Donald S. Sweetkind

Lake-Level History of Lake Michigan for the Past 12,000 Years: The Record From Deep Lacustrine Sediments

Abstract Collection and analysis of an extensive set of seismic-reflection profiles and cores from southern Lake Michigan have provided new data that document the history of the lake basin for the past 12,000 years. Analyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic, isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changes during this recent part of the lakes history. The post-glacial history of lake-level changes in the Lake Michigan basin begins about 11.2 ka with the fall from the high Calumet level, caused by the retreat of the Two Rivers glacier, which had blocked the northern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water from glacial Lake Agassiz (about 10.6 ka), during which deposition of the distinctive gray Wilmette Bed of the Lake Michigan Formation interrupted deposition of red glaciolacustrine sediment. Lake level then continued to fall, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewa low phase, lake level was about 80 m lower than it is today in the southern basin of Lake Michigan. The rise of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bay outlet, resulted in a prominent, planar, transgressive unconformity that eroded most of the shoreline features below present lake level. Superimposed on this overall rise in lake level, a second influx of water from Lake Agassiz temporarily raised lake levels an unknown amount about 9.1 ka. At about 7 ka, lake level may have fallen below the level of the outlet because of sharply drier climate. Sometime between 6 and 5 ka, the character of the lake changed dramatically, probably due mostly to climatic causes, becoming highly undersaturated with respect to calcium carbonate and returning primary control of lake level to the isostatically rising North Bay outlet. Post-Nipissing (about 5 ka) lake level has fallen about 6 m due to erosion of the Port Huron outlet, a trend around which occurred relatively small (± ∼2 m), short-term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the sed-imentological and sediment-magnetic properties of the sediments.

Chlorine-36 data at Yucca Mountain: statistical tests of conceptual models for unsaturated-zone flow.

An extensive set of chlorine-36 (36Cl) data has been collected in the Exploratory Studies Facility (ESF), an 8-km-long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport in the unsaturated zone (UZ) at this site. At several locations, the measured values of 36Cl/Cl ratios for salts leached from rock samples are high enough to provide strong evidence that at least a small component of bomb-pulse 36Cl, fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, was measured, implying that some fraction of the water traveled from the ground surface through 200-300 m of unsaturated rock to the level of the ESF during the last 50 years. These data are analyzed here using a formal statistical approach based on log-linear models to evaluate alternative conceptual models for the distribution of such fast flow paths. The most significant determinant of the presence of bomb-pulse 36Cl in a sample from the welded Topopah Spring unit (TSw) is the structural setting from which the sample was collected. Our analysis generally supports the conceptual model that a fault that cuts through the nonwelded Paintbrush tuff unit (PTn) that overlies the TSw is required in order for bomb-pulse 36Cl to be transmitted to the sample depth in less than 50 years. Away from PTn-cutting faults, the ages of water samples at the ESF appear to be a strong function of the thickness of the nonwelded tuff between the ground surface and the ESF, due to slow matrix flow in that unit.

Hydrologic models of modern and fossil geothermal systems in the Great Basin: Genetic implications for epithermal Au-Ag and Carlin-type gold deposits

The Great Basin region in the western United States contains active geothermal systems, large epithermal Au-Ag deposits, and world-class Carlin-type gold deposits. Temperature profiles, fluid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems associated with gold mineralization share many common features, including the absence of a clear magmatic fluid source, discharge areas restricted to fault zones, and remarkably high temperatures (>200 °C) at shallow depths (200–1500 m). While the plumbing of these systems varies, geochemical and isotopic data collected at the Dixie Valley and Beowawe geothermal systems suggest that fluid circulation along fault zones was relatively deep (>5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small (<2.5‰) shifts from the meteoric water line (MWL). Many fossil ore-forming systems were also dominated by meteoric water, but usually exhibit δ18O fluid-rock interactions with larger shifts of 5‰–20‰ from the MWL. Here we present a suite of two-dimensional regional (100 km) and local (40–50 km) scale hydrologic models that we have used to study the plumbing of modern and Tertiary hydrothermal systems of the Great Basin. Geologically and geophysically consistent cross sections were used to generate somewhat idealized hydrogeologic models for these systems that include the most important faults, aquifers, and confining units in their approximate configurations. Multiple constraints were used, including enthalpy, δ18O, silica compositions of fluids and/or rocks, groundwater residence times, fluid inclusion homogenization temperatures, and apatite fission track anomalies. Our results suggest that these hydrothermal systems were driven by natural thermal convection along anisotropic, subvertical faults connected in many cases at depth by permeable aquifers within favorable lithostratigraphic horizons. Those with minimal fluid δ18O shifts are restricted to high-permeability fault zones and relatively small-scale (~5 km), single-pass flow systems (e.g., Beowawe). Those with intermediate to large isotopic shifts (e.g., epithermal and Carlin-type Au) had larger-scale (~15 km) loop convection cells with a greater component of flow through marine sedimentary rocks at lower water/rock ratios and greater endowments of gold. Enthalpy calculations constrain the duration of Carlin-type gold systems to probably <200 k.y. Shallow heat flow gradients and fluid silica concentrations suggest that the duration of the modern Beowawe system is <5 k.y. However, fluid flow at Beowawe during the Quaternary must have been episodic with a net duration of ~200 k.y. to account for the amount of silica in the sinter deposits. In the Carlin trend, fluid circulation extended down into Paleozoic siliciclastic rocks, which afforded more mixing with isotopically enriched higher enthalpy fluids. Computed fission track ages along the Carlin trend included the convective effects, and ranged between 91.6 and 35.3 Ma. Older fission track ages occurred in zones of groundwater recharge, and the younger ages occurred in discharge areas. This is largely consistent with fission track ages reported in recent studies. We found that either an amagmatic system with more permeable faults (10−11 m2) or a magmatic system with less permeable faults (10−13 m2) could account for the published isotopic and thermal data along the Carlin trend systems. Localized high heat flow beneath the Muleshoe fault was needed to match fluid inclusion temperatures at Mule Canyon. However, both magmatic and amagmatic scenarios require the existence of deep, permeable faults to bring hot fluids to the near surface.

Geophysical framework of the northern San Francisco Bay region, California

We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ∼175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which influenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude significant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward–Rodgers Creek–Maacama fault system suggests that this system reactivated older basement structures, which appear to influence seismicity patterns in the region.

Deep permeable fault-controlled helium transport and limited mantle flux in two extensional geothermal systems in the Great Basin, United States

This study assesses the relative importance of deeply circulating meteoric water and direct mantle fluid inputs on near-surface 3 He/ 4 He anomalies reported at the Coso and Beowawe geothermal fields of the western United States. The depth of meteoric fluid circulation is a critical factor that controls the temperature, extent of fluid-rock isotope exchange, and mixing with deeply sourced fluids containing mantle volatiles. The influence of mantle fluid flux on the reported helium anomalies appears to be negligible in both systems. This study illustrates the importance of deeply penetrating permeable fault zones (10 −12 to 10 −15 m 2 ) in focusing groundwater and mantle volatiles with high 3 He/ 4 He ratios to shallow crustal levels. These continental geothermal systems are driven by free convection.

Structural geology of the proposed site area for a high-level radioactive waste repository, Yucca Mountain, Nevada

Geologic mapping and fracture studies have documented the fundamental patterns of joints and faults in the thick sequence of rhyolite tuffs at Yucca Mountain, Nevada, the proposed site of an underground repository for high-level radioactive waste. The largest structures are north-striking, block-bounding normal faults (with a subordinate left-lateral component) that divide the mountain into numerous 1–4-km-wide panels of gently east-dipping strata. Block-bounding faults, which underwent Quaternary movement as well as earlier Neogene movement, are linked by dominantly northwest-striking relay faults, especially in the more extended southern part of Yucca Mountain. Intrablock faults are commonly short and discontinuous, except those on the more intensely deformed margins of the blocks. Lithologic properties of the local tuff stratigraphy strongly control the mesoscale fracture network, and locally the fracture network has a strong influence on the nature of intrablock faulting. The least faulted part of Yucca Mountain is the north-central part, the site of the proposed repository. Although bounded by complex normal-fault systems, the 4-km-wide central block contains only sparse intrablock faults. Locally intense jointing appears to be strata-bound. The complexity of deformation and the magnitude of extension increase in all directions away from the proposed repository volume, especially in the southern part of the mountain where the intensity of deformation and the amount of vertical-axis rotation increase markedly. Block-bounding faults were active at Yucca Mountain during and after eruption of the 12.8–12.7 Ma Paintbrush Group, and significant motion on these faults postdated the 11.6 Ma Rainier Mesa Tuff. Diminished fault activity continued into Quaternary time. Roughly half of the stratal tilting in the site area occurred after 11.6 Ma, probably synchronous with the main pulse of vertical-axis rotation, which occurred between 11.6 and 11.45 Ma. Studies of sequential formation of tectonic joints, in the context of regional paleo-stress studies, indicate that north- and northwest-striking joint sets formed coevally with the main faulting episode during regional east-northeast–west-southwest extension and that a prominent northeast-striking joint set formed later, probably after 9 Ma. These structural analyses contribute to the understanding of several important issues at Yucca Mountain, including potential hydrologic pathways, seismic hazards, and fault-displacement hazards.

EFFECTS OF HYDROTHERMAL ALTERATION ON THE MAGNETIZATION OF THE OLIGOCENE CARPENTER RIDGE TUFF, BACHELOR CALDERA, SAN JUAN MOUNTAINS, COLORADO

Intracaldera Oligocene Carpenter Ridge Tuff fills the Bachelor caldera in the central San Juan caldera complex and hosts mineral deposits of the Creede mineral district. The Carpenter Ridge Tuff and unaltered portions of its intracaldera Bachelor Mountain Member, have strong, high-coercivity, reverse magnetizations with average magnetic susceptibility (MS) and natural remanent magnetization (NRM) of 6 × 10−3 volume SI and 8 A/m, respectively. Oxide phenocrysts in these rocks are titanomagnetite and ilmenite; however, magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and Curie temperatures between 580°C and 620°C. Much of the intracaldera tuff was affected by potassic metasomatism, a type of hydrothermal alteration characterized by addition of K and loss of Ca and Na, between 27.3 Ma and 25.1 Ma. Potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz, oxidation of the original oxide phenocrysts to hematite and rutile, and consequent suppression of MS and NRM by a factor of 5. Also present, however, are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed; values of MS and NRM are similar to those of outflow tuff. Such rocks are suspected of containing secondary magnetite on the basis of (1) Curie and thermal unblocking temperatures at and below 580°C, indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed, and (2) observation in one sample of magnetite rimming cores of hematite+rutile. The presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry, although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition. These altered rocks may record a change from a pervasive alteration by alkaline, oxidizing fluids to a vein-controlled alteration dominated by more reduced fluids in which magnetite was stable.

Three-dimensional geologic modeling of the Santa Rosa Plain, California

New three-dimensional (3D) lithologic and stratigraphic models of the Santa Rosa Plain (California, USA) delineate the thickness, extent, and distribution of subsurface geologic units and allow integration of diverse data sets to produce a lithologic, stratigraphic, and structural architecture for the region. This framework can be used to predict pathways of groundwater flow beneath the Santa Rosa Plain and potential areas of enhanced or focused seismic shaking. Lithologic descriptions from 2683 wells were simplified to 19 internally consistent lithologic classes. These distinctive lithologic classes were used to construct a 3D model of lithologic variations within the basin by extrapolating data away from drill holes using a nearest-neighbor approach. Subsurface stratigraphy was defined through the identification of distinctive lithologic packages tied, where possible, to high-quality well control and to surface exposures. The 3D stratigraphic model consists of three bounding components: fault surfaces, stratigraphic surfaces, and a surface representing the top of pre-Cenozoic basement, derived from inversion of regional gravity data. The 3D lithologic model displays a west to east transition from dominantly marine sands to heterogeneous continental sediments. In contrast to previous stratigraphic studies, the new models emphasize the prevalence of the clay-rich Petaluma Formation and its heterogeneous nature. Isopach maps of the Glen Ellen Formation and the 3D stratigraphic model show the influence of the Trenton Ridge, a concealed basement ridge that bisects the plain, on sedimentation; the thickest deposits of the Glen Ellen Formation are confined to north of the Trenton Ridge.

Innovative Environmental Tracer Techniques for Evaluating Sources of Spring Discharge from a Carbonate Aquifer Bisected by a River

Littlefield Springs discharge about 1.6 m³ /s along a 10-km reach of the Virgin River in northwestern Arizona. Understanding their source is important for salinity control in the Colorado River Basin. Environmental tracers suggest that Littlefield Springs are a mixture of older groundwater from the regional Great Basin carbonate aquifer and modern (post-1950s) seepage from the Virgin River. While corrected ¹⁴C apparent ages range from 1 to 9 ka, large amounts of nucleogenic ⁴He and low ³He/⁴He ratios suggest that the carbonate aquifer component is likely even older Pleistocene recharge. Modeled infiltration of precipitation, hydrogeologic cross sections, and hydraulic gradients all indicate recharge to the carbonate aquifer likely occurs in the Clover and Bull Valley Mountains along the northern part of the watershed, rather than in the nearby Virgin Mountains. This high-altitude recharge is supported by relatively cool noble-gas recharge temperatures and isotopically depleted δ²H and δ¹⁸O. Excess (crustal) SF₆ and ⁴He precluded dating of the modern component of water from Littlefield Springs using SF₆ and ³H/³He methods. Assuming a lumped-parameter model with a binary mixture of two piston-flow components, Cl⁻ /Br⁻, Cl⁻ /F⁻, δ²H, and CFCs indicate the mixture is about 60% Virgin River water and 40% groundwater from the carbonate aquifer, with an approximately 30-year groundwater travel time for Virgin River seepage to re-emerge at Littlefield Springs. This suggests that removal of high-salinity sources upstream of the Virgin River Gorge would reduce the salinity of water discharging from Littlefield Springs into the Virgin River within a few decades.

Geology and geochemistry of volcanic centers within the eastern half of the Sonoma volcanic field, northern San Francisco Bay region, California

Volcanic rocks in the Sonoma volcanic field in the northern California Coast Ranges contain heterogeneous assemblages of a variety of compositionally diverse volcanic rocks. We have used field mapping, new and existing age determinations, and 343 new major and trace element analyses of whole-rock samples from lavas and tuff to define for the first time volcanic source areas for many parts of the Sonoma volcanic field. Geophysical data and models have helped to define the thickness of the volcanic pile and the location of caldera structures. Volcanic rocks of the Sonoma volcanic field show a broad range in eruptive style that is spatially variable and specific to an individual eruptive center. Major, minor, and trace-element geochemical data for intracaldera and outflow tuffs and their distal fall equivalents suggest caldera-related sources for the Pinole and Lawlor Tuffs in southern Napa Valley and for the tuff of Franz Valley in northern Napa Valley. Stratigraphic correlations based on similarity in eruptive sequence and style coupled with geochemical data allow an estimate of 30 km of right-lateral offset across the West Napa-Carneros fault zones since ∼5 Ma. The volcanic fields in the California Coast Ranges north of San Francisco Bay are temporally and spatially associated with the northward migration of the Mendocino triple junction and the transition from subduction and associated arc volcanism to a slab window tectonic environment. Our geochemical analyses from the Sonoma volcanic field highlight the geochemical diversity of these volcanic rocks, allowing us to clearly distinguish these volcanic rocks from those of the roughly coeval ancestral Cascades magmatic arc to the west, and also to compare rocks of the Sonoma volcanic field to rocks from other slab window settings.