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Marvin A. Lanphere

Publications and source records attributed to Marvin A. Lanphere.

At least 19 recordsLinked to original sources
USGS2014

Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska

We have identified six major belts and two nonbelt occurrences of plutonic rocks in Glacier Bay National Park and Preserve and characterized them on the basis of geologic mapping, igneous petrology, geochemistry, and isotopic dating. The six plutonic belts and two other occurrences are, from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous (145.5–99.6 Ma) granodiorite and tonalite of the Muir-Chichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet (65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, quartz diorite, and granite of the Muir-Fairweather felsic-intermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) layered gabbros of the Crillon-La Perouse mafic belt; and (8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz syenite of the Tkope belt. The rocks are further classified into 17 different combination age-compositional units; some younger belts are superimposed on older ones. Almost all these plutonic rocks are related to Cretaceous and Tertiary subduction events. The six major plutonic belts intrude the three southeast Alaska geographic subregions in Glacier Bay National Park and Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr Inlet Suture Zone (which contains the Border Ranges Fault Zone), and (3) the Central Alexander Archipelago. Each subregion includes rocks assigned to one or more tectonic terranes. The various plutonic belts intrude different terranes in different subregions. In general, the Early Cretaceous plutons intrude rocks of the Alexander and Wrangellia terranes in the Central Alexander Archipelago subregion, and the Paleogene plutons intrude rocks of the Chugach, Alexander, and Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture Zone, and Central Alexander Archipelago subregions.

Alaska↗
USGS2013

Geologic map of the east half of the Lime Hills 1:250,000-scale quadrangle, Alaska

This map is compiled from geologic mapping conducted between 1985 and 1992 by the U.S. Geological Survey as part of the Alaska Mineral Resource Assessment Program. That mapping built upon previous USGS work (1963–1988) unraveling the magmatic history of the Alaska–Aleutian Range batholith. Quaternary unit contacts depicted on this map are derived largely from aerial-photograph interpretation. K-Ar ages made prior to this study have been recalculated using 1977 decay constants. The east half of the Lime Hills 1:250,000-scale quadrangle includes part of the Alaska–Aleutian Range batholith and several sequences of sedimentary rocks or mixed sedimentary and volcanic rocks. The Alaska–Aleutian Range batholith contains rocks that represent three major igneous episodes, (1) Early and Middle Jurassic, (2) Late Cretaceous and early Tertiary, and (3) middle Tertiary; only rocks from the latter two episodes are found in this map area. The map area is one of very steep and rugged terrain; elevations range from a little under 1,000 ft (305 m) to 9,828 ft (2,996 m). Foot traverses are generally restricted to lowermost elevations. Areas suitable for helicopter landings can be scarce at higher elevations. Most of the area was mapped from the air, supplemented by direct examination of rocks where possible. This restricted access greatly complicates understanding some of the more complex geologic units. For example, we know there are plutons whose compositions vary from gabbro to granodiorite, but we have little insight as to how these phases are distributed and what their relations might be to each other. It is also possible that some of what we have described as compositionally complex plutons might actually be several distinct intrusions.

Alaska↗
USGS2013

Paleomagnetic correlation and ages of basalt flow groups in coreholes at and near the Naval Reactors Facility, Idaho National Laboratory, Idaho

Paleomagnetic inclination and polarity studies were conducted on subcore samples from eight coreholes located at and near the Naval Reactors Facility (NRF), Idaho National Laboratory (INL). These studies were used to characterize and to correlate successive stratigraphic basalt flow groups in each corehole to basalt flow groups with similar paleomagnetic inclinations in adjacent coreholes. Results were used to extend the subsurface geologic framework at the INL previously derived from paleomagnetic data for south INL coreholes. Geologic framework studies are used in conceptual and numerical models of groundwater flow and contaminant transport. Sample handling and demagnetization protocols are described, as well as the paleomagnetic data averaging process. Paleomagnetic inclination comparisons among NRF coreholes show comparable stratigraphic successions of mean inclination values over tens to hundreds of meters of depth. Corehole USGS 133 is more than 5 kilometers from the nearest NRF area corehole, and the mean inclination values of basalt flow groups in that corehole are somewhat less consistent than with NRF area basalt flow groups. Some basalt flow groups in USGS 133 are missing, additional basalt flow groups are present, or the basalt flow groups are at depths different from those of NRF area coreholes. Age experiments on young, low potassium olivine tholeiite basalts may yield inconclusive results; paleomagnetic and stratigraphic data were used to choose the most reasonable ages. Results of age experiments using conventional potassium argon and argon-40/argon-39 protocols indicate that the youngest and uppermost basalt flow group in the NRF area is 303 ± 30 ka and that the oldest and deepest basalt flow group analyzed is 884 ± 53 ka. A south to north line of cross-section drawn through the NRF coreholes shows corehole-to-corehole basalt flow group correlations derived from the paleomagnetic inclination data. From stratigraphic top to bottom, key results include the following: * The West of Advanced Test Reactor Complex (ATRC) flow group is the uppermost basalt flow group in the NRF area and correlates among seven continuously cored holes in this study under surficial sediments. The West of ATRC flow group is also found in coreholes near the ATRC, the Idaho Nuclear Technology and Engineering Center (INTEC), and in corehole USGS 129. * The ATRC Unknown Vent flow group correlates among seven continuously cored holes in this study underlying the West of ATRC flow group and a sedimentary interbed. Additional paleomagnetic inclination and stratigraphic data derived from the NRF coreholes changed the previously reported interpretation of the subsurface distribution of this basalt flow group. The ATRC Unknown Vent flow group also is found in coreholes near the ATRC and INTEC. * The Central Facilities Area (CFA) Buried Vent flow group correlates among all eight coreholes in the NRF area. It also is found in coreholes near the CFA and the Radioactive Waste Management Complex (RWMC) to the south. This basalt flow group is thickest near the CFA, which may indicate proximity to the vent. The State Butte flow group is found below the CFA Buried Vent flow group in the four northern NRF coreholes. It correlates to the State Butte surface vent located just northeast of the NRF. It is not found in coreholes south of the NRF. * The Atomic Energy Commission (AEC) Butte flow group is found in coreholes USGS 133, NRF 6P, and NRF 7P. It probably underlies coreholes NRF B18-1, NRF 89-05, and NRF 89-04, but those coreholes were not drilled deeply enough to penetrate the flow group. The AEC Butte flow group vent is exposed at the surface near the ATRC, and its flows are found in many coreholes near the ATRC and INTEC. The AEC Butte flow group abruptly pinches out against the Matuyama Chron reversed polarity flows of the East Matuyama Middle flow group between coreholes NRF 7P and NRF 15. * The East Matuyama Middle flow group correlates between coreholes NRF 15 and NRF 16 and may correlate to coreholes NPR Test/W-02 and ANL-OBS-A-001. * The North Late Matuyama flow group correlates among coreholes USGS 133, NRF 6P, NRF 7P, NRF 15, and NRF 16. It probably underlies coreholes NRF B18-1, NRF 89-05, and NRF 89-04, but those coreholes were not drilled deeply enough to penetrate the flow group. The vent that produced the North Late Matuyama flow group may be located in the general NRF area because it is thickest near corehole NRF 6P. * The Matuyama flow group is found in coreholes in the southern INL from south of the RWMC to corehole USGS 133 and may extend north to corehole NRF 15. The Matuyama flow group is thickest near the RWMC and thins to the north. * The Jaramillo (Matuyama) flow group is found in corehole NRF 15, which is the deepest NRF corehole, and shows that the basalt flow group is thick in the subsurface at NRF. This flow group is thickest between the RWMC and INTEC and thins towards the ATRC and NRF.

Idaho↗
USGS2012

Paleomagnetism and K-Ar ages of volcanic rocks from Long Valley caldera, California

Paleomagnetic measurements and K‐Ar age determinations on volcanic rocks from Long Valley caldera, California, have enabled further refinement of eruptive activity within this large silicic volcanic center. K‐Ar age determinations show that postcaldera volcanic eruptions began 0.73 m.y. ago and continued periodically until about 50,000 years ago. The eruptions were not temporally random but tended to occur in distinct episodes separated by periods of quiescence. Volcanism in the western half of the caldera was particularly intense between 0.15 and 0.50 m.y. ago, when many units ranging in composition from basalt to rhyolite were erupted. An average of the virtual geomagnetic poles for 33 units from the caldera yields a paleomagnetic pole at 89.7°N, 138.4°E, (α 95 = 5.1°), which is indistinguishable from the earth's rotational axis. The ancient geomagnetic field dispersion about this mean pole is 16.0°, with upper and lower limits of 19.3° and 13.6°, respectively.

California↗
USGS2011

Paleomagnetic correlation of surface and subsurface basaltic lava flows and flow groups in the southern part of the Idaho National Laboratory, Idaho, with paleomagnetic data tables for drill cores

Paleomagnetic inclination and polarity studies have been conducted on thousands of subcore samples from 51 coreholes located at and near the Idaho National Laboratory. These studies are used to paleomagnetically characterize and correlate successive stratigraphic intervals in each corehole to similar depth intervals in adjacent coreholes. Paleomagnetic results from 83 surface paleomagnetic sites, within and near the INL, are used to correlate these buried lava flow groups to basaltic shield volcanoes still exposed on the surface of the eastern Snake River Plain. Sample handling and demagnetization protocols are described as well as the paleomagnetic data averaging process. Paleomagnetic inclination comparisons between coreholes located only kilometers apart show comparable stratigraphic successions of mean inclination values over tens of meters of depth. At greater distance between coreholes, comparable correlation of mean inclination values is less consistent because flow groups may be missing or additional flow groups may be present and found at different depth intervals. Two shallow intersecting cross-sections, A-A- and B-B- (oriented southwest-northeast and northwest-southeast, respectively), drawn through southwest Idaho National Laboratory coreholes show the corehole to corehole or surface to corehole correlations derived from the paleomagnetic inclination data. From stratigraphic top to bottom, key results included the (1) Quaking Aspen Butte flow group, which erupted from Quaking Aspen Butte southwest of the Idaho National Laboratory, flowed northeast, and has been found in the subsurface in corehole USGS 132; (2) Vent 5206 flow group, which erupted near the southwestern border of the Idaho National Laboratory, flowed north and east, and has been found in the subsurface in coreholes USGS 132, USGS 129, USGS 131, USGS 127, USGS 130, USGS 128, and STF-AQ-01; and (3) Mid Butte flow group, which erupted north of U.S. Highway 20, flowed northwest, and has been found in the subsurface at coreholes ARA-COR-005 and STF-AQ-01. The high K20 flow group erupted from a vent that may now be buried south of U.S. Highway 20 near Middle Butte, flowed north, and is found in the subsurface in coreholes USGS 131, USGS 127, USGS 130, USGS 128, USGS 123, STF-AQ-01, and ARA-COR-005 ending near the Idaho Nuclear Technology and Engineering Center. The vent 5252 flow group erupted just south of U.S. Highway 20 near Middle and East Buttes, flowed northwest, and is found in the subsurface in coreholes ARA-COR-005, STF-AQ-01, USGS 130, USGS 128, ICPP 214, USGS 123, ICPP 023, USGS 121, USGS 127, and USGS 131. The Big Lost flow group erupted from a now-buried vent near the Radioactive Waste Management Complex, flowed southwest to corehole USGS 135, and northeast to coreholes USGS 132, USGS 129, USGS 131, USGS 127, USGS 130, STF-AQ-01, and ARA-COR-005. The AEC Butte flow group erupted from AEC Butte near the Advanced Test Reactor Complex and flowed south to corehole Middle 1823, northwest to corehole USGS 134, northeast to coreholes USGS 133 and NRF 7P, and south to coreholes USGS 121, ICPP 023, USGS 123, and USGS 128. Evidence of progressive subsidence of the axial zone of the ESRP is shown in these cross-sections, distorting the original attitudes of the lava flow groups and interbedded sediments. A deeper cross-section, C-C- (oriented west to east), spanning the entire southern Idaho National Laboratory shows correlations of the lava flow groups in the saturated part of the ESRP aquifer. Areally extensive flow groups in the deep subsurface (from about 100-800 meters below land surface) can be traced over long distances. In cross-section C-C-, the flow group labeled "Matuyama" can be correlated from corehole USGS 135 to corehole NPR Test/W-02, a distance of about 28 kilometers (17 miles). The flow group labeled "Matuyama 1.21 Ma" can be correlated from corehole Middle 1823 to corehole ANL-OBS-A-001, a distance of 26 kilometers (16 miles). Other flo

Idaho↗
USGS2010

Tertiary volcanic rocks from the southern Pannonian Basin, Croatia

The Tertiary volcanic rocks of the southern part of the Pannonian Basin are related to a sequence of tectonic events that occurred along the northern margin of the Dinarides when subduction ceased and after uplift of the region in middle Eocene time. The oldest rocks are andesites and dacites erupted during an Egerian to Eggenburgian marine transpression phase. In early Miocene time (about 17 Ma), rifting started, probably caused by the rise of the mantle, resulting in E-W extension and strike-slip faulting leading to formation of the Pannonian Basin. The initial rifting was accompanied by trachyandesitic volcanism of Karpathian age, followed by Badenian-age (15 to 13 Ma) volcanic activity producing basalts, andesites, and dacites. During these volcanic phases the Pannonian Basin, or the Paratethys, probably was periodically connected with the Mediterranean Sea. The final period of volcanism (about 9 to 7 Ma) produced alkalic basalts and basalts and occurred in fresh-water environments. These volcanic rocks have had a complex petrological evolution, related to an active continental margin. Egerian-Eggenburgian andesites and dacites can be explained by upper crustal assimilation with fractional crystallization (ACF) of primary basalt magma. The three younger associations originated by partial melting of rocks from both the upper mantle and the lower crust. Their evolution was accompanied by crustal contamination.

Carpathian Basin, Pannonian Basin↗
USGS2008

The Pleistocene eruptive history of Mount St. Helens, Washington, from 300,000 to 12,800 years before present

We report the results of recent geologic mapping and radiometric dating that add considerable detail to our understanding of the eruptive history of Mount St. Helens before its latest, or Spirit Lake, stage. New data and reevaluation of earlier work indicate at least two eruptive periods during the earliest, or Ape Canyon, stage, possibly separated by a long hiatus: one about 300-250 ka and a second about 160–35 ka. Volcanism during this stage included eruption of biotite- and quartz-bearing dacite domes and pyroclastic flows in the area west of and beneath the present-day edifice, accompanied by the deposition of set C tephras. Ape Canyon-stage rocks are compositionally similar to younger Mount St. Helens dacite. The Cougar stage, about 28-18 ka, was probably the most active eruptive stage in Mount St. Helens’ history before the Spirit Lake stage. During the Cougar stage, a debris avalanche buried the area south of the present-day edifice, and voluminous pyroclastic flows, dacite domes, tephra, and a large volume pyroxene andesite lava flow were erupted. Two tephra sets, M and K, were deposited midway through this stage. Swift Creek-stage deposits were emplaced in two phases, beginning about 16 ka and ending about 12.8 ka. During the first phase, set S tephras and three large fans and at least one smaller fan of dacitic fragmental material were deposited on the northwest, west, south, and southeast flanks of Mount St. Helens. The fans are dominated by lithic pyroclastic-flow deposits associated with dome building but include both primary and reworked material from pumiceous pyroclastic flows and lahars. One Swift Creek-age dome on the west flank of the volcano has been located, and others must have been nearby. During the second phase, set J tephras were deposited, but no pyroclastic flows or domes are known to be associated with the andesitic set J tephras. Preliminary petrographic analysis of these older rocks suggests that the volcano’s magmatic system was simpler during the Ape Canyon stage than during subsequent stages and that the magmatic system has evolved from relatively simple to more complex as the volcano matured. Compositional cycles as envisioned by C.A. Hopson and W.G. Melson for the Spirit Lake stage probably did not occur during the Ape Canyon stage but developed later during the Cougar and Swift Creek stages.

Washington↗
USGS2007

Metamorphic and structural history of continental crust at a Mesozoic collisional margin, the Ruby terrane, central Alaska

The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the south‐central part of the Ruby terrane, demonstrate that the low‐angle schistose fabric formed under high‐ P /low‐ T conditions, at peak conditions of 10.8‐13.2 kbar and 425‐550° C, consistent with the rare occurrence of glaucophane. White mica 40 Ar/ 39 Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 ± 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high‐ P/T metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500° C between 118 ± 3 and 109 ± 1 Ma. Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar to that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon‐Tanana Upland. The current orientation of the Ruby terrane at an oblique angle to these other crustal blocks and to the Cordilleran trend is due to post‐collisional tectonic processes that have greatly modified the original continental margin.

Alaska↗
USGS2006

40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy

The Italian volcano, Vesuvius, erupted explosively in AD 79. Sanidine from pumice collected at Casti Amanti in Pompeii and Villa Poppea in Oplontis yielded a weighted-mean 40 Ar/ 39 Ar age of 1925±66 years in 2004 (1σ uncertainty) from incremental-heating experiments of eight aliquants of sanidine. This is the calendar age of the eruption. Our results together with the work of Renne et al. ( 1997 ) and Renne and Min ( 1998 ) demonstrate the validity of the 40 Ar/ 39 Ar method to reconstruct the recent eruptive history of young, active volcanoes.

Vesuvius↗
USGS2006

Argon geochronology of Kilauea's early submarine history

Submarine alkalic and transitional basalts collected by submersible along Kilauea volcano's south flank represent early eruptive products from Earth's most active volcano. Strongly alkalic basalt fragments sampled from volcaniclastic deposits below the mid-slope Hilina Bench yield 40 Ar/ 39 Ar ages from 212 ± 38 to 280 ± 20 ka. These ages are similar to high-precision 234 ± 9 and 239 ± 10 ka phlogopite ages from nephelinite clasts in the same deposits. Above the mid-slope bench, two intact alkalic to transitional pillow lava sequences protrude through the younger sediment apron. Samples collected from a weakly alkalic basalt section yield 138 ± 30 to 166 ± 26 ka ages and others from a transitional basalt section yield 138 ± 115 and 228 ± 114 ka ages. The ages are incompatible with previous unspiked K–Ar studies from samples in deep drill holes along the east rift of Kilauea. The submarine birth of Kīlauea volcano is estimated at < 300 ka. If the weakly alkalic sequence we dated is representative of the volcano as a whole, the transition from alkalic to tholeiitic basalt compositions is dated at ∼ 150 ka.

Hawaiʻi↗
USGS2006

The Late-Holocene evolution of the Miseno area (south-western Campi Flegrei) as inferred by stratigraphy, petrochemistry and 40 Ar/ 39 Ar geochronology:Chapter 6 in Volcanism in the Campania Plain — Vesuvius, Campi Flegrei and Ignimbrites

This study on terrestrial and marine successions increases the understanding of the Late-Holocene volcanological and stratigraphical evolution of the south-western part of Campi Flegrei caldera. Stratigraphic data derived from field studies of two major tuff vents located along the coastal zone, namely Porto Miseno and Capo Miseno, clearly indicate that the Porto Miseno tuff ring slightly predates the Capo Miseno tuff cone. 40 Ar/ 39 Ar step-heating experiments, carried out on fresh sanidine separates from pumice samples, yielded a plateau age of 5090±140 yr BP for Capo Miseno and 6490±510 yr BP for Porto Miseno vent, thus confirming field observations. The volcanoclastic input derived from this recent and intense eruptive activity played a major role in the inner-shelf stratigraphic evolution of the Porto Miseno Bay deposits that have been drilled up to 40 m depth off the crater rim. The cored succession is characterised by transgressive marine deposits (mostly volcanic sand) with two intercalated peat layers (t 1 and t 2 ), dated at 3560±40 yr BP and 7815±55 yr BP ( 14 C), respectively, interbedded with a 1–5 m thick pumice layer (tephra C). Peat layers have been chronostratigraphically correlated with two widespread paleosols onland while petrochemical analyses allowed us to correlate tephra C with the Capo Miseno tuff cone deposits. The results presented in this study imply a Late-Holocene volcanic activity that is also well preserved in the marine record in this sector of the caldera where a new chronostratigraphic reconstruction of the eruptive events is required in order to better evaluate the hazard assessment of the area.

Campi Flegrei caidera↗
USGS2003

Geochronology and eruptive history of the Katmai volcanic cluster, Alaska Peninsula

In the Katmai district of the Alaska Peninsula, K&ndash;Ar and 40 Ar/ 39 Ar ages have been determined for a dozen andesite&ndash;dacite stratocones on the arc front and for 11 rear-arc volcanoes, 10 of which are monogenetic. Tied to mapping and stratigraphic studies, our dating emphasized proximal basal lavas that rest on basement rocks, in order to estimate ages of inception of each polygenetic cone. Oldest among arc-front cones is Alagogshak Volcano (690&ndash;43 ka), succeeded in the Holocene by the active Mount Martin cone. Mount Mageik consists of four overlapping subedifices, basal lavas of which give ages of 93, 71, and 59 ka, and Holocene. The three small prehistoric cones of Trident Volcano yield ages of 143, 101&ndash;58, and 44 ka. Falling Mountain and Mount Cerberus, dacite domes near the 1912 Novarupta vent, are related compositionally to the Trident group and give ages of 70 ka and 114 ka. Mount Katmai, which underwent caldera collapse in 1912, consists of two subedifices that overlapped in space and time, and is the only arc-front center here to include basalt and rhyolite; one cone began by 90 ka, the other by 47 ka. Snowy Mountain also consists of two contiguous cones, which started around 200 and 171 ka, respectively, the younger remaining active into the Holocene. Devils Desk, the only mafic cone on the arc front, was short-lived at about 245 ka. In the rear-arc, (1) Mount Griggs produced mafic-to-silicic andesite in several episodes between 292 ka and the Holocene; (2) the Savonoski River cluster includes a Pliocene dacite dome and five small mafic cones (390&ndash;88 ka); (3) Gertrude Creek cone (49.8% SiO 2 ) yields an age of 500 ka; and (4) the Saddlehorn Creek cluster includes five Pliocene basalt-to-andesite remnants. Eruptive volumes were reconstructed, permitting estimates of average eruption rates for edifice lifetimes. Since the mid Pleistocene, total volume erupted along the arc front here is 210&plusmn;47 km 3 and in the rear-arc 39&plusmn;6 km 3 , of which Mount Griggs alone accounts for 35&plusmn;5 km 3 . Most productive has been Mount Katmai at 70&plusmn;18 km 3 , yielding a rate of &sim;1 km 3 /kyr, followed by Mount Mageik (0.33 km 3 /kyr) and Mount Griggs (0.3 km 3 /kyr since 50 ka).

Alaska↗
USGS2003

Pleistocene tephrostratigraphy and paleogeography of southern Puget Sound near Olympia, Washington

Our detailed mapping in the south Puget Sound basin has identified two tephras that are tentatively correlated to tephras from Mount St. Helens and Mount Rainier dated ca. 100-200 ka and 200 ka, respectively. This, plus the observation that fluvial and lacustrine sediments immediately underlying the Vashon Drift of latest Wisconsin age are nearly everywhere radiocarbon infinite, suggests that glacial and nonglacial sediments of more than the past five oxygen-isotope stages are exposed above sea level. Distal lacustrine advance outwash equivalent to the Lawton Clay in the Seattle area is conspicuously absent. Instead, a thick (>120 ft) glaciolacustrine silt below the Vashon sediments contains dropstones and is radiocarbon infinite. Elsewhere, coarsegrained advance Vashon outwash rests unconformably on radiocarbon-infinite non-glacial sediments. These relationships may imply that late Pleistocene tectonic activity has modified the paleotopography and stratigraphy of the south Puget Sound area.

Washington↗