Roger H. Morin

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Evaluation and Application of the Transient-Pulse Technique for Determining the Hydraulic Properties of Low-Permeability Rocks—Part 2: Experimental Application

The transient-pulse technique is a well-established laboratory method for determining the permeability of hydraulically tight rocks. Although graphical solutions to this test make it possible to evaluate both the permeability and the specific storage of a rock specimen, the attendant procedures are relatively complicated. Often, the expression introduced by Brace et al. (1968) is typically used to interpret the experimental results and arrive at a value for permeability only. In Part 1 of this study, the general solution for the transient-pulse test is extended to consider quantitatively the transient distributions of hydraulic head and hydraulic gradient within the specimen and to examine the validity of using the solution presented by Brace et al. (1968) under these conditions. Based on a series of parametric studies, some theoretical and practical considerations related to the design of a transient-pulse test are also provided. In Part 2, a relatively convenient and general approach to calculating the specific storage of a specimen from a transient-pulse test is presented and its efficiency is demonstrated through the application of this approach to experimental investigations.

Reservoir-scale fracture permeability in the Dixie Valley, Nevada, geothermal field

Wellbore image data recorded in six wells penetrating a geothermal reservoir associated with an active normal fault at Dixie Valley, Nevada, were used in conjunction with hydrologic tests and in situ stress measurements to investigate the relationship between reservoir productivity and the contemporary in situ stress field. The analysis of data from wells drilled into productive and non-productive segments of the Stillwater fault zone indicates that fractures must be both optimally oriented and critically stressed to have high measured permeabilities. Fracture permeability in all wells is dominated by a relatively small number of fractures oriented parallel to the local trend of the Stillwater Fault. Fracture geometry may also play a significant role in reservoir productivity. The well-developed populations of low angle fractures present in wells drilled into the producing segment of the fault are not present in the zone where production is not commercially viable.

In situ measurements of fluid flow in DSDP Holes 395A and 534A: Results from the Dianaut Program

The DIANAUT program provided the first opportunity to directly measure vertical fluid flow in ocean boreholes by means of a high resolution thermal flowmeter. Measurements of volumetric flow rate were obtained in DSDP (Deep Sea Drilling Project) Holes 395A and 534A. These results identified a total flow of 2300 L/hr of seawater entering Hole 395A from the seafloor that diminished to about 550 L/hr at a depth of 251 meters below seafloor (mbsf), indicating that approximately 3/4 of the original downward flow had exited the borehole and entered the open formation across the upper 140 m of basement. This information allows the upper oceanic crust at this site to be delimitated into three hydrologic units, with basalt permeabilities of 3.0 × 10−14 m2 near the sediment/basement interface decreasing sharply as a function of depth to values much less than 10−16 m2 below 440 mbsf. It is estimated that approximately 108 L of seawater have entered this well since it was drilled in 1975. Quantitative flow measurements in Hole 534A were inconclusive because of technical problems with the flowmeter packer. Nevertheless, results showed that borehole fluid was moving upward and out into the open ocean at a rate on the order of a few hundred liters per hour, roughly one order of magnitude less than that determined for Hole 395A and moving in the opposite direction. There is good correlation between these field measurements and the attendant temperature logs from each well, and the results provide strong evidence of important mass-transport processes associated with the diverse submarine hydrologic systems in the upper oceanic crust.

Heat Flow and Hydrologic Characteristics at the AND-1B borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica

The Antarctic Drilling Program (ANDRILL) successfully drilled and cored a borehole, AND-1B, beneath the McMurdo Ice Shelf and into a fl exural moat basin that surrounds Ross Island. Total drilling depth reached 1285 m below seafl oor (mbsf) with 98 percent core recovery for the detailed study of glacier dynamics. With the goal of obtaining complementary information regarding heat fl ow and permeability, which is vital to understanding the nature of marine hydrogeologic systems, a succession of three temperature logs was recorded over a fi veday span to monitor the gradual thermal recovery toward equilibrium conditions. These data were extrapolated to true, undisturbed temperatures, and they defi ne a linear geothermal gradient of 76.7 K/km from the seafl oor to 647 mbsf. Bulk thermal conductivities of the sedimentary rocks were derived from empirical mixing models and density measurements performed on core, and an average value of 1.5 W/mK ± 10 percent was determined. The corresponding estimate of heat fl ow at this site is 115 mW/m 2 . This value is relatively high but is consistent with other elevated heat-fl ow data associated with the Erebus Volcanic Province. Information regarding the origin and frequency of pathways for subsurface fl uid fl ow is gleaned from drillers’ records, complementary geophysical logs, and core descriptions. Only two prominent permeable zones are identifi ed and these correspond to two markedly different features within the rift basin; one is a distinct lithostratigraphic subunit consisting of a thin lava fl ow and the other is a heavily fractured interval within a single thick subunit.

Modeling regional salinization of the Ogallala aquifer, Southern High Plains, TX, USA

Two extensive plumes (combined area .1000 km 2 ) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to .500 mg l 21 . Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the modeled plume area is in the range of 10 24 ‐10 25 m 3 day 21 , and the TDS concentrations are typically .3000 mg l 21 . Regions of increased salinity, identified within the Whitehorse Group (evaporite unit) underlying the Ogallala aquifer, are controlled by the structure and thickness variations relative to the recharge areas. Distinct flow paths, on the order of tens of km to .100 km in length, and varying flow velocities indicate that the salinization of the Ogallala aquifer has been a slow, ongoing process and may represent circulation of waters recharged during Pleistocene or earlier times. On-going pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping. Because the distribution of saline ground water in this region of the Ogallala aquifer is heterogeneous, careful areal and vertical characterization is warranted prior to any well-field development. q 2000 Elsevier Science B.V. All rights reserved.

Permeability Profiles in Granular Aquifers Using Flowmeters in Direct-Push Wells

Numerical hydrogeological models should ideally be based on the spatial distribution of hydraulic conductivity (K), a property rarely defined on the basis of sufficient data due to the lack of efficient characterization methods. Electromagnetic borehole flowmeter measurements during pumping in uncased wells can effectively provide a continuous vertical distribution of K in consolidated rocks. However, relatively few studies have used the flowmeter in screened wells penetrating unconsolidated aquifers, and tests conducted in gravel-packed wells have shown that flowmeter data may yield misleading results. This paper describes the practical application of flowmeter profiles in direct-push wells to measure K and delineate hydrofacies in heterogeneous unconsolidated aquifers having low-to-moderate K (10(-6) to 10(-4) m/s). The effect of direct-push well installation on K measurements in unconsolidated deposits is first assessed based on the previous work indicating that such installations minimize disturbance to the aquifer fabric. The installation and development of long-screen wells are then used in a case study validating K profiles from flowmeter tests at high-resolution intervals (15 cm) with K profiles derived from multilevel slug tests between packers at identical intervals. For 119 intervals tested in five different wells, the difference in log K values obtained from the two methods is consistently below 10%. Finally, a graphical approach to the interpretation of flowmeter profiles is proposed to delineate intervals corresponding to distinct hydrofacies, thus providing a method whereby both the scale and magnitude of K contrasts in heterogeneous unconsolidated aquifers may be represented.

Geothermal state of DSDP Holes 333A, 395A AND 534A: Results from the Dianaut Program

Heat-flow values determined from the DIANAUT program provide insight into the geothermal state of three DSDP holes situated on the western flank of the Mid-Atlantic Ridge. Very low values (20 mW/m2 max.) observed in 3.5-m.y.-old crust at Hole 333A decrease with depth and indicate persistent basement cooling due to cold water circulation in the porous level of the upper oceanic crust. Cold ocean bottomwater continues to invade the upper level of fractured 7.3-m.y.-old basement at Hole 395A, but temperature v. time records show large-scale temperature fluctuations, probably associated with borehole convection, at 500 mbsf and stable temperatures, probably indicating a slow return to thermal equilibrium, at 600 mbsf. The 154-m.y.-old crust at Hole 534A exhibits a relatively steady thermal state with a heat flow close to theoretical values for a conductively cooling plate; small variations in the thermal gradient recorded in this well probably represent very slow and subtle movement of borehole fluids. The close comparison of these latest results with those obtained several years previously, demonstrates the presence of relatively stable thermal regimes.

Drilling the central crater of the Chesapeake Bay Impact Structure: A first look

The late Eocene Chesapeake Bay impact structure is a well-preserved example of one of Earths largest impact craters, and its continental-shelf setting and relatively shallow burial make it an excellent target for study. Since the discovery of the structure over a decade ago [Edwards et al., 2004; Poag et al., 2004], test drilling by U.S. federal and state agencies has been limited to the structures annular trough (Figure 1). In May 2004, the U.S. Geological Survey (USGS) drilled the first scientific test hole into the central crater of the Chesapeake Bay impact structure in Cape Charies,Virginia (Figure 1). This partially cored test hole, the deepest to date, penetrated postimpact sediments and impact breccias to a total depth of 823 m.

Theoretical Evaluation of the Transient Response of Constant Head and Constant Flow-Rate Permeability Tests

A theoretical analysis is presented that compares the response characteristics of the constant head and the constant flow-rate (flow pump) laboratory techniques for quantifying the hydraulic properties of geologic materials having permeabilities less than 10−10 m/s. Rigorous analytical solutions that describe the transient distributions of hydraulic gradient within a specimen are developed, and equations are derived for each method. Expressions simulating the inflow and outflow rates across the specimen boundaries during a constant-head permeability test are also presented. These solutions illustrate the advantages and disadvantages of each method, including insights into measurement accuracy and the validity of using Darcys law under certain conditions. The resulting observations offer practical considerations in the selection of an appropriate laboratory test method for the reliable measurement of permeability in low-permeability geologic materials.