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Gl-OLOG'< ,.. WA H ~·GTO~r,-98604 OLYMP!A. :::, t\l

\IASHINGTON STATE DEPART"lPff OF M/l.TURAL RESOURCES DIVISIOr! OF GEOLOGY A:rn EAP.TH RESOUP.CES

Ceoloric mapping o1 the We~atchee area

by

Randall Gresens

1J7J

Open File ~eport ?C-6

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LEGEND

Geology by Randell Gresens, 1975

Quaternary-Recent, undifferentiated.

Quaternary-Recent, landslide. Block pattern on erosional cut through old landslide.

Quaternary, terrace deposit. Predominantly thinly laminated, unconsol­idated clay and silt. Bedding commonly is \ -2 inches thick. Occassional fine crossbedding is confined within thin laminae. Ripple-marking may occur between laminae, but more commonly bedding planes are remarkably f~at and smoot~, often accentuated by concordant mica flakes. The clay­silt sequence is clearly a lake deposit .

. The upper 20 feet or so of the terrace contains cl ays and silts with less regular beddin9. Sandy layers, with or without crossbedding, may be present. Sand, silt, or cl ay layers may be stained with li monite.

Pebbly, poorly sorted, unstratified all uvium may form the upper terrace near the borders of the terrace with bedrock, and it may ma ke up most of the exposed terrace in the extreme up -canyon extensions of the terrace. Cobbles and pebbles are generally plutonic and metamorphic rocks foreign to the immediate area .

Tertiary, Columbia River Basalt. Probably Yakima Basalt.

~nconformity~

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Tertiary, zones of hydrothermal alteration. Si li cification and/or hematization of predominantly sedimentary rocks.

Tertiary, intrusive roc ks. Predominantly andesite, but ranging from diabase to rhyolite.

Tertiary, Wenatchee Formation.

Twp, Pitcher Member . White arkosic pebble conglomerate. Contains clasts of (1) silicic volcanic rocks ranging from porphyritic andes ite to rhyolite (flow-banded rhyolite and pumice occur local ly); (2) vein quartz and black chert; and (3) metamorphic rocks, including fine­grained metasiltstone(?), quartzite. Poorly sorted; poorly cemented.

Twdg, Dry Gulch Member . Li ght gray to light brown, yellow-weathering, thinly laminated, s l ightly arkosic, fine-grai ned sandstone to siltstone. Minor crossbedding occurs locally. Top and bottom usua lly gradational into a 5 to 20 feet thick, reddish brown, fissi le shale.

Sma ll percentages of dark, fine-grained lithic fragments generally are present in sandstone. Muscovite flakes may occur in sandstone, but not in the associated shale. Small blebs of limonite are common in the sandstone, and hematite concretions, often with white interiors, may occur locally. The sandstone forms bo l d outcrops.

Leaf fossils are present locally. The unit is probably la ke sed.iment.

Twsu, Squi 1 chuck Member, upper sandstone. Predomi nantly a grayish­white, coarse quartzose sandstone. Locally it is a quartz pebble conglomerate. Muscovite is almost always present is small amounts; lithic fragments are rare or lacking. Locally it may resemble the finer-grained sandstones of Twsl. Corm,only includes interbeds of blue-gray shale or siltstone similar to Twsl. The upper sandstone unit may be absent from the Squilchuck Member.

Twsl, Squilchuck Member, lowe~ ~~al e . Predominantly a bl ue-gray shale to s iltstone. Muscovite flakes almost always present in amounts up to two percent. Locally oxidized to a red color , parall el to bedding, giving ri se to variegated shlaes. Buff-co lored, quartzose sandstone interbeds , 1 to 10 feet thick , are common. Sandstone and quartz pebbl e conglomerate resembling Twsu may occur as channel filli ngs i n shale, and shale chips are coITTT10n in sandstone interbeds. Sandstone interbeds corrrnonly exhibit fluviatile crossbedding. The interbeds form lenses that vary greatly in abundance in thickness. At Dry Gulch, thick sandstone interl ayers resembling Twsu are mined for quartz sand. The shale locally appears to have a tuffaceous component, and white volcanic ash layers up to six inches thick occur as interbeds.

At the contact with the overlying Twsu, there usually is a prominant 3 to 5 feet thick zone of strongly oxidized shale, giv ing rise to a dis­tinct deep red co lor . Where sandstone interbeds are abundant in Twsl (as at Dry Gulch), the oxidized zone may be a guide to choosing the contact be tween Twsu and Twsl.

Twsb, Squi l chuck Member, basal sandstone. A buff-colored, medium­grained, quartzose sandstone, s imilar to the interbeds in Twsl, that corm,only is at the base of the Wenatchee Formation. May have a thin basal conglomerate , with clasts rangi ng f rom Swauk Formation to s ilicic vol cani c rocks derived from the Camas Formation. Us ually has impress ive fluvi atil e crossbedding. Varies greatly in thickness, from absent to 80 feet thick.

Generally, the units of the Wenatchee Formation do not co ntain biotite or calcite. However, either may be present in Twsb, especi ally in the basal conglomerate .

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Tertiary, Camas Formation. Generally a grayis h-white to yellowish­white, biotite-bearing, cal cite-cemented, medium- grained, arkosic sandstone . Thin pebble conglomerate layers contain clasts of predomin­antly quartz and s ili cic vol cani c rocks ranging from porphyri t i c andesite to rhyolite; occasional clasts of chert, metamorphic rocks, or mudsto ne( ?) may be present. Clasts of plutonic granite and/or granitic gneiss may locally be abundant, but appear to be mo re weathered t han other types of clasts . Coarse conglomerate beds, with cl asts of quartz and si l icic vol cani c rocks up to 6 inches in diameter, are knovm.

Sandstone occurs in massi ve 20 to 60 feet thick beds in which fluviatile crossbedding i s common . Dark brown, poo rly consol idated shal es in 1 to 30 feet t hi ck beds do not crop out well . Loca l ly the sandstone may be coarse-grained and quartzose ,. resembling sandsto ne in the Wenat chee Formation . The presence of abundant biotite and/ or occasional cl asts of s ilici c vol canic rocks usually serves to distinguis h quartzose Camas sandstone fro m sandstone of the Wenatchee Format ion, but t hese criteria may be l acking. Quartzose sandstone may become t he dominant rock type covering several square miles .

The s tippl ed pattern indi cates a zone of deep res idual so il developed on the weathered fo r mation, but co ntai ning recogni zabl e reli ct bedding .

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s Tertiary, Swauk Formation. Generally a li ght- to dark-gray, cal cite­cemented, medium- to coarse-grained, arkosic sandstone . Massive conglomerate beds, wi th cl as t s up to 3 feet in diameter are common. The cl asts are almost exclus i vely plutoni c granite (sorre pegmatit i c) , grani te gnei ss , and quartz . Some dar k, fi ne-g rained metamorphi c rocks and quartzite cl asts may occur. Vol cani c cl asts are absent.

The beds typi cally range from 2 to 6 feet t hi ck, and sometimes exhi bit a crude graded bedding. Cro ss bedding i s rare and i s restri ct ed to small cross beds in minor si ltstones that may occur at t he tops of graded beds . Individual beds are us ual ly poorly sorted . Fragmental, brecci a- like un its containing weathered, angul ar rock f ragment s in a matr i x of hi gh clay content are probably mud-flows .

The rock i s toug h and well -indurated i n fres h out crop , although it generall y weather s readily and do es not crop out well. Cal cite vei ni ng i s ubiquitous .

The stippl ed pattern i nd icates a so il profi l e devel oped on t he formatio n under t he unco nformity with the Wenatchee Format ion.

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