Boggs S. Principles of Sedimentology and Stratigraphy

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13.3 Sequence Stratigraphy

457

level for streams that dra to the ocean. Changes in a stream's graded profile

can either create or remove accommodation space. Such changes can include

changes  discharge, sediment supply, channel form, and uplift, as well as the po-

sition of the bayle (sea level). Application of sequence-stratigraphy concepts to

nonmarine systems is beg actively researched but is still controversial. The gen-

eral view is that the lower reaches (100-150 km) of fluvial systems are most likely

to be greatly affected by base-level changes and that it is this portion of fluvial sys-

tems that is most likely to be preserved in the stratigraphic record (see Shanley

and McCabe, 1994, and Vcent, Macdonald, and Gutteridge, 1998).

Global Sea-Level Analysis

General Principles. One of the most controversial applications of sequence

stratigraphy concepts is the analysis of ancient sea levels. As discussed through

out this book, changes  sea level have an important bearing on sedimentation

pattes. Studies of sea-level changes have special relevance with respect to analy

sis of cyclic successions  the stratigraphic record. Sea-level changes through time

have been studied particularly intensively by P. R. Vail and his associates at the

Exxon research laboratory in Houston (e.g., Va il, Mitchum, and Thompson, 1977a,

1977b; Haq, Hardenbol, and Vail, 1988). These authors used seismic data and sur

face outcrop data to tegrate occurances of coastal onlap, marine (deep-water)

onlap, baselap, and toplap to a model that involves asymmetric cycle oscilla

tions of relative sea level.

Vail and his group inferred changes in relative sea level by reference to

coastal onlap charts. These charts were constructed by estimating from seismic

profiles the magnitude of sea-level rise, as measured by coastal aggradation (the

thickness of coastal sediments deposited during sea-level rise). The amount of sea

level drop is determed by measuring the magnitude of downward shifts in

coastal onlap, that is, the elevation (vertical) difference between the point of max

imum coastal onlap reached at maximum sea level and the point of maximum sea

level fall, which is determined from the seismic records by the position where the

next (younger) onlap unit lies above the unconformable surface produced during

the sea-level fall (Vail, Mitchum, and Thompson, 1977a; Vail, Hardenbol, and

To dd, 1984). The procedures used  constructg a relative coastal onlap chart

om coastal and mare sequences are illustrated  Figure 13.18.

The first step volves analysis of sequences such as those shown as units A

through E of Figure 13.18A. Sequence boundaries, areal distributions, and the

presence or absence of coastal onlap and toplap are determined by tracing reflec

tions on seismic profiles. Available age controls from well data are used to estab

lish the geologic-time range of each sequence. An environmental analysis is also

made from seismic and other available data to distinguish coastal facies from ma

ne facies. The second step is to construct a chronostratigraphic (time-stratigraphic)

chart of the sequences, a procedure first described by Wheeler (1958). Both stratal

surfaces and unconformities give time-stratigraphic information. Because they are

depositional surfaces, the seismic response to strata surfaces are assumed to be

chronostratigraphic reflectors. In addition, because seismic reflectors are isochro

nous (have the same age everywhere), they can cross lithologic boundaries. That is,

the seismic reflections from a given surface may extend laterally through a variety

of lithofacies. Seismic reflectors may be traced continuously, for example, through a

shelf system, over the shelf edge, and downward through an equivalent slope sys

tem. Unconformities are not isochronous surfaces; however, strata below an un

conformity are older than strata above it. Therefore, strata between unconformities

constitute time-stratigraphic units. After determining the ages of depositional

sequences, such as those shown on the stratigraphic cross section in Figure

13.18A, from well-control or other information, workers plot the stratigraphic

458

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

gu 13.18

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D ASTAL DEPOSITS

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t COASTAL AGGAOAT:ON

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Diagram illustrating the proce

dures used by Exxon geologists

for constructing a regional cycle

of relative coastal onlap. [After

Vail, P. R., R. M. Mitchum, Jr., and

S. Thompson, Ill, 1977, Seismic

stratigraphy and global change of

sea level, Part 3: Relative changes

of sea level from coastal onlap, in

Payton, C. E. (ed.), Seismic strati

gaphy-Applications to hydrocar

bon exploration: Am. Assoc.

Petroleum Geologists Mem. 26,

Fig. 13, p. 78, reprinted by per

mission of PG, Tulsa, Okla.]

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information against geologic time to construct a chronostratigraphic correlation

chart (Fig. 13.18B). Such charts are referred to as Wheeler diagrams. The final step

the procedure is to identify cycles of relative coastal onlap in each seismic se

quence, measure the magnitude of aggradation and seaward downshifts in coastal

onlap that result from relative se and fall of sea level, and plot these changes and

sea-level stdslls against geologic time as shown in Figure 13.18C. Thus, the

magnitude of coastal aggradation is a measure of a relative rise in sea level; a rela

ve standstill is indicated by coastal toplap; and seaward shifts in coastal onlap

indicate a relative sea-level fall. The plots of relative coastal onlap are repeated for

each sequence (cycles A though E of Fig. 13.18C) to complete the relative coastal

onlap chart. Finally, the changes in relative coastal onlap are used as the basis of

inferring changes in relative sea level. For simplicity, it is assumed that there has

been

no subsidence of the margin and that the bed-thinning eects of compaction

under deep burial have been considered (original thickness restored).

The time interval occupied by a relative se and fall of sea level, as ter

preted from a coastal onlap chart such as that shown in Figure 13.18C, constitutes

cycle of relave sea-level change in a region. Vail, Mitchum, and Thompson

Plioc.

Pieistoc.

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Oligocene

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Jurassic

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13.3 Sequence Stratigraphy

459

Figure 13.19

Eustatic sea-level curves for Phanerozoic

time. (A) Hallam, 1984; (B) Vail, Mitchum,

and Thompson, 1977b. [From Hallam, A.,

1984, Pre-Quaternary sea-level changes:

Ann. Rev. Earth and Planetary Sciences, v.

12, Fig. S, p. 220, reprinted by permission.]

(1977b) correlated regional cycles and used this information to construct compos

ite charts of global cycles of relative sea-level change. ey interpreted these cy

cles to be worldwide and apparently to be controlled by absolute or eustatic

sea-level changes. Figure 13.19 shows the chart published by Vail, Mitchum, and

Thompson (1977b); a somewhat different chart constructed by Hallam (1984) from

mpilations of continental flooding is shown also for comparison. Upon publica

tion, the Exxon coastal onlap charts, and the relative sea-level curves inferred from

these charts, immediately generated lively interest, discussion, and controversy

among geologists. They continue to be a focus of controversy.

Reliability of Sea-level Analysis from sequence-Stratigraphic Data.

Following publi

cation of the coastal onlap curves of Vail, Mitchum, and Thompson (1977b), several

workers (e.g., Brown and Fisher, 1980; Kerr, 1984; Miall, 1986) challenged the basic

premise of the Vail group that these curves primarily reflect eustatic changes in

sea level, and they pointed out that tectonism (basin subsidence) and rates of sed

imt 5pply also affect these curves. Nonetheless, interest by the geologic com

munity in sea-level analysis continued at a high level. The publication in 1988 of

the SEPM Special Publication Sea-Level Changes: An Integrated Approach, edited by

Wilgus et a!., is an indication of this interest. This volume contains several articles

on analysis of sea-level changes as well as articles dealing with sea-level changes

460

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Figu 13.20

and sequence stratigraphy. In this volume, Hag, Hardenbol, and Vail (1988) pre

sent a new generation of coastal onlap curves and corresponding eustatic sea-level

curves for the Mesozoic and Cenozoic (originally published by the same authors

in 1987) that have been the subject of considerable subsequent discussion and con

troversy (e.g., Christie-Blick, Mountain, and Miller, 1990; Hallam, 1998; all,

1991, 1992, 1994, 1997; Walker, 1990). Haq, Hardenbol, and Vail (1988) indicate at

these new curves are based on well-log and outcrop data as well as seismic data

and have greater resolution than that obtainable from seismic data alone. These

highly

detailed curves are not reproduced in full here; however, a much simplified

portion of the late Te rtiary and Quateary curve is shown in Figure 13.20 to illus

trate the coastal onlap chart and show the nature of the second-order and thi

order sea-level cycles interpreted from this chart Figure 13.21 shows a more

complete, but still simplified, part of the cycle chart for the Te rtiary.

Christie-Blick, Mountain, and Miller (1990) provide a comprehensive cri

tique of seismic stratigraphy and its usefulness in sea-level analysis and interpre·

tation of the stratigraphic record. With respect to the Haq, Hardenbol, and V

(1988) curves, Christie-Blick et a!. are particularly conceed by what they call the

uncritical interpretation of all second- and third-order boundaries as resultg

from eustatic sea-level changes. They matain that amplitudes of eustatic fluctu

ations cannot be inferred from seismic stratigraphic data alone because coastal

aggradation (the vertical component of onlap) is primarily a result of basin subsi

dence, not sea-level rise. Furthermore, downward shifts  onlap reflect only the

rate of sea-level fall  relation to the rate of bas subsidence. They suggest that

the large component of basin subsidence cannot be easily or objectively removed

to derive the smaller eustatic signal. Christie-Blick et a!. point out further that an

other major limitation of the global onlap chart is the uncertainties about the li

bration of many boundaries to the geologic time scale. Another potential problem

in interpretation arises from the operation of autocyclic mechanisms (Chapter 12),

such as delta switching, that can generate small-scale cycles. Miall (1991, 1992,

1994) also concludes that the implied precision of the Mesozoic-Cenozoic global

Coastal onlap

3rd ord er sea-level

cycles

Age

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years

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Coastal onlap and long-term (second-order)

and short-term (third-order) sea-level curves

for the late Tertiary and Quaternary, as de

termined by Haq et al., 1988. [Redrawn

from Haq, B. U., J. Hardenbol, and P. R. Vail,

1988, Mesozoic and Cenozoic chronostratig

raphy and cycles of sea-level change, in Wil

gus, C. K., et al. (eds.), Sea-level changes:

----- 1

---

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An integrated approach: Soc. Econ. Paleon

tologists and Mineralogists Spec. Pub. 42,

Fig. 14, p. 94, reproduced by permission of

SEPM, Tulsa, Okla.]

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13.3 Sequence Stratigraphy

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• S£££  IN

REY10US CYC CHARTS

CENOZOIC·. B. U. HAO, J. HAENBOL, R R. VAIL

R. C. WRIGHT, L E. STOVER, G. BAUM. T. LOUTIT.

A. GOMBOS. T DAV' ES, C. PR.UM, K. ROMINE.

H. POSAMENTIER, R. JAN DuCHENE

cycle chart of Haq, Hardenbol, and Vail (1988) is not justified. He maintains that

e chart is not an independently tried and tested global standard and that the im

plied precision is unsupportable because it is greater than that of the best available

chronostratigraphic techniques, such as those used to construct the global stan

dard time scale (Chapter 15).

In spite of these criticisms, interest in sea-level analysis and sequence stratig

raphy in general remains high. To quote Plint et aL (1992), "The Exxon sea-level

curve is here to stay, although it is likely to undergo progressive evolution and re

nement as more data are gathered and better chronostratigraphic control be

comes available." Over the next several years, geologists all over the world will

surely be watching the outcome of the sea-level controversy. Hallam (1998) points

out that relative sea levels can be studied by using an alteate approach that em

ploys facies analysis. Fundamentally, this approach involves estimating relative

sea-level changes by studying shallowing-upward successions  carbonate and

siliciclastic rocks.

Figure 13.21

Simplified global sequence

chart for part of the Te r

tiary and Quaternary. The

polarity column shows the

geomagnetic time scale

(explained in Section

13.4); series and stages are

discussed in Chapter 15.

[After Haq, B. U., ). Hard

enbol, and P. R. Vail, 1987,

Chronology of fluctuating

sea levels since the Triassic:

Science, v. 235, Fig. 2,

by the AMS. As redrawn

by Vail et al., 1991.]

462

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

Summa Remar. The sequence concept produced a revolution in stratigraphy

during the 1980s and early 1990s and gave rise to the term sequence stratigraphy,

defined as the study of rock relationships within a chronostratigraphic (time

stratigraphic} framework of repetive, genetically related strata bounded by sur

faces of erosion or nondeposition or their correlative conformities (Van Wa goner

et al., 1988). Numerous geologists have embraced the sequence stratigraphy con

cept, and it is currently widely applied, particularly within the peoleum industry,

to a variety of stratigraphic successions. Since the initial concept was proposed,

numerous papers on sequence stratigraphy have been presented at scientific

meetings and published in the geological literature. The concept has not, however,

been without its detractors, as menoned. It has been cricized, among other

things, for being largely theoretical and introduced without specific, worked-out

examples; for inadequate attention to problems of scale; and for an overemphasis

on eustatic sea-level cycles without due regard to the effects of local tectosm and

sediment supply. Also, it is diicult to apply the sequence concept to nonmarine

and deep-sea sediments, which may not be subdivided into well-defined uncon

formity-bounded units.

Nonetheless, the sequence-stratigraphic concept is now widely embraced

and has certainly revived interest in stragraphy and produced many new ideas.

The aspect of sequence stratigraphy that deals with global eustasy has been the

most severely criticized part of the theory. As discussed, many geologists simply

do not believe that reliable global sea-level charts can be constructed from 

quence-stratigraphic data. On the other hand, most geologists agree that relative

sea-level changes do affect the geometry and facies distribuon of sediments de

posited on continental margins and within sedimentary basins. Such facies can 

effectively studied and interpreted by using outcrop and subsurface data within a

sequence-stratigraphic framework. Thus, sequence stratigraphy appears to be a

useful tool for stratigraphic analysis even if the global nature of sea-level curves

produced by the Exxon group cannot be proven.

Galloway (1989) proposed an alternative method for defining sequences

based on the concept of repetitive episodes of progradation punctuated by peri

ods of transgression and ooding of depositional surfaces. Galloway's sequences

are bounded by features formed during maximum submergence, in contrast to the

Exxon sequences, which are bounded by features formed during maximum emer

gence. At this time, the Exxon scheme for defining sequences appears to be more

widely understood and used.

13.4 MAGNETOSTRATIGRAPHY

General Principles

Magnetic stratigraphy, or magnetostratigraphy, is a relatively new branch of

stratigraphy developed largely since about the middle 1960s. The principles of

magnetic stratigraphy were initially applied to the study of volcanic rocks and

sediments younger than about 5 million years. Magnetic stratigraphic techniques

have now been applied to much older rocks, and a detailed magnetic polarity time

scale has been extended to the Jurassic and parts of the older record. Magnetic

stratigraphy came about through the discovery that magnetic iron-rich minerals

in igneous and sedimentary rocks can preserve the orientation or field direction of

Earth's magnetic field at the time the rocks were formed. During the cooling of

molten rock, iron-bearing minerals become magnetized in alignment with Earth's

magnetic field as they cool through a critical temperature of about 500°C-600°C

(for magnetite), the Curie point. As they approach this temperature, the influence

of the magnetic field exerts itself, and the atomic-scale magnetic fields within the

13.4 Magnetostratigraphy

463

crystal lattices of the minerals begin to line up parallel to one another and to the

direction of e magnetic lines of force around Earth. With further cooling, these

atoms become locked into this orientation, and each mineral in essence becomes a

small magnet, having polarity consistent wi Earth's magnetic field. These mag-

netic properties are retained for geologically long periods of time unless the rocks

are again heated to near the Curie point. Therefore, this residual magnetism 

called remanent magnetism, or thermal remanent magnetism (TRM).

During deposition of sediments, small magnetic mineral grains are able to

rotate in the loose, unconsolidated sediment on the depositional surface and thus

align themselves mechanically with Earth's magnetic field. This (statistically) pre

ferred orientation of magnetic minerals in sedimentary rocks imparts bulk mag

netic properties to the rocks, referred to as depositional remanent magnetism or

deital remanent magnetism. Because sediment grains can be disturbed by bio

turbating organisms or by physical and chemical processes during burial and dia

genesis, the magnetization of sedimentary rocks is less stable, as well as weaker,

than that of volcanic lavas. The study of remanent magnetism in rocks of various

ages to determine the intensity and direction of Earth's magnetic field in the geo

logic past is called paleomagnetism.

Remanent magnetism is measured by instruments called magnetometers.

Early magnetometers were capable of making paleomagnetic measurements only

in ieous rocks and hily magnetized iron-bearing red sediments. Modem su

perconducting magnetometers can measure the magnetism  much more weakly

magnetic sediments, including carbonates. Remanent magnetism is complex and

can include secondary magnetism caused by prolonged eects of Earth's present

magnetic field or by chemical changes owing to alteration of one magnetic miner

al to another. Demagnetization techniques are available for destroying this sec

ondary magnetic effect in the laboratory so that the primary magnetization can be

measured. It is this primary magnetic component, which records Earth's geomag

netic field at the time volcanic or sedimentary rocks formed, that is of interest in

satigraphic studies.

The significance of primary remanent magnetism for stratigraphic studies

stems from the fact that Earth's maetic field has not remained constant through

out geologic history but has frequently reversed. The geomagnetic eld is gerat

ed in some poorly understood way by the motion of highly conducting nickel-iron

fluids in the outer part of Earth's core; this motion is assumed to be controlled by

thermal convection and by the Coriolis force generated by Earth's rotation, consti

tuting what is called a self-exciting dynamo (e.g., Butler, 1992; Merrill, McElhin

ny, and McFadden, 1996). Studies of the remanent magnetism in igneous and

sedimentary rocks show that the dipole (main) component of Earth's magnetic

field has reversed its polarity at irregular intervals from Precambrian time on

ward, apparently because of instabilities in outer-core convection (Merrill, McEl

hinny, and McFadden, 1996). ' hen Earth's magnetic field has the present

orientation, it is said to have normal polarity. When this orientation changes 180°,

it has reversed polaty.

Figure 13.22 illustrates diagrammatically the magnetic lines of force around

Earth during normal and reversed polarity epochs. Note that during a time of nor

mal polarity the magnetic lines of force appear to "flow" out of Earth at the geo

graphic South Pole, bend around Earth, and reenter at the geographic North Pole.

Thus,

a compass needle hinged vertically (swings up and down) would point

down (into Ear) at the North Pole and up (out of Earth) at the South Pole. The

flow direction of the magnetic lines of force, and the direction of a compass nee

dle, would reverse during an episode of reversed polarity.

Reversals of Earth's maetic field are recorded in sediments and igneous

rocks by patterns of normal and reversed remanent magnetism. e direction of

magnetization of a rock is defined by its north-seeking magnetization. If the

464

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

(A) NORMAL

(B) REVERSE

Geographic Noh

Figure 13.22

Schematic representation of Earth's magnetic

field during episodes of (A) normal and (B) re

versed polarity. [After Wylie, P. )., 1976, The

way the Earth works: john Wiley and Sons,

Inc., Fig. 9.1, p. 120, reprinted by permission.]

north-seeking magnetization of rocks points toward Earth's present magnetic

north pole, the rock is said to have normal-polarity magnetization. If the north

seeking magnetization points toward the present-day south maetic pole, the

rock has reversed-polarity magnetization, or reversed polarity. Thus, sedimenta

ry and igneous rocks that display bulk remanent magnetic properties of the same

magnetic polarity as the present maetic field of Earth have normal polarity,

whereas those that have the opposite magnetic orientation have reverse polarity.

These geomagnetic reversals are contemporaneous worldwide phenomena.

Thus, they provide unique stratigraphic markers in igneous and sedimentary

rocks. The reversal process is thought to take place over a period of 1000-10,000

years (e.g., Clement, Kent, and Opdyke, 1982). The intensity of the maetic field

decreases by 60-80 percent over a period of about 10,000 years preceding reversal.

The actual reversal requires about 1000-2000 years, followed by a buildup of 

tensity for the next 10,000 years (Cox, 1969). The last unquestioned reversal of the

magnetic field took place approximately 700,000 years ago, although a brief rever

sal

or excursion of the field may have occurred about 20,000 years ago.

[Note: The intensity of Earth's magnetic field has been declining over the past

few centuries. It has decreased by 10 percent  the past 300 years, and the rate of

decline is increasing. It is possible that the magnetic field will fall to nearly zero in

the next millennium, leading scitists to speculate that another magnetic reversal

is in the making (Nova, 2003). Think what that would do to compass dictions!]

the early years of paleomaetic study, intervals of reversed or normal po

larity lasting 100,000 years or more were called epochs and those having a dura

tion of about 10,000-100,000 years were called events. Geomagnetic polarity

reversals

are now known to occur on a much broader spectrum of time scales

ranging from less than 10,000 to gater than 10 million years. Maetic stratigra

phy is based on these changes in polari recorded in sediments or volcanic rocks

that produce recognizable patterns of alternating-polarity stratigraphic units that

can be used for chronological and correlation purposes.

Sampling, Measuring, and Displaying Remanent Magnetism

To determine remanent magnetism  sedimentary rocks, geologists commonly re

move samples from the field for subsequent laboratory analysis (see McElhinny

and

McFadden, 2000, and Ta uxe, 2002, for details), although techniques have been

developed to measure remanent magnetism in well bores (e.g., Bouisset and Au



gustin, 1993). Three kinds of samples may be taken:

1. Samples cored with a portable drill. Cores taken by this method are com

monly 2.5 em in diameter and 6-12 em long. Before cores are broken out of the

rock on the outcrop, the orientation of the cores must be determined and

13.4 Magnetostratigraphy

465

marked on the sample. The inclination (dip) of the core axis is determined and

the azimuth of the core axis (deviaon om geographic north) is measud by

use of a magnetic and/ or sun compass.

2. Oriented block (hand) samples. These samples, which are broken om the

outcrop with a hammer, are easier to obtain an core samples but present

more difficulties with later orientation in the laboratory instruments.

3. Cores of lake- or ocean-bottom samples. Sediment cores, commonly obtained

by piston coring apparatus, are assumed to penetrate the sediment vertically

but are azimuthly unoriented.

In e laboratory, the remanent magnesm of a rock specimen is measured by

means of a magnetometer. Several types of magnetometers (balanced fluxgate, cy

genic, astac, spier) are used (Hailwood, 1989; Butler, 1992). Three orthogo

nal

components of magnetism are commonly measured; these three components

are then combined to give the diction and intensity of the magnetic vector of the

specimen. Because most rocks carry different components of magnetization that

have been acquired at different times, the signature of secondary maetism must

 rem oved to reveal the primary remanent magnetism. Secondary magnetization

may be removed by progressive demagnetization methods such as alteating

field demagnezation, thermal demagnetization, and chemical demagnetization.

Once primary remanent magnetism has been measured, vector dictions in

paleomagnetism are described in terms of the following:

1. Inclination (with respect to component in the original bed at the collecting

station)

2. Declination (with respect to geographic north) (Fig. 13.23)

Inclination is called positive if downward directed and negative if upward direct

. Positive inclination in the Northern Hemisphere indicates normal polarity;

negative inclination means reversed polarity. Inclination directions are opposite

lative to the polarity in the Southern Hemisphere. Inclination and declinaon to

gether define the geomagnetic field vector (F in Fig. 13.23). Inclination is a func

on of the latitude at which the rock specimens formed, and declination shows the

deviation of the ancient paleomaetic pole from the geographic pole. The polaty

data are commonly displayed graphically in polarity-reversal satigraphic

columns by plotng intervals of normal polarity in black and intervals of reversed

Geographic noh

Vertical

component

Magnetic north

: (horizontal component)

re 13.23

Description of the direction of the geomagnetic field. The total magnetic

field vector F can be divided into a vertical component and a horizontal

component. The angle 0 is the declination, the azimuthal angle between

magnetic north and geographic north. The angle I is the inclination (dip),

the angle between the horizontal and F.

466

Chapter 13 I Seismic, Sequence, and Magnetic Stratigraphy

gu 13.24

Late Cenozoic (Pliocene-Pleistocene) geomagnetic polarity time

scale. Each horizontal line in the columns labeled normal polarity,

intermediate polarity, or reversed polarity represents an igneous

rock for which both K-Ar age and geomagnetic polarity have been

determined. Auxiliary information from marine magnetic anomaly

profiles and deep-sea core paleomagnetism have also been used.

Black pattern in the dominant-polarity column indicates normal

polarity; white indicates reversed polarity. Arrows indicate disputed

short polarity intervals or geomagnetic "excursions"; numbers to

the right of the polarity column indicate interpreted ages of polari

ty boundaries. [After Maniken, E. A., and G. B. Dalrymple, 1979,

Revised geomagnetic polarity time scale for the interval 0-5 m.y. B.

P. : jour. Geophysical Research, v. 84, Fig. 3, p. 624.]

K-Ar

Age

(Ma)

0.0

1.0

2.0

3.0

4.0

5.0

�







·�



-E







i





i



z 

i



•







•



•

!

�

·�





.







Age

Polarity

Pola

�



(Ma)

event

epoch





Laschamp ?

Blake ?

l











1.67



Olduvai

�

1.87



Reunion

�x· ?









polarity in white (e.g., Fig. 13,24). Dating of the polarity intervals by radiometric and

biochronologic methods forms the basis of the maetic polarity me scale, de

scribed in the next paragraphs. Data may be plotted also as virtual geomaetic pole

(VGP) latitudes. e VGP represents the position of the eective north geomagnetic

pole calculated fm both inclination and declination information (Hailwood, 1989).

Magnetic Polarity Time Scales

The concept of remanent maetism is well known to today's students of geology;

however, only a few studies of rock magnetism had been made prior to the 1960s.

The basic principles of magnetostratigraphy were developed in the early and

middle 1960s in the remarkably short time of about five years by two groups of