The Role of Downhole Measurements in Marine Geology and ...

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324 ß Goldberg: DOWNHOLE MEASUREMENTS 35, 3 / REVIEWS OF GEOPHYSICS SCIENTIFIC APPLICATIONS [e.g., Hays et al., 1976; Shackelton et al., 1984]. Due to the gravitational pull of the Sun and Moon on the equatorial There are few areas of marine geology and geophysics bulge of the Earth, our planet wobbles slightly as it spins, that cannot benefit from logging and downhole measure- defining its precession. This wobble results in climate ments. Data are recorded in situ and usually in a con- cycles that last 19,000-23,000 years. The Earth's tilt, or tinuous profile to the bottom of the borehole, providing obliquity, varies with a period of 41,000 years, and its researchers with an accurate proxy of the geology at orbit varies from nearly circular to 95 % eccentricity with various depths. Exploration of the structure, deforma- periods of 95,000 and 413,000 years, affecting its distance tion, and stresses in the Earth's crust that are accessible from the Sun. These periodicities have been observed in by drilling has often been supplemented by downhole a number of marine and terrestrial sedimentary environmeasurements and proven to be critical in understand- ments. Pliocene and Pleistocene marine sediments have ing subsurface phenomena. The Earth's environment demonstrated the clearest correlation between sedimenlikewise has been investigated by scientific drilling and tary layers and Milankovitch periodicities because of the downhole measurements to discern details of the sedi- high recovery of material that can be accurately dated ment record over time. In the following sections, recent using marine core sections. Climate cycles are not limresearch studies that highlight the application of down- ited to the younger sediments, however; coring in both hole measurements to topics related to the Earth's en- Cretaceous and Triassic rocks shows evidence of Mivironment and Earth's crust are discussed. lankovitch cycles [Arthur et aL, 1984; Fischer, 1986; Olsen, 1986]. Paleoclimate Milankovitch orbital variations may alter the deposi- Because Earth's climate has large effects on the tional environment considerably. Spectral analysis of the ocean's chemistry, circulation, and biological productiv- continuous and regular sampling that log data provide ity, changes in the global environment leave a recogniz- can resolve Milankovitch periodicities and thus sedimenable signature in the deep-sea sedimentary record. How tation rates for a known depth interval. In cores, sedithe global climate system has changed and the causes of mentation rates are typically determined by estimating these changes therefore may be deduced from downhole the elapsed time between biostratigraphic, radioisotopic, data if these signatures can be measured. Short- and or magnetostratigraphic markers that can be identified. long-term changes in such variables as temperature, This requires an absolute timescale, which can be suphumidity, and wind patterns, the influx of energy from plied by correlating Milankovitch periodicities with the the Sun, sea level, water temperature, and oceanic up- cores. Using the continuous logs to identify cycles, a welling can all affect the mineralogy, porosity, and grain timescale can be extended millions of years into the past size of sediment layers deposited on the seafloor. Most with greater precision than can be achieved with the downhole log responses are affected by sediment poros- core-based methods alone. Log data may be used to ity, which is closely related to the mineralogy and grain illustrate major trends and rapid changes in the sedisize, both of which can vary dramatically as ocean up- mentation rate by shifts in the wavelength of their variwelling patterns and mean water temperature alter bio- ation with depth in areas with variable or discontinuous logic productivity. The amount of clay incorporated in accumulation or poor age control from core data [Mothe sediment is also often determined by climate and will linie et al., 1990]. Jarrard and Arthur [1989] describe a strongly affect the mineralogy and porosity. The natural method to determine sedimentation rates using log data gamma ray log is particularly sensitive to these variations that requires neither precise biostratigraphy nor an abin clay content. If the temporal changes can be ade- solute timescale. They constrain the sedimentation rate quately resolved, the geophysical and geochemical mea- to the Milankovitch orbital periods by computing spectra surements taken downhole can serve as proxies for cli- of log data and identifying peaks in the continuous mate change in the sediment record. Using downhole spectra that have a constant spacing ratio between them. logs in paleoclimate studies has the significant advantage The use of spectra for interpreting precise sedimentaof measuring several independent parameters that all tion rates between time markers identified in cores has reflect climatic changes throughout the stratigraphic se- proven to be a powerful technique in paleoclimate studies. quence. In addition, the continuous and uniform sam- In 1989, a site in the Sea of Japan was drilled through pling of downhole records allows for rigorous statistical 2.8 m.y. of fine sediment layers to a depth of 459 m analyses that are not possible using short or incomplete below the seafloor. Ingle et al. [1990] describe the recovdata from core sections. ered core as dark opal-rich layers with organic carbon Of the numerous interrelated factors driving climatic interlayered with clay-rich sediments that are derived changes, few are generally accepted to be more impor- from aeolian dust blown from loess deposits in China. tant than the variation in Earth's tilt and orbital cycles The layers alternate about every 4 m. Variation in nataround the Sun. Milankovitch [1941] was the first to ural gamma radioactivity versus age in this hole is shown recognize that periodicity in Earth's orbit and tilt alters in Figure 8, where the age-depth conversion has been the solar energy flux falling at any given latitude in cycles computed from a sedimentation rate that was conlasting thousands to hundreds of thousands of years strained by paleomagnetic reversals and biostratigraphy
35, 3 /REVIEWS OF GEOPHYSICS Goldberg: DOWNHOLE MEASUREMENTS ß 325 recorded from the core. Increases in gamma radiation reflect high clay content of the continental clay-rich layers, which have abundant K and A1. These cyclical signatures are apparent in other downhole data; the density and resistivity logs reflect the lower porosity of the continental clays. DeMenocat et at. [1992] compare the gamma ray log and a marine 8SO record to illustrate the strong correlation with variations in northern hemisphere ice volume [e.g., Raytoo et at., 1989]. The concentration of 8SO has often been used as a proxy for paleoclimatic changes in ice volume, temperature, and sea level because it has a lower concentration in fresh water from polar ice than in seawater. When polar temperatures decrease, ice volume increases and sea level drops; in warmer periods, ice volume decreases and sea level rises. The increased gamma radioactivity in the clays in the Sea of Japan is associated with long periods Age (Ma) of drought when loess deposits erode and are transported eastward by prevailing winds. When the Asian climate warms, the major deposition in the Sea of Japan is much lower in radioactive K as well as A1, and the deposits are interpreted to be fluvial. The most prominent periodicity of the climatic variation in the Sea of Japan is 41,000 years, corresponding 2.0 with the predicted Milankovitch cycle caused by changes in Earth's tilt. Figure 9 illustrates how the downhole logs from the Sea of Japan can be used to determine characteristic periodicities for a time interval. Note that 2.5 different instruments vary in their time resolution. All i,-1,,, i,,, i,, ,i three resistivity logs show the characteristic 41,000-year 100 60 20 periodicity, as well as variability at shorter periods, using two of the three different tools, the shallow resistivity log 798B Gamma Ray with 0.75-m resolution and the FMS log with 5-mm Incr. terrigenous content resolution (see Figure 4). Worthington [1990] described a relationship based on the Nyquist periodic sampling Figure 8. Correlation of the natural gamma radiation log in ODP Hole 798A in the Sea of Japan with the marine sO condition between the instrument resolution h (in record from the North Atlantic [from deMenocal et al., 1992]. meters), the sedimentation rate s (in meters per million The depth scale of the log was converted to age using the years), and the cycle period -r (in millions of years) to be paleomagnetic reversal sequence shown. The fine-scale correh = s-r/2. For a given sedimentation rate, climate cycles lation between the logs confirms that aridity during glacial below this resolution limit of the instrument will be periods cause terrigeneous deposits in the Sea of Japan. aliased and appear at lower frequencies. The deep resistivity log has 2-m vertical resolution and requires a sedimentation rate of at least 20 m/m.y. to detect eccen- periods in the range of 2000-10,000 years are not well tricity cycles of 95,000-123,000 or 413,000 years and at understood, but they have been linked to cycles in ocean least 100 m/m.y. to detect precession cycles of 19,000- circulation, iceberg rafting in surface waters, and solar 23,000 years. With the FMS tool the minimum sedimen- radiation [e.g., Dansgaard et al., 1982; Henrich, 1989]. tation rate required for detection of 19,000- to 23,000- The ability to predict these short-period, non-Milankoyear precession cycles is 22 m/m.y., which is commonly vitch events that cause abrupt changes in climate may exceeded in many nonerosive marine environments. Us- benefit society significantly if potential hazards due to sea ing the FMS, cycles at even finer scales can be detected. level rise, drought, or solar radiation can be prevented. For example, the 7-m interval of the FMS log in Figure In summary, the wide variety of properties measured 9 corresponds approximately with a 60,000-year interval using logs can detect periodic changes due to climate or shown in the deep and shallow resistivity logs and re- other driving forces continuously and at extremely fine solves cycles with suborbital periods as short as 6000 scale. Detection methods using downhole data are powyears. Peaks in the spectrum represent the 41,000-year erful, rapid, and complete compared with laboratory cycles as well as the lower-magnitude peaks at suborbital analyses on core samples and are independent of core periods of 6000, 8000, and 12,000 years [deMenocat and recovery, which is rarely continuous in the deeper bore- King, 1995]. The causes and consequences of suborbital holes. 1.0 1.5 18 Glacial Interglacial Jar 5 4.5 4 3.5 3 I''''1 .... I''''1''''1 Old -- ß
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