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GEOLOGY, SOILS, AND GEOMORPHOLOGY OF THE WALNUT GULCHEXPERIMENTAL WATERSHED, TOMBSTONE, ARIZONA

W. R. OSTERKAMP, U.S. Geological Survey, National Research Program, 1955 E 6th St, Ste 115, Tucson,AZ 85719

ABSTRACT The surface geology of the Walnut Gulch Experimental Watershed (WGEW), Tombstone, Arizona, is dominated by fan

deposits, but in southern and southeastern parts of WGEW a complex history of tectonism has resulted in igneous-intrusive andvolcanic rocks, and highly disturbed Paleozoic and Mesozoic rocks in the Tombstone Hills. Soils, which are dominantly sand andgravel loams that vary from deep and well drained to thin and immature, are reflective of the rocks on which they formed. Largelandforms are mostly dissected pediments and erosion surfaces, and hills of the volcanic and carbonate rocks. Episodic faultingthat began in Precambrian time has resulted in complex geologic and geomorphic conditions that remain poorly understood owingto Basin and Range structural and depositional processes. Small-scale landforms of the watershed are individual hills, undissectedremnants of alluvial fans (fan terraces), basin floors, alluvial fans, and recent alluvial sediment of stream channels, flood plains,and terrace-inset deposits. This paper combines the results of previous studies with recent field investigations and analysis of aerialphotography to yield a summary of watershed conditions in support of ongoing research.

INTRODUCTIONThe Walnut Gulch Experimental Watershed

(WGEW) is part of the U.S. Department of Agriculture(USDA) Agricultural Research Service's (ARS) South-west Watershed Research Center (SWRC), Tucson, AZ(Fig. 1). Walnut Gulch is a major tributary of the upperSan Pedro River, entering it from the east. The 150-km2

watershed is equipped with 88 rain gages and 15 runoffflumes in 12 intensively studied subwatersheds; num-bered flumes, such as number 10, cited herein refer tothe drainage area of the respective subwatersheds desig-nated by a five-digit number starting with 63.0 (Fig. 1).A principal goal is to relate rainfall, runoff, and sedi-ment yield to land use through erosion modeling. Tomeet this goal, basic knowledge of watershed character-istics is essential. Information describing geologic (rocktypes, structural relations, and history), landform- andgeomorphic-processes, and soil relations is no longeradequate to meet the research objective. An investiga-tion was initiated in 1996 to compile expanded baselineinformation of watershed characteristics. The field-based investigations have been augmented by the use ofaerial photography, imagery analysis and GIS techni-ques, and by integrating available information withrecently published data and interpretations of those data. Accompanying maps of WGEW depict soil distribu-tions and geomorphic features resulting from erosionaland depositional processes. Aerial photography, 1:24,000scale, was used for the mapping; additional analysis wasbased on 1:5000 orthophotographs. Rock exposures,sediment, and landforms constituting topographic reliefin WGEW were the focus of the mapping. Field studiesof the geology and geomorphology examined rock andsoil exposed on hillslopes and at river banks, gullies,and road cuts. Mapped contacts are based on field obser-vations and previous geologic investigations, but areinferred where masked by soil, vegetation cover, or

human activities. Separate deposits of conglomerate andoverlying alluvium in the watershed are interpreted fromcharacteristics of tectonic disturbance, soil development,degree of cementation, particle-size distribution, andsource rocks contributing to the deposits.

GEOLOGYThe geology of WGEW is expressed by consoli-

dated rocks and fan and alluvial deposits that range inage from Precambrian to Recent. Except for moderndeposits, all of the rock units have been complexlyfaulted and folded during a series of tectonic episodesthat resulted in a Basin and Range physiography,emplacement of igneous intrusive and extrusive rocks,and the occurrence of related mineral deposits of theTombstone Hills, south of Tucson (Fig. 1).

Rock Units Rock types in WGEW include lithified sedimentary,

plutonic, and volcanic rocks, and fan deposits and allu-vium, with varying degrees of calcrete cementation,derived from weathering of exposed rocks (Fig. 2). Thesedimentary, plutonic, and volcanic rocks range in agefrom Precambrian through late Cenozoic and were dis-placed by moderate to major tectonism. Detailed des-criptions of the petrology, age, geologic history, andchemistry of these formations are reported by, amongothers, Gilluly et al. (1954), Gilluly (1956), Bryant(1968), Drewes (1981), and Force (1996).

Lithified Sedimentary, Plutonic, and Volcanic Rocks.The oldest rock unit of the Walnut Gulch Basin is anunnamed, Precambrian gneissic granite exposed near theheadwaters area of the Dragoon Mountains (Gilluly1956, p. 13). The sheared granite forms much of thegrass-covered pediment at the base of the DragoonMountains. Stratigraphically higher and exposed only in

OSTERKAMP, W. R. 2008. GEOLOGY, SOILS, AND GEOMORPHOLOGY OF THE WALNUT GULCH EXPERIMENTAL WATERSHED, TOMBSTONE, ARIZONA.JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE 40(2):136-154.

GEOLOGY, SOILS, AND GEOMORPHOLOGY OF WALNUT GULCH WATERSHED g OSTERKAMP 137

Figure 1. Walnut Gulch Experimental Watershed location maps.

138 GEOLOGY, SOILS, AND GEOMORPHOLOGY OF WALNUT GULCH WATERSHED g OSTERKAMP

a north-south band about a kilometer west of MilitaryHill (mostly in sections 14, 15, 22, 23, and 27, T. 20 S.,R. 22 E.) (Fig. 1c) is the middle- to late-Cambrian BolsaQuartzite, a littoral, transgressive-sea sand and graveldeposit (Bryant 1968). Crossbedding is common in theformation, the lower part of which typically is coarsegrained and rich in feldspar, whereas the finer-grainedupper part contains little feldspar (Krieger 1968). Themost erosion-resistant formation of the Tombstone area,the Bolsa Quartzite forms high ridges and part of thesouthwestern basin divide (Fig. 2).

Marine limestones, with interbedded shale, sand-stone, and dolomite, were deposited intermittently fromlate-Cambrian through Permian time; in ascending orderthey are the Abrigo (late Cambrian), Martin (Devonian),and Escabrosa (early Mississippian) Limestones, and theHorquilla Limestone (Pennsylvanian), Earp Formation(Pennsylvanian/Permian), Colina Limestone (Permian),and Epitaph Dolomite (Permian), which comprise theNaco Group. These Paleozoic carbonate rocks weremoved to their present positions in late-Cretaceous time

by regional occurrences of widespread Laramide over-thrusting (Drewes 1981) and form hillslopes and erosionsurfaces near the western and southern limits of WGEW.

Outcroppings of Abrigo Limestone and MartinLimestone, which is easily eroded and forms gentleslopes (Gilluly et al. 1954), are at the same north-striking folds where the Bolsa Quartzite occurs. Theupward sequence continues eastward with the EscabrosaLimestone at the top of Military Hill and a larger area ofHorquilla Limestone, which fills a faulted syncline,immediately to the east. The crinoid-rich, thick-beddedEscabrosa Limestone resists erosion and forms conspic-uous, poorly vegetated cliffs of the higher hills (Gillulyet al. 1954, Bryant 1968). The Horquilla Limestone isthe most widespread of the Paleozoic carbonate forma-tions, underlying most of the eastern part of the Tomb-stone Hills; owing to numerous soft, thin shale beds, iterodes readily to gently sloping hills (Bryant 1968). Theuppermost formations of the Naco Group, the EarpFormation, the Colina Limestone, and the Epitaph Dolo-mite, are exposed only in small areas of the Tombstone

Figure 2. Geologic map of the Walnut Gulch Experimental Watershed.


Hills. The Earp Formation is on a part of the down-thrown block of the high-angle Prompter thrust fault, akilometer north of Military Hill. The Colina Limestoneis widespread and resistant to erosion in the TombstoneHills (Gilluly et al. 1954) and is faulted to the surfacenorth of Walnut Gulch a kilometer upstream from Flume2. Exposures of the lower part of the Epitaph Dolomiteare resistant to erosion and form cliffs, whereas theupper part has thin shale and limestone beds that readilyerode (Gilluly et al. 1954). The aptly named EpitaphDolomite forms much of Comstock Hill (and nearby“Boothill”) northwest of Tombstone and also outcropsimmediately north of the Colina Limestone near Flume 2.

A small area at the base of the southern DragoonMountains is underlain by the Gleeson Quartz Monzo-nite, an easily weathered, coarse-grained stock rich inquartz, plagioclase, hornblende, and biotite. An erosion-resistant alaskitic facies of the Gleeson Quartz Monzo-nite occurs in the uppermost WGEW and supports anoak woodland canopy. Largely by radiometric dating(K-Ar) of biotite in rock samples from the TombstoneHills and Dragoon Mountains, Hayes and Drewes(1968) assigned a mid-Jurassic age to plutons such asthe Gleeson Quartz Monzonite; this conclusion wassupported by Anderson (1968), who determined an ageof 178±5 M years from K-Ar dating of muscovite takenfrom the monzonite. Later, Drewes (1976) used K-Ardating to establish an early Jurassic age for the GleesonQuartz Monzonite.

Beds of the late Jurassic to early Cretaceous BisbeeGroup unconformably overlie the overthrust carbonaterocks of the Tombstone Hills in the southwestern part ofWGEW (Hayes and Drewes 1968, Force 1996). Reacti-vation of Precambrian age, northwest-trending faults inearly Mesozoic time caused increased relief near thefaults. Basal deposits resulting from the renewed move-ments were thick, coarse conglomerates and sandstones.Widely distributed and generally alternating arkosicsandstones, deltaic sandstones, mudstones, and lime-stones grade upwards in the Bisbee Group, reflectinglowered energy conditions (Drewes 1981, Force 1996).

The principal hosts for the silver deposits andrelated minerals of the Tombstone mining district arebeds of the Bisbee Group offset by high-angle faults andinjected by quartzitic veins (Force 1996). Where therm-ally altered in the Tombstone Hills, strata of the BisbeeGroup are resistant to erosion and may form ridges, butotherwise the beds weather to rounded hills less prom-inent than those of nearby calcareous rocks (Gilluly1956). Most outcroppings of the Bisbee Group inWGEW are alined roughly from a small patch about 8km southeast of Tombstone to larger areas of exposure2 km southeast of and directly south of Tombstone. Alarge area of Bisbee Group sub-parallels Walnut Gulchsouth and west of Tombstone, and the most northerlyoutcroppings are related to faulting adjacent to WalnutGulch upstream 2 to 3 km from Flume 2 (Fig. 2).

Named for Uncle Sam Hill on the divide 5 kmsouthwest of Tombstone, the Uncle Sam Porphyry is aresistant quartz-latite to quartz-monzonite porphyryunderlying much of the southwestern Tombstone Hillsand extending northward at least to Flume 2 (Fig. 2).Using radiometric dating, Marvin et al. (1973) deter-mined a late Cretaceous age for the Uncle Sam Por-phyry. In the Tombstone Hills the Uncle Sam Porphyryerodes to rugged escarpments, but to the southwest it isexposed as dissected pediment. Much of the Uncle SamPorphyry is an extrusive rock body that locally overliesthe Schieffelin Granodiorite and thus may be equivalentin age (Force 1996). With its quartz, feldspar, andcorundum phenocrysts, it intrudes older, underlyingrocks of the Tombstone area but to the southwest ofWGEW the porphyry is cut by and thus is of similar ageor slightly older than adjacent emplacements ofSchieffelin Granodiorite (Gilluly 1956).

The feldspar-rich, quartz-poor Schieffelin Grano-diorite locally grades to a quartz monzonite (Gilluly1956). It is closely related in age to the ore deposits 1 to4 km southwest and west of Tombstone. As are otherrock units of WGEW, the Schieffelin Granodiorite isroughly oriented northwest, alined with the fault systemof the Tombstone Hills. The northernmost exposure, atWalnut Gulch upstream from Flume 2, overlies faultedPaleozoic rocks. Although a K-Ar date of biotite fromthe Schieffelin Granodiorite gave a late Cretaceous agesimilar to the Uncle Sam Porphyry (Creasey and Kistler1962), its stratigraphic position above the Uncle SamPorphyry and below the oldest fan deposits of WGEWindicates that it is slightly younger than the Uncle SamProphyry. Owing to relatively high susceptibility tochemical breakdown, the Schieffelin Granodioriteweathers to subdued erosion surfaces that slope gen-erally northeastward from the Tombstone Hills towardWalnut Gulch.

Several small masses of resistant rhyolite intrude thePaleozoic limestones and form topographic highs andpart of the basin divide in the southern part of WGEW.The unnamed intrusions are sills and dikes up to 150 min thickness (Gilluly 1956). Biotite from the rhyoliteyielded a K-Ar date of 63 M years, indicating a veryearly Paleocene age (Creasey and Kistler 1962). Therhyolite intrusions overlie complexly folded and faultedbeds of Paleozoic carbonate rocks, mostly of the ColinaLimestone, 6 to 7 km south of Tombstone in section 31,T. 20 S., R. 23 E.

The S O Volcanics, named for exposures at S ORanch (not shown) 13 km east of Tombstone, are thickquartz-latite tuffs and hornblende-andesite flows that aredistributed along the southeastern basin margin fromStockton Hill (section 7, T. 20 S., R. 24 E.) westwardnearly 7 km (Fig. 2). Andesite flows, with black horn-blende phenocrysts up to 30 mm in length, formrounded but prominent hills and mesas; elsewhere, rela-tively soft tuffs of the lower S O Volcanics erode readily


and thus are rarely exposed. A sample from the tuffmember yielded a K-Ar date of about 47 M years, ormid-Eocene in age (Marvin et al. 1973). The S OVolcanics are low to intermediate in density and contrib-ute to a gravity low beneath exposures east of the Tomb-stone Hills (Spangler 1969).

The youngest of the volcanic rocks in WGEW is anolivine basalt exposed along Walnut Gulch a kilometernortheast of Tombstone. The age of the basalt is notknown, but because it intrudes fan deposits of likelyMiocene age and is well weathered, a late Miocene orearly Pliocene age is assumed. The small volcanic bodyis one of several of late Cenozoic age between the Dra-goon Mountains and the Tombstone Hills that implymovement of lava along otherwise concealed fault zones(Drewes 1981).

Fan Deposits and Alluvium. Poorly to well-cementedalluvial deposits in WGEW include the Emerald Gulchand Gleeson Road Conglomerates (also termed fanglom-erates), the Jones Ranch Alluvium, and unconsolidatedstream alluvium (Fig. 2). Because exposures of the allu-vial deposits are small, they often are mapped as undif-ferentiated alluvium. The names Emerald Gulch Con-glomerate, Gleeson Road Conglomerate, and JonesRanch Alluvium, which were suggested by M. A.Alonso as alternatives to her previous designations ofAlluvium I, II, and III (Alonso 1997), are used hereinand are intended for application only to local deposits;they have not been submitted for approval as formalgeologic names.

The Emerald Gulch Conglomerate is named forexposures along lower Emerald Gulch east of Tomb-stone. It is the oldest of the alluvial beds, equivalent inage to the deformed fan deposits of Eocene throughearly Miocene age of Melton (1965) and likely to theMiocene age Pantano Formation of the Santa Cruz RiverBasin (Brown et al. 1966, Pool and Coes 1999),including the Tucson area (Fig. 1a). The Emerald GulchConglomerate is virtually limited to channel bottoms,principally at an unnamed draw heading at the southerndivide of WGEW; outcroppings extend from 0.5 kmdownstream from a stock pond (#12, in section 20, T. 20S., R. 23 E.) to the site of Flume 15 (NW ¼ of section 7,T. 20 S., R. 23 E.), immediately upstream from WalnutGulch. Minor exposures, probably displaced to the sur-face by faulting, occur near Flumes 8 and 9. The con-glomerate has massive, 1-to-2-m thick gray to whitebeds of gravel and cobbles separated by thin, sandyinterbeds. It is well cemented with sandy calcrete ofprobable ground-water origin and contains clasts aslarge as 0.8 m of limestone and sandstone derived fromthe Paleozoic and Mesozoic sections and smallerfragments of volcanic rocks and flint.

Dissected beds of the mid- to late Cenozoic GleesonRoad Conglomerate, correlative with the Gila Conglom-erate of Gilluly (1956), the undeformed basin fill ofMelton (1965), and upper and lower basin fill of Brown

et al. (1966), are widespread in WGEW. The lowermostbeds of the Gleeson Road Conglomerate are inferred tobe correlative to the lower basin fill of Brown et al.(1966) in the Sierra Vista area (Fig. 1b), and the upperpart of the Gleeson Road Conglomerate is probablyequivalent both stratigraphically and in age to the Plio-Pleistocene upper basin fill of Brown et al. (1966). Tothe west and northwest, along the axis of the San PedroRiver Valley (Fig. 1b), the upper part of the GleesonRoad Conglomerate grades into fine-grained fluviatileand lacustrine beds of the Plio-Pleistocene St. DavidFormation (Gray 1965, Melton 1965).

The Gleeson Road Conglomerate is named for Glee-son Road, which traverses beds and soils derived fromconglomerate eastward between Tombstone and thetown of Gleeson. Most of WGEW is directly underlainby the Gleeson Road Conglomerate, exceptions being inthe Tombstone Hills where carbonate, clastic, and vari-ous igneous rocks are at the surface, and in the south-eastern part of the drainage basin, where S O Volcanicsare exposed. The Gleeson Road Conglomerate varies inthickness from veneers overlying near-surface bedrockto at least 900 m in the north-central part of WGEW(Spangler 1969). Although Melton (1965) described itas undeformed, locally the fanglomerate is extensivelyfractured where underlying faults have reactivated, andat larger areal scales the formation is tilted owing to lateCenozoic fault-block movement (Stewart 1980).

Terraces of the conglomerate are underlain by amature red to brown soil that elsewhere in the San PedroRiver Valley (Melton 1965, Haynes 1968) has an age ofabout 30,000 years. Pronounced 20th century gullying ofthe mostly massive, undeformed conglomerates andpoorly cemented sand and silt partings of the GleesonRoad Conglomerate is widespread. Most near-surfacestrata contain less than 10% carbonate (Breckenfeld1994), and thus have low resistance to mechanicalerosion. Abundant bedforms, channel-fill and alluvial-plain depositional sequences, and hydromorphic paleo-sols within the conglomerate indicate that WGEW, priorto settlement, was characterized by less variable dis-charges, deeper channels, and higher ground-waterlevels than those of the current drainage system.

Clasts of the Gleeson Road Conglomerate mostly arederived from nearby bedrock. Most clasts in eastern partsof WGEW are from plutonic or volcanic rocks, whereaslimestone clasts predominate in the southern part. Thistendency is modified where paleostreams transportedcoarse sediment away from the local source. Vegetationis highly variable, but grasses are typically most densewhere clasts of the S O Volcanics are abundant.

The Jones Ranch Alluvium (Fig. 2), the cienegadeposits of Melton (1965) and part of the pre-entrench-ment alluvium of Brown et al. (1966), is transitional inage between the Gleeson Road Conglomerate andHolocene alluvium. The color is generally brownish-pink in contrast to brownish-gray of most Holocene


deposits. The Jones Ranch Alluvium includes the oldestinset deposits of the present drainage system. Thus, itrepresents late Quaternary fan and terrace strata of silt,sand, and gravel that mostly are topographically higherthan the most recent channel deposits and that werepartially removed by late 19th and 20th century erosion.The channel, flood-plain, and terrace deposits of theJones Ranch Alluvium are up to 3 m thick, may becapped by a paleosol, and, having little or no carbonatecement, are easily eroded. Included also in the JonesRanch Alluvium are fan deposits along mountain-frontfaults, such as those at Jones Ranch near the headwatersof Walnut Gulch. Where fault-scarp deposition hasoccurred, the Jones Ranch Alluvium may be tens ofmeters thick.

The youngest beds of the watershed are mostly lateHolocene flood-plain, bar, and channel deposits of sandand gravel. This alluvium partially refills incisionsdeveloped by post-development gully erosion. Most ofthe deposits are bars and terraces up to 2 m above mod-ern stream channels, but locally, such as at the TobosaSwale (section 21, T. 20 S., R. 23 E.) and Cowan Ranch(section 12, T. 20 S., R. 23 E.), mid- to late Holoceneswamp deposits occupy closed depressions caused bylate-Quaternary faulting. The alluvial and paludal(swamp) deposits typically support dense grass. Whererecent gully erosion has exposed dark, carbonate-richpaludal beds at Cowan Ranch, radiocarbon datingyielded an age of about 5,200 years.

Structural Geology and Geologic HistoryUnlike many areas of the Basin and Range Province,

much of WGEW is dominated by sedimentary rocks ofthe Paleozoic and Mesozoic sections, several granitic andgneissic intrusions of various ages, and a range ofvolcanic rocks related to block-fault tectonism. Cementedfan deposits and alluvium, typical of the Basin andRange, are at the surface in much of WGEW, but mostlyas small-to-moderate thicknesses overlying bedrock.

Structural features in WGEW principally are prod-ucts of tectonic episodes of Precambrian, early andmiddle Mesozoic, late Mesozoic to early Cenozoic, andmid-to-late Cenozoic times (Drewes 1981). Althoughfolds and faults in Precambrian granitic rocks are diffi-cult to recognize owing to reactivation of crustalstresses, the large-scale features, including the plutons,remain as prominent structural features reflecting earlytectonism. Precambrian rocks were deformed furtherduring Mesozoic time both by deep plutonic emplace-ments and by the intrusion at shallower levels of dikesand related tabular rocks.

The tectonism that resulted in the present Basin andRange physiography of southeastern Arizona began atthe start of the Mesozoic Era. In early Triassic throughearliest Cretaceous time, rocks of the watershed andadjacent areas were compressed, causing block faultingand a second period of intrusion by igneous masses such

as the Gleeson Quartz Monzonite, the Uncle Sam Por-phyry, and the Schieffelin Granodiorite. Related to igne-ous activity in the Tombstone Hills area was theemplacement along existing faults of mineralized quartzveins and porphyry dikes (Force 1996). The Mesozoictectonism culminated in late Cretaceous time withregional overthrust faulting.

Starting in the early Tertiary, the last major tectonicevent yielding the present topography of the WalnutGulch area was relaxation of compressional forces thatcaused the Mesozoic faulting and igneous activity. Thechange caused southwest to northeast tension andrenewed block faulting typical of the Basin and RangeProvince. This latest tectonic period extended into Holo-cene time and yielded Paleocene rhyolite flows and theEocene S O Volcanics. In all cases of tectonism,exposed rocks were eroded, and the sediment wasdeposited as alluvial fans or valley fill (Drewes 1981).

The effects of tectonic events of the Walnut Gulcharea, including volcanic activity and igneous intrusion,have been very complex. Faults and folds in rocks of alate Cretaceous thrust plate that partially form theTombstone Hills are numerous and are only summarizedhere, and in other parts of WGEW many structural fea-tures are no doubt covered and thereby concealed byyounger rocks or more recent tectonic events. Details ofthe structural geology of WGEW were described in pre-viously cited reports, especially those of Gilluly (1956),Drewes (1981), and Force (1996); a regional perspectivewas given by Menges and Pearthree (1989).

Three major fault systems are related to the tectonicevents of the watershed. The first has had recurringmovements beginning in Precambrian time. The faultsare mostly high-angle normal shears oriented northwest,one of which appears to extend from southeast of Bisbeeto the northeast flank of the Mule Mountains, passingeast of Tombstone and Benson before entering theTucson Basin (Drewes 1981).

The second set includes the Mesozoic compressionalblock faults and large-scale, very low-angle late-Cretaceous thrust faults that moved Paleozoic andMesozoic strata, locally with Precambrian crystallinerocks, northeastward up to 200 km. At least two over-thrusts related to plate tectonics, indicated at field sitesbut not fully verified by Drewes (1981), appear to havecovered WGEW and adjacent areas and moved rocksthat comprise the Tombstone Hills into their presentpositions. The eastern edge of the second, the Cochisethrust plate, abuts the southwestern Dragoon Mountainsand contains numerous folds and high-angle faultsassociated with the overthrust movements (Drewes1981). The faults are common along the Dragoon Moun-tains, but many are concealed by sediment or are poorlyexposed. Only one of the thrust faults, forming thewestern extent of the small outcropping of Precambriangneissic granite, is apparent within the narrow head-water area of WGEW (Gilluly 1956, p. 13).


Regional faults of the third set are extensional, havehad continuing movement since Oligocene time, andresulted in the Basin and Range topography of areasfrom Oregon south through Nevada into Arizona, NewMexico, and northern Mexico. The high-angle detach-ment faults mostly trend north, displace older structuralfeatures, and determine the landscape of the Basin andRange Province. In southeastern Arizona, the dominanttrend is northwest, as shown by the alinement of theDragoon Mountains and the Mule Mountains. None ofthe major Basin and Range faults is known to traversethe Walnut Gulch area, but several secondary faults mayoffset rocks of the watershed.

The uppermost part of WGEW is in the DragoonMountains, a prominent northwest-southeast range ofsoutheast Arizona. Gilluly (1956) interpreted the Dra-goon Mountains to be an individual fault-block rangeand identified and mapped a Dragoon fault as a suite ofmostly high-angle thrust faults, some poorly exposed,along the western base of the range. Later, Drewes(1981) interpreted the high-angle faults to be secondaryfeatures of the large, compressive Cochise thrust plate;his investigations suggested that, because rocks of theCochise thrust plate do not exhibit major fracturing,Cenozoic tensional block faulting did not occur in theunderlying rocks. Based on the interpretations ofDrewes (1981), therefore, it is inferred that the southernDragoon Mountains and basement rocks beneath theTombstone Hills east of Tombstone are parts of thesame Basin and Range detachment block.

Many fracture zones beneath erosion surfaces of thewatershed are complexes of high-angle thrust faults andnormal faults, some of which displace Paleozoic beds ofthe Tombstone Hills. The largest, most well-knownfaults, such as the Prompter Fault about 2 km south ofTombstone, strike north-northwest. Less frequently,prominent faults strike northeast or, as does the maintrace of the Prompter Fault, nearly east-west; examplesthat may be en-eschelon sets that also exhibit strike-slipmovement are near Military Hill south of Tombstone(Gilluly 1956, pl. 5). Most of the steep faults are closelyrelated to larger-scale Basin and Range tensionalfaulting or to igneous activity during the latter part ofthe tectonism. The combined overthrusting and exten-sional warping resulted in numerous small folds, seem-ingly randomly oriented, and several larger folds thaterode to steep escarpments of the Paleozoic and Meso-zoic rocks. Consequently, strikes and dips measured onfolded Paleozoic rocks in the Tombstone Hills byGilluly (1956) showed no apparent pattern.

From logs of deep wells, Gilluly (1956) inferred aconcealed, east-trending fault or fault zone north of theTombstone Hills, the northern, downthrown side ofwhich has a much greater thickness of fan deposits thanis present to the south. Spangler (1969) confirmed theconclusion with seismic profiles, one indicating a high-angle normal fault in the southwest corner of section 35,

T. 19 S., R. 22 E.; the southern, upthrown block isSchieffelin Granodiorite beneath 120 m of GleesonRoad Conglomerate, whereas the north block has alarge, undetermined thickness of conglomerate. Comple-mentary with these observations, a prominent gravity“low,” indicating thick alluvium, extends northwestfrom Walnut Gulch about 10 km east of Tombstone(Spangler 1969). Small extrusions of basalt, such as thatnortheast of Tombstone (Fig. 2), and apparent fault con-trol of the Walnut Gulch channel downstream fromFlume 6 are consistent with the well-log and seismic-profile evidence. Gilluly (1956) interpreted the fault toseparate Basin and Range blocks, implicitly suggestingwhy separate erosion surfaces are apparent west of theDragoon Mountains and in areas within and adjacent tothe Tombstone Hills.

About 80% of WGEW is underlain by largelyunknown thicknesses of fan deposits. The seismic-profile data of Spangler (1969), however, which mainlywere from the lower part of the watershed along WalnutGulch, indicate that thicknesses of the Gleeson RoadConglomerate south of Walnut Gulch are mostly lessthan 100 m, but north of Walnut Gulch they typicallyexceed 200 m. Beds of the fan deposits, especially of theEmerald Gulch Conglomerate, have been altered by neo-tectonic folding and small-scale faulting, by carbonate(calcrete) deposition, and locally by hydrothermalcementation. Typically, these fan deposits, or fanglom-erates, are veneered by 1 to 6 m of Quaternary alluvialgravel that also may be well cemented by calrete(Gilluly 1956, Alonso 1997).

The episodes of Pliocene to Recent tensional stressin southern Arizona (Stewart 1980) tilted and faulted fandeposits and alluvium. The faults control channel shapesand positions of several stream reaches and the sites offormer swamp deposition (Cooley 1968, Menges andPearthree 1989, Alonso 1997). The faults also mayaffect transmission loss during streamflow, and thusground-water recharge. In places, Quaternary faulting inWGEW has resulted in the deposition of fluvial andpaludal beds of the Jones Ranch Alluvium in contactwith older fan deposits and volcanic rocks. Downthrownfault blocks have caused local areas of subsidence, andthese swale areas have become sites of swamp depositsup to 2 m in thickness.

Mapping by Force (1996) of faults, folds, and dikesof the Tombstone Hills mining district, part of which isin WGEW, identified structures in the Bisbee Group andintrusive rocks of similar age along which hydrothermalmineralization occurred. Many of the structures strikeslightly east of north to N 40EE, and most show little orno relation to the drainage network. Faults strikingN75EE or north to N15EE along the channel ofsubwatershed 63.015 displace beds of the Gleeson Roadand Emerald Gulch Conglomerates and thereby deter-mine channel positions and outcroppings of bedrock.Faults dipping 40, 75, and 50 degrees along channels of


subwatersheds 63.001 and 63.009 displace the GleesonRoad Conglomerate and thus have influenced depositionof the Jones Ranch Alluvium. Many small faults in thePlio-Quaternary deposits along the San Pedro Rivershow that the area remains tectonically active.

SOILS Soil, as a product of natural hydrologic and geomor-

phic processes, is a layered mass of minerals and, gener-ally, organic matter and rock fragments that differs fromthe parent material (rocks) from which it is derived interms of morphology, physical and chemical characteris-tics, organisms, and organic content. The layers, or hori-zons, that comprise a soil are of variable thickness (asalso are soil bodies), are typically but not alwaysunconsolidated, and differ from each other in terms ofdegree of alteration that has occurred during theweathering process of the underlying parent material(Joffe 1949). The objectives of this part of the WGEWinvestigations were to interpret which variables mostsignificantly influence soil types and distribution in thewatershed, and to summarize soil characteristics relativeto watershed conditions of climate, time, geology, land-forms, and vegetation. In the WGEW, and many areasof southeastern Arizona, geology is inferred to exert amajor control on soil distribution, maturity, thickness,and permeability. Most soils of the watershed are uncon-solidated, but locally near-surface soil horizons may bemoderately to well consolidated owing to the depositionof calcrete.

Soil SurveysThe Natural Resource Conservation Service (NRCS)

is responsible for conducting and publishing soil surveysin the United States. The presentation of soils informa-tion has evolved from printed descriptions of pedons,tables of soil physical and chemical characteristics, andmaps of soil series distributions to internet-based geo-graphical information systems (GIS) such as STATSGOand SSURGO. The first soil survey of the WGEW wasconducted by the NRCS in the late 1960s (Gelderman1970) and contained pedon descriptions and locations of21 soil map units. Physical and chemical properties ofthe soil series of the map units became available in 1974(USDA Soil Conservation Service 1974). Currentlythree GIS soil surveys are available for the WGEW:STATSGO, consisting of three soil map units; SSURGO,consisting of 18 soil map units; and a more detailed sur-vey (Breckenfeld 1994) that is based on the SSURGOdata and consists of 25 soil map units <http://www.tucson.ars.ag.gov/dap/>.

Determinants of Soil CharacteristicsTypically soils are regarded as products of geology

and the weathering processes to which rock types aresubjected. The length of time that biochemical weather-ing and erosion act on a parent material is also a prin-

cipal determinant of soil development and thus maturity.Because topography and biology, particularly vegetationand land use, affect erosion and related hydrologic andgeomorphic processes and are interactive with anddependent on geology and climate, often they too areviewed as fundamental determinants of soil genesis.More specifically, the slopes and slope lengths of land-forms that define the topography of a watershed are con-trols of weathering rates and erosion and sedimentmovement from hillsides to bottomlands, and frombottomlands of the watershed to downstream parts of thedrainage network. Thus, soils of the WGEW vary withcomponents of the small-scale landforms of the water-shed and their geologic characteristics.

The detailed soil surveys of the WGEW (Gelderman1970, Breckenfeld, 1994) have demonstrated that soiltypes are functions of local geomorphic features, andthat many soils are immature owing to semiarid climate,slow biochemical weathering, and rampant post-settlement rill and gully erosion in fan deposits northand east of Tombstone (Graf 1983). Where acceleratederosion of the last century has not stripped the upperhorizons, soils tend to be thick, mature loams rich insand and gravel and of high carbonate (calcrete) content.Virtually all of the soils reflect the underlying rocksfrom which they developed.

Basic controls of soil formation in the WGEWinclude a semiarid climate, an incomplete vegetationcover, and landform surfaces that have been exposed toolittle time to permit weathering to deep, mature soils.Accordingly, soils of the watershed are mostly poorlydeveloped and strongly indicative of the rock types fromwhich they evolved. The warm, semiarid climate ofWGEW results in relatively slow biochemical reductionof bedrock. Soils of Holocene age, therefore, are typi-cally coarse, permeable, and poorly developed. Surfaces,such as those of fan terraces, that were first exposed toweathering processes prior to Holocene time when theclimate may have been more moist than now, are deeper,more mature, and generally more argillaceous than theyounger soils.

The amount of time that a rock or deposit of rockfragments (such as fan deposits) is exposed to a set ofclimatic and biological conditions determines the tex-ture, composition, and extent to which a soil developson the rock or rock-deposit surface. In the WGEW, timehas been inconsequential relative to soil-forming pro-cesses in areas of bare rock. In contrast, where surfacesof fan terraces remain and have been exposed toweathering processes throughout the late Cenozoic andQuaternary periods, time has been sufficient to yielddeep, argillic soils, even where climatic conditions havebeen generally arid to semiarid. Nowhere in the water-shed has time been adequate, under prevailing climates,to yield clayey soils, rich in iron and aluminum oxidesand hydroxides, that are indicative of long-term warm,moist conditions.


A comparison of the soils map of the WGEW (Fig.3) with a map of the geology (Fig. 2) suggests that areasof soil groups correlate well with bedrock outcroppingsand colluvial veneers on bedrock, fan deposits, and allu-vium. Specifically, areas of plutonic-rock exposures areunderlain by shallow, quickly drained gravelly sand orclay loams, and hilly areas of volcanic rocks are cappedby moderately deep, poorly permeable cobbly clayloams. Moderately permeable, shallow to very shallowcobbly loams are associated with limestone and dolo-mite of the Tombstone Hills. Soils on fan terraces of theGleeson Road Conglomerate are typically very deep,poorly permeable gravelly sandy loams, whereas youn-ger soils on steeper slopes of dissected beds of the Glee-son Road Conglomerate are deep, moderately permeablesandy loams and clay loams. Soils of mid-to-lateHolocene alluvium are deep, well drained, and highlypermeable sand loams (Breckenfeld 1994).

Remnant surfaces of the WGEW are interrupted byescarpments with thinner and less mature soils that slopedown toward the channels that have dissected the fandeposits. Where capped by fan terraces, the fan depositsare the uppermost beds of the Gleeson Road Conglom-

erate, and the large-scale surfaces that have been dis-sected are either pediments or erosion surfaces. Mid-Holocene to recent accumulations of basin-fill, alluvial-fan, and flood-plain deposits described by (Breckenfeld1994) in the WGEW are restricted to partially closedbasins, locales adjoining upland bedrock surfaces, andterrace and inset sediment, sand and gravel bars, andstream gravel within fan incisions. These deposits,which are grouped as the Jones Ranch Alluvium and lateHolocene alluvium originate from mountains, hills, andother up-slope sources, and generate permeable, veryimmature, sandy-loam soils that may be susceptible tocovering or modification by subsequent episodes ofchannel erosion or sedimentation.

In the WGEW the distribution and density of plantspecies appear to be more dependent on moisture availa-bility and slope conditions than they are on geology.Vegetation, therefore, is a control that, like soils, varieswith climate, landform, and external stresses such asland use. Of the factors that control soil development,vegetation is likely the least correlated with soil dis-tribution in much of semiarid southeastern Arizona.

Figure 3. Simplified map showing distribution of soil groups in the WGEW; adapted from Breckenfeld (1994).


Summary of Soil GroupsTable 1, compiled from Breckenfeld (1994), is a de-

tailed list of soil map units in the WGEW with arealextents and textural classes, and Figure 3 is a simplifiedsoils-distribution map, based on the pedon descriptionsand soil map units of Table 1. The soil groups indicated inFigure 3 combine many of the map units of Table 1,partly to reduce map complexity and partly to illustrate theclose relation that the soil units have with geology (Fig. 2).

The Baboquivari-Combate-Bodecker Group consistsof permeable, immature soils formed on late Holocenechannel, flood-plain, and alluvial-terrace deposits in allparts of the watershed. Mature, poorly transmissive soilsof the Forrest-Bonita Group are derived principally fromearly to mid-Holocene cienega and inset deposits of theJones Ranch Alluvium and weathered side-slope allu-vium of recently dissected fan deposits. Large amountsof clay and silt result in the bottomland soils of theForrest-Bonita Group being mostly clay and silt loams.

Deep sandy gravel loams of the Blacktail-Elgin-Stronghold-McAllister-Bernardino Group occur on bedsof the Gleeson Road Conglomerate. The generally deep,mature soils of this group have developed slowly inareas of the upper, eastern part of the watershed wherethe Tombstone surface and Whetstone pediment (plana-tion surfaces on the fan deposits) are incompletely dis-sected. In the lower, western part of the watershed, soilsthat have formed on the erosion surfaces of Jones RanchAlluvium and the Gleeson Road and Emerald GulchConglomerates are in the Luckyhills-McNeal Group.Because soils of the group also are derived largely fromfanglomerate beds, and because the Tombstone surfaceand the Whetstone pediment are more dissected in thewestern half of the watershed than elsewhere, the soilsof the Luckyhills-McNeal Group tend to be sandy andgravelly loams that are immature compared with soilswhere rilling and gully erosion have been less extensive.An A horizon of these soils is typically absent, having

Table 1. Map units, areal extent, and textural class of soils in the WGEW based on Breckenfeld(1994).

Map unit Area (ha)Percent of total area Textural Class

Baboquivari-Combate complex 543 3.67 sandy loamBlacktail gravelly sandy loam 245 1.66 gravelly sandy loamBudlamp-Woodcutter complex 65 0.44 very gravelly sandy loamChiricahua very gravelly clay loam 147 0.99 very gravelly sandy loamCombate loamy sand 106 0.72 loamy sandElgin-Stronghold complex 1,504 10.16 very gravelly fine sandy loamEpitaph very cobbly clay loam 242 1.63 very cobbly clay loamForrest-Bonita complex 140 0.95 fine sandy loamGraham cobbly clay loam 284 1.92 cobbly clay loamGraham-Lampshire complex 244 1.65 very cobbly loamGrizzle coarse sandy loam 81 0.55 coarse sandy loamLampshire-Rock outcrop complex 385 2.60 very cobbly loamLuckyhills loamy sand 68 0.46 loamy sandLuckyhills-McNeal complex 4,255 28.75 very gravelly sandy loamMabray-Chiricahua-Rock outcrop complex 495 3.35 very cobbly loamMabray-Rock outcrop complex 838 5.66 extremely cobbly loamMcAllister-Stronghold complex 1,358 9.17 gravelly fine sandy loamMonterosa very gravelly fine sandy loam 284 1.92 very gravelly fine sandy loamRiverwash-Bodecker complex 171 1.15 sandSchiefflin very stony loamy sand 393 2.66 very stony loamy sandStronghold-Bernardino complex 760 5.13 very gravelly loamSutherland very gravelly fine sandy loam 674 4.55 very gravelly fine sandy loamSutherland-Mule complex 182 1.23 very gravelly fine sandy loamTombstone very gravelly fine sandy loam 1275 8.62 very gravelly fine sandy loamWoodcutter gravelly sandy loam 62 0.42 gravelly sandy loam


been removed by late-Quaternary erosion (Breckenfeld1994).

Soils of the Sutherland-Mule-Tombstone Grouphave developed from weathering of clastic rocks of theBisbee Group (Fig. 2) and from conglomerate bedsderived from it in areas adjacent to the Tombstone Hillsand in upper parts of the watershed. The Sutherland-Mule-Tombstone soils are very gravelly, mature loamsthat typically contain well developed pedogenic calcrete.

Soils that have developed directly on and besideexposures of volcanic rocks, igneous and carbonaterocks of the Tombstone Hills, and igneous and meta-morphic rocks in the uppermost part of the watershedinclude, respectively, those of the Epitaph-Graham-Grizzle Group, the Mabray-Chiricahua-Rock-Schief-felin-Lampshire-Monterosa Group, and the Budlamp-Woodcutter Group. The soils of these groups stronglyreflect the rock types from which the soils formed; theyhave little organic matter and are almost everywhere lessthan 0.2 m in thickness (Breckenfeld 1994). Volcanic-terrain soils of the Epitaph-Graham-Grizzle Group, forexample, are mostly thin, clay-rich loams containingabundant gravel and cobble clasts of basalt or andesiteand tuff derived from the S O Volcanics. Most soils ofigneous and carbonate rocks in the Tombstone Hills, theMabray-Chiricahua-Rock-Schieffelin-Lampshire-Monterosa Group, are very immature, shallow graveland cobble loams; exceptions are clay and gravelly clayloams of the Chiricahua Series (Breckenfeld 1994),which forms on the Bolsa Quartzite. In headwater areasof the watershed are shallow clay-, sand-, and gravel-loam soils of the Budlamp-Woodcutter Group that occurabove monzonite and gneissic granite.

GEOMORPHOLOGY AND PHYSIOGRAPHICCHARACTERISTICS AND PROCESSES

The WGEW is in the Basin and Range physio-graphic province, which presently dominates much ofsouthwestern North America and was formed through aseries of tectonic events starting in Precambrian timeand culminating in the Tertiary Period (Table 2). Thewatershed can be described generally as an activelyeroding alluvial-fan surface; however, the geomorph-ology of the watershed is complex. The landforms andlandform surfaces of the watershed are products of geo-logic conditions in the watershed, the physiography ofsoutheastern Arizona, the geologic (particularly oro-genic and epeirogenic) history of the area, and the pro-cesses of soil formation, erosion, sedimentation, pedi-mentation, and stream incision.

Walnut Gulch is a major tributary of the upper SanPedro River (Fig. 1b), entering it from the east. The earl-iest maps and descriptions of the physiography, geology,and landforms of the area including Walnut Gulch andthe Walnut Gulch watershed were produced duringregional explorations in advance of intensive Europeansettlement (Wheeler 1875), and later to support mineral

exploration (Smith 1997). The SWRC maintains a set of1:5000-scale ortho-topo maps covering the watershedand 1:1000-scale ortho-topo maps covering subwater-sheds; these maps provide detail of the drainage networkand small-scale landforms of the WGEW.

The investigation that was initiated in 1996 toexpand baseline information of watershed characteristicsincluded an evaluation of geomorphology based on pre-viously published work and new maps were drawn.Mapping was accomplished through field investigationsaugmented by 1:24,000-scale aerial-photograph inter-pretations and 1:5000-scale GIS techniques. Rock expo-sures, alluvial deposits, and landforms constituting topo-graphic relief in the watershed were the focus of themapping. Field studies of the landforms and geomorphicprocesses examined erosional and depositional surfaceson hillslopes, fan terraces, and at river banks, gullies,and road cuts. Separate deposits of conglomerate andoverlying alluvium in the watershed were interpretedfrom characteristics of tectonic disturbance, soil textureand development (Fig. 3), degree of carbonate cementa-tion, particle-size distribution, and source rocks (Fig. 2);the geomorphic interpretations presented in this sectionare consistent with previous interpretations, with a fewnoted exceptions.

Geomorphology The effects of tectonic activity, weathering, and

erosion on the sedimentary, plutonic, and volcanic rocksof the Walnut Gulch watershed are exhibited by itslarge-scale landforms and dissected erosion surfaces(Fig. 4). At the upper end of the watershed, crystallinerocks vulnerable to chemical weathering underlie anarea of pediment along the west flank of the DragoonMountains (Fig. 1b). High-relief areas of the TombstoneHills, south of Tombstone (Fig. 1b) are sites of bedrockexposure that directly reflect the complex history ofcrustal disturbance coupled with variations in resistanceto rock weathering, erosion, and soil erodibility. Mostlyin the southeastern part of the watershed, the S O Vol-canics weather and erode to rounded hills, and dis-section of fanglomerate beds in northern parts of thewatershed show the effects of late-Cenozoic regionaluplift and base-level adjustment. Geomorphic results ofgeologic events are summarized in Table 2.

Physiography The San Pedro River Basin, including the Walnut

Gulch watershed, is in the Basin and Range Physio-graphic Province. Mountains of the Basin and RangeProvince typically are large fault blocks of Paleozoicsedimentary rocks and younger igneous-intrusive andvolcanic rocks. The troughs separating the tensionallyconstructed fault blocks are filled with Tertiary age bedsof silt, sand, and gravel derived from erosion of themountain blocks.


Owing to late-Mesozoic movement of thrust platesover faulted rocks of the San Pedro River Valley (Fig.1b) and subsequent covering of the plates in manyplaces by fan deposits, some Basin and Range blockfaults remain obscured. A conceptualization by Stewart(1980), however, suggests that the southern DragoonMountains signify a horst, the San Pedro River occupiesa graben, and at least two half-graben blocks separatethe mountains and the river. A series of stair-steps abovethe river resembles large-scale stream terraces in thispart of southeastern Arizona. This pattern may indicateremoval of fan deposits from fault-block surfaces bylate-Quaternary downcutting of the San Pedro River(Cooley 1968), possibly corroborating the model pro-posed by Stewart (1980).

Large-Scale Landforms During and after episodes of Cenozoic deformationand faulting, the fan deposits and adjacent bedrock areaswere erosionally planed to pediments sloping gentlyfrom mountain fronts toward the San Pedro River. Thepediments resulted from long-term wearing back andbeveling at the bases of the fault blocks; thus, the pedi-ments were surfaces of sediment transport, slowly erod-ing into and over the bedrock. The early stage of pedi-mentation was one of planation of sediment deposited asthe Emerald Gulch and Gleeson Road Conglomerates andwas followed by late-Cenozoic incision of the conglom-erates and local re-deposition of the sand and gravel asthe Jones Ranch Alluvium and Holocene alluvium.

The exposure of pre-Cenozoic rocks by Basin andRange faulting caused rapid erosion and deposition of

Table 2. Summary of geologic events in the Walnut Gulch Experiment Watershed and the resulting geomorphic effects.

Geologic Time Geologic Events Geomorphic Effects CommentsPrecambrian Plutonic activity;

folding, faultingInitiation of fracturepatterns

At least one in watershed

Paleozoic Marine transgression Flat clastic and carbonaterocks

Mostly marine deposits

Mesozoic Compressional blockfaultng

Fault-block relief, erosion Deposition, Bisbee Group

Igneous activity Plutonic rocks, TombstoneHills

Mineralization at faults

Erosion of fault blocks Deposition of BisbeeGroup

Clastics fining upwards

Mesozoic, late Regional overthrustfaulting

Formation of TombstoneHills

Due to plate tectonics

Initiation of TombstoneSurface

Erosion, fan deposits

Tertiary Tensional blockfaulting

Basin and Rangetopography


Rhyolite; hills of S OVolcanics


Development, DragoonPediment

Surface of sedimenttransport

Development, WhetstonePediment

Surface sedimenttransport

Tertiary, late Tilting, faulting of fandeposits

Start, much of drainagenetwork

Deposition inset sediment

Epeirogenic uplift Incision by San PedroRiver

Headward erosion

Deposition of insetalluvium

Jones Ranch Alluvium

Holocene, late Drought; humansettlement

Renewed channel incision By runon from above


the Gleeson Road Conglomerate. Accompanying theerosion processes was development during the Plioceneof the Tombstone Surface and the Whetstone Pediment(Menges and Pearthree 1989) (Fig. 4). The magmas andmineral-rich veins in the Tombstone Hills, which duringMesozoic and early Cenozoic time had moved upwardalong the older complex of faults (Drewes 1981), alsowere partially beveled by erosional processes. In the Walnut Gulch area Bryan (1926) identified(1) a Tombstone Pediment, largely a surface (Fig. 4) onvariable thicknesses of Gleeson Conglomerate veneeringeroded bedrock in the Tombstone Hills south of WalnutGulch, and (2) a Whetstone Pediment (Fig. 5), whichslopes westward from the northern and central Dragoon

Figure 4 Map showing major geomorphic features of the Walnut Gulch Experimental Watershed including bedrock areas ofmountains and hills, areas of erosional surface and pediment, and a high-angle normal fault.

Figure 5. Photograph southward showing sparse grasses and shrubs on the upperWhetstone Pediment in the foreground and the Tombstone Hills in the distance; the darkband between them, in front of the Tombstone Hills, is vegetation on the TombstoneSurface.

Note - the following text was omitted from the published version of this paper: the Gleeson Road Conglomerate. Accompanying the erosion processes was development during the Pliocene of the Tombstone Surface and the Whetstone Pediment (Menges and Pearthree, 1989) (fig. 4). The magmas and mineral-rich veins in the Tombstone Hills, which during Mesozoic and early Cenozoic time had moved upward along the older complex of faults (Drewes, 1981), also were partially beveled by erosional processes. In the Walnut Gulch area Bryan (1926) identified (1) a Tombstone Pediment, largely a surface (fig. 4) on variable thicknesses of Gleeson Conglomerate veneering eroded bedrock in the Tombstone Hills south of Walnut Gulch, and (2) a Whetstone Pediment (fig. 5), which slopes westward from the northern and central Dragoon


Mountains, is 15 to 30 m lower than the Tombstone Sur-face, and formed on tilted beds of the Gleeson RoadConglomerate. Headward extension by the WalnutGulch drainage network dissected and isolated bedrockexposures and deeply incised the Tombstone Surfaceand rocks of the thrust plate upon which it formed.

The Whetstone Pediment merges gradationally withthe higher Tombstone Surface north of Walnut Gulch,possibly as subtly developed fan terraces associated withthe east-west faulting as inferred by Gilluly (1956) andSpangler (1969). Gray (1965) proposed that the Tomb-stone Surface developed in the mid-Pleistocene, fol-lowed by erosion resulting in the Whetstone Pedimentin pre-Sangamon time. Alternatively, the Paleozoic andMesozoic rocks, combined with later igneous intrusions,that were thrust into the present Tombstone Hills areaformed a topographically high area. Erosion, much moreas incision than of pedimentation, since the overthrustevents occurred no doubt reduced the extent of bedrockexposures. The landforms of the Tombstone Hills, how-ever, are fundamentally different from those of dissectedfan deposits to the north, and generally do not includesurfaces of sediment transport. Distinguishing the fea-ture as a pediment, as did Bryan (1926), therefore, maybe erroneous.

Well logs and seismic profiles (Spangler 1969) fromnear Flume 1 (Fig. 1c) show that about 50 m of con-glomerate overlie Uncle Sam Porphyry, suggesting thatthe area is part of the Tombstone Hills complex.Schieffelin Granodiorite at Flume 2 and well recordsshowing it at shallow depth also indicate TombstoneHills complex. The seismic studies of Spangler (1969),however, suggest that the Naco Group is at shallowdepth near Flume 2. One to 2 km downstream, the Bis-bee Group is faulted to within 70 m of the surface. A100-m well south and downstream of Flume 6 (Fig. 1c)penetrated only Gleeson Road Conglomerate, suggestingthat it is on the north, downthrown side of a fault and onthe Whetstone Pediment. Locally, as in the Lucky Hillsarea (Fig. 1c), gullies deeply incise the Whetstone Pedi-ment, much of the erosion having occurred during thelast 130 years. In higher parts of the watershed, betweenthe Dragoon Mountains and Tombstone, dissection hasbeen less intense, especially recently, than it has been inlower parts of the watershed. In the San Pedro Rivertrough, both the Whetstone Pediment and the Tomb-stone Surface are covered by Holocene alluvium.

Pediments and other surfaces of planation in arid orsemiarid regions typically develop gentle slopes on bed-rock or older, partially consolidated, alluvial deposits. Inthe Walnut Gulch watershed, most areas of pediment areon tilted strata of the Gleeson Road Conglomerate thatwere beveled by erosion into a gently sloping surfacethat later was dissected by stream-channel incision pro-gressing eastward from the San Pedro River. Other pedi-ments, near to and adjoining the Dragoon Mountains,the Tombstone Hills, and exposures of volcanic rocks,

expand largely by headward fluvial erosion into thebases of bedrock hills, forming and maintaining abruptand slowly receding fronts or escarpments. Thus, bothprocess sets of pedimentation yield low-relief surfacesof uniformly gentle slope, or ones that are slightly con-cave upward, on which erosion is minimal and the sedi-ment supplied to the upper margin of the surface fromthe bedrock exposures moves downslope with little orno permanent storage. Channel incision of a pedimentmay occur following renewed uplift of the bedrock areaor by downcutting of the principal stream at the down-slope end of the pediment, causing a lowering of baselevel. In the Walnut Gulch watershed, lowering of theSan Pedro River has resulted in headward extension anddissection of pediment surfaces along Walnut Gulch andits tributaries.

The Dragoon Pediment (Fig. 4), the lower limit ofwhich is inferred, occurs as a relatively narrow bandalong the western base of the Dragoon Mountains and inthe Walnut Gulch watershed has developed on quartzmonzonite and sheared gneissic granite that originallymay have been rock exposures of the mountain front.The Tombstone Surface, as previously described, is acomplex of erosional terraces capping variable thick-nesses of fan deposits overlying Paleozoic and Mesozoicrocks that form the Tombstone Hills. The TombstoneHills area generally is at higher elevation than surround-ing areas, resulting in relatively high-energy conditionsand deep incision of the fan deposits adjacent to exposedbedrock. Thus, areas underlain by Gleeson Road Con-glomerate around the Tucson Hills are regarded here tobe erosion surfaces and not surfaces of transport (por-tions of a pediment). The structurally lower Whetstone Pediment lies tothe north and east of the Tombstone Surface. The transi-tion between the two is indistinct but generally is nearWalnut Gulch and closely parallels the long-active,northwest-trending, high-angle fault that was interpretedby Gilluly (1956) and mapped by Drewes (1981) (Fig.4). The Whetstone Pediment is entirely on fan depositsand throughout the watershed has been dissected by adrainage network initiated by mid-to-late Cenozoicextensional faulting (Menges and Pearthree 1989) andenhanced by regional epeirogenic uplift during late-Quaternary time. The eastern component of the pediment, the upperWhetstone Pediment (Fig. 6), is partially dissected,mostly as a result of erosive runon from the DragoonMountains and upper parts of the pediment. The westernportion, termed the Dissected Whetstone Pediment, hasbeen well dissected by runon from higher parts of thepediment, headward extension of tributaries due to lateQuaternary lowering of the San Pedro River (Cooley1968), and renewed river and tributary incision fol-lowing concentrated livestock grazing and relatedhuman stresses on the channel system beginning about130 years ago (Fig. 4). The narrow zone separating the


two components of the pediment closely conforms to thechange from soils of the Blacktail-Elgin-Stronghold-McAllister-Bernardino Group to those of the Lucky-hills-McNeal Group (Fig. 3). Owing probably to thedegrees of channel incision and dissection in the twocomponents, the easternmost exposures of EmeraldGulch Conglomerate are at the boundary separating thetwo parts. Plant cover also appears to reflect theintensity of erosion, the upper Whetstone Pedimentbeing dominated by grasses and by trees at higherelevations, whereas the Dissected Whetstone Pedimenttypically has sparse grasses but abundant whitethornacacia (Acacia constricta) and creosotebush (Larreatridentata).

Small-scale Landforms Landforms of the Walnut Gulch watershed werecategorized by Breckenfeld (1994) as hills and moun-tains (including isolated or individual hills or moun-tains), fan terraces, alluvial fans, basin floors, and floodplains. These landforms are products of fluvial erosion,deposition, and related hillslope processes, and hencethey and the soils that veneer them reflect late Quaternaryclimate and climate variability. A unique suite of soiltypes is associated with each landform category. Hills and mountains in the Basin and Range Physio-graphic Province of southeastern Arizona range fromsteep, site-specific erosional features that supply sedi-ment from bare rock surfaces to upland surfaces of lowto moderate slope upon which erosion is less intense andgenerally thin argillic (enriched in silicate-clay) soils

may accumulate. Slope steepness is largely a function ofthe ability of a bedrock type to resist chemicalweathering, and the intensity by which a hill or moun-tain has been affected by faulting and folding. Principalexamples of this type of landform in the Walnut Gulchwatershed are small areas of granitic and gneissic rocksof the Dragoon Mountains, rounded hills formed of theS O Volcanics in the southeastern part of the watershed,and surfaces underlain by mostly carbonate, volcanic,and igneous-intrusive rocks in the Tombstone Hills. Fan terraces, as defined by Breckenfeld (1994), areremaining surfaces of alluvial fans that have had streamincision since the end of fan deposition. The remnantsurfaces, therefore, overlie generally mature argillicsoils and are interrupted by escarpments with thinnerand less mature soils that slope down toward thechannels that have dissected the fan deposits. As pre-viously described, the fan deposits that are capped byfan terraces are the uppermost beds of the Gleeson RoadConglomerate; the large-scale surfaces that have beendissected are the Whetstone Pediment and the Tomb-stone Surface. Mid-Holocene to recent accumulations of basin fill,alluvial fan, and flood-plain deposits described byBreckenfeld (1994) in the Walnut Gulch watershed arerestricted to partially closed basins, locales adjoiningupland bedrock surfaces, and terrace and inset sediment,sand and gravel bars, and stream gravel within fan inci-sions. These deposits, which are grouped as the JonesRanch Alluvium and late Holocene alluvium, originatefrom mountains, hills, and other up-slope sources, and

Figure 6. View to the northeast showing the northern Dragoon Mountains in the middleright. The horizon on the left, extending to the right in front of the mountains, is thesurface of the upper Whetstone Pediment, in front of which, in mid-picture, is a maturedrainage incising beds of the Gleeson Road Conglomerate. Vegetation is dominantlygrasses and creosotebush (Larrea tridentata).


generate permeable, very immature, sandy-loam soils thatmay be susceptible to covering or modification by sub-sequent episodes of channel erosion or sedimentation.

Drainage Development and Geologic Controls on Erosion The large-scale crustal disturbances, that started insoutheastern Arizona in Precambrian time and have con-tinued to the present, have controlled drainage patternsof the area; each tectonic pulse altered the stream net-work that previously had prevailed. The present drain-age patterns of the San Pedro and Walnut Gulch Basinswere imposed initially by the extensional faulting thatbegan in mid-Cenozoic time (Menges and Pearthree1989). Regional epeirogenic uplift in late-Quaternarytime caused incision by the San Pedro River, whichresulted in elevated energy conditions along tributaries,including Walnut Gulch (Cooley 1968). The combinedeffects of (1) base-level lowering by the river, (2) head-ward erosion by tributaries, (3) downstream erosion byrunoff from the Dragoon Mountains and the TombstoneHills, (4) structural control of stream channels, and (5)recent landscape stress possibly due to drought, floods,and human settlement explain why the Whetstone andTombstone Surfaces of the Walnut Gulch Basin andelsewhere are now deeply incised. The Walnut Gulch watershed is atypical of thoseheading in mountains. The uppermost part is anoma-lously small and narrow due to the tectonic history,especially of the thrust faulting that moved older rocksnortheastward onto younger rocks (Drewes 1981). Thedrainage divide at the southeastern edge of the water-shed is largely determined by S O Volcanics, and expo-sures of the Naco and Bisbee Groups and the Uncle SamPorphyry in the Tombstone Hills largely define thesouthwestern divide. The northern drainage divide is theresult of long-term drainage-network evolution, but alsomay suggest separate fault blocks. Stream-channel positions in the Tombstone Hillsarea mostly have been determined by the complex offaults and folds, which have been altered further byigneous activity and hydrothermal changes to adjacentrocks. The positions of much of Javelina Draw, forexample, which enters Walnut Gulch from the south insection 32, T. 19 S., R. 22 E. (Fig. 1c), appears to bedetermined by faults and possibly folding. Drainage-basin evolution in the northern part of the watershedunderlain by Gleeson Road Conglomerate has beenstrongly affected by the same fault systems that controldrainage patterns elsewhere, but conclusive field evi-dence for many of the faults is lacking. Reaches of Walnut Gulch where fault control hasbeen established by field observations or is stronglysuspected owing to channel morphology and alignmentinclude (1) sites of abrupt shift in channel direction fromnorth-northwest to west-southwest back to north-northwest immediately south of the basalt exposure and

upstream of Flume 6 (Gilluly 1956, Drewes 1981), (2)a straight, northwest-trending 1.5-km length immed-iately downstream from Flume 6, (3) the area of NacoGroup exposures upstream from Flume 2 (Gilluly 1956),and (4) short, straight channel lengths oriented west,then north, downstream from Flume 2. Fractures clearlycontrol channel position along a tributary to WalnutGulch at Flume 5 (Alonso 1997).

Recent Erosion, Sedimentation, and Geomorphic Research Few interpretive studies of sediment yield fromwatersheds of the Southwest, including that of WalnutGulch, are available. An investigation by Lane et al.(1997) on watershed processes that control sedimentyield includes data from the WGEW. Because the dataused in their analyses were similar to those consideredherein, results also were similar. Ideally, validatedsediment-yield data from the WGEW and similar water-sheds of the Southwest can be related to measurementsof sediment released by hillslope erosion to permit esti-mates of sediment budgets, including fluxes of sedimentwithin a watershed and changes of sediment storage asa function of time.

Recent analyses of atmospherically depositedcesium-137 on the shrub-dominated Lucky Hills sub-watershed have indicated patterns and rates of soilerosion and redistribution of sediment relative to similaranalyses for a grass-dominated area. Elevated hillslopeerosion rates in the shrub-dominated subwatershed werelargely attributed to vegetation and were correlated withrock in the upper soil profile; they were not correlated,however, with slope or land curvature (Nearing et al.2005, Ritchie et al. 2005). Field experiments to quantifyplot-scale hillslope erosion rates have been the focus ofrainfall simulations (Paige et al. 2003). Simulations con-ducted across a range of sites on the watershed revealedstrong associations between rainfall and soil and covertypes. Rock fragments significantly affect hillslope ero-sion on the watershed where rock cover, or desert pave-ment, has developed as water has moved small soil par-ticles downslope while leaving the rock fragments on thesurface (Simanton and Toy 1994, Simanton et al. 1994). Sediment yields from small watersheds have beenquantified through accumulation surveys of sediment instock tanks starting in the late 1950s. Sediment accumu-lation records of 30 to 47 years recently were updatedand evaluated for sub-watersheds ranging in area from0.35 to 1.6 km2. Within the 150-km2 watershed, sedi-ment yield from the sub-watersheds ranged from about63 to 375 (metric) tons per square kilometer per year (tkm-2 yr-1), with a mean of 175 t km-2 yr-1 and a standarddeviation of 125 t km-2 yr-1. Although sediment yieldswere temporally and spatially variable, with the excep-tion of runoff volume, no significant relations werefound to explain sediment-yield variability; characteris-tics of channel-network development, however, prob-


ably influence sediment transport and storage dynamics(Nichols 2006). In addition to plot, hillslope, and small-watershedresearch, the watershed is instrumented to measure sedi-ment flux at small flumes draining areas of 0.5 to 11.2ha (hectares). Prior to the mid-1980s, fluvial sedimentwas collected at several large flumes. Sediment exportrates from eight unit-source subwatersheds recentlywere evaluated for the period 1995 through 2005. Thedata were used to develop statistical relations betweenflow characteristics and sediment concentrations, andbetween total event sediment exports to event runoffcharacteristics (Nearing et al. 2007). In 2002, researchto quantify the contributions of coarse sediment(Nichols 2004) to total sediment load was initiated andpit traps were installed below the overfall of flumes tomeasure runoff at the outlets of two small subwater-sheds. Preliminary results of this ongoing researchindicate that as much as 15% of the sediment transportedduring a flow event is not sampled (Nichols 2003). Compilations of sediment-discharge data (Oster-kamp 1999) do not adequately characterize sediment-yield variations in the Walnut Gulch watershed, butinvestigations by Renard et al. (1993) indicate that amajor control of sediment-yield variation in recentdecades has been land use. Gully erosion in the LuckyHills area of the northern part of the watershed (Fig. 1c),for example, probably began due to heavy grazing andhigh-magnitude storms in the late 1800s and early1900s. Photographs suggest that channel incision in theLucky Hills was intense in the 1930s and that channelerosion remains active 70 years later. Although difficultto document, variation in geology, thus soils, very likelyinfluences sediment yields in the Walnut Gulch water-shed. Research recently has been conducted to quantifythe influence of geomorphology on soil erodibility(Rhoton et al. 2007). Computed sediment yields, for varying periods,from 15 sub-basins of the watershed vary from 40 to 370t km-2 yr-1 (Osterkamp 1999). The highest yields were inthe northern watershed where gully erosion continues toincise fan deposits. The lowest yields also were from fandeposits in the northern watershed at sites not yetdegraded by gully erosion. Sparse data from anunnamed tributary to Walnut Gulch heading near thesouth-central basin divide suggest that sediment yieldsfrom areas of the Naco Group and the S O Volcanicsare low, approximately 50 to 60 t km-2 yr-1. Recent research to understand the geomorphicevolution of the main stem of Walnut Gulch hasrevealed a pattern of increasing vegetation and narrow-ing of primary flow paths within the broader alluvialchannel. Since the 1970s, these changes have beencoincident with reductions in the number and magn-itudes of floods. Cyclic patterns of channel narrowingand widening and aggradation and degradation areanticipated in response to periods of drought and above-

average precipitation. The cycles are important controlsof short-term sediment transport and storage within thechannel network. Understanding the causes of erosion, measuringsediment movement, and developing a process basedunderstanding of erosion, transport, and deposition arefundamental research goals in the Walnut Gulch Experi-mental Watershed. Imposed disturbance of the last 130years has been a major determinant of erosion and sedi-ment flux, hillside and bottomland sediment storage, andits removal from storage in the drainage network. Theeffects of geology and soils, topography, semiarid cli-mate, and native Desert Plains Grassland vegetation,however, also strongly influence sediment movement inthe watershed and are more easily quantified than is theeffect of land use. In sub-basins, therefore, where human disturbanceis minimal but where surface geology is dominated bya small range of rocks types, discharge data are vitalresources upon which other watershed research relies.Especially useful could be flow and sediment-concentra-tion data from subwatersheds throughout the basin thatare underlain primarily by (1) Paleozoic carbonate rocks(mostly in the Tombstone Hills), (2) S O Volcanics (inthe southeast), (3) the Bisbee Group (in the southwest),(4) the Schieffelin Granodiorite (in the west), and (5) theUncle Sam Porphyry (in the extreme southwest).Expanding the current instrumentation network to furtherthe direct collection of water and sediment-dischargedata, supplemented with measurements of sediment storedin reservoirs and time-integrated changes of sedimentstorage along stream channels, seems mandatory for theacquisition of variable-source flux information supportingother research in the watershed.

SUMMARY STATEMENT The geology and thus the landforms of the Walnut

Gulch Experimental Watershed have been very com-plex, and an understanding of the events that led to thecomplexity helps explain the mineralization of theTombstone Hills, the unique form and drainage patternof the watershed, and especially why rainfall/runoff rela-tions and sediment yields of the watershed are highlyvariable. The synopsis of the geology and geomorphicand physiographic characteristics provided here is basedpartly on basin-specific field observations of rock out-croppings, soil and vegetation distributions, and geo-morphic surfaces, but mostly on published reports ofareas in the American Southwest larger than WGEW.Data provided in the reports are more detailed than werepossible to collect for this investigation. Some of thosereports, cited previously, have contributed substantiallyto understanding the geology of the Tombstone area.All, however, became dated upon publication. A reason-ably complete geologic knowledge of WGEW, there-fore, has not yet been achieved. Nevertheless, eachstudy adds to the fund of information, and the generali-


zations provided herein will be modified as future inves-tigations document the geologic history of the areabetter than now. Meanwhile, it is hoped that this sum-mary can help guide near-term activities for other fieldinvestigations and erosion-modeling efforts dependenton geologic information, and thus provide the founda-tion for progress in those studies.

ACKNOWLEDGMENTSMuch of the field investigation to confirm published

sources of information was conducted by Maria AngelesAlonso, then a post-doctoral student from Madrid, Spain, anddescriptions of some alluvial deposits are her interpretations(Alonso 1997). Field investigation was aided by SharonBiedendbender, Ecologist, USDA Forest Service, Sierra Vista,AZ, and by T. J. Toy, Professor, University of Denver. Maps,and the digitizing of field data, were prepared by Sudhir RajShrestha, University of Wyoming, and by Jared Buono andCarl Unkrich, Agricultural Research Service, Tucson, AZ. S.N. Miller, University of Wyoming, helped with the interpreta-tion of field observations and compiled the geologic and geo-morphic data that were supplied to him and converted thosedata into maps. D. J. Breckenfeld shared his extensive know-ledge of soils of the WGEW and provided the informationnecessary to construct the table and map that describe soils ofthe watershed. M. H. Nichols provided valuable help with thecontent and writing of the sections on geomorphology, andwith J. J. Stone, provided essential bibliographic informationand editing. These professionals, and several others of theAgricultural Research Service, Tucson, gave terrific supportfor this effort and that support is very much appreciated.

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NEARING, M. A., M. H. NICHOLS, A. KIMOTO, and J. C.RITCHIE. 2005. Spatial patterns of soil erosion anddeposition in two small, semiarid watersheds. Jour-nal of Geophysical Research 110:F04020,doi:10.1029/2005JF000290.

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