WASHINGTON GEOLOGICAL SURVEY MAP SERIES 2017-01 Geologic Map of the Littlerock 7.5-minute Quadrangle GEOLOGIC MAP OF THE LITTLEROCK 7.5-MINUTE QUADRANGLE, Geologic Map of the Littlerock 7.5-minute Quadrangle, Pamphlet accompanies map 123°07ƍ30Ǝ R4W R3W 05ƍ00Ǝ 02ƍ30Ǝ 123°00ƍ00Ǝ 47°00ƍ00Ǝ 47°00ƍ00Ǝ Thurston County, Washington Evc Qgt Evc Qgt Qaf Qls Qgo Qgog Qgo THURSTON COUNTY, WASHINGTON G2 Evc Evc Qgt Michael Polenz, Jessica L. Vermeer, Gabriel Legorreta Paulín, Evc ? ? Qls T18N Download the G40-41,48 Qa Qoa Evc ? Evc Qgaf Jeffrey H. Tepper, Shannon A. Mahan, and Recep Cakir T18N Qa T17N Qgo T17N Qgo? Evc Qaf Qgic Qgog Qgt ? G10 Qa June 2017 Evc Qgo ? publication ? Qaf C19-20 G14 Qgic Michael Polenz—Washington Geological Survey, Department of Natural Resources Evc Qgo ? Qgog? Qls Qa Qa G3 ? Qoa Qgic C26 Qgic Jessica L. Vermeer—Washington Geological Survey, Department of Natural Resources Qgoaf Evc Qaf Qgag C4 Qoa Evc QEcf QEcf Qgo Qgic Gabriel Legorreta Paulín—Universidad Nacional Autónoma de México, Ciudad Universitaria Del Coyoacán Qls Qa ? S1 Qaf MAJOR FINDINGS GEOLOGIC SYMBOLS G34 ? Qgoaf Qgog Jeffrey H. Tepper—University of Puget Sound G58-59 Qgo • A new ~47.4 Ma laser-ablation U-Pb date on zircon from a slightly reworked airfall tuff provides a close maximum Mass-wasting landforms (overlay pattern)—Landforms that suggest mass movement on unstable Magnetic 47°02' Evc ? Qaf Qgt limiting age for the McIntosh Formation. anomalies 122 Qge Qgo? slopes, but evidence for landslide deposits is inconclusive. Evc Qaf Qp (nanoteslas) Evc Qgo 40 39 ° Shannon A. Mahan—U.S. Geological Survey, Denver Federal Center Qaf Qgo • A new 46 Ma Ar/ Ar date suggests that basalt east of the Black Hills is younger than the Crescent Formation in 45 ? Mima mounds (overlay pattern)—Dark brown to black, organic-rich sand to pebbly sand loam soil in 15.1 ' Qgo the Black Hills and older than gabbroic intrusive rocks east of the Littlerock quadrangle. Chemical similarities 35 Qaf Evc Qgic 8.6 regularly spaced round to oval mounds about 2 to 6 ft high and 10 to 30 ft across, with little or no " Qa suggest that all of these volcanic rocks shared the same mantle source(s). 5.2 ? Qge Qp Recep Cakir—Washington Geological Survey, Department of Natural Resources G56-57 G13 Qgic comparable soil between mounds. The black color is mostly due to ash from burned organic matter. 3.1 Evc Qaf Qge G12 • Ice covered the east half of the Littlerock quadrangle during the Vashon glaciation. It was preceded by at least one NTRMounds rest on Vashon proglacial outwash terraces with a highly permeable substrate of mostly 1.7 0.8 Qge Qoa Qaf ice incursion that reached higher elevations farther west in the Black Hills. Each ice incursion appears to have northern-sourced pebbles and cobbles. Mapped where landforms and (or) field-observed deposits indicate 0.1 Evc Qp Qgag ? Qp Qgo impounded a south-draining lake in the Waddell Creek valley, with an earlier lake reaching at least 800 ft elevation the presence of such mounds. Their enigmatic origin is discussed below (Mima Mounds in -0.7 Qa -1.3 Qaf Qpd and the Vashon lake reaching close to 465 ft. Qgo Paleoenvironmental Interpretation section). -2.0 Qp -2.8 ? G16 Qgt ? Qaf Qgim • Geochemical traits separate Olympic Mountains–style Crescent basalt in the southern third of Littlerock quadrangle -3.9 C31 Qgic Qgt Qa Qge -5.5 ? from Willapa Hills–style basalt farther north, likely due to separate eruptive centers. Field relations suggest that the Contact—solid where location accurate; long-dashed where approximate; short-dashed ? -8.2 Evc Qge southern basalts are stratigraphically above those farther north. where inferred -18! ? Evc -14.3 Evc -68,-31 ? Qgag • Although Possession ice (MIS 4) never came close to the Littlerock quadrangle, new luminescence dates on fluvial U -269 ! Qoa Qa ? ? High-angle dip-slip fault—inferred; identity or existence questionable, location ! ! ! A deposits imply that Possession meltwater, like Vashon meltwater, drained through the Black River valley, D Aero- -4,-67 ? QEcf Qgic concealed; relative motion shown by U and D -14 ! Evc Qls ? suggesting that the pre-Vashon landscape was similar to the modern southern Puget Lowland. magnetic 17,66 ! Qgt Abstract—No. 387-2 (297674): LITTLEROCK QUADRANGLE -130 A new 47.4 Ma U/Pb zircon date from tuff in McIntosh Fm. marine sedimenta- Qge max spots 58,64 Qge Qa ? Geologic unit too thin to show as a polygon—solid where location accurate; Qp strong QEcf G54-55 long-dashed where approximate; short-dashed where inferred; queried where identity Qp ? ry rocks that overlie Crescent Fm. basalt in the southern Black Hills constrains Qge or existence questionable ? DESCRIPTION OF MAP UNITS We present a new 1:24,000 scale geologic map of the Littlerock 7.5-minute Qoa ? Qge Qgog Qgic weak 78 C21 Evc Qgic (see pamphlet for detailed map unit descriptions) ? ? Continental ice limit, late Wisconsinan—solid where location accurate; long-dashed Evc C30 Qge the transition from Siletzia volcanism and accretion to marine sedimentation. Qpd ? Quaternary Unconsolidated Deposits where approximate; short-dashed where inferred; dotted where concealed; queried quadrangle, which straddles the eastern Black Hills and southern Puget G52-53 Bedrock Qge ? Ev where identity or existence questionable elevation (ft) Qoa 8 Qpd c Qaf Qgic? POSTGLACIAL DEPOSITS -18 Qp Qge Qge !-18 seismic C Qgic ? Continental ice limit, pre–late Wisconsinan—long-dashed where approximate; o Lowland southwest of Olympia and was previously mapped at smaller Qp Holocene Nonglacial Deposits survey a Evc Qge l short-dashed where inferred; queried where identity or existence questionable C Qaf Qp Qp Qa ? C28 Modified land—Boulders, cobbles, pebbles, sand, silt and clay in varied amounts; locally derived, but -14 water re We mapped glacial deposits from the Vashon stade and at least one prior gla- e C22 Qa Qa Qaf ml AAƍ Cross section line well k scales. More detailed geologic mapping is needed for informed land man- Qpd C29 ? redistributed to modify topography. Qoa Qp Qp F Evc C5 Qge a G38-39, W1 Qpo1 ? Fluvial terrace—solid where location accurate; short-dashed where inferred; queried where uult 47 W3 Qp Evc K lt ciation that was slightly more extensive; these glaciations created ice Evc Evc Peat—Organic and organic-rich sediment; includes peat, gyttja, muck, silt, and clay; typically in closed 57ƍ30Ǝ S2 Qp 57ƍ30Ǝ identity or existence questionable; hachures point down slope Mima Prairie o agement; forestry, agriculture, recreation, ecologically sensitive areas, and ? G15 Qp p C52 Qoa depressions. basin ia Ev G11 Qaf h Evc c Geophysical data collection line F N Qa ? Qpd a QEcf W2 QEcf S u e dammed lakes in the Waddell Creek valley. New OSL and IRSL dates on fluvial ? Qp c w W4 Qp Evc Landslide deposits—Cobbles, pebbles, sand, silt, clay, boulders, and diamicton of basalt, siltstone, and a lltt development increasingly compete for natural resources. We support the Qoa Qa Qls ? Former shoreline or marine limit—identity or existence questionable, location accurate m a C34 ? Qge sandstone in varied amounts; clasts angular to rounded; unsorted; mostly loose, jumbled, and unstratified; m u ? Evc Qp o k 42 Inclined bedding—showing strike and dip n u ? locally retains primary bedding and compaction. Fault C m sediments suggest that Possession (MIS4) meltwater drained south via the Qge ? ? r ? Qp e F new map with field work, lidar, subsurface exploration data, aeromagnetic Small, minor inclined joint—showing strike and dip " ek Qp 79 F a 20 u Pleistocene to Holocene Nonglacial Deposits ' a l Qgo Qoa Qgag uultl t 40 39 Qa Qgim Small, minor vertical or near-vertical joint—showing strike 10 t Black River Valley, although Possession ice never came close to the map area. Qp ° data, geophysical explorations, geochemistry, and new Ar/ Ar, detrital Qpd Alluvium—Pebbles, cobbles, sand, silt, clay, peat, and boulders in varied amounts; along streams and on ? Qp Qgo Qoa Qge Qa Qoa Inclined asymmetric (Z-shaped, clockwise sense of shear) minor fold hinge—showing bearing and plunge 123 Qgic flood plains; gray, or dark brown due to organic matter, locally iron-stained red or yellowish brown; loose; 8 C45 ? thinly bedded to massive; unit Qoa where relict. Inclined flow bands, lamination, layering, or foliation in igneous rock—showing strike and dip 46°40' zircon U/Pb, luminescence and radiocarbon dates. Qp W5 15 Qp Qgt 0 4 8 mi Alluvial fan deposits—Pebbles, sand, silt, cobbles, and boulders in varied amounts; gray to brown; loose; 90 Shear—showing strike and dip Geochemistry and clay mineral identification by XRD of thick, deeply weath- ? Qp Qgic Qaf Qp Qgo ? W6 subangular to rounded; moderately to poorly sorted; stratified to poorly stratified; form concentric lobes. Qa 55 Shear—showing strike Qgo Qp Figure M2. Aeromagnetic image of the Littlerock quadrangle and vicinity. Faults are from Walsh and others (1987). Max spots mark New bedrock geochemistry confirms that the Black Hills are cored by Cres- Qp ? W10 Qgic GD9 W14 inflections in the aeromagnetic anomalies and tend to form linear clusters (that commonly coincide with topographic lineaments). Mima QEcf ? W15 Age site, fossil ered, clay-rich soils in the Black Hills suggest that most of the Black Hills are Qp W7 Evc Prairie basin (informally named here) is delineated based on an aeromagnetic trough. No measurements were made where the Ev W9 PLEISTOCENE GLACIAL DEPOSITS c Qp GD4 magnetic trough is strongest, and basin depth likely exceeds the lowest estimated levels from our seismic depth stations (82 m [270 ft] Qp W13 Qgo Age site, U-Pb, uranium-lead cent Formation basalt.