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  • GEOLOGY PLEISTOCENE GLACIAL DEPOSITS … · Sample no. SiO2 Al2O3 TiO2 FeO† MnO CaO ... 18/2W...
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Budd Inl et Lake Lake St.Clair St.Clair Yelm McIntosh McIntosh Lake Lake Rainier OLYMPIA Pattison Pattison Lake Lake Shelton Black Hills Offutt Offutt Lake Lake McIntosh Lake Offutt Lake Maytown N i s q u a l l y Black Black Lake Lake Lake Lake Lawrence Lawrence Summit Summit Lake Lake Summit Lake D e s c h u t e s R N i s q u a lly R e a c h T o t t e n I n l e t American American Lake Lake Lake St.Clair Pattison Lake Black Lake Lake Lawrence American Lake Ri v e r P u y a l l u p Orting Spanaway R i v e r O H O P V A L L EY Ta n w a x C r e e k Ta n w a x C r e e k TACOMA Tenino Grand Mound Cr Cr Cr R Sk o ok u mch u c k R Sk o ok u mch u c k J o h n s o n J o h n s o n Maytown quadrangle Figure 4. Lidar shaded-relief image of the southern Puget Lowland showing preliminary lidar mapping of areas where Mima mounds can be identified. This mapping was done with low-resolution lidar, but higher resolution mapping is currently under way. The mounded areas are outlined in red. Note the association of mounded areas with major outwash channels (Fig. 3). Steilacoom M c L a n e C r Qgoy3 Qgos Qgos Qgo n4 Qgo n4 Qgo y4 Qgoy4 Qgo n4 Qgo n2 Qgo n2 Qgo n1 Qgo n3 Qgo o1? Qgo o2 Qgo o3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y4 Qgo y1 Qgo y1 Qgo y1 Qgo y2 Qgo y2 and and Qgo y2 and Qgoy3 Q g o y 3 Budd Inlet Lake St. Clair YELM YELM LOBE LOBE Yelm YELM LOBE YELM LOBE RAINIER CHANNEL RAINIER CHANNEL TENALQUOT PRAIRIE ROCKY PRAIRIE Tenino STONY POINT STONY POINT CHANNEL CHANNEL McIntosh Lake OLYMPIA OLYMPIA LOBE LOBE INTERLOBE TERR AIN INT ERLOBE TERRAIN Grand Mound YELM LOBE YELM LOBE YELM CHANNEL YELM CHANNEL OLYMPIA LOBE OLYMPIA LOBE CHANNEL CHANNEL Rainier Deschutes R. OLYMPIA McCORKLE McCORKLE CHANNEL CHANNEL CHAMBERS PRAIRIE Scatter Creek Henderson Inlet kettle train Pattison Lake East Olympia Chain Hill Qgos Qgos ap p r o x . i c e lim it Black Hills Budd Inlet kettle train Nisqually kettle train Offutt Lake Tempo Lake Sheehan Lake Qgos Maytown N i s q u a l l y C h eh a l i s R B l a c k R i v e r Black Lake Lake Lawrence E l d Inlet Summit Lake D e s c h u t e s R Maytown quadrangle MIMA PRAIRIE VIOLET PRAIRIE Cr M u ck GRAND MOUND PRAIRIE i i s s s s W ad d e l l C r e e k Scatter Cr C h eh a l i s R N i s q u a l l y D e s c h u t e s R Ri v e r P u y a l l u p R i v e r C h eh a l i s R River 0 0 1 mi 1 km Figure 3. Lidar shaded-relief image showing progressive meltwater paths during ice withdrawal from the southern Puget Lowland. Modified from Walsh and Logan (2005). Not all of the outwash units in the East Olympia quadrangle (Walsh and Logan, 2005) are present in the Maytown quadrangle. MnO*10 P 2 O 5 *10 TiO 2 CAB IAT MORB OIT OIA 40 50 60 70 0 5 10 15 20 MgO + CaO SiO 2 35 40 45 50 55 60 65 70 75 0 2 4 6 8 10 12 14 16 Na 2 O + K 2 O Picro- basalt Basalt Basaltic andesite Andesite Dacite Rhyolite Trachyte Trachydacite Trachy- andesite Basaltic trachy- andesite Trachy- basalt Tephrite basanite Phono- tephrite Tephri- phonolite Phonolite Foidite SiO 2 Figure 2. A. Chemical variation diagram in weight percent showing the chemical affinities of Maytown and East Olympia igneous rocks and Crescent Formation basalt. Labels on MnO/TiO 2 /P 2 O 5 diagram are from Mullen (1983). CAB, calc-alkaline basalts; IAT, island arc tholeiites; MORB, mid-ocean ridge and marginal basin basalts; OIT, ocean island tholeiites; OIA, ocean island alkalic basalts. B. Rocks in the Maytown and East Olympia quadrangles follow a notable differentiation trend in this diagram. C. TAS diagram (Le Maitre and others, 2002) illustrates the classification of the Crescent, Maytown, and East Olympia rocks. EXPLANATION A B C Crescent Formation, Tumwater quadrangle (Walsh and others, 2003b) Crescent Formation, Tumwater quadrangle (this study) Northcraft Formation, Bucoda quadrangle, Alice’s Restaurant (this study) Northcraft Formation, Bucoda quadrangle, Skookumchuck quarry (this study) Northcraft Formation; East Olympia quadrangle, Tempo Lake (Walsh and Logan, 2005) Northcraft Formation, East Olympia quadrangle, McIntosh Lake (Walsh and Logan, 2005) Unit Eig, East Olympia quadrangle (Walsh and Logan, 2005) Unit Eig on Littlerock Road (locality 1) Unit Eig gabbro/diorite (locality 2) Figure 1. Three-dimensional view of the southern Puget Lowland, including the northern part of the Maytown quadrangle (red). This view was generated using subsurface interpretation of water well logs from Drost and others (1998), modified by Walsh and others (2003b). Note the increase in depth to bedrock to the east. Explanation of Subsurface Units Vashon till (Quaternary) Vashon advance outwash (Quaternary) Pre-Vashon drift (Quaternary) Bedrock and older Pleistocene deposits (Tertiary–Quaternary) Water Qgt Qgt Qga Qgp TQu Qga Qgp TQu vertical exaggeration 3X 0 1 2 3 4 5 mi 0 0 0 1 2 mi 0 1 2 3 km Table 1. Geochemical analyses results. Sample numbers represent the location of the sample; for instance, 17/2W 27.23 indicates that the sample is located in township 17 north, range 2 west, section 27, approximately 0.2 mi east of the southwest section corner and 0.3 mi north of the southern section boundary. Localities 1 and 2 are on this map. The rest are from adjacent quadrangles. Analyses were made at the Washington State University GeoAnalytical Laboratory. Instrumental precision is described in Johnson and others (1998). Values for oxides are given in percent; single elements in parts per million (ppm). Total Fe is expressed as FeO. Elements with * are by ICP-MS. All others are by XRF. Sample symbols correspond to sample symbols in Figure 2. Sample no. SiO 2 Al 2 O 3 TiO 2 FeO† MnO CaO MgO K 2 O Na 2 OP 2 O 5 Total Ni Cr Sc* V Ba* Rb* Sr* Zr* Y* Nb* Ga Cu Zn Pb* La* Ce* Th* Nd* U* Pr* Sm* Eu* Gd* Tb* Dy* Ho* Er* Tm* Yb* Lu* Hf* Ta* Cs* 17/2W 30.47A (loc. 1) 48.54 16.3 3.18 13.89 0.21 10.14 4.61 0.12 2.85 0.16 100 62 67 33.2 351 24 1.6 377 116 35.62 7.26 18 396 130 0.35 6.26 18.12 0.27 17.76 0.1 3.23 5.79 2.12 6.98 1.19 7.27 1.42 3.7 0.49 2.88 0.43 3.68 0.47 0.05 17/2W 27.46 (loc. 2) 56.19 12.92 2.38 14.01 0.23 5.61 3.35 0.8 3.74 0.76 100 0 0 23.6 70 140 10.7 337 373 84.05 29.26 23 132 181 1.05 24.24 70.56 1.15 59.62 0.38 11.69 17.13 5.58 18.49 3.02 17.95 3.44 8.76 1.18 6.95 1.01 10.71 1.73 0.08 15/1W 11.57 62.91 15.07 1.58 6.83 0.14 4.71 1.96 1.38 4.98 0.43 100 0 1 18.3 125 340 32.7 327 315 34.99 21.44 18 117 92 4.73 34.34 74.27 5.14 38.47 1.61 9.41 8.47 2.33 7.99 1.23 7.18 1.4 3.61 0.51 2.99 0.47 8.11 1.47 0.68 15/1E 6.11 59.87 16.45 1.51 7.65 0.18 5.47 2.45 1.06 5.02 0.34 100 0 1 21.6 140 255 17.9 388 181 30.14 15.76 20 85 104 4 19.62 44.13 1.88 25.63 0.63 5.99 6.11 2.13 6.31 1.01 6.12 1.22 3.16 0.45 2.7 0.42 4.76 1 0.1 18/2W 60.05 49.39 15.52 2.13 11 0.14 11.81 7.03 0.29 2.49 0.2 100 57 76 45 316 55 2.5 243 112 27.34 10.39 16 168 99 2.91 9.69 23.34 0.9 15.9 0.27 3.38 4.57 1.67 5.21 0.91 5.65 1.12 3 0.42 2.49 0.38 3.13 0.67 0.02 18/2W 60.06 49.64 15.47 2.07 9.91 0.17 12.56 7.12 0.32 2.55 0.2 100 63 96 45.4 308 63 3.3 246 111 27.85 10.13 16 162 90 3.21 10.04 24.23 0.87 16.2 0.26 3.44 4.63 1.7 5.35 0.92 5.79 1.15 3.04 0.43 2.56 0.39 3.1 0.64 0.02 18/2W 65.59 49.41 15.88 2.01 11.44 0.16 11.29 6.49 0.27 2.83 0.22 100 51 124 40 299 80 2.3 234 123 28.05 10.92 18 157 88 0.67 10.34 25.02 0.93 17.07 0.28 3.62 4 .75 1.73 5.37 0.93 5.75 1.14 3.07 0.43 2.53 0.38 3.38 0.7 0.05 38 11 10 4 15 14 63 20 5 11 1 2 Qa Qa Qa af af Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgos Qgo Qgo Qgt Qgt Qp Qp Qp Qgt Qaf Qaf Qgt Qp Qaf Qa Qa Qa Qa Qp Qp Qp Qp Qp Qp Qp Qgo k Qgo k Qgo k Qgos r Qa Qgos r ml Qgo y4 Qgo y4 Qgo y4 Qa Qgt Qgt Qgt Qgt Qgt Qp Qgo n3 Qgo n3 Qgo n3 Qgo n3 Qgo n3 Qgt Qgt Qgt Qp Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qp Qp Qp Qp Qp Qp Qp Qp Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgoy3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qgo y3 Qp Qp Qp Qp Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qgt Qp Qp Qp Qp Qgt Qp Qp Qp Qp Qp Qgt o2 Qgt o2 Qgt o2 Qgt o2 Qaf Qgt o2 Qgt o2 Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qa Qga Qga Qga Qga Qga Qga Qgon3 Qgo y3 Qgos Qgos Qgos Qgos Qgos Qgos ml Qgo n3 Qgo n3 Qgo Qgo Qgo Qgo Qgo Qga Qga Qgt o2 Qp Em m Em m Em m Em m Em m Em m Em m Em m Emm Em m Qls Qls Qls Qls Qmw Qgt o2 Qgt o2 Qgt o2 ml Qp Qaf Qgo y3 Qp Qp Qp Qp Qp Qp Qp Qp Qp Qp Qp Qp Qls Qmw Qls Qls Qgt Qp Qgt di Qgt di Qgt di Qgt di Qgt di Qgt di Qgt di Qgt di Qgt di Qaf Qaf Qaf Qaf Qaf Qaf Qaf Qaf Qaf Qaf Qa Qa Qa Qa Evc n Evc n Evc n Qgo y3 Eig Eig Eig Eig Qgo y3 Qgo y3 Qgo y3 Qa Qp Qp Qaf Qaf Qgo Qp Qaf Qaf Qaf Qaf Qaf Qaf Qp Qaf Qaf Qaf ml ml Qp af af af af af af af Qaf Qaf Qgo kb Qgos Qgos Qgos Qgos Qgos Qgos Qgt Qp Qgo kb Qp Qgok Em m ml ml Qaf Qaf Qaf Qp Qgo Qgo Em m Qgo Qgo y3 Qgt Qgt Qgt Qp Qgt Qgt Qgt Qgp Qgp Qgp Qgo k Qgo k Qgo k Qgo k Qgo k Qgos Qgos Qp Qp ml Qgos Qgt Qgo k Qgo kb Qgo kb Qgo k Qgo kb Qgo k Qgo kb Qgo k Em m Qgt Qgt o2 Qgos Qgos Qgo Qgt Qgo k Qgo k Qp Qgt Qgo y3 Qgo y3 Qgo y3 Em m Em m Qgo y3 Qgo k Qa Em m Qgo y3 Qgo y3 Qgo n3 Qgo y3 Qa Qgos Qgos Qgas Em m Qgo y3 Qga Qa Qa Qp Qgt o2 Qa Qa Qa Qgos Qgos Qgos Qgos Qgas Qgas Qgas Qgo y3 Qgo y3 Evc n Qa Qa Qgos 122°52¢30² 47°00¢00² 123°00¢00² 47°00¢00² R 3 W R 2 W T 17 N T 16 N 55¢00² 55¢00² 55¢00² 57¢30² 122°52¢30² 46°52¢30² 123°00¢00² 46°52¢30² 57¢30² 57¢30² 57¢30² R 3 W R 2 W T 17 N T 16 N 55¢00 ² 101 Olympia TUMWATER LACEY NISQUALLY LITTLE ROCK MAYTOWN EAST OLYMPIA TENALQUOT PRAIRIE 5 5 8 SUMMIT LAKE 7000 FEET 1000 1000 0 2000 3000 4000 5000 6000 0.5 1 KILOMETER 1 0 0.5 1 0 1 MILE SCALE 1:24,000 Disclaimer: This product is provided ‘as is’ without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular use. The Washington Department of Natural Resources and the authors of this product will not be liable to the user of this product for any activity involving the product with respect to the following: (a) lost profits, lost savings, or any other consequential damages; (b) the fitness of the product for a particular purpose; or (c) use of the product or results obtained from use of the product. This product is considered to be exempt from the Geologist Licensing Act [RCW 18.220.190 (4)] because it is geological research conducted by the State of Washington, Department of Natural Resources, Division of Geology and Earth Resources. © 2009 Washington Division of Geology and Earth Resources supplemental contour at 10 feet Lambert conformal conic projection North American Datum of 1927; to place on North American Datum of 1983, move the projection lines approximately 21 meters north and 103 meters east as shown by crosshair corner ticks Base map from scanned and rectified U.S. Geological Survey Maytown 7.5-minute quadrangle, 1990 Shaded relief generated from a lidar bare-earth digital elevation model (available from Puget Sound Lidar Consortium, http://pugetsoundlidar.ess.washington.edu/); sun azimuth 315°; sun angle 45°; vertical exaggeration 6x Digital cartography by Anne C. Heinitz, Elizabeth E. Thompson, J. Eric Schuster, and Isabelle Y. Sarikhan Editing by Katherine M. Reed and Jaretta M. Roloff Production by Jaretta M. Roloff This geologic map was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program contour interval 20 feet APPROXIMATE MEAN DECLINATION, 2008 MAGNETIC NORTH TRUE NORT H 17 ° Suggested citation: Logan, Robert L.; Walsh, Timothy J.; Stanton, Benjamin W.; Sarikhan, Isabelle Y., 2009, Geologic map of the Maytown 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-72, 1 sheet, scale 1:24,000. WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCES GEOLOGIC MAP GM-72 Maytown 7.5-minute Quadrangle February 2009 Geologic Map of the Maytown 7.5-minute Quadrangle, Thurston County, Washington by Robert L. Logan, Timothy J. Walsh, Benjamin W. Stanton, and Isabelle Y. Sarikhan February 2009 MAJOR FINDINGS • Ice retreat after the Vashon glacial maximum may have been extremely rapid. • Mima mound formation may have been ice marginal and temporally associated with ice-front retreat. • Late-stage glacial drainage patterns control the distribution of geologic materials. • Igneous rocks in the quadrangle may be derived from at least two sources. • Northeast-trending late Eocene dikes intrude sedimentary bedrock. • Eocene volcanic rocks invaded McIntosh Formation marine sands and silts and are locally preserved as peperite. http://www.dnr.wa.gov/AboutDNR/Divisions/GER/ GEOLOGY Glacial ice and meltwater deposited drift and carved extensive areas of the southern Puget Lowland into a complex geomorphology that provides insight into latest Pleistocene glacial processes. Throughout the map area the many streamlined elongate hills (drumlins) reveal the direction of ice movement. Mima mounds (Washburn, 1988) cover parts of the quadrangle. During field mapping, lidar imagery was used to interpret landform origins. Water well and geotechnical boring logs were used to characterize the underlying geologic materials and estimate their thicknesses. Water well data modified by Walsh and others (2003b) from Drost and others (1998) was used to create a three-dimensional view of part of the Maytown quadrangle and surrounding area (Fig. 1). Figure 1 shows the thicknesses of Quaternary and bedrock units. Bedrock is sporadically exposed in the hills in the southern half of the quadrangle. These outcrops are the Eocene McIntosh Formation (unit Em m ), which is primarily thinly bedded marine siltstone and sandstone and was mapped and named by Snavely and others (1951, 1958) southeast of the project area. These sedimentary rocks were altered along their contacts with a palagonite-rich volcanic breccia (unit Evc n ) that formed when late Eocene lava flowed onto and sank into, or “invaded” the sediments of the McIntosh Formation. The invasive nature of these volcanic rocks can be seen in a roadcut on the west side of Interstate 5 about a mile north of the Maytown exit (the north edge of sec. 5, T16N R2W) where concordant and discordant contacts and a breccia of mixed palagonite, sandstone, and siltstone (peperite) are exposed. In the central part of the quadrangle, coarse-grained dikes, intermediate in chemical composition between gabbro and diorite (unit Eig; locality 1) cut the McIntosh Formation (unit Em m ). The dikes may represent feeder conduits for the invasive volcanic rocks (unit Evc n ), but, as noted by Walsh and Logan (2005), the volcanic rocks may be younger than the intrusive rocks. A rock borrow pit due west of Offutt Lake in the East Olympia quadrangle (Walsh and Logan, 2005) was excavated in a porphyritic volcanic rock with a chilled margin in contact with a coarse-grained mafic intrusive rock type, unit Eig. This unit symbol has been carried over into the Maytown quadrangle to emphasize the chemical similarities between certain rocks in the Maytown quadrangle and the East Olympia quadrangle. Although different in appearance in hand specimens and thin sections, unit Eig in both quadrangles has mineralogy (mainly plagioclase feldspar and clinopyroxene) similar to gabbro and diabase of the Crescent Formation. However, the chemistry of unit Eig is different (Table 1, Fig. 2) (Phillips and others, 1989) from that of Crescent Formation rocks. An Ar/Ar age of 38.76 ±2.50 Ma (Walsh and Logan, 2005) indicates that unit Eig is too young to be Crescent, but it is about the same age as the volcanic rocks of Grays River (Walsh, 1987; Phillips, 1987a) and the Northcraft Formation, both of which are present nearby. As noted in Walsh and Logan (2005), unit Eig rocks are transitional tholeiitic to calc-alkaline, like the volcanic rocks of Grays River and the Northcraft Formation (Fig. 2). Even though unit Eig (locality 2) in the East Olympia quadrangle contains large (1–2 cm) augite crystals similar to those found in some volcanic rocks of Grays River in the Doty Hills, 20 mi southwest of the Maytown quadrangle (Walsh and others, 1987), we suggest the East Olympia and Maytown intrusive rocks (unit Eig) belong to the Northcraft Formation because of their geographic proximity and chemical affinity (Fig. 2). The Pleistocene history of the Puget Lowland was described in detail by Bretz (1913). He noted a “western lobe” and an “eastern lobe” of the “Puget Sound Glacier” separated by the Black Hills northwest of the map area. Mapping by Logan (2003) and Logan and Walsh (2004) refined the location of the ice margin. Bretz (1913) also recognized an interlobe terrain near the Nisqually River (Fig. 3). This interlobe area, characterized by abundant eskers, kettles, deranged drainage patterns, and generally higher elevations than surrounding ground moraine, was continuous from the Steilacoom area northeast of the Maytown quadrangle (Fig. 4) to the area of maximum ice extent south of the town of Rainier (Fig. 3). The continuity of the interlobe terrain is interrupted only by subsequent outwash channels that dissect the feature near the lower reaches of the Nisqually River valley just east of Lake St. Clair (Fig. 3). Noble and Wallace (1966) referred to the lobes that were separated by the interlobe terrain as the Yelm lobe and the Olympia lobe. Walsh and Logan (2005) described and labeled outwash trains from the Yelm and Olympia lobes in their map of the East Olympia quadrangle, and we use their outwash train designations in the Maytown quadrangle. At the glacial maximum, about 13,500 yr B.P. (Walsh and others, 2003a), Olympia lobe ice impinged on and partially spilled over the hills north of Tenino (Walsh and Logan, 2005; Bretz, 1913) (Fig. 3). Vashon ice of the Olympia lobe flowed across the entire Maytown quadrangle, leaving ice-scoured, striated till deposits covering older glacial deposits (units Qgp, Qga, and Qgas) and bedrock (units Emm, Evcn, and Eig) on the hills there. The hills (some cored by bedrock and others cored by advance or older glacial deposits) and water well data for the Maytown quadrangle provide evidence that the preglacial geomorphology differed in amount of relief and location of drainages from the relatively flat valleys and low hills that exist there today. As the ice front began to retreat but remained at about the latitude of Chain Hill (Fig. 3), meltwater was directed from the Yelm lobe south and westward through Bretz’s Stony Point channel (east and south of the quadrangle) and across the prairie occupied by Scatter Creek west of Tenino and into the Mima mound– covered Rocky Prairie in the southeast corner of the Maytown quadrangle. Some of the meltwater flowed under stagnant ice southwest of Rocky Prairie (Fig. 3), but when the hills were cleared of ice, meltwater cut its way around many of the hills there and deposited recessional outwash gravels (unit Qgo y3 that originated from the Yelm lobe per Walsh and Logan, 2005) around their bases. Upon further Olympia and Yelm lobe ice-front retreat, later meltwater streams found pathways farther north where units Qgo y4 , Qgo n3 (Yelm lobe and Nisqually area, respectively, following Walsh and Logan, 2005), and Qgos were deposited. Meltwaters also exposed some advance sediments (unit Qga) in the central map area. Low sand hills formed either as dunes or more likely as ripples orthogonal to meltwater flow directions (unit Qgos r ). As the ice front retreated still farther north and glacial Lake Russell (Bretz, 1913) formed, drainage from that lake, ice-marginal streams, and an ancestral Deschutes River flowed southwestward through the Black River depression to the Chehalis River until Lake Russell was no longer impounded. When Lake Russell found a lower outlet into the Strait of Juan de Fuca (via glacial Lake Leland) and abandoned the Black River channel, the Deschutes flowed northward into Puget Sound through the path of least resistance, which was a keeled dead-ice terrain with numerous kettles along the river’s current valley (Figs. 3 and 4). The stream that deposited unit Qgo y3 was probably fed by a glacial outburst flood (Tanwax flood of Pringle and Goldstein, 2002; Pringle and others, 2000). It was strong enough to carry and deposit boulders, many as large as 3 ft in diameter, in the Deschutes River valley and possibly icebergs or ice calved from the ice front near Offutt Lake. The icebergs were aligned with the southerly turn of the channel and covered or surrounded by unit Qgo y3 to form the concave-to-the-south kettle field that includes Offutt Lake (Fig. 3 and Walsh and Logan, 2005). The floodwaters were blocked by ice that occupied the south half of the Maytown quadrangle, including the western part of Rocky Prairie. As a result, outwash flow was slowed dramatically, forming a temporary lake that caused a delta to prograde westward across Rocky Prairie. As the hydraulic head increased in the Rocky Prairie area, water began flowing beneath the ice in sec. 12, T16N R2W, eventually breaching the ice dam and continuing its flow westward to the Black River and eventually to the Chehalis River (Fig. 3). The ice blockage of Rocky Prairie and the McCorkle channel was short lived, and so was the outwash flow through those areas. As the glacier melted back to the north near Sheehan Lake (sec. 18, T17N R1W, about a mile east of the Maytown quadrangle), meltwater could then flow northward through to a lower base level, eventually cutting downward through the terrace represented by unit Qgo y4 . Ice must have continued to impinge on the hills southeast of Sheehan Lake, causing ice-marginal meltwater from the Nisqually and Lake St. Clair area (unit Qgo n4 ) (Fig. 3) to flow south through sec. 21, T17N R1W, into the Deschutes River valley, eventually leaving unmatched terraces of outwash gravel in that section. The ice that blocked the flow from the east probably occupied a depression that was a continuation of present-day Budd Inlet (Fig. 3). Evidence of this is a train of kettles that reaches from the modern southern tip of Budd Inlet to at least as far south as Sheehan Lake and probably almost to Offutt Lake. The modern Deschutes River flows through many of these kettles to reach Puget Sound. Kettle trains similar to the one south of Budd Inlet also formed south of Nisqually Reach (forming Lake St. Clair) and south of Henderson Inlet (forming Pattison Lake) as glacial ice occupying these depressions was buried by recessional outwash of the Olympia lobe (Fig. 3). Continued retreat of the ice front allowed ice-marginal streams originating in the Nisqually and Lake St. Clair area and farther east from glacial Lake Puyallup to flow westward across the northern half of the map area, cutting channels through the Vashon ground moraine (unit Qgt) and into underlying advance outwash (unit Qga). As the energy of these streams waned, they eventually deposited sand and gravel, followed by sand and silt (unit Qgos), around the remaining stagnant ice blocks. When the ice melted away, the Budd Inlet, Henderson Inlet, and Nisqually kettle chains were formed. Soils capping bedrock tend to be reddish brown, but most of those capping glacial outwash deposits are very dark brown to black. The soils covering unit Qgo y3 are formed into mounds about 2 to 6 ft high and 10 to 30 ft across. The mounds are referred to as Mima mounds and have been extensively studied by previous workers (summarized in Washburn, 1988, and discussed in Bretz, 1913). Unit Qgo y3 is the only unit within the Maytown and East Olympia quadrangles on which the Mima mounds formed (Walsh and Logan, 2005; Pringle and Goldstein, 2002). The mounds must have formed very shortly after unit Qgo y3 was deposited because (1) they did not form within kettles, which clearly formed after unit Qgo y3 was deposited (kettles crosscut meltwater channels in unit Qgo y3 ); (2) they appear to be partially buried by alluvial fans near the margins of the unit Qgo y3 channel; and (3) they did not form in unit Qgo y4 channels that cut the surface of unit Qgo y3 . Regionally, Mima mounds are associated with major outwash channels (Fig. 4). Preliminary investigations using lidar imagery and reconnaissance field checking reveal that the oldest outwash train is located south of the Maytown quadrangle, in and to the west of Violet Prairie (Fig. 3). The second oldest is the Qgo y3 train that is probably temporally associated with the Mima Prairie mounds, west of the Maytown quadrangle. A broad area through the Fort Lewis and Muck Creek vicinity is covered by mounds on top of coarse outwash gravels, and sporadic groups of mounds are found farther to the northeast near Spanaway. These mounds are also found in sharply cut channels. These four major episodes of outwash deposition and mound formation occurred as the ice front retreated from the southern Puget Lowland. If the channels were formed as the result of seasonal floods, then the ice may have retreated from the Grand Mound area (Fig. 3) to Tacoma in about four years. It is possible that the retreat was much faster if the channels were formed by continual outburst flooding during a rapid breakup of the ice along the ice margin. Such rapid ice breakup can be seen today in the Antarctica. We have been unable to locate datable material in these deposits, however, so the chronology of retreat is poorly constrained. Snavely and others (1958) mapped broad southward-plunging folds in bedrock southeast of the map area. The few attitudes measured on sparse outcrops during this study indicate that the bedrock is gently folded with dips generally less than 10 degrees; however, there was not enough data to define the nature of the folds, nor were any faults noted. DESCRIPTION OF MAP UNITS Quaternary Unconsolidated Deposits HOLOCENE NONGLACIAL DEPOSITS Fill—Engineered fills of unknown materials associated with Interstate 5; shown only where fill placement is extensive. Modified land—Soil, sediment, or other geologic material reworked by excavation and (or) redistribution to modify the topography. Only major gravel pits are shown as unit ml, although there are large regraded areas associated with golf courses and housing developments, especially in the northern half of the map area. Landslide—Silty sandy gravel; generally loose, jumbled; tan to gray; few or no discernible sedimentary structures; surfaces generally undulatory. Most landslides are along steep sides of outwash channels and may have occurred shortly after ice retreated from the map area. Colluvium—Soil and glacial sand and gravel; deposited by soil creep and shallow raveling on hill slopes; entirely postglacial. The unit is mapped where it is thick enough to mask underlying geologic strata. Alluvium—Silt, sand, and gravel deposited by streams; includes some lacustrine deposits and organic materials, such as peat. Most alluvium is very fine due to low stream gradients in the map area. Peat—Peat, muck, silt, and clay in and adjacent to closed depressions, marshes, and wetlands. Alluvial fan—Silt, sand, and gravel deposited at the confluence of upland streams with outwash channels and terrace edges; commonly capped by dark brown to black, mucky loamy soils. PLEISTOCENE GLACIAL DEPOSITS Vashon Recessional Outwash, Nisqually/Lake St. Clair Source Five trains of recessional outwash from the Nisqually/Lake St. Clair area have been noted by Walsh and Logan (2005). They are, from youngest to oldest, units Qgos, Qgo n4 , Qgo n3 , Qgo n2 , and Qgo n1 . Only units Qgos and Qgo n3 are extensive enough to be mapped in the Maytown quadrangle. Vashon recessional outwash sand and silt—Sand and silt with minor gravel interbeds; tan to brown; clasts moderately to well rounded; generally well sorted; clasts and grains consist of northern-source plutonic and metamorphic rocks and polycrystalline quartz carried by Vashon ice, and porphyritic volcanic rock from the Cascade Range 60 mi to the east; thickness varies from about 4 to 20 ft. This unit covers much of the north half of the quadrangle and was probably deposited by meltwater derived from stagnant ice south of Lake St. Clair (Fig. 3) and drainage from glacial Lake Puyallup farther east, possibly grading in elevation to glacial Lake Nisqually (Bretz, 1913) and glacial Lake Russell. Fairly small patches of locally derived ice-contact sand are also included in this unit. Vashon recessional outwash sand and silt, ripples—Sand and silt with minor gravel interbeds; tan to brown; clasts moderately to well rounded; generally well sorted; clasts and grains consist of northern-source plutonic and metamorphic rocks and polycrystalline quartz carried by Vashon ice, and porphyritic volcanic rock from the Cascade Range; varies in thickness from about 4 to possibly greater than 20 ft. When artificially illuminated from the southwest (using lidar), the gently rolling surface of this unit resembles large sand ripples with their long axes perpendicular to the direction of meltwater flow. Vashon kettle-bottom silt and peat—Silt and peat in flat bottoms of kettles. Some kettle bottoms may consist of sand and gravel with or without a fine cover. The unit is generally found in large-diameter kettles where buried ice must have been thick relative to its depth of burial. Vashon recessional outwash sand in kettle walls—Sand and minor gravel; tan to gray; moderately to well rounded and sorted; mostly northern-source plutonic and metamorphic rock carried by Vashon ice and minor porphyritic volcanic rock from the Cascade Range. In the northeastern part of the map, where exposed near the Deschutes River valley, this unit consists of gravelly sand and is generally coarser farther upstream. In the southwest corner of the map, the kettles are in gravel and are evidence that ice was probably rafted at least that far by meltwater associated with unit Qgo y3 . Vashon recessional outwash gravel, train 3—Sand and gravel; tan to gray; moderately to well rounded; consists of plutonic and metamorphic clasts transported from the north by Vashon ice and deposited by meltwater. This channel deposit is at a slightly higher elevation than the younger channels containing units Qgo n4 in the East Olympia quadrangle and Qgos (Fig. 3). The channel that contains unit Qgo n3 (Fig. 3) is nearly horizontal, with a slight westward gradient at its northern end, and was probably formed by meltwater derived from stagnant ice south of Lake St. Clair. This unit was identified from lidar imagery and, in the Maytown quadrangle, the unit is very sandy, caps low till hills, and forms bars. Vashon Recessional Outwash, Yelm and Olympia Lobe Sources There are four major trains of recessional outwash from the Yelm lobe (Fig. 3). They are represented, from youngest to oldest, by units Qgo y4 , Qgo y3 , Qgo y2 , and Qgo y1 . Meltwater from the Olympia lobe incised into units Qgo y1 and Qgo y2 , depositing units Qgo o1 through Qgo o3 (Walsh and Logan, 2005). Subsequent meltwater from the Yelm channel truncated both the earlier Yelm and Olympia lobe outwash channels and deposited units Qgo y3 and Qgoy4. The descriptions below are of those outwash trains found in the Maytown quadrangle. Vashon recessional outwash, Yelm lobe, Rainier channel, Deschutes River—Loose sand and gravel; grains and clasts well rounded; moderately to well sorted; restricted to channels that cut unit Qgo y3 and forms terraces that grade to an outlet in the northeast corner of the map where, at the time of this unit’s deposition, glacial ice had melted back to a point north of Hartsuck Road (now named 93rd Ave SW). This unit contains fewer boulders than unit Qgo y3 . The outwash originated from both the Rainier channel of the Yelm lobe and the upper Deschutes River (Fig. 3). Vashon recessional outwash, Yelm lobe, Rainier channel, Tanwax Creek–Ohop Valley flood—Bouldery and cobbly sand and gravel; clasts and grains well rounded; moderately to well sorted; capped by silty Mima mounds. Boulders are both Cascade-derived andesite and plutonic and metamorphic rock deposited by Vashon ice. Regional geomorphology indicates that the waters that deposited the unit came from both the Rainier channel of the Yelm lobe (Fig. 3) and the Tanwax Creek–Ohop Valley glacial outburst flood (Fig. 4) (Pringle and Goldstein, 2002; Pringle and others, 2000). This unit traces the path of the Deschutes River when it flowed from the McIntosh Lake area northwestward through Offutt Lake; ice west of Offutt Lake blocked the flow, forcing it south toward Tenino (Fig. 3). The Tenino flow was short lived, and its path is delineated by the Offutt Lake kettle chain. As the ice melted, the prehistoric Deschutes River flowed from Rocky Prairie westward through the area bounded by Beaver Creek in the south to Salmon Creek farther north. Vashon recessional outwash sand and gravel—Sand and gravel; tan to gray; moderately to well sorted and rounded; consists of plutonic and metamorphic lithic fragments deposited by Vashon meltwater that created outwash channels and terraces of various elevations, but formed after glacial ice retreated. Lithologically, the unit is difficult to distinguish from advance outwash where there are no intervening layers of till; however, it contains no glassy volcanic porphyry from the Cascade Range. Vashon Drift Vashon till covered by recessional outwash from the Olympia lobe, Tenalquot Prairie channel—Outwash-modified till; gray; compact; unsorted mixture of clay through boulder-size plutonic and metamorphic rocks deposited by Vashon ice as ground moraine, commonly shaped into drumlins. The unit is covered by gray to tan, northern source–dominated, pebbly sandy gravel deposited by glacial melt- water, probably from the Olympia lobe (Fig. 3); drumlin relief is subdued by the sediment cover. This unit covers a terrace along the southern part of the map area that was probably formed by the same glacial meltwaters that formed the terrace containing unit Qgo o2 , based on similarities of surface elevations of the terraces. Vashon till, dead-ice terrain—Gravelly till associated with eskers and kettles and loose sand and gravel. This unit was deposited beneath stagnant ice at the snout of the Olympia lobe. Angular glacial erratic boulders, most of coarse- grained intrusive rock types, are disseminated on the surface of this unit. Vashon till, drumlinized ground moraine—Clay, silt, sand, and gravel; gray to tan; unstratified; compact; unsorted; deposited directly by glacial ice; nearly everywhere in sharp contact with underlying units; low permeability and porosity; sand and smaller grains very angular; pebble- to boulder-size clasts commonly striated and faceted; boulders generally disseminated and rare; sporadic interbeds of sand and gravel. The surface of this unit is characterized by streamlined drumlins and striations that are generally hundreds to thousands of feet long. Angular glacial erratic boulders, mostly plutonic, are on the surface of this unit. Vashon advance outwash—Sand and gravel and lacustrine clay, silt, and sand; gray to light brown; compact; grains well rounded; mostly polycrystalline quartz, plutonic, and minor metamorphic lithic grains; deposited during Vashon glacial advance. The unit is most easily distinguished from recessional outwash where Vashon till lies between the outwash units. This unit may also contain pre-Vashon drift (unit Qgp) at its base, but poor outcrop due to raveled terrace scarps prevented differentiation of these units. Vashon advance sandy outwash—Sand and silt with minor gravel immediately underlying unit Qgt; grains well rounded; mostly polycrystalline quartz, plutonic, and minor metamorphic lithic grains; deposited during Vashon glacial advance. The unit is exposed only in the southern part of the map area. Pre-Vashon Drift Pre-Vashon drift—Sand and gravel with some clay and silt; light brown or gray to yellowish brown to brick red; generally compact; clasts well rounded; dominated by northern-source polycrystalline quartz, plutonic, and minor metamorphic grains. The unit crops out in an area near the southern margin of the quadrangle that was mapped by Noble and Wallace (1966) as Logan Hill Formation, but the purely northern provenance makeup of the clasts indicates that this may be Double Bluff Drift (Easterbrook and others, 1967; Lea, 1984). Tertiary Sedimentary and Igneous Rocks Northcraft Formation? (late Eocene)—Volcanic breccia; dark-gray to black, palagonitic groundmass; fragments mostly euhedral; common twinned and zoned plagioclase phenocrysts and euhedral twinned augite phenocrysts in glassy matrix with minor ghost grains of another mineral, possibly an amphibole. The unit is in concordant and discordant contact with volcanic lithic sandstone and siltstone of the McIntosh Formation (unit Emm) and has thermally altered the sedimentary rocks at the few contacts observed during mapping. In the Interstate 5 roadcut near Maytown, palagonite, sandstone, and siltstone form a chaotically mixed breccia (peperite) characteristic of lava flows exploding in wet sediment. Such timing would suggest that the invasive rocks are part of the Northcraft Formation. Intrusive rocks (late Eocene)—Gabbroic to dioritic dikes (basaltic and basaltic andesite composition); black to dark gray-green, ophitic, subophitic, and inter- granular; coarse-grained. Weakly pink to green pleochroic augite makes up about 40% of the rock with an equal amount of oligoclase feldspar grains; an opaque iron-oxide mineral with graphic texture comprises about 15%. Secondary minerals include disseminated heulandite and veins of calcite. Table 1 shows that the unit’s chemistry is fairly silicic despite the abundance of augite. Chemical comparisons are illustrated in Figure 2. An Ar/Ar age of 38.76 ±2.50 Ma obtained by Walsh and Logan (2005) in correlative rocks to the east and the fact that these rocks appear to intrude the McIntosh Formation indicate that they are too young to be Crescent Formation and are more likely to belong to the Northcraft Formation or a correlative of the Grays River volcanics (Walsh and Logan, 2005). However, no exposures of contacts between the dikes and the McIntosh Formation or with the volcanic invasive rocks were observed during this study. McIntosh Formation (late middle Eocene)—Volcanic lithic marine sandstone and siltstone; fine- to medium-grained; gray-green; most grains angular or euhedral twinned and (or) zoned feldspar; green to colorless amphibole grains commonly disseminated throughout the rock; grains typically in a dirty brownish yellow, fine-grained matrix with opaque and altered minerals. Although no macrofossils were found in the Maytown quadrangle, the unit contains Tertiary shallow-water benthic Foraminifera that are not age-diagnostic (W. W. Rau, Washington Division of Geology and Resources, retired, oral commun., 1983). Snavely and others (1951) mapped this unit to the southeast near Centralia, 11 mi south of the quadrangle. The unit’s thickness in the map area is unknown. GEOLOGIC SYMBOLS Contact Whole rock geochemistry sample location—Number is (loc. no.) under ‘Sample no.’ in Table 1 Inclined bedding— Showing strike and dip ACKNOWLEDGMENTS This project was made possible by the U.S. Geological Survey National Geologic Mapping Program under award no. 07HQAG0142. Geochemical analyses were performed at the Washington State University GeoAnalytical Laboratory. We thank Kitty Reed and Jari Roloff for their editorial reviews of this report and Anne Heinitz, Eric Schuster, Chuck Caruthers, and Liz Thomson for their assistance with cartographic issues. REFERENCES CITED Bretz, J H., 1913, Glaciation of the Puget Sound region: Washington Geological Survey Bulletin 8, 244 p., 3 plates. Drost B. W.; Turney, G. L.; Dion, N. P.; Jones, M. A., 1998, Hydrology and quality of ground water in northern Thurston County, Washington: U.S. Geological Survey Water-Resources Investigations Report 92-4109 [Revised], 230 p. Easterbrook, D. J.; Crandell, D. R.; Leopold, E. B., 1967, Pre-Olympia Pleistocene stratigraphy and chronology in the central Puget Lowland, Washington: Geological Society of America Bulletin, v. 78, no. 1, p. 13-20. Johnson, D. M.; Hooper, P. R.; Conrey, R. M., 1998, XRF analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead: Washington State University GeoAnalytical Laboratory, 25 p. Le Maitre, R. W. (editor); Streckeisen, A.; Zanettin, B.; Le Bas, M. J.; Bonin, B.; Bateman, P.; Bellieni, G.; Dudek, A.; Efremova, S.; Keller, J.; Lamere, J.; Sabine, P. A.; Schmid, R.; Sorensen, H.; Woolley, A. R., 2002, Igneous Rocks—A classification and glossary of terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks: Cambridge University Press, 2nd ed., 236 p. Lea, P. D., 1984, Pleistocene glaciation at the southern margin of the Puget lobe, western Washington: University of Washington Master of Science thesis, 96 p., 3 plates. Logan, R. L., 2003, Geologic map of the Shelton 1:100,000 quadrangle, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-15, 1 sheet, scale 1:100,000. [http://www.dnr.wa.gov/geology/pdf/ofr03-15.pdf] Logan, R. L.; Walsh, T. J., 2004, Geologic map of the Summit Lake 7.5-minute quadrangle, Thurston and Mason Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2004-10, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/Publications/ger_ ofr2004-10_geol_map_summitlake_24k.pdf] Mullen, E. D., 1983, MnO/TiO 2 /P 2 O 5 —A minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis: Earth and Planetary Science Letters, v. 62, p. 53-62. Noble, J. B.; Wallace, E. F., 1966, Geology and ground-water resources of Thurston County, Washington; Volume 2: Washington Division of Water Resources Water-Supply Bulletin 10, v. 2, 141 p., 5 plates. Phillips, W. M., 1987a, Geochemistry and areal extent of the Grays River volcanic rocks, southwest Washington and adjacent Oregon [abstract]: Eos (American Geophysical Union Transactions), v. 68, no. 52, p. 1815. Phillips, W. M., compiler, 1987b, Geologic map of the Mount St. Helens quadrangle, Washington and Oregon: Washington Division of Geology and Earth Resources Open File Report 87-4, 59 p., 1 plate, scale 1:100,000. Phillips, W. M.; Walsh, T. J.; Hagen, R. A., 1989, Eocene transition from oceanic to arc volcanism, southwest Washington. In Muffler, L. J. P.; Weaver, C. S.; Blackwell, D. D., editors, Proceedings of workshop XLIV— Geological, geophysical, and tectonic setting of the Cascade Range: U.S. Geological Survey Open-File Report 89-178, p. 199-256. Pringle, P. T.; Goldstein, B. S., 2002, Deposits, erosional features, and flow characteristics of the late- glacial Tanwax Creek–Ohop Creek Valley flood—A likely source for sediments composing the Mima mounds, Puget Lowland, Washington [abstract]: Geological Society of America Abstracts with Programs, v. 34, no. 5, p. A-89. Pringle, P. T.; Goldstein, B. S.; Anderson, N. R., 2000, Tanwax Creek–Ohop Valley late-glacial flood—Evidence that discharge from an ice-dammed lake in the Carbon River Valley was augmented by a temporary landslide dam, Puget Lowland, Washington [abstract]. In Washington Department of Ecology; Washington Hydrologic Society; U.S. Geological Survey, Program and abstracts from the 3rd symposium on the hydrogeology of Washington State: Washington Department of Ecology, p. 85. Snavely, P. D., Jr.; Brown, R. D., Jr.; Roberts, A. E.; Rau, W. W., 1958, Geology and coal resources of the Centralia–Chehalis district, Washington, with a section on Microscopical character of Centralia–Chehalis coal, by J. M. Schopf: U.S. Geological Survey Bulletin 1053, 159 p., 6 plates. Snavely, P. D., Jr.; Rau, W. W.; Hoover, Linn, Jr.; Roberts, A. E., 1951, McIntosh Formation, Centralia– Chehalis coal district, Washington: American Association of Petroleum Geologists Bulletin, v. 35, no. 5, p. 1052-1061. Walsh, T. J., compiler, 1987, Geologic map of the Astoria and Ilwaco quadrangles, Washington and Oregon: Washington Division of Geology and Earth Resources Open File Report 87-2, 28 p., 1 plate. Walsh, T. J.; Korosec, M. A.; Phillips, W. M.; Logan, R. L.; Schasse, H. W., 1987, Geologic map of Washington—Southwest quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-34, 2 sheets, scale 1:250,000, with 28 p. text. Walsh, T. J.; Logan, R. L.; Polenz, Michael; Schasse, H. W., 2003a, Geologic map of the Nisqually 7.5-minute quadrangle, Thurston and Pierce Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-10, 1 sheet, scale 1:24,000. [http://www. dnr.wa.gov/geology/pdf/ofr03-10.pdf] Walsh, T. J.; Logan, R. L.; Schasse, H. W.; Polenz, Michael, 2003b, Geologic map of the Tumwater 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-25, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/ Publications/ger_ofr2003-25_geol_map_tumwater_24k.pdf] Walsh, T. J.; Logan, R. L., 2005, Geologic map of the East Olympia 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-56, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/Publications/ger_gm56_geol_ map_eastolympia_ 24k.pdf] Washburn, A. L., 1988, Mima mounds—An evaluation of proposed origins with special reference to the Puget lowlands: Washington Division of Geology and Earth Resources Report of Investigations 29, 53 p. 38 1 af Eig Em m Evc n Qa Qga Qgas Qgo Qgo k Qgo kb Qgo n3 Qgo y3 Qgo y4 Qgos Qgos r Qgp Qgt Qgt di Qgt o2 Qls Qmw Qp ml Qaf

GEOLOGY PLEISTOCENE GLACIAL DEPOSITS … · Sample no. SiO2 Al2O3 TiO2 FeO† MnO CaO ... 18/2W 60.05 49.39 15.52 2.13 11 0.14 11.81 7.03 0.29 2.49 0.2 100 57 76 45 316 55 2.5 243

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Page 1: GEOLOGY PLEISTOCENE GLACIAL DEPOSITS … · Sample no. SiO2 Al2O3 TiO2 FeO† MnO CaO ... 18/2W 60.05 49.39 15.52 2.13 11 0.14 11.81 7.03 0.29 2.49 0.2 100 57 76 45 316 55 2.5 243

Budd Inlet

Lake Lake St.ClairSt.Clair

Yelm

McIntoshMcIntosh Lake Lake

Rainier

OLYMPIA

PattisonPattisonLakeLake

Shelton

BlackHills

OffuttOffuttLakeLake

McIntosh Lake

OffuttLake

Maytown

NisquallyBlackBlackLakeLake

LakeLakeLawrenceLawrence

Summit SummitLakeLake SummitLake

DeschutesR

Nisqually ReachTotten

Inlet

AmericanAmericanLakeLake

Lake St.Clair

PattisonLake

BlackLake

LakeLawrence

AmericanLake

River

Puyallup

OrtingSpanaway

RiverOHOP

VALLEY

Tanwax

Creek

Tanwax

Creek

TACOMA

Tenino

GrandMound

CrCrCr

R

Skookumchuck

R

Skookumchuck John

son

John

son

Maytown quadrangle

Figure 4. Lidar shaded-relief image of the southern Puget Lowland showing preliminary lidar mapping of areas where Mima mounds can be identified. This mapping was done with low-resolution lidar, but higher resolution mapping is currently under way. The mounded areas are outlined in red. Note the association of mounded areas with major outwash channels (Fig. 3).

Steilacoom

McL

ane

Cr

Qgoy3

QgosQgos

Qgo n4

Qgo n4

Qgoy4

Qgoy4

Qgon4

Qgon2Qgon2

Qgon1Qgon3

Qgoo1

?

Qgo

o2

Qgo

o3

Qgoy3

Qgoy3Qgoy3

Qgoy3 Qgoy3

Qgoy3 Qgoy4

Qgo

y1

Qgo

y1

Qgo

y1Qgo

y2

Qgoy2

and

and

Qgo

y2

and

Qgoy3

Qgoy3Budd Inlet

Lake St.Clair

YELMYELMLOBELOBE

Yelm

YELM LOBEYELM LOBERAINIER CHANNELRAINIER CHANNEL

TENALQUOTPRAIRIE

ROCKY PRAIR

IE

Tenino

STONY POINTSTONY POINTCHANNELCHANNEL

McIntoshLake

OLYMPIAOLYMPIALOBELOBE

INT

ER

LO

BE

T

ER

RA

I NI N

TE

RL

OB

E

TE

RR

AI N

GrandMound

YELM LOBEYELM LOBEYELM CHANNELYELM CHANNEL

OLYMPIA LOBEOLYMPIA LOBECHANNELCHANNEL

Rainier

Deschutes R.

OLYMPIA

McCORKLEMcCORKLECHANNELCHANNEL

CHAMBERS PRAIRIE

Scat

ter

Cree

k

Henderson Inlet kettle train

PattisonLake

EastOlympia

ChainHill

Qgos

Qgos

ap

prox. ice limit

BlackHills

Budd Inlet kettle train

Nis

qual

ly k

ettle

tra

in

OffuttLake

TempoLake

SheehanLake

Qgos

Maytown

Nisqually

Chehalis R

Black

Rive

r

BlackLake

LakeLawrence

El

d Inlet

SummitLake

DeschutesR

Maytown quadrangle

MIMA P

RAIRIE

VIOLET PRAIRIE

CrMuck

GRAND MOUND PRAIRIE

iissss

Waddell

Creek

Scatter Cr

Chehalis R

Nisqually

DeschutesR

River

Puyallup

River

Chehalis R

River

0

0 1 mi

1 km

Figure 3. Lidar shaded-relief image showing progressive meltwater paths during ice withdrawal from the southern Puget Lowland. Modified from Walsh and Logan (2005). Not all of the outwash units in the East Olympia quadrangle (Walsh and Logan, 2005) are present in the Maytown quadrangle.

MnO*10 P2O5*10

TiO2

CAB

IAT

MORB OIT

OIA

40 50 60 700

5

10

15

20

MgO

+ C

aO

SiO2

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Na 2

O +

K2O

Picro-basalt Basalt

Basalticandesite

AndesiteDacite

Rhyolite

Trachyte

TrachydaciteTrachy- andesite

Basaltic trachy- andesiteTrachy-

basalt

Tephritebasanite

Phono-tephrite

Tephri-phonolite

Phonolite

Foidite

SiO2

Figure 2. A. Chemical variation diagram in weight percent showing the chemical affinities of Maytown and East Olympia igneous rocks and Crescent Formation basalt. Labels on MnO/TiO2/P2O5 diagram are from Mullen (1983). CAB, calc-alkaline basalts; IAT, island arc tholeiites; MORB, mid-ocean ridge and marginal basin basalts; OIT, ocean island tholeiites; OIA, ocean island alkalic basalts. B. Rocks in the Maytown and East Olympia quadrangles follow a notable differentiation trend in this diagram. C. TAS diagram (Le Maitre and others, 2002) illustrates the classification of the Crescent, Maytown, and East Olympia rocks.

EXPLANATION

A

B C

Crescent Formation, Tumwater quadrangle (Walsh and others, 2003b)

Crescent Formation, Tumwater quadrangle (this study)

Northcraft Formation, Bucoda quadrangle, Alice’s Restaurant (this study)

Northcraft Formation, Bucoda quadrangle, Skookumchuck quarry (this study)

Northcraft Formation; East Olympia quadrangle, Tempo Lake (Walsh and Logan, 2005)

Northcraft Formation, East Olympia quadrangle, McIntosh Lake (Walsh and Logan, 2005)

Unit Eig, East Olympia quadrangle (Walsh and Logan, 2005)

Unit Eig on Littlerock Road (locality 1)

Unit Eig gabbro/diorite (locality 2)

Figure 1. Three-dimensional view of the southern Puget Lowland, including the northern part of the Maytown quadrangle (red). This view was generated using subsurface interpretation of water well logs from Drost and others (1998), modified by Walsh and others (2003b). Note the increase in depth to bedrock to the east.

Explanation of Subsurface Units

Vashon till (Quaternary)

Vashon advance outwash (Quaternary)

Pre-Vashon drift (Quaternary)

Bedrock and older Pleistocene deposits (Tertiary–Quaternary)

Water

Qgt

Qgt

Qga

Qgp

TQu

Qga

Qgp

TQuvertical exaggeration 3X

0 1 2 3 4 5 mi

0

0

0 1 2 mi

0 1 2 3 km

Table 1. Geochemical analyses results. Sample numbers represent the location of the sample; for instance, 17/2W 27.23 indicates that the sample is located in township 17 north, range 2 west, section 27, approximately 0.2 mi east of the southwest section corner and 0.3 mi north of the southern section boundary. Localities 1 and 2 are on this map. The rest are from adjacent quadrangles. Analyses were made at the Washington State University GeoAnalytical Laboratory. Instrumental precision is described in Johnson and others (1998). Values for oxides are given in percent; single elements in parts per million (ppm). Total Fe is expressed as FeO. Elements with * are by ICP-MS. All others are by XRF. Sample symbols correspond to sample symbols in Figure 2.

Sample no. SiO2 Al2O3 TiO2 FeO† MnO CaO MgO K2O Na2O P2O5 Total Ni Cr Sc* V Ba* Rb* Sr* Zr* Y* Nb* Ga Cu Zn Pb* La* Ce* Th* Nd* U* Pr* Sm* Eu* Gd* Tb* Dy* Ho* Er* Tm* Yb* Lu* Hf* Ta* Cs*

17/2W 30.47A (loc. 1) 48.54 16.3 3.18 13.89 0.21 10.14 4.61 0.12 2.85 0.16 100 62 67 33.2 351 24 1.6 377 116 35.62 7.26 18 396 130 0.35 6.26 18.12 0.27 17.76 0.1 3.23 5.79 2.12 6.98 1.19 7.27 1.42 3.7 0.49 2.88 0.43 3.68 0.47 0.05

17/2W 27.46 (loc. 2) 56.19 12.92 2.38 14.01 0.23 5.61 3.35 0.8 3.74 0.76 100 0 0 23.6 70 140 10.7 337 373 84.05 29.26 23 132 181 1.05 24.24 70.56 1.15 59.62 0.38 11.69 17.13 5.58 18.49 3.02 17.95 3.44 8.76 1.18 6.95 1.01 10.71 1.73 0.08

15/1W 11.57 62.91 15.07 1.58 6.83 0.14 4.71 1.96 1.38 4.98 0.43 100 0 1 18.3 125 340 32.7 327 315 34.99 21.44 18 117 92 4.73 34.34 74.27 5.14 38.47 1.61 9.41 8.47 2.33 7.99 1.23 7.18 1.4 3.61 0.51 2.99 0.47 8.11 1.47 0.68

15/1E 6.11 59.87 16.45 1.51 7.65 0.18 5.47 2.45 1.06 5.02 0.34 100 0 1 21.6 140 255 17.9 388 181 30.14 15.76 20 85 104 4 19.62 44.13 1.88 25.63 0.63 5.99 6.11 2.13 6.31 1.01 6.12 1.22 3.16 0.45 2.7 0.42 4.76 1 0.1

18/2W 60.05 49.39 15.52 2.13 11 0.14 11.81 7.03 0.29 2.49 0.2 100 57 76 45 316 55 2.5 243 112 27.34 10.39 16 168 99 2.91 9.69 23.34 0.9 15.9 0.27 3.38 4.57 1.67 5.21 0.91 5.65 1.12 3 0.42 2.49 0.38 3.13 0.67 0.02

18/2W 60.06 49.64 15.47 2.07 9.91 0.17 12.56 7.12 0.32 2.55 0.2 100 63 96 45.4 308 63 3.3 246 111 27.85 10.13 16 162 90 3.21 10.04 24.23 0.87 16.2 0.26 3.44 4.63 1.7 5.35 0.92 5.79 1.15 3.04 0.43 2.56 0.39 3.1 0.64 0.02

18/2W 65.59 49.41 15.88 2.01 11.44 0.16 11.29 6.49 0.27 2.83 0.22 100 51 124 40 299 80 2.3 234 123 28.05 10.92 18 157 88 0.67 10.34 25.02 0.93 17.07 0.28 3.62 4.75 1.73 5.37 0.93 5.75 1.14 3.07 0.43 2.53 0.38 3.38 0.7 0.05

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SCALE 1:24,000

Disclaimer: This product is provided ‘as is’ without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular use. The Washington Department of Natural Resources and the authors of this product will not be liable to the user of this product for any activity involving the product with respect to the following: (a) lost profits, lost savings, or any other consequential damages; (b) the fitness of the product for a particular purpose; or (c) use of the product or results obtained from use of the product. This product is considered to be exempt from the Geologist Licensing Act [RCW 18.220.190 (4)] because it is geological research conducted by the State of Washington, Department of Natural Resources, Division of Geology and Earth Resources.

© 2009 Washington Division of Geology and Earth Resources

supplemental contour at 10 feet

Lambert conformal conic projectionNorth American Datum of 1927; to place on North American Datum of 1983,

move the projection lines approximately 21 meters north and 103 meters east as shown by crosshair corner ticks

Base map from scanned and rectified U.S. Geological Survey Maytown 7.5-minute quadrangle, 1990

Shaded relief generated from a lidar bare-earth digital elevation model (available from Puget Sound Lidar Consortium, http://pugetsoundlidar.ess.washington.edu/); sun azimuth 315°; sun angle 45°; vertical exaggeration 6x

Digital cartography by Anne C. Heinitz, Elizabeth E. Thompson, J. Eric Schuster, and Isabelle Y. Sarikhan

Editing by Katherine M. Reed and Jaretta M. RoloffProduction by Jaretta M. Roloff

This geologic map was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program

contour interval 20 feet

APPROXIMATE MEANDECLINATION, 2008

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Suggested citation: Logan, Robert L.; Walsh, Timothy J.; Stanton, Benjamin W.; Sarikhan, Isabelle Y., 2009, Geologic map of the Maytown 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-72, 1 sheet, scale 1:24,000.

WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCESGEOLOGIC MAP GM-72

Maytown 7.5-minute QuadrangleFebruary 2009

Geologic Map of the Maytown 7.5-minute Quadrangle, Thurston County, Washingtonby Robert L. Logan, Timothy J. Walsh, Benjamin W. Stanton, and Isabelle Y. Sarikhan

February 2009

MAJOR FINDINGS

• Ice retreat after the Vashon glacial maximum may have been extremely rapid.• Mima mound formation may have been ice marginal and temporally associated with

ice-front retreat.• Late-stage glacial drainage patterns control the distribution of geologic materials.• Igneous rocks in the quadrangle may be derived from at least two sources.• Northeast-trending late Eocene dikes intrude sedimentary bedrock.• Eocene volcanic rocks invaded McIntosh Formation marine sands and silts and are

locally preserved as peperite.

http://www.dnr.wa.gov/AboutDNR/Divisions/GER/

GEOLOGY

Glacial ice and meltwater deposited drift and carved extensive areas of the southern Puget Lowland into a complex geomorphology that provides insight into latest Pleistocene glacial processes. Throughout the map area the many streamlined elongate hills (drumlins) reveal the direction of ice movement. Mima mounds (Washburn, 1988) cover parts of the quadrangle.

During field mapping, lidar imagery was used to interpret landform origins. Water well and geotechnical boring logs were used to characterize the underlying geologic materials and estimate their thicknesses. Water well data modified by Walsh and others (2003b) from Drost and others (1998) was used to create a three-dimensional view of part of the Maytown quadrangle and surrounding area (Fig. 1). Figure 1 shows the thicknesses of Quaternary and bedrock units.

Bedrock is sporadically exposed in the hills in the southern half of the quadrangle. These outcrops are the Eocene McIntosh Formation (unit Emm), which is primarily thinly bedded marine siltstone and sandstone and was mapped and named by Snavely and others (1951, 1958) southeast of the project area. These sedimentary rocks were altered along their contacts with a palagonite-rich volcanic breccia (unit Evcn) that formed when late Eocene lava flowed onto and sank into, or “invaded” the sediments of the McIntosh Formation. The invasive nature of these volcanic rocks can be seen in a roadcut on the west side of Interstate 5 about a mile north of the Maytown exit (the north edge of sec. 5, T16N R2W) where concordant and discordant contacts and a breccia of mixed palagonite, sandstone, and siltstone (peperite) are exposed.

In the central part of the quadrangle, coarse-grained dikes, intermediate in chemical composition between gabbro and diorite (unit Eig; locality 1) cut the McIntosh Formation (unit Emm). The dikes may represent feeder conduits for the invasive volcanic rocks (unit Evcn), but, as noted by Walsh and Logan (2005), the volcanic rocks may be younger than the intrusive rocks.

A rock borrow pit due west of Offutt Lake in the East Olympia quadrangle (Walsh and Logan, 2005) was excavated in a porphyritic volcanic rock with a chilled margin in contact with a coarse-grained mafic intrusive rock type, unit Eig. This unit symbol has been carried over into the Maytown quadrangle to emphasize the chemical similarities between certain rocks in the Maytown quadrangle and the East Olympia quadrangle. Although different in appearance in hand specimens and thin sections, unit Eig in both quadrangles has mineralogy (mainly plagioclase feldspar and clinopyroxene) similar to gabbro and diabase of the Crescent Formation. However, the chemistry of unit Eig is different (Table 1, Fig. 2) (Phillips and others, 1989) from that of Crescent Formation rocks. An Ar/Ar age of 38.76 ±2.50 Ma (Walsh and Logan, 2005) indicates that unit Eig is too young to be Crescent, but it is about the same age as the volcanic rocks of Grays River (Walsh, 1987; Phillips, 1987a) and the Northcraft Formation, both of which are present nearby. As noted in Walsh and Logan (2005), unit Eig rocks are transitional tholeiitic to calc-alkaline, like the volcanic rocks of Grays River and the Northcraft Formation (Fig. 2). Even though unit Eig (locality 2) in the East Olympia quadrangle contains large (1–2 cm) augite crystals similar to those found in some volcanic rocks of Grays River in the Doty Hills, 20 mi southwest of the Maytown quadrangle (Walsh and others, 1987), we suggest the East Olympia and Maytown intrusive rocks (unit Eig) belong to the Northcraft Formation because of their geographic proximity and chemical affinity (Fig. 2).

The Pleistocene history of the Puget Lowland was described in detail by Bretz (1913). He noted a “western lobe” and an “eastern lobe” of the “Puget Sound Glacier” separated by the Black Hills northwest of the map area. Mapping by Logan (2003) and Logan and Walsh (2004) refined the location of the ice margin. Bretz (1913) also recognized an interlobe terrain near the Nisqually River (Fig. 3). This interlobe area, characterized by abundant eskers, kettles, deranged drainage patterns, and generally higher elevations than surrounding ground moraine, was continuous from the Steilacoom area northeast of the Maytown quadrangle (Fig. 4) to the area of maximum ice extent south of the town of Rainier (Fig. 3). The continuity of the interlobe terrain is interrupted only by subsequent outwash channels that dissect the feature near the lower reaches of the Nisqually River valley just east of Lake St. Clair (Fig. 3). Noble and Wallace (1966) referred to the lobes that were separated by the interlobe terrain as the Yelm lobe and the Olympia lobe. Walsh and Logan (2005) described and labeled outwash trains from the Yelm and Olympia lobes in their map of the East Olympia quadrangle, and we use their outwash train designations in the Maytown quadrangle.

At the glacial maximum, about 13,500 yr B.P. (Walsh and others, 2003a), Olympia lobe ice impinged on and partially spilled over the hills north of Tenino (Walsh and Logan, 2005; Bretz, 1913) (Fig. 3). Vashon ice of the Olympia lobe flowed across the entire Maytown quadrangle, leaving ice-scoured, striated till deposits covering older glacial deposits (units Qgp, Qga, and Qgas) and bedrock (units Emm, Evcn, and Eig) on the hills there. The hills (some cored by bedrock and others cored by advance or older glacial deposits) and water well data for the Maytown quadrangle provide evidence that the preglacial geomorphology differed in amount of relief and location of drainages from the relatively flat valleys and low hills that exist there today. As the ice front began to retreat but remained at about the latitude of Chain Hill (Fig. 3), meltwater was directed from the Yelm lobe south and westward through Bretz’s Stony Point channel (east and south of the quadrangle) and across the prairie occupied by Scatter Creek west of Tenino and into the Mima mound– covered Rocky Prairie in the southeast corner of the Maytown quadrangle. Some of the meltwater flowed under stagnant ice southwest of Rocky Prairie (Fig. 3), but when the hills were cleared of ice, meltwater cut its way around many of the hills there and deposited recessional outwash gravels (unit Qgoy3 that originated from the Yelm lobe per Walsh and Logan, 2005) around their bases. Upon further Olympia and Yelm lobe ice-front retreat, later meltwater streams found pathways farther north where units Qgoy4, Qgon3 (Yelm lobe and Nisqually area, respectively, following Walsh and Logan, 2005), and Qgos were deposited. Meltwaters also exposed some advance sediments (unit Qga) in the central map area. Low sand hills formed either as dunes or more likely as ripples orthogonal to meltwater flow directions (unit Qgosr). As the ice front retreated still farther north and glacial Lake Russell (Bretz, 1913) formed, drainage from that lake, ice-marginal streams, and an ancestral Deschutes River flowed southwestward through the Black River depression to the Chehalis River until Lake Russell was no longer impounded. When Lake Russell found a lower outlet into the Strait of Juan de Fuca (via glacial Lake Leland) and abandoned the Black River channel, the Deschutes flowed northward into Puget Sound through the path of least resistance, which was a keeled dead-ice terrain with numerous kettles along the river’s current valley (Figs. 3 and 4).

The stream that deposited unit Qgoy3 was probably fed by a glacial outburst flood (Tanwax flood of Pringle and Goldstein, 2002; Pringle and others, 2000). It was strong enough to carry and deposit boulders, many as large as 3 ft in diameter, in the Deschutes River valley and possibly icebergs or ice calved from the ice front near Offutt Lake. The icebergs were aligned with the southerly turn of the channel and covered or surrounded by unit Qgoy3 to form the concave-to-the-south kettle field that includes Offutt Lake (Fig. 3 and Walsh and Logan, 2005). The floodwaters were blocked by ice that occupied the south half of the Maytown quadrangle, including the western part of Rocky Prairie. As a result, outwash flow was slowed dramatically, forming a temporary lake that caused a delta to prograde westward across Rocky Prairie. As the hydraulic head increased in the Rocky Prairie area, water began flowing beneath the ice in sec. 12, T16N R2W, eventually breaching the ice dam and continuing its flow westward to the Black River and eventually to the Chehalis River (Fig. 3).

The ice blockage of Rocky Prairie and the McCorkle channel was short lived, and so was the outwash flow through those areas. As the glacier melted back to the north near Sheehan Lake (sec. 18, T17N R1W, about a mile east of the Maytown quadrangle), meltwater could then flow northward through to a lower base level, eventually cutting downward through the terrace represented by unit Qgoy4. Ice must have continued to impinge on the hills southeast of Sheehan Lake, causing ice-marginal meltwater from the Nisqually and Lake St. Clair area (unit Qgon4) (Fig. 3) to flow south through sec. 21, T17N R1W, into the Deschutes River valley, eventually leaving unmatched terraces of outwash gravel in that section.

The ice that blocked the flow from the east probably occupied a depression that was a continuation of present-day Budd Inlet (Fig. 3). Evidence of this is a train of kettles that reaches from the modern southern tip of Budd Inlet to at least as far south as Sheehan Lake and probably almost to Offutt Lake. The modern Deschutes River flows through many of these kettles to reach Puget Sound. Kettle trains similar to the one south of Budd Inlet also formed south of Nisqually Reach (forming Lake St. Clair) and south of Henderson Inlet (forming Pattison Lake) as glacial ice occupying these depressions was buried by recessional outwash of the Olympia lobe (Fig. 3). Continued retreat of the ice front allowed ice-marginal streams originating in the Nisqually and Lake St. Clair area and farther east from glacial Lake Puyallup to flow westward across the northern half of the map area, cutting channels through the Vashon ground moraine (unit Qgt) and into underlying advance outwash (unit Qga). As the energy of these streams waned, they eventually deposited sand and gravel, followed by sand and silt (unit Qgos), around the remaining stagnant ice blocks. When the ice melted away, the Budd Inlet, Henderson Inlet, and Nisqually kettle chains were formed.

Soils capping bedrock tend to be reddish brown, but most of those capping glacial outwash deposits are very dark brown to black. The soils covering unit Qgoy3 are formed into mounds about 2 to 6 ft high and 10 to 30 ft across. The mounds are referred to as Mima mounds and have been extensively studied by previous workers (summarized in Washburn, 1988, and discussed in Bretz, 1913). Unit Qgoy3 is the only unit within the Maytown and East Olympia quadrangles on which the Mima mounds formed (Walsh and Logan, 2005; Pringle and Goldstein, 2002). The mounds must have formed very shortly after unit Qgoy3 was deposited because (1) they did not form within kettles, which clearly formed after unit Qgoy3 was deposited (kettles crosscut meltwater channels in unit Qgoy3); (2) they appear to be partially buried by alluvial fans near the margins of the unit Qgoy3 channel; and (3) they did not form in unit Qgoy4 channels that cut the surface of unit Qgoy3. Regionally, Mima mounds are associated with major outwash channels (Fig. 4). Preliminary investigations using lidar imagery and reconnaissance field checking reveal that the oldest outwash train is located south of the Maytown quadrangle, in and to the west of Violet Prairie (Fig. 3). The second oldest is the Qgoy3 train that is probably temporally associated with the Mima Prairie mounds, west of the Maytown quadrangle. A broad area through the Fort Lewis and Muck Creek vicinity is covered by mounds on top of coarse outwash gravels, and sporadic groups of mounds are found farther to the northeast near Spanaway. These mounds are also found in sharply cut channels.

These four major episodes of outwash deposition and mound formation occurred as the ice front retreated from the southern Puget Lowland. If the channels were formed as the result of seasonal floods, then the ice may have retreated from the Grand Mound area (Fig. 3) to Tacoma in about four years. It is possible that the retreat was much faster if the channels were formed by continual outburst flooding during a rapid breakup of the ice along the ice margin. Such rapid ice breakup can be seen today in the Antarctica. We have been unable to locate datable material in these deposits, however, so the chronology of retreat is poorly constrained.

Snavely and others (1958) mapped broad southward-plunging folds in bedrock southeast of the map area. The few attitudes measured on sparse outcrops during this study indicate that the bedrock is gently folded with dips generally less than 10 degrees; however, there was not enough data to define the nature of the folds, nor were any faults noted.

DESCRIPTION OF MAP UNITS

Quaternary Unconsolidated Deposits

HOLOCENE NONGLACIAL DEPOSITS

Fill—Engineered fills of unknown materials associated with Interstate 5; shown only where fill placement is extensive.

Modified land—Soil, sediment, or other geologic material reworked by excavation and (or) redistribution to modify the topography. Only major gravel pits are shown as unit ml, although there are large regraded areas associated with golf courses and housing developments, especially in the northern half of the map area.

Landslide—Silty sandy gravel; generally loose, jumbled; tan to gray; few or no discernible sedimentary structures; surfaces generally undulatory. Most landslides are along steep sides of outwash channels and may have occurred shortly after ice retreated from the map area.

Colluvium—Soil and glacial sand and gravel; deposited by soil creep and shallow raveling on hill slopes; entirely postglacial. The unit is mapped where it is thick enough to mask underlying geologic strata.

Alluvium—Silt, sand, and gravel deposited by streams; includes some lacustrine deposits and organic materials, such as peat. Most alluvium is very fine due to low stream gradients in the map area.

Peat—Peat, muck, silt, and clay in and adjacent to closed depressions, marshes, and wetlands.

Alluvial fan—Silt, sand, and gravel deposited at the confluence of upland streams with outwash channels and terrace edges; commonly capped by dark brown to black, mucky loamy soils.

PLEISTOCENE GLACIAL DEPOSITS

Vashon Recessional Outwash, Nisqually/Lake St. Clair Source

Five trains of recessional outwash from the Nisqually/Lake St. Clair area have been noted by Walsh and Logan (2005). They are, from youngest to oldest, units Qgos, Qgon4, Qgon3, Qgon2, and Qgon1. Only units Qgos and Qgon3 are extensive enough to be mapped in the Maytown quadrangle.

Vashon recessional outwash sand and silt—Sand and silt with minor gravel interbeds; tan to brown; clasts moderately to well rounded; generally well sorted; clasts and grains consist of northern-source plutonic and metamorphic rocks and polycrystalline quartz carried by Vashon ice, and porphyritic volcanic rock from the Cascade Range 60 mi to the east; thickness varies from about 4 to 20 ft. This unit covers much of the north half of the quadrangle and was probably deposited by meltwater derived from stagnant ice south of Lake St. Clair (Fig. 3) and drainage from glacial Lake Puyallup farther east, possibly grading in elevation to glacial Lake Nisqually (Bretz, 1913) and glacial Lake Russell. Fairly small patches of locally derived ice-contact sand are also included in this unit.

Vashon recessional outwash sand and silt, ripples—Sand and silt with minor gravel interbeds; tan to brown; clasts moderately to well rounded; generally well sorted; clasts and grains consist of northern-source plutonic and metamorphic rocks and polycrystalline quartz carried by Vashon ice, and porphyritic volcanic rock from the Cascade Range; varies in thickness from about 4 to possibly greater than 20 ft. When artificially illuminated from the southwest (using lidar), the gently rolling surface of this unit resembles large sand ripples with their long axes perpendicular to the direction of meltwater flow.

Vashon kettle-bottom silt and peat—Silt and peat in flat bottoms of kettles. Some kettle bottoms may consist of sand and gravel with or without a fine cover. The unit is generally found in large-diameter kettles where buried ice must have been thick relative to its depth of burial.

Vashon recessional outwash sand in kettle walls—Sand and minor gravel; tan to gray; moderately to well rounded and sorted; mostly northern-source plutonic and metamorphic rock carried by Vashon ice and minor porphyritic volcanic rock from the Cascade Range. In the northeastern part of the map, where exposed near the Deschutes River valley, this unit consists of gravelly sand and is generally coarser farther upstream. In the southwest corner of the map, the kettles are in gravel and are evidence that ice was probably rafted at least that far by meltwater associated with unit Qgoy3.

Vashon recessional outwash gravel, train 3—Sand and gravel; tan to gray; moderately to well rounded; consists of plutonic and metamorphic clasts transported from the north by Vashon ice and deposited by meltwater. This channel deposit is at a slightly higher elevation than the younger channels containing units Qgon4 in the East Olympia quadrangle and Qgos (Fig. 3). The channel that contains unit Qgon3 (Fig. 3) is nearly horizontal, with a slight westward gradient at its northern end, and was probably formed by meltwater derived from stagnant ice south of Lake St. Clair. This unit was identified from lidar imagery and, in the Maytown quadrangle, the unit is very sandy, caps low till hills, and forms bars.

Vashon Recessional Outwash, Yelm and Olympia Lobe Sources

There are four major trains of recessional outwash from the Yelm lobe (Fig. 3). They are represented, from youngest to oldest, by units Qgoy4, Qgoy3, Qgoy2, and Qgoy1. Meltwater from the Olympia lobe incised into units Qgoy1 and Qgoy2, depositing units Qgoo1 through Qgoo3 (Walsh and Logan, 2005). Subsequent meltwater from the Yelm channel truncated both the earlier Yelm and Olympia lobe outwash channels and deposited units Qgoy3 and Qgoy4. The descriptions below are of those outwash trains found in the Maytown quadrangle.

Vashon recessional outwash, Yelm lobe, Rainier channel, Deschutes River—Loose sand and gravel; grains and clasts well rounded; moderately to well sorted; restricted to channels that cut unit Qgoy3 and forms terraces that grade to an outlet in the northeast corner of the map where, at the time of this unit’s deposition, glacial ice had melted back to a point north of Hartsuck Road (now named 93rd Ave SW). This unit contains fewer boulders than unit Qgoy3. The outwash originated from both the Rainier channel of the Yelm lobe and the upper Deschutes River (Fig. 3).

Vashon recessional outwash, Yelm lobe, Rainier channel, Tanwax Creek–Ohop Valley flood—Bouldery and cobbly sand and gravel; clasts and grains well rounded; moderately to well sorted; capped by silty Mima mounds. Boulders are both Cascade-derived andesite and plutonic and metamorphic rock deposited by Vashon ice. Regional geomorphology indicates that the waters that deposited the unit came from both the Rainier channel of the Yelm lobe (Fig. 3) and the Tanwax Creek–Ohop Valley glacial outburst flood (Fig. 4) (Pringle and Goldstein, 2002; Pringle and others, 2000). This unit traces the path of the Deschutes River when it flowed from the McIntosh Lake area northwestward through Offutt Lake; ice west of Offutt Lake blocked the flow, forcing it south toward Tenino (Fig. 3). The Tenino flow was short lived, and its path is delineated by the Offutt Lake kettle chain. As the ice melted, the prehistoric Deschutes River flowed from Rocky Prairie westward through the area bounded by Beaver Creek in the south to Salmon Creek farther north.

Vashon recessional outwash sand and gravel—Sand and gravel; tan to gray; moderately to well sorted and rounded; consists of plutonic and metamorphic lithic fragments deposited by Vashon meltwater that created outwash channels and terraces of various elevations, but formed after glacial ice retreated. Lithologically, the unit is difficult to distinguish from advance outwash where there are no intervening layers of till; however, it contains no glassy volcanic porphyry from the Cascade Range.

Vashon Drift

Vashon till covered by recessional outwash from the Olympia lobe, Tenalquot Prairie channel—Outwash-modified till; gray; compact; unsorted mixture of clay through boulder-size plutonic and metamorphic rocks deposited by Vashon ice as ground moraine, commonly shaped into drumlins. The unit is covered by gray to tan, northern source–dominated, pebbly sandy gravel deposited by glacial melt- water, probably from the Olympia lobe (Fig. 3); drumlin relief is subdued by the sediment cover. This unit covers a terrace along the southern part of the map area that was probably formed by the same glacial meltwaters that formed the terrace containing unit Qgoo2, based on similarities of surface elevations of the terraces.

Vashon till, dead-ice terrain—Gravelly till associated with eskers and kettles and loose sand and gravel. This unit was deposited beneath stagnant ice at the snout of the Olympia lobe. Angular glacial erratic boulders, most of coarse- grained intrusive rock types, are disseminated on the surface of this unit.

Vashon till, drumlinized ground moraine—Clay, silt, sand, and gravel; gray to tan; unstratified; compact; unsorted; deposited directly by glacial ice; nearly everywhere in sharp contact with underlying units; low permeability and porosity; sand and smaller grains very angular; pebble- to boulder-size clasts commonly striated and faceted; boulders generally disseminated and rare; sporadic interbeds of sand and gravel. The surface of this unit is characterized by streamlined drumlins and striations that are generally hundreds to thousands of feet long. Angular glacial erratic boulders, mostly plutonic, are on the surface of this unit.

Vashon advance outwash—Sand and gravel and lacustrine clay, silt, and sand; gray to light brown; compact; grains well rounded; mostly polycrystalline quartz, plutonic, and minor metamorphic lithic grains; deposited during Vashon glacial advance. The unit is most easily distinguished from recessional outwash where Vashon till lies between the outwash units. This unit may also contain pre-Vashon drift (unit Qgp) at its base, but poor outcrop due to raveled terrace scarps prevented differentiation of these units.

Vashon advance sandy outwash—Sand and silt with minor gravel immediately underlying unit Qgt; grains well rounded; mostly polycrystalline quartz, plutonic, and minor metamorphic lithic grains; deposited during Vashon glacial advance. The unit is exposed only in the southern part of the map area.

Pre-Vashon Drift

Pre-Vashon drift—Sand and gravel with some clay and silt; light brown or gray to yellowish brown to brick red; generally compact; clasts well rounded; dominated by northern-source polycrystalline quartz, plutonic, and minor metamorphic grains. The unit crops out in an area near the southern margin of the quadrangle that was mapped by Noble and Wallace (1966) as Logan Hill Formation, but the purely northern provenance makeup of the clasts indicates that this may be Double Bluff Drift (Easterbrook and others, 1967; Lea, 1984).

Tertiary Sedimentary and Igneous Rocks

Northcraft Formation? (late Eocene)—Volcanic breccia; dark-gray to black, palagonitic groundmass; fragments mostly euhedral; common twinned and zoned plagioclase phenocrysts and euhedral twinned augite phenocrysts in glassy matrix with minor ghost grains of another mineral, possibly an amphibole. The unit is in concordant and discordant contact with volcanic lithic sandstone and siltstone of the McIntosh Formation (unit Emm) and has thermally altered the sedimentary rocks at the few contacts observed during mapping. In the Interstate 5 roadcut near Maytown, palagonite, sandstone, and siltstone form a chaotically mixed breccia (peperite) characteristic of lava flows exploding in wet sediment. Such timing would suggest that the invasive rocks are part of the Northcraft Formation.

Intrusive rocks (late Eocene)—Gabbroic to dioritic dikes (basaltic and basaltic andesite composition); black to dark gray-green, ophitic, subophitic, and inter- granular; coarse-grained. Weakly pink to green pleochroic augite makes up about 40% of the rock with an equal amount of oligoclase feldspar grains; an opaque iron-oxide mineral with graphic texture comprises about 15%. Secondary minerals include disseminated heulandite and veins of calcite. Table 1 shows that the unit’s chemistry is fairly silicic despite the abundance of augite. Chemical comparisons are illustrated in Figure 2. An Ar/Ar age of 38.76 ±2.50 Ma obtained by Walsh and Logan (2005) in correlative rocks to the east and the fact that these rocks appear to intrude the McIntosh Formation indicate that they are too young to be Crescent Formation and are more likely to belong to the Northcraft Formation or a correlative of the Grays River volcanics (Walsh and Logan, 2005). However, no exposures of contacts between the dikes and the McIntosh Formation or with the volcanic invasive rocks were observed during this study.

McIntosh Formation (late middle Eocene)—Volcanic lithic marine sandstone and siltstone; fine- to medium-grained; gray-green; most grains angular or euhedral twinned and (or) zoned feldspar; green to colorless amphibole grains commonly disseminated throughout the rock; grains typically in a dirty brownish yellow, fine-grained matrix with opaque and altered minerals. Although no macrofossils were found in the Maytown quadrangle, the unit contains Tertiary shallow-water benthic Foraminifera that are not age-diagnostic (W. W. Rau, Washington Division of Geology and Resources, retired, oral commun., 1983). Snavely and others (1951) mapped this unit to the southeast near Centralia, 11 mi south of the quadrangle. The unit’s thickness in the map area is unknown.

GEOLOGIC SYMBOLS

Contact

Whole rock geochemistry sample location—Number is (loc. no.) under ‘Sample no.’ in Table 1

Inclined bedding—Showing strike and dip

ACKNOWLEDGMENTS

This project was made possible by the U.S. Geological Survey National Geologic Mapping Program under award no. 07HQAG0142. Geochemical analyses were performed at the Washington State University GeoAnalytical Laboratory. We thank Kitty Reed and Jari Roloff for their editorial reviews of this report and Anne Heinitz, Eric Schuster, Chuck Caruthers, and Liz Thomson for their assistance with cartographic issues.

REFERENCES CITED

Bretz, J H., 1913, Glaciation of the Puget Sound region: Washington Geological Survey Bulletin 8, 244 p., 3 plates.

Drost B. W.; Turney, G. L.; Dion, N. P.; Jones, M. A., 1998, Hydrology and quality of ground water in northern Thurston County, Washington: U.S. Geological Survey Water-Resources Investigations Report 92-4109 [Revised], 230 p.

Easterbrook, D. J.; Crandell, D. R.; Leopold, E. B., 1967, Pre-Olympia Pleistocene stratigraphy and chronology in the central Puget Lowland, Washington: Geological Society of America Bulletin, v. 78, no. 1, p. 13-20.

Johnson, D. M.; Hooper, P. R.; Conrey, R. M., 1998, XRF analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead: Washington State University GeoAnalytical Laboratory, 25 p.

Le Maitre, R. W. (editor); Streckeisen, A.; Zanettin, B.; Le Bas, M. J.; Bonin, B.; Bateman, P.; Bellieni, G.; Dudek, A.; Efremova, S.; Keller, J.; Lamere, J.; Sabine, P. A.; Schmid, R.; Sorensen, H.; Woolley, A. R., 2002, Igneous Rocks—A classification and glossary of terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks: Cambridge University Press, 2nd ed., 236 p.

Lea, P. D., 1984, Pleistocene glaciation at the southern margin of the Puget lobe, western Washington: University of Washington Master of Science thesis, 96 p., 3 plates.

Logan, R. L., 2003, Geologic map of the Shelton 1:100,000 quadrangle, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-15, 1 sheet, scale 1:100,000. [http://www.dnr.wa.gov/geology/pdf/ofr03-15.pdf]

Logan, R. L.; Walsh, T. J., 2004, Geologic map of the Summit Lake 7.5-minute quadrangle, Thurston and Mason Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2004-10, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/Publications/ger_ ofr2004-10_geol_map_summitlake_24k.pdf]

Mullen, E. D., 1983, MnO/TiO2/P2O5—A minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis: Earth and Planetary Science Letters, v. 62, p. 53-62.

Noble, J. B.; Wallace, E. F., 1966, Geology and ground-water resources of Thurston County, Washington; Volume 2: Washington Division of Water Resources Water-Supply Bulletin 10, v. 2, 141 p., 5 plates.

Phillips, W. M., 1987a, Geochemistry and areal extent of the Grays River volcanic rocks, southwest Washington and adjacent Oregon [abstract]: Eos (American Geophysical Union Transactions), v. 68, no. 52, p. 1815.

Phillips, W. M., compiler, 1987b, Geologic map of the Mount St. Helens quadrangle, Washington and Oregon: Washington Division of Geology and Earth Resources Open File Report 87-4, 59 p., 1 plate, scale 1:100,000.

Phillips, W. M.; Walsh, T. J.; Hagen, R. A., 1989, Eocene transition from oceanic to arc volcanism, southwest Washington. In Muffler, L. J. P.; Weaver, C. S.; Blackwell, D. D., editors, Proceedings of workshop XLIV—Geological, geophysical, and tectonic setting of the Cascade Range: U.S. Geological Survey Open-File Report 89-178, p. 199-256.

Pringle, P. T.; Goldstein, B. S., 2002, Deposits, erosional features, and flow characteristics of the late- glacial Tanwax Creek–Ohop Creek Valley flood—A likely source for sediments composing the Mima mounds, Puget Lowland, Washington [abstract]: Geological Society of America Abstracts with Programs, v. 34, no. 5, p. A-89.

Pringle, P. T.; Goldstein, B. S.; Anderson, N. R., 2000, Tanwax Creek–Ohop Valley late-glacial flood—Evidence that discharge from an ice-dammed lake in the Carbon River Valley was augmented by a temporary landslide dam, Puget Lowland, Washington [abstract]. In Washington Department of Ecology; Washington Hydrologic Society; U.S. Geological Survey, Program and abstracts from the 3rd symposium on the hydrogeology of Washington State: Washington Department of Ecology, p. 85.

Snavely, P. D., Jr.; Brown, R. D., Jr.; Roberts, A. E.; Rau, W. W., 1958, Geology and coal resources of the Centralia–Chehalis district, Washington, with a section on Microscopical character of Centralia–Chehalis coal, by J. M. Schopf: U.S. Geological Survey Bulletin 1053, 159 p., 6 plates.

Snavely, P. D., Jr.; Rau, W. W.; Hoover, Linn, Jr.; Roberts, A. E., 1951, McIntosh Formation, Centralia– Chehalis coal district, Washington: American Association of Petroleum Geologists Bulletin, v. 35, no. 5, p. 1052-1061.

Walsh, T. J., compiler, 1987, Geologic map of the Astoria and Ilwaco quadrangles, Washington and Oregon: Washington Division of Geology and Earth Resources Open File Report 87-2, 28 p., 1 plate.

Walsh, T. J.; Korosec, M. A.; Phillips, W. M.; Logan, R. L.; Schasse, H. W., 1987, Geologic map of Washington—Southwest quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-34, 2 sheets, scale 1:250,000, with 28 p. text.

Walsh, T. J.; Logan, R. L.; Polenz, Michael; Schasse, H. W., 2003a, Geologic map of the Nisqually 7.5-minute quadrangle, Thurston and Pierce Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-10, 1 sheet, scale 1:24,000. [http://www. dnr.wa.gov/geology/pdf/ofr03-10.pdf]

Walsh, T. J.; Logan, R. L.; Schasse, H. W.; Polenz, Michael, 2003b, Geologic map of the Tumwater 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-25, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/ Publications/ger_ofr2003-25_geol_map_tumwater_24k.pdf]

Walsh, T. J.; Logan, R. L., 2005, Geologic map of the East Olympia 7.5-minute quadrangle, Thurston County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-56, 1 sheet, scale 1:24,000. [http://www.dnr.wa.gov/Publications/ger_gm56_geol_ map_eastolympia_ 24k.pdf]

Washburn, A. L., 1988, Mima mounds—An evaluation of proposed origins with special reference to the Puget lowlands: Washington Division of Geology and Earth Resources Report of Investigations 29, 53 p.

38

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Tritium Spike 1.Control Chart! Pro Statistics for file: F:\SPIKES\SPIKE1 .CCP n =15 Mean = 43.0773 Std Dev = 6.6850 %CV = 15.52 Min = 31.4600 Max = 51.3700

Tritium Spike 1.Control Chart! Pro Statistics for file: F:\SPIKES\SPIKE1 .CCP n =15 Mean = 43.0773 Std Dev = 6.6850 %CV = 15.52 Min = 31.4600 Max = 51.3700

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