Pleistocene Glaciation in the Southern Part of the North ...

Pleistocene Glaciation in the Southern Part of the North ...

Pleistocene Glaciation in the Southern Part of the North Cascade Range, Washington By, Stephen C. Porter Presented by Erin Eakins ESS 433 Autumn, 2007 Previous Studies Earliest references were from studies by pioneers in900, 1 1904,1906,19141916 Threeice sheets in the IcicleCreek Drainage Basinwereconsideredthe only glacial sequences in the area Crandell traced alpine glaciers along the rangeand correlated themwith the Puget Lobe in the adjacent lowland. Stratigraphic Usage

Sediments resulting from an ice advance were collectively mapped as drift including till, erratics outwash, lacustrine sediments, and ice-contact stratified drift. Glacial-stratigraphic units have been used to designate the principle glacial and nonglacial episodes. The bulk of preserved interglacial deposits are constituted of loess, colluvium, alluvium, and landslide sediments. Criteria Of Age 1)Those that are time dependent and permit relative ages to be assigned on the basis of a progressive change in degree of weathering, erosion, mass wasting, or extent of vegetation cover with increasing age of deposits 2) Those that are essentially independent of time and are based largely on spatial relationships or physical characteristics of the drifts Weathering Rinds

Mean thicknesses of weatheringrinds developedon clastsof Teanaway Basalt were measured for each drift sheet. Although the rangesin rind thicknessfor the Thorp, Kittitas, and LakedaleDriftsare mutually exclusive, valuesfor the tow members of the ttitas Ki Drift overlap, as dothoseof the threelower members of the La kedale Drift. Loess Thickness Maximum thickness of loess on Kittitas and Lakedale Drifts adjacent to the Yakima River and the lower reaches of its principle tributaries appears to vary systematically with the

age of the underlying drift. The mean measurements at sites where loess is regarded as relatively thick ranges from 3.5 m on moraines and terraces of Swauk Prairie age to only 0.7 m on those of the Domerie. Soils Soil profiles on each major drift sheet were sampled at 10 cm intervals. Subsequent laboratory determinations included grain-size analyses, pH, percentage of magnetic minerals, and moist color. Grain-size was clay-silt, pH tends to be slightly acidic (5-7), there were not significant percentages of magnetic

minerals, the younger soils tend to be yellowishbrown and the older ones a reddish color. Moraine and Terrace Relationships Both the relative spacing of moraines and their relative distance from cirques provide a crude basis for comparison of moraine succession in adjacent valleys, but direct correlations between valleys are possible by directly tracing outwash terraces that border the Yakima River and its principle tributaries upstream to moraines or groups of moraines belonging to each of the three major drift sheets.

Upper Yakima River Drainage Basin *Possible Pre-Thorp Drift Thorp Drift Kittitas Drift 1) Swauk Prairie Member 2) Indian John Member Post-Kittitas Soil Lakedale Drift 1) Bullfrog Member 2) Ronald Member 3) Domerie Member 4) Hyak Member 5) Lakedale Loess Holocene Sediments Post-Lakedale Soil Possible Pre-Thorp Drift Although neither glacial sediments nor glacial landforms of unequivocalpreThorpage werefoundduringthis study, some eviden ce pointsto sucha possible d

early ice dvance. avance.Mean thicknessof weathering rinds on Teanaway Basalt erratics at four siteshave the combined mean of2.78. Thesesamplesapparently represent a populatio n distinctfrom those obtained fro m the ThorpDrift (which has a mean of 1.96).Thesehigh rind valuesfoundalong the perimeterof the entire ty of theThorp Driftcould mean thatan olderdrift was deposited below it.Thesewouldhave been derived from pre-Thorp glaciersthat were probably shorter thanthoseof subsequ ent advances,for neithertill nor terracesof pre-Thorp agewerefoundat or beyond the Tho

rp and SwaukPrairie terminal m oraines. Thorp Drift The oldest unequivocal glacial sediments in the upper Yakima River drainagebasincrop out nearThorp and provideevidenceof an ancientadvance of valleyglaciersto a point at least65 km east of the present Cascade divide. In areas no t subjectedto later glaciation, the original morphologyof the drift has been str onglymodifiedby erosion and mass wasting, so that many of the relat ive-age criteria employed to differentiateand subdivideyoungerdrift sheets could not bepplied a to Thorp

deposits. x Ecept in and beyond the terminal zone, where subdued oraines m and adissectedoutwash train are present, the extent of Thorp ice is inferredlargely from er ratic stones. Kittitas Drift Large valley glaciers deposited this drift near the upper end of the KittitasValley. Weatheringvaluesfall betweenthoseof Thorp Driftand Lakedale Drift and indicate iKttitas ice advancesoccurred during a e sparate and distinctglacial age. Although eathering w rinds themsel ves do not permit subdivisionof the drift sheet, landfo

rm associations clearly demons trate at leasttwo major terminal fluctuations f the o mainYakima valleyglaciergivingus the SwaukPrairie and Indian John mem bers. Swauk Prairie Member Kittitas till extends downvalleyas far as Swauk Prairie and Thorp Prairie, which are bro ad beltsof moraines, marking terminal posit ions of two lobes of theYakima valley glacier. The morainecomplexat Swauk Prairieis composedof sixarcuate ridges of sto ny till irregularlymantled with loess. Most tillasts cl are

rounded pebbles and cobbleswhich were probably strea m gravels reworked by theadvancingglacier. Indian John Member Evidence for a significant advanceof the Yakima valleyglacier followingits recession from the oraines m at Swauk Prairie is foundat Indian John Hill. Although Indian John drift conceivablyrepresentsa separate glacialage, its weathering characteristicsare nearly indistinguishable from thoseof the Swauk Prairie Member, suggesting thatit probably formed duringa major readvanceafter theSwauk Prairieinterval but during the same

glaciation. Post-Kittitas Soil The post-Kittitas soil is the most distinctive soil-stratigraphic unit in the upperYakima River valley and, where recognized, constit utes the bestsingle criterionfor separating the Kittitas and Lakedaledrift sheets.The interval during which the po st-Kittitas soil developedapparently was marked by cessatio n of loess deposition in relati vely stable slope condition s. No sediments datingto thisinterval comparesclosely in characterwith the presentinterglacial which, under nat ural conditions,is a time of dominan

t slopestability, soil formation, andmoderate stream degradation. Lakedale Drift Valley floors throughout much of the higher parts of th Yakima River drainage basin are mantledith w comparatively un-weatheredand little-eroded drifthat t retains much of its ginal ori constructional mo rphology. The Lakedaledrift is dividedin to the Bullfrog Memb er, the RonaldMember, the Domerie Memb er,the Hyak Member, and the L akedaleLoess.Although weathering rinds and other relative-age

criteria canbe usedto distinguish subunits of the La kedale drift, the primary basis for subdivisonwas the outwash terraces that are traceableto morainesin eachof the threemajor tributaryvalleysnearthe headof the Yakima River. Bullfrog Member The outermost Lakedale moraines form two arcs across the main valley, delimiting theformercoalescenttermini of two ice lobes.Each moraineis compound an d consistsof two distinct crests; twotills,separated by a layer of coarsegravel,are exposedin the inner of the two terminalloops of the Yakima valleyice lobe,implying two distinct advances.The basal sedimentsare predominantly pebble gravel with lenses

of sand, an d theydisplay cut-and-fill stratification throughout. They grade up-section intooarser c cobble-boulder gravel. The upward coarseningof the gravel bodyis similarto that of the Indian John te rrace; it is believed to reflect progressi ve aggradation along the valleyfloor duringadvanceof the glacier. Ronald Member A prominent moraine loop about 1 km northwest of the Bullfrog member, the Ronald membermarks the positionof the Cle Elumvalleyglacier duringa subsequ ent haltor readvance.A high outwash terrace connectsthe two Ronald moraines and can be traced

through the Bullfrog moraines,beyond long which se veral lon g terraceremnants flank theYakima River. The Outwash consists largely of rounded cobble pebblegravel and is mantledwith about 1 m of loess. The oraines m have laminated lay c in theinteriorthat is deformedinto a seriesof faulted overturnedfolds followed by deltaic sands thatgrade upward into pe bble gravel. The stratigraphic sequence indicates tha t proglacial lake sediments were deformedby a readvanceof the glacier following itstreat re from the

Ronald moraine. Domerie Member Prominent moraine systems surround the lower end of Kachess Lake, Swamp Lake, and KeechelusLake, indicating substantial retrea t of the Yakimavalley glacier afterthe LateRonaldadvance. Each ofthe four main glacierterminal positions is mar ked by a moraine complex composed as of many as six differentridges.The innermost of these morainesis composedof silty, stony till, in places consisting largely offormed de lake sediments.The Domerievalley trains occuras prominent terraces below each ofthe major moraine comple xes and can be tracedat least as far as the gorge southof Lookout

Mountain.Reconstruc ted profiles indicatedhat t thetwo higherterraces are paired;therefore, eachpresumably represents a constructionalface sur gradedon a moraine, andit records an interval of readvanceor terminal Hyak Member The Hyak member represents a late stillstand or readvance of glaciers in the higher parts of the CascadeRange. Moraines occur throu gh a distanceof 3.2 km betweenyak H and Snoqualmie Pass; at theirdownvalley end, they grade into alow outwash terrace that slopes southeast to het shoreof KachessLake. The morainecomplex

consists of a series of ridges arated sep by swalesand closeddepressio ns, many containing smallogs b or pondsand consist largely of crudely tratified s flowtill, e venlylaminated lacustrine sediments, and fluvial sand and gravel. This assemblage of sediments and morphology suggests that debrismantled te rminal ice fothe Hyak glacier became stagna nt and meltedout differential ly. By contrast, the innermost the morainecomplexare sharp-crested, arcuateoops l

that cross the valleyat Lakedale Loess Like Kittitas loess, Lakedale loess is coarse textured with 50% sand, 30% silt, and 20% lay c by weight.Loessis finergrainedsouthwestof the Yakima River and at increasingdistances northeast of the valleycenter, implying thatthe dominantwindsduringthe loess deposition were southwester ly. Loess related to theakedale L ice advancesmantlesmuchof the landscape do wnvalleyfrom the Domerie end morainesand is especially prominent between Swauk Prairieand

Easton.Throughthis reach,it covers outwash terraces of Lakedaleage and formsa discontinuous mantleof variable thickness on en mo rainesand adjacent hillslopes. Down valleyit commonly overliesKittitas loessand thinly mantles Thorp outwash. Holocene Sediments Postglacial alluvium is confined largely to modern flo odplains which range from only afew meters to as much as 2 km as in the case of the Yakima River. The alluviumof even major streams is probably this and consists mostly of poi nt-bar and channel

gravels derived by reworking sediments, andverbank o siltsand lacustrine ays. cl Other majorbodies of alluvium occur asluvial al fans betweenKachessLake and Nelson. Post-Lakedale Soils Soils formed on Lakedale Drift are weakly de velopedand contrast markedlywith post-Kittitassoil. Weatheringprofilesdevelopedin Lakedaleloesson outwashterraces tend tobe deeperwith increasing age of te rraces. Becauseeolian sedimentation persist ed on all terracesuntilthe termination of the

Domerie d avance, the apparent deeper weathering may indicate progressi ve accretionof eolian sediment while soil was forming. Drift of the Puget Lobe Sublobes of the glacier pushed into the lower ends of the we st-draining Cascadevalleys, block ing streams and depositing drift. The Snoqualmie Embayment,a reentrant in the Cascades formed where the valleys of the South and iddle M Forks of the Snoqualmie Ri ver and theCedar River meetthe PugetLowland,

contains anarray of landformsand sedimentsformedduringat least two such incur sions of northernice. Pre-Vashon Drift Excavations in 1972-73 along I-90 near the bottomof the massive South Fork morainalembankmentexposed evenlylaminatedlake sedimentsat an altitudeof 300 m. The sedim ents contain pe aty wood fragmentswith a radiocarbon ageof 50,000 yr; they therefore antedate Vashon Drift in the adjacentlowland. The sediments very likely date to pre-Fraserponding in the Snoqualmie Emb ayment associated iw th an advanceof the Puget Lobe into the southernPuget

Lowland. Vashon Drift The position of the glacier margin during the maximum dvance a of the FraserGlaciationin the Snoqualmie Embaymentis marked by a high, massive deltamoraine complexthat crosses the lower end s of the principlevalleys.The top of the drift body in each valleystands at about 500 m, but it descends gradually both eastand south. The bulkof the exposed sedi ment consistsof coarse boulder andobble c gravel along the

proximalfaceof the morainal embankment. Chronology and Regional Correlations If the provisional moraine correlations across the range crest are valid,then the Domerie Member dates approximatelyto the timeof the Vashon Drift deposition. Thetratigraphic s relationships suggest that histis a c lose minimum age for Hyak till and that the Hyak advanceprobably culminationno more than se veral centuriesearlier. This means the y Hak Member may correlate with Sumas Drift in the northern Puget Lowland. McNeely Drift Mount at

Rainier, whichis overlain by a tephra layer more than8,750 year old, may also correlate ith w SumasDrift and represent an dvance a of glaciersin the southern Cascadesat thwas approximately contemp oraneouswith the Hyak advanceat SnoqualmiePass. If the inferredcorrelation of the Domerie Member with Vashon Drift is correct,then the Bullfrog and Ronald

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