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THE REGOLITH GEOLOGY OF THE MDRS STUDY AREA

Jonathan Clarke

Introduction Regolith is everything between fresh rock and fresh air and comprises the land surface of the earth and all planetary bodies with a solid surface. Understanding the regolith is an important component to any aspect of science that needs to understand the earth’s surface. It is therefore important to soil science, environmental geology, geomorphology, planetary exploration, land system studies, soil ecology, hydrology, civil engineering, and mineral exploration. Key references for regolith studies include Taylor and Eggleton (2001), Ollier and Pain (1996) and Eggleton (2002). These books should be available in university libraries or through interlibrary loan. Merrits et al. (1998) is a useful reference that covers related aspects of environmental geology, and a copy is available in the MDRS hab. In the MDRS study area understanding the regolith is important to understanding the genesis of the landforms and how these may, and may not, be valid analogues for Mars, the relationship between regolith and landforms, the relationship between regolith materials and microbiomes, and for engineering issues including slope stability, efficacy of the leach field, and surface trafficability. This report will provide an introduction to the regolith terrain concept and outline the main regolith terrain systems I have observed during Expedition One. The regolith terrain concept Traditionally earth scientists have mapped either the geomorphology (shape of the landscape) or the regolith materials (transported cover, soils etc.) The regolith terrain concept integrates the two, so that the landscape as a whole is divided up into a series of systems that are composed of various combinations of landforms and regolith materials. This has some similarities to land system mapping practiced by land management groups, but is more geologically focused. A key part of such an approach is regolith terrain mapping. I have written an introduction to regolith terrain mapping in the MDRS field area as a complement to this paper (Clarke 2003). I urge anyone interesting in the regolith at MDRS to read this report. Two useful scales have been identified for mapping; these include the map scale (for features of the 10 m to the km scale, and the site scale, typically 10 cm to 10 m. Map scale regolith terrain systems in the MDRS field area I have divided the MRDS field area at the map scale into 18 main regolith terrain systems. References to the bedrock geology are from Stokes (1986), Chronic (1990) and Fillmore (2000). All three references are available in the MDRS hab. A summary of the stratigraphic is contained in Table 1.

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Table 1

Stratigraphy of the MDRS area Formation Member Age Lithology Mancos Shale Emery Late Cretaceous Yellow fluvial to marginal marine sand Blue Gate Carbonaceous and pyritic marine shale Ferron Yellow fluvial to marginal marine sand, local

coal in upper part Tunuck Carbonaceous and pyritic marine shale Dakota Late Cretaceous Calcareous cross-bedded channel filling

sandstone, conglomeratic sandstone, and conglomerate with local oyster reefs

Morrison Late Jurassic Brushy Basin Red brown clays and shales with lesser white

and green beds, minor green tuffs, red-brown sandstones, and anhydrite or carbonate cemented nodules

Salt Wash White, cross-bedded sandstone and conglomeratic sandstone

Curtis Late Jurassic Thin bedded red-brown shales with beds of nodular gypsum and cross cutting gypsum veins. Thin sandstone lenses towards top

Entrada Early Jurassic Thickly bedded cross-bedded brown sandstone with lesser interbedded brown shales

Dissected plains of cracking clays. This regolith terrain system is formed on the Brushy Basin Member of the Morrison Formation. Cracking or swelling clays (smectites) are typically formed in arid to sub-arid environments, such as prevailed during the deposition of this unit. The PIMA indicates the presence of montmorillinite (magnesium smectite) and minor nontronite (iron smectite). The plains immediately to the east of the hab are typical of this system (Figure 1). Coarse clear gypsum fragments are minor. Clay plains often exhibit patchy efflorescence of sulphate and halite, especially when the surface dries out after rain or in areas of groundwater seepage or surface water ponding. Vegetation is absent, possibly because of the high salinity. Repeated setting and drying cycles are probably responsible for the concentration of rock fragments on the surface. Smooth plains of non-cracking clays. Non swelling clays form in deeply weathered environments (kaolinite, halloysite) or as a result of marine diagenesis (illite). The main areas characterised by non-swelling clays in the region are those underlain by the marine sediments of the Tununk and Blue Gate Members of the Mancos Shale Formation that form hab “bench” west of “hab rim” and Factory Bench west of Skyline Rim (Figure 2). Clay plains often exhibit patchy efflorescence of sulphate and halite, especially when the surface dries out after rain or in areas of groundwater seepage or surface water ponding. During weathering there has been only limited formation of large clear fragments of regolith gypsum, largely through remobilization from diagenetic veins and beds. Salt efflorescences are present, and vegetation is sparse.

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Dissected plains of non-cracking clays. This regolith terrain unit mainly occurs on the northern part of Factory bench in the vicinity of Coal Mine Wash. It this area it is developed on the Tununk Member, however this unit is also locally developed in other areas where the stratigraphy is dominated by non swelling clays, including to the east of the hab in the Salt Wash Member of the Morrison Formation (Figure 3). Fragments of clear gypsum are common in surface exposures of this unit and has formed during weathering by formation of sulphate by oxidation of sulphides such as pyrite and marcasite disseminated throughout the host shale. Salt efflorescences are present, and vegetation is sparse. Dissected plains of weathered non-cracking clays. Significant development of weathered profiles are rare in the MDRS field area, I have only observed thick development (more than 1 m) of saprolitic material to the north of Factory Butte and in the Coal Mine Wash area. They have formed on weathered Tununk Member. Dissected plains of gypcreted weathered non-cracking clays. The gypcrete is locally present as duricrust cap on weathered Tununk Member on the dissected plains north and west of factory Butte. Dissected slopes of cracking clays. This regolith terrain unit is very well developed along “Hab rim” to the west of the hab and on the small hills. The main stratigraphic unit on which this regolith terrain unit is developed is the Brushy Basin Member of the Morrison Formation, although local, small scale slopes of cracking clays are also present in the Salt Wash Member (Figure 4). In the MDRS field area the montmorillinite and nontronite clays are mostly oxidized and therefore lacking in diagenetic pyrite. During weathering there has been only limited formation of large clear fragments of regolith gypsum, largely through remobilization from diagenetic veins and beds, or through precipitation of cyclic salts in groundwater. Minor salt efflorescences are present, and vegetation is sparse. Dissected slopes of non-cracking clays. Such surfaces are very well developed in the vicinity of Factory Butte, North Caineville Mesa, and the foot of Skyline Rim. The first two examples are developed on the Blue Gate Member, while the third is developed on the Tununk Member. Local dissected slopes are developed on the relatively thin interbeds of non-cracking clays in the Salt Wash Member (Figure 5). Fragments of clear gypsum are common in surface exposures of this unit and has formed during weathering by formation of sulphate by oxidation of sulphides such as pyrite and marcasite disseminated throughout the host shale. Minor salt efflorescences are present, and vegetation is sparse. Dissected sandstone slopes. This regolith terrain unit is characterised by very steep slopes and escarpments. Dissected sandstone slopes form the prominent landforms of the study area, including “hab ridge” (Dakota Formation), Skyline Rim (Ferron Sandstone), Caineville Mesa and Factory Butte (both Emery Sandstone). The visually impressive washes and canyons are also developed in sandstone, including Salt and Neilson Washes (Ferron Sandstone), Tank Wash and Lith and White Rock Canyon (all Salt Wash Member). The importance of this regolith terrain unit is under emphasized on maps because of the tendency of sandstone slopes to form vertical or near-vertical escarpments that form caps to dissected clay slopes (Figure ). The skeletal lithic soils support sparse vegetation of perennial and ephemeral shrubs, herbs, and grasses. Dissected sandstone plains. These features have developed on Factory Bench (Ferron Sandstone) and east of the hab (Salt Wash Member). They are interpreted to form the top of Factory Butte and the Caineville Mesas (Figure 6). The topography of sandstone plains is often quite rough and may be variably mantled by aeolian sand (Figure ).

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Washes. Ephemeral streams flowing across the MDRS field area have eroded a number of deep washes. These include the formally named White Rocky Canyon, Salt Wash, Tank Wash and Neilson wash and the informally named “Candor Chasma” and “Lith Canyon” (Figure 7). Sediments in washes are typically various mixtures of sand, gravel, and cobbles. They may be moderately vegetated, possibly because of better drainage. Colluvial fans are closely associated with alluvial fans but are dominated by gravity flow processes, especially mud flows. Gravity falls and slides and fluvial processes are minor components. As a result they tend to be steeper than alluvial fans and richer in fine-grained material so that the sediments are matrix supported. Colluvial and alluvial fans are end members of a spectrum of fan deposits. Curiously no medium or large sale alluvial fans were found in the MDRS field area, although small (metre scale) alluvial fans are common at the foot of rills, gullies, and washouts. Colluvial fans are common along the edges of washes, canyons, mesas, buttes, and rims. They are typically of limited extent in the MDRS field area (Figure 8). Talus deposits are formed by rock fall and slides and are dominated by very coarse (cobble to boulder sized) debris with very little matrix. They occur at the foot of escarpments such as Skyline Rim, and may form talus cones, gully fills, and aprons. Like colluvial fans, they are important process indicators, despite their limited extent. (Figure 9). Aeolian plains. A sandy source is needed to develop deposits of windblown sand. The sand-dominated stratigraphic units such as the Emery and Ferron Sandstones and the Salt Wash Member (Figure 10). Although in principle sand can be transported by wind for 10’s or 100’s of km, in practice the sand usually stays close to source. There are no major deposits of windblown sand in the MDRS field area, although to the north east of Hanksville there are extensive sand dunes, both crescentic and barchanoid, in the San Rafael Desert. These sands are derived from the Entrada Formation, a Jurassic aeolian sandstone. In the MDRS field area aeolian sand deposits consist of sand ripples, mounds and small dunes or thin topography mantling sheets over sandstone. Local windblown deposits also develop along channels and washes, which are also rich in sand. Deflationary lags are common between these small aeolian deposition features. Aeolian sands are moderately vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of better drainage and lower salt content to the soil. Channels. Washes are characteristic of fluvial systems cutting through dissected areas of moderate to high relief. Where these debouche onto plains less confined channel systems ten to develop. In the MDRS are the largest fluvial systems, the Freemont River to the south and Muddy Creek to the north east, are still partly confined. As is common in arid environments, these major streams show strong seasonal variation in flow and have a braided morphology. Smaller streams are ephemeral and are also braided, with well developed bars (Figure 11). Sediments in channels are typically various mixtures of sand, gravel, and cobbles. Channel sediments are often sparsely vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of reasonable soil drainage. Floodplains of silty sand. Floodplains are not well developed in the study area, except along the Freemont River (Figure 12) and Muddy Creek. Localised flood plains are developed in both channels and washes, typically along the insides of meanders.

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Relict gravel deposits on rises. There are a range of relict gravel deposits. Some of these are locally derived or lags formed by deflation. There are also local patches of coarse gravel and cobbles that contain exotic clasts composed of a range of igneous rocks, limestones, and chert. These are very well founded and bear no relation to current topography. They are interpreted as dissected remnants of high level terraces or streams, either from an ancestral Fremont River, alluvial fans from the Henry Mountains to the south, or Muddy Creek (Figure 13). Relict gravel deposits are moderately vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of better drainage and lower salt content to the soil. Site scale regolith terrain systems in the MDRS field area In addition to the above units, which are also represented at the site scale, there are a multitude of smaller regolith terrains at the smaller scale. Most of these I will not describe in detail, as they represent local and scale variations. The more important features I outline below. Pockets, pans, and basins on cracking clay slopes. These are small to medium sized features that form smooth flat areas on gentle to smooth slopes and are surrounded by raised surfaces of cracked clay (Figure 14). The pockets, pans, and basins are floored by silty clay and have ephemeral pools of water after rain. Small rills may lead into and from them. A few have downflow pipes within them. Their genesis is obscure, may have be related to the formation of terraces on clay slopes. Volcanic material, commonly exposed on slopes. Green tuffaceous sediments form lenticular beds in both the Brushy Basin Member and the Dakota Formation (Figure 15). These tend to be more resistant to erosion than the enclosing sediment and shed talus down slope. Organic rich bedrock. Coals are found in the upper part of the Ferron Sandstone and crop out along the margins of the appropriately named Coal Mine Wash (Figure 16). Weathered coals are characterised by abundant sulphate, both clear gypsum and yellow jarosite (KFe3(SO4)2(OH)6), and by low soil pH. This contrasts with other areas of the MDRS field area which have alkaline soils. Sulphate rich bedrock, commonly exposed on slopes. The Brushy Basin Member (Figure 17) and the Curtis Formation contain beds and nodules of gypsum and anhydrite that formed during early diagenesis. Some of these beds are quite large and are significant on the site scale as a source of sulphate for sulphur bacteria and for remobilization through the host rock and regolith. Gypsum and anhydrite that formed during deposition and diagenesis is typically cloudy or translucent and is easily distinguished from the clear gypsum formed during weathering. Efflorescences are thin surface layers of white salts. In the MDRS they are mostly gypsum or anhydrite, with secondary halite and calcite (Figure 18). Efflorescences are especially common on the cracking clays of Brushy Basin member and may be the reason why this unit lacks vegetation and forms the Mars-like landscape next to the hab. The source of the salts in the member Piping is a groundwater phenomenon characteristic of cracking clays. Infiltrating water is channeled along the cracks, the concentrated subterranean flow eroding vertical and horizontal pipes which discharge lower down the slope. At the MDRS site the ridge behind the hab consists of the Brushy Basin member of the Morrison Formation overlain by a cap of Dakota Sandstone. Piping is common and often spectacular, the inflow and outflow pipes vary from a few cm in size to more than 2 m (Figures 19 and 20). The role of piping in slope erosion along hab ridge is complex. Piping can

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develop on flat surfaces, but is rare, usually at least some precursor gully is necessary. Small pipes then develop and are presumably progressive enlarged. The entrances to some of the largest are covered by vegetation, indicating that these areas are significantly moister than the surrounding vegetation free slopes. Collapse of pipes leads to gully enlargement and even blocking of gullies, when the collapsed part of the gully is lower than the formerly downstream portions. Regolith processes in the MDRS field area Weathering appears minor in the MDRS field area. Only in the area north of Factory Butte are reasonably thick weathered profiles developed. Elsewhere weathering appears to be restricted to thin disturbed veneers on clay surfaces and poorly developed and locally developed yellow-brown goethitic mottles in the Dakota Formation. Slope and scarp retreat is the main landscape shaping process. Most of the major landforms are shaped by parallel retreat of slopes beneath resistant cap rocks of shallowly dipping sandstone. One the cap rock is removed the softer underlying clay-rich rocks are lowered to form spires or mammaliated hills. Runoff and fluvial processes appear to be the main process that initiates dissection of the landscape and a major way of exporting eroded material from the area. The entire region falls into the catchment of the Dirty Devil River and its two tributaries, the Fremont River and Muddy Creek. Fluvial incision is important on all sandstone slopes and many clay slopes. Aeolian processes are the other main process exporting sediment from the area. Its effectiveness may be less than that of runoff, because aeolian sands tend to be confined to areas of sandy substrate. The role of dust transport is still unquantified, but dust storms are known in the area and may be significant as both a means of exporting weathered clays and for the importation of cyclic salts. Mass movement forms both talus and colluvial deposits. While very local, mass movement is extremely important on the steepest slopes. The mantled of cracked clay is extremely mobile, extremely during and after rain. Mass movement may be the most significant transport mechanism on these slopes. Sapping occurs when seeping groundwater erodes material. Classic landforms of groundwater sapping include piping, box canyons and theatre headed valleys. Sapping is an especially interesting process in the MD0RS field area because it is believed to be an important geomorphic agent on Mars. In the field area sapping by groundwater seeping along the contacts between sandstones and underlying shales undermines the resistant sandstone caps. These moist areas are also favourable for microorganism growth because they are generally moister than the surround rock. Sapping of areas with a hard cap produces theatre headed and flat floored valleys or box canyons, such as those in the Curtis Formation along the road to Hanksville (Figure ). Piping is also very important in the MDRS field area, especially on cracking clays, and some pipes are large enough for people to climb into. Some theatre headed valleys also developed on clay slopes. Groundwater evaporation forms the surface salt efflorescences. The high levels of surface salt concentrations that these indicate may be the reason why the surfaces formed on the Brushy Basin Member are free of vegetation. When the surface dries out after range there is a great increase in the abundance of efflorescence. Why the salt is then lost over time (until the next rain) is unclear. Possibilities include deflation and consumption by animals. Salt crystalisation in areas of groundwater

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seepage also weakens the rock allowing it to be washed or blown away and is an important component to sapping. Trafficability One of the immediate applications of regolith studies is the relationship between the regolith type and the ability of vehicles and pedestrians to traverse the surface. This is important not only for safety reasons but because of damage to maintained tracks and to sensitive rangeland soils. Clay slopes These plains are difficult to traverse on foot when wet and very difficult in a vehicle. They present a significant bogging hazards, tracks are damaged, and there are safety issues should control be lost. Movement by vehicle should only be attempted under such conditions in an emergency. Twenty four to 48 hours is usually sufficient to allow the surface to dry out for foot traffic. More time (at least 72 hours) should be allo0wed to for vehicle tracks to dry out. Clay plains These plains are difficult to traverse on foot when wet and very difficult in a vehicle. They present a significant bogging hazards, tracks are damaged, and there are safety issues should control be lost. Movement by vehicle should only be attempted under such conditions in an emergency. Twenty four to 48 hours is usually sufficient to allow the surface to dry out for vehicle movement on established tracks. Sandstone surfaces Sandstone surfaces can be traversed under almost all conditions. Because they are normally interspersed with areas of clay surfaces, routes to and from sandstone surfaces need to be treated with the same care after rain as clay surfaces. Depositional surfaces Extreme care must be exercised when crossing areas of windblown sand and alluvial sediments in washes and channels. Except where there is a maintained track, these should be attempted only by 4WD vehicles with this capability engaged and driven by experienced drivers. Because windblown sands are vulnerable to disturbance, cross country driving should be undertaken in these areas should be carried out only in an emergency. Sediments in washes pose similar challenges. Although superficially dry, they may overlie wet sediment or mud, providing tracks for the unwary driver. Further research Microbiology. Microorganisms play a role in most regolith processes. The relationship between regolith type, moisture, and different microbial communities is well worth exploring and will provide insight into microbial processes in the study area and possible roles played by microorganisms on Mars. Regolith processes. I suggest the following regolith processes as being of particular interest in quantification. They are the dust budget, rates of down slope movement on cracking clay slopes,

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sediment budgets on ephemeral streams, and monitoring of discharge from pipes. All these projects are long term and will require the establishment and monitoring of stations over several years by different crews. Another interesting question is what some slopes developed on the cracking clays of the Brushy Basin Member undergo erosion through development of runoff gullies and others through groundwater sapping and formation of theatre headed valleys. Finally the genesis of the pockets, basins and pans on clay slopes needs to be elucidated. Gravel provenance in the relict deposits will help determine whether they are formed by ancestral streams of the Freemont and Muddy Creek, or by derivation from the Henry Mountains. Determining the provenance will illuminate the palaeodrainage of the area and the geomorphic and tectonic development of the region. Impact of MDRS on regolith properties. Preliminary studies of soil pH (N. Wood, pers. comm.) indicate that the soil has been acidified for a distance of several 10’s of metres surround the hab. Mapping of soil pH, phosphate, and nitrate using commercial soil test kits will quantify the degree to which MDRS has changed the surface chemistry and help devise strategies to minimize this. Any Mars base will have to have minimal contamination of the environment and a study at MDRS will provide baseline data on how much contamination results from a site in a terrestrial analogue. Drainage properties of the immediate MDRS site are very important is disposal of waste water through the leach field. The smectite-rich regolith is very unsuitable for waste water disposal because of its swelling properties and low permeability. Engineering investigations into the hydrology, chemistry and physics of the regolith material in the leach field may lead to more effective disposal of waste water which currently ponds on the surface and is not only unsightly, but presents a traffic and environmental hazard. Trafficability. After rain the vehicle parking area is almost impassible to vehicles and foot traffic. Ways should be investigated for temporary or permanent structures that reduce this problem. These might include moveable walkways of perforated planking and mats of various types. While mud on Mars is unlikely, disruption of the surface layer on Mars is likely to produce an analogous problem with dust.

References Chronic, H. 1990. Roadside geology of Utah. Mountain Press Publishing Company, Missoula,

Montana, 325p (especially pp 93-107). Clarke, J. D. A. 2003. Regolith terrain mapping in the MDRS field area. Can be downloaded from Eggleton, R. A. 2002. The regolith glossary. CRC LEME, Floreat Park, Perth, Australia. Eggleton, R. A. and Taylor, G. 2001. Regolith geology and geomorphology. John Wiley & Sons. Fillmore, R. 2000. The geology of the parks, monuments, and wildlands of southern Utah. University

of Utah Press, Salt Lake City, 268 p (especially pp53-101 and pp151-179). Merritts, D., De Wet, A., and Menking, K. 1998. Environmental geology. W.H. Freeman & Co.,

New York, 452 p.

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Ollier, C. D. and Pain, C. 1996. Regolith, soils, and landforms. John Wiley & Sons. Stokes, W. L. 1987. Geology of Utah. Utah Museum of Natural History and Utah geological and

mineral survey, Salt Lake City, 280 p (especially chapters 13 and 14).

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Figures

Figure 1. Dissected plain of cracking clay, “hab bench”.

Figure 2. Smooth plain of non-cracking clay, “Skyline bench”

Figure 3. Dissected plain of non-cracking clay, Factory Bench.

Figure 4. Dissected slope of cracking clays, west of hab

Figure 5. Dissected slope of non-cracking clay, near Factory Butte

Figure 6. Dissected sandstone plain, south east of hab.

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Figure 7. Wash north of hab

Figure 8. Colluvial fans

Figure 9. Talus at head of box canyon.

Figure 10. Aeolian plain, east of hab

Figure 11. Channel north of hab

Figure 12. Floodplain of Fremont River

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Figure 13. Relict gravels.

Figure 14. Basin on clay slope, “hab rim”.

Figure 15. Green volcanic sediment, “hab rim”.

Figure 16. Outcropping coal in Coal Mine Wash.

Figure 17. Nodular anhydrite bed behind hab.

Figure 18. Halite and gypsum efflorescence, Factory Bench

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Figure 19. Large down flow pipe, Lith Canyon.

Figure 20. Large out flow pipe, “hab rim”.