A paleomagnetic study of remagnetized Upper Jurassic red beds from Chihuahua, northern Mexico

307

Physics of the Earth and Planetary Interiors, 62 (1990) 307—322 Elsevier Science Publishers By., Amsterdam

A paleomagnetic study of remagnetized Upper Jurassic red beds from Chihuahua, northern Mexico Emilio Herrero-Bervera

1,

Jaime Urrutia-Fucugauchi

2

and M.A. Khan

School of Ocean and Earth Science and Technology, Hawaii Institute of Geophysics. University of Hawaii, 2525 Correa Roa4 Honoluh~,Hawaii 96822, (U.S.A.) 2 1.aboratorio de Paleomagnetismo, Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Delegacion Coyoacan,

04510 Mexico D.F. (Mexico)

(Received September 9, 1989; revision accepted March 6, 1990)

ABSTRACT Herrero-Bervera, E., Urrutia-Fucugauchi, J. and Khan, M.A., 1990. A paleornagnetic study of remagnetized Upper Jurassic red beds from Chihuahua, northern Mexico. Phys. Earth Planet. Inter., 62: 307—322. Paleomagnetic results from 75 samples (132 specimens) collected from 20 beds from a — 55-rn thick sequence of Upper Jurassic red beds from La Casita Formation, northern Mexico are reported. Detailed partial thermal (up to 680°C) and alternatin~field (up to 320 mT) demagnetizations reveal different behaviors of vectorial composition and magnetic stability. Before tilt correction, the directions cluster around the dipolar and present-day field directions for northern Mexico. The dominant well-grouped remanent magnetization directions after tilt correction are steeply dipping and lie far apart from Mesozoic—Cenozoic directions reported for northern Mexico and North America. The high-coercivity magnetization components apparently reside predominantly in red pigment hematite. Polished section observations show a more complex mineral assemblage, characterized by abundant red pigment and well-rounded opaque hematite, titanomagnetite and ilinenite grains. Titanomagnetites are less abundant and show a fresh angular appearance with low oxidation class (C-i and C-2). Most samples are characterized by the high-coercivity present field magnetization directions, which are attributed to red hematite pigment recently magnetized. Some samples in contrast show two or more magnetization components. High-temperature magnetization components (i.e. those observed between 550—600 °Cand 680°)show reversed polarity. However, vector plots are noisy, and directional changes during demagnetization are often accompanied by increases in intensity and are not consistent between samples. Results of incremental partial thermal demagnetization on the low-field magnetic susceptibility amsotropy have been studied by heating samples to successively higher temperatures and measuring the anisotropy after cooling in air from each temperature increment. The results conform to the pattern generally expected for a primary depositional fabric with predominantly oblate-shaped susceptibilities, and with k 3 directions close to the bedding pole and k1 directions lying in the bedding plane and preferentially aligned in a SW—NE direction as a result of possible currents acting at the time of deposition. The available evidence indicates that these red beds have been remagnetized and that most of the magnetization was acquired during recent (Brunhes Chron) times.

1. Introduction

Elston and Purucker, 1979; Purucker et aL, 1980),

Red beds commonly present relatively strong, easily measurable remanent magnetizations and consequently are extensively used for paleomagnetic studies (e.g. Irving, 1964; Turner, 1980). Although they have provided reliable paleomagnetic results (e.g. Collinson and Runcorn, 1960; Baag and Heisley, 1974; Heisley and Steiner, 1974;

many critical aspects of processes involved in the acquisition of the paleomagnetic record still remain poorly known (e.g. Larson and Walker, 1975; Collinson, 1980; Turner, 1980; Walker et al., 1981; Heisley and Herrero-Bervera, 1985; Larson and Walker, 1985; Maslanyj and Collinson, 1988). Detailed investigations of particular red bed formations have provided valuable information. Never-

0031-9201/90/$03.50

© 1990

—

Elsevier Science Publishers BY.

308

C. HERRERO-BERVERA ET AL.

~

~

JURASSIC

(~~• l\

~

\ “~e/, d~0,,O,~ ~ SAMPLING )~ ~ SITE \‘4 ~.

~‘\

~

~\

(

~

“.~,

°ør

~mIJ

0

a I06W

103’30, I

2930 N

05’ 29’30

c:UT~0~ LA CASITA GROUP Oi 0

5

tO

IS

2Ok~

N MN CONCHOt

rnvu—....

_—JS

SAMPtING 5T~~

7_,

oe CUA11~P~~ç•,j,,)

j74

29

-

29’

C GRANUO DAM

TIANCAS ALDAMA

CHIHUAHUA I06’W

I 105’ 30

b

lao’

Fig. 1. (a) Summary of observed (solid arrows) and expected (dashed arrows) paleomagnetic declinations for northern Mexico. Expected declination is computed from stable North America data. Sources of Mexican data are: Nairn (1976), Gose et al., (1982) and Cohen et al., (1982). Major left-lateral faults suggested are shown by major discontinuous lines (figure adapted from Urrutia-Fucugauchi, 1984). Location of paleomagnetic site is shown by the square. (b) Schematic representation of outcrops of La Casita red beds with location of sampling site.

309

REMAGNETIZED UPPER JURASSIC RED BEDS, NORTHERN MEXICO

theless, because red beds can contain more than one stable rema.nent magnetization and various carriers of remanent magnetization formed at various times (e.g. Turner, 1980; Walker et al., 1981), paleomagnetic studies of a specific formation require careful evaluation of the history of remanent magnetization acquisition. The study reported in this paper was initially undertaken in an attempt to derive a magnetic polarity stratigraphy and to estimate the paleomagnetic field for the Late Jurassic of northern Mexico. The results obtained strongly suggest that the paleomagnetic record of these Jurassic red beds has been affected by remagnetization. The experimental results are used to assess the processes and timing of the dominant remanent magnetization acquisition event. 2. Geology and sampling A thick sequence of conglomerates, sandstones and siltstones of the Kimmeridgian—Portlandian La Casita Formation (Imlay, 1943, 1953) is well exposed in the area of Placer de Guadalupe, Chihuahua (Fig. 1). Sampling was restricted to a fresh exposure of dark red to brown arkosic sandstones in a road cut which represents about 50—55 m of vertical section. A total of 84 samples orientated in situ using a Brunton compass were collected from 20 beds as hand samples or cores. Between two and five independent samples were collected from each horizontal to investigate the within-bed directional dispersion and magnetic characteristics of the rock. Bedding plane measurements were taken at regular intervals and later averaged out. This resulted in a mean estimate of bedding plane of 86.6° strike, 42.8° dip (with associated statistical parameters (Fisher, 1953) of N = 7, k = 59, and a95 = 7.9°). Folding of the strata took place during the Laramide orogeny in the Late Cretaceous—Early Tertiary. Nine samples were lost from the study during subsequent transport and handling in the laboratory. A total of 132 specimens of variable height (mostly 2.2 cm) and 2.5 cm diameter were sliced from the remaining 75 samples. The sample collection was split into two sets and measured independently by the authors at their respective institutions.

3. Methods The intensity and direction of natural remanent magnetization (NRM) were measured using either a DIGICO spinner magnetometer or an ScT cryogenic magnetometer. The stability and vectorial composition of NRM were investigated by stepwise thermal and alternating field (AF) demagnetization. Thermal demagnetization was accomplished in a conventional non-inductively wound electrical furnace, the samples being cooled in a region where the residual field was less than 10 nT. Temperature was monitored in the furnace by means of Platinum thermocouples that were referred to an ice bath and read on a Beckman digital multimeter and a Love temperature controller. Thermal demagnetization experiments were carried out in 7—10 steps up to 680°C, and AF demagnetization was carried out in 10 steps up to 100 mT and for one specimen in 17 steps up to 320 mT. Bulk susceptibility was measured on a low-field susceptibility bridge. Isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) acquisition experiments and AF demagnetization of maximum IRM and ARM were completed on a few selected specimens. IRM was given in 22 steps up to a maximum field of 760 mT and ARM was given in five to six steps up to a maximum AC coaxial field of 80—100 mT with a direct field of 0.05 mT. Maximum ARM was later AF demagnetized in steps. Anisotropy of magnetic susceptibility (AMS) of some samples coming mainly from the basal portion of the sequence was measured on an anisotropy attachment to a DIGICO meter. Care was taken to avoid shape sample problems by selecting 2.2-cm long specimens. Polished section observations were completed on a Zeitz ore petrography microscope using the oil immersion technique.

4. Results Initial directions referred to the present horizontal are scattered around the present dipolar field and geomagnetic field directions for the sampling site (Fig. 2a). Directions within each bed present variable degrees of grouping, with a95 and

310

E. HERRERO-BERVERA CT AL.

NRM

NRM 0°

0° •l7~ 118

/

/ /

•1I 8~~I3

~4

/

I

/ i I

\

.-~“~io —16.1

a

17

•~~‘2O

S

14

5

I

I

IS

270°I

~‘

\

90°

I

270°

I

I

•II

~~ I~

(a)

1800

(b)

I

90°

I

180°

PRESENT HORIZONTAL

PALAEOHORIZONTAL

Fig. 2. Initial NRM directions for each bed (numbers 1—20) plotted in equal area projections before (a) and after (b) tilt correction. The dipolar field direction is shown by the cross, and the present-day geomagnetic field by the triangle.

k parameters (Fisher, 1953) ranging from 2.3° to 36° and 710° to 8°, respectively. Scatter increased towards the top of the section (bed num-

bers 1 and 20 correspond to the base and top of section, respectively) where beds 3, 9, 10, 13, 16, 17, 19 and 20 present a95 values > 10°. The between-bed scatter is not high, with an overall

E 0.5

NE •.~

I

10

I

°

NV

10

I

~

05

J/J005

05 RB6.4

~0

V

:E: ~•::~

34/m

~ •~•~:~ ~::

200

300

400

500

600

700

600

700

‘C

20

J0’109610

°

100

25

4~0

Fig. 3. Orthogonal vector plot (top) of directional and intensity

~

C 0

changes (in sample coordinates) during AF demagnetization (three-axes stationary demagnetization). Normalized intensity changes (bottom) for same specimen.

-~ 100

200

300

400

500

C

Fig. 4. Examples of changes in intensity during thermal demagnetization. Note the different scales for both diagrams.

311

REMAGNETIZED UPPER JURASSIC RED BEDS. NORTHERN MEXICO

hA

2.2A

1.1 A

~50 500

£

1

I.2A

400

z

~

~

4.IA

\~l0

3.2A

21l26’c

10.2

661

2.3A

6;

Fig. 5. Examples of orthogonal vector plots for typical thermal demagnetization series. --s, projections on to the horizontal plane; •—., projections on to the vertical plane. Intensities are normalized to initial NRM intensity. Numbers indicate demagnetization steps in °C.

a95

= 13.5°and k = 30 for all sites. Correction for structure results in steeper inclinations (Fig. 2b). Directions after tilt correction are far away from Mesozoic—Cenozoic paleomagnetic directions for Mexico and North America (Urrutia-Fucugauchi, 1979, 1984). These observations suggest that NRM directions predominantly reflect a secondary magnetization acquired in recent times. The relative scatter observed within and between beds (Table 1) suggests that other components of magnetization may also be present. AF demagnetization of pilot specimens from different beds up to 100 mT and one up to 320 mT (Fig. 3) resulted in a slight decrease in initial intensity and little change in direction, indicating high-coercivity minerals as the main magnetic cartiers (hematite).

Thermal demagnetization proved to be more effective in investigating the vectorial composition of the magnetization which is not surprising for red sediments (e.g. Ouliac, 1976). With demagnetization, most samples show a decrease in intensity up to 680°C with median destructive temperatures around 500°C (Fig. 4). The low-field susceptibility generally showed a slight steady increase during thermal treatment. NRM directions generally remained without appreciable changes up to 500—550°C. For some samples, extrapolation of linear trends in the vector plots moved close to the origin (Fig. 5), but in most cases apparent undefined components were still present (Figs. 5 and 6). The directions observed above 500°C were inconsistent from sample to sample, and the within- and between-beds scatter in-

a HERRERO-BERVERA ET AL.

312

3.1

6.IA

9.IA

£

6~0

£

N

~

~

£

S

~

~

N

~

6.2A r £ .1 6~ ~

4.3B ~

~b01o1:~c

lOlA N ~

00

680 ~4

210

N

£

680

N

551

~

£

6B

650

~

550

~ \8~C

Fig. 6. Examples of orthogonal vector plots for thermally demagnetized specimens showing changes at high-temperature steps (specimens mainly from the upper section). Symbols as in Fig. 5.

creased with the demagnetization (Fig. 7 and Table 1). Some samples show evidence of high-blocking temperature components of reverse polarity, e.g. in some vector plots, vector trends clearly bypass the origin (Figs. 5 and 6) and although measured directions seem erratic in most samples, in three cases the directions suggest the reverse polarity components (Fig. 8). The red beds are mainly formed by subangular quartz grains cemented by red pigment hematite and calcite. The red hematite pigment is also found as thin pellicles coating detrital quartz grains. Most of the pigment is very fine-grained or amorphous to permit distinction of hematite crystals. There seem to be no preferred alignments or discernible fabrics. A few fragments (1%) of intermediate extrusive igneous rocks are also present. There is no observable microscopic preferred

orientation of grains; although macroscopically, cross-bedding and other sedimentological features could be observed. In addition to the red hematite pigment, opaque hematite grains, titanomagnetite and ilmemte are present. Titanomagnetites are less abundant, present a fresh angular appearance and low oxidation state (C-i and C-2; Haggerty, 1976) (Fig. 9). Opaque hematite grains are well rounded and show a pitted surface, which suggest a detrital origin (Fig. 9). IRM acquisition curves (Fig. 10) do not saturate up to the maximum field available of 760 mT, which is consistent with the predominance of fine-particle hematite which usually present high coercivities (300—1800 mT, Dunlop, 1972). The curvature of the IRM acquisition curve near the origin is convex away from the vertical axis (Fig. 10) which argues against any significant fraction

313

REMAGNETIZED UPPER JURASSIC RED BEDS. NORTHERN MEXICO

550°C

NRM

270°

~

90°

270°

E°I’

90°

180°

180°

630°C

665°C

270°

90’

180’

270°

0

LOWER

£

PRESENT

. UPPER

90°

18

Fig. 7. Directions plotted before tilt correction for specimens from the upper section (beds 11—20) for initial NRM and after 5500, 630° and 665°C thermal demagnetization. Open circles represent downward inclinations and closed circles represent upward inclinations. Dipolar and present-day field directions are given by the cross and triangle, respectively.

of magnetite or maghemite in the samples (Dunlop, 1972). The presence and relative amount of black crystalline specularite are more difficult to estimate. Specularite occurs as detrital particles which may then carry a primary detrital remanence (Steiner, 1983), and presents intermediate coercivities 100—300 mT; Dunlop, 1972). Observations on polished sections confirm the presence of wellrounded opaque hematite grains in the samples, however, it is not clear what its role is as NRM carrier. We should point out that the presence of speculante is no guarantee of detrital magnetiza(—

tion. It frequently forms as a result of the in situ oxidation of titanomagnetite. The relatively low maximum field available for the IRM experiments was insufficient for a full investigation of the coercivity spectrum of samples, however, the coercivity characteristics are compatible with the presence of specularite (they may also reflect variation in grain size of impurities of the hematite pigment). AF demagnetization of the maximum IRM shows that IRM is softer than NRM (Fig. 10) which may suggest a low-intermediate coercivity mineral.

314

E. HERRERO-BERVERA ET AL.

00

550

/

/

I

.~

I

/

/

1

.00

I

~4A

1

650

\ ,dI665

\\ \

/

/f~

____ ‘,,500

~‘ /

650625\

/

o~5

680 ~

8O__-°°~625

665

680a—...----L 650.---_’~ “

1800 Fig. 8. Equal area projection of directional changes of specimens showing apparent reverse components with certain consistency (specimens from three different beds). Solid symbols represent downward inclination and open symbols represent upward inclination. Numbers represent steps in °C.

Magnetic susceptibility anisotropy measured in nine samples (Figs. 11 and 12) shows a magnetic fabric compatible with a primary depositional origin (Graham, 1966; Hamilton and Rees, 1970, Tauxe et al., 1990), with predominantly oblate susceptibility ellipsoids, minimum (k3) susceptibility axes orientated normal to the bedding plane and maximum (k1) susceptibility axes lying in the bedding plane and aligned in a SW—NE direction (Fig. 12) (Urrutia-Fucugauchi, 1981). The AMS was also measured after heating the samples to 100, 200, 300, 400, 500 and 600°C(Fig. 11). The directional pattern remains the same, but the scatter is greatly reduced after heating the samples to 600°C. Heating apparently results in changes in the magnetic mineralogy, giving a magnetic fabric which can be more accurately measured and

is better defined upon temperature (UrrutiaFucugauchi, 1981). The magnetic fabric may be explained by hematite coating on quartz and other non-magnetic particles, where the magnetic fabric is then controlled by the original depositional fabric although the hematite coating is produced at a much later time. Alternatively it could be associated with plate-like detrital magnetic grains, therefore representing a primary depositional fabric. It is likely that paramagnetic minerals do play an important role in rocks characterized by a weak susceptibility such as these red beds. The pattern points to a depositional fabric with horizontal foliation planes, and a cluster of the k3 susceptibility axes does not conclusively support a detrital remananent magnetization/ post-detrital remanent magnetization (DRM/PDRM). Micro-

REMAGNF.TIZED UPPER JURASSIC RF.D BEDS. NORTHERN MEXICO

315

Fig. 9. (a) Subhedral titanomagnetite grain showing its elongated shape and ore petrology low C-i oxidation class. In the background, red hematite pigment is coating quartz grains. (b) Titanomagnetite grain showing ilmenite lamellae (oxidation class C-2). (c) Opaque detrital hematite grains showing a well-rounded shape and pitted surface. (d) Fragments of intermediate igneous rocks.

316

E HERRERO-BERVERA ET AL.

Fig. 9 (continued).

scopic observations do not show any preferred alignment of quartz grains or of the ferromagnetic fraction.

Effects of deformation resulting from folding (and strain) on the directions of remanent magnetization do not appear severe. Beds are folded with

317

REMAGNETIZED UPPER JURASSIC RED BEDS, NORTHERN MEXICO

dips of about 43°, k1 susceptibility axes are on the bedding plane and NRM directions are norma! to bedding plane. NRM directions do not appear deviated towards the maximum k1 directions. Thermal demagnetization at temperatures below 550—600°C resulted in a decrease of the within- and between-bed scatter, possibly as a result of removal of viscous components. Heating

12

x

—8W—

(1 0-°)

8 (a) 4 I

K, 1K3

______________________

(b)

1.0 1.2

~:

oc

~

i/Jo

I

1.2 1.1

1.1

I

1.1

0.8

IRM

1.0 oc

0.7

1.2

RB 6.4

0.6

K2/K3

1.1 0.5

I

0

20

I

I

40

I

60

I

80

100

ml

______

______

(ci)

1.0 100 200 300 400 500 600

1.!

Fig. 11. Summary of anisotropy of magnetic susceptibility 1.0

ui0

(AMS) results. Diagrams give the low-field susceptibility and magnetic anisotropy parameters (i.e. anisotropy degree k1/k3, lineation k1/k2 and foliation k2/k3) as a function of temperature. Samples were heated in 100°Csteps up to 600°C. The bulk susceptibility and magnetic anisotropy were measured after each heating run (see also Fig. 12).

IRM

0.8 0.7 .

RB 5.2

0.6 0.5

I

0

I

I

I

20

I

I

40

I

60

I~.

80

I

to higher temperatures, however, resulted in a marked increase in the scatter (Fig. 7) particularly for samples from the upper section (numbers 10— 20 in Fig. 13). Histograms of NRM intensity (e.g. Fig. 14) constructed after each heating step show a departure from a roughly log-normal distribution (with a tendency for bi-modal distribution at high temperatures) and a much larger dispersion (par-

100

ml

RB 5.2

4,

0

0

0

0

3.2

o

0

E

0

S

RB 6.4

I 0

~

~ •

.6

o•

ticularly above 665°C) in intensity values. This correlates with the changes in direction, which are more scattered after demagnetization at high tern-

0.8 0

0

SI

80

160

240

320

400

480

560

640

720

800

ml Fig. 10. Examples of isothermal remanent magnetization (IRM) acquisition curves (bottom) and intensity changes during AF demagnetization of maximum IRM and initial NRM (top two diagrams).

peratures (Fig. 7). This indicates the presence of vanous components of remanent magnetization residing in different magnetic minerals. Mean directions (Table 1) for most beds estimated from the high-temperature demagnetizing steps generally remained close to the present dipolar direc-

318

E. HERRERO-BERVERA ET AL.

20°C

200°C

0

90W(.IIIII

II~~+I

1111

III.)90E

90W(-III

II

iS

I~~I

aJ

•

/

IISI.~OE

a)

-

\a

/

-

/ -

a

180

400° C

-

180

600°C

0

(~~d)

a0w(_i

I

I

I

I

I

lI~

I

I

I

I

I Ij90E

/I

\~ \~: 180

90W(— I I

I

I

I

I

i.I~~I

I

I

I

I

I I

-

/

I~90E

/I

180

Fig. 12. Directions of (AMS) principal axes (maximum as triangles and minimum as circles) as a function of temperature. Circumference represents bedding plane in the equal area plot. Observe that k 3 axes are normal to bedding plane and k1 axes lie in the plane orientated SW—NE. The AMS directional pattern appears better defined after heating the sample to 600°C. The directional pattern corresponds well with that expected for a depositional magnetic fabric.

tion (referred to the present horizontal, Fig. 13), although the scatter was considerably higher (compare Figs. 2 and 13).

5. Discussion The process and time of acquisition of remanence(s) in red beds are presently subjects of much interest among paleomagnetists (e.g. Turner, 1980; Walker et a!., 1981; Helsley and HerreroBervera, 1985; Maslanyj and Coffinson, 1988). There seem to be two major opposing views. On

one hand it is proposed that red beds can carry a DRM or PDRM which reflects the Earth’s magnetic field at or soon after deposition of the sediments (e.g. Baag and Helsley, 1974; Purucker et a!., 1980). This view is supported by agreement of paleopoles with contemporaneous data derived from other lithologies, or with corresponding segments of apparent polar wander paths and also by detailed magnetostratigraphical investigations (e.g. Steiner and Helsley, 1975a, b; Lienert and Helsley, 1980; Herrero-Bervera and Helsley, 1983). On the other hand, it is proposed that remanence in red beds is a chemical remanent magnetization

319

REMAGNETIZED UPPER JURASSIC RED BEDS. NORTHERN MEXICO

630° C

630°C

0°

0° •l7~

/7 / I 270°

~

6

•l2

/7

•I0

3

•13

I

I

/1

20\

I4

~I9

I

•

1

J

•I7

90°

2700

•~ 16

I

I

II

15s~

2cr\

13

I

I

)

90°

i~

180°

(a)

(b)

PRESENT HORIZONTAL

18o° PALAEOHORIZONTAL

Fig. 13. Mean directions of magnetization for each bed (numbers 1—20) observed after thermal demagnetization at high-temperature steps referred to present horizontal (a) and paleohorizontal (b). Note that between-bed scatter increases with the bed number (higher position in sequence).

(a) .io

-5.0

(b)

Log ~ 685C

10

J:-4.7

~

~,

O~o.44 5

/

_____________________ -5.5

Larson and Walker, 1982). Turner (1979, 1980) has proposed a model of evolution of red beds in which he distinguishes three major types of red bed paleomagnetic records. Type A red beds, are characterized by apparently reliable paleomagnetic poles and magnetostratigraphical results often

-4.0

-4.5

-5.0

~

L

yi

__________ 4.0

(CRM) often consisting of multiple phases acquired over long periods of time and rarely reflects the Earth’s magnetic field at the time of deposition of sediments (e.g. Walker et al., 1981;

3~5

09

Fig. 14. Histograms of NRM intensity for the specimens of the upper section before demagnetization (a) and after thermal demagnetization at 665°C(b). Note the tendency for bi-modal distribution after heating at high temperatures. (Vertical axis denotes number of specimens.)

showing zones of normal and reverse polarities separated by intermediate zones which can be correlated laterally over large distances. The directions agree well with those determined from coeval igneous rocks and stabihty tests (e g fold test or conglomerate test) indicate that remanences are stable for long penods Type A red beds probably reflect the Earth’s magnetic field at, or shortly after, deposition. Type B red beds are characterized by complex paleomagnetic records with multiphase magnetizations. In stratigraphic studjes, discrete zones of truly normal or reverse polarity are rarely found, and the scatter of directions is generally high. In some cases, the results may

320

relate to the time of deposition, but the record is difficult to retrieve. Finally, type C red beds are characterized by an apparently simple paleomagnetic record, with umvectorial remanences, well-grouped directions and high stability during thermal, AF or chemical demagnetization. However, the direction is often widely divergent from that expected for the age of the sediments, Field tests indicate post-folding magnetizations, and directions referred to the present horizontal are close to those for a time much younger than the time of deposition, and very often close to the present-day field. Our paleomagnetic results of the La Casita red beds may well be explained in terms of Turner’s model. The upper section of the sequence (i.e. approximately beds 14—20) corresponds mainly to type B red beds, and the lower section of the sequence corresponds mainly to type C red beds. The dominant remanence observed (that isolated below 550—600°Cand 320 mT during demagnetization) was probably acquired during recent times. An argument against a Late Jurassic (or any pre-Eocene time) origin of the characteristic remanence observed in these red beds comes from the comparison of directions before and after tilt correction (Figs. 2 and 12). Although we were unable to locate a site for a fold test, the relatively high dip of the strata (-- 43°),permits us to compare the directions before and after tilt correction clearly, with both observed and expected directions for the site from other paleomagnetic studies of Mexico and North America (UrrutiaFucugauchi, 1979, 1984). Expected directions estimated from the North American apparent polar wander path (APWP) present shallow incinations and northerly declinations. Directions reported for other Late Jurassic formations from northern Mexico also show northerly declinations and steeper inclinations. Mean direction for the Antimonio Formation (Sonora State) is D = 350°, I = 34° a95 = 12° (Cohen et a!., 1982), and for the Zuloaga Formation (Coahuila State) is D = 350°, 1= 44°, a95 = 8° (Gose et al., 1982). In contrast, the tilt-corrected direction for the La Casita Formation presents much steeper incinations (Figs. 2 and 12). If they were reflecting a Late Jurassic (or pre-Early Tertiary) paleofield

E. HERRERO-BER VERA ET AL.

direction, then they would imply a northerly paleolatitudinal position for this area of northern Mexico and a considerable (highly unlikely) southward tectonic displacement. Thus, remagnetization after folding, particularly in recent times, is considered a more likely possibility. The dominant remanence in the red beds is possibly a CRM residing in fine-particle hematite (with high resistance to AF demagnetization) which is present as pigment coating and cementing other minerals. Most of the remanence may have been acquired during relatively recent times as indicated by the agreement between the present field direction and the red bed directions. The multicomponent magnetizations documented for some of the beds and the occurrence of reverse polarity high-temperature components indicate a long period of long remanence acquisition, which is characteristic of CRM acquisition in red sediments (e.g. Larson and Walker, 1975, 1982; Walker et a!., 1981). A viscous remanent magnetization (VRM) acquired during the Brunhes Chron and residing in the fine-grained hematite (which can present high coercivities, > 300 mT; Dunlop and Sterling, 1977) cannot be ruled out completely. With respect to the high-blocking temperature components or apparent directional instability present during thermal demagnetization, we notice that similar behavior has been reported for other Jurassic sedimentary formations from the western United States. Results for the Swift, Rierdon, Morrison and Summerville formations have been summarized by Steiner (1980). The interpretation is in terms of weak geomagnetic field intensity and of frequent reversals of polarity with a larger amount of reverse polarity than of normal polarity. La Casita Formation covers the Kimmeridgian—Portlandian, an interval which may also be characterized by frequent polarity reversals (Larson and Hilde, 1975; Ogg and Steiner, 1990), and part of the results could also be interpreted in terms of reverse components of magnetization (although incompletely separated). The red beds investigated, however, appear to carry a much younger (mainly acquired during Brunhes times) paleomagnetic record, and thus, unfortunately no implications for the Late Jurassic magnetostratigraphy can be derived.

REMAGNETIZED UPPER JURASSIC RED BEDS, NORTHERN MEXICO

Acknowledgements Financial support for this study to E. HerreroBervera and M.A. Khan was provided by the Hawaii Institute of Geophysics, as well as NSF grant EAR-8916597 to E. Herrero-Bervera and by a grant from Umon Texas Petroleum to M.A. Khan. We also thank Silvia Gonzalez for her assistance with the petrographic analyses. Constructive and valuable comments by Dr. M. Steiner and unknown refrees are gratefully acknowledged. Hawaii Institute of Geophysics contnbution No. 2287.

321 Helsley, C.E. and Herrero-Bervera, E., 1985. Reply. J. Geophys. Res., 90: 2063—2065. Helsley, C.E. and Steiner, M., 1974. Paleomagnetism of the Lower Triassic Moenkopi Formation. Geol. Soc. Am. Bull., 85: 457-464. Herrero-Bervera, E. and Helsley, CE., 1983. Paleomagnetism of a polarity transition in the Lower (?) Triassic Formation, Wyoming. J. Geophys. Res., 88: 3506—3522. Imlay R.W., 1943. Upper Jurassic ammonites from the Placer de Guadalupe District, Chihuahua, Mexico. J. Paleontol., 17: 527—544. Imlay, R.W., 1953. Las Formaciones Jurasicas de Mexico. Bol. Soc. Geol. Mexicana, 16: 1-63. I~g E., 1964. Palaeomagnetism and its application to geological and geophysical problems. Wiley, New York, 399

References

Larson, R.L. and Hilde, T.W.C., 1975. A revised time scale of magnetic reversals for the Early Cretaceous and Late Jurassic. J. Geophys. Res., 83: 2586—2594. Larson, E.E. and Walker, T.R., 1975. Development of CRM

Baag, C.G. and Helsley, C.E., 1974. Evidence for penecontemporaneous magnetization of the Moenkopi Formation. J. Geophys. Res., 79: 3308—3320. Cohen, K.K., Anderson, T.H. and Schmidt, V.A., 1982. Preliminary results: Paleomagnetism of Mesozoic units from Northwest Sonora and their tectonic implication for Northem Mexico. In: J. Urrutia-Fucugauchi (Editor), Paleomagnetism and Tectonics of Middle America and Adjacent Regions I. Geof. Intern., 20: 219—233. Collinson, D.W., 1980. An investigation of the scattered remanent magnetization of the Dunnet Head Sandstone. Geophys. J. R. Astron. Soc., 62: 393—402. Collinson, D.W. and Runcorn, S.K., 1960. Polar wandering and continental drift: Evidence from paleomagnetic observations in the United States. Geol. Soc. Am. Bull., 71: 915—958. Dunlop, D.J., 1972. Magnetic mineralogy of unheated and heated red sediments by coercivity spectrum analysis. Geophys. J. R. Astron. Soc., 27: 37—55. Dunlop, Di. and Stirling, J.M., 1977. ‘Hard’ viscous remanent magnetization (VRM) in fine grain hematite. Geophys. Res. Lett., 4: 163—166. Elston, D.P. and Purucker, M.E., 1979. Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona. J. Geophys. Res., 84: 1653—1665. Fisher, R., 1983. Dispersion on a sphere. Proc. R. Soc. London Ser. A, 217: 295—305. Gose, W.A., Belcher, R.C. and Scott, G.R., 1982. Paleomagnetic results from northeastern Mexico: Evidence for large Mesozoic rotations. Geology, 10: 50—54. Graham, J.W., 1966. Significance of magnetic anisotropy in Appalachian sedimentary rocks. Am. Geophys. Union, Geophys. Monogr., 10: 627—648. Haggerty, S.E., 1976. Oxidation of opaque mineral oxides in basalts. Oxide Min. (Mm. Soc. Am), 3: 1—100. Hamilton, N. and Rees, A.I., 1970. The use of magnetic fabric in paleo current estimation. In: S.K. Runcom (Editor), Palacogeophysics. Academic Press, New York, pp. 445—464.

during early stage of red bed formation in Late Cenozoic sediments, Baja, California. Geol. Soc. Am. Bull., 86: 639— 650. Larson, E.E. and Walker, T.R., 1982. A rock magnetic study of the Lower massive sandstone, Moenkopi Fm. (Triassic), Gray Mountain area, Arizona. J. Geophys. Res., 87: 4819— 4836. Larson, E.E. and Walker, T.R., 1985. Comment on ‘Paleomagnetism of a polarity transition in the Lower Triassic (?) Chugwater Formation, Wyoming,’ by Emilio Herrero-Bervera and Charles E. Helsley. J. Geophys. Res., 90: 2060—2062. Lienert, BR. and Helsley, C.E., 1980. Magnetostratigraphy of the Moenkopi Formation at Bear Ears, Utah. J. Geophys. Res., 85: 1475—1480. Maslanyj, M.P. and Collinson, D.W., 1988. A magnetic and petrological study of some Permian aeolian red sandstones. Geophysical J., 92: 421—430. Nairn, A.E.M., 1976. A paleomagnetic study of certain Mesozoic formations in northern Mexico. Phys. Earth Planet. Inter., 13: 47—56. Ogg, J.G. and Steiner, M.B., 1990. Late Jurassic and Early Cretaceous magnetic polarity time scale. Proc. 2nd Jurassic Stratigraphy Conf., Lisbon, Portugal 1988 (Danish Geol. Surv.) in press. Ouilac, M., 1976. Removal of secondary magnetization from natural remanent magnetization in sedimentary rocks: Alternating field or thermal demagnetization technique? Earth Planet. Sci. Lett., 29: 65—70. Purucker, M.E., Elston, D.P. and Shoemaker, EM., 1980. Early acquisition of characteristic magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountam, Arizona. J. Geophys. Res., 85: 997—1012. Steiner, M.B., 1980. Investigation of the geomagnetic field polarity during the Jurassic. J. Geophys. Res., 85: 3572— 3586. Steiner, M.B., 1990. Early and Middle Jurassic magnetic polarity time scale. Proc. 2nd Jurassic Stratigraphy Conf., Lisbon, Portugal, 1988 (Danish Geol. Surv.) in press.

322 Steiner, M.B., 1983. Detrital remanent magnetization in hematite. J. Geophys. Res., 88: 6523—6539. Steiner, M.B. and Heisley, CE., 1975a. Reversal pattern and apparent polar wander for the Late Jurassic. Geol. Soc. Am. Bull., 86: 1537—1543. Steiner, M.B. and Helsley, C.E., 1975b. Late Jurassic magnetic polarity sequence. Earth Planet. Sci. Lett., 27: 108—112. Turner, P., 1979. The palaeomagnetic evolution of continental red beds. Geol. Mag., 116: 289—301. Turner, P., 1980. Continental Red Beds. Elsevier, Amsterdam, 562 pp. Tauxe, L., Constable, C., Stokking, L. and Badgley, C., 1990. Use of anisotropy to determine the origin of characteristic remanence in the Siwalik red beds of Northern Pakistan. J. Geophys. Res., 95: 4391—4404.

E HERRERO-BERVERA ET AL.

Urrutia-Fucugauchi, J., 1979. Preliminary apparent polar wander path for Mexico. Geophys. J. R. Astron. Soc., 56: 227—235. Urrutia-Fucugauchi, J., 1981. Preliminary results on the effects of heating on the magnetic susceptibility anisotropy of rocks. J. Geomagn. Geoelectr., 33: 411—419. Urrutia-Fucugauchi, J., 1984. On the tectonic evolution of Mexico: Paleomagnetic constraints. In: R. van der Voo, C.R. Scotese and N. Bonhommet (Editors), Plate Reconstruction From Paleozoic Paleomagnetism. Am. Geophys. Union, Geodyn. Ser., 12: 29—47. Walker, T.R., Larson, E.E. and Hoblitt, R.P., 1981. The nature and origin of hematite in the Moenkopi Formation (Tnassic), Colorado Plateau: A contribution to the origin of magnetism in red beds. J. Geophys. Res., 86: 317—333.