, spores, and other organic walled

microfossils, on sediments trapped within the headscarp graben of the Surprise Valley Landslide Complex in an attempt to determine the paleoclimate when the megalandslide occurred. Most sediments sampled were reddish, indurated shallow lacustrine deposits. Previous researchers have assumed that sediments in the arid Southwest have been oxidized, destroying the palynomorphs. Sampling and analysis has yielded palynomorphs describing both paleo and recent environments.

Landslides are inherently difficult to date because secondary movements disrupt the processes

required for accurate dating using direct methods, resulting in biases towards more recent secondary slide events. The analysis of local sediment trapped by landslides provides an additional tool to indirectly date sequences of landslippage dating back tens, or even hundreds of thousands of years. Identification of palynomorphs complements dates obtained using other methods by providing insights into paleoenvironment/paleoclimate during each movement sequence. INTRODUCTION

The Colorado Plateau region of the southwestern United States contains areas of sharp topographic relief, often experiencing elevation changes of 1,000 meters or greater over short distances. Much of this extreme relief has been created by incision of the Colorado River and its tributaries as the surrounding plateaus were elevated during the last four Ma. Canyon cutting releases the horizontal stresses locked into the bedrock by removing lateral support within the thick sequences of strata. Overlying strata have also been eroded from the region, resulting in a high degree of overconsolidation in the remaining bedrock materials, especially shale. Shale materials tend to exhibit plastic creep as it is unloaded. Overconsolidated shales also tend to dilate and lose shear strength as they undergo concentrated strain. It has recently been demonstrated that shales can transition to residual strengths via plastic creep without ever being loaded to peak strengths (Mesri and Shahien, 2003).

The Colorado Plateau is comprised of relatively flat lying alternating sedimentary sequences,

dominated by sandstone, limestone, siltstone and shale. Incision of the plateau has created deep

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canyons, especially where massive units are exposed. Some of the most prominent erosional escarpments include those formed by the Navajo Sandstone in Zion Park and the Redwall Limestone in Grand Canyon. As competent units are incised into underlying shales, these softer formations are subject to plastic creep. River anticlines parallel many of the deepest channels where these floor in overconsolidated shales, allowing dilation of pre-existing systematic regional joints. This dilation is most obvious near cliff faces, where more brittle strata lying above the shales exhibit evidence of lateral translation towards the incised canyons. This is especially noticeable in the central Grand Canyon, as described by Huntoon and Elston (1979). The volumetric strain associated with such shale creep usually results in a marked loss of shear strength through the process of strain softening (Trollope, 1973), often spawning landslides. This phenomenon is common in the Bright Angel, Chinle, Mancos, and other shales in the region. Rogers and Pyles (1980) presented test data that suggested the micaceous Bright Angel Shale lost two-thirds of its mobilized shear strength through saturation.

Numerous composite megalandslides (defined as landslides having a total volume of at least

10 million cubic meters) have formed due to suspected strain softening in shales. The majority of the landslides in question appear to be prehistoric, likely, middle to late Pleistocene in age (< 700 ka). In most cases, surface drainage has been re-established across landslide surfaces and the original landslide mass appears dormant. Other factors, which are presently not active, in addition to strain softening, may have contributed to the genesis of these landslides as very few slides appear to have been reactivated during the Holocene (<11 ka).

Landslides in arid to semi arid climes are inherently hard to date using direct methods, such as

cosmogenic dating (Nichols et. al, 2006) or lichenometry. Landslide complexes often remain active and experience subsequent movements long after their initial failure. Dating of secondary movements within a landslide complex results in the dating of that movement, not the initial failure, thus giving a young bias to any dates. On the other hand, landslide surfaces may also consist of material exposed long before movement, yielding an old age bias. Lichenometry involves the dating of rockfall, landslide, or earthquake events based on the size of lichens growing on rocks, which is calibrated through established growth rates established for a particular region. Lichenometry also requires features be undisturbed and it has an effective use to around 500 years before present (Bull and Brandon, 1998), limiting its use to the dating of relatively recent features.

A large number of megalandslides of the Colorado Plateau region have disrupted surface

drainages, forming temporary impoundments behind landslide dams and within headscarp grabens on the many of the larger landslides. Some impoundments persisted long enough to capture fine grained lacustrine sediments, often interfingered with debris flow deposits and colluvial talus, depending on proximity to exposed headscarps. Due to their considerable age (> 20 ka), the majority of these closed depressions have either breached or been dissected via headward erosion of local ephemeral watercourses. Channels are often incised through the accumulated materials after breachment, exposing thick sequences of post-slide sediment. The headscarp graben of the Deer Creek Landslide Complex in Grand Canyon is a closed basin about 320 m by 800 m which is still accumulating sediments, as shown in Fig. 1.

Another indirect method to date periodic episodes of landsliding is to analyze sediments trapped by the initial slide event. Not all landslides in the Colorado Plateau form impoundments

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so those locations with suitable conditions must be identified. Ideally, these impoundments have been breached and incised by erosion, exposing the full sequence of sediments without their being obscured or covered by talus. Full exposure of these deposits allows recovery of lacustrine samples near the lowest horizons of the graben fill, which would have been deposited soon after the initial or parent landslide occurred (some of these graben fills can be > 100 m thick). If one cannot access the lowest horizons, we might assume the impoundment filled and breached in a relatively short period, geologically speaking, and any horizon might yield a relatively accurate date, within some band of time. Dates for these kinds of materials are usually determined using optically stimulated luminescence (OSL) methods.

In addition to providing potentially datable materials, temporary impoundments capture

palynomorphs, organic walled microfossils including pollens and spores, which can be indicative of climatic conditions at the time of failure, up through the present. The Colorado Plateau region experienced a cooler and more humid climate during the glacial periods of the Pleistocene, resulting in a greater amount of precipitation throughout this presently arid and semi-arid region (Anderson et. al, 2000). Increased precipitation would elevate groundwater levels throughout the region, resulting in reduced slope stability near the edges of escarpments. Ahnert (1960) was one of the earliest workers who described the mass wasting of the Colorado Plateau and opined that their character reflects formation during a wetter paleoenvironment.

Palynology has been sparingly employed to evaluate timing and paleoclimates of large

landslide complexes. Adam (1975) conducted a successful study of the Weeks Creek Landslide near San Francisco, CA using carbon 14 dating and palynology. The record, which was recovered by coring through sediments accumulated in the slide’s headscarp graben, was surprisingly complete for the last 3,000 years, with the exception of the the last 100 years when humans introduced cattle grazing in the area.

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Figure 1. The main headscarp graben of the Deer Creek Landslide, located to the west of Surprise Valley, is filled with fine-grained lacustrine deposits interfingered with colluvial talus. It has not yet breached and continues to accumulate sediments. USE OF PALYNOLOGY TO COMPLEMENT THE ANALYSIS AND DATING OF MEGALANDSLIDES

Prior researchers commonly believed palynology, the study of organic walled microfossils, to be of limited use in the hot, arid climate of the southwestern U.S. It was assumed that the dry environment allowed for the desiccation of sediments and oxidation of any palynomorphs, making the method unsuitable for most sites in the region (Oboh-Ikuenobe, 2003). The sediments in question are often red in color, reinforcing the belief that they have been oxidized, thus destroying any entrained palynomorphs.

Since the parent materials in the region are often red, we wondered if significant oxidation

might not have occurred since the re-deposition of the sediments in landslide related features. Sediments were recovered from several locations throughout the Grand Canyon and Vermilion Cliffs for palymorph identification. The first location selected was in central Surprise Valley, an old landslide headscarp graben in the central Grand Canyon.

The Surprise Valley Landslide Complex is the oldest and most recognized landslide complex

in the central Grand Canyon. This slide complex was initially described by Ford et al (1974) and Huntoon (1975). It has been estimated to have an age of between 574,000 to 1,136,000 years based on minimum and maximum rates of incision within this portion of Grand Canyon (Savage, 2002). This landslide consists of multiple translated and backrotated blocks and has over 600 meters of relief. The basal slip surface of the landslide floors in the Cambrian age Bright Angel Shale while the headscarp progresses upwards through the Muav, Temple Butte, Redwall, and Supai Formations along a series of regional systematic joints. Prior to its dissection by stream incision, this complex included a closed basin measuring roughly 1,000 by 2000 m developed within a massive headscarp graben. The basin allowed for the accumulation of an extensive

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deposit of fine-grained, reddish lacustrine sediments in what is now central Surprise Valley. Headward erosion from Bonita Creek, a tributary of the Colorado River, has breached the basin and excavated a channel through the thick sequence of well indurated sediments. Rogers and Pyles (1980) were the first to recognize and map the areal extent of these lacustrine materials. This graben is of such size that sediment layers visibly dip towards the center of the basin (Fig. 2). A typical exposure of these dissected lacustrine sediments is shown in Fig. 3.

Samples recovered from the central Surprise Valley contained palynomorphs including 18

pollens and spores, freshwater algae, and fungal hyphae. Palynomorphs from the Surprise Valley sample are listed in Table 1. Of particular interest is pollen from Picea (large grain), a species of spruce that is presently restricted to colder, high-latitude tundra environments. The presence of this palynomorph indicates the climate in the region was much cooler than present immediately after the Surprise Valley slide event. Adam (1975) believes that pollens recovered in sediment cores generally do not blow in from far away, indicating that this palynomorph is locally derived. Such information provides insights into when and why this landslide occurred as cold weather species thrived at much lower latitudes during the glacial maxima of the Pleistocene. The Colorado Plateau experienced a more humid climate during glacial periods, a factor that may have played a major role in the genesis of such large landslides throughout the region.

Table 1. Palynomorphs within the central Surprise Valley deposit: Three groups of palynomorphs are present: 1.) Pollen and spores:

Ephedra fragilis (Ephedraceae)

Sequioia (Taxodiaceae)

Ambrosia (ragweed) Equisetum Solaniaceae Anemopsis californica (Saururaceae)

Mitella? (Saxifragaceae) Taxodium and one unidentified genus of Taxodiaceae

Annoniaceae? Picea (spruce) large grain 2.) Freshwater algae: Bryophyte spore Pinus (pine) Botryococcus and algal

clusters are the dominant palynomorphs in both samples.

Chrysoplenium (dominant pollen)

Podocarpaceae (specimens similar to fossil Zonalapollenites)

3.) Fungi:

Cruciferaceae Quercus (oak) Fungal hyphae are present. Dodocatheon? (Primulaceae)

Saxifraga (Saxifragaceae)

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Figure 2. The main headscarp graben of the Surprise Valley Landslide Complex formed a closed basin that accumulated a ~40 m thick sequence of fine grained lacustrine sediments until it was breached by the headward erosion of Bonita Creek. These sediment beds gently dip towards the depoaxis of the old graben.

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Figure 3. Reddish lacustrine sediments were deposited in the main headscarp graben of the Surprise Valley Landslide Complex when this feature was a closed basin. Headward erosion of Bonita Creek subsequently breached the depression and excavated a channel, exposing tens of meters of fine grained sediment. Samples recovered here contained 18 pollens and spores, one freshwater algae, and fungal hyphae.

Samples were also recovered from the headscarp graben of the nearby Deer Creek Landslide Complex, which is located ~4 km west of Surprise Valley, for palynomorph identification. This closed basin has yet to breach and continues to accumulate fine-grained lacustrine sediments and colluvial talus raveling from the steep headscarp (Fig 1). Since this deposit is not incised, one can only sample the uppermost (recent) portions of the material to a depth of around 2 meters, using a hand sampler (Fig. 4). Palynomorphs included numerous species from the genus Ephedra, which is not surprising given that Ephedra is one of the most common palynomorphs in Holocene Grand Canyon sediments. It is possible that older deposits with a different assemblage of diagnostic palynomorphs are present at depths presently inaccessible due to the limitation of using hand sampling methods.

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Figure 4. Hand sampling and analysis of the reddish lacustrine sediments filling the main headscarp graben of the Deer Creek Landslide revealed a modern assemblage of palynomorphs. Older palynomorphs diagnostic of the climate when the landslide occurred are likely buried deeper but are not accessible via hand sampling. The depth of this deposit is unknown, but limiting geometry suggests a depth approaching 100 m.

The Deer Creek landslide also appears to have temporarily dammed the Colorado River, resulting in the diversion of the channel to the south of the modern landslide, just downstream from the mouth of Deer Creek. This dam appears to have impounded a sizable lake, resulting in the accumulation of at least 52 m of fine grained lacustrine sediment (Fig. 5) which appear to have been deposited in a reducing environment (Webb et. al, 2005). These buff colored fine grained deposits are located on the south side of the Colorado River, about 4 km upstream of the Deer Creek slide, at the mouth of Owl Eyes Canyon. Webb et. al (2005) dated this deposit to be around 21 ka in age using both infrared-stimulated luminescence (IRSL) and optically stimulated luminescence (OSL) dating, within estimates of age for the Deer Creek Landslide Complex (Savage, 2002). This date falls close to the date of the Last Glacial Maximum (20ka) when climate in the southwest was much cooler and moister than at present (Musgrove et. al, 2001). A dissected terrace of boulders and cobbles (mapped as Quaternary terrace gravels), is located downstream of the present day landslide complex. Billingsley (2005) believes that the catastrophic failure of a landslide dam spawned by the Deer Creek to have been responsible for the massive debris flow that deposited these terraces.

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Figure 5. A 52 m thick sequence of buff colored lacustrine sediments blankets the slopes in lower Owl Eyes Canyon, along the south side of the Colorado River just upstream of Granite Narrows, near River Mile 134.5. These have been dated at 21 ka and are likely related to a lake impounded by a temporary dam formed by the Deer Creek Landslide.

Samples of this sediment were also recovered by the authors for pending palynology analysis. The deposition of any palynomorphs in this reducing environment makes the material ideal for their preservation. Although these samples were deposited in a non-oxidizing environment, potential problems exist with the analysis of palynomorphs from these sediments. These sediments were captured due to the damming of the main stem of the Colorado River, not a side canyon tributary with a limited drainage area. At this location the Colorado River watershed drains over 260,000 square kilometers (100,000 square miles). This large area includes diverse landscapes ranging from alpine regions of the Rocky Mountains and high plateaus to deserts. Palynomorphs in this sediment may not represent a localized environment, so they may not be diagnostic of climatic conditions at the time of this 21 ka age landslide.

CONCLUSIONS AND IMPLICATIONS

Palynology provides an additional tool to earth scientists studying the genesis of prehistoric megalandslide complexes common to the uplifted plateaus of the western United States. Although dating is rarely conducted using actual palynomorphs, these microfossils provide a means to aid in estimating climatic conditions immediately after landslide events. If palynomorph assemblages are different from those in modern samples, that information helps to compliment and validate dates determined by other methods, such as OSL or IRSL. In the case of the lacustrine sediments trapped in the Surprise valley Landslide Complex in the Grand Canyon, this method helped us determine that the climate was cooler and, likely, much wetter at the time of landslippage. They may also provide clues about the influence of temporary reservoirs inundating the slopes of these megalandslides, cased by lava dams 70 to 80 km downstream (Fenton et al., 2004).

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This additional information provides researchers insights into possible causes of these landslides, such as a wetter climate elevating groundwater, simple saturation of the overconsolidated shales, or rapid drawdown produced by sudden drainage of temporary impoundments. These findings can be included in geomechanical models to replicate paleo environmental and climatic conditions with greater validity than pure assumptions alone.

The use of palynology is not without limitations. Not all megalandslides create conditions

allowing for the capture of lacustrine sediment. Landslides must create conditions suitable to the capture of sediment packages, such as a landslide dam and/or closed basins created on the landslide surface itself. If sediment is present, it must not be too oxidized, which results in the destruction of the palynomorphs. Sediment captured by landslides should also be from a limited watershed. Sediment from large areas is more likely to contain palynomorphs from a wide range of environments, leading to greater difficulty in the determination of potential local paleoclimates. REFERENCES Adam, D.P., 1975, A Late Holocene Pollen Record from Pearson’s Pond, Weeks Creek

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Ahnert, F., 1960, The Influence of Pleistocene Climates Upon the Morphology of Cuesta Scarps on the Colorado Plateau, Annals of the Association of American Geographers, vol. 50, no. 7, p. 139-156.

Anderson, R.S. Betancourt, J.L., Mead, J.I, Hevly, R.H, Adam, D.P., 2000, Middle- and late-Wisconsin paleobotanic and paleoclimatic records from the southern Colorado Plateau, USA, Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 158, p. 25-43.

Billingsley, George H., 2005, United States Geological Survey, Flagstaff, AZ, Personal Communication.

Bull, W.B., and Brandon, M.T., 1998, Lichen Dating of Earthquake-Generated Regional Rockfall Events, Southern Alps, New Zealand, Geological Society of America Bulletin, vol. 110, no. 1, p. 60-84.

Fenton, C.R., Poreda, R.J., Nash, B.P., Webb, R.H., and Cerling, T.E., 2004, Discrimination of Five Pleistocene Lava-Dam Outburst-Flood Deposits, Western Grand Canyon, Arizona: Journal of Geology, volume 112, pp. 91–110

Ford, T.D., Huntoon, P.W., Breed, W.J., and Billingsley, G.H., 1974, Rock Movement and Mass Wasting in the Grand Canyon: in W.J. Breed and E. Roat, eds., Geology of the Grand Canyon: Museum of Northern Arizona, Flagstaff, pp. 116-128.

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Huntoon, P.W. and Elston, D.P., 1979, Origin of the River Anticlines, Central Grand Canyon, Arizona, United States Geological Survey, Geological Survey Professional Paper 1126-A, 9 p.

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and Engineering, Rolla, MO, Personal Communication. Rogers, J. D., and Pyles, M. R., 1980, Evidence of Cataclysmic Erosional Events in the Grand

Canyon of the Colorado River, Arizona: Proceedings of the Second Conference on Research in the National Parks, San Francisco: National Park Service, Washington, D.C., Physical Sciences, v. 5, pp. 392-454. Online: http://web.umr.edu/~rogersda/grand_canyon_research/

Savage, J., 2002, Landslide history and river diversions in Central Grand Canyon, Arizona, Masters Thesis, Colorado School of Mines, Golden, CO, 119 p.

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