Calcium Carbonate Cementation of Alluvial Fans in Southern Nevada

LAURENCE H. LATTMAN Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221

ABSTRACT INTRODUCTION

Several types of calcium carbonate cement The Las Vegas Basin of southern Nevada occur on alluvial fans in southern Nevada. is flanked by coalescing alluvial fans of various Secondary calcium carbonate deposits of lithologies. These fans are composed of detritus pedogenic origin begin as coatings on pebble- which may be dominantly carbonate, andesite size and larger clasts, and progress through and basalt, rhyolite and rhyolitic tuff, or calcic horizons to petrocalcic horizons and siliceous sedimentary rock. All fans have been laminar layers. Nonpedogenic deposits include cemented by calcium carbonate to some degree. some laminar layers, gully-bed cementation, The carbonate and andesite fans show the and case hardening. best-developed cementation and the rhyolite The extent and development of cementation fans and those composed dominantly of sili- are greatest on fans composed of carbonate and ceous sediments have markedly less well- basic igneous rock detritus, less on fans built developed cementation. of siliceous sedimentary detritus, and least on The calcium carbonate cement has generally fans composed of acid igneous rock material. been called "caliche" and is presumed to be In a single fan, the best-developed cementation due primarily to pedogenic processes (Gile is in the poorly sorted layers of alluvium con- and others, 1966). The present study reveals taining more than 25 percent material coarser that the cementation occurs by pedogenic and than pebbles. The major source of calcium nonpedogenic processes with resulting different carbonate deposited on noncarbonate fans is forms and distributions of the cement. It is apparently wind-blown silt and sand and, as the purpose of the study to establish a geo- a result, fans of noncarbonate detritus are best morphically significant distinction among types cemented downwind of playas high in car- of calcium carbonate cement, to ascertain the bonates. major factors influencing formation and distri- Only fans composed of carbonate and basic bution of these various types, and to determine igneous rock detritus show clearly defined, the influence of the different types on the older, abandoned surfaces and extensive areas erosion of alluvial fans in southern Nevada of the strongest forms of cementation; petro- and adjacent areas. calcic horizons and laminar layers are found only on these surfaces. It is suggested that GENERAL GEOLOGY formation and preservation of these surfaces Physiography are contingent upon development of strong Las Vegas Basin (Fig. 1) is at the south- carbonate cementation. eastern end of Las Vegas Valley and alluvial Petrocalcic horizons and laminar layers ap- fans are distributed roughly radially around parently formed over a wide climatic range, it except at the northwestern side. Las Vegas and variation of the other factors controlling Valley is a northwest-trending basin of the the degree of cementation, therefore, appears Basin and Range province and is flanked on to govern the formation and preservation of both sides by bahadas made of coalescing abandoned surfaces. alluvial fans.

Geological Society of America Bulletin, v. 84, p. 3013-3028, 10 figs., September 1973 3013

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Mountains (Fig. 1) southeast of Las Vegas are Tertiary igneous rocks, dominantly rhyolite and rhyclitic tuff. The clasts in the surface and near-surface deposits of the fans derived from these ranges directly reflect the bedrock lithology. Below the fan surfaces, however, an older Tertiary unit, the Muddy Creek Forma- tion (Longwell and o:hers, 1965) may be present. The present study deals with surface and near-surface cementation and therefore is concerned with fan lithologies that are con- trolled by the lithology of the range from which each fan is derived.

CALCIUM AND MAGNESIUM CARBONATE CEMENTATION An extensive literature exists on calcium carbonate cementation (caliche) in arid and semi-arid regions. Geologists have contributed to these studies (Bretz and Horberg, 1949; Brown, 1956; Gardner, 1972), and soil scientists Figure 1. Location map. The numbers refer to have been concerned mostly with the origin locations mentioned in text. 1 = Kyle Canyon fan; of the caliche or its significance as an indicator 2 = McCollough fan; 3 = River Mountain fan; of climatic and geomorphic history. The pres- 4 = Eldorado Valley. ent investigation is concerned with some The highest point in the Las Vegas area is aspects of origin and with the role of the Charleston Peak in the Spring Mountains cementation in modern fan processes. In the (Fig. 1) with an elevation of ~3,650 m; the discussion below, soil-science terms are used center of Las Vegas Basin has an elevation of where possible, and new terms are introduced ~640 ra. where needed to emphasize the geomorphic Typically, precipitation in this area is a significance. function of elevation. At McCarren Airport, In all cases, in the study area, the secondary Las Vegas, elevation 662 m, the annual precipi- cement studied contained more than 80 per- tation is between 10 and 13 cm; and at the cent calcium carbonate, the remainder being Desert National Wildlife Refuge in Las Vegas primarily magnesium carb'Jnate, siliceous ma- Valley, 29 km northwest of Las Vegas, the terial, and organic detritus. annual precipitation is 18 to 25 cm at an elevation of about 915 m. In the Spring Moun- Types of Carbonate Cementation tains, the Kyle Canyon Ranger Station at an Despite different origins and distributions, elevation of 2,195 m records an annual pre- all carbonate cements are chemically similar, cipitation of about 50 cm. Average annual and it has not been possible in this study to temperature, also a function of elevation, separate them on the basis of origin by chemi- varies from 18°C at Las Vegas to 10°C at the cal analysis alone (Table 1 summarizes the Kyle Canyon Ranger Station. types of cement). Calcic Horizons. Pedologists define a calcic Bedrock and Fan Geology horizon as a soil horizon "of secondary carbon- The pre-Quaternary geology of the Las ate enrichment that is more than 6 inches thick, Vegas area has been mapped by Longwell and has a calcium carbonate equivalent content of others (1965). Three of the mountain ranges more than 15 percent, and has at least 5 mapped are of direct interest to this study. percent more calcium carbonate equivalent Over 85 percent of the Spring Mountains than the C" (Soil Survey Staff, 1967, p. 34). (Fig. 1) are composed of Paleozoic carbonate If no C horizon is present, the definition of a rocks, the McCollough Mountains (Fig. 1) calcic horizon is similar to the above except south of Las Vegas are Tertiary igneous rocks, that it contains more than 5 percent by volume mostly andesite and basalt, and the River of identifiable secondary carbonates. The geo-

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TABLE 1. SUMMARY OF CHARACTERISTICS OF CARBONATE CEMENT IN SOUTHERN NEVADA

Type Origin Characteristics

Calcic Pedogenic Continuous and widely distributed (80 to 100 Highly variable thickness and amount horizon percent of a fan surface) of cement

Petrocal cic Pedogenic Continuous but less widely distributed than A late stage in development of calcic horizon calcic horizons (30 to 80 percent of a fan horizons. Hard and nonslaking. May surface) be formed from calcic horizons by case hardening of exposures

Laminar Pedogenic Generally discontinuous and of restricted Laminated deposit consisting of >90 horizons and distribution (10 to 50 percent of fan surface) percent carbonate. Forms on top of nonpedogenic petrocalcic horizons and in topographic lows on fan surface. Hard and nonslaking

Case Nonpedogenic Restricted to surface exposures on vertical Forms rapidly on carbonate-rich hardening faces. Surface hardening also occurs on material, especially calcic horizons. gently sloping exposures Decreases inward from surface of exposure

Gully-bed Nonpedogenic Restricted to beds and lower banks of fan washes Resembles laminar layers but generally cementation more crudely laminated. Hard and nonslaking

Caliche Nonpedogenic Extensive on abandoned fan surfaces underlain by Angular rubble derived by mechanical rubble laminar layers and petrocalcic horizons breakup

logical literature commonly uses the term laminar horizon or layer is commonly found "caliche" to refer to such horizons. on top of a petrocalcic horizon. Gile and others (1966) point out that Some petrocalcic horizons are present in the calcium carbonate accumulation in soil starts study area and are restricted to carbonate or as pebble coatings and goes through a series andesite-basalt parent materials. of sequential steps leading through interstitial Laminar Horizon. The laminar horizon or carbonate fillings to a "plugged" horizon layer (Fig. 2A) consists of essentially pure which may develop a "laminar horizon" at (>90 percent) carbonate with minor amounts the top. The same authors (1965, p. 74) define of clay, organic matter, silica as chert, and a K-fabric as one in which a "fine-grained sand and silt grains. Soil scientists (Gile and authigenic carbonate occurs as an essentially others, 1966) consider that it forms from continuous medium. It coats or engulfs, and downward-percolating water when this water commonly separates and cements skeletal peb- is stopped by a relatively impermeable zone bles, sand and silt grains." The K horizon is such as a plugged horizon, a petrocalcic hori- defined as containing "90 percent or more by zon, or bedrock (a laminar horizon on bedrock volume of K-fabric in its most prominent sub- is considered as a petrocalcic horizon by soil horizon (K2)." The K horizon, which may be scientists). soft or hard, is a late stage of development of a The laminar layer has one of the lowest calcic horizon. vertical permeabilities of any natural calcic In the present study area, calcic horizons layer (R. Cooley, Desert Research Institute, ranging from pebble coatings to plugged hori- 1972, oral commun.) and plays an important zons are found on alluvial-fan deposits of car- role in fan geomorphology, discussed later. bonate and andesite-basalt lithologies. On The laminar horizon, as noted above, is rhyolite, rhyolitic tuff, and noncalcareous sedi- believed to be due to pedogenic processes, and ments, only pebble coatings or thin discon- the lack of clastic material in the laminar tinuous stringers of calcareous cement are horizon may be explained by a lifting process. found. Gile and others (1966, p. 355) state "The Petrocalcic Horizon. A petrocalcic horizon laminar horizon is a new horizon in the soil. . . . is a special case of a calcic horizon and is defined The overlying horizons have been displaced (Soil Survey Staff, 1967, p. 8) as a continu- upward from the position now occupied by ously cemented or indurated calcic horizon. the laminar horizon." Evidence from the It is a late stage in the development of calcic present study, presented later, suggests that soils. This horizon is extremely hard when dry, some geomorphically significant laminar layers very firm to extremely firm when moist, and are nonpedogenic and form by accretion at the fragments from it do not slake in water. A surface of the ground rather than within a soil.

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Figure 2A. A laminar horizon overlying a petro- with Fig. 5B). Scale is in centimeters, calcic horizon. The pebbles are carbonate (compare

Case Hardening. Case hardening is a non- Caliche Rubble. Caliche: rubble is a term pedogenic form of cementation developed on here applied to broken, angular fragments of vertical or near vertical exposures. It has been calcium carbonate cement and cemented described in detail by Lattman and Simonberg gravel littering a surface (Fig. 2B). It is derived (1971) and is apparently formed by surface from surface or near-surface; petrocalcic hori- water flowing over steep exposures, causing zons or laminar layers, and is; the result of such solution of calcareous fines and redeposition as layers undergoing modern destruction. The cement. It forms very rapidly (within a few extreme angularity of the fragments, which years) and can be so hard that the surface layer range in size from about 2.5 i:o 30 cm, indicates breaks through limestone clasts. Case harden- that mechanical rather than chemical processes ing occurs on colluvium and forms most rapidly are the cause of the disintegration. in poorly sorted layers. It also forms on allu- Caliche rubble covers extensive parts of fans vium, particularly along modern drainage in the study area and leads to the conclusion lines. It is developed on nearly all steep expo- that destruction of calcium carbonate-rich sures in fans composed of carbonate detritus layers dominates over pedogenic calcium car- and, in other fans, forms primarily at the bonate deposition on the older fan surfaces exposed edges of calcic horizons. today. Case hardening, being a surface process NONPEDOGENIC DEPOSITS OF results in exposures which exhibit decreasing CARBONATE CEMENT cementation inward from the exposed face. Calcic horizons which are soft on fresh expo- The pedogenic deposition of carbonate sure may appear to be petrocalcic horizons in cement—calcic horizons, petrocalcic horizons, vertical section where case hardened on older, and some laminar horizons o:: layers—has been natural exposures. discussed thoroughly by soil scientists (Gile In addition to case hardening of steeply and others, 1966). Case hardening and some sloping exposures, calcic horizons exposed on laminar layers are not pedogenic, but may horizontal or gently sloping surfaces also have considerable geomorphic significance. exhibit a surface hardening with or without a laminar layer. These surface-hardened horizons Laminar Layer have a hard crust about 2.5 to 10 cm thick in Laminar layers from 0.5 to 25 cm thick are the study area and become softer and more widely distributed on the carbonate fans flank- chalky in depth. The upper parts of these ex- ing the Spring Mountains ard on the McCol- posed horizons are best classified as petrocalcic lough fan which is composed of andesite and horizons. basalt detritus. These rock types, in addition

Figure 2B. Caliche rubble and andésite boulders rubble. Directly beneath the rubble is a petrocalcic overlying a petrocalcic horizon on an old surface of horizon ~32 cm thick (hammer in foreground) and the McCollough fan. Note the angularity of the caliche free of coarse clastic material.

to showing similar laminar layers in all of Spring Mountains range from about 0.5 to southern Nevada and adjacent states, are also about 10 cm thick. Directly beneath many of the lithologic types to which petrocalcic hori- these layers the gravels are poorly cemented zons are restricted in the study area. and should be classified as calcic horizons. It is The laminar layers in southern Nevada were here argued that the laminar layers form on all found at or within less than 30 cm of the and in silt layers of low permeability which are present surface except on slopes of over 10°, present immediately under nearly all of the on which they may be buried under as much modern fan surfaces. Such silt layers have been as 1 m of uncemented colluvium. Where the reported by Denny (1965) and attributed to laminar layer is close to the surface beneath a weathering. gently sloping fan interfluve, it is most com- A typical section of a silt layer is shown in monly found immediately below an unce- Figure 3. In the study area, there are almost mented desert pavement generally less than no pebbles, weathered or unweathered, within about 10 cm thick. Most importantly, on some the silt layers, and it is believed that this layer fan interfluves extensive areas of laminar layers is due to freeze and thaw of water. At the do not lie on bedrock or on petrocalcic or Desert National Wildlife Refuge in Las Vegas cement-plugged horizons. Other laminar lay- Valley, data indicate that during the months ers, from 2 to 15 cm thick, cover the beds and of December, January, and February the lower sides of gullies. The present study leads to diurnal temperature fluctuates above and be- the conclusion that a hard, nonslaking laminar low freezing an average of 35 times. December layer can form on the topographic surface of through March is also the time of maximum fans from sheetwash and gully flow and is con- precipitation. During the winter of 1968-1969, trolled by low-permeability layers of fine sedi- for example, three 1-in. snowfalls were recorded ment (silt and clay). at the refuge. At Tule Springs Park in the Laminar Layer on Interfluves. Laminar center of Las Vegas Valley, concrete has been layers on the interfluves of fans flanking the damaged by freeze and thaw. There is, there-

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fore, a mechanism for expelling the larger an uncemented, 2.5-cm-thick desert pavement pebbles and producing a relatively pebble-free cn the Kyle Canyon fan reveals an underlying zone of fine material immediately beneath the layer of pebbles with a sand matrix. This base fan surfaces. Silt is removed from the surface contains carbonate pebbles, and quartz, chert, by deflation, leaving a desert pavement over feldspar, and carbonate sand and silt grains, the silt layer. strongly cemented by calcium carbonate. Car- bonate cement fills pore spaces in the layer as ARMOR OF CALICHE SIZES FRAGMENTS, PEBBLES AND. BROKEN PEBBLES. SUT 1 PEBBLE RANGE calcite prisms up to 0.24 mm long and oriented O.J - 5 CM. MOST AND SAND MATRIX 2 5-5 CM 2.5 CM at right angles to the pore walls. The contact MOSTLY HIGHLY CALCAREOUS CP MEOIUM TO SAND AND SILT. FINE SAND. between this base and the overlying laminar SCATTERED o AND SILT. PEBBLES PEBBLES GEN- layer is sharp. The laminar layer is composed Ù ERALLY 2 5 CK' SU8-R0UNDED PEBBLES PEBBLES of units averaging about 3.8 mm thick con- AND SOME BOULDERS ¿7- GENERALLY MATRIX S'Ml^AR TO 2.5 CM sisting of very fine, interlocking, equant crys- ZONE ABOVE SOIL CALICHE oo. ° o a MAY BE PRESENT tals of calcite arrayed in wavy, convolute Figure 3. Diagrammatic cross section of a typical bands. These wavy bands average 0.44 mm silt layer on the Kyle Canyon fan. thick and occur as segregated 2- and 6-micron calcite crystals. The individual units also con- At many places on the surfaces of the Spring tain varying quantities, up to 28 percent, of Mountain fan, laminar layers are forming sand-size particles of detrital quartz, calcite, today in broad, very shallow (2.5 to 15 cm feldspar, and chert. Some microscopic struc- deep), closed depressions which hold standing tures (Fig. 4B) are present which suggest water after runoff (Fig. 4A). These depressions organic activity, possibly algae. are floored by the silt layer with, sometimes, On the youngest (lowest) surface of the a very thin (about 1 mm thick) clay layer on Spring Mountain fan are isolated patches of top of the silt. The origin of these shallow laminar layers that occur as thin, lenslike depressions is not certain. Several areas approxi- sheets. Such sheets may be separated vertically mately 2 m across were stripped of desert pave- from each other by as mucli as 30 cm and over- ment by hand exposing the underlying silt lap each other by several meters. These laminar layer to surface processes. Within a few weeks, layers overlie silt layers which may be very deflation had produced depressions 3 to 6 cm weakly cemented and are not plugged or petro- deep in these areas. This suggests that some of calcic horizons. the depressions may be caused by deflation of From these observations, it is concluded silt layers following disturbance or removal of that laminar layers up to 15 cm thick at and desert pavement by man, animals, or sheet- below the Spring Mountain fan surfaces have wash. formed by deposition in broad (up to tens of A microscopic study of a typical laminar meters), shallow, closed topographic depres- layer from one such depression now covered by sions which are floored by silt or clay. Such

Figure 4A. A deposit of calcium carbonate forming J today in a small basin on the surface of the Kyle Canyon fan. The deposit is laminated and, except for Figure 4B. Photomicrograph of a laminar layer being soft, closely resembles laminar layers. Hammer showing a structure which may be organic, possibly in right middleground. algae.

depressions are common today and are appar- remainder of the bed. Former gully beds lined ently part of a fan surface crossed by anastomos- by laminar layers are exposed in the banks of ing rills and washes. Carbonate and siliceous some modern downcutting drainage lines silt and sand, in part of eolian origin, are (Fig. 5D). incorporated in the layers as well as fine ma- In the case of laterally shifting drainage terial and scattered pebbles brought in by lines, gully-bed cementation is found to extend sheetwash. beneath the present bank for distances of up Laminar Layers on Gully Beds and Banks. to 6 m. If the drainage line remains in one plan The larger drainage lines on fans derived from position for a time, the cementation can extend carbonate and andesitic-basaltic source areas several meters up the sides. are commonly floored by an irregular laminar Laminar Layer on Steeper Slopes. In some layer on the bed and partway up the banks areas, thin laminar layers occur on the sides of (Fig. 5A). This layer is here called gully-bed interfluves well above modern drainage. These cementation. The laminations in these layers layers are usually covered by modern colluvium are crude and poorly developed (Fig. 5B). which is not an overlying pedogenic horizon Commonly numerous pebbles and cobbles are (Fig. 6). These sloping layers are not related contained within the layer. Gully-bed cemen- to plugged or petrocalcic horizons and their tation occurs whether or not a gully is flowing origin is not clear. They may have been de- on a plugged or petrocalcic horizon and there- posited on the banks of gullies when the gullies fore may not be of pedogenic orgin. If there is were at a higher level and now remain after no plugged or petrocalcic horizon, the gully- gully incision, or they may be due to evapora- bed cementation is underlain by fine sand, silt, tion of thin films of water adhering to the or clay that was deposited in the bed of the surface after runoff. The geomorphic impor- gully (Fig. 5C). The cementation starts as tance of these layers is to reduce infiltration coatings on cobbles and pebbles embedded in and increase runoff on the slopes on which a silt or sand matrix and then forms over the they occur.

Figure 5A. Gully-bed cementation near the head boulders are andesite. The sediments underlying the : a small fan in the McCollough Mountains. The cementation are not plugged or cemented.

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Figure 5B. Gully-bed cementation from Kyle Can- Fig. 2A). Scale is in centimeters. yon fan. Note the crude laminations (compare with Caliche Rubble. Caliche rubble occurs DISTRIBUTION OF THE VARIOUS where petrocalcic horizons or laminar layers CEMENT TYPES are exposed at the surface. Petrocalcic horizons and superjacent laminar layers are developed Development of carbonate cementation in only on the older, higher fan surfaces of carbon- southern and south-central Nevada is appar- ate and andesitic- basaltic fans. As the older ently related to three basic factors: fan com- surfaces are present almost exclusively in the position and texture, availability of wind-blown proximal and medial parts of the fans in south- carbonate silt and sand, aid age of the fan ern Nevada, extensive caliche rubble is only surface (Gile and others, 1966). present in these parts of the fans. Fan Texture and Composition The rubble can be seen in process of formation from the petrocalcic horizons and laminar Throughout the study area the development layers today (Fig- 2B). The previously given of cement varies widely even within the same evidence of freeze-thaw activity is adduced to basin. All fans derived from carbonate bedrock support the idea that this process can produce show calcic horizons and, on the older surfaces, the caliche rubble. The rubble, as previously they possess petrocalcic horizons. Gully-bed noted, is angular and shows very little evidence cementation and interfluve laminar layers are of active solution. widespread. All of the carbonate detritus fans The impermeability of petrocalcic horizons thus cemented exhibit a wide range in clast and laminar layers has reduced modern calcic size from silt to boulders and, in general, soil formation by downward-percolating water pebbles and coarser clasts make up more than on the older surfaces; caliche rubble indicates 60 percent of the noncement material in the that the petrocalcic horizons and laminar surface and near-surface laysrs of alluvium. layers on the older fan surfaces are in process Fans composed of noncarbonate sedimentary of destruction in southern Nevada today. rock detritus generally show less well-developed

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Figure 5D. View of the wall of a large, modern marked by white layers of gully-bed cementation wash on the Kyle Canyon fan. Notice two former (arrows). Man standing in left center for scale, channels exposed in upper half of the wall and both

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percent as CaO). This lack of difference has caused rejection of the idea that calcium for the cement on andesite fans came from weathering of feldspars in the andesite. Fans composed of acid igneous material, for example, the fan extending northwestward from the River Mountains (Fig. 1) on the southeastern side of the La:; Vegas Basin, are almost always very poorly cemented, showing little more than a few scattered, coated pebbles and weak calcic horizons. Even where, as in Las Vegas Basin, large quan tities of calcareous Figure 6. A crude laminar layer over 30 m above dust are available, the cementation is very modern drainage. The alluvium beneath the layer is weak. Such fans, especially if derived from very weakly cemented and pervious. The laminar layer tuffaceous and rhyolitic rocks, have less than truncates the underlying alluvium and is overlain by about 10 percent material of greater than modern colluvium. pebble size in the upper layer; whereas andesite and basalt fans contain over 65 percent ma- cementation than carbonate detritus fans. Such terial coarser than pebbles i:i the upper layer. noncarbonate fans, for example, those along The reason for the difference in cementation the east side of Las Vegas Valley, may show of fans composed of acid and basic igneous rock extensive pebble coatings and calcic horizons detritus is not understood and is the subject of but none which possessed petrocalcic horizons a further investigation. It is suggested that the were found in southern and south-central amount of coarse clasts in the detritus may play Nevada. Isolated patches of laminar layers on a role in this difference because these clasts interfluves, and gully-bed cementation occur exhibit the earliest deposits of calcium car- on these fans but only if, as discussed below, bonate as rinds. wind-blown carbonate fines were distributed on the fan surface. Well-developed, near-surface Availability of Wind-blown Carbonate calcic horizons are found in noncarbonate fans Silt and Sand in poorly sorted deposits possessing 50 percent Carbonate cementation of noncalcareous fan or more material larger than pebbles. Well- deposits has led many workers to conclude that sorted deposits of noncalcareous, sand-size ma- an external carbonate source is needed. The terial are weakly cemented although they may several possible sources of such carbonate have exhibit case hardening. been reviewed by Gardner '1972). Soil scien- Fans composed of basic igneous rock detritus, tists (for example, Flach and Smith, 1969) commonly andesite or basalt, have a wide attribute the source to wind-blown dust and the range in degree of cementation. The McCol- present study confirms this concept throughout lough fan (Fig. 1) extending northward from southern and south-central Nevada. The most the McCollough Mountains is very well ce- strongly developed cementation on fans not mented, possessing extensive areas of petro- composed of carbonate detri tus always occurs calcic horizons, thick laminar layers, and gully- downwind of playas on who<:e upwind side are bed cementation. On the other hand, fans carbonate ranges and fans. An excellent ex- extending from the southeastern side of the ample is Hot Creek Valley in south-central McCollough Mountains into Eldorado Valley Nevada. The northwestern f ank of this valley (Fig. 1) are very weakly cemented. Through- is composed of carbonate mountain ranges and out the study area, the degree of cementation the southwestern and eastern flank is composed of such weakly cemented fans is apparently a of igneous rocks. The only well-developed function of wind-transported calcareous fines. calcic horizons and gully-bed cementation Analyses of andesite boulders from the Mc- occur on the carbonate fans in the north- Collough fan show that the amount of calcium western part of the valley, and on the igneous in the weathered rind (average is 7.15 percent fans in the northeastern part which are down- as CaO) is not significantly different at the wind of a playa containing over 25 percent 0.05 level than the amount of calcium in the carbonate material. The strong cementation of fresh interior of the boulder (average is 7.35 the fans extending into the Las Vegas Basin

from the northern side of the McCollough more extensive and better developed cementa- Range has already been noted as well as the tion than well-sorted, noncalcareous fans con- lack of cementation on the south side of the taining less than about 20 percent pebble-size same range in Eldorado Valley. Samples of and coarser clasts. The role of coarse clasts in wind-blown dust in Las Vegas Basin contain development of cement is still under investiga- about 25 to 28 percent acid-soluble material tion but it is suggestive that the earliest form (after extraction of salt by distilled water), of calcareous cement on fans is coatings on the whereas dust in Eldorado Valley contains lower sides of gravel particles. about 4 to 5 percent acid-soluble material. The development of calcareous cement is Some of this difference may be due to human greater with increasing age of fan surfaces on activity. the same fan. This is apparently true over a The prevailing winds in western Nevada are wide area of the southwestern United States. from the southwest, west, and northwest. There are no extensive carbonate bedrock out- ROLE OF CEMENTATION IN THE crop areas in the state west of approximately GEOMORPHIC EVOLUTION OF FANS long 118°30' W. and nowhere west (upwind) Those alluvial fans surrounding the Las of this line was there found any carbonate Vegas area which have extensive carbonate cementation beyond light pebble coatings or cementation (Kyle Canyon fan, McCollough local, very weakly developed calcic horizons. fan, and so on) show three or more distinct In eastern Nevada, extensive carbonate bed- and well-preserved surfaces. Adjacent fans rock outcrops occur and noncarbonate fans which do not have such cementation (River downwind of these outcrops commonly show Mountain fan, fans in Eldorado Valley) show well-developed cementation including plugged only a single surface with a few, small, scattered horizons. outliers which cannot be correlated with It is concluded that the source of calcium specific surfaces. An excellent discussion of carbonate for cementation of fans made of fan surfaces is given by Bull (1964). The Kyle noncarbonate detritus is wind-blown dust and, Canyon fan is discussed in detail below but less commonly, drifting sand derived from the conclusions apply to other cemented fans playas, especially playas flanked at least in in southern Nevada. part by carbonate fans. The Kyle Canyon fan (Fig. 1) extends north- eastward from the Spring Mountains. It heads Age of Fan Surface in Kyle Canyon and reaches to the center of Throughout the study area the older (high Las Vegas Valley. Three surfaces can be distin- fan) surfaces exhibit thicker calcic horizons, guished (Fig. 7), including the modern (no. 3) and petrocalcic horizons occur only on older surface (Dolliver, 1968; Lattman, 1971). surfaces. A similar relationship was found by The three surfaces may be fitted by curves Gile and others (1966) in southern New of the form: y = aJ>x (Fig. 8) with highly Mexico. Soil mapping in the Las Vegas and significant regression coefficients greater than Eldorado Valley areas (Langan and others, — 0.92. Extrapolated to the playa at the center 1967) shows Petrocalcic Calciorthid soils re- of Las Vegas Valley, the three surfaces were stricted to older surfaces and Typic Calciorthids apparently graded to a slightly fluctuating base on the younger surfaces. A more detailed dis- level. The history of the deposits at the foot cussion of age of fan surfaces and degree of of this fan has been studied by Haynes (1967). cementation is given below for the Kyle The three surfaces may be distinguished Canyon fan. easily on aerial photographs by color (color photographs) or gray tone (black and white Summary photographs) and each higher, older surface Calcium carbonate cementation throughout shows a progressively lighter buff color or a south and south-central Nevada is well devel- progressively lighter gray tone. The lighter oped on fans composed of carbonate detritus, color or tone is due to exposed petrocalcic and on some noncarbonate fans downwind of horizons, laminar layers, caliche rubble, or playas which are flanked in part by carbonate combinations of these. fans. Poorly sorted, noncalcareous fan deposits The oldest, highest surface (no. 1 of Figs. possessing over 50 percent pebble-size and 7 and 8) shows very hard petrocalcic horizons coarser material in the surface layer show much or surface-hardened calcic horizons everywhere

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NUMBER 3 SURFACE (ACTIVE MODERN PAVEMENT *JJB OHAIftlGC I

I NUMBER Ji SURFACE

Ì I NUMBER ? SURFACE

NUMBER I SURFACE

ISS11 flEOROCI! ^ ACTIVE ORA IMAGE LINE

- - VERTICAL SCARP

x - RAVED ROAD

LINPAVEO ROAD

Figure 7. Gcomoiphic map of the Kyle Canyon fan.

at or within about 1 m of the surface and the The modern (no. 3) surface exhibits pebble risers below this surface are strongly case coatings and a calcic horizon, but no petro- hardened (Lattman and Simonberg, 1971). calcic horizon is present. Isolated patches of Although difficult to determine, the thickness laminar layers occur. Gully-bed cementation of cement underlying this surface appears to is found as isolated patches to extensive sheets vary from about 0.5 to 2 m, and the upper 20 along the major fan-building washes. cm is nonslaking. Caliche rubble litters the As shown in Figure 8, the surfaces converge surface and only a few scattered patches of downfan but, everywhere on the fan where the laminar layers are preserved. The cemented surfaces may be distinguished, they are sepa- horizons and layers are apparently being rated from each other by steep to vertical, destroyed today. case-hardened risers which range in height The well-preserved, intermediate surface from about 60 cm at the dis;tal end to about (no. 2) also shows petrocalcic or surface- 90 m at the proximal end of the fan. hardened calcic horizons extensively covered at, or close to, the surface by laminar layers Evolution of the Kyle Canyon Fan Surfaces (Fig. 9). Some caliche rubble occurs over the The excellent preservation of surfaces 1 and 2 entire surface. Thickness of the cemented is apparently due to the strongly cemented horizons and layers varies from about 30 to layers at or just below the surface (Fig. ¡.OA). about 150 cm. The cemented layers are also Except for local talus cones and colluvial cover undergoing destruction on this surface, but extending from the flanking canyon sides, the the extensive preservation of laminar layers present surfaces are stripped on the cemented indicates that the destruction has not proceeded horizons which formed just below the original as far as on the older surface. surface. That little local denudation has oc-

2500- (JJy = 2649.06 e

£2) y= 2524.02e" 2 250- )y = 246l.57e

2000- SURFACES SHOWN AS DOTS WHERE < EXTRAPOLATED — 1750 in £r Eslt # ** -*' • V • * 4 - •-, UJ1- 1500- £ ------*-- * 2 Figure 10A. View up the Kyle Canyon fan showing 1250- surface 2. Notice strongly cemented layer just below the topographic surface. About halfway down the 1000- far wall is a layer of gully-bed cementation (arrow) underlain by a case-hardened horizon. This layer can be traced downstream only as far as a bedrock barrier.

KILOMETERS FROM HEAD OF CANYON points and little incision occurs in the surfaces Figure 8. Curves fitted to the three surfaces of the upstream from these points. The nickpoints Kyle Canyon fan. Data for curves from Lattman are generally developed on the petrocalcic (1971). horizon or overlying laminar layer but in a curred on surfaces 1 and 2 since they were few cases where no petrocalcic horizon is abandoned by through-flowing drainage is present the nickpoint is maintained by gully- suggested by Figure 8 and the very close fit bed cementation. Lack of incision upstream (r > —0.92) of the actual surfaces to the from the nickpoints has helped preserve broad, curves. Note also the similarity of the curve flat areas of the older surfaces. for the cemented (no. 2) surface and that for Modern, through-flowing washes are incised the uncemented, modern (no. 3) surface. into the older surfaces and, in downcutting, Surfaces 1 and 2 are being dissected today have encountered bedrock barriers (Fig. 7). by rills and gullies which head on these surfaces As a result, local, intermediate surfaces (no. 2a) and drain to the through-flowing fan-building have developed below surface 2 and exhibit washes. All of these rills and gullies have nick- well-developed calcic horizons, rarely surface

. -. , * fi»

- Vf ••. V • • « Figure 9. Laminar layers on surface 2 overlying on which the intervening alluvium is not well ce- well-cemented layers of alluvium. Several laminar mented. Exposure in this view is due to scraping of layers at different levels may be seen (arrows), a re- surface by bulldozer. lation that is also found on the modern (no. 3) surface

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Figure 10B. View across modern wash incised into by gully-bed cementation (arrow) unconformably surface 2 of the Kyle Canyon fan. The higher, former overlying the fan alluvium. course of this wash is marked in the middle foreground

hardened and without laminar cap. These developed the steep, case-hardened valley sides locally developed surfaces are, of course, of the main drainage and the local surfaces younger than surface 2 and formed during a associated with bedrock barriers. shorter time interval. Apparently they did not progress to the stage of development of a Other Fans in Southern Nevada petrocalcic horizon. Nickpoint retreat of rills Development of fan suriaces and drainage and gullys on these surfaces is maintained by lines similar to the Kyle Canyon fan are found gully-bed cementation. on the well-cemented McCollough fan. On uncemented fans of the Las Vegas Basin, no Kyle Canyon Drainage distinct, fan-wide, older surfaces are preserved The main drainage of Kyle Canyon is today. It is concluded that, in southern Ne- incised between case-hardened walls, and its vada, carbonate cementation in the form of higher, former, unincised course is marked by petrocalcic horizons, laminar layers, and gully- a layer of gully-bed cementation (Fig. 10B). bed cementation underlying older surfaces and Upstream from local bedrock barriers the controlling nickpoint retreat along drainage sidewalls of this drainage are marked by aban- lines is a necessary requisite: for the preserva- doned layers of gully-bed cementation formed tion of these surfaces. These three types of during cessation of, or very slow, downcutting cementation are best developed on fans com- (Fig. 10A). As such layers control nickpoint posed of coarse carbonate and basic igneous retreat in incising drainage today (Fig. 10C), rock clasts, less well developed on fans derived it appears that the main drainage incised by from noncarbonate sedimentary rocks, and nickpoint retreat was probably initiated by not developed on fans mads of detritus from the stream capture commonly associated with acid igneous rock ranges. Ths older surfaces are alluvial fans (Lattman, 1971). Each local bed- therefore extensively preserved on the car- rock barrier which temporarily halted down- bonate and basic igneous rock fans, preserved cutting caused development of another local in small patches on other sedimentary rock layer of gully-bed cementation with subse- fans, and not preserved on acid igneous rock quent retreat of a new nickpoint. Thus was fans.

Figure IOC. Modern gully on surface 2 of Kyle cementation visible in left middleground and the gully Canyon fan. The nickpoint is formed by gully-bed walls downstream from the nickpoint are case hardened.

Climatic Controls on Cementation tion of the cemented layers. Whether or not incision of surfaces is initiated by changes in As shown on Figures 7 and 8, surface 1 on climate, the calcic soils and cementation on the Kyle Canyon fan extends from near the the surfaces are apparently not accurate indi- head of Kyle Canyon (where the surface is at cators of Pleistocene climatic fluctuations. an elevation of about 2,590 m) well out into These soils may, therefore, be difficult to use Las Vegas Valley where remnants are pre- regionally for correlation of surfaces, a difficulty served at elevations as low as 1,370 m. Rainfall which is increased by variation of cementation near the head of Kyle Canyon today is about due to different fan lithologies and availability 50 cm per yr and is about 20 cm per yr in Las of eolian material. Vegas Valley. Throughout the entire extent of surface 1, the cementation is similar and is Age of Cementation being destroyed. There is no evidence that the Gile and others (1966), reporting on a study original cementation differed noticeably from in southern New Mexico, have noted that point to point on the surface despite the range plugged horizons (their Stage III) and plugged in rainfall that must have existed at the time horizons with overlying laminar layers (their the cement formed. It appears, therefore, that Stage IV) occur on older, late Pleistocene sur- extensive calcium carbonate cementation oc- faces in gravelly material and on nongravelly curred on some fans in southern Nevada over a material only on mid-Pleistocene surfaces. No wide rainfall range, a wider range than is plugged or petrocalcic horizons with laminar usually postulated for local variation due to layers are found in nongravelly material in Pleistocene climatic changes. It is concluded southern Nevada. that Pleistocene climatic changes probably did The present study indicates that laminar not significantly affect the areal distribution layers may not only be an indicator of late of calcium carbonate deposition in southern stage in the soil-forming process but may Nevada. locally be of nonpedogenic origin. Additionally, On the other hand, incision of the main the oldest surface on the Kyle Canyon fan has drainage below a fan surface does stop extensive almost no laminar layer today, apparently due cementation and initiates a period of destruc- to its removal as caliche rubble because isolated

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fragments of laminar layers are found loose or Bi ll, W. B., 1964, Geomorphology of segmented the surface. On this surface, the calcic horizon alluvial fans in western Fresno County, Cali- fornia: U.S. Geol. Survey Prof. Paper 352-E, is surface hardened to a petrocalcic horizon p. E89-E128. which is also being removed as caliche rubble. Denny, C. S., 1965, Alluvial fans in the Death The depth of surface hardening here, about Valley rsgion, California and Nevada, U.S. 20 cm, is independent of microrelief or thick- Geol. Survey Prof. Paper 466, 62 p. ness of cementation. This suggests that surface Dolliver, C. V., 1968, Late Tertiary and Quater- hardening is continuing to develop downward nary history of Kyle Canyon, Spring Moun- to form a petrocalcic horizon as the hardened tains, Nevada [Ph.D. thesis]: University Park, material is being removed from surface 1 Pennsylvania State Univ., 95 p. today. It is concluded that, on fans built of Flach, K. W„ and Smith, G. D„ 1969, The new carbonate and basic igneous rock detritus in system of soil classification applied to arid-land soils, in McGinnies, W. G., and Goldman, southern Nevada, plugged and petrocalcic B. J., eds., Arid lands ,.n perspective: Am. horizons (but not laminar layers) will continue Assoc. Adv. Sci. Pub. and Tucson, Arizona to be formed as material is removed from Univ. Press, p. 59-73. abandoned fan surfaces. The surface will be Gardner, R. G., 1972, Origin of the Mormon preserved until consumed by nickpoint and Mesa caliche, Clark County, Nevada: Geol. scarp retreat. Soc. America Bull., v. 83, p. 143-156. From these observations, it must be con- Gile, L. H., Peterson, F. F., and Grossman, R. B., cluded that the presence or absence of a 1965, The K horizon: A master soil horizon of carbonate accumulaticn: Soil Sci., v. 99, laminar layer or a petrocalcic horizon is not a p. 74-82. reliable indicator of age and cannot be used ——- 1966, Morphological and genetic sequences for regional correlation or dating of surfaces. of carbonate accumulation in desert soils: Soil Sci., v. 101, p. 347-360. ACKNOWLEDGMENTS Haynes, C. V., 1967, Quaternary geology of the The study of the various types of cementa- Tule Springs area, Clark. County, Nevada: tion and the factors influencing their distribu- Nevada State Mus. Anthropological Papers, tion is part of an investigation supported by a no. 13, pt. 1, 104 p. grant from Army Research Office—Durham. Langan, L. N., Larsen, L. I., and George, C. J., 1967, Soil survey of Las Vegas and Eldorado The Kyle Canyon fan had been previously Valleys area, Nevada: Soil Conservation Serv- studied under a grant from Air Force Cam- ice, U.S. Dept. Agriculture, ser. 1957, no. 23, bridge Research Laboratories. 87 p. Desert Research Institute, Las Vegas, ex- Lattman, L. H., 1971, Geomorphology of the east tended logistic support and Burke Maxey and flank of the Spring Mountains, Nevada: Air Richard Cooley of the Institute gave gener- Force Cambridge Research Laboratories, Rept. ously of their experience and knowledge of AFCRL-71-0326, 89 p. Nevada. Finn Bronner of Army Research Lattman, L. H., and Simonberg. E. M., 1971, Case- Office—Durham, and Warren Grabau of Wa- hardening of carbonate alluvium and col- terways Experiment Station—Vicksburg, vis- luvium, Spring Mountains, Nevada: Jour. Sed. Petrology, v. 41, no. I, p. 274-281. ited the writer in the field, were helpfully hard Longwell, C. R., Pampeyan, E. H., Brower, B., to convince, and gave valuable suggestions. and Roberto, R. J., 1965, Geology and mineral Wayne Pryor, University of Cincinnati, did deposits of Clark County, Nevada: Nevada the thin-section analysis and William Bull, Bur. Mines Bull. 62, 218 p. University of Arizona, read the manuscript and Soil Survey Staff, 1967, Supplement to soil classi- made valuable suggestions. fication system (7th approximation): Soil Con- servation Service, U.S. Dept. Agriculture, REFERENCES CITED 207 p. Bretz, J. H„ and Horberg, L., 1949, Caliche in southeastern New Mexico: Jour. Geology, v. 57, p. 491-511. Brown, C. N., 1956, The origin of caliche on the northeastern Llano Estacado, Texas: Jour. MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY Geology, v. 64, p. 1-5. 17. 1973

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