Vol 21,No.4 Investigations . Service . Information Winter1991 Geologic Insights into Flood Hazards in Piedmont Areas of Arizona by Philip A. Pearthree Arizona Geological Survey Proper managementof flood hazards inpiedmontareas ofArizona isbecoming increasinglyimportantasthe State's pop­ ulation grows and urban areas expand. In the Basin and Range Province of the western United States, piedmonts (liter­ ally, "thefoot ofthe mountains") are the low-relief, gentlyslopingplainsbetween the mountain ranges and the streams or playasthat occupy thelowestportions of \~ the valleys. ~uch o.f southern, cen:ral, ..' andwesternAnzona iscomposedofpied­ monts, and they comprise most of the developable land near the rapidly ex­ panding population centers of the State. Viewedfrom above,piedmontsofAr­ izona are complex mosaics composed of alluvial fans and stream terraces of dif­ ferent ages that record therecent geolog­ ic history of an area. Alluvial fans are generallycone-shapeddepositionalland­ forms that emanate from a discrete source and increase in width downslope; adja­ centfan surfaces may merge downslope to form a continuousalluvialapron. Allu­ Figure 1. Aerial photograph of the western piedmont of the White Tank Mountains, which shows vial fans represent periods of net aggra­ alluvial surfaces of different ages. The approximate ages of the deposits in thousands of years (ka) dation, whenlarge amounts of sediment are as follows: Y, younger than 10 ka; M2, 10 to 150 ka; MI, 150 to 800 ka; 0, older than 800 were removed from mountain areas and ka (br = bedrock). The arrows point to relatively large drainages that head in the mountains and deposited on adjacent piedmonts. The flow from right to left across the piedmont. The areas of recent alluvialjan activity (labeled AF) Quaternary Period (roughly the past 2 along these drainages are identified by extensive young deposits (Y) and distributary channel million years) has been characterizedby patterns, which consist ofstreams that branch and flow out ofa larger stream. Old, inactive alluvial repeated changes in global climate. Pe­ fans (units MI, M2, and 0) are characterized by a lighter color and tributary drainage patterns, which consist ofstreams thatflow into a larger stream. These old fans compose much ofthe piedmont riods ofalluvial-fandepositioninArizona and have been isolated from floods associated with the larger drainages for more than 10,000 years. were probably due to climate changes that increased the amount of sediment suppliedto streamsfrom mountainslopes Piedmont areas subject to active allu­ is little topographic relief to confine the and possibly decreased the capacity of vial-fanfloodingareofparticularconcern floodwaters. If development on pied­ streams to transport sediment across from a floodplain-managementperspec­ monts occurs without regard to the dis­ piedmonts (Bull, 1991). Stream terraces tive. Piedmonts inArizona are typically tributionofactive alluvialfans, lives and are steplike landforms that are typically drainedbya few relativelylarge streams inset below adjacent fan surfaces. They thatheadinadjacentmountains andmany representformer floors ofstream valleys smaller streams that head on the pied­ ALSO IN THIS ISSUE thatwereabandonedasthestreamsdown­ mont (Figure 1). Active alluvial fans Oil and Gas News 6 cut even further. Terraces thus exist in along the larger streams may be subject National Geologic-Mapping Program 7 areas wherethelong-term trendhasbeen to widespread inundation, local high­ Mt. Pinatubo 9, 10 for streams to entrench themselves into velocityflow, anddrastic changesinchan­ New AZGS Publications 11 olderdeposits. nelpositions duringfloods becausethere propertymaybe put at risk. Itis preferable and less expensive patterns may occur. If waters take a new path during a flood, tomitigate theseflood hazardsbeforeanarea is developedthan a channel that seemed insignificant can grow in size and ca­ to deal with them afterwards, when floodwaters are lapping pacity. Human alterations to natural stream systems on pied­ at doorsteps. ' montsmayhave a profoundimpactonthecourseoffloodwaterSj . The long history of stream behavior and flooding recorded ill-advised obstruction or diversion of natural channels ma. in the geology and geomorphology of piedmonts provides an cause adjacent areas to receive the brunt of floodwaters. For invaluableperspectiveonfloodplain-managementissues. Floods all of these reasons, local, State, and Federal floodplain-man­ leave physical evidence of their occurrence in the form of agement officials in the United States have come to realize the alluvial deposits. Characteristics of large floods that have importanceof adequatelydefining andmanagingfloodhazards occurred during the past few years may be reconstructed in in piedmont areas. some detail because evidence of their impact on the landscape Traditional methods of definingregulatoryfloodplains typ­ is fresh. Over hundreds or thousands of years, the impact of icallyinvolvefoursteps: (1) makingtheassumptionthatchannel individualfloodsismore difficultto resolve,butthecumulative beds and banks are fairly stable; (2) estimating the size of the effects ofmany floods are recorded in the geologyand geomor­ flood thathas a 0.01 probability of occurring in any given year phologyof a piedmont. Geologicalstudies canaddress several on a particular stream (the lOO-year flood); (3) routing the 100­ key issues: (1) How large are the extreme floods on particular yearflooddownstream usingsomepreferredhydrologic model; drainages?; (2) Whichportions ofpiedmontsareprone to flood­ and (4) determining the area thatwillbe flooded andhow deep ing, especially alluvial-fan flooding?; (3) Do the positions of the floodwaters will be at any flooded locality. These proce­ channels on alluvial fans typically change during floods?; and dures, however, are inappropriate for defining areas that are (4) What are the depths and velocities of floodwaters during prone to alluvial-fanflooding. As outlinedabove, channels on alluvial-fanfloods? alluvial fans mayor may not be stable dUring large floods. In TheArizona Geological Survey (AZGS) is engaged incoop­ addition,floodwatersmayspreadso widelythataccuratemodel­ erative efforts withlocalfloodplain-management agencies, the ing of their extent and depth is difficult. Furthermore, the University of Arizona, and the Arizona Department of Water available data for estimating the sizes of 100-year floods on Resources. Thesestudies combinegeologic investigationswith drainagesinArizona aremodestatbest; differentmethods yield more traditional hydrologic analyses to delineate flood-prone dramatically different sizes (e.g., House, 1991). areas more accurately and to understand better the flooding Hydrologists and engineers have proposed several models processes on piedmonts in Arizona. to simulate alluvial-fanflooding. Themostwidely used model is one thatis mandatedby the FederalEmergencyManagement FLOODPLAIN-MANAGEMENT ISSUES Agency (FEMA) to set flood-insurance rates. This FEMA allu­ vial-fan methodology (AFM) is based on several simplifying Theprincipalobjectiveoffloodplain-managementagenciesis assumptions aboutflow behaviorduringalluvial-fanfloods: (1) to prevent humans and their property from being exposed to floodwaters affect only part of an alluvial fan during anyone undue risks from flooding. These agencies must also maintain flood; (2) floodwaters are conveyedin one or more channels of _ credibilitywiththepersons whom they regulate andincludein a specific and predictable depth and width; and (3) these their purview only the areas that are truly at risk of being channels canform anywhere onthe alluvialfanatthebeginning flooded. Conceptsoffloodplainmanagementarefirmly rooted oforduringa flood (Dawdy, 1979; FEMA, 1985). TheAFMuses in the disciplines of hydrology and civil engineering; geologic the width of the channels that are supposed to form during a information typically has not been used in flood-hazard eval­ flood and the total width of the alluvial fan to determine 100- uations. Because flooding in pied­ mont areas may be quite complex, however, standard hydrologic or engineering methods for accurately assessing and managing flood haz­ ards are of questionable value. Crit­ ical technical issues, such as deter­ mining the extent and character of floodingonpiedmonts,cannotbeade­ quately addressed without integrat­ y inghydrologic methodsandgeologic investigations. Streamsthat cross piedmontareas ~c:& ~ in Arizona have several characteris­ Y \,1 - tics thatmake them particularlyhaz­ ~< C-/ C::;M2 M2 ardous to life and property. As is Y - \ o typicalinthedesert, piedmontstreams ~ M2 flow infrequently,lendinga false sense a ~ ..AiO)~ ~ of security to persons who live near washesoronactivealluvialfans. Large piedmontfloods are usually generat­ Figure 2. Surficial geologic maps that illustrate differences in the extent ofyoung deposits and alluvial­ edbyintenseprecipitationinadjacent fan flooding hazards on two piedmonts in Arizona. The map-unit designations are the same as those mountains. Piedmontdwellingsmay used in Figure 1; additional map units are as follows:MO, 500 to 1,000 ka alluvial fans; and Tbf, thus become flooded even if it has dissected basin-fill deposits older than 1,000 ka. (a) The southwestern piedmont of the Sierra Estrella Mountains in central Arizona is mostly covered with young alluvial deposits less than 10,000 years rainedverylittleinthoseareas. During old (map unit Y), which indicates that alluvial-fan flooding is an important process on this piedmont alluvial-fanflooding,floodwaters are (from Demsey, 1989). (b) The southwestern piedmont of the Eagle Tail Mountains in southwestern free to spread outand inundatewide Arizona is almost entirely 'composed ofalluvial fans and terraces more than 10,000 years old (map units areas, and drastic changesinchannel M2, Ml, 0, and Tbf). No active alluvial fans exist on this piedmont (from Demsey, 1990).

2 Arizona Geology, vol. 21,no.4,Winter1991 Figure 3. Generalized map of the southern piedmont of the Tortolita Mountains. This map contrasts areas that have been flooded during the past few thousand years with the limits ofactive alluvial fans, as o 1 mi defined by the AFM mandated by FEMA. The dark-gray areas are II--~I--'I /~' .•urfaces that are 'younger than 5,000 years and that are considered to o 1 km , be flood prone based on geologic data and FEMA estimates. The light­ gray areas are surfaces that are older than 5,000 years (in many areas, much older) and that are considered to be flood prone by FEMA, but have not been flooded for a long time. Discrepancies between geologic data and the FEMA alluvial-fan areas are most pro­ nounced in the southeastern portion of the piedmont. The approximate area between the outermost threads of flow during the 1988 alluvial-fan flood on Wild Burro Wash is shown by the hachured pattern. The lighter hachures indicate areas that were probably affected by the flood but were not mapped in detail. The area that was affected by this flood coincides very closely with the flood-prone areas determined from geologic data.

year flood-flow depths and velocities at any point on the fan. FEMA regulations include all areas subject to inundation of 6 inches or more during the 100-year flood in the 100-year floodplain. Onpiedmonts in Arizona, the Surfaces less than extent of the 100-year floodplain has been 5,000 years old defined using the assumptions of the AFM Surfaces greater than 5,000 and topographicinformation(Fuller, 1990). years old, within FEMA alluvial­ Application of the AFM inArizona has fan flood zones generateda stormofcontroversy,however, Surfaces greater than 5,000 years old, outside FEMA alluvial­ becauseithas designatedbroadportions of o fan flood zones piedmonts as potential sites of inundation Approximate limits of extreme bythe100-yearflood. Three questionshigh­ alluvial-fan flood of July 1988 lightthe technicalaspects ofthis controver­ .. sy: (1) Has the AFM been applied to areas .. that are actually subject to alluvial-fan flooding?; (2) Are the oftheoriginaldepositional topography, suchas bars (ridges) of _ fundamental assumptions of the AFM concerning thebehavior coarse deposits, swales (troughlike depressions) where low of floodwaters on alluvial fans realistic?; and (3) Are the sizes flows passedbetweenbars, anddistributarychannelnetworks of the 100-year floods used in the AFM realistic? Geologic (networks thatbranchdownstream),whichare characteristic of investigations of piedmont areas can provide an independent active alluvial fans (Figure 1). Young fan surfaces also show assessment of each of these questions. minimaldevelopmentofsoil, desertpavement,androckvarnish and are basically undissected. Very old alluvial-fan surfaces, GEOLOGYAND GEOMORPHOLOGY onthe other hand, havenotbeensubject to large-scaleflooding OF PIEDMONTS IN ARIZONA for hundreds of thousands of years. These surfaces are char­ acterizedbywell-developedsoilswithclay- andcalcium-carbon­ Thephysicalcharacteristics ofalluvial surfaces (alluvialfans ate-rich horizons, well~developeddendritic stream networks andstream terraces) onpiedmonts maybe used to differentiate that are entrenched several meters below the fan surface, and them by age. Alluvial surfaces are typically deposited by the strongly developed varnish on surface rocks. Old alluvial-fan larger drainages that cross a piedmont; thus, the initial surface surfacesmayalso havesmooth,closelypackeddesertpavements features areshapedbylarge-scaledepositionalprocesses. When betweentheentrencheddrainages. The agesofalluvial surfaces surfaces are isolated from further deposition or reworking by in the southwestern United States may be roughly estimated large streams, they are gradually modified over thousands of based on these surface characteristics, especially soil develop­ years by other processes, which operate very slowly and on a ment (Gile and others, 1981; Bull, 1991). smallerscale. These modifyingprocessesinclude(1) small-scale The distribution, character, and relative abundance of sur­ erosion and deposition thatsmooththe original surface topog­ faces of different ages clarify the long-term history of stream raphy; (2) bioturbation,thechurningofsedimentsbyorganisms, behavior on piedmonts, which, in turn, illuminates the nature which obliterates depositional structures; (3) development of and distributionofpotentialfloodhazards. Thebroadspectrum soils, primarily throughaccumulationofsilt, clay, and calcium of stream behavior on the piedmonts of Arizona is illustrated carbonate; (4) developmentofsurficialgravelpavements (desert by variations in the areal extent of young deposits. Some pavements) above zones of accumulated silt and clay; (5) ac­ piedmonts have extensive deposits of Holocene age (less than cumulation of rock varnish on surface gravel; (6) development 10,000 years old), which indicates that the alluvial fans have of tributary dendritic (treelike) stream networks on surfaces; been recently active; other piedmonts have few Holocene de­ and (7) entrenchment of these stream networks below original posits (Figure 2). The extent of active alluvial fans depends on depositional surfaces and subsequent surface dissection. the rock types in the adjacent mountains and on the stability Alluvialsurfaces ofsimilar agehave a characteristic appear­ ofbase-levelatthelowerendofthepiedmont. Manypiedmonts ance because they have undergone similar postdepositional inArizona,however, showsimilarpatternsoflong-termerosion modifications,andtheyaredistinctly differentfrombothyoung­ and deposition. The upper piedmont areas near the mountain er and older surfaces. Young (less than a few thousand years rangesare dominatedbyabandonedalluvialfans ofPleistocene old) alluvial-fansurfaces, for example, stillretainclear evidence age (greater than 10,000 years old); active stream systems are

Arizona Geology, vol. 21, no. 4, Winter1991 3 ------..

5~------, flood-insurance-rate maps identify broad areas as beingflood Legend 120cms prone thathavenotbeenfloodedfor 5,000 years ormore (Figure A. SJackwater Deposit .... :: 100 ems 4 3). The most serious discrepancies are in the southeastern portionofthepiedmont,where youngdeposits associatedwith ~ ~:,e~~::~~~,",~,"""'~"""'''~/ o. the larger drainages are very limited. The results of the AZG.ZI ~3 .,. •. .s geologic-mappingeffortclearlyimplythattherearesubstantive c o problems with the AFM itself or with the manner in which it ~2 was applied to the Tortolita piedmont. Because of these prob­ ill lems, large portions of the piedmontwere included in the 100- year floodplain that are not flood prone. By integratinggeologic evidence withhydrologic flow mod­ els, geologists can estimate the sizes of the largest floods that oL....~__.J_-L-----l_-'-----L_ _'__...... L_'______.L~~__L~_ _'______'_----' have occurred along streams during the past tens, hundreds, o 10 20 30 40 50 60 70 80 or thousands of years. Large floods leave behind evidence that Distance (meters) maybeusedto estimate theheightofthewaterduringthe flood. This evidenceincludesfine-grained sedimentandfloateddebris Figure 4. Illustration of the integration of geologic data with the HEC­ deposited in areas of slow water flow, as well as scour lines 2 hydrologic model to estimate the size of a paleoflood. Physical evidence offlooding is used to estimate the highest level ofthe flood. Water-surface where floodwaters eroded older sediments (Baker, 1987). profiles generated by the HEC-2 model using different discharge rates (in Paleofloods are commonly reconstructed in bedrock canyons cubic meters per second, or ems) are compared with the high-water marks and upper piedmont areas, where the floods were confined by to estimate paleoflood discharges (from House, 1991). stable valley sides and the channel beds were relatively un­ scoured. To reconstruct these floods, researchers use the HEC­ 2hydrologic flowmodel (Hydrologic EngineeringCenter, 1982) entrenched well below these fans, and young deposits are to generate water-surfaceprofiles ofdifferentdischarges. They restricted to channels and low terraces. Young deposits are then compare these profiles with the geologic evidence of the commonlymore extensive in the middle and lower portions of water-surface elevation above the channel bed (Figure 4). The the piedmonts, which indicates that these are areas of uncon­ mostreasonableestimates of themaximumpaleoflooddischarge fined distributary flow and alluvial-fan activity. The areas in are those that best fit the geologic evidence. Arizona that are subject to alluvial-fan flooding, therefore, are House (1991) reconstructedpaleoflooddischarges onthefive typically located in the middle and lower piedmont areas. largest streams that cross the southern portion of the Tortolita piedmont. The purpose of these studies was to evaluate the GEOLOGIC STUDIES OF PIEDMONT validity of the 100-year discharges that were used in the AFM FLOOD HAZARDS IN ARIZONA to determine flood hazards on the piedmont. The AFM 100- year discharges were determined through the use of a rainfall- A Geologic studies of piedmonts in southern and central Ar­ runoff model based on idealized, intense rainfall events and ~ izona have provided a variety of useful information on the estimates ofwaterrunoff instream channels (Zeller,1979). The character and extent of flood hazards. AZGS geologists have results of this model, however, are suspect because of uncer­ mapped piedmontsindetailto defineflood-prone areas. Finan­ tainties inthe size, duration, andintensityofrainfall events that cial supportfor these efforts camefrom thePima CountyFlood generate large floods in these drainages, as well as uncer­ Control District, Flood Control District of Maricopa County, tainties in the parameters used to obtain the runoff estimates. Arizona Department of Water Resources, and U.S. Geological The geologic recordoffloodingin eachoftheTortolita drainages SurveyCOGEOMAP program. Otherinvestigations ofthe size is at least several hundred years long, yet the largest recon­ and character of piedmont flooding have been undertaken structedpaleofloods are substantiallysmaller thanthe100-year­ cooperatively by the AZGS and University of Arizona Depart­ flood estimates obtained from the rainfall-runoff model. This ment of Geosciences, with financial support from the Pima discrepancy suggests that the 100-year discharges used in the County Flood Control District and National Science Foun­ flood-hazard assessment of the Tortolita piedmontare unreal­ dation. The following paragraphs summarize the results of isticallylarge. these investigations. To realistically assess the hazards of alluvial-fan flooding, As discussedabove, surficialgeologic mapping delineates the researchersmustunderstandthe characterofwaterflow during extent of geologically young deposits, thus revealing the areas these floods. Flood-hazard assessments mustinclude answers that have been subject to alluvial-fan flooding in the recent to the following questions: (1) How deep and fast are water geologic past (during the past few thousand years). AZGS flows during floods on alluvial fans?; (2) Do channel patterns geologistshavemappedindetail the surficialdepositsinseveral commonly change during fan floods or are they relatively areas ofsouthernandcentralArizona (McKittrick, 1988; Jackson, stable?; (3) How much of the fan area is affected by relatively 1989, 1990a,b; Field and Pearthree, 1991b). They have also deep, high-velocitychannelized flow?; and (4) How important conducted specific rnapping projects to evaluate flood hazards is shallower, less hazardous sheet flooding? Researchers have on the Tortolita piedmont north of Tucson (Pearthree and usedseveralhydrologic models ofalluvial-fanfloodingto assess others, 1991) and on the piedmonts around the White Tank floodhazards (e.g., theAFMmandatedbyFEMA). Itis difficult Mountains west of Phoenix (Field and Pearthree, 1991a). to determine how closely these models approximate reality, Thegeologic assessmentofflood-prone areas on the Tortolita however, because information on alluvial-fan floods is scant. piedmont is particularly interesting because this is one of the AZGS geologistsmadea detailedreconstructionofflow pattems few areas inArizona where the AFMhas been used to generate duringa veryrecent, extremealluvial-fanflood, which affected flood-insurance-rate maps (FEMA, 1989). The implications of partof the southernpiedmontof the Tortolita Mountains. This ~) geologic investigations, therefore, may be directly compared study provides a real data set that may be used to test the wr withthe alluvial-fanboundaries determined throughthe AFM. models'predictions. The geologic data and the alluvial fans depicted on the flood­ AZGS geologists discovered fresh evidence (channel scour insurance-rate maps,however, are substantiallydifferent. The and deposition, damaged vegetation, plant flotsam, and fine-

Arizona Geology,vol. 21,no.4,Winter1991 r~.

Flow depth By deciphering this history, geologists canhelp identify flood­ 2000 (centimeters) proneareas, estimate the sizesofthe largestfloods thatarelikely to occur, and illuminatethenature of flood flow onpiedmonts. o no flow c;; 1600 By integrating geologic data with hydrologic models, re­ Do to 30 searchers can improve these models and make more accurate, _I 1200 realistic assessments of flood hazards on piedmonts. AZGS [] 30 to 100 ;; geologists have outlined the areas that are potentially subject "0 .~ 800 .100+ to alluvial-fan flooding bymapping indetail young sediments C depositedbystreamsonseveralpiedmontsinArizona. Alluvial­ L!.. '" 400 fan areas determined by these geologic studies differ signifi­ cantlyfrom thosedeterminedbythemodel thatFEMAmandates for establishingflood-insurancerates. This suggests thateither -1 -0.5 0 0.5 1.5 2 2.4 2.9 3.4 3.8 themodelorthemannerinwhichithasbeenappliedinArizona is incorrect. Geologists canalso estimate the sizes of the largest Distance from fan apex (kilometers! floods duringthe pastfewhundredyearsbyintegratingphysical evidence left by the floods with hydrologic models. These Figure 5. Histogram that shows the distribution offlood-flow depths during paleoflood investigations may be used to check the validity of the 1988 Wild Burro flood on the southern piedmont of the Tortolita hydrologic models that rely solely on idealized rainfall and Mountains. The areas that were covered by three depths of flow were runoffparameters. AZGS geologists reconstructedindetail the determined from 11 transects made across the flood zone, starting above flow during a recent, extreme alluvial-fanflood. The informa­ the fan apex. The area of relatively deep flow (greater than 30 centimeters, tion on flow paths, depths, and velocities should be used to or 1 foot) remained fairly constant downfan. The flow diverged rapidly below the fan apex, however; the number of distinct flow paths separated evaluate how closely the models represent conditions dUring a by dry areas increased, as did the total flooded area (from Pearthree and realflood. others, in prep.). REFERENCES

grained slackwater deposits) of a very largeflood that affected Baker, V.R, 1987, Paleoflood hydrology and extraordinary flood events: Journal of much of the active alluvial fan of Wild Burro Wash (Figure 3; Hydrology, v. 96, p. 79-99. Pearthree and others, in prep.). Datable material in the flood Bull,W.B.,1991, Geomorphicresponses to climatic change: New York, OxfordUniver­ sity Press, 326 p. deposits, reports of flood damage at the lower end of the Dawdy, D.R, 1979, Flood frequency estimates on alluvial fans: American Society of piedmont, and weather radar records were used to determine Civil Engineers, Journal ofthe Hydraulics Division, v.105, no. HYII, p. 1407-1413. that the flood occurred inJuly 1988. The maximum discharge Demsey, KA., 1989, Geologic map ofQuaternary and upper Tertiary alluvium in the at the apex of the alluvial fan, which was estimated through PhoenixSouth30' x60' quadrangle,Arizona: Arizona Geological Survey Open-File Report89-7,scale 1:100,000. the use ofpaleofloodtechniques (see above),is thelargestflood __1990, Geologic map ofQuaternary and upperTertiary alluviumintheLittle Horn recordedfor a drainage ofthis sizeinsouthernArizona. AZGS Mountains 30' x 60' quadrangle, Arizona: Arizona Geological Survey Open-File geologists mapped flow paths anddepths on the alluvial fanin Report90-8,scale 1:100,000. the field, using large-scale aerial photographs with detailed Federal Emergency ManagementAgency (FEMA), 1985, Guidelines and specifications for study contractors: 155 p. topographic contours that were constructed before the flood. __1989, Flood insurancerate maps,Pima County,Arizona (revised),panels 040073 Flow depths and velocities in channels were reconstructed at 1015C,1020C,and 1025 C: scale 1:12,000 and 1:24,000,3 sheets. about 20 locations on the fan. . Field,J.J., andPearthree,P.A.,1991a, Geologic mappingofflood hazardsinArizona: An The results ofthis studyhaveseveralramificationsforflood­ example from the WhiteTankMountains area,Maricopa County: Arizona Geolog­ hazard analysis on alluvial fans. The portion of the piedmont ical Survey Open-File Report 91-10, scale 1:24,000,4 sheets. __ 1991b, Surficial geology around the White Tank Mountains, central Arizona: that was flooded closely coincides with deposits that are less Arizona Geological Survey Open-File Report 91-8, 9 p., scale 1:24,000, 9 sheets. than5,000 years oldand associatedwithWildBurro Wash. The Fuller, J.E., 1990, Misapplication of the FEMA alluvial fan model: A case history, in detailedsurficial geologic mapping, therefore, accuratelydelim­ French, RH., ed.,Hydraulics/hydrology ofarid lands: AmericanSociety ofCivil itedflood-prone areas (Figure 3). Floodflow onthe alluvialfan Engineers National Conference, San Diego, 1990, Proceedings, p. 367-372. Gile, L.H.,Hawley,J.W.,and Grossman,RB.,1981,Soils andgeomorphologyofthe Basin was very complex. From relatively few paths at the fan apex, and Range area of southern New Mexico -- Guidebook to the Desert Project: New the flow quicklybecamemore complicated downfanand even­ Mexico Bureau of Mines and Mineral Resources Memoir 39,222 p. tually split into 42 paths separated by dry areas (Figure 5). House, P.K., 1991, Paleoflood hydrology of the principal canyons of the southern Tor­ Although deep channelized flow and shallow sheet flow were tolita Mountains, southeastern Arizona: Arizona Geological Survey Open-File Report91-6,31 p. bothimportant, flow that was less than 30 centimeters (1 foot) HydrologicEngineering Center,1982,HEC-2watersurfaceprofiles program user'smanual: deep was much more widespread. Areas that were not inun­ U.S. Army Corps of Engineers, 40 p. dated between the outermost flow paths became larger down­ Jackson, G.W., 1989, Surficial geologic maps of the northeastern, southeastern, and fan, composing more thanhalf of the fan area at the lowermost southwesternportions ofthe Tucsonmetropolitan area: ArizonaGeologicalSurvey Open:File Report 89-2, 6 p., scale 1:24,000,7 sheets. limitofmapping. Localflowwithinchannels was muchdeeper __1990a,Quaternarygeologic mapoftheCoronadeTucson7.5' quadrangle,Arizona: and faster than the AFM predicted. Channels that existed Arizona Geological Survey Open-File Report 90-3, 6 p., scale 1:24,000. before the flood conveyed most of the floodwaters, and very __1990b, Surficial geologic maps ofthe Picacho basin: Arizona Geological Survey little change in preflood channel patterns occurred during the Open-File Report 90-2, 9 p., scale 1:24,000,5 sheets. flood. This clearly implies that areas in or adjacent to existing McKittrick,M.A., 1988,Surficial geologic maps oftheTucsonmetropolitanarea: Arizona Geological Survey Open-File Report 88-18, 7 p., scale 1:24,000, 12 sheets. channelsonalluvialfanshavemuchhigherflood potentialthan Pearthree,P.A., Demsey,K.A., Onken,Jill, andVincent,KR,1991,Geomorphicassess­ areas away from channels. ment of fluvial behavior and flood-prone areas on the Tortolita piedmont, Pima County,Arizona: ArizonaGeologicalSurv"yOpen-FileReport91-11,scale 1:12,000.' Pearthree, P.A., House, P.K, and Vincent, KR; In'prep., Detailed reconstruction ofan CONCLUSIONS extreme a,lluvial-fanflood ontheTortolitapiedmont,PimaCounty,Arizona.: Ariz0tla Geological Survey Open-File Reporh".,· .• «> Geologic analyses ofpiedmontsinArizonacansupplyunique Zeller, M.E.,1979, Hydrology manual for engineeringdesign and flood plain manage­ and invaluable insights into the character and extent of pied­ ment within Pima County, Arizona: Pima County DepartmentofTransportati0rt montflooding. A longhistoryofstreambehavior and flooding and Flood Control District, 122 p. is preservedin the geology and geomorphologyofa piedmont.

Arizona Geology,vol. 21, no. 4,Winter1991 5 .- .

memberCommissionretainsthesamepowers andduties to hold REGULATION OF OIL AND GAS hearings, approverules, andsetpolicy,butstaffsupportisnow provided by the AZGS. IN ARIZONA The Oiland Gas ProgramAdministrator handles the day-to- day administrative and regulatory functions re.quired i~ th~) by Steven L. Rauzi drilling,production,andundergroundstorageof 011, gas,hehum~' Arizona GeologicalSurvey andgeothermalresources. Specific activities include collec~ing compliancebonds;issuingpermits; maintainingwelllogs,fIles, In July 1991, the Arizona Geological Survey (AZGS~ :was and samples; maintaining production, injection, and under­ assignedstatutoryresponsibilityfor carryingouttheprovlSlons ground-storagerecords; reviewing,revising, andadoptingrules; of the Oil and Gas ConservationAct, under the oversightof the taking minutes of all meetings and hearings; and answering Arizona Oil and Gas ConservationCommission. The Commis­ correspondence on oil and gas matters. The Program Admin­ sion,whichconsists offive appointed officials, was established istrator also provides expertise on the petroleum geology of in 1959 by Governor Fannin. The first organizational meeting Arizona, in accordance withthe statutoryprovision to encour- of the Commission was held on July 8,1959. Before the Com­ age and promote the oil and gas industry in Arizona. mission was established, the State Land Commissioner was Promotingthe oil and gas potential ofArizona datesback to responsible for administering and enforcing the provisions of 1970 when the Commission started to conduct fundamental the act. geol~gicstudiesrelatedto oil, gas, andhelium. TheCommission Subsequent amendments to the act of 1951 broadened the expanded its staff of geologists and began compiling regi~nal statutory authority ofthe Commissionto include theregulation informationonoilandgasgeologyinthe Stateo SubsequentfIeld of the following: (1) all drilling, development, andproduction reconnaissanceresulted inthepublicationof a series ofregional of oil, gas,helium, andgeothermalresources; and (2) all under­ andlocalmaps. Theseinclude geothermalgradient, structural, groundstorageofoil,liquifiedpetroleumgas (LPG), andnatural geologic, and geophysical maps. Many of the maps are accom­ gas. The Commissionalsohas sta~toryauthorityto h?ldpublic panied by well descriptions and reports on the petroleum hearings, approve rules, set pohcy, and promote 011 and gas geology of a region, county, or producing field. activity in Arizona. Current oil and gas publications include the informative Under the enablingstatute, commissioners are appointed to Arizona Well Location Map and Report. This report is a 28-page a 5-year term by the Governor with the advice of and confir- tabulationofallwells drilledfor oil, gas,helium, orgeothermal resources in Arizona. The wells are plotted on two accompa­ nying maps, a 1:667,000-scale map of the entire State and a 1:127,000-scalemap ofproducing oiland gas fields inArizona. Theheliumfields eastofHolbrookare also includedonthelatter map. The most recent oil and gas publication is Proterozoil: Hydrocarbon Source Rock in Northern Arizona and Southern Utah, &. whichdescribes the distributionofthe recentlyrecognized, late .. Precambrian source rock in northern Arizona and southern Utah. A chart titled Oil and Natural Gas Occurrence in Arizona concisely summarizes the petroleum geology of Arizona. Formoreinformationonoilandgas regulations,publications, and related activities, contact Steven L. Rauzi, Oil and Gas ProgramAdministrator,Arizona GeologicalSurvey,845 N. Park Ave., Suite 100, Tucson, AZ 85719; tel: (602) 882-4795.

OIL AND GAS NOTES Figure 1. Members ofthe Oil and Gas Conservation Commission and AZGS A permit to drill was issued in September to United Gas support staff review the oil and gas rules and budget and discuss idle wells Search of Tulsa, Oklahoma. The company plans to drill the at their September meeting. Front row, left to right: Barbara H. Murphy well, the Mohave County No.1, about 15 miles south of St. (Commissioner), Jan c. Wilt (Commission Chairperson), and Katherine L. George, Utah, in T. 41 N., R. 11 W., sec. 10. The proposed Mead (Assistant Attorney General). Back row, left to right: Pamela J. total depth is 6,500 feet. Lott (Secretary), Larry D. Fellows (Director of the AZGS and State DryMesa CorporationofFarmington, New Mexico,recom­ Geologist), J. Dale Nations (Commissioner), James E. Warne (Commission pleted an oil well in the Dry Mesa field as a gas producer Vice-Chairperson), and Steven L. Rauzi (Oil and Gas Program Admin­ inthePennsylvanianParadoxFormation. MerrionOiland Gas istrator). Corporation, also of Farmington, recompleted a gas well in the East Boundary Butte field as an oil producer in the Mississippian Leadville Limestone. mation by the Senate. Current members include Jan C. Wilt of About 258,000 acres in Arizona are currentlyleased for oil Tucson, Chairperson; James E. Warne, Jr. of Phoenix, Vice­ and gas exploration; 198,000 of these are Federal leases and Chairperson; Archie Roy Bennett of Prescott; Barbara H. 60,000 are State leases. Recent leasing activity in northern Murphyof Glendale; andDr. J. DaleNations ofFlagstaff(Figure Arizonaispartly drivenby a growinginterestinthe oilpoten­ 1). The State Land Commissioner serves as ex officio member tial of Precambrian rocks. United Gas Search picked up the and, thus, has no voting privileges. Since 1959, 32 individuals parcels for its Mohave County No.1 well at the Bureau of have served the citizens of Arizona by accepting appointment Land Management (BLM) lease sale in June 1991. Premco to the Commission. Western, Inc. ofDallas, Texas, successfully bid ontwo parcels \ Since it was established in 1959, the Commissionhas main­ near the Mohave County No. 1 well at the BLM lease sale in .. December 1991. .. tained a staff and office in Phoenix. In July 1991, that office was merged into the AZGS and moved to Tucson. The six-

6 Arizona Geology, vol. 21,no.4,Winter1991 Arizonans Support National Geologic-Mapping Program

liTo stimulateSta~e~theproductionofgeologicmap informationinStat~, Program Objectives _theUnited throughthe cooperationofFederal, and The objectives of the geologic mapping program shall include academic partlclpants," members of the 102d Congress mtro­ (1) Determinationof the Nation's geologic framework through ducedtwobills, S.1179 andH.R. 2763,in1991. Thebillsestablish systematic developmentofgeologic maps, to becontributed a national geologic-mapping program, which includes a State to a national geologic-map data base, at scales appropriate geologic-mapping component. The U.S. Geological Survey to the geologic settingandtheperceivedapplication, for the (USGS) would be the lead Federal agency. At the National purpose of resolving issues related to land-use manage­ Governors ConferenceheldinSeattleinAugust1991, governors ment, assessment, utilization and conservation of natural unanimouslypasseda resolutionthatstronglysupportednation­ resources, ground-water management, and environmental al legislation to build the Nation's geologic-map database. protection; S. 1179, H.R. 2763, and the governors' resolution emphasize (2) Establishment of a geologic mapping association whose the importance of geologic mapping to modern society and the cooperating partners coordinate to identify national prior­ need for more extensive and detailed map coverage of the ities and to develop the national geologic-map data base; Nation. If S. 1179 and H.R. 2763 are both passed and funded, (3) Developmentofcomplementarygeophysical, geochemical, the rate at which geologic maps are completed will be signif­ geochronologic, and paleontologic data bases that provide icantly increased. Excerpts from the bills and the full text of value-addeddescriptiveand interpretiveinformationto the the resolution are reprinted below. geologic-map data base; (4) Application of cost-effective mapping techniques that as­ SENATOR DECONCINI COSPONSORS S.1179 semble, produce, translate, and disseminate geologic-map S. 1179, the Geologic Mapping Act of 1991, was introduced information and that render such information of greater on May 23 by Senator Bennett Johnston (D-LA) and originally application and benefit to the public; [and] cosponsoredbySenatorsJeff Bingaman(D-NM) andLarryCraig (5) Development of public awareness for the role and appli­ (R-ID). Additional cosponsors included Senator Dennis cation of geologic-map information to the resolution of DeConcini (D-AZ). The bill was referred to the Committee on national issues of land-use management. Energy and Natural Resources. S. 1179 was passed by the committee in earlyNovember, butdid notreach the full Senate CONGRESSMEN STUMP AND KOLBE before the December recess. COSPONSOR H.R. 2763 H.R. 2763, the National Geologic Mapping Act of 1991, was Findings introducedonJune25 byCongressmanNickRahall(D-WV) and The Congress finds and declares the following: originally cosponsored by Congressmen Barbara Vucanovich (1) Geologic maps are the primary data base for virtually all (R-NV), BillBrewster (D-OK), andDaveMcCurdy (D-OK). More applied and basic Earth-science investigations, including than30 additionalcosponsorsincludedCongressmenBob Stump exploration for and development of mineral, energy, and (R-AZ) and Jim Kolbe (R-AZ). The bill was referred to the water resources; land-use evaluationand planningfor en­ Committee on Interior and Insular Affairs. H.R. 2763 passed vironmentalprotection; recognitionand mitigationofgeo­ the House by a voice vote on November 19. logic hazards; design and construction of infrastructure requirementssuchas utilitylines,transportationcorridors, Findings and surface-water impoundments; and basic research into The Congress finds and declares that the composition, structure, and history of Earth materials (1) During thepast2 decades, theproductionofgeologic maps and formation processes. has been drastically curtailed. (2) AlISO Statesrequirebasic geologic-mapinformationto plan (2) Geologic maps are the primary data base for virtually all and execute decisions that affect the social and economic applied and basic earth-science investigations, including welfare of the public and private sectors. (a) exploration for and development of mineral, en­ (3) Despite the pivotal role that geologic maps play in the ergy, and water resources; portrayal and dissemination of geologic information, the (b) screening and characterizing sites for toxic and Nationhasnever committeditsel£to a sustained, systematic nuclear waste disposal; effortto builda comprehensivenationalgeologic-map data (c) land-use evaluationand planningfor environmen- base; instead, scientific efforthasbeendirected away from tal development, preservation, and quality; the acquisition of long-term baseline information and (d) earthquake hazards reduction; toward the solution of short-term single-issue problems. (e) predictingvolcanic hazards; (4) A comprehensive,nationwideprogramofgeologic mapping (f) designand constructionofinfrastructurerequire­ based on Federal, State, and private efforts is essential to ments such as utility lifelines, transportation cor­ systematicallybuildtheNation's geologic-map databaseat ridors, and surface-water impoundments; a pace that responds to increasing demand for data nec­ (g) reducinglosses from landslides and otherground essary for the long-term needs of the Nation. failures; (h) mitigating effects of coastal and stream erosion; Purpose (i) siting of critical facilities; and The purpose of this Act is to expedite the production of a (j) basic earth-science research. .. geologic-map information base for the Nation which can be (3) Federal agencies, State and local governments, prIvate m­ appliedto resolutionofissues relatedto land-usemanagement, dustry, and the general public depend on the information assessment, utilization and conservation ofnatural resources, provided by geologic maps to determine th~ extent ~f ground-water management, and environmental protection. potentialenvironmentaldamagebefore embarkmgonproJ-

ArizonaGeology,vol. 21,no.4,Winter1991 7

* ---~ .

ects that could lead to preventable, costly environmental Geologic-map coverage of the Nation, however, is critically problems orlitigation. insufficient and out of date to meet the demands of private, (4) The combined capabilities of State, Federal, and academic industrial, and government-agency users. The Nation's gover- groups to provide geologic mapping are not sufficient to nors express their strongsupportfornationallegislationto build . meet the present and future needs of the United States for the Nation's geologic-map database through a program to be~ national security, environmental protection, and energy implementedinequitypartnership betweenthe States (throughW": self-sufficiency of the Nation. their geological surveys or other designated agencies) and the (5) States are willing to contribute 50 percent of the funding Federal government(through theU.S. Geological Survey). The necessary to complete the mapping of the geology within program must be sufficiently funded at both the Federal and the State. Statelevels to permitachievingcompletegeologic-map coverage (6) Thelack ofpropergeologic mapshasled to thepoordesign for the Nation at an appropriate level of detail within a rea­ of such structures as dams and waste-disposal facilities. sonably short period of time. (7) Geologic mapshave provenindispensableinthe searchfor needed fossil-fuel and mineral resources. (8) A comprehensivenationwideprogramofgeologic mapping PROFESSIONAL MEETINGS is required in order to systematically build the Nation's geologic-map databaseata pacethatresponds to increasing Tucson Gem & Mineral Show. Annual exhibit, February 12-16, demand. Tucson, AZ. Contact Tucson Gem & Mineral Show Committee, P.O. Box 42543, Tucson, AZ 85733; tel: (602) 322-5773.

Purpose Mineral Resources. Annual meeting, February 24-27, Phoenix, AZ. The purpose of this Act is to expedite the production of a Contact Meetings Dept., Society for Mining, Metallurgy, and Explo­ geologic-map data base for the Nation, to be locatedwithin the ration, Inc., P.O. Box 625002, Littleton, CO 80162; tel: (303) 973-9550. UnitedStates Geological Survey, which canbe applied to land­ use management, assessment, and utilization, conservation of Arizona-Nevada Academy of Science and the Southwestern and Rocky Mountain Division of the American Association for the naturalresources, groundwatermanagement, and environmen­ Advancement of Science. Joint meeting, May 17-21, Tucson, AZ. tal protection. Special session on May 19: "Air, Land, and Water in the Canyons ofthe Colorado: Values, Conflicts, and Resolution." Special evening Program Objectives program features Carl Sagan. Contact Sandra Brazel, Office of The objectives of the geologic mapping program shall include Climatology,ArizonaStateUniversity, Tempe,AZ85287-1508; tel: (602) (1) DeterminingtheNation's geologicframework throughsys- 965-6265. tematic developmentofgeologic maps, to be contributedto thenational geologic-map data base, ata scale ofl:100,OOO, Sunset Review of AZGS Completed with supplemental maps at scales appropriate to the geo­ logic setting and the perceived applications; The Joint Natural Resources and Agriculture Committee of (2) Developmentofa complementarynationalgeophysical-map Reference held a Sunset Review of the Arizona Geological database, geochemical-map database,anda geochronologic Survey (AZGS) on October 8, 1991 and recommended that the AZGS be continued for another 10 years. The review covered and paleontologic data base that provide value-added the period from July 1, 1977 to June 30, 1991. Senator Gus descriptive and interpretive information to the geologic­ Arzberger and Representative Susan Gerard co-chaired the map data base; committee. Other members were Senators Ann Day, John E. (3) Application of cost-effective mapping techniques that as­ Dougherty, Nancy L. Hill, and James J. Sossamon, and Rep­ semble, produce, translate, and disseminate geologic-map resentatives Hemy Evans, Kyle W. Hindman, Richard "Dick" information and that render such information of greater Pacheco, and Greg Patterson. Those who testified in behalf of application and benefit to the public; and the AZGS were Larry D. Fellows, AZGS Director and State (4) Development of public awareness for the role and appli­ Geologist; James A. Briscoe, President, JABA Inc.; Charles P. cation of geologic-map information to the resolution of Miller, President, Miller Resources, Inc.; and James D. Sell, national issues of land-use management. Manager, Southwestern Exploration Dept., ASARCO, Inc. GOVERNORSYMINGTON SUPPORTS RESOLUTION SPEAKERS WANTED The following resolution was supported by Governor Fife Professionals who wish to share their knowledge ofhow mineral and Symington and other governors at the National Governors energy resources are formed, how and where they are located, and Conference in August 1991. why they are important to modern society may contact the Southern Geologic maps are a principalsource ofcritical earth-related Arizona Earth Science Education Coalition (SAESEC). SAESEC is information required by Federal, State, and local government composed of professionals from the Tucson Section of the American Institute of Mining, Metallurgical, ahd Petroleum Engineers (AIME), agenciesandtheprivatesector. Theyare essentialfornumerous Arizona Geological Society, AmericanInstitute of Professional Geol­ assessments, evaluations,and decisionsrelated to the economic ogists,SouthwestMineralsExplorationAssociation,Tucson/SanManuel developmentandmaintenance oftheenvironmentofthe Nation. Women's Auxiliary of AIME, and Mining Club of the Southwest. These maps provide vital information needed for land-use SAESEC receives requests for free speakers from local student, civic, planning. In particular, they are indispensable for locating social, andseniorcitizengroups. Any geologistorrelatedprofessional disposal sitesformunicipal,hazardous, andradioactivewastes; who is interestedin donating time and talentto this speakers' bureau should contact Claire E. Roscoe, Manager, Mining Club ofthe South­ locatingandprotectingsurface-waterandgroundwaterresourc­ west, P.O. Box 27225, Tucson, AZ 85726; tel: (602) 622-6257. es; locatinganddevelopingmineralandenergyresources; reduc­ ing the risks from earthquakes, landslides, and ground-failure hazards; predicting hazards from volcanoes and from stream andshorelineerosion; sitingcritical emergencyfacilities; routing highwaysandpublic-utilitylines; andinvestigatingbasic earth­ science matters.

Arizona Geology, vol. 21,noA,Winter1991 ------.._------..~.-.,----~------~~~~----- ...... _--"--"""'-"~~--

Pinatubo Generates More Brilliant Sunrises and Sunsets May Cause Cooler Global Temperatures and Higher Skin-Cancer Risks

by Evelyn M. VandenDolder climatic fluctuations are so muchlargerinany given area, such Arizona Geological Survey asArizona, andbecause the meanglobal temperaturenaturally varies by0.2 0 C (Luhr, 1991). Inaddition, an EINino, a periodic Althoughmore than10,000kilometers (6,200 miles) separate warming of ocean waters, is developing in the Pacific Ocean. the United States from the Philippines, the eruption of Mount This eventwillwarm theEarthfor abouta year,further masking Pinatubo inJune 1991 may affectboth the climate and the skin­ the climatic effects of the volcanic haze (Monastersky, 1991). cancer risk in Arizona (Monastersky, 1991). The ash cloud that One scientistbelieves that the eruption of Mount Pinatubo and now circles the globe has already created the most brilliant resultantreflection ofsunlightare actually inducingtheEI Nino sunrises and sunsets in the State in recent years. event and will shift the jet stream over the north Pacific farther After more than 2 months of souththis winter,increasingpre­ intensified seismicity, deforma- cipitation and ending the 5-year tion,anddischargeofsmallsmoky droughtin California (Geotimes, plumes,MountPinatubobeganto 1991). Despite the natural vari­ erupt on June 9. The largest ation in global temperatures and explosions occurred on June 14 the surfaceheating of theEI Nino and 15, creating an ash and gas event, otherscientistsstillbelieve cloud as high as 20 miles into the that theglobal-scalecoolingofOS stratosphere (Geotimes, 1991; C will be large enough to be no­ GlobalVolcanismNetwork,1991). ticeable (Luhr, 1991). By July 7, the cloud had circled The sulfur dioxide aerosols the globe (Figure 1). By mid­ may also alter the chemistry of August, as shown on satellite thestratosphere, thinningthepro­ images, the thickest cloud layer tective ozonelayer that surrounds extended to 20 0 north and south the globe and allowing more ul­ .a of the Equator, or near the lati­ traviolet radiation to reach the .. tudes of Mexico City and Rio de Earth's surface. This could in­ Janeiro. A thinner layer extend­ crease the risk of skin cancer, edto 35 0 northlatitude, oralmost especially in the mid- and high as far north as Flagstaff (Associ­ latitudes (Monastersky, 1991). ated Press, 1991). Based on sat­ Figure 1. Images from a polar-orbiting satellite of the U.S. Some scientists estimate that the ellite and aircraft measure­ National Oceank qnd Atmospheric Administration (NOAA). This aerosols couldcausea 15-percent ments, MountPinatubo wasprob­ satellite includes a high-resolution radiometer, which can measure reductioninozone values during ably thelargestvolcanic eruption the reflected sunlightfrom dust and haze in the atmosphere. Such measurements, however, can be obtained only for areas above the the winter and a 6- to 8-percent ofthecentury, spewingmore than ocean on cloudless days. The top image was compiled from May reduction during the summer in twice the amount of ash and gas 28 through June 6, before the eruption ofMount Pinatubo. Most the mid-latitudes, which include as the 1982 eruption of El Chi­ of the reflectivity in this image is due to airborne dust from the Arizona. Thisincreasedradiation chbninMexico. MountPinatubo Saharan and Arabian Deserts. The bottom image was compiled would increase the skin-cancer couldcontinue to eruptintermit­ from July 25 through August1, weeks after the aerosol cloud from risk and may cause several thou­ tently for several years (Associ­ Mount Pinatubo hadcircled the globe. Images courtesy ofNOAA. sand more cases of melanoma in ated Press, 1991). the United States during the next The haze that encircles the Earth includes both volcanic ash few decades (Monastersky, 1991). The effect of the eruption and sulfur dioxide aerosols. These aerosols formed when on the ozone layer, however, is debatable. Some scientists millions of tons ofsulfur dioxide gasfrom the eruptionreacted believe that the data and computer models used to determine with water vapor in the stratosphere and created tiny drops the level of ozone depletion are insufficient to make such (aerosols) ofsulfuric acid, whichare also containedinacid rain predictions. causedbyair pollution. Scientists estimatethatthe aerosolswill Whatever the effects of the Mount Pinatubo eruption, all stay in the stratosphere for 2 to 3 years before they fall to the scientists studying this event agree that it has provided, and Earth's surface (Monastersky, 1991). will continue to offer during the next few years, a wealth of The haze filters sunlight, creating magnificent sunrises and information onvolcanic processes. sunsets. Italso absorbs sunlight and reflects itbackinto space, which could cool the Earth's climate. In 1992 and 1993, after REFERENCES thehaze disperses andbecomes more evenly distributedaround Aldhous, Peter, 1991, Before and after: Nature, v. 352, p. 651. the Earth, mean global temperatures could decline by 0.5 0 C AssociatedPress,1991,MountPinatuba's haze girdlesglobe; volcanicash, gasmay cool 0 the Earth: The Arizona Republic, August 14, p. A4. (about 1 F; Aldhous, 1991; Luhr, 1991). The eruption of EI Geolimes, 1991, Could Pinatubo abate drought, cause EI Nino?: v. 36, no. 9, p. 11. ~ Chich6n lowered global temperatures by a few tenths of a Global Volcanism Network, 1991, Geologic phenomena;June 12: Geolimes, v. 36, no. .. degree for 2 to 3 years (Associated Press, 1991). Although the 9,p.25. Luhr, J.P., 1991, Volcanic shade causes cooling: Nature, v. 354, p. 104-105. temperature decrease due to Mount Pinatubo could last for 2 Monastersky,Richard, 1991, Pinatubo'simpactspreadsaroundtheglobe: Science News, to 4 years, the decline may not be noticeable because normal v. 140, no. 9, p. 132-133.

9 For the Classroom Calculating the Height of the Figure 1. Pinatubo Global Ash Cloud

by Jon E. Spencer Arizona Geological Survey

The ash cloud produced by the Pinatubo volcanic eruption spreadquickly around the Earthand, bymid-August 1991, was responsibleforbrilliant sunrises and sunsets inArizona. At 30 minutes after sunset, a brilliant orange to red glow dominated the western sky. The glow had a fairly distinct, horizontal top, above which was significantly darker sky. This boundary de­ scended to the horizon; glowset occurred 40 minutes after sunset. Glowriseoccurred40 minutesbeforesunrise. Although the distinctive boundary at the top of the orange glow had becomemorediffusebyNovember,itwillprobablystillbevisible well into 1992. A similar brilliant glow was also seen after the C = Center of Earth eruption of Krakatau in 1883. This article describes a method to calculate theheightofthe top ofthe ashcloudfrom onesimple measurement: the time between sunset and glowset (or glow­ rise and sunrise). Figure 2. The glowis causedbysunlightstrikingthe airborne ash. The Earth's shadow falls on the ash, and the edge of the shadow p travels around the Earth as the Earth turns. An observer sees the sun set when sunlight travels along a line that is tangent a (2) Law of sines: to the Earth at the point of the observer (if there are no large mountains obscuring the distanthorizon). As showninFigure y -.!!.- =~ =_c_ 1, glowset for the observer at point A occurs at a later time, a sina: sin~ siny when the edge of the Earth's shadow crosses the airborne ash b atpoint B. Note thatat glowset atpointA, an observer atpoint D would see sunset. Thus, during the time betweensunset and glowset (here designated m, as measuredinminutes), the Earth (1) a + p + Y = 180° (3) Pythagorean theorem has rotated an amount equal to angle DCA, which is here As shown here, (right triangles only): y = 90° (right triangle), designated (Xl (alpha 1). Given that the Earthrotates 360° every 24 hours and that there are 1,440 minutes per 24-hour day, the therefore, a + p = 90° angle (Xl may be calculated as follows: therefore, C = Va2+b2

Ash-cloud height hI' in kilometers, is then equal to BC - r. The angle (X2 is equal to (Xl divided by 2. This calculated ash-cloud height is accurate for an observer To calculate ash-cloudheight(designatedhI inFigure1) using at theEarth'sequator, butincreasinglylargeradjustments must trigonometry, we mustknow two variables that define triangle bemade as one approaches the poles. This is because at sunset ABC in Figure 1. One is angle (X2' which we calculated above; at higher latitudes, the sun does not descend vertically to the the other is the length of line AC (designated r in Figure 1), horizon but at an angle. Near the poles, the sun does not set which is the radius of the Earth (6,370 kilometers). We must atall duringspringandsummermonths, buttravels essentially also use in our calculations the three properties of triangles horizontally around the sky near the horizon. The ash-cloud outlinedinFigure2. Firstwedetermine angle 8(theta). Because height corrected for latitude (h ) is determined simplyby mul­ the triangle is a right triangle, angle 8 is equal to 90° minus (X2' 2 tiplying the calculatedash-cloudheight (hI) bythe cosine of the Next, we calculate the length ofline AB (designated x in Figure latitude, as follows: 1). According to the law of sines, h =hi x cos(latitude) x r 2 --=-- sintt2 sinO This is a fairly accurate determination of the height of the top of the ash cloud. In September 1991, glowset occurred 40 therefore, minutes after sunset in Tucson, which is at latitude 33° north. rxsintt x=-----=-2 Applying the above equations revealed that the height of the sinO top of the ash cloud was 24.3 kilometers, or about 80,000 feet. This calculation is accurate at times near the spring and fall Next, from the Pythagorean theorem, we calculate the length Ai equinoxes, but is slightly inaccurate at times near the winter ., of line BC, as follows: and summer solstices. A calculation to correct for the time of BC=Vx2 xr2 year, however, is beyond the scope of this article.

Arizona Geology, vol. 21,no.4,Winter 1991 New AZGS Publications

j hefollowing!mblicationsmaybepurchasedfrom theArizona gests that such models do not represent the natural processes

• Geological Survey (AZGS), 845 N. ParkAve., #100, Tucson, AZ insmaller watersheds, such as the Tortolita Mountains. Using 85719. Orders are shippedby UPS; a street address is required the slackwater-deposit/paleostage-indicator (SWD-PSI) tech­ for delivery. Allordersmustbeprepaidbycheckormoneyorder nique, the author reconstructed the paleofloodhistory of three payable inU.S. dollars to the Arizona Geological Survey. Add canyons in the Tortolita Mountains and found major discrep­ shippingandhandlingcharges, listedbelow,to your totalorder: ancies between his results and those obtained by FEMA. The In the United States: 20.01-30.00,add5.75 50.01-100.00,add 10.25 reportdescribes paleofloodhydrology andits useinthe current $1.01-$5.00, add $2.00 30.01-40.00,add 6.50 Over 100.00,add 12% study and recommends that it be used to check the validity of 5.01-10.00, add 2.50 40.01-50.00,add 8.00 Other countries: Re­ theoreticallyderived, regulatoryflood-magnitude estimates used 10.01-20.00,add4.50 quest price quotation in engineering design and floodplain management.

Slaff, Steven, 1991, Earth-fissure activitynearBradyandPicacho The following 10 Contributed Maps were donated by Karl E. pumpingplants,Tucson aqueduct, CentralArizonaProject,Pinal Karlstrom, formerly of the DepartmentofGeology atNorthern County, Arizona: Open-File Report 91-1, 43 p., scale 1:24,000, Arizona University. The areas were mapped by geology grad­ 2 sheets. $10.50 uate students as part of their master's theses. The AZGS seeks Earthfissures are surface and subsurface tension cracks that high-quality mylars of geologic maps of areas within Arizona, develop in unconsolidated and semiconsolidated sediments. compiledfrom originalresearch. Please contacttheAZGSifyou Hundreds of fissures exist west of the Picacho Mountains in wish to donate items to its Contributed Map series. Picacho basin. This report focuses on the portion of the basin that includes the Brady and Picacho pumping plants and parts Puis,D.D.,1985, Geologicmapandbalancedcross-sectionofthe ofthe Tucsonaqueduct, Santa Rosa canal, and CentralArizona northern Mazatzal Mountains, central Arizona: Contributed Irrigation and Drainage District (CAIDD) canal. The author Map CM-91-B, scale 1:10,000,2 sheets. $5.50 identified and mapped earth fissures in the study area and evaluated the potentialfor future fissure development to dam­ Sherlock, S.M., 1986, Geologic map and cross-section ofEarly age the Central Arizona Project (CAP). Changes in the local Proterozoicrocks ofMcDonaldMountain - BreadpanMountain water-tableelevationwereexaminedto seewhethertheyprovide area, northern SierraAnchas, Gila County,Arizona: Contributed a qualitative indication of present and future fissure-activity Map CM-91-C, scale 1:10,000,2 sheets. $5.00 rates, and large-format photographs were tested as a fissure­ identificationtool. Theprojectwas primarilyfundedby the U.S. Argenbright,D .N.,1985, Geologic mapandProterozoic structure Bureau of Reclamation. ofCrazyBasin area, Yavapai County,Arizona: ContributedMap CM-91-D, scale 1:10,000. $4.00 e Schmidt, Nancy, Reynolds, S.J., and Horstman, K.C., 1991, AZGEOBIB: A preliminary listofreferences on the geology of Brady, T.B., 1987, Geologic map ofthe Sheep Basin Mountain Arizona: Open-File Report 91-4,302 p. $27.00 area, northern SierraAnchas, Gila County,Arizona: Contributed The largest bibliography on Arizona geology, AZGEOBIB Map CM-91-E, scale 1:10,000. $5.00 contains approximately 11,600 references pertinent to the ge­ ology of Arizona. Although the references are primarily geo­ Roller,J.A., 1986, Geologic map and cross-section ofthe central logicalinfocus, thebibliographyisa usefulsource ofinformation Mazatzal Mountains, central Arizona: Contributed Map CM­ notonlyfor geologists,butalso for educators, librarians,hydrol­ 91-F, scale 1:10,000,2 sheets. $6.00 ogists, archaeologists, and environmental and land-use plan­ ners. In its preliminary form, AZGEOBIB is released both as D arrach, Mark, 1987, Geologicmap and Proterozoic structureof a text file (MS-DOS disk) and as a hard-copy printout. the Cleatorshear zone, Yavapai County,Arizona: Contributed Map CM-91-G, scale 1:3,000. $4.50 Gilbert, W. G., 1991, Bedrock geology ofthe eastern Gila Bend Mountains, Maricopa County,Arizona: Open-FileReport91-5, Labrenz, M.L., 1988, Geologic map of the western halfof the 13 p., scale 1:24,000. $5.00 Young quadrangle, northern SierraAnchas, andgeologic mapof The eastern Gila Bend Mountains are composed of Early the Marsh Creek area, Diamond Butte andYoung quadrangles, Proterozoic crystalline rocks and overlying, mid-Tertiary, con­ Gila County,Arizona: ContributedMap CM-91-H,scale 1:24,000 tinental clastic and volcanic rocks of the Sil Murk Formation. and 1:10,000, 2 sheets. $4.00 Slightly more thanhalf of the bedrock in the eastern part of the range is composed of granitic rocks. Except for a few scattered Wessels, R.L., 1990, Geologic map ofthe Jakes Corner area and exposures, themid-Tertiaryrocks crop outinthe southwestern geologic cross-section ofEarly Proterozoic rocks ofthe Jakes part of the map area. Earlier studies suggest that the area has Corner area, northern Sierra Anchas, Gila County, Arizona: low potential for mining barite, copper, molybdenum, lead, Contributed Map CM-91-I, scale 1:10,000,2 sheets. $8.50 zinc, silver, gold, uranium, thorium, and rare-earth elements. The project was supported by the Cooperative Geologic Map­ Doe, M.F., 1991, Geologic map ofthenorthernMazatzal Moun­ pingProgram (COGEOMAP) betweentheAZGS andU.S. Geo­ tains andgeologic cross-sections ofthe southfork ofDeadman logicalSurvey. Creek and Barnhardt - Shake Tree Canyon, central Arizona: Contributed Map CM-91-J, scale 1:24,000,2 sheets. $5.25 House, P.K., 1991,Paleofloodhydrologyoftheprincipalcanyons ofthe southernTortolitaMountains, southeasternArizona: Open- Albin, A.L., 1991, Geologic map ofthe Peacock Mountains and ..File Report 91-6,31 p. $5.00 southern GrandWash Cliffs, includingPeacock Peak, Antares, .. To derive 100-yeardischarge estimatesfor developingflood- Hackberry, Valentine, and the southernhalfoftheMusicMoun­ insurance-rate maps, agencies such as the Federal Emergency tain SE andMilkweedCanyon SW7.5-minute quadrangles,north- ManagementAgency(FEMA) have traditionallyusedstandard , western Arizona: Contributed Map CM-91-K, scale 1:24,000. theoretical rainfall-runoff models. This study, however, sug- $5.25

Arizona Geology, vol. 21,no.4,Winter1991 11 STAFF NOTES as well as geologists in Yavapai County. Duringthe fall, Philip A. Pearthreegave Heparticipatedinthe Minerals andMining several presentations on geologic haz­ NewEmployees ClusteroftheArizona Strategic Planning ards in Arizona. He co-led a field trip Emily CreighDiSantehas beenhired as for Economic Development Committee. (withKirkR. VincentandP. Kyle House_ AdministrativeAssistantIattheArizona He participated in seismic safety meet­ for the Arizona Hydrological Society t9 Geological Survey (AZGS) and serves as ings sponsored by the Arizona Division examine evidence ofpiedmontflooding. Editorial Assistant. She brings to the job ofEmergencyServices andwasa panelist He lectured on seismic hazards in the morethan11 yearsofexperienceinwriting at a meeting on environmental steward­ Basin and Range Province at Arizona and technical editing, as well as 4 years ship sponsored by the HAZWaste Soci­ State University and the University of of experience managing her own desk­ ety. Larry also participated in meetings Arizona. Phil also gave a geologic per­ top-publishingbusiness. Emilyhas a B.A. oftheEnvironmentalEducationTaskForce spectiveonseismichazards to theArizo- degree in international studies and an and theInteragencyCommittee onEnvi­ na Seismic SafetyCouncilandto planners M.A. degree in teaching English as a ronmental Education and helped orga­ and emergency-response personnel in second language. She speaks Spanish nize the SouthernArizona EarthScience PimaCounty. fluently. Education Coalition. He gave two pre­ sentations on the geology of the Verde At the annual meeting of the Geological Pamela J. Lott is the new Secretary for basin and led a field trip as part of the SocietyofAmerica andassociatedsociet­ the AZGS Oil and Gas Program. Her 12 third annual Verde River Days held at ies in October, Jon E. Spencer gave an years ofworkexperienceincludegeneral Dead Horse Ranch State Park in Cotton­ oral presentation titled "Miocene Base­ office work, bookkeeping, paralegal du­ wood. Larry received the 1991 Distin­ andPrecious-MetalMineralizationAsso­ ties, secretarial duties for an oil and gas guished Community Service Award at ciatedwithBasinBrines and Detachment company, and managing her own busi­ the fall conference oftheArizona Science Faults,West-CentralArizona,"whichwas ness. Pam has a B.S. degree in social TeachersAssociation(ASTA). coauthored by John T. Duncan and Ste­ science and has been approved by the phen J. Reynolds. Jon also served as American BarAssociation as a paralegal Thomas G. McGarvinled several work­ cochairmanofa full-daysymposiumtitled professional. shops and field trips during the fall. He "Cenozoic Extension in the Cordillera: Richard A. Trapp,GeologistII,is thenew presented a workshop titled "The Logic Geometry, Timing, Mechanisms, and AZGS database manager and backup inGeologicProcesses" to educatorsatthe Regional Controls." In December, he geologistfor theOilandGasProgram. He fall conferences ofASTA andtheArizona gave a talk titled "Mineral Deposits As­ has been a computer specialist and sys­ Associationfor LearninginandAboutthe sociatedwithDetachmentFaultsinWest­ tems analyst for 9 years and has experi­ Environment, secondaryschool teachers CentralArizona" attheArizona Confer- enceincomputerizinggeologic databases. in the Marana school district, and mem­ ence oftheAmericanInstituteofMining, Rickhas B.S. andM.S. degrees ingeology bers of the TucsonEstates' Nature Club. Metallurgical, andPetroleumEngineers. e and has coauthored 11 publications. He held two additional workshops for Marana-districtteachers: "N0 Rocks, No r----Arizona Geology ------, Recent Activities Ice Cream" and classroom methods for Vol. 21,No. 4 Winter1991 Larry D. Fellows represented the AZGS presentinggeologic concepts to students State of Arizona: Governor Fife Symington at the annual meeting of the directors of in grades K-6. Tom taught docents at ArizonaGeological Survey State geological surveysinwesternstates Tohono ChulParkinTucsonaboutbasic Director & State Geologist: Larry D. Fellows and at the midyear business meeting of geologic conceptsandthegeologic setting Editor: Evelyn M. VandenDolder the Association of American State Geol­ ofthe Tucsonarea. Heledtwo field trips Editorial Asst. & Designer: Emily Creigh DiSante ogists. He testified at the SunsetReview for teachers that focused on the geology lllustrator: PeterF. Corrao hearing of the AZGS conducted by the ofthe Tucson,Santa Catalina, andRincon Copyright © 1991 Arizona Geological Survey Arizona Legislature. Larry met with the Mountains and the Tucson basin. Tom AZGS advisory committees for environ­ receivedthe1991 DistinguishedCommu­ mentalandengineeringgeology, mineral nity Service Award at the ASTA fall resources, and earth science education, conference. Printedomecycled paper

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