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Geology topics

K.R. Ludwig

Publications and source records attributed to K.R. Ludwig.

At least 19 recordsLinked to original sources
USGS2017

Paleoclimatic Inferences from a 120,000-Yr Calcite Record of Water-Table Fluctuation in Browns Room of Devils Hole, Nevada

The petrographic and morphologic differences between calcite precipitated below, at, or above the present water table and uranium-series dating were used to reconstruct a chronology of water-table fluctuation for the past 120,000 yr in Browns Room, a subterranean air-filled chamber of Devils Hole fissure adjacent to the discharge area of the large Ash Meadows groundwater flow system in southern Nevada. The water table was more than 5 m above present level between about 116,000 and 53,000 yr ago, fluctuated between about +5 and +9 m during the period between about 44,000 and 20,000 yr ago, and declined rapidly from +9 to its present level during the past 20,000 yr. Because the Ash Meadows groundwater basin is greater than 12,000 km2 in extent, these documented water-table fluctuations are likely to be of regional significance. Although different in detail, water-level fluctuation recorded by Browns Room calcites generally correlate with other Great Basin proxy palcoclimatic data.

Quaternary Research↗
USGS2017

Duration and structure of the past four interglaciations

Reanalysis and additional dating of the Devils Hole ??18O paleotemperature record confirm that the last interglaciation in the Great Basin (the continental analog of marine isotopic substage 5e) lasted ???22,000 yr, consistent with the Vostok paleotemperature record which suggests a duration of ???19,000 yr for this event in Antarctica. The three preceding interglaciations in the Devils Hole record (analogs of marine isotopic substages 7e, 9c, and 11c) range from 20,000 to 26,000 yr in duration. A ???20,000-yr duration for the last interglaciation is consistent with TIMS uranium-series dated sea-level high stands. Thus, the widely held view that interglaciations were of 11,000-to 13,000-yr duration and constituted only about 10% of mid-to-late Pleistocene climatic cycles needs reexamination. The warmest portion of each interglaciation in the Devils Hole time series is marked by a ??18O plateau, signifying apparent climatic stability for periods of 10,000-to 15,000-yr duration. ?? 1997 University of Washington.

Quaternary Research↗
USGS2017

Sr-Isotope record of Quaternary marine terraces on the California coast and off Hawaii

Strontium-isotopic ratios of dated corals have been obtained from submerged reefs formed during Quaternary glacial periods off the Hawaiian islands. These data, combined with data from deep-sea sediments, tightly constrain the secular variation of marine 87Sr 86Sr for the past 800,000 yr. Although long-term trends are apparent, no significant (>0.02???), rapid (<100,000 yr) excursions in 87Sr 86Sr were resolved nor did we observe any samples with 87Sr 86Sr greater than that of modern seawater. Strontium in mollusks from elevated marine terraces formed during interglacial periods on the southern California coast show resolvable and consistent variations in 87Sr 86Sr which, when compared to the trend of Quaternary marine 87Sr 86Sr, can be used to infer uplift rates and define approximate ages for the higher terraces. The Sr-isotope age estimates indicate that uplift rates vary among crustal blocks and were not necessarily constant with time. No contrast in Sr-isotopic ratios between similar-age Hawaiian and California fossils was observed, confirming that any change in marine 87Sr 86Sr from glacial to interglacial periods must be small. A realistic appraisal of the potential of Sr-isotope stratigraphy for chronometric applications in the Quaternary suggests that the technique will be limited to relatively coarse distinctions in age. ?? 1992.

Quaternary Research↗
USGS2011

The chronology for the d18O record from Devils Hole, Nevada, extended into the Mid-Holocene

This report presents the numeric values for the chronology of the paleoclimatically relevant mid-to-late Pleistocene record of the ratios of stable oxygen isotope (delta18O) in vein calcite from Devils Hole, Nev., which recently had been extended into the mid-Holocene. Dating was obtained using 230Th-234U-238U thermal ionization mass spectrometry. Devils Hole is a subaqueous cave of tectonic origin, which developed in the discharge zone of a regional aquifer in south-central Nevada. The primary groundwater recharge source area is the Spring Mountains, the highest mountain range in southern Nevada [altitude 3,630 meters (m)], approximately 80 kilometers to the east of the cavern. The walls of the open fault zone comprising the cave system are coated with dense vein calcite precipitated from the through-flowing groundwater. The calcite, up to 40 centimeters (cm) thick, contains a continuous record of the sequential variation of the composition of stable oxygen isotopes in the ground water over time. The vein calcite has also proven to be a suitable material for precise uranium-series dating via thermal ionization mass spectrometry utilizing the 230Th-234U-238U decay clock. Earlier work has presented data from the Devils Hole core DH-11, a 36-cm-long core of vein calcite recovered from a depth of about 30 m below the water table (about 45 m beneath the ground surface). The DH-11 core provided a continuous record of isotopic oxygen variation from 567,700 to 59,800 years before present. Recent work has extended this record up to 4,500 years before present, into the mid-Holocene epoch.

Open-File Report↗
USGS2002

Uranium-series dating of pedogenic silica and carbonate, Crater Flat, Nevada

A 230Th-234U-238U dating study on pedogenic silica-carbonate clast rinds and matrix laminae from alluvium in Crater Flat, Nevada was conducted using small-sample thermal-ionization mass spectrometry (TIMS) analyses on a large suite of samples. Though the 232Th content of these soils is not particularly low (mostly 0.1-9 ppm), the high U content of the silica component (mostly 4-26 ppm) makes them particularly suitable for 230Th/U dating on single, 10 to 200 mg totally-digested samples using TIMS. We observed that (1) both micro- (within-rind) and macro-stratigraphic (mappabe deposit) order of the 230Th/U ages were preserved in all cases; (2) back-calculated initial 234U/238U fall in a restricted range (typically 1.67??0.19), so that 234U/238U ages with errors of about 100 kyr (2??) could be reliably determined for the oldest, 400 to 1000 ka rinds: and (3) though 13 of the samples were >350 ka, only three showed evidence for an open-system history, even though the sensitivity of such old samples to isotopic disruption is very high. An attempt to use leach-residue techniques to separate pedogenic from detrital U and Th failed, yielding corrupt 230Th/U ages. We conclude that 230Th/U ages determined from totally dissolved, multiple sub-mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples, chemical enhancement of 238U/232Th, or isochrons. Copyright ?? 2002 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗
USGS1997

Data from Devils Hole Core DH-11

This report presents the stable isotope values measured in Devils Hole Core DH-11 and interpolated ages at the depth the samples were taken, as analyzed in a recent publication by Winograd and others (1997).

Nevada↗
USGS1996

Coral ages and island subsidence, Hilo drill hole

A 25.8-m-thick sedimentary section containing coral fragments occurs directly below a surface lava flow (the ∼1340 year old Panaewa lava flow) at the Hilo drill hole. Ten coral samples from this section dated by accelerator mass spectrometry (AMS) radiocarbon and five by thermal infrared multispectral scanner (TIMS) 230 Th/U methods show good agreement. The calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef. This sedimentary unit is underlain by a 34-m-thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 14 C date of 10,340 years. The age-depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole or 2.5–2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age-depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30.9 m of lava of which 11 m was above sea level. This surface has now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the coastline but could extensively modify the offshore shelf.

Journal of Geophysical Research B: Solid Earth↗
USGS1994

Calculation of 230ThU isochrons, ages, and errors

If analytical errors are responsible for the scatter of points on a 230 Th- 234 U- 238 U isochron diagram, the isochron should be fitted by a technique that 1. weights the points according to their analytical errors and error correlations, and 2. either takes into account the presence of some of the same data in two coupled XY isochrons or (equivalently) uses a single, three-dimensional XYZ isochron. A method based on maximum-likelihood estimation is presented that fulfills these requirements, and the relevant equations for errors in age and initial 234 U 238 U "> 234 U 238 U are given. Equations for estimating the necessary isotope-ratio errors and error-correlations for both alpha-spectrometric and mass-spectrometric data are also developed.

Geochimica et Cosmochimica Acta↗
USGS1994

Thorium-230 ages of corals and duration of the last interglacial sea-level high stand on Oahu, Hawaii

Thorium-230 ages of emergent marine deposits on Oahu, Hawaii, have a uniform distribution of ages from ∼114,000 to ∼131,000 years, indicating a duration for the last interglacial sea-level high stand of ∼17,000 years, in contrast to a duration of ∼8000 years inferred from the orbitally tuned marine oxygen isotope record. Sea level on Oahu rose to ≥1 to 2 meters higher than present by 131,000 years ago or ∼6000 years earlier than inferred from the marine record. Although the latter record suggests a shift back to glacial conditions beginning at ∼119,000 years ago, the Oahu coral ages indicate a near present sea level until ∼114,000 years ago.

Hawaii↗
USGS1994

Chaotic deposition by a giant wave, Molokai, Hawaii

A coral-basalt breccia-conglomerate is exposed >60m above present sea level and nearly 2km inland from the present shoreline on the southwest side of East Molokai Volcano. This deposits was apparently laid down by a giant wave that broke over an outer reef, similar to the present fringing reef, and advanced as a turbulent bore over the back-reef flat, picking up a slurry of carbonate-rich debris and depositing it on the slopes inland as the wave advanced. U-series dating of coral fragments indicates that the age of this deposit is 240-200 ka. This giant wave was most likley caused by one of the many large submarine landslides that have been identified on the lower slopes of the major Hawaiian Islands.

Hawaii↗
USGS1993

234U/238U as a ground-water tracer, SW Nevada-SE California

The 234U/238U ratio of uranium in oxidizing ground waters is potentially an excellent ground-water tracer because of its high solubility and insensitivity to chemical reactions. Moreover, recent advances in analytical capability have made possible very precise uranium-isotopic analyses on modest (approx.100 ml) amounts of normal ground water. Preliminary results on waters from SW Nevada/Se California indicate two main mixing trends, but in detail indicate significant complexity requiring three or more main components.

California, Nevada↗
USGS1992

U-Pb dating of uranium deposits in collapse breccia pipes of the Grand Canyon region

Two major periods of uranium mineralization are indicated by U-Pb isotope dating of uranium ores from collapse breeeia pipes in the Grand Canyon region, northern Arizona. The Hack 2 and 3, Kanab North, and EZ 1 and 2 orebodies apparently formed in the interval of 200 + or - 20 Ma, similar to ages inferred for strata-bound, Late Triassic-hosted uranium deposits in southern Utah and northern Arizona. Samples from the Grand Canyon and Pine Nut pipes, however, indicate a distinctly older age of about 260 Ma. The Pigeon, Orphan, and Arizona-1 deposits were apparently mineralized before 220, 186, and 169 Ma, respectively, but no useful upper age limits can be inferred. There is no evidence in the U-Pb isotope data for uranium mineralization related to Laramide tectonism, mid-Tertiary volcanism, or late Tertiary uplift. The clustering in ages for a variety of uranium deposits at about (or slightly younger than) the age of the lower part of the Chinle Formation (Late Triassic) suggests that uranium in these deposits may have been derived by leaching from volcanic ash in the Chinle and mobilized by ground-water movement resulting from changing hydrologic gradients caused by regional uplift to the southwest. Pb isotope ratios of galenas in mineralized pipes are more radiogenic than those of sulfides from either uranium-poor pipes or occurrences away from pipes. This isotopic contrast suggests that fluids which passed through the pipes had interacted with the Proterozoic basement, possibly through the vertical fractures which influenced the location and evolution of the pipes themselves. Regardless of the source of the common Pb in the uranium-bearing pipes, the generally distinct Pb isotope composition of their galenas (compared to those of barren pipes and nonpipe sulfides in the region) may provide a useful exploration guide.

Economic Geology↗
USGS1992

Mass-spectrometric 230Th-234U-238U dating of the Devils Hole calcite vein

The Devils Hole calcite vein contains a long-term climatic record, but requires accurate chronologic control for its interpretation. Mass-spectrometric U-series ages for samples from core DH-11 yielded 230 Th ages with precisions ranging from less than 1,000 years (2σ) for samples younger than ∼140 ka (thousands of years ago) to less than 50,000 years for the oldest samples (∼566 ka). The 234 U/ 238 U ages could be determined to a precision of ∼20,000 years for all ages. Calcite accumulated continuously from 566 ka until ∼60 ka at an average rate of 0.7 millimeter per 10 3 years. The precise agreement between replicate analyses and the concordance of the 230 Th/ 238 U and 234 U/ 238 U ages for the oldest samples indicate that the DH-11 samples were closed systems and validate the dating technique in general.

Nevada↗