Paleomagnetism of Sedimentary Rocks

Process and Interpretation

Kenneth P. Kodama

• Publisher: Wiley-Blackwell
• Year: 2012
• ISBN:
  9781118384152
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This book describes the paleomagnetism of sediments and sedimentary rocks, how sediments and sedimentary rocks become magnetized, and how the physical and chemical processes involved can affect the accuracy of paleomagnetism.

Topics covered include depositional and post-depositional remanence acquisition, the detection and correction of compaction-caused inclination shallowing, reduction diagenesis of magnetic minerals, chemical remagnetization, and rotation of remanence by grain-scale rock strain. The book also has a chapter on environmental paleomagnetism, including examples of the new technique of high-resolution rock magnetic cyclostratigraphy and its application to sedimentary sequences.

By emphasising the accuracy of sedimentary paleomagnetism and the magnitude of post-depositional processes that can affect it, the book will be invaluable in the geologic interpretation of sedimentary paleomagnetic data.

Paleomagnetism of Sedimentary Rocks will be welcomed by paleomagnetists, students of paleomagnetism and all Earth scientists who use sedimentary paleomagnetic data in their research. Additional resources for this book can be found at: www.wiley.com/go/kodama/paleomagnetism.

1

The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability

THE IMPORTANCE OF SEDIMENTARY PALEOMAGNETISM

Sediments and sedimentary rocks, both lacustrine and marine, are important targets for paleomagnetists who want to answer questions about global and regional tectonics, about paleoclimate, and about the behavior and history of the Earth’s magnetic field. Of the 9259 results reported in the Global Paleomagnetic Database in 2009, 4971 (54% of them) were attributed to sedimentary rocks. Although estimates indicate that sedimentary rocks make up only about 8% of the total volume of the Earth’s crust (Buchner & Grapes 2011), they are ubiquitous in the thin veneer of crustal rock available to geologists.

There are two significant reasons why sedimentary rock is an important target for paleomagnetic studies. The first is that sedimentary rocks give a nearly conti­nuous record of the geomagnetic field. This is critically important to paleomagnetic studies because it allows undisputed time averaging of the secular variation of the geomagnetic field and the application of the geomagnetic axial dipole (GAD) hypothesis. The GAD hypothesis, that the Earth’s magnetic field has been a dipole at the center of the Earth oriented parallel to the Earth’s rotation axis, is central to the widespread and successful use of paleomagnetism in the Earth sciences. Without it, paleomagnetism would probably not be a subdiscipline of geology. Using the GAD hypothesis, paleomagnetists can calculate the paleolatitudes of rocks and the amount of vertical axis rotation that may have occurred in an area. The amount of time needed to adequately average the effects of secular variation is not easily determined. In fact, some paleomagnetists would argue that the departure of the time-averaged field from the GAD field is caused by a bias in secular variation that persists for millions of years, particularly back in the Paleozoic or Precambrian. Based on the behavior of the Earth’s field over the past 5 million years, when plate motions would not be large enough to affect the observation of geomagnetic secular variation, averaging over several thousand years is generally considered a long enough time to ensure that a GAD field is observed.

The paleomagnetism of igneous rocks is much stronger than that of sedimentary rocks, so it is more robust and withstands the effects of remagnetization more easily than that of sedimentary rock paleomagnetism. However, just averaging the magnetizations of a pile of lava flows is no guarantee that enough time has passed to adequately average the effects of geomagnetic secular variation. For instance, data from the Hawaiian Volcano Observatory (Kauahikaua et al. 1998) show that the number of flows erupted in Hawaii per thousand years over the past 12,000 years varies from 1 to 11 in any given 1000 year period. Based on the sequence of flows erupted over the past 12,000 years and assuming that about 3000 years is needed to adequately average secular variation and obtain the GAD field, anywhere from 6 to 17 flows should be measured for a paleomagnetic study that can be reliably used to reconstruct paleolatitude or for other tectonic applications. Since the volcanic history in any particular region isn’t known in detail, most workers use the amplitude of the circular standard deviation of virtual geomagnetic poles (VGPs) derived from lava flows to estimate whether secular variation has been adequately averaged. The behavior of the geomagnetic field over the past 5 million years is the only guide to the amount of secular variation, i.e. the amplitude of the circular standard deviation, expected if a sequence of igneous rocks has faithfully recorded secular variation.

Although there can be unrecognized unconformities and hiatuses in any sequence of sedimentary rocks, collecting samples from a thick stratigraphic section gives confidence that enough time has been sampled to average paleosecular variation. Knowing the average sediment accumulation rate from magnetostratigraphy, rock magnetic cyclostratigraphy, fossils, or from radiometric control can give assurance that secular variation has been averaged; even without this information, knowing the typical sedimentation rate for different lithologies can however guide sampling strategy and data interpretation (Table 1.1).

Table 1.1 Typical sediment accumulation rates for sedimentary rocks

Sedimentary environment of deposition	Average sediment accumulation rate	Sampling thickness to average secular variation
Deep marine	1 cm/1000 years	∼3 cm
Near-shore marine	0.1–1 m/1000 years	∼0.3–3 m
Continental lacustrine	1 m/1000 years	∼3 m

There is another reason why the continuity of sedimentary paleomagnetic records is important to paleomagnetists. Recent sediments (marine and lacustrine) and high-fidelity records from ancient sedimentary rocks allow the detailed observation of geomagnetic field behavior. A continuous record of Earth’s magnetic field behavior is critical for understanding the generation of the geomagnetic field and for providing constraints on models of the geodynamo. The best constraints on geodynamo models come from records of transitional field behavior during polarity transitions (Merrill et al. 1996). There is a rich array of data from marine sediments showing the behavior of the field during the most recent polarity transition, the Brunhes-Matayama, some 780,000 years ago. Clement (1991) was the first to show preferred longitudinal bands of virtual geomagnetic pole paths during the Brunhes-Matuyama polarity transition using the paleomagnetism of marine sediments. The accuracy of this result was questioned and then modified by observations from igneous rocks, but it was the continuity of the sedimentary paleomagnetic record that was critical to the Clement’s initial observation. Recent work on sedimentary records of secular variation of the geomagnetic field at high latitudes (Jovane et al. 2008) shows field behavior close to the so-called tangent cylinder (latitudes >79.1°), a cylinder parallel to the Earth’s rotation axis that includes the inner core of the Earth. These workers collected 682 samples from a 16 m long marine core at 69.03°S in the Antarctic showing that the dispersion of secular variation was high (about 30°) during the past 2 myr suggesting vigorous fluid motion in the outer core. This kind of record would be nearly impossible to obtain from igneous rocks.

Finally, continuous records of geomagnetic field paleointensity variations from marine and lacustrine sediments not only allow another constraint on geodynamo models, but they can also provide a way to correlate and date marine sediments globally. The best example of this is Valet’s work (Guyodo & Valet 1996, 1999; Valet et al. 2005) on constructing stacked relative paleointensity records S_int200, S_int800 and S_int2000 over the past 200 thousand, 800 thousand and 2 million years, respectively.

The second important reason for the large number of sedimentary paleomagnetic results in the Global Paleomagnetic Database is that paleohorizontal can be unequivocally determined from sedimentary rocks. Knowing the paleohorizontal may seem trivial, but it is not always straightforward to determine for igneous rocks and it is absolutely critical for determining the paleolatitude of the rocks from the paleomagnetic vector, assuming a GAD field. Only the bedding of sedimentary rocks unambiguously gives the ancient horizontal. Intrusive igneous rocks provide no record of the paleohorizontal; it must be detected indirectly, sometimes by the rare occurrence of layered early crystallized minerals (Cawthorn 1996) or by techniques like the aluminum-in-hornblende paleobathymetric technique (Ague & Brandon 1996). One example of this approach comes from the paleomagnetic study of the Cretaceous Mt Stuart batholith. The paleomagnetism of the Mt Stuart batholith provides an important paleomagnetic data point in the argument for large-scale translation of Baja British Columbia along western North America’s continental margin (Cowan et al. 1997). However, its anomalous paleomagnetic inclination could just as easily be explained by wholesale tilting of the batholith after it was magnetized. The small amount of tilt of the batholith, 7° according to the aluminum-in-hornblende paleobathymeter (Ague & Brandon 1996), argues for tectonic transport. It is not as convincing as paleohorizontal obtained from sedimentary bedding however, thus leaving the tectonic transport interpretation ambiguous. Another example of how the paleohorizontal of intrusive igneous rocks can only be determined indirectly comes from a paleomagnetic study of the Eocene Quottoon plutonic complex in the Coast Mountains of British Columbia. In this case, 12–40° tilting of the pluton is inferred from a regular decrease in exhumation age from west to east across the pluton determined by a transect of K–Ar ages (Butler et al. 2001). In the Quottoon study the amount of tilting has a large effect on the interpretation of the paleomagnetic inclination of the rocks.

Extrusive igneous rocks of course have a much better control on the paleohorizontal, either from direct measurement of the layering in the lava flows or from the bedding of sedimentary rocks intercalated between the flows, but there can still be ambiguities. One concern to paleomagnetists is the problem of initial dip of extrusive igneous rocks, particularly of highly viscous volcanic flows such as andesitic rocks. Strato-volcano edifices can have initial, non-tectonic dips up to 35–42°. Even low-viscosity basaltic flows can have dips as large as 12° (MacDonald 1972; Francis 1993; Gudmundsson 2009). These unrecognized dips contribute error to the measurement of the paleohorizontal for extrusive igneous rocks and hence in the paleolatitude determined from these rocks. One good example of the possibility of unrecognized tilt in igneous rocks comes from the work of Kent & Smethurst (1998) in which the frequency distribution of inclinations from the Global Paleomagnetic Database are binned into different time periods (Cenozoic, Mesozoic, Paleozoic and Precambrian) to see if they are consistent with the GAD hypothesis throughout Earth’s history. The Cenozoic and Mesozoic data have inclination frequency distributions consistent with the GAD, but the Paleozoic and Precambrian bins have more low inclinations than predicted by random sampling of the GAD. Even sedimentary inclination shallowing is ruled out as the cause because exclusively igneous results show the effect. While Kent and Smethurst speculate that octupolar geomagnetic fields contributed to the dipole in these distant times, Tauxe & Kent (2004) point out that uncorrected and unaccounted-for dips of igneous rocks, both extrusive and intrusive, could also explain the effect.

Sediments and sedimentary rocks therefore have an important role to play in paleomagnetic studies because of the continuous record they provide and because their bedding planes give an unequivocal record of the ancient horizontal.

ENVIRONMENTAL MAGNETIC RECORD FROM SEDIMENTS AND SEDIMENTARY ROCKS

The continuity or near continuity of the sedimentary record, particularly when compared to igneous rocks, also makes sedimentary rocks an important target for paleoenvironmental studies. In environmental magnetic studies, the magnetic minerals of sedimentary rocks can record paleoenvironmental conditions. While sedimentary paleomagnetism uses directional and intensity records of the geomagnetic field for correlation, dating, and paleolatitude information, environmental magnetism uses parameters that measure the concentration of magnetic minerals, magnetic particle grain size, and magnetic mineralogy of sedimentary rocks as proxies of the paleoenvironment. Several important books have been written on environmental magnetism (Thompson & Oldfield 1986; Evans & Heller 2003) and the reader is referred to them for a more complete treatment. Chapter 8 will however introduce and cover rock magnetic cyclostratigraphy in detail, which uses environmental magnetic principles. Rock magnetic cyclostratigraphy is an exciting new use of mineral magnetic measurements that provides high-resolution chronostratigraphy for sedimentary rock sequences.

Astronomically driven climate cycles are known to be recorded by sedimentary sequences lithologi­cally (Hinnov 2000), but the environmental mag­netics of the rocks can be a very sensitive detector of Milankovitch-scale climate variations. The rock magnetic record becomes particularly important when climate-driven lithologic changes are difficult to identify in sedimentary rocks (Latta et al. 2006). Ultimately, rock magnetic cyclostratigraphy can provide 20 kyr resolution, much better than even the best magnetostratigraphy that records even the shortest geomagnetic polarity chrons. Pioneering efforts by Ellwood (e.g. Ellwood et al. 2010, 2011) looking at magnetic susceptibility variations in stratigraphic type localties have not focused exclusively on the magnetic response to astronomically driven climate cycles. Measurements that examine the concentration variations of only depositional remanent magnetic minerals (magnetite or hematite) can be more straightforward to interpret than susceptibility measurements that respond to concentration variations of diamagnetic (calcite, quartz), paramagnetic (iron-bearing silicates), and remanent magnetic minerals. Concentration variations of remanent magnetic minerals therefore have the potential to provide cleaner records of global climate cycles, either run-off variations from the continents or global aridity.

THE EVIDENCE FOR HIGH-QUALITY PALEOMAGNETIC DATA FROM SEDIMENTARY ROCKS

A main focus of this book is the accuracy of sedimentary paleomagnetic records, including some processes that can cause inaccuracies and biases in sedimentary paleomagnetism. The starting point of this discussion must however be the understanding that sediments and sedimentary rocks can and do provide very high-quality and accurate records of the Earth’s magnetic field throughout geologic time. The intent of this book is not to give the impression that there are insurmountable problems with sedimentary paleomagnetic data. Rather, the aim is to discuss some of the very important inaccuracies that can arise in sedimentary paleomagnetic data. These inaccuracies tend to be essentially second-order effects, but prevent paleomagnetism from achieving its full potential as an important tool for the Earth sciences.

The evidence that young sediments can provide a good record of the geomagnetic field is plentiful. I will show some examples, but it is by no means an exhaustive list. In doing so I will also give an estimate of the repeatability of these sedimentary records of the geomagnetic field; this is not so much a rigorous measure of the accuracy of the sedimentary paleomagnetic recorder, but a way of estimating the precision of the very best sedimentary paleomagnetic recorders. This is probably the only way to understand the accuracy of the paleomagnetism since, for most cases, the true direction and intensity of the geomagnetic field is not known. In this approach I will rely on studies in the scientific literature that report on multiple records from cores that sample sediments, both lake and marine, of the same age. The best records come from the most recent sediments which have not yet been appreciably affected by post-depositional processes, chemical and physical, that can affect the magnetization’s accuracy and precision. Some of these studies report the scatter in inclination and declination down-core, others simply show plots of the agreement between multiple records of the field from which the scatter can be estimated. I do not calculate statistical parameters from these records but simply show, from digitizing the plots, the range of scatter in inclination and declination. The point of this exercise is to give the reader a better feeling for the repeatability of paleomagnetic records of the field and hence an estimate of their accuracy.

Before embarking on an examination of multiple records of the recent geomagnetic field, we will briefly consider how sediments become magnetized parallel to the Earth’s field. The process by which sediments become magnetized is called the depositional or detrital remanent magnetization (DRM) process. In this process individual iron oxide, sub-micron-sized magnetic particles, typically magnetite (Fe₃O₄), become oriented so their magnetic moments are statistically biased toward the ambient Earth’s magnetic field during deposition. There is one basic theory for how this process occurs in nature, and all modifications of this theory will be covered in detail in Chapter 2. Geologist...

Table of contents

• Cover
• Dedication
• Title page
• Copyright page
• 1 The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability
• 2 The Magnetization Mechanism of Sediments and Sedimentary Rocks: Depositional Remanent Magnetization
• 3 Post-Depositional Remanent Magnetization
• 4 Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause
• 5 How to Detect and Correct a Compaction-shallowed Inclination
• 6 Post-Depositional Diagenesis and Chemical Remanent Magnetization
• 7 Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts
• 8 Magnetization of Sediments and the Environment
• 9 The Magnetization of Sedimentary Rocks: Processes and their Interpretation
• Glossary of Paleomagnetic and Rock Magnetic Acronyms
• References
• Index