MAPPING OF MAJOR LATEST PLEISTOCENE TO HOLOCENE ERUPTIVE EPISODES FROM THE GLACIER PEAK VOLCANO, WASHINGTON—A RECORD OF LAHARIC INUNDATION OF THE PUGET LOWLANDS FROM DARRINGTON TO THE PUGET SOUND
123° 122° 121° 49° Benjamin W. Stanton and Joe D. Dragovich, Washington Department of Natural Resouces, Division of Geology and Earth Resources, 1111 Washington St SE, Olympia, WA 98504-7007, [email protected] and [email protected] map area Bellingham SAN WHA TCOM 20 JUAN Volcanic hazards in the Pacific Northwest are typically associated with more visible stratovolcanoes, such as Mount Rainier. Glacier Peak, a North Cascade dacitic stratovolcano near Darrington, Washington, has produced at least three large eruptive episodes See Figure 2 14 path of lahars since the culmination of the last continental glaciation with each episode likely lasting a few hundred years. Data from recent detailed geologic mapping, new C ages, stratigraphic relations, clast petrographic analyses, geochemical analyses, and laharic SKAGIT to lower Skagit Valley ISLAND sand composition indicate that three large eruptive episodes occurred in the latest Pleistocene and Holocene—the information refines and expands on the pioneering work of Beget (1981). These voluminous events traveled up to 135 km downvalley of the path of lahars to lower Stillaguamish Valley Glacier Peak edifice and reached the Puget Sound via the ancient Skagit and Stillaguamish deltas. Glacier Peak has erupted dacite of similar composition throughout its history. Hypersthene-hornblende-(augite)-phyric vesicular lahar clasts found from Glacier Peak to La volcano 48° SNOHMISH Conner, Washington have similar geochemistry to dacite flows sampled on the volcano. Radiocarbon dating of the White Chuck assemblage shows an eruptive episode of ~11,900 yrs B.P. This implies ice occupation during part of the White Chuck event and is 5 further supported by mapped deposits of the White Chuck assemblage that directly overlie recessional outwash along high inset terraces. Charcoal obtained from the Kennedy Creek assemblage yielded 14C ages of ~5,000 yrs B.P. Near La Conner, Washington, 14 KITSAP Seattle KING the Kennedy Creek lahar runout is locally interbedded with mid-Holocene estuarine and deltaic sediments of the Skagit delta. The Chocolate Creek assemblage yielded many C ages of ~1,800 yrs B.P. This assemblage occurs as the lowest and most inset lahar terrace near Darrington. Log-bearing flood deposits near Padilla Bay are 120 km from the edifice and contain dacitic sands and silts. This may be the most distal deposit of the Chocolate Creek complex. Many cities downvalley of Glacier Peak are built on Figure 1. Location of the study area as shown by the gray-shaded area (Figure 2). The arrows depict the river terraces comprised of lahar, lahar runout (hyperconcentrated flood deposits) and volcanic alluvium. Although Glacier Peak is generally not visible from urban areas, it is an active volcano that periodically erupts in an explosive manner. This could affect paths of Glacier Peak lahars and lahar runout that Figure 18. Glacier Peak edifice as viewed aerially from the north. inundated the Skagit and Stillaguamish River valleys. several downstream communities and disrupt Interstate 5. The dashed lines are county borders. Description of Map Units
122°30´ 122°00´ Figure 6. Locations of radiocarbon age date sample sites. Qgv QUATERNARY SEDIMENTARY RX RX 122°30´ Qgoe Qvsk 122°00´ DESCRIPTION OF MAP UNITS Qaf AND VOLCANIC DEPOSITS For specific latitude and longitude coordinates, measured Qls RX Figure 2. This geologic map of the study area illustrates the extent of Samish Qa Rx QUATERNARY SEDIMENTARY Qgv Bay conventional radiocarbon and calibrated ages refer to Table 1. Qgoe Holocene Nonglacial Deposits AND VOLCANIC DEPOSITS Rx RX Mt Josephine Qgt Qvlc vi Qls Detailed sample descriptions are given in Table 2 (see pamphlet). exposed Glacier Peak lahar assemblages. The area bounded by the Rx Qa Rx Qgv Alluvium, undivided Holocene Nonglacial Deposits Qg Qg Refer to Figure 2 for geologic unit legend. Qa Concrete Qgv Qvl Qa rectangle represents Figure 3, which provides a more complete picture c Qa All laharic assemblages assumed in Qvl on this map. Samish Sk Qvl agit Qa River Qg Qvlc Padilla Sauk Mt Bay Alluvium, undivided v Qls 22 Sedro-Woolley Qg Qa Qgoe Rx 48°30´ Bay 25 48°30´ of the lahar assemblages in the Darrington vicinity. Qgv Qls er Qgle Qa Alluvial fan deposit Qg Riv Radiocarbon age sites; locations B´ 27 Skagit Burlington * Qa Approximate mid-Holocene Mt Josephine Qgv Qgv Qaf with “CS” followed by a number * 20 * * Qgtvi * * Iron Mt sea level and location of the Late Pleistocene and Holocene Glacier Qa 26 Qvl Qvl refer to critical age sites referenced Skagit delta front at about Qgoe 21 in stratigraphic columns. For all Peak Volcanic and Sedimentary Deposits Qgoe Qls Qvl *Mount Vernon 530 Qa 5,000 yrs B.P. Qgle Qa Landslide complexes radiocarbon ages refer to Table 1. Volcanic assemblages, undivided, Qls Qg 19 RX Qg Qls 14 RX Qa Rx Qls Qg RX including lahar and pyroclastic Qgv Qvl * Qvl 0 5 10 miles La Conner CS-3 Concrete deposits near Glacier Peak edifice Rx Qa * Mt Chaval Qvs (16.1 km) Rx Qgoe k 12 Qg 1:305,000 Qgoe Qvl Devil’s Mt 18° 106,667 (Pleistocene to Holocene) Skagit Delta 10 Rx Qa Older alluvium (cross sections only) 16 RX H * T * * H R Qvlk Geochemical sample sites; many Qgv Qoa CS-2 T O Rx Qvl R S N Qvl APPROXIMATE MEA N O c k
Dacite flows, domes, breccia, * N
a * C
u g u Skagit 15 DECLINATION, 2006 I u i * E T t Qg E R CS-1 U Bay N ive sites contain mutiple samples. Mt Higgins 28 R r and tephra deposits (Qvp) east T G Sauk Mt Qvd Qvl 13 Qg A Qvl Qls Late Pleistocene and Holocene Glacier CS-7 Qg 9 Qvl M k Qvlk Qg Qgtvi Padilla v Qvl iver * of the Suiattle River near Glacier c Qa No R Radiocarbon age sites; locations Peak Volcanic and Sedimentary Deposits rth Fk Qvl Qvs Qg Qa ish Green Mt Bay Qls Qgoe k Stilaguam Qa 32 * Sedro-Woolley Qa Qvlc Peak (Pleistocene to Holocene?) Qvl * Qg 30 48°30´ er Qvlc Chocolate Creek Assemblage (Holocene) * ** * iv 48°30´ with “CS” followed by a number Qgtvi *34 11* Qg Darrington t R Qa 23 agi Qa Qg * Qvl * * Sk Qa CS-6 Qgo refer to critical age sites referenced Chocolate Creek Assemblage e Non-cohesive lahar—Silty sandy Stillaguamish CS-5 RX Su Qvlc Rx Rx iattle River * Delta Qvl Qg c Qgv gravel to gravelly sand; few cobbles * (Holocene)—Non-cohesive lahar Qgle Qvs Qvlc in stratagraphic columns (1-7); see w Whitehorse Mt Whitechuck Mt 18 u * and rare boulders; compact and dacite- * CS-4 Qa * Qa Port Qvl Qvlc Figure 6 for locations of other rich; fine to coarse-ash matrix Susan Arlington 24 Burlington Qa Qvd u Kennedy Creek Assemblage Qg RX Qvl Qvd Qgv 33 * Iron Mt radiocarbon sites. * W Kennedy Creek Assemblage (Holocene) hite C * * hu Figure 19. Suiattle fill as viewed aerially from the southwest. ck Qvl u (Holocene)—Volcanic sediments, Rive Qvl Qvlk Qgv RX r Qvl S Ice k Qgoe Rx au Qls Qg k u and non-cohesive lahar; tentaively Volcanic sediments, undivided— R Qls Qgo Qvsw i City or town. e v 8 u Qvlc Qa e Glacier Qvs r * Hyperconcentrated flood deposits, S * correlates hyperconcentrated flood k 29 Peak o * Qgtvi u th * Qvl Mountain or Peak. B lahars, and volcanic alluvium; medium- F Qvd 17 deposits and volcanic alluvium in k S Rx tilla Ice Qvl RX guami k Qaf Qa to coarse-grained sand, gravelly sand, sh Rive the lower Skagit valley Qgtvi Marysville r RX Rx Qvtw RX and cobbly sandy gravel; loose; Pilchuck Mt Qg Mount Vernon Qaf Rx 31 Qls dacite-rich Qg Sloan Pk White Chuck Assemblage Qls RX * Qa RX Qg Vesper Pk Qvs Non-cohesive lahar—Silty sandy gravel 48°00´ 122°30´ Qg 122°00´ 121°30´ 121°00´ 48°00´ Qvlw (late Pleistocene)—Volcanic k Qa Qa Rx Qvlk to gravelly sand; rare cobbles and * Qvl sediments and non-cohesive lahar Qa u Darrington Vicinity Map Qa boulders; compact and dacite-rich; fine * Pleistocene Glacial Deposits Qgv Qls to coarse-ash matrix (See Figure 3) Qvsw Qaf Table 1. Radiocarbon age dates (below and right). La Conner u Qvtw Deposits of the Fraser glaciation White Chuck Assemblage (late Pleistocene) Rx Qaf Qvlk Mt Chaval SITE # CONVENTIONAL SITE # CONVENTIONAL Qg and older glacial and nonglacial Qls Volcanic sediments, undivided— SAMPLE # GEOLOGIC 14 SAMPLE # GEOLOGIC 14 SKAGIT CO ON LATITUDE LONGITUDE C AGE (BP) ON LATITUDE LONGITUDE C AGE (BP) u Qvlc deposits, undivided Qgoe Qvs (LAB #) UNIT (LAB #) UNIT SNOHOMISH CO w Hyperconcentrated flood deposits, lahars, MAP(S) [CALENDAR AGE] MAP(S) [CALENDAR AGE] Qvl u SKAGIT CO and volcanic alluvium; medium- to BT-1 03-1C-1 2,200 ±60 c Devil’s Mt SNOHOMISH CO 8 Qa 48°07'15"N 121°04'08"W ≤200 22 Qvlc 48°30'06"N 122°23'55"W CS-3 Qgv (W-4274) (178839) [264 ±81 calBC] TERTIARY INTRUSIVE AND MESOZOIC coarse-grained sand, sandy gravel, and BT-2 03-1C-4 2,260 ±40 * Qgoe Qa Qvl * Qa Qvlc 8 48°07'15"N 121°04'08"W ≤200 22 c 48°30'06"N 122°23'55"W Skagit Delta * Qls (W-4268) (178840) [306 ±67 calBC] u Qa cobbly sandy gravel; loose; dacite-rich TO PALEOZOIC LOW-GRADE Qvlw Rx u Rx Qgv Rx Qvs 01-73E-1 200 ±40 05-33J 2,260 ±60 k 9 Qa 48°17'58"N 121°33'09"W 22 Qvlc 48°30'06"N 122°23'55"W CS-2 (209512) [300 ±73 calBC] Whidbey * * METAMORPHIC ROCKS Qls Pumice—Terrace-capping pumiceous (161229) [1,736 ±70 calAD] * Qls 01-9L-1 250 ±70 02-85K 2,270 ±60 u Qg 10 Qa 48°22'14"N 122°33'19"W 23 Qaf 48°15'52"N 121°58'37"W Whidbey Formation Qaf v Qvtw ash, ash-lapilli, and lapilli deposit (160052) [1,666 ±127 calAD] (171371) [306 ±73 calBC] Qgv Qaf Island Qvl 530 01-9K-1 310 ±60 BT-9 2,820 ±60 c Qa forming a ‘pumice plain’ 10 Qa 48°22'16"N 122°33'19"W 24 Qvl 48°10'29"N 121°19'46"W Qvlw RX Bedrock, undivded Rx Qaf (160051) [1,568 ±67 calAD] (W-4266) [988 ±79 calBC] Skagit Qvl 01-49G 330 ±50 98-90A 3,820 ±70 u CS-1 Qgv 11 Qa 48°16'42"N 122°40'50"W 25 Qoa 48°29'36"N 122°05'49"W Qa (159615) [1,556 ±62 calAD] (125993) [2,277 ±112 calBC] Rx Qvs Non-cohesive lahar—Cobbly to Bay u Qls k Qvsk Mt Higgins Qvsk TP-1 425 ±75 98-90D 4,120 ±70 u Qaf Qvl Qa 26 Qoa 48°29'44"N 122°05'50"W Qvlw u * Qgoe w bouldery gravelly sand, commonly with 12 ~48°22'24"N ~122°28'15"W u Qa Rx (SK-33B) [1,517 ±81 calAD] (125994) [2,708 ±118 calBC] Qg a trace of ash; compact and dacite-rich 01-49F 580 ±60 DBD-44 4,790 ±80 v 13 Qa 48°16'55"N 122°40'00"W 27 Qvlk 48°30'00"N 122°5'00"W Qvlw Qvlk Qvlw Qaf CS-u Qvlw (159614) [1,360 ±43 calAD] (USGS-880) [3,537 ±106 calBC] Qvlc Qls Rx Qgl Qvlc e Pleistocene Glacial Deposits 99-105D-2 660 ±50 BT-10 5,020 ±100 14 Qoa 48°25'36"N 122°24'33"W 28 Qvlk 48°17'01"N 121°46'25"W * (142314) [1,336 ±42 calAD] (W-4279) [3,822 ±105 calBC] u Qgo Deposits of the Fraser Glaciation, Everson Interstade u u u e Qa TP-3 670 ±75 02-107H1 5,090 ±50 u Qvlk 15 Qoa ~48°20'19"N ~122°20'17"W CS-5 Qvlk 48°13'06"N 121°34'35"W u Qvl Qgoe r * Qvlc u w Green Mt Qls (SK-59) [1,328 ±52 calAD] (177000) [3,879 ±64 calBC] * ve u * u u u Suiattle Riv Qa i Qvlk * * Qvl e Qaf Qgl R k r Qvlk e Recessional outwash, undivded—Sand, h CS-6* Qvsk Qls 99-105D-1 710 ±70 BT-11 5,120 ±90 N is *Qvl 14 Qoa 48°25'36"N 122°24'33"W 29 Qvlk 48°16'15"N 121°11'33"W orth Fk aum k (136116) [1,306 ±62 calAD] (W-4275) [3,909 ±105 calBC] Stillag Qvlw u Qvl Rx Qgoe sandy gravel, gravelly sand, and sandy c Qgl Qgoe * Qvlk Qvl e TP-2 820 ±50 01-95A 5,190 ±70 Whidbey u Qls cobbly gravel with rare boulders 16 Qoa ~48°21'00"N ~122°22'57"W CS-6 Qvlk 48°16'41"N 121°46'22"W u Qgv (SK-37) [1,206 ±45 calAD] (164347) [4,013 ±107 calBC] u Qvlw Qvsk * Qgoe Qgv Formation u Darrington Qa BT-4 1,080 ±50 02-106C1 5,460 ±110 Qvlw Qg Qgoe 17 Qvd 48°07'15"N 121°04'10"W CS-5 Qvlk 48°13'06"N 121°34'35"W A´ v Recessional glaciolacustrine deposits— (W-4276) [950 ±47 calAD] (172586) [4,272 ±135 calBC] Qvsk Qvl Qgoe BT-3 1,100 ±50 BT-12 5,520 ±250 Figure 20. Lahar deposit from Critical Radiocarbon Site 2 (CS-2). Qls Qgl Silty clay, clay, silty sand, and sand with 18 Qvd 48°02'14"N 121°02'14"W 29 Qvlk 48°06'48"N 121°11'33"W Camano Qvlw u Qgv e (W-4614) [936 ±47 calAD] (W-4214) [4,357 ±279 calBC] Qgv CS-5 Rx local dropstones BT-5 1,130 ±70 02-25E-1 8,040 ±40 Island u Qa 18 Qvd 48°02'14"N 121°02'14"W 30 Qaf 48°17'10"N 121°49'36"W Qvlw u Qgoe Qg (W-4568) [889 ±80 calAD] (171170) [6,958 ±97 calBC] Stillaguamish Qvl Qvlk Qg v Rx v Qls BT-7 1,150 ±50 BT-13 8,380 ±90 Delta Rx Qgo 18 Qvd 48°02'14"N 121°02'14"W 31 Qg 48°03'05"N 121°09'50"W S e Deposits of the Fraser Glaciation, Vashon Stade (W-4613) [880 ±69 calAD] (W-4277) [7,420 ±100 calBC] * u 530 a BT-6 1,220 ±60 B-2-8 8,580 ±90 u k 18 Qvd 48°02'14"N 121°02'14"W 32 Qoa 48°15'44"N 121°35'33"W Qaf (177994) [7,647 ±87 calBC] R Qls Ice contact deposits—Sandy gravel, (W-4567) [802 ±75 calAD] Whitehorse Mt iv Qls TP-4 1,540 ±60 B-2-9 8,850 ±50 e Whitechuck Mt Qgt Qoa 32 Qoa 48°15'44"N 121°35'33"W Qgtvi vi gravelly sand, and bouldery cobbly 19 ~48°25'13"N ~122°26'31"W r Qvs Rx (SK-81) [517 ±66 calAD] (177846) [8,011 ±139 calBC] k Qa Qls Rx gravel, locally with interlayered beds of 02-39F 1,620 ±60 B-1-7 9,680 ±50 Qls CS-1 Qvlc 48°18'31"N 121°25'57"W 32 Qoa 48°15'43N 121°35'35W CS-4 silty sand or sand (171171) [440 ±74 calAD] (177993) [9,073 ±135 calBC] Port u 01-7E-1 1,620 ±70 DBD-42-B 10,350 ±175 A CS-2 Qvlc 48°20'45"N 121°32'44"W CS-7 Qvlw 48°17'00"N 122°01'30"W Susan Qvlw Arlington Qvl u Vashon glacial deposits, undivided— (158065) [435 ±84 calAD] (BETA 1714) [10,174 ±362 calBC] BT-14 11,420 ±150 Qg 01-55M-2 Qvl 1,700 ±70 Qvl Qvl v Till, advance outwash, and advance CS-3 c 48°21'26"N 121°30'34"W 33 w 48°10'41"N 121°27'36"W k Qvlc * u (159616) [335 ±78 calAD] (W-4616) [11,237 ±161 calBC] Qvlw * glaciolacustrine deposits 01-84F-1 1,780 ±60 B-2-15 11,600 ±60 Qvl * Figure 3. A simplified geologic map of the Darrington 7.5-minute quadrangle CS-4 Qvlc 48°10'25"N 121°20'05"W 32 w 48°15'44"N 121°35'33"W * W (161231) [247 ±83 calAD] (177995) [11,424 ±124 calBC] hite DBD-47-B 1,790 ±75 BT-15 11,670 ±160 C TERTIARY INTRUSIVE AND MESOZOIC 20 Qvl 48°29'30"N 121°17'00"W 34 Qvlw 48°16'15"N 121°45'18"W hu u (from Dragovich and others, 2002) located in Snohomish and Skagit Counties, (USGS-866) c [239 ±93 calAD] (W-4283) [11,506 ±192 calBC] ck Ri u u ver TO PALEOZOIC LOW-GRADE 01-78D-2 1,830 ±70 B-2-18 SAUK 11,940 ±40 CS-2 Qvl 48°20'45"N 121°32'44"W 32 Qvlw 48°15'44"N 121°35'33"W u Washington. Inset terraces are shown along the Sauk and Suiattle Rivers METAMORPHIC ROCKS (161230) c [200 ±92 calAD] (178088) [11,757 ±163 calBC] u BT-8 1,860 ±70 u 17 Qvlc 48°07'15" N 121°04'09"W Qvl portrayed by hachures on the downslope side. Rocks, undivided—Includes Eocene (W-4265) [158 ±77 calAD] Note: All samples collected by J. Dragovich except: BT-## (Beget, 1982), TP-# and (Thompson, 01-84F-4 1,980 ±60 1978), DBD samples collected by D. Dethier. Rx CS-4 Qvlc 48°10'25"N 121°20'05"W Qvl Cross sections A-A´ and B-B´ are shown in Figures 4 and 5 respectively. intrusions, Jurassic Darrington Phyllite, (161232) [15 ±64 calAD] Jurassic semischist of Mount Josephine, 99-107F Qvl 2,070 ±60 u ** 21 c 48°27'45"N 122°20'02"W u u Jurassic Shuksan Greenschist, and (137753) [89 ±75 calBC] * Glacier Peak u Permian to Devonian metasedimentary u B rocks of the Chilliwack group B' SSW NNE u City of Darrington 700 charcoal sample yielded 700 S radiocarbon age of Figure 7. Simple timeline showing all known Glacier Peak volcanic assemblages since the present. K= 1000 yr BP (Before Present) ou A A' 1700 ±70 yr B.P. th F 650 650 CCM = Crystal Creek Mudflow; WCA = White Chuck Assemblage; KCA = Kennedy Creek Assemblage; L= unnamed lahars of Crystal Creek; k Stillag SW NE
u kettle amish River 600 Qvlk 600 600 600 CCA = Chocolate Creek Assemblage; SGA = South Guardian Assemblage; and YL = young unnamed lahars of Chocolate Creek.
B7 Suiattle River
B8 Hwy 530
B6 B14
City Well City Well Darrington B16 2 charcoal samples yielded B3 B1
B2 Qvs Qvsk k 550 radiocarbon ages of Qvlc 550
Marysville 550 550 Qgle W257,271,271B (composite well) (composite 1620 ±70 yr B.P. and Sauk River Qa W280 road
W274-277B (composite well) (composite W279 500 1830 ±70 yr B.P., respectively. Qgtvi Qa 14,000 yr BP 12K 10K 8K 6K 4K 2K present 500 500 Qvsw 500 Pilchuck Mt Qvlk Qoa wood pumice Qoa logs Qa Qvlc 450 * 450 Qgle 450 450 e Qvlc Qgl Suiattle River
Elevation (feet) Qgtvi Qvsk Elevation (feet) Qgoe Suiattle River Qvlc Sloan Pk 400 400 400 Hwy 530 400 Sauk River (Hwy 530) Qgoe Qa Sauk Rive r Bridge bend in section
Elevation (feet) Elevation Rx Qoa Qa Qvsk 350 Qgtvi 350 350 Qgv 350 Qgv Qgv Qgoe Rx 300 300 300 Qgle 300 Figure 21. Lahar terrace composed of Chocolate Creek Assemblage above Suiattle River. Vesper Pk vertical exaggeration 50x vertical exaggeration 50x CCM WCA KCA L CCA SGA YL 48°00´ 122°30´ 122°00´ 121°30´ 121°00´ 48°00´ Figure 4. Cross Section A-A' was constructed using Snohomish Figure 5. Cross-section B- B' shows the relationship of inset terraces along the Sauk River and also County Public Works geotechnical data (collected in February, illustrates ice blockage during the eruption of the White Chuck Assemblage. 2003), thin section analysis of mounted sand samples from selected intervals in all three borings, and radiocarbon dates obtained from five samples. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. 122°30´ 122°00´ Figure 8. Sample site locations of dacite clasts or tephra deposits Figure 9. Geochemical data for dacite clasts and dacite flows from multiple geologic Figure 10. This graph compares the calendric ages of samples from the three largest volcanic assemblages RX RX Critical radiocarbon site 02-39F (CS-1) Critical radiocarbon sites 01-7E-1 and 01-78D-2 (CS-2) Critical radiocarbon site 01-55M (CS-3) Critical radiocarbon site 01-84F-1 and 01-84F-4 (CS-4) Critical radiocarbon sites 02-106C-1 and 02-107H-1 (CS-5) Critical radiocarbon site 01-95A (CS-6) Critical radiocarbon site DBD-42-B (CS-7) RX analyzed for glass compositions. For specific latitude and units of varying ages. Geologic units include clasts from various lahars, Glacier Peak known to have erupted from Glacier Peak in postglacial time. (NW ¼ of NE ¼ of Sec. 31, T. 33 N., R. 11 E.) (NE ¼ of SW ¼ of Sec.17, T. 33 N., R.10 E.) (SE ¼ of SW ¼ of Sec. 10, T. 33 N., R.10 E.) (NW ¼ of SW ¼ of Sec. 13, T. 31 N., R.11 E.) (SE ¼ of SE ¼ of Sec. 25, T. 32 N., R. 9 E.) (SE ¼ of NE ¼ of Sec. 9, T. 32 N., R. 8 E.) (SW ¼ of NW ¼ of Sec. 10, T. 32 N., R.7 E.) Samish Bay Thickness Elevation (m) longitude coordinates and weight percentages see Table 3 dacite clasts from glacial outwash, Glacier Peak dacite clasts from Holocene alluvium, Terrace top Thickness Elevation (m) Thickness Elevation (m) Thickness Elevation (m) RX Mt Josephine Thickness Elevation (m) Thickness Elevation (m) White Chuck Road Thickness Elevation (m) Terrace top Terrace top White Chuck Assemblage Kennedy Creek Assemblage Chocolate Creek Assemblage Terrace top 115.8 Terrace top 548.5 115.0 61.0 in pamphlet. For sample descriptions see Table 4 in pamphlet. as well as dacite flows from the Glacier Peak edifice. We include here Glacier Peak 225.5 158.2 183.0 Years calBC Years calBC Years calAD Lahar (Unit Qlvw): subangular, gray dacite Alluvium (Unit Qa): sand with wood, roots, Qg Qg in a fine sand to gravel matrix with white, Concrete Refer to Figure 2 for geologic unit legend. lahars from the ~110Ka Whidbey Formation. Data shows the consistency of Glacier 30cm and some charcoal Qa 1.5m gravel-sized pumice and subangular cobble- Qa 1,000 12,000 10,000 5,000 3,000 2,000 1 4,000 1,000 Sk All laharic assemblages subsumed into Qvl on this map. Peak volcanic products over time (~200 ka). This compositional consistency is also 0 1,000 Fill: gravelly sand Fill: silty sand with wood, organic material and sized lacustrine ripups; a charcoal sample Qvl agit River Lahar (Unit Qlvc): angular to subangular, a trace of gravel 6m Padilla Sauk Mt Alluvium (Unit Qa): silty clay, mottled brown 1m obtained in a pumice-rich diamicton likely Sedro-Woolley Qg reflected in the petrographic consistency of the dacite. Lahar (Unit Qlvc): angular, gray, gravel to red and gray gravel- to cobble-sized dacite 48°30´ Bay 48°30´ 40cm with roots and trace of cobbles cobble-sized dacite in a matrix of sand; associated with this terrace to the north had r in a matrix of sand 3.9m Qg Rive BT-12 BT-8 1.5m Lahar (Unit Qvl ): subangular gravel to a minimum radiocarbon age of 10,350 ±175 Skagit Glass probe locations; refer to BT-14 white pumice prevalent Forest duff; localized k Burlington Qa cobbles in a matrix of ashy fine to coarse yr B.P. 3 Paleosol: charcoal in a matrix of silt; a sample 1.5m L Table 3 for results (see pamphlet). B-2-18 BT-11 01-84F-1 5cm sand; consists mostly of vesicular red dacite Qa Iron Mt See of charcoal had a radiocarbon age of 1,620 ±60 yr B.P. Qvl Qvl The center of the square is the 20cm Lahar (Qvl ): angular, red and gray gravel- Glacial outwash (Unit Qgoe): cobbly B-2-15 02-106C-1 01-84F-4 Figure 14. Alluvium (Unit Qa): Silty clay, mottled brown with roots c Pumiceous silt to silty sand; probable Qvl approximate sample site location. to boulder-sized dacite in a matrix of sand gravelly sand to sandy gravel; some cross- Mount Vernon catostrophic flood deposit related to Qvlk 15cm 2.0m and silt (3m) bedding notable Qg Qg BT-15 02-107H-1 02-39F Alluvium (Unit Qa): Sandy gravel to sand RX Paleosol: charcoal in a matrix of orange silt Qg RX RX DBD-42B Qvl 0 5 10 miles * BT-10 01-55M-2 to silty fine sand; a sample of charcoal had a 6.25cm Paleosol: charcoal in a matrix of orange silt; 1m La Conner K radiocarbon age of 1,620 ±70 yr B.P.; Paleosol: peat; logs and sticks and brown Charcoal in sandy silt; a sample had a Glaciolacustrine (Unit Qgle): clay; finely Mt Chaval (16.1 km) 6cm 8m 1:305,000 another sample of charcoal yielded an age a sample of charcoal had a radiocarbon age peaty mats; a sample of peat had a radiocarbon age of 5,190 ±70 yr B.P. laminated with rare dropstones; gray Qg 01-95A 01-7E-1 of 1700 ± 70 yr B.P. Qvl Devil’s Mt of 1,830 ± 70 yr B.P. radiocarbon age of 1,780 ±60 yr B.P. Skagit Delta 75cm RX B DBD-44 01-78D-2 Older alluvium (Unit Qoa): silty sand; planar C 18° Lahar (Qvlc): subangular gravel to cobble- Paleosol or overbank organic silt: charcoal in Q bedding I Alluvium (Unit Qa): fine to coarse sand; sized white pumice in an ashy matrix; a matrix of sandy silt; a sample of charcoal had Skagit P H T Qg G Qvl H See discussion on page #. DBD-47B 1.0m becomes angular to subrounded, red and 0.3m R T Lahar (Unit Qvlc): Subangular gravel to cobbles a radiocarbon age of 5,460 ±110 yr B.P.; another O Bay Mt Higgins E APPROXIMATE MEA N R planar cross-bedded N
O 1.3m H gray, gravel to boulders of dacite in a fine F N Qvl Qg DECLINATION, 2006 C sample had a radiocarbon age of 5,090 ±50 yr B.P. Qg O I (1x boulder 0.5m) in a matrix of ashy fine to coarse
E * Qvl T 99-107F E M U to coarse sand and ash matrix; coarsening r N Age range of White Chuck Rive R Direct age date, center point represents sand; consists mostly of semivesicular red dacite; Older alluvium (Unit Qoa): cobbly sandy Till (Unit Qg): clayey, sitly, pebbly sand with No N T G Glacial outwash (Qgoe): bouldery to cobbly rth Fk Qvl A 1.3m Qa Qvl Green Mt M gravel to sandy gravel; planar cross-bedded; downwards 1.3m Qoa cobbles and boulders, gray-blue Qg Stilaguamish Assemblage; becomes lighter crude normal grading Older alluvium (Unit ): sandy to cobbly gravels with interbeds and lenses of sand; Qvl measured age; range represents ± error 03-1C-1 2m Qg J Qg Darrington as certainty decreases 113.3 discontinuous sand lenses notable Paleosol: wood and peat in ash; a sample of gravel; clast supported with a dacite concentration localized cross-bedding Qa Qg S Qvl 03-1C-4 peat had a radiocarbon age of 1,990 ±60 yr of approximately 35 percent R Lower limiting age, center point represents B.P. Stillaguamish Qa RX Su Age range of Kennedy Creek D iattle River Unknown; covered by slough to Sauk River Delta measured age; range represents ± error 05-33J Older alluvium (Unit Qoa): fine to medium sand; Qg Assemblage (~5.5m) 0.5m Whitehorse Mt planar bedded Whitechuck Mt A >2m Qa 154.6 Lahar (Unit Qvl): angular to subrounded, 177 .0 Port Qvl Upper limiting age, center point represents Age range of Chocolate Creek Susan Arlington Unknown; covered by slough to Suiattle 2.8m red and gray, gravel- to boulder-sized dacite RX Qvd measured age; range represent ± error Unknown; covered by slough Qg Qvl Qvd Assemblage River (~7m) in a matrix of silt and sand; grades to clast- W supported; pumice fragments noted hite C huck R Qvl RX iver S Ice 6.1m au 13.5m Qg k R total i v e Unknown; covered by slough r Glacier S o Peak u th F Qvd k S 222.4 tilla Ice 147.6 RX guami sh Rive 540.0 Continues approximately 23m to sand and gravel Marysville r RX Suiattle River 107.8 Suiattle River Glaciolacustrine (Unit Qg): clay; laminated 111.0 Qg RX White Chuck River North Fk Stillaguamish River mine floor Pilchuck Mt Sauk River approximately 30m below and convoluted, blue-gray Sloan Pk approximately 1 km north Qg RX Qa RX Qg Vesper Pk 0.9m 170.0 48°00´ 122°30´ Qg 122°00´ 121°30´ 121°00´ 48°00´ Qa Sauk River 33.5 North Fk Stillaguamish River