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Post-Glacial Lahars of the Sandy River Basin, Mount Hood, Oregon

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Post-Glacial Lahars of the Sandy River Basin, Mount Hood, Oregon Kenneth A. Cameron and Patrick Pringle, USGSCascades Volcano Observatory VancouverWashinoton 98661 Post-GlacialLahars of the SandyRiver Basin, MountHood, Oregon Abstract Vithin the last I0,000 years,three significant lahar'producingperiods have occurr€dar Mount Hood, Oregon.The Timberline eruptire period occurred belween 1400and 1800years BP. It wasby far the nosr voluminousof lhe three periods,producing enoughclastic debris to bury the glacialtopography ofthe southwestface of the mountain beneatha snootb debris fan. Snaller debris fans were forned in the upper reachesof the Sandy and Salmon Rivers. Timberline-agelahars trareled the length of the Zigzag and SandyRivers, a disrancein ercesso{90 krn. Thesejahars and associatedfluvial depositshelped forrn flat-floored valleys near the confluenceof these rivers and cr€ated a deha at the mouth of the Sandy at the Colunbia River. The flons probably atlained depthsofg to 12 m above modern river level for nost of their passage.Pyroclastic flows of this age traveled at least 13 km frorn the rent a.€a at Crat€r Rock along the Zigzag or Litle Zigzag Rirers. Between400 and 600 yearsBP, ihe Zigzag erupriveperiod produceddeposirs along the middle reachesof the Zigzag River and the upper Sandy River. Bouldert labars and flurial depositscreated a terrace B to I0 n above present river lerel along the Zigzag and veneereda Timberlin€-aget€rrace on the Sandy River. The Old Maid eruptive period occured b€tw€€n lB0 and 2?0 years BP. A single lahar flowed down the Sandy River at least as {ar as Brightwood,30 }n from Crarer Rock. Sand d€posilsthat may b€ related to this erent are found down to the nouth of the SandyRiver. A single lahar also flowed at least l8 km down th€ Zigzag River. The lahars,which were probably about 9 n deep,buried maiure cedar forestsalong both rivers. lntroduction to a discussionof the depositsin the Sandyand Zigzag drainagehasins (Figure l). Thesebasins Since the end of the Fraser alpine glaciatiod werechosen due to the concentrationof popula- abour 10,000years ago, three significant lahar- tion and recent intensedevelopment, which producing eruptive periods have occurred at would exacerbatethe effectsof future eruptions. Mount Hood. The lahars were confined to river basinsoriginating on the southwestflank of the CraterRock mountain.The older, Timberline eruptiveperiod occuried 1400ro lB00 yearsBP (Crandell,1980), A compositedome known as Crater Rock lies at the Zigzag eruptive period 400 ro 600 yearsBP, the apex of the smooth debris fan ruhich gives and the more recent Old Maid eruptive period the southwestside of Mount Hood its distinctive occurredlB0 ro 270 yearsBP (Crandell,1980). profile (Figure 2). This massof amphibole-rich Depositsfrom the Timberlineeruptive period (in- phyric dacite sits atop the modern vent, and its formally termed Timberline-agein this report) extrusion was the probable causative force can be tracedthe length of the Zigzagand Sandy behindlahar generation. Rivers to the Columbia River. Flows from the Field studiesand air-photointerpretation Zigzag eruptive period (informally termed showthat the Crater Rock dome is composedof Zigzag-agein this report)have been identified at leastthree lobes(Figure 3). This is basedupon only in the niddle Zigzag basin and in the up- degreeof alteralion.lexture. and fabric conlrasts per Sandybasin at Old Maids Flat. Depositsfrom betweendifferent areasof the dome.The dacite (informally the Old Maid eruptive period termed of LobeI hasan oxidized,brick red or pink col- 0ld Maid-agein this report)have been identified ored matrix, but phenocrystsof plagioclaseand from the upper Sandy basin downstreamto 2.5 amphiboleare unaltered.Localized areas of high km belov the confluenceof the Sandy and Sal- fumarolic activity havecreated patches of white mon Riversnear the hamletof Brightwoodand and yellowalteration and encrustation.Spires of at a single locality below the confluenceof the whitish,pumiceous rock rise 5 to l0 m abovethe Zigzag ard Little Zigzag Rivers. surfaceof the domeon the north-northwestside Although the lahars from the three eruptive of the lobe,The generalsurface is very irregular periods affected all drainageson the soulhwest and angularwith local relief of over I m. The flank of Mount Hood,this paperwill be limired lobe showsno obviousfabric. NorthweslScience. Vol. 60, \o. 4, lq86 225 T J 1: Figure l. Generallocation nap for the Sandy and Zigzag River basinr. .:{& Figure 2. South*est flank o{ Mount Hood shoving the smooth surfac€ of the main debris fan forin€d during rhe Tinb€rlin€ eruptive p€riod, wifi Crater Rock at its apex.Mount Adans, Washington,is in the background.Photograph by Ellen Cameron. 226 Cameronand Pringle SLrm mrt .RATERAREAMAp .€>S)t "I K\<N'"T" /=_N;:::,.2tr:_>-7 ' -- - (./----,r ).. /? t:':t{'}-ta' €<r/Il-41 .*/^*$oWi/.o)). "<.\ _.w ^o' r"'-\y/ ;ii *.___j ,N\\J ;t --)' l,\))"n' ,' t1,,:.1 ' 4a'21 , ,/) Figure 3 Map vi€{ of C.ater Rock and su.rounding area shosing location of the rhree lobes.Dots representfunrarotic areas. "X" indicales spines on the dome. Dashed lin€s indicate inferred linirs ot rhe breadcrusr-bonb teohra Dlun€. Lobe 2 has an unaltered grey matrix and a Two main regionsof fumarolic activity flank much smoothersurface texture with Iocal relief Crater Rock and bound CoalmanGlacier (Figure of underI m. It possessesa general fabric in the 3). Temperaturesof the fumaroles(B4oC to 90oC) form of platyjointing that dipsto the southwest are at or belowlocal boiling point. Hydrother- (downslope) and is the result of flor.vwithin the mal alteration has reducedthe surrounding lobe during extrusion.It is separatedlocally from fragmentaldebris to sticky yellow,red and blue- Lobe I by deepcrevasses eroded along the con- grey clay.Both fumarolicareas remain snow-free tact. No current fumarolic activity was seenon throughout the winter despite their elevation Lobe 2. (3200m) and preventCoalman Glacier frorn join- ig Zigzagand White River Glaciers. Lobe 3 appearsto be the youngestarea of the dome.Whether it is a true lobe with exten- TimberlineEruptive Period sivetalus coveror a depositof angularclastic debriscould not be deterrnineddue to snowcover Timberline-agelahars are the mostvoluminous and lack of penetratingexposures. Nevertheless, of the postglacial lahar sequences.They form Lobe3. as it l'ill be calledfor convenience,is the bulk of the valley-fill depositsin the Sandy an areaof grey,vesicular, unaltered, angular rub- and Zigzagbasins and the broad fan that extends ble.Individual clasts average 0.5 m in diameter, from CraterRock to the baseof the nourrarn. but a few iange up to 2 m and all sizescommonly Pyroclasticflow depositsof this age have been showthermally related prismatic fracture pat- found as far as Barlow Campground on the terns.No current fumarolicactivity wasobserved ZlgzagRiver (El2 NEI/4 Secl5 T3S RBE),l3 on this lobe. km from the vent area, and lahar and hyper- Post-ClacialLahars of the SandyRiver Basin,Mount Hood, 0reqon 227 concentratedrunout depositsas far as the mouth abruptly to bouldery lahars and flood deposits of the SandyRiver at the townof Troutdale,S0 of the upper half of the sequence.Here clasts km from the source area. make up 40 to 70 percent of the flow units. Averageclast size is 20 to 50 cm in diameterwith Ageand Orlgin rare bouldersto 2 m. Individualflows are I to 1.5m thick. Lensesof beddedfluvial sandand entrained Radiocarbondating, of organic debris gravel20 to 50 cm rhick and I to 4 m long are in the lahars,dates the Timberlineflows at 1440 occasionallyfound in the upper half. Weather- (Table + 155 to l?80 t 200 years BP l). ing of the top unit extendsdownrvard for at least in Depositsof Timberline age are found all 40 cm at all sitesmeasured on the fan, wilh 40 drainageson the southwestside of the mountain to 70 cm of weatheringtypical for depositsof this (upper from the SalmonRiver reachesonly) west age in the Sandy River basin (Crandell, 1980). to the Sandy River. The exhusionand mass wasringof Crater Rock producedthe pyroclastic The timberline-agedeposits of the debrisfan flowsand lahars.Prismatically jointed boulders areveneered by sand-and gravel-sizedangular arecommon throughout the lower75 percentof particlesof grey,phyric dacite. This is inferred the sequencebut rare in the upper25 percent, to be air-fall materialfrom smallevents reported in mid l800's (Folsom, suggestingthat lahar generationaccompanied to have occurred the I dome building. 1970).This depositis rarelyover cm thick and lies directly on the ground surface. Areal D stributionand General Stratigraphy Belowthe main debrisfan and aboveOld Maids Flat the SandyRiver flows through a nar- The main debrisfan (Figure4) is boundedon the row gorge cut through the Pliocene volcanic Illumination Ridge, on the eastby the noth by rocks of the SandyGlacier volcano (Wise, 1969) lateral moraine of White River Glacier,on right and the Pleistoceneand Holocenedeposits of the Multorpor Mountain and on the the south by Mount Hood cone.The high gradientand nar- southwestby Laurel Hill. The actualthickness row nature of the gorgeprecludes the accumula- exposuresof over of the depositsis unknown,but tion of more than a traceof deposits. l5 m occur along Highway26 at Laurel Hill (SEl/4 SWI/4 Sec 14 T3S R8E). Exposures At the mouth of the canyon,a wideningvalley measuredfrom aerialphotographs at the head- and decreasinggradient have allowed the forma- cuts of the Sandy River and Rushing Water tion of a debris fan at Old Maids Flat which is (Figure Creekare in excessof 100rn in height.These
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