The occurrence and origin of Late Paleozoic ...gilder/Re... · new applications of the paleomagnetic method to other areas of Earth science, particularly in studies of the physical

THE OCCURRENCE AND ORIGIN OF LATE PALEOZOIC

REMAGNETIZATION IN THE SEDIMENTARY ROCKS

OF NORTH AMERICA

Chad McCabe

Department of Geology and Geophysics Louisiana State University, Baton Rouge

R. Douglas Elmore School of Geology and Geophysics University of Oklahoma, Norman

Abstract. Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are

in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergo- ing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in dif-

ferent settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite-bearing sandstones and carbonates. In magnetite- beating carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain- building events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids.

INTRODUCTION

Despite numerous investigations spanning four decades, the paleomagnetic data base for the early and middle Paleozoic of North America remains sparse and the subject of controversy. A major cause of the slow progress in this area has been the development of the paleomagnetic method itself. Advances in laboratory procedures, data analysis techniques, and instrumentation have occurred continuously since the late 1940s when the first investigations of Paleozoic rocks in North America were undertaken

by John Graham. Older results, particularly those which were not based on stepwise demagnetization, are now viewed with suspicion, and many older studies have been repeated using modem techniques. Technological progress notwithstanding, the full potential of the paleomagnetic method in Earth science has not been realized, particularly for Paleozoic and earlier times. The greatest obstacle in this regard has been our incomplete understanding of the processes by which rocks become magnetized. Although we can now separate magnetization components acquired in different magnetization events with a high degree of confidence, we often do not fully understand the geologic context of the magnetization events. The processes by

which different rock types acquire a magnetization at the time of their formation are reasonably well understood. However, for the Paleozoic sedimentary rocks of North America, the most persistent and troublesome question concerns the possibility of partial or complete remagnetization at some time well after the depositional event.

Impodance of Remagnetization in Earth Science Research conducted during the 1980s has yielded

unequivocal evidence that sedimentary rocks over a large portion of North America (and Europe) were remagnetized during the Pennsylvanian and Permian periods. The exact areal extent and ultimate cause(s) of this remagnetization remain subjects of continuing controversy and active investigation. The general recognition of the massive scale of the late Paleozoic remagnetization event in North America has contributed to a minor revolution in Paleozoic

paleomagnetism. A comparison of reviews of the Paleozoic data base for North America by Van der Voo [1981, 1989] indicates the magnitude of this revolution: many of the paleopoles from pre-Pennsylvanian rocks cited by Van der Voo [1981] as "reliable" had been shown to be late Paleozoic remagnetizations by the time of his 1989 review. The recognition of the prevalence of remagnetiza-

Copyright 1989 by the American Geophysical Union.

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471 ß

Reviews of Geophysics, 27, 4 / November 1989 pages 471-494

Paper number 89RG02844

472 ß McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION 27, 4/REVIEWS OF GEOPHYSICS

tion has stimulated new interest in Paleozoic apparent polar wander for North America and has underscored the necessity for detailed laboratory experiments and field tests (e.g., fold test and conglomerate test) to bracket magnetization acquisition ages. Recent investigations have produced new poles of demonstrated early and middle Paleozoic age, thus defining previously unrecognized drift of North America with respect to the pole (see Van der Voo [1989] for a review).

The causes for late Paleozoic remagnetization have become the focus of much interest and controversy. From a paleomagnetic point of view, it is obviously essential that the causes of remagnetization be fully understood. If reliable petrologic or geological criteria can be established for predicting the presence or absence of remagnetization, then our effectiveness in finding primary magnetizations will be increased significantly. In addition, a full understanding of the remagnetization phenomenon will result in new applications of the paleomagnetic method to other areas of Earth science, particularly in studies of the physical and chemical changes that sedimentary rocks undergo following deposition (i.e., diagenesis). Remag- nitization is tangible evidence of an important chemical and/or thermal event in the aliagenetic history of a rock unit. Paleomagnetic data, combined with other kinds of geologic information, can provide essential constraints on the nature and age of such events. Although our knowledge is still far from complete, recent research, reviewed below, has uncovered a number of possible links between the remagnetization phenomenon and other important geologic features and processes.

History of the Remagnetization Hypothesis Although generally accepted by the paleomagnetic

community only recently, a late Paleozoic remagnetization event affecting Europe and North America has been suspected by some authors since the mid-1960s [e.g., Chamalaun and Creer, 1964; Creer, 1964, 1968]. Creer [1968] pointed out (1) that many European and North American paleomagnetic poles derived from early Paleozoic rocks were very similar to poles from late Paleozoic rocks and (2) that the global paleomagnetic data set for the early Paleozoic was inconsistent with standard Pangea reconstructions, which at the time were considered by some to be valid for the early Paleozoic. Creer suggested that the apparent inconsistency could be resolved by assumiing that many European and North American Paleozoic rocks were remagnetized near the end of Paleozoic time, whereas early Paleozoic magnetizations from the Gondwana continents had been preserved. The proposed mechanism of remagnetization was the secondary (i.e., diagenetic or authigenic) development of iron oxides in Laurasian red beds while the continent occupied the tropical weathering zone during late Paleozoic time.

The southern continents, it was argued, escaped remagnetization because they were located nearer the south pole, where colder climates prevailed and oxidative aliagenesis was less severe. The theory of this remagnetization event and its origin as articulated by Creer [1968] became known as the "remagnetization hypothesis."

Creer's remagnetization hypothesis was rejected by many paleomagnetists on the basis of both geologic and paleomagnetic arguments, and probably a bit of wishful thinking as well. For example, McElhinny [1973] pointed out that the reasoning which led Creer to suspect widespread remagnetization was flawed because it was assumed that Pangea existed throughout the Paleozoic. He noted that geological evidence of middle and late Paleozoic sutures within Pangea (e.g., the Caledonides and Uralides in Europe) indicates that the supercontinent was not in existence prior to the late Paleozoic. McElhinny [1973] did allow that some European rocks were probably remagnetized during the late Paleozoic but suggested that remagnetization was a consequence of the Hercynian Orogeny rather than climate. In another study, conducted in North America, McElhinny and Opdyke [1973] presented evidence from the Ordovician Trenton Limestone

that cast doubt on Creer's hypothesis. The Trenton magnetization direction was similar to those from North American red beds considered by Creer to be remagnetized. However, the Trenton magnetization carrier was shown to be magnetite, which could not have formed by oxidative aliagenesis as proposed by Creer [McElhinny and Opdyke, 19731.

It has been known for many years that the directions of the late Paleozoic paleomagnetic field in North America are unique for late Paleozoic and younger time. Pennsyl- vanian and Permian directions are southerly to southeasterly and shallow in most of the United States, and the paleomagnetic north poles are located in the vicinity of 45øN latitude and 120øE longitude. The global data set indicates that polarity is almost always reversed during this time, and a Pennsylvanian-Permian interval of predomi- nanfly reversed fields has become known as the Kiaman Reversed Superchron. These known characteristics of the late Paleozoic field were used in some of the early demagnetization studies of North American red beds to infer that

partial remagnetization or overprinting (i.e., the addition of a secondary remagnetization to the depositional one) had occurred during the late Paleozoic [e.g., Irving and Op- dyke, 1965; Roy et al., 1967].

During the 1970s, fold test results were cited as very strong evidence in addition to magnetization direction and polarity for late Paleozoic remagnetization in some deformed rock units. The paleomagnetic fold test is used to bracket the age of acquisition of a magnetization with respect to the age of deformation. If paleomagnetic directions cluster significantly better after a tilt correction is applied, the test is said to be positive, and the magnetization

27, 4/REVIEWS OF GEOPHYSICS McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION ß 473

in question was acquired prior to the folding event. Con- versely, if the directions cluster better before tilt correction, the test is negative, and a postfolding remagnetization is demonstrated. For example, Steiner [1973] reported a negative fold test in Arbuckle Group carbonates of Ok- lahoma. The results of this study indicated a late Paleozoic remagnetization, which was attributed to thermal overprinting during the Arbuckle Orogeny (i.e., mountain- building event). In a study of the Ordovician Juniata sandstone of the central Appalachians, Van der Voo and French [1977] isolated two ancient magnetizations. One of the magnetization components failed the fold test and had directions very close to the expected Pennsylvanian- Permian direction. Van der Voo and French [1977]

pointed out that remagnetization in orogenic belts was likely to involve thermal and chemical processes acting during deep burial, rather than simple near-surface weathering as advocated by Creer [1968]. A similar partial remagnetization was demonstrated in the Silurian Rose Hill Formation [R. B. French and Van der Voo, 1977; A.

N. French and Van der Voo, 1979]. Despite these results, late Paleozoic remagnetization was still considered by many workers to be a rare phenomenon.

Probably the most important stimulus for the recognition of the true importance of remagnetization was the proposal of Alleghenian (Pennsylvanian or Permian) megashear between Laurentia and the allochthonous terranes of

the northern Appalachians [e.g., Kent and Opdyke, 1978]. This idea was originally suggested by a paleomagnetically determined paleolatitude difference in Devonian and Mis- sissippian rocks between stable North America and "Acadia" (i.e., the Canadian Maritime-coastal New Eng- land region). The megashear proposal was criticized on a number of grounds. For example, Roy and Morris [1983] noted that the discrepancy might be due to the inadvertent comparison of magnetizations of different age, specifically remagnetized results from stable North America with older magnetizations from "Acadia." Subsequent paleomagnetic work on Appalachian red beds showed that this was indeed the case [e.g., Irving and Strong, 1984; Kent and Opdyke, 1985]. Meanwhile, late Paleozoic remagnetizations of a very different type were demonstrated to exist in some Appalachian limestones [e.g., Scotese et al., 1982; McCabe et al., 1983]. The limestones differed from the

red beds in that the remagnetization was carried in magnetite rather than hematite. Other studies demonstrated

that remagnetizations in some units could be related to specific diagenetic events [e.g., Elmore et al., 1985].

Further investigations indicated that partial or complete late Paleozoic remagnetizations residing in both magnetite and hematite are present in sedimentary rocks over a huge area of North America. Remagnetization has turned out to be the rule rather than the exception in pre-late Paleozoic sedimentary rocks. The phenomenon has now been observed in the late Paleozoic orogenic belts and their

foreland basins, in the western interior, and in the mid-

continent region as well.

THE OCCURRENCE OF LATE PALEOZOIC

REMAGNETIZATION IN NORTH AMERICA

This section presents a brief review of the key investigations that have yielded evidence for late Paleozoic remagnetization in pre-Pennsylvanian sedimentary rocks of North America. Different lithologies are treated under separate subheadings. The results of the studies cited are summarized in Table 1. Sampling localities are shown in Figure 1, and the paleomagnetic poles are depicted in Fig- ure 2o

Sandstones

The general recognition of the prevalence of remagnetization in Appalachian red beds was hampered for many years by what appeared to be strong evidence against remagnetization in two key rock units. Studies of the Mis- sissippian Mauch Chunk [Knowles and Opdyke, 1968] and Devonian Catskill [Van der Voo et al., 1979] red beds yielded statistically positive fold tests, thus suggesting pre-Alleghenian magnetizations despite paleomagnetic poles similar to Permo-Carboniferous ones. Confidence in the Catskill result was increased by the report of a virtually identical pole from the flat-lying Catskill of New York State [Kent and Opdyke, 1978]. Several investigations of sedimentary carbonates of Devonian and Mississippian age appeared to confirm the Mauch Chunk and Catskill poles [e.g., Martin, 1975; Elston and Bressler, 1977; Kent and Opdyke, 1979; Scott, 1979]. As a result of these red bed and carbonate studies many paleomagnetists believed, mis- takenly, that the paleomagnetic field for stable North America changed very little during the interval Devonian through Permian.

Comparison of paleopoles from the Mauch Chunk, Catskill, and carbonate units with Devonian/Mississippian poles from terranes adjacent to Laurentia indicated a discrepancy in paleolatitude. This led to the idea of A1- leghenian sinistral megashear between Laurentia on the one hand and "Acadia," southern Britain, and Europe on the other [e.g., Kent and Opdyke, 1978; Van der Voo et al., 1979; Van der Voo and $cotese, 1981; Kent, 1982]. Other workers raised both geological and paleomagnetic objec- tions to the late Paleozoic megashear hypothesis. One view held that the paleolatitude discrepancy cited in favor of megashear was not the result of tectonic displacement, but rather was due to the comparison of remagnetized (i.e., Pennsylvanian or Permian) data from stable North America with Devonian/Mississippian data from the adjacent terranes [e.g., Roy and Morris, 1983; Irving and Strong, 1984, 1985; Briden et al., 1984]. Convincing evidence for this point of view was found in a paleomagnetic study of


TABLE 1. Selected Pre-Pennsylvanian Paleozoic Sedimentary Rocks From North America That Are Suspected or Demonstrated to Have

Stratigraphic Paleomagnetic •p, •m Fold Rock Unit Age North Pole or A95 Test Reference

CARBONATES WITH REMAGNETIZATION PROBABLY •SIDING IN MAGNETITE

St. Joe Limestone, Arkansas M Barnett Formation, Texas M Greenbrier Limestone, Pennsylvania M Leadville Formation, Colorado M Salem Limestone, Indiana M Columbus Limestone, Ohio 1D

* Helderberg-Onondaga, New York D Helderberg, New York 1S-eD Helderberg, New York-Virginia 1S-eD Trenton Limestone, New York-

Quebec-Ontario Knox Group, Tennessee

? Oaxaca Sediments, Mexico Appalachian Basin Carbonates,

Tennessee

• Allentown Dolomite, Pennsylvania 1C Bonneterre Dolomite, Missouri 1C Nolichucky Formation, Tennessee 1C

õ Upper Arbuckle Group, Oklahoma eO Leithsville Formation, Pennsylvania mC

39øN, 132øE .... 49øN, 119øE 2, 3 -- 43øN, 131 øE 4, 8 Post 46øN, 123øE 4, 8 -- 45øN, 125øE .... 46øN, 120øE 3, 1 -- 49øN, 115øE 4 -- 50ON, 129øE 2 -- 49øN, 115øE 9 Syn

Scott [1979] Kent and Opdyke [1979] Chen and Schmidt [ 1984] Horton et al. [1984] Van der Voo and McCabe [ 1985a] Martin [1975] Kent [ 1985] Scotese et al. [ 1982] Scotese [1985]

mO 53øN, 127øE 3 -- eO 40øN, 126øE 8 Syn eO and 1Pz 51øN, 125øE 4 Post

McCabe et al. [ 1984] Bachtadse et al. [1987] McCabe et al. [1988]

OandM 48øN, 128øE 5 -- 47øN, 114øE 4, 8 Syn 43øN, 126øE 3, 5 -- 40ON, 120øE .... 30øN, 147øE 8 Post 53øN, 113øE 5, 8 Post?

McCabe et al. [1989] Stead and Kodama [ 1984] Wisniowiecki et al. [1983] Gillett [ 1982] Elmore et al. [1988] Stead and Kodama [ 1984]

CARBONATES WITH •MAGNETIZATION RESIDING IN HEMATITE

Martin Formation, Arizona Temple Butte Formation, Arizona St. George and Table Head,

Newfoundland

Arbuckle Carbonates, Oklahoma Kindblade Formation, Oklahoma Taum Sauk Limestone, Missouri Peerless, Colorado Royer Dolomite, Oklahoma Muav Limestone, Arizona

1D 56øN, 109øE 2, 1 1D 53øN, 115øE 2, 1

Elston and Bressler [1977] Elston and Bressler [1977]

e, mO 45øN, 138øE 4, 7 eO 46øN, 129øE 2, 3 eO 43 ON, 128 øE 2, 4 1C 37øN, 133øE 2, 5 1C 40øN, 135 øE 2, 4 1C 42øN, 130øE 2, 4 mC 55øN, 110øE 3, 2

__

Post

Post --

Deutsch and Prasad [ 1987] Steiner [1973] Elmore et al. [ 1985] Dunn and Elmore [ 1985] Peck et al. [1986] Nick and Elmore [1988] Elston and Bressler [1977]

Key to rock ages: Pz Paleozoic D

M Mississippian S

Preceding letters signify:

Devonian O Ordovician

Silurian C Cambrian

e early m middle 1 late

Fold test result, where applicable, indicates age of the magnetization with respect to local Alleghenian folding. A95 or ¾, •Srn de- r'me the semi-angle(s) in degrees of the ellipse of 95% confidence about the mean pole.

the Mississippian Deer Lake Group sediments on the North American foreland of western Newfoundland [Irving and Strong, 1984]. The Deer Lake magnetization was found to be composed of two ancient components, one postfolding and of Pennsylvanian/Permian age and the other prefolding and presumably of depositional origin. Irving and Strong [1984] showed that their results were inconsistent with the

Alleghenian megashear hypothesis and that the Mauch Chunk and Catskill poles were probably based on remagnetizations. Figure 3 shows the excellent agreement of the Deer Lake overprint, Mauch Chunk, Catskill, and carbonate magnetizations with results of known Kiaman Su- perchron age.

Additional detailed studies of the Mauch Chunk [Kent

and Opdyke, 1985] and Catskill [Miller and Kent, 1986a, b] data confirmed the conclusion of Irving and Strong [1984] that remagnetization had affected these units. These investigations showed that the Mauch Chunk and Catskill magnetizations were multicomponent and revealed why the earlier studies of these rocks had yielded mislead- ing fold tests. The Mauch Chunk magnetization reported by Knowles and Opdyke [1968] had been calculated from the vector remaining after demagnetization at 550 ø or 600øC, which failed to resolve satisfactorily either of the two ancient components present. Although the contribution of a prefolding magnetization resulted in a statistically positive fold test, the paleomagnetic direction obtained was a composite and therefore devoid of geological signifi-


Suffered Remagnetization During the Late Paleozoic Kiaman Reversed Superchron

Stratigraphic Palcomagnetic õp, õm Fold Rock Unit Age North Pole or Ao5 Test

SILICICLASTIC ROCKS-SECONDARY MAGNETIZATIONS

Mauch Chunk (North Limb), Pennsylvania

Mauch Chunk (South Limb), Pennsylvania

Taggard Redbeds, Pennsylvania Deer Lake, Newfoundland Catskill (North Limb), Pennsylvania Catskill (South Limb), Pennsylvania Catskill, New York Perry, New Brunswick 1D Terrenceville, Newfoundland Andreas, Pennsylvania eD Bloomsburg (North Limb),

Pennsylvania Bloomsburg (South Limb),

Pennsylvania Rose Hill, Maryland-West Virginia Juniata, Pennsylvania-Virginia Moccasin, Tennessee

ô Bourinot Sediments, Nova Scotia

IM 50øN, 112øE 6 Syn

1M 52øN, 130øE 4 Syn mM 45øN, 122øE 4, 8 Post eM 47øN, 140øE 5, 10 Post 1D 48 ON, 124øE 5 Syn 1D 43øN, 127øE 5 Syn 1D 47øN, 117øE 5 --

41øN, 133øE 2, 5 Post 1D 44øN, 117øE 4, 7 Post?

58øN, 122øE 5 Syn

1S 52øN, 103øE 5, 9 Syn

1S 43øN, 118øE 5, 10 Syn mS 45øN, 117øE 2, 4 -- 10 47øN, 129øE 8 Post mO 41øN, 125øE 3, 6 Syn? mC 37øN, 126øE 2, 4 Post

Red Mountain, Alabama Red Mountain, Alabama Neda and Queenston, Iowa-

Wisconsin-Illinois

PA LF•OZOIC IRONSTONES

S 38øN, 132øE 2, 4 -- S 38øN, 132øE 4 Pre

10 45øN, 132øE 8, 16

* A negative fold test was reported, but the age of the folding may be either Acadian or Alleghenian.

'• Rocks sampled include limestones, red beds, and igneous intrusives.

• Synfolding magnetization demonstrated by Kodama [1988]. õ Interpreted as rotated tectonically. Not plotted in Figure 2. ô Magnetization generally unblocks below 580øC and may

reside in magnetite.

Reference

Kent and Opdyke [1985]

Kent and Opdyke [1985] Chen and Schmidt [ 1984] Irving and Strong [1984] Miller and Kent [1986a] Miller and Kent [1986b] Kent and Opdyke [1979] Stearns and Van der Voo [1987] Kent [1982] Miller and Kent [1988a]

Kent[1988]

Kent[1988] French and Van der Voo [ 1979] Van der Voo and French [ 1977] Watts et al. [1979] Johnson and Van der Voo [ 1985]

Hodych et al. [ 1985] Perroud and Van der Voo [ 1984]

Kean [1981]

cance [Kent and Opdyke, 1985]. Figure 4 shows stepwise thermal demagnetization data obtained by Kent and Op- dyke [1985] for two Mauch Chunk samples. Two ancient magnetization directions, one primary and one a late Paleozoic overprint, are clearly resolved in the data. In the case of the Catskill, Miller and Kent [1986a] showed that

the fold test presented by Van der Voo et al. [1979], although indeed statistically positive, was best interpreted as a magnetization acquired during the deformation event (i.e., synfolding). Miller and Kent [1986a, b] also isolated a prefolding magnetization from some of their Catskill samples.

Recent pal•magnetic studies of a number of Ap- palachian red bed units have now revealed a general pat-

tern of remagnetization in this lithology that was first well documented in the Bloomsburg red beds [Irving and Op- dyke, 1965; Roy et al., 1967] and the Juniata Sandstone [Van der Voo and French, 1977]. Two ancient magnetizations are typically resolved by detailed thermal demagnetization, both residing in hematite. The first component revealed during progressive thermal treatment is a magnetization that is removed, or unblocks, by treatment at in- termediate temperatures (e.g., about 350 ø to 550øC in the example given in Figure 4). Application of the paleomagnetic fold test indicates that this magnetization was acquired either during or after Alleghenian folding. The palcomagnetic directions of this secondary magnetization are southerly to southeasterly with very shallow inclina-

• o oo oe I I

130 120 110 100 90 80 70 60

50

40

30

20

10 50

Figure 1. Map of North America showing the sampling locations of the studies listed in Table 1. For

studies covering a large area, an average site locality was determined for purposes of this map. Symbols are as follows: solid circles, sandstones; open squares, magnetite- bearing carbonates; solid triangles, hematite-bearing carbonates; and open hexagons, ironstones.

Figure 2. Paleomagnetic north poles from the investigations listed in Table 1. Symbols are keyed to lithologic type as in Figure 1. Heavy curve is the late Paleozoic and Mesozoic apparent polar wander path for North America using poles given by Van der Voo [1989]. The letter symbols refer to ages on the reference path as follows: M, Mississippian; 1P, Pennsyl- vanian; P, Permian; T R, Triassic; J, Jurassic; and K, Cretaceous.

tions, virtually identical to expected directions for the Pen- nsylvanian or Permian. Magnetization polarity is over- whelmingly reversed, consistent with acquisition during the Pennsylvanian-Permian Kiaman Reversed Superchron. The second ancient magnetization revealed by thermal treatment in these Appalachian red beds is a prefolding magnetization, often with mixed polarities, with unblocking temperatures in a narrow range near the Curie point of hematite (i.e., 680øC). The prefolding magnetization is sometimes quite difficult to isolate: it is not present in some specimens, and in others the natural magnetization is obscured by spurious magnetizations acquired during thermal treatment [e.g., Kent and Opdyke, 1978].

The partial remagnetizations of the Silurian Bloomsburg [Irving and Opdyke, 1965; Roy et al., 1967] and Or- dovician Juniata [Van der Voo and French, 1977] sandstones have been confirmed in recent studies of those

units by Kent [1988] and Miller and Kent [1989]. Mag- netization overprints of late Paleozoic age that reside in hematite have now been found to occur in most of the Ap- palachian sandstones that have been subject to detailed study (Table 1). In a few cases [Chen and Schmidt, 1984; Stearns and Van der Voo, 1987] a prefolding magnetization could not be isolated, and a complete late Paleozoic remagnetizafion was reported. Siliciclastics of the Cambrian Bourinot Group [Johnson and Van der Voo, 1985] are unusual in that the late Paleozoic overprint in these rocks appears to reside in magnetite.

Compared to paleomagnetic studies of Paleozoic sandstones in the Appalachians, there have been fewer and less conclusive studies of sandstones in other areas in

North America. For example, Watts et al. [1980] report that Cambrian sandstones (e.g., Hickory and Lion Sandstone members of the Riley Formation) in Texas con-

27, 4 / REVIEWS OF GEOPHYSICS McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION ß 477

rain magnetizations that although initially interpreted to be Cambrian in age, were probably at least partially remagnetized during the late Paleozoic. McCabe et al. [1988] re-

BF '--"4+3øS + o q. "'-- • TH - 4_2 S 1-+4øS • • + o

port that Paleozoic sandstones in southern Mexico are remagnetized, possibly as a result of events during the late Paleozoic.

Figure 3. Portion of North America drawn on a Kiaman paleolatitude grid with results of paleomagnetic studies from selected Mississippian and Devonian units suspected of being late Paleozoic remagnetizations. Arrows point in the direction of the observed declination, and numbers indicate observed paleo- latitudes. Note good agreement with the Kiaman grid in all cases. Letter symbols are codes for the formations studied. BF and SJ are remagnetized carbonate results; MC is from Mauch Chunk and CA from Catskill red beds. DH is the overprint from the Deer Lake red beds of western (cratonic) Newfoundland, and TH is the overprint from the Terrenceville red beds from the Avalon zone of eastern Newfoundland. Figure after Irving and Strong [1984, 1985].

300 ø • 400 ø

•NRM

,•,•550 o600ø 5000

450 ø

H2

W Up 550 ø

500 ø

NRM

350 ø 400 ø

450 ø 660 ø

670 ø

W Up

600 ø

620 ø 640 ø

• • : N Figure 4. Demagnetization diagrams from the Mauch Chunk red beds [after Kent and Opdyke, 1985]. Open symbols are projections of endpoints of the magnetization vec- tors (after the indicated thermal demagnetization treatments) on a north-south vertical plane; solid symbols are the projections onto the horizontal plane. Linear trajectories of the vector endpoints indicate removal of single magnetization components during demagnetization. The depositional magnetization unblocks above 640øC and is present with both normal and reversed polarities. The magnetization removed between about 300 ø and 550øC is the reversed polarity late Paleozoic overprint, which has directions that

Down are southerly and shallow. The coordinate system used is not corrected for bedding tilt; the best clustering of the overprint directions is obtained with a 50% tilt correction, suggesting that this magnetization was acquired during Alleghenian folding. The depositional magnetization is prefolding.

,80 ø

Down z1


Carbonates

The Paleozoic carbonates of North America have turned

out to be very disappointing for apparent polar wander studies because of the prevalence of complete late Paleozoic remagnetization. Nonetheless, studies of the carbonate remagnetizations have provided important new insights into magnetization processes in sedimentary rocks and have added an unexpected twist to our understanding of the remagnetization phenomenon: the carbonate remagnetizations frequently reside in magnetite, rather than hematite as is generally the case for red beds. Therefore any general explanation for widespread late Paleozoic remagnetization must account for the possibility of different remagnetization mechanisms in different settings. The physical mechanism of remagnetization in magnetite- bearing carbonates has proven to be controversial. It may be due to chemical diagenesis resulting in the development of new magnetite [Scotese et al., 1982; McCabe et al., 1983] or thermal activation of preexisting magnetite [Kent, 1985]. The evidence for these remagnetization mechanisms will be discussed in later sections.

Prior to the early 1980s, carbonate magnetizations residing in magnetite had usually been interpreted as an early- acquired remanence [e.g., McElhinny and Opdyke, 1973; Martin, 1975]. Evidence cited for an early acquisition included (1) the very fact that magnetite was the carrier, since at the time it was thought that magnetite in sediments must have a detrital origin, and (2) the observation of a "sedimentary" magnetic fabric (as determined from magnetic anisotropy studies) in several Appalachian limestones, which also suggested a depositional origin for the carriers of magnetization [Graham, 1966; McElhinny and Opdyke, 1973; Kent, 1979]. Despite these arguments, the similarity of the carbonate paleopoles with Pennsy!- vanian-Permian ones was a disturbing indication that remagnetization might have occurred. This dilemma was discussed by Kent [1979] in his paper on the Devonian Onondaga Limestone of New York State. Kent showed that a "sedimentary" magnetic fabric as defined by Graham [1966] was well developed in his samples. However, the magnetization clearly postdated tectonic folding in the Hudson Valley. Unfortunately, the age of the Hudson Valley deformation is ambiguous: it could be either Acadian (Devonian) or Alleghenian (Pennsyl- vanian-Permian). Kent [1979] discussed the possibility of Permian remagnetization but could not rule out a Devonian remagnetization age because of this uncertainty.

Alleghenian remagnetization in an Appalachian carbonate was more clearly established following study of the Devonian Helderberg Group [Scotese et al., 1982; McCabe et al., 1983; Scotese, 1985]. A fold test on the

Helderberg magnetization in the central Appalachians (Figure 5) revealed for the first time what was to become a commonly observed feature of the Appalachian remagnetizations in both carbonates and red beds: the best

grouping of directions is obtained after only partial correction for bedding flit, suggesting that the magnetization was acquired during folding [McCabe et al., 1983; $cotese, 1985]. Subsequent research into the effects of strain on magnetization directions has indicated that such observations do not necessarily prove a synfolding magnetization [e.g., van der Pluijm, 1987; Kodama, 1988]. However, Kodama [ 1988] showed that the effects of strain in the Cambrian Allentown Dolomite in Pennsylvania could not fully explain the observed "syn-Alleghenian" fold test result in that unit. He concluded that the Al-

lentown magnetization was probably indeed synfolding in age and found no basis for discarding the conclusions of McCabe et al. [1983] for the Helderberg magnetization age.

A fold test suggesting a syn-Alleghenian magnetization has also been obtained for the Ordovician Knox Dolomite

of east Tennessee [Bachtadse et al., 1987]. Post-

Alleghenian fold tests have been reported from the Mississippian Greenbrier Limestone [Chen and Schmidt, 1984] and the Cambrian Leithsville Formation [Stead and Kodama, 1984]. Several other carbonate units in the

Appalachians and the Appalachian foreland basin have yielded paleomagnetic poles indicating a late Paleozoic magnetization age, although no field tests were available with which to strictly bracket the acquisition age with respect to Alleghenian folding (Table 1). A late Paleozoic magnetization residing in magnetite has been reported in the Cambrian Bonneterre Dolomite in Missouri which has

been used to constrain the timing of the Mississippi Valley-type mineralization within the unit [Wisniowiecki et al., 1983].

Remagnetizations in magnetite-beating carbonates are reportedly common in the midcontinent region, but no results have yet been fully published. The Mississippian Salem Limestone in Indiana carries a late Paleozoic

remagnetization [Van der Voo and McCabe, 1985a]. In addition Marshak et al. [ 1989] report that the remagnetization is present in Paleozoic carbonates from several midcontinent states and that extracted magnetite grains appear to have a diagenetic origin.

The remagnetization in some North American carbonates is carried by hematite. Postfolding late Paleozoic magnetizations residing in hematite have been found in Cambro-Ordovician carbonate units in southern Oklahoma

[e.g., Steiner, 1973; Elmore et al., 1985; Cochran and Elmore, 1987]. Although the magnetization in these carbonates was initially interpreted as a thermal overprint due to the Arbuckle Orogeny [Steiner, 1973], low burial temperatures based on paleotemperature estimates (conodont alteration index <1.5) would appear to eliminate the possibility of a thermoviscous origin [e.g., Elmore et al., 1985]. Most of these magnetizations are interpreted as chemical remanent magnetizations related to specific diagenetic events such as dedolomitization. The Cambrian

27, 4/REVIEWS OF GEOPHYSICS McCabe and Elmore' LATE PALEOZOIC REMAGNETIZATION ß 479

Up

Down

½ U• Up ß

Tilt Correction '• t0.•.111t•orrect,on '•

Bedding Strike

Down Down

Figure 5. Fold test results fxom Helderberg (Devonian) car- d bonates fxom the Valley and Ridge province of the central Ap- 60 palachians (redrawn from data given by McCabe et al. [1983] and Scotese [1985]). (a) Site mean paleomagnetic directions 50 without any correction for bedding tilt. Note vertical (equal area)

projection. All directions shown are on the upper (i.e., southern) 40 hemisphere. Symbols are as follows: circles are directions fxom fold limbs with northwesterly dips; diamonds are directions from southeasterly dipping beds; the square is the average bedding strike. (b) Projection and symbols as in Figure 5a but directions 20

are corrected for 60% of the present bedding dip. (c) Directions fully corrected for bedding tilt. (d) Plot of precision parameter (k) 10 versus percent correction for bedding tilt for the same site mean data. Note that best clustering of directions occurs at 60% unfolding, suggesting a syn-Alleghenian acquisition of the magnetization.

K 30

i i i i

00 1•) •0 3•) 40 5•0 60 70 8•) 9'0 100 % UNFOLDING

Peerless Formation in Colorado carries a late Paleozoic

component that apparently resides in several forms of secondary hematite [Peck et al., 1986]. The hematite occurs on and intergrown with dolomite rhombs and also replaces glauconite and other grains. In contrast to most other remagnetized units, the Peerless magnetization is present with both normal and reversed polarities. Other late Paleozoic carbonate remagnetizations residing in hematite are listed in Table 1. Paleozoic ironstones in the

midcontinent and in Alabama also have a late Paleozoic

magnetization residing in hematite (Table 1) [Kean, 1981; Perroud and Van der Voo, 1984; Hodych et al., 1985].

THE PHYSICAL MECHANISM OF

REMAG NETIZATION

Late Paleozoic remagnetizations in different lithologies and settings have been explained in a variety of ways. In general, proposed remagnetization mechanisms fall into one of two categories: chemical remanent magnetization (CRM) and thermoviscous remanent magnetization

(TVRM). Chemical remagnetization in different rock units has been inferred to be a consequence of the chemical action of several kinds of fluids, including meteoric (atmosphere-derived) waters, solute-rich waters derived from deep levels of sedimentary basins (basinal brines), and hydrocarbons. Thermoviscous magnetization may be due to conduction of heat during burial and uplift. Alternatively, TVRM may result from advective heat transfer following the introduction of hot fluids.

Chemical remanent magnetization occurs at temperatures below the Curie point during the precipitation of authigenic magnetic phases or during replacement of precursor minerals by diagenetic magnetic phases. The remanence is generally thought to be in the direction of the ambient field and is acquired as the critical size for single-domain behavior is attained by the growing mineral grains. Demonstration of this magnetization mechanism in natural samples is simple in principle but is often difficult in practice. First, a magnetization must be linked with an observable magnetic phase. Next, it must be shown that the magnetic phase in question is a secondary one, due to either mineral transformations or the de novo growth of


new mineral grains. Finally, tertiary effects such as overprinting due to thermal processes must be ruled out.

Thermoviscous magnetization is a consequence of prolonged heating to some temperature below the Curie temperature and is a combination of a partial ther- moremanence and a viscous remanence. Documentation

of a TVRM mechanism requires a knowledge of the thermal history of the rock unit and of the blocking temperature-relaxation time relationship appropriate for the assemblage of magnetic grains present.

Chemical Remanent Magnetization in Red Beds The origin of magnetization and timing of remanence

acquisition in red beds continues to be a controversial subject. The debate focuses on whether the magnetization was acquired "soon" or "long" after deposition. Recent work on Triassic or younger red beds has been summarized elsewhere [e.g., Tauxe et al., 1987] and will not be discussed in this paper. The magnetization in red beds is generally considered to be due to both chemical and depositional magnetization processes [e.g., Purucker et al., 1980; Walker et al., 1981].

In the case of the Appalachian red beds, there appears to be little doubt that the remagnetization is a CRM. It has long been recognized that the red pigment in most red beds is authigenic hematite, and the physical properties of the Appalachian red bed remagnetizations indicate that the carrier mineral is indeed hematite [e.g., Kent and Opdyke, 1985]. Kent and Miller [1987] have shown that the

thermal maturities of Appalachian red beds are not high enough to have permitted thermally activated remagnetization according to the blocking temperature-relaxation time relationship for hematite derived by Pullaiah et al. [1975]. Moreover, the experimental results of Kent and Miller [1987] indicate that Appalachian red bed samples conform to the predictions of Pullaiah et al. [1975], thus lending further support for chemical remagnetization.

In contrast to studies of Triassic rocks, there has been

little effort to relate the petrographic characteristics of Paleozoic red beds to the magnetization in the rocks. As a result, little is known about the hematites that carry each of the two commonly observed magnetization components. Studies of Triassic rocks have documented various types of authigenic or replacement hematite, some of which are relatively early and some which apparently formed relatively late [e.g., Walker et al., 1981]. It would be interesting to compare the various types of hematite reported from Triassic units with those in the remagnetized Paleozoic red beds. This type of work could help to elucidate the mechanism of CRM acquisition and help to constrain the nature of fluids that may have been involved in hematite authigenesis. Petrographic as well as geochemical studies are needed to better understand the origin of this chemical remagnetization event.

Chemical Remagnetization in Magnetite-Bearing Carbonates

Until recently, the presence of magnetite in a sedimentary rock (inferred from magnetic properties) was usually interpreted as evidence for a detrital origin of the magnetite and an early acquisition of the characteristic remanence, provided significant heating did not occur (see reviews by Verosub [1977] and Lowrie and Heller [1982]). Although a few reports of authigenic or diagenetic magnetite could be found in the literature [e.g., Friedman, 1954; Donovan et al., 1979], it was generally assumed that low-temperature, secondary magnetite was an unusual occurrence.

It is now recognized that magnetotactic bacteria are one source of early, postdepositional magnetite [e.g., Frankel et el., 1979; Kirschvink and Chang, 1984]. Another, quite different occurrence of secondary magnetite was first suggested following paleomagnetic and petrogmphic studies of Paleozoic carbonates from New York State, the central Appalachians, and Missouri [McCabe et al., 1983; Wisniowiecki et al., 1983]. In these cases it was inferred

that the magnetite was a diagenetic product and the magnetization a chemical remanence of late Paleozoic age. The evidence presented by McCabe et al. [1983] for a chemical remanence model is as follows: (1) the characteristic magnetization directions are to the SSE and shallow, consistent with the Pennsylvanian or Permian expected direction; (2) all directions are reversed, consistent with a Kiaman Superchron magnetization; (3) the magnetization appears to be syn-Alleghenian in the folded sites; (4) magnetic extracts contain crystal aggregates of end-member magnetite with spheroidal and botryoidal morphologies, consistent with a diagenetic origin (Figure 6); and (5) available thermal maturity indicators (i.e., conodont alteration index; vitrinite reflectance index) suggest that thermal conditions have never reached a level sufficient to reset the magnetization in preexisting magnetite according to the predictions of the Pullaiah et el. [1975] blocking temperature-relaxation time relationship for magnetite.

Subsequent studies revealed that the occurrence of magnetite with diagenetic textures was common in remagnetized Paleozoic limestones and dolomites in the United States and Canada [McCabe et al., 1984; Horton et al., 1984; Freeman, 1986; Bachtadse et al., 1987; Elmore et al., 1988]. In another study, McCabe et al. [1985] investigated the origin of magnetic fabric in the Trenton Limestone. McElhinny and Opdyke [1973] had previously interpreted the Trenton fabric as being "sedimentary," thus suggesting a depositional origin for the magnetic carriers, an interpretation that is apparently in conflict with the diagenetic appearance of magnetite groins extracted from the Trenton Limestone and other remagnetized limestones. Using a new method for determining the


..................... igii?;i?•i Figure 6. Scanning electron micrograph of magnetite spheres extracted from the remagnetized Bonneterre Dolomite (Cambrian, Missouri). X ray diffraction results showed that the individual spheres are magnetite, and qualitative energy-dispersive analysis showed that the only cation present in the spheres as a major or minor element is iron [McCabe et al., 1983]. The angular grains are bits of matrix dolomite. The grain morphology and composition suggest a diagenetic origin for the magnetite. The Bonneterre remagnetization was documented by Wisniowiecki et al. [1983], and the figure was published previously by McCabe et al. [1987]. Scale bar indicates 10 gm.

loo

95

90

85

80 500

Buffalo Rochester

400 300

Syracuse

0 a6o o

clistance [km]

20

15

lO

Figure 7. Magnetite-fflite correlation in an east-west transect of the Onondaga Lime- stone of New York State [from McCabe et al., 1989]. The transect begins near Albany, New York (0 km), and proceeds westward to a point in Ontario just west of Buffalo, New York. The variation in anhysteretic susceptibility (k•,, triangles), reflecting magnetite concentration [Jackson et al., 1988], is compared with the variation in percent illite (circles) in a bentonite horizon [Johnsson, 1986]. The excellent agreement of the two parameters was interpreted to indicate that illitization and magnetite authigenesis were controlled by the same diagenetic factors and that they occurred at the same time. Illitiza- tion is known to have occurred during Al- leghenian time [McCabe et al., 1989].

magnetic fabric residing only in magnetite (anisotropy of anhysteretic susceptibility), McCabe et al. [1985] showed that the Trenton fabric was probably due, at least in part, to tectonic rather than sedimentary processes.

The conclusion of McCabe et al. [1983] that thermally activated remagnetization was unlikely in the Devonian carbonates of New York State has proven to be controversial. Kent [1985] showed that the blocking temperature-relaxation time relationship of Pullaiah et al. [1975] was not appropriate for these Devonian carbonates and that thermoviscous remagnetization could not be discounted given the observed thermal maturity in eastern upstate New York. The evidence for thermoviscous

remagnetization in this lithology will be discussed in the next section.

Circumstantial evidence for chemical magnetization in the Onondaga Limestone of central upstate New York was reported by Jackson et al. [1988] in a study of regional variations in certain rock magnetic parameters. Relative magnetite concentration, inferred from low-field and anhysteretic susceptibility, was found to vary in a regular fashion along an east-west traverse across the state (Figure 7). Magnetite content is very low in western New York near Buffalo but increases in a regular fashion to a maximum near Syracuse. Farther east, magnetite content decreases again. Jackson et al. [1988] point out that

482 ß McCabe and Elmore.' LATE PALEOZOIC REMAGNETIZATION 27, 4/REVIEWS OF GEOPHYSICS

magnetite content is strongly covariant with the degree of diagenetic illitization of mixed-layer clays in a bentonite horizon within the Onondaga [Johnsson, 1986] and conclude that magnetite authigenesis and illitization were coeval events that were caused by the same set of diagnetic factors. Johnsson [1986] had previously argued that illitization had occurred during deep burial in Alleghenian time, and other K-bentonites of the Appalachian Basin have yielded Alleghenian K/At ages [Elliott and Aronson, 1987]. Jackson et al. [1988] concluded from these considerations that (1) most of the magnetite in the Onondaga of central New York must be authigenic and (2) magnetite authigenesis and remagnetization are both Alleghenian events, thus supporting a CRM model for the remagnetization mechanism.

In another study, McCabe et al. [1989] cited evidence of chemical remagnetization in Paleozoic carbonates of the southern Appalachian Basin. Paleomagnetic studies indicated that late Paleozoic remagnetization was clearly present in Ordovician carbonates of the Nashville dome, as well as early and late Paleozoic carbonates in the overthrust belt of east Tennessee. However, more weakly magnetized Mississippian carbonates now exposed between the Nashville dome and the overthrust belt

showed no evidence for late Paleozoic remagnetization, and a dual-polarity magnetization of probable alepositional age was present at one locality. The thermal maturity of sites that were remagnetized had the same low value as those that were not (conodont alteration index <1.5), suggesting that remagnetization was not due to TVRM. Rock magnetic results indicated that, on average, there is more magnetite in samples from the remagnetized localities than in the nonremagnetized Mississippian samples (Figure 8). McCabe et al. [1989] concluded from these observations that magnetite authigenesis and remagnetization are related in this setting and inferred a chemical origin for the observed remagnetizations.

As we have seen, studies of magnetite-bearing Paleozoic carbonates indicate that at least some of the magnetite present in samples is secondary and that at least some of the remagnetizations could be CRMs. However, the geochemical pathways that lead to magnetite formation are not understood. Several different geochemical scenarios have been proposed, some based on studies of post- Paleozoic rocks. For example, Hornafius [1984] suggested that meteoric waters may cause oxidation of pyrite to magnetite in the Monterey Formation in California. Oxidation of siderite by fluids to produce authigenic magnetite has also been proposed [e.g., Ellwood et al., 1988]. Hart and Fuller [1988] report a CRM residing in authigenic magnetite in the Monterey Formation, and associate the magnetite with fluids that caused late stage dolomite diagenesis.

Studies of Paleozoic carbonates have yielded evidence for other geochemical mechanisms for magnetite

• 30

z "' 20

10

0 20 40

40

• 30 .

z

20 lO

o 20 40 60 80 1 oo 120 1 z•o 160 SIRM INTENSITY

Figure 8. Histogram of isothermal remanent magnetization at 0.3 T, reflecting magnetite content, for (a) Mississippian samples f•om Tennessee and Alabama that show no evidence for late

Paleozoic remagnetization and (b) Tennessee samples f•om sites that were remagnetized during the Kiaman Superchron. Mag- netization units are milliamperes per meter. Figure from McCabe et al. [1989].

authigenesis. McCabe et al. [1989] have proposed that magnetite authigenesis in the Appalachian Basin was triggered by the introduction of potassium-rich brines during Alleghenian time. This mechanism was suggested by several observations: (1) an episode of late Paleozoic potassium metasomatism appears to have occurred within the basin [Hearn and Sutter, 1985; Hearn et al., 1987]; (2) Alleghenian illitization of Ordovician bentonites in the southern Appalachian Basin seems to be related to the

introduction of exotic potassium, rather than burial heating [Elliott and Aronson, 1987]; (3) potassium is required for the diagenetic illitization of detrital smectites, and the reaction can release iron [Boles and Franks, 1979]; and (4) magnetite content and degree of illitization are strongly covariant in New York State limestones [Jackson et al., 1988]. McCabe et al. [1989] suggest that the iron necessary for magnetite authigenesis is locally derived from detrital clays within the carbonates but that the reaction is triggered by the introduction of potassium in exotic brines. Another proposed geochemical mechanism is via the presence of hydrocarbons. The relationship of


crude oil to magnetite authigenesis in Paleozoic rocks is well documented in a few instances [e.g., Elmore et al., 1987] and will be discussed in a separate section.

Because of the low concentrations of magnetite in remagnetized carbonates (typically about 10 ppm), direct observation and characterization of the magnetic carriers has been extremely difficult. In the past, it has only been possible to study the carriers as magnetic isolates prepared from insoluble residues. Although this kind of work has been a great help in inferring the origin of the magnetite grains, the study of the carriers out of their natural context is obviously less than ideal. A recent abstract by Suk et al. [1988] reports finding authigenic magnetite in situ in samples of the Ordovician Knox Group carbonates. This important development promises to clarify the relationship between the magnetic minerals and other rock components and to constrain further the origin and relative age of the magnetite carriers.

Thermoviscous Remagnetization in Magnetite-Bearing Rocks

Published blocking temperature-relaxation time relationships such as the one of Pullaiah et al. [1975] are based on a simplified treatment of the Ndel [1949] single-domain theory of viscous magnetization. Such relationships are often depicted graphically using thermal activation nomograms (e.g., Figure 9), which can be readily used to predict the laboratory unblocking temperature for a TVRM acquired at a given time and temperature. The possibility that a particular magnetization is due to TVRM is tested by choosing geologically constrained estimates of the temperature and duration of TVRM acquisition, and then the nomogram is used to predict the maximum laboratory unblocking temperature (typically for 1 hour heating) of the magnetization being tested, provided it is a TVRM. If the observed maximum laboratory unblocking temperature of the magnetization in question exceeds this prediction, then the magnetization is inferred to be due to processes other than TVRM.

However, recent studies have demonstrated that the

predictions of Pullaiah et al. [1975] significantly underes- timate laboratory unblocking temperatures of TVRMs in some magnetite-beating rocks [e.g., Middleton and Schmidt, 1982; Kent, 1985; Jackson and Van der Voo, 1986], indicating that the Pullaiah et al. [1975] nomograms are not fully applicable to the problem of TVRM blocking and unblocking. Kent's [1985] elegant study of the Devonian carbonates of New York State provides a good example. Using a conglomerate test on Devonian limestone clasts collected from a Pleistocene

glacial deposit, Kent [1985] deduced that a universally present magnetization aligned with the present field was a viscous magnetization acquired since the last glaciation. In addition, he showed that the maximum unblocking temperatures of this magnetization exceeded values

1Ga I ' ' C I ' I ' ' 10 a q.

10a . 10aa "'.. "- "'.

• 10a "'-. ".. *'... lrn ø. 'øo

ld '.

100s

lsL I , I\ , I \ xl • \1 I 50 150 250 350 450 550

TEMPERATURE (øC)

Figure 9. Therma! activation nomogr•m sitowing the b]ocking temgerat•re-rehxation time rehtionshig (solid curves) œor magnetite according to ?ullaiah et al. []9?5]. inferred acquisition (A) and observed unblocking (B) conditions for a recent viscous remanence. The observations follow a trend (dotted curve) that is not consistent with the predictions of Pullaiah et al. [1975] but is consistent with the Middleton and Schmidt [1982] nomogram (not shown for clarity). Inferred maximum burial conditions (C) and predicted maximum unblocking temperatures (D) for the resulting TVRM using the Middleton and Schmidt [1982] relationship. The predicted unblocking temperature is similar to observed values (about 500øC), and so the remagnetization could be a TVRM if the Middleton and Schmidt [ 1982] relationship is valid for the carriers of the late Paleozoic remagnetization. Pre- dicted maximum unblocking conditions according to Pullaiah et al. [1975] (E), suggesting that the observed characteristic magnetization is not due to TVRM. Figure after Kent [1985].

predicted by Pullaiah et al. [1975] for a room temperature viscous magnetization acquired in 10,000 years (dotted curve AB in Figure 9). However, this magnetization did behave in accord with the predictions of a proposed blocking temperature-relaxation time relationship based on a lognormal distribution of grain sizes [Walton, 1980; Middleton and Schmidt, 1982]. Using the Middleton and Schmidt [1982] thermal activation nomograms, the observed thermal maturity range of the New York Devonian carbonates, and an assumed duration of burial of 100 Ma, Kent [1985] inferred that the late Paleozoic

remagnetization also present in the samples could be a thermoviscous remanence, since the predicted and observed maximum unblocking temperatures are both about 500øC (dotted curve CD in Figure 9).

These and other experimental results clearly indicate that demagnetization temperatures of TVRMs in magnetite-beating rocks are often much higher than those predicted by Pullaiah et al. [1975]. However, it has been pointed out that the Walton [1980] theory is not applicable to the problem of laboratory unblocking and that its apparent concordance with experimental results is fortuitous [Enkin and Dunlop, 1988; Worm et al., 1988; Worm and Jackson, 1988]. Therefore, even though


Walton's approach appears to fit the very limited amount of experimental data available for some carbonates, there is no reason to suppose that it will work equally well for all magnetite-beating rocks over all temperature and time ranges.

Worm and Jackson [1988] have developed a more exact approach to the application of Ndel [1949] theory and have shown that the approximations used by Pullaiah et al. [1975] should result in reasonably good predictions of TVRM acquisition and decay for single-domain grains regardless of grain volume. It appears that the discrepancy between N6el theory and experimental results is due to the prevalence of pseudo-single-domain and multidomain grains in natural samples, to which N•el theory probably does not apply [Enkin and Dunlop, 1988; Worm and Jackson, 1988]. Developing a sound theoretical framework for predicting the TVRM characteristics for pseudo- single-domain and multidomain grains will be difficult, and it might involve fundamentally different processes than those which control single-domain blocking and unblocking [e.g., Moon and Merrill, 1986]. Until such a theory is available, predictions of the presence or absence of TVRM for a given thermal history must be considered speculative.

A partial solution to the question of thermoviscous remagnetization in magnetite-bearing limestones of the Appalachian Basin has been recently proposed by Jackson et al. [1989] and Jackson [1989]. They cite evidence based on hysteresis studies that the distribution of magnetite grains in their samples is bimodal, consisting of multidomain and single-domain fractions. We note that this is a departure from the conventional wisdom based on unblocking temperatures [e.g., Kent, 1985], Lowrie-Fuller tests [e.g., McElhinny and Opdyke, 1973], and magnetic extract analyses [e.g., McCabe et al., 1983], which appear to indicate the predominance of multidomain and pseudo- single-domain magnetite grains in this rock type. Further, Jackson [1989] suggests that the single-domain fraction lacks shape anisotropy, indicating equant grains consistent with a diagenetic origin. Jackson [1989] proposes that the well-known two-component natural remanence in remagnefized carbonates is composed of a TVRM of recent origin residing in multidomain grains such as the ones seen in extract studies, and a CRM of late Paleozoic age residing in single-domain magnetites. If confirmed by future research, these findings indicate that N6el theory may be used to evaluate the possibility of remagnetization due to TVRM in this rock type (e.g., dotted curve CE in Figure 9) and that the late Paleozoic remagnetization in magnetite-bearing carbonates is probably the result of chemical processes in most Appalachian settings.

Another problem in evaluating the possibility of TVRM is that details of the thermal history of particular rock units are frequently not available. Thermal maturities based on

vitrinite refl•tance or conodont alteration index are often

used to test for the possibility of TVRM, but there are serious drawbacks to this practice. The most important is that the results yield integrated time-temperature rather than thermal history. That is, a long exposure to a relatively low temperature can yield the same thermal maturity as a short exposure to a higher temperature. Therefore, in order to estimate paleotemperature for a given thermal maturity, one must assume an exposure time. Further, it is possible that the observed thermal maturity is the result of multiple heating events, some of which may have long preceded the magnetization event in question. Additional data can help to constrain the thermal history further. For example, apatite and zircon fission- track cooling ages and fluid inclusion data should be considered if they are available. Considerations of physical stratigraphy can also constrain burial history if there is a sufficient sedimentary record preserved. Unfortunately, existing methods cannot provide an absolutely definitive thermal history, unless that history is exceedingly simple.

Some insight into the likelihood of TVRM in different settings can be gleaned from geological considerationso For example, Kent [1985] pointed out that the Onondaga magnetization in the Hudson Valley is clearly postfolding but that some of the samples have a well-defined magnetic fabric of tectonic origin. Further, Sierra et al. [1989] cite evidence that some of the Hudson Valley samples they studied have a magnetic fabric residing in magnetite that was acquired in a stress field that predated the one that gave rise to the Hudson Valley folding. These observations might be interpreted as suggesting that at least some of the magnetite was present before folding and is therefore older than the magnetization, thus favoring a thermoviscous remagnetization mechanism. On the other hand, it is probable that replacive or void-filling secondary magnetite will reflect, at least to some extent, any preexisting sedimentary or tectonic fabric that is present. Until the acquisition of magnetic anisotropy in remagnetized carbonates is more fully understood, arguments for particular modes of remagnetization based on anisotropy data must be regarded as tentative.

Geological arguments can also provide insight into possible heating mechanisms. For example, McCabe et al. [1989] noted that TVRM due simply to burial and uplift is unlikely in upstate New York, since uplift there occurred much later than the observed late Paleozoic remagnetiza- tiono According to the apatite fission-track results of Johnsson [1986], uplift and cooling through the -100øC isotherm took place during Mesozoic time. Nonetheless, thermoviscous remagnefizafion is possible if hot fluids provided the heat and if the resulting thermal anomalies decayed rapidly [Dorobek, 1989; McCabe et al., 1989].

TVRM must be considered a viable remagnetization


mechanism in areas such as the Hudson Valley of upstate New York, where the thermal maturity is rather high (conodont alteration index 4.5) and where maximum burial is likely to have occurred during Alleghenian time. In such areas, an additional increment of heat provided by hot, exotic fluids might have resulted in the acquisition of TVRM as the fluids cooled. On the other hand, it is

difficult to imagine any heating mechanism that could cause a TVRM in near-surface rocks, especially in the midcontinent region, far away from any likely source of hot fuids. We therefore regard chemical authigenesis to be responsible for the remagnefizafion in magnetite-bearing carbonates from areas such as the southern Appalachian Basin and the midcontinent region, where very low thermal maturities indicate that the rocks presently exposed have not seen even moderately high temperatures.

Chemical and thermoviscous remagnefizafion mechanisms are not mutually exclusive processes, and it is probable that both were important in the remagnetization of magnetite-bearing carbonates. The problem, given the present state of our knowledge, is determining which mode of remagnefizafion is the dominant one in individual cases. Until more definitive constraints are available, the Hudson

Valley and midcontinent regions may be regarded as end-member settings, with thermoviscous remagnefization processes permissible in the former and chemical processes probably dominant in the latter. A more definitive assessment of the relative importance of the two remagnetization mechanisms in different settings is likely to be possible in the not too distant future. If the conclusions of Jackson [1989] are confirmed, and the magnetite grains that carry the characteristic remanence in remagnetized carbonates acquire TVRM in accordance with single- domain theory, then the possibility of TVRM can be confidently determined in any region for which the thermal history is reasonably well constrained.

Chemical Remagnetization in Hematite-Bearing Carbonates

Many Paleozoic carbonate units contain secondary, late Paleozoic magnetizations of reversed polarity that reside in hematite. Most of these magnetizations are interpreted as CRMs acquired during hematite authigenesis [e.g., Chen and Schmidt, 1984; Elmore et al., 1985; Deutsch and

Prasad, 1987]. Several studies have used high unblocking temperatures observed in the laboratory and independent paleotemperature estimates (e.g., conodont alteration index) for the rocks to suggest that thermoviscous remagnetization is unlikely [e.g., Elmore et al., 1985]. Additional research has shown that there are various

diagenetic processes that can cause acquisition of a secondary chemical magnetization.

Some of these late Paleozoic CRMs residing in hematite have been related to specific diagenetic events such as

dedolomitization (i.e., calcite replacing dolomite). For example, Elmore et al. [1985] describe a Permian CRM in the Ordovician Kindblade Formation in the Arbuckle

Mountains (southern Oklahoma) that resides in hematite that is associated wth dedolomite. The hematite formed as

a byproduct of the replacement of ferroan dolomite by calcite. The dedolomitization was apparently caused by oxidizing fluids high in calcium content that migrated through the formation during or after uplift of the Arbuckle Mountains.

A Permian CRM has also been reported from dedolomitized rocks which occur in a aliagenetic halo around karst collapse features in the upper Cambrian Royer Dolomite in the Arbuckle Mountains [Nick and Elmore, 1988; Elmore et al., 1989]. Dolomite not affected

by the weathering processes that produced the karst feature contains an apparent early Paleozoic magnetization residing in magnetite. The distribution of the aledolomite and associated CRM around the karst feature and isotopic results from the aledolomite (calcite) suggest that meteoric fluids caused dedolomifization. This is a case where

weathering processes [i.e., Creer, 1968] did cause remagnetization.

Several studies have also documented how fluids can

control acquisition of a late Paleozoic CRM. For example, hematite Liesegang bands found around fractures in the lower Ordovician Kindblade Formation in Oklahoma

contain a late Paleozoic CRM, whereas limestones without

the bands contain an unstable or weak magnetization [Cochran and Elmore, 1987]. Fluids that emanated from

the fractures caused precipitation of the Liesegang bands and acquisition of the CRM.

In the Taum Sauk Limestone (upper Cambrian, Missouri), Dunn and Elmore [1985] report a late Paleozoic magnetization associated with burrowed limestones, whereas unburrowed micrific limestone contains an

apparent early Paleozoic magnetization. Authigenic hematite is intergrown with the calcite that fills the burrows, which suggests coprecipitation of the two phases. The burrows apparently provided pathways for fluids which caused precipitation of hematite and acquisition of the CRM. Although the time of remanence acquisition in the unburrowed limestone is problematic because the magnetization is not constrained by field tests, the pole position is similar to poles from other units that are interpreted to be early Paleozoic in age [e.g., Van der Voo, 1989]. If the magnetization was acquired relatively early, several origins are possible. For example, it could be a alepositional, or early postdepositional remanent magnetization. Alternatively, it could be a CRM related to hematite replacement of a goethite precursor during early aliagenesis [e.g., Channell et al., 1982].

Although it is evident that fluids can cause remagnetizafion, the origin of the fluids is an important issue. In the

486 ß McCabe and Elmore.' LATE PALEOZOIC REMAGNETIZATION 27, 4 ! REVIEWS OF GEOPHYSICS

A NON- MINERALIZED MINERALIZED BRECCIATED

ZONE ZONE ZONE

Figure 10. Cross section showing folded Viola Formation in southern Oklahoma with mineralized fractures. Basinal fluids

caused the Permian remagnetization in an alteration halo around the mineralized fi'actures and veins [Elmore et al., 1989]. The

Viola away fi'om the mineralized and extensively fi'actured zone contains an older magnetization residing in magnetite. Arrows locate sampling sites.

Kindblade and Taum Sauk formations the fluids could

have been basinal or meteoric. Preliminary results from vein-filling calcites and associated lower Paleozoic carbonates in the Arbuckle Mountains in southern

Oklahoma suggest a relation between basinal fluids and a CRM in hematite [Elmore et al., 1989]. Fluid inclusion

studies suggest that most of the calcites formed from fluids that had salinities of 10-20 wt % equivalent NaC1 and high divalent cation content and were relatively hot (>50øC). Oxides (hematite, goethite) and sulfides (sphalerite, marcasite) are also found in the calcites. Paleomagnetic results from limestones in an alteration halo around the

veins indicate the presence of a Kiaman magnetization residing in hematite (Figure 10). Limestones away from the veins contain an older magnetization that resides in magnetite. Although it is likely that there was some mixing with more dilute fluids, the evidence suggests that basinal fluids caused remagnetization around the veins which were the conduits for fluid flow. Away from these conduits, however, the limestones were apparently impermeable, and the basinal fluids could not enter the rock and cause remagnetization.

Some Paleozoic ironstones also contain a late Paleozoic

magnetization in hematite that has been related to dehydra- tion of iron hydroxides to hematite [Kean, 1981], perhaps induced by mild heating as a result of burial [Hodych et al., 1985]. Hematite that replaced fossils also contains a late Paleozoic magnetization in Clinton-type iron ores [Perrout and Van der Voo, 1984].

It is evident that remagnetization of limestones by hematite is not necessarily pervasive and, in fact, is commonly related to specific aliagenetic features and/or conduits for fluid flow. If the objective of a study is to constrain the timing of a diagenetic event, then these are the types of rock that should be investigated. On the other hand, some lithologies (e.g., unburrowed Taum Sauk Limestone) apparently escaped remagnetization in the late Paleozoic, perhaps because the rocks were cemented, which prevented the introduction of the remagnetizing fluids. These lithologies are the most likely to retain a

alepositional or early chemical magnetization and are therefore the best targets for apparent polar wander studies.

Chemical Remagnetization Due to Hydrocarbons As previously stated, late Paleozoic secondary mag-

netizations that reside in magnetite have been reported from several units that have been shown to contain

authigenic magnetite. Although there are several possible mechanisms for the formation of authigenic magnetite, one mechanism that has received considerable attention is a

relationship with hydrocarbons. Establishing a relationship between hydrocarbons and authigenic magnetite not only has implications for understanding remagnetization mechanisms but could also lead to an approach to date or constrain the timing of hydrocarbon migration by isolating the magnetization carried by authigenic magnetite.

Although the formation of authigenic magnetite has been linked to a reducing event created by hydrocarbon migration in several studies [e.g., Donovan et al., 1979; Bagin and Malumgan, 1976], the proposed relationship was neither clearly established nor adequately tested. Donovan et al. [1979] reported magnetite in altered Permian clastic rocks above the producing horizons at the Cement oil field, Oklahoma, and hypothesized that hydrocarbon-related brines migrated from depth upward along faults causing reduction of hematite and iron oxyhydroxides to magnetite. This work, however, has become controversial. For example, Reynolds et al. [1985] noted that the magnetite reported by Donovan et al. [ 1979] from Cement exhibits synthetic textures suggesting that it represents contamination from drilling. Reynolds et al. [1984, 1985] also reported ferrimagnetic pyrrhotite from the Cement field and suggested that it precipitated as a result of hydrocarbon-related fluids.

A recent outcrop study of the altered Permian Rush Springs Sandstone (Guadalupian) at Cement, Oklahoma, suggests that a CRM residing in authigenic magnetite is present [Elmore and Leach, 1989]. A magnetization with both normal and reversed directions was isolated from

samples of carbonate-cemented "bleached" sandstones

27, 4 /REVIEWS OF GEOPHYSICS McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION ß 487

that contain hydrocarbons. The pole position suggests acquisition of the remanence in the late Permian or early Triassic. Demagnetization and rock magnetic results suggest that magnetite carries the remanence, although pyrrhotite is also present. The magnetization is interpreted as a CRM based on the lack of stratigraphic control of the distribution of the normal and reversed directions and on

the presence of magnetic spheres interpreted to be authigenic magnetite. The results of the study support the hypothesis that the chemical conditions created by the hydrocarbons can cause precipitation of magnetite (as well as pyrrhotite) and that paleomagnetic analysis can be used to constrain the time of hydrocarbon migration.

Elmore et al. [1987] and McCabe et al. [1987] report the occurrence of magnetic spheres interpreted to be authigenic magnetite in samples of solid bitumen. Iron was the only element identified in the spheres using energy dispersive X ray analysis. X ray diffraction patterns obtained using Fe k-alpha radiation show that magnetic extracts contain mostly magnetite. Polished grain mounts examined in reflected light also indicate that the spheres are magnetite. The spheres are similar to those extracted from some carbonate units that have been interpreted as authigenic magnetite [e.g., McCabe et al., 1983]. Experi- ments on the acquisition of isothermal remanent magnetization also suggest that the magnetization in the solid bitumen is dominated by a low-coercivity phase such as magnetite. Chromatograms of the saturated hydrocarbon fraction of the bitumen show depletion of the C15+ hydrocarbons, which is typical of biodegraded hydrocarbons. The microbial degradation of the hydrocabons may have produced the magnetite as a mineral by-product.

A paleomagnetic, rock magnetic, peltographic, and geochemical study of the hydrocarbon-saturated speleothems in southern Oklahoma also indicates that there is a

relationship between hydrocarbons and a chemical magnetization that resides in magnetite [Elmore et al., 1987; Elmore and Crawford, 1989]. The speleothems, which are composed of light and dark calcite bands, occur in caves of karst origin in the Ordovician Kindblade Formation. Ver- tebrate fossils interbedded with the deposits indicate that they are Permian in age. The dark bands contain primary hydrocarbon inclusions and possess over an order of magnitude stronger natural magnetization than the lighter bands, which do not yield stable decay during demagnetization. Alternating field and thermal demagnetizations of specimens of the dark calcite reveal a Permian direction of magnetization. The results of rock magnetic experi- menu, and the fact that maximum unblocking temperatures are generally below 580øC, suggest that the dominant component resides in magnetite (the Curie temperature of pure magnetite is 580øC). The presence of authigenic magnetite spheres in magnetic extracts of the dark calcites supports a chemical origin for the magnetization, and shal-

low burial depths reduce the possibility of a thermoviscous magnetization. The occurrence of primary hydrocarbon inclusions suggests that hydrocarbons seeped into the caves during precipitation of the speleothems and were trapped in the calcite crystals. The relationship between intensity of magnetization and hydrocarbon abundance leads to the inference that chemical conditions created by the hydrocarbons caused precipitation of authigenic magnetite and acquisition of the associated chemical remanence.

Gas chromatographic (GC) analysis of the C15+ aliphatic fraction isolated from some of the speleothems indicates that biodegradation is slight [Elmore et al., 1987]. Analysis by GC and gas chromatography/mass spectrometry (GC/MS) also suggests that the probable source of the speleothem oil is the Woodford Formation (M. Engel and S. Imbus, personal communication, 1989). The speleothems are found in deformed limestones that are adjacent to and faulted over the deep Anadarko Basin. The oil was generated from the Woodford in the deep basin, and it migrated up into the limestones during and after deformation [Cardott and Lambert, 1985]. Although not due to remagnetization, the remanence in the speleothems was apparently caused by hydrocarbons expelled as a result of deformation.

Several limestone units that are hydrocarbon beating also contain authigenic magnetite and associated CRMs. For example, the Phosphoria Formation at Sheep Mountain in Wyoming is hydrocarbon impregnated and contains a CRM residing in authigenic magnetite that is probably Cretaceous in age [Benthein and Elmore, 1987]. The results from the Phosphoria are consistent with the interpretation that the chemical conditions created by the hydrocarbons caused precipitation of the authigenic magnetite and acquisition of the associated CRM. The magnetization in some hydrocarbon-impregnated Arbuckle Group (lower Ordovician) limestones in Oklahoma is also secondary (post-Pennsylvanian based on a fold test) and resides in magnetite [Elmore et al., 1988]. The late Paleozoic is the time of hydrocarbon migration in these rocks [Cardott and Lambert, 1985], so the results are con-

sistent with the hydrocarbon/magnetite connection. Ellwood and Crick [1988] also report a late Paleozoic magnetization residing in possible authigenic magnetite from the hydrocarbon-impregnated Boggy Formation (Pennsylvanian), a carbonate unit exposed in southern Oklahoma.

Several studies have reported authigenic magnetite in oil-saturated sandstones [e.g., Kilgore and Elmore, 1989; Ellwood and Crick, 1988]. Although a stable magnetization has not been reported from these hydrocarbon- impregnated sandstones, the results from some units have implications for diagenetic and paleomagnetic studies. For example, hydrocarbon-impregnated sandstones in the Tri-

488 ß McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION 27, 4 /REVIEWS OF GEOPHYSICS

assic Chugwater Formation in Montana contain authigenic magnetite, but the sandstones have significantly lower magnetic intensifies than the relatively unaltered red beds, presumably because the chemical conditions created by hydrocarbons caused dissolution of the hematite [Kilgore and Elmore, 1989]. Although some magnetite does apparently form, there is an overall decrease in magnetization associated with the removal of the hematite. The results

have obvious implications in the use of aeromagnetic sur- veys as a method to search for anomalous concentrations of authigenic magnetic minerals associated with hydrocarbons [e.g., Donovan et al., 1979].

Although the results described above provide empirical evidence for a relationship between hydrocarbons and a remanence residing in authigenic magnetite, a geochemical mechanism for the formation of the magnetite can only be speculated on at this time. The fact that the solid bitumen is degraded suggests that the microbial attack of hydrocarbons is one possible diagenetic pathway that could lead to precipitation of authigenic magnetite [Elmore et al., 1987; McCabe et al., 1987]. Biodegradation of hydrocarbons has also been suggested as causing the precipitation of other authigenic minerals such as pyrite and pyrrhotite [Sassen, 1980; Sassen et al., 1989]. Although biodegradation is one possibility for the precipitation of magnetite, other mechanisms related to the presence of hydrocarbons cannot be ruled out. The fact that the C15+ hydrocarbons in the speleothems are not as extensively degraded as the solid bitumen suggests that biodegradation may not be the only pathway for the formation of diagenetic magnetite [Elmore et al., 1987]. Berner [1964], for example, showed that the formation of diagenetic magnetite is thermodynamically possible for several reactions. Future work will hopefully provide new information on the specific chemical conditions created by the hydrocarbons that cause magnetite authigenesis.

There are several possible sources of the iron that is needed for the formation of authigenic magnetite. kon within the host rock is one possible source. The hydrocarbons themselves may have provided the iron, as iron is found in oil [Ellrich et al., 1985]. Fluids associated with

hydrocarbons also contain iron [Kharaka et al., 1985] which could contribute to the formation of authigenic magnetite.

ULTIMATE CAUSES: REMAGNETIZATION DUE TO

THE MIGRATION OF FLUIDS DURING OROGENY

Approximately 20% of the total volume of unmetamor- phosed sedimentary rocks of the Earth's crust consists of water [Hanor, 1979]. Pore waters of different types are known to exist, each containing differing quantities of dis- solved species and having the potential for involvement in

a variety of water-rock interactions. Some sedimentary waters are of meteoric origin, some are trapped formation waters, and others may be released during igneous, metamorphic, or diagenetic processes. The chemistry of pore waters evolves as water-rock interactions take place. Hydrocarbons and associated fluids form another class of sedimentary pore fluids that may be involved in diagenetic reactions [e.g., Sassen, 1980; Elmore et al., 1987; Surdam et al., 1989]. The movement of fluids from place to place within sedimentary basins can result in the transfer of heat and chemically active species to new sites of active diagenesis. It is becoming increasingly apparent that the different kinds of sedimentary fluids may cause remagnetization under the proper circumstances.

Them are a number of striking features of the remagnetizations discussed in this paper. In the first place, they occurred in a relatively brief interval of time, mostly during the -60 Ma long Kiaman Reversed Superchron. This suggests that there may have been a single ultimate cause for the remagnetizations, perhaps somehow related to the orogenic events that affected much of the continent at the time. However, at least a few different remagnetization mechanisms were certainly involved, and the affected rocks range in tectonic setting from the late Paleozoic omgenic belts to the continental interior. How then, can there be a single cause for all of these remagnetizations?

McCabe et al. [1983, 1984] speculated that chemical remagnetization in magnetite-bearing carbonates might have been due to the lateral migration of basinal fluids, perhaps including hydrocarbons, from the orogenic belts towards the craton. This idea has since been invoked for

chemical remagnetization in red beds [Oliver, 1986; Miller and Kent, 1988b] and for thermoviscous magnetization in magnetite-bearing rocks [Miller and Kent, 1988b]. Al- though still speculative, this "orogenic fluids" hypothesis is an attractive one in that it could reasonably explain many of the observed remagnetizations in terms of a single ultimate cause.

Several recent studies have documented relationships between remagnetizations and fluids that migrated as a result of orogenic activity. For example, hydrocarbons that migrated during or after deformation in southern Ok- lahoma caused acquisition of a chemical remanence in magnetite [Elmore et al., 1987; Elmore and Crawford, 1989]. Elmore et al. [1989] recently documented that warm, saline basinal fluids can cause remagnetization. In addition, several recent papers dealing with the regional pattern of the Appalachian remagnetizations cite circumstantial evidence supporting the orogenic fluids hypothesis. For example, Miller and Kent [1988b] noted that many Appalachian red bed and carbonate remagnetizations are synfolding, which suggests a relation with tectonic activity. In addition, the apparent ages of remagnetization in the Appalachians are older in the south and


younger in the north [Miller and Kent, 1988b]. This trend is consistent with the south to north progression in the timing of Alleghenian deformation [Rodgers, 1967] and is therefore also consistent with the suggestion that fluids associated with tectonic activity caused remagnetization [Miller and Kent, 1988b].

In the studies of Jackson et al. [1988] and McCabe et al. [1989], which were described earlier, it was argued that the regional patterns of magnetite authigenesis and remagnetization in the Appalachian Basin carbonates could be explained by the migration of warm, chemically active fluids from the orogenic zone toward the craton. McCabe et al. [1989] proposed that both the observed patterns of illitization and magnetite content in the Devonian carbonates of New York State are a consequence of the migration of potassium-bearing fluids from the orogenic zone to the southeast. In the southern Appalachian Basin the observed pattern of magnetite concentration and remagnetization suggests that aquifers carrying the magnetite-forming fluids may have been confined to the lower Paleozoic part of the section and that fluid movement was primarily lateral rather than vertical [McCabe et al., 1989]. The in- ferrer path of the remagnetizing fluids in this region is shown in Figure 11.

The lateral migration of sedimentary fluids during foreland basin evolution has long been suspected as a possible origin for Mississippi Valley-type ore deposits [e.g., Dozy, 1970]. Using a simple numerical model, Sharp [1978] has shown that this "lateral secretion" hypothesis may be geologically reasonable under certain conditions. The proposed mechanism for fluid expulsion is the development of overpressure during rapid sedimentation in the foreland basin setting. In Sharp's [1978] model, fluids are periodically expelled laterally from overpressured zones via faults and permeable strata. Other physical mechanisms of fluid expulsion related to orogenic events have been proposed. For example, Oliver [1986] suggested that

lateral compression due to the movement of thrust sheets could drive fluids toward the craton. Bethke [1986] has advocated gravitational flow from mountain highlands via aquifers to the adjacent foreland as the most likely mechanism to provide the high flow rates that appear to be required. Because sedimentary basins frequently contain regionally extensive shales or other impermeable strata, different mechanisms of fluid flow may operate on different fluid types at different levels.

Although the physical mechanism of lateral fluid expulsion from orogenic belts is not yet well constrained, there is a growing body of evidence for a major aliagenetic event of Alleghenian age in eastern North America that could have been a consequence of the cratonward migration of sedimentary fluids. The evidence includes (1) the A1- leghenian K/Ar and Ar/Ar ages that have been obtained from diagenetic K-feldspars [Hearn and Sutter, 1985; Hearn et al., 1987] and K-bentonites [Elliott and Aronson, 1987] in the Appalachian Basin; (2) the distribution of oil fields, Mississippi Valley-type ore deposits, and remagnetized rocks on the Appalachian foreland [Oliver, 1986]; and (3) evidence of the forceful injection of hot fluids in Appalachian Basin limestones during or following A1- leghenian deformation [Dorobek, 1989].

It is evident that most of the secondary magnetizations discussed in this review are associated with orogenic activity. Although lateral migration of fluids expelled during orogenesis may have caused remagnetization in some cases, the evidence is largely circumstantial, and other processes can also cause remagnetization. It is clear, however, that deformation can produce the conduits (e.g., faults and fractures) for fluid flow [e.g., Elmore et al., 1989]. Vertical movement of fluids along these conduits might be the driving force behind remagnetization in some settings. In others, exposure to meteoric fluids during uplift might have resulted in remagnetization during the late Paleozoic [e.g., Nick and Elmore, 1988].

w

NASHVILLE DOME

CHATTANOOGA SHALE

ORDO VICIAN CARBONIFEROUS

E

VALLEY AND RIDGE

Figure 11. Cartoon of the southern Appalachian Basin in cross section showing the inferred migration route of remagnetizing fluids [McCabe et al., 1989]. The fluids are assumed to be generated within the orogen and driven westward through permeable Ordovician strata. It is in-

ferred that the Chattanooga Shale prevented migration of the fluids into the overlying Carboniferous limestones.

490 ß McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION 27, 4 /REVIEWS OF GEOPHYSICS

In recent years there has been a great deal of general interest in the possible role of orogenic fluids in causing a number of different kinds of important geologic features, including remagnetization. Paleomagnetic and rock magnetic investigations will no doubt have an important role in the evolution of thinking on the subject, since such studies might be used to map fluid pathways, to provide age estimates of migration events, and to constrain the chemistry and/or temperature of the fluids involved.

CONCLUSIONS

From a paleomagnetic point of view, the remagnetizations discussed in this paper have proved significant. For example, Van der Voo [1979] used the postfolding magnetizations from the Juniata and Rose Hill red beds to put an upper bracket on the age of Alleghenian folding in the central Appalachians. This was a very useful result, since geological constraints for the upper age of the Alleghenian event in the region are rather loose. In another study, Van der Voo and McCabe [1985b] pointed out that the many high-quality determinations of the late Paleozoic remagnetization might provide the best available estimate of the position (if not the age) of the North American apparent polar wander path during the Kiaman S uperchron. A glance at Figure 2 will show that the remagnetization poles lie somewhat to the north of the apparent polar wander path defined by the smaller number of poles from Pennsyl- vanian and Permian rocks. This suggests a somewhat more northerly paleoposition of North America, which has important implications for the early configuration of Pangea.

The temporal association between the late Paleozoic remagnetizations and orogeny is very clear, if not fully understood. Orogenic events may be important in causing remagnetizations in orogenic belts other than late Paleozoic ones. For example, the younger remagnetizations that have been reported from units in the Rocky Mountains [e.g., Schwartz and Van der Voo, 1984; Ben- thien and Elmore, 1987; Globerman and Irving, 1988] and in Alaska [e.g., Plumley et al., 1981] can be correlated with the timing of major orogenic activity in those regions. In the Appalachians, Taconic (Ordovician)or Acadian remagnetization events have been suspected in some places, and they may be more common than is currently believed [e.g., Tucker and Kent, 1988; Miller and Kent, 1989; Hodych, 1989].

Additional work is clearly needed to better establish a general relationship between orogeny and remagnetization. It is particularly important that we obtain a better understanding of the basic aquifer architecture in sedimentary basins and of the physical mechanisms that may drive fluids from place to place. Additional studies of regional patterns of remagnetization could be productive in this regard [e.g., Miller and Kent, 1988b; McCabe et al., 1989]. Stud-

ies relating remagnetization to specific conduits for fluid flow [e.g., Elmore et al., 1989] are also needed. In cases of suspected chemical remagnetization, petrographic and geochemical studies are needed to establish temporal and spatial relationships between secondary magnetic phases and other observable evidence for diagenesis [e.g., Elmore et al., 1985]. Authigenic magnetic phases also need to be related to specific secondary magnetizations. Research of this nature is difficult and time consuming but is essential in constraining the origin of remagnetization in individual cases.

The role of thermoviscous processes in causing remagnetization is far from clear at the present time. Theoretical and experimental studies are needed to better constrain the TVRM acquisition characteristics of different natural as- semblages of magnetite carders. In addition, the detailed thermal histories necessary for evaluating the likelihood of thermoviscous remagnetization in particular rock units are, for the most part, not available. Studies of secondary fluid inclusions offer particular promise in constraining the temperature and composition of fluids that may have moved through late Paleozoic aquifers. For example, Dorobek [1989] has obtained estimates of filling temperature, salinity, and relative age with respect to deformation of secondary two-phase fluid inclusions in some Siluro- Devonian carbonates of the central Appalachians. Addi- tional, more detailed studies of this type are urgently needed before the true role of thermoviscous processes can be ascertained. Additional information on burial histories

from apatite and zircon fission-track cooling ages will also be helpful.

One perplexing aspect of the late Paleozoic remagnetizations is the tendency for different magnetic minerals to form in different lithologies. Although hematite-beating remagnetized carbonates are not uncommon, most of the studies in the Appalachians have found that magnetite is the carrier of the remagnetization in this rock type. In contrast, hematite is almost always the carrier of the remagnetization in siliciclastic rocks. The spatial distribution of the two remagnetization types also indicates a lithology dependence: in cases where remagnetized carbonates and sandstones are present within the same locality, the carbonates tend to contain magnetite and the sandstones hematite as the carder of the overprint [e.g., Chen and Schmidt, 1984; McCabe et al., 1988]. Miller and Kent [1988b] have pointed out that there seems to be no difference in magnetization age with respect to folding between the two rock types in the central and southem Ap- palachians. These observations can be interpreted as suggesting that the same fluids might result in different kinds of remagnetization in different rock types. If this very speculative idea is true, it could be an important clue to understanding the late Paleozoic remagnetization phenomenon. For example, it may indicate that all of the magnetite remagnetizations are TVRMs, whereas the red


bed ones are all CRMs. Although perhaps possible, we regard this as unlikely, since TVRM appears to be unlikely in some carbonate settings (e.g., the midcontinent). On the other hand, these observations could be explained by the differing chemical environments afforded by different rock types. For example, the iron sulfides and organic matter commonly present in carbonates might act as oxygen sinks, thus creating more reducing conditions that might tend to favor the authigenic formation of magnetite rather than hematite. Alternatively, the buffering effect of carbonate could be important in determining the stable iron oxide phase. Experimental studies of iron-mineral aliagenesis under different chemical conditions need to be applied to the problem of remagnetization in order to help choose between different scenarios.

In just a few years, a great deal has been learned about the phenomenon of widespread remagnetization during the late Paleozoic. However, a number of major issues per- taining to remagnetization mechanisms and the ultimate cause(s) of remagnetization remain unresolved. Once we have a more fully developed understanding of the remagnetization phenomenon, our ability to discover primary magnetizations in Paleozoic or older rocks will be increased significantly. New applications of the paleomagnetic method are also likely, particularly in studies of carbonate and siliciclastic diagenesis, and perhaps also in probing the ancient plumbing of sedimentary basins.

ACKNOWLEDGMENTS. We thank Mike Jackson, Jeff

Hauor, John Miller, Rob Van der Voo, and Ben van der Pluijm for comments on an earlier draft of this paper. Supported by Na- tional Science Foundation grants EAR-8816826 (to C.M.) and EAR-8617597 (to R.D.E.).

J. A. Jacobs was the editor responsible for this paper. He thanks Dennis V. Kent for assistance in evaluation of its techni-

cal content and J. Graham Cogley for serving as the cross- disciplinary referee.

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R. D. Elmore, School of Geology and Geophysics, Univer- sity of Oklahoma, Norman, OK 73019.

C. McCabe, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803.

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