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Paleomagnetic stratigraphy of Pliocene Centerville Beach section, northern California
Earth and Planetary Science Letters, 34 (1977) 381-386 O Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
381
I5]
PALEOMAGNETIC STRATIGRAPHY OF PLIOCENE CENTERVILLE BEACH SECTION, NORTHERN C A L I F O R N I A J. ROBERT DODD, JUDSON MEAD Department of Geology, Indiana University, Bloomington, Indiana (USA) and ROBERT J. STANTON, Jr. Department o f Geology, Texas A & M University, College Station, Texas (USA) Received October 4, 1976 Revised version received January 20, 1977 Samples from the upper two-thirds of the approximately 1900 m thick Neogene section exposed south of Centerville Beach on the northern California coast have predominantly reversed detrital remanent magnetism. Fossil evidence suggests a lower Plioeene through lower Pleistocene age for the section. The combined paleomagnetic and fossil data indicate that a large part of the section was deposited during the Matayama reversed epoch (2.4-0.7 million years ago). Samples from correlative sections exposed a few kilometers inland from the Centerville Beach section have a predominantly normal polarity and appear to have been remagnetized. The Centerville Beach section is important because it may serve as a standard with which to compare both other on-land Pliocene sections from western North America and nearby Deep Sea Drilling Project cores.
1. Introduction Although magnetostratigraphy has mainly been used for correlating deep-sea cores [1,2] a number o f onland Neogene sections have also been studied. For example, strata in New Zealand have been correlated with deep-sea sections with a high degree o f certainty [3,4]. Similar studies have also been conducted in Japan [5], Italy [6,7], and the United States [8]. Magnetostratigraphy is potentially more useful for correlation of shallow marine and continental sections exposed on land than it is for correlation o f deep-sea cores. The relative uniformity o f the deep-sea environment makes correlation o f oceanic cores b y standard paleontologic techniques easier than is usually the case with sections o f shallow-water sediments. Paleomagnetic correlation o f on-land sections may in some cases be preferable to correlation b y fossils because, ideally, it is independent o f environmental factors which
strongly influence the distribution o f fossils. However, environmental factors may be important in determining whether the polarity record is preserved or complicated by diagenesis [7]. Two of the greatest difficulties in paleomagnetic correlation of shallow marine and non-marine sections are finding sections with the original polarity record adequately preserved and correlating polarity changes in sections which contain discontinuities in sedimentation. This second problem can be resolved by combining paleontologic data with the incomplete paleomagnetic sequence in order to identify specific polarity changes. For example, if paleontologic data indicate that a section is lower Pliocene in age, the polarity reversals in that section can then be matched with lower Pliocene reversals in the standard magnetostratigraphic section for more precise correlation. The problems of paleontologic correlation and the potentials o f paleomagnetic correlation are well il-
382
lustrated in the Neogene of California. Thick sections of Neogene sedimentary rocks were deposited in several basins and in a broad range of environments [9]. Because of the marked geographical provincial differences and because of the great thicknesses of strata deposited during the relatively short time interval in which lineages might have evolved, biostratigraphic zonation is coarse, although the fossils have' been studied extensively. Thus magnetostratigraphy would appear to be a valuable means of improving correlations within the California Neogene. We have studied the Centerville Beach section because it is an especially good starting point in establishing the magnetostratigraphy of the outcropping Pliocene section and in tying it to that of the adjacent Pacific, where the magnetostratigraphy has been well established from deep-sea cores [2,10].
2. Location and stratigraphy The 1900 m of terrigenous sedimentary rocks which crop out at Centerville Beach (Figs. 1 and 2) were deposited in the Humboldt (or Eel River) Basin, one of several basins of Neogene age on the Pacific Coast. The section has been studied by several previous workers [11-15]. It consists of a series of northwarddipping beds continuously exposed in sea cliffs. The outcrops are nearly continuous and essentially unweathered because the cliffs are being actively eroded by the surf. The rocks, which range from mudstone to coarse sandstone, are predominantly siltstone and very fine sandstone. Recent work by Piper et al. [15] and Ingle [13] indicates that the sediments were deposited in shoaling basin environments grading from a 2000 m deep basin plain during deposition of the Pullen Formation to a shelf less than 100 m deep during deposition of the upper part of the upper Rio Dell Formation. Much of the lower part of the section consists of turbidite deposits. Most earlier workers determined an upper Miocene through Pliocene age for the Pullen, Eel River, and Rio Dell Formations. However, Ingle [13], on the basis of the planktonic foraminifera, has shown that the upper part of the Rio Dell Formation, comprising about 40% of the section, is Pleistocene in age so that the section is generally younger than previously recognized.
LOCATION • MAP
HUMBOLDT BAY
eFortuno
N/
Ferndale
A
B Rio Dell
N l
0 I
5 I
0
I
5
I
10 Miles I I0
1
15Kin
I
Fig. 1. Index map showing the location of the Centerville Beach section and the approximate location of sections at Wildcat Ridge (A), Price Creek (B), and Scotia (C),
3. Method Oriented rock specimens were collected at approximately 25-m stratigraphic intervals through the Centerville Beach section (Fig. 2). A cube approximately 3 cm on a side was cut from each sample and the paleomagnetic field direction measured with a spinner magnetometer with a capability of measuring samples with a minimum intensity of 1 X 10 -6 emu. Four randomly selected samples were subjected to stepwise, tumbler-type demagnetization in an alternating magnetic field of increasing peak intensity to determine the optimum demagnetization field. All other samples were magnetically cleaned in a lO0-Oe field to remove viscous magnetization. 4. Results The intensity and orientation of magnetization of the samples subjected to stepwise demagnetization
383 120
I AO
I
I
i
200
300
400
I00
Centerville Beech
8O
~ 6o 40 2O
LEGEND
O
100
Peak Alternating F i e l d (Oe)
Hookston Fm.
Fig. 3. Percent of the natural remanent magnetization (NRM) remaining in four samples from Centerville Beach after demagnetization in an alternating magnetic field of increasing peak intensity. The sample numbers refer to sites shown in Fig. 2.
Rio Dell Fm. lupper, middle, and lower member)
Eel River Fm.
Pullen Fro.
False Cape shear zone Formation contact 20 .>-Bedding attitude
Trdm
Sample site
O I
I
2000 [
I
I
1
I
Feet
I
I
15--i 10--
Trdl
,5~_~ j~34
Ter
1OOO M e t e r s i
Tp
fc
~ .
Fig. 2. Geologic map of the Centerville Beach, California area showing location of samples collected for this study. The geology is taken from Ogle [ I 1].
did not change markedly with increasing peak fields suggesting that the magnetization is quite stable (Fig. 3); however, the scatter of the polarity direction of reversed samples is reduced somewhat by the demagnetization process. The intensity of magnetization of many of the samples was less than 1 × 10 - 6 emu and could not be measured. As a result, spacing of usable samples was greater than the 25-m spacing of collected samples. In fact, all of the samples from the Pullen Formation and the lower part of the Eel River Formation were too weakly magnetized for paleomag. netic measurement. Three reversed and three normal zones are present in the Centerville Beach section (Fig. 4) with the number of reversed samples exceeding the number of normal samples by a factor of more than two to one. The stratigraphic position of magnetic reversals is accurate within 100 m, the maximum thickness between bounding samples. Spacing between samples is greater in normal intervals than reversed intervals, so the possibility of undetected short reversed intervals within normal intervals isgreater than for undetected short normal intervals within the reversed intervals. Correlation with the standard magnetostratigraphic time scale of the Centerville Beach section requires additional, paleontologic data because the section is not continuous to the present. The upper portion of the section with samples that could be measured, is
384 Sample NO,
DegreesIn¢linotion
_
9C~
60"
30"
30 ~
60" E
Intensity emu X Io -6
+ 90" ~
0 J
8243 41-4059 ----
3O-28--
- t200 -
Middqe
R
~2
~N~
* - 900
24-21-2O--
8-7--
GAuss ~
- 150G
Upper
36-34--
151613--
d o.
• 1800
48~:~0 _--
~
....
-
- 600
:x \ \ \ x
EEL ?
RWER
- 300
~
R
<
~N~ R BRUNHES~.~"
PULLEN
I 500
i
L
L
Thickness
Fig. 4. Inclination of the magnetic vector and magnetic intensity of samples from the Centerville Beach section. Sample numbers correspond to sites shown in Fig. 2. The formation names are shown with the intensity curve. The interpreted polarity epochs are printed vertically next to the polarity column. Polarity events are shown with the inclination curve. The asterisk beside the thickness line marks the PliocenePleistocene boundary as recognized by Ingle [ 13 ].
upper Pliocene to lower Pleistocene [13], the interval of geologic time including the Matayama reversed epoch in the standard polarity section. As samples at Centerville Beach are predominantly reversed, we suggest that these rocks were largely deposited during the Matayama reversed epoch. The normal polarity interval in the middle Rio Dell Formation apparently represents the Olduvai event. This correlation agrees very well with the location of the Pliocene-Pleistocene boundary on the basis of foraminifera as determined by Ingle [13] at about 1000 m (Fig. 4) because the base of the Pleistocene is generally defined as occurring at the base of the Olduvai event [16]. The upper normal interval in the Centerville Beach section is in the correct stratigraphic position to correspond to the Jaramillo event, and the lower normal interval correlates with the upper portion of the Gauss normal epoch. A plot of the thickness of the section against the age of the interpreted polarity changes (Fig. 5) suggests a nearly constant sedimentation rate. This would seem to support our interpretation of the magnetostratigraphy. The relatively high sedimentation rate of 70 cm per year is comparable to that found in similar Neogene sections in New Zealand [3] and accords well with the inferred turbidite origin of much of the section [15].
~
i 1000
J
,
1
1500
(m)
Fig. 5. Diagram showing sedimentation rate for the Centerville Beach section. The paleomagnetic polarity boundaries from Fig. 4 are plotted against the ages of the interpreted polarity changes taken from Opdyke [21. The slope of the line indicates a sedimentation rate of about 70 cm per 1000 years.
Based on earlier paleontologic interpretations which placed the lower Pullen Formation in the upper Miocene and the remainder of the section entirely in the Pliocene [11,18], we had previously concluded that the sediments of the Centerville Beach section had been deposited largely during the Gilbert reversed epoch [17]. The subsequent change in our interpretation, necessitated by Ingle's [13] recent paleontologic work, emphasizes the difficulty in matching reversals in an unknown section with those in the standard section, and the importance of combining magnetostratigraphic and biostratigraphic data.
5. Magnetostratigraphy of other sections in the Humboldt Basin Sections exposed a few kilometers inland from Centerville Beach on Wildcat Ridge, along Price Creek, and along the Eel River at Scotia and Rio Dell (Fig. 1) were also studied. These sections, which are up to 2400 m thick, include additional lower and middle Pleistocene strata above the Rio Dell Formation. At least the upper parts of these sections were probably deposited in a somewhat shallower environment than the strata of the beach section [11 ]. Although the sections at Wildcat Ridge, Price Creek, and Scotia are correlative with that at Centerville
385 N
•
•
0 0 0 0
•@
0
•
0
0
0 °
000
0
0
+
\
°
Before correction for structure
/
Fig. 6. Equal area stereographic projection of the orientation of magnetic polarity of samples from the upper part of the Scotia section. The open circles show the orientation of the samples after a correction has been made to indicate what the orientation would be if the strata were horizontal (i.e., undeformed). All samples plot in the lower hemisphere.
Beach, the polarity of samples from these sections is predominantly normal. These samples probably have been remagnetized or possibly the original remanent magnetization, although still present, is obscured by later viscous magnetic overprint which we have not been able to remove. In order to determine if the samples have been diagenetically altered, we compared the measured sample polarity of the steeply dipping strata before and after correction was made for the post-depositional deformation (Fig. 6). If the rock had been diagenetically remagnetized after deformation, the uncorrected magnetic polarity should correspond to the magnetic field which existed at the time of alteration. That field should have had approximately the same orientation as the modern field or the reverse of the modern field. The results strongly suggest post-deformation alteration. The apparent magnetic alteration of the samples from the inland sections is difficult to explain. The samples from the Scotia section were taken along rapidly eroding river banks and appear as fresh as those from Centerville Beach. Erosion at Price Creek
is probably not quite so rapid, but outcrops and samples appear to be fresh. The samples from Wildcat Ridge were from roadcuts, at positions up to 15 ft below the original land surface, well below the depth of any normal surface sampling method. We have also determined the polarity of Pliocene samples from the Coalinga region of central California and they also seem to be magnetically altered. They mostly have normal polarity, show a rapid drop in magnetic intensity with increased demagnetization field intensity compared to the slight drop for the Centerville Beach samples, and show an orientation parallel to the modern field even when collected from dipping beds. Our results contrast with reports in the literature of other sections from similar settings which appear to contain the original paleomagnetic orientation [3,4,8]. Subtle chemical alteration of the paleomagnetic orientation of rocks from both deep-sea cores and sections exposed on land has been described [7,8,19]. Alteration may result from post-depositional changes in Eh and pH, resulting in formation of new magnetic minerals oriented in the magnetic field existing at the time of the alteration [7]. Johnson et al. [19] and l_arson and Walker [20] suggest post-depositional low-temperature oxidation of the magnetic mineral may also result in loss of detrital remanent magnetization. Detailed mineralogic and petrologic study of the apparent ly altered samples from the inland sections as compared to the Centerville Beach section might provide information about the diagenetic process. In any event, the Centerville Beach section should make an excellent standard with which to compare future resuits from on-land marine Pliocene sections in the Pacific Coast area.
6. Conclusions The remanent magnetic stratigraphy of the Pliocene section exposed along Centerville Beach is preserved and can be correlated with the standard magnetostratigraphic time scale by concurrent use of biostratigraphic data. This section provides a reference section for comparison with the existing biostratigraphic framework for on-land sections in the California Pliocene and with the sequence established in nearby Deep Sea Drilling Project cores. Hopefully, in the future the magnetostratigraphy can be deter-
386 mined for other on-land sections in the California Neogene improving our understanding o f the time relationships b e t w e e n these sections.
Acknowledgements The authors wish to thank Edward L. Crisp, Diane Suchomel, and Steven R. Y o u n g for helping in the preparation and running o f samples, Charles Helsley for his suggestions and Ken K o d a m a for c o m m e n t i n g on the manuscript. This w o r k was supported by N S F research grants G A - 3 0 8 7 8 ( D o d d ) and G A - 3 0 7 4 6 (Stanton).
References 1 A.V. Cox, R.R. Doell and G.B. Dalrymple, Reversal of the earth's magnetic field, Science 144 (1964) 1537. 2 N.D. Opdyke, Paleomagnetism of deep sea cores, Rev. Geophys. Space Phys. 10 (1972) 213. 3 J.P. Kennett and N.D. Watkins, Late Miocene-early Pliocene paleomagnetic stratigraphy, paleoclimatology, and biostratigraphy in New Zealand, Geol. Soc. Am. Bull. 85 (1974) 1385. 4 B.R. Lienert, D.A. Christoffel and P. Vella, Geomagnetic dates on a New Zealand Upper Miocene-Pliocene section, Earth Planet. Sci. Lett. 16 (1972) 195. 5 H. Nakagawa, N. Niitsuma and I. Hayasaka, Late Cenozoic geomagnetic chronology of the Boso Peninsula, Japan J. Geol. Soc. 75 (1969) 267. 6 H. Nakagawa, N. Niitsuma and C. Elmi, Pliocene and Pleistocene magnetic stratigraphy in Le Castella area, southern Italy - a preliminary report, Quat. Res. 1 (1971) 360. 7 N.D. Watkins, D.R. Kester and J.P. Kennett, Paleomagnetism of the type Pliocene/Pleistocene boundary section at Santa Maria de Catenzaro, Italy, and the problem of post-depositional precipitation of magnetic minerals, Earth Planet. Sci. Lett. 24 (1974) 113.
8 N.M. Johnson, N.D. Opdyke and E.H. Lindsay, Magnetic polarity stratigraphy of Pliocene-Pleistocene terrestrial deposits and vertebrate faunas, San Pedro Valley, Arizona, Geol. Soc. Am. Bull. 86 (1975) 5. 9 R.D. Reed, Geology of California (American Association of Petroleum Geologists, Tulsa, Okla., 1933) 355 pp. 10 A.V. Cox, Geomagnetic reversals, Science 163 (1969) 237. 11 B.A. Ogle, Geology of the Eel River Valley area, Humboldt County, California, Calif. Div. Mines Bull. 164 (1953) 128 pp. 12 W.F. Faustman, Paleontology of the Wildcat Group at Scotia and Centerville Beach, California, Calif. Univ. Publ. Geol. Sci. 41 (1964) 97. 13 J.C. Ingle Jr., Late Neogene paleobathymetry and paleoenvironments of the Humboldt Basin, northern California, in: The Neogene Symposium, A.E. Fritsche, H. TerBest, Jr. and W.W. Wornardt, eds., Soc. Econ. Paleontol. Mineral., Pac. Sec. (1976) 53. 14 J.C. Ingle, Jr., Pliocene planktonic foraminifera from northern California and paleo-oceanographic implications, Geol. Soc. Am. Spec. Paper 121 (1969) 130. 15 D.J.W. Piper, W.R. Normark and J.C. Ingle, Jr., The Rio Dell Formation: a Plio-Pleistocene basin slope deposit in northern California, Sedimentology 23 (1976) 309. 16 W.A. Berggren and J. van Couvering, The late Neogene: biostratigraphy, biochronology, and paleoclimatology of the last 15 million years in marine and continental sediments, Palaeogeogr. Palaeoecol., Palaeoclimatol. 16 (1974) 1. 17 J.R. Dodd, J. Mead and R.J. Stanton, Jr., Paleomagnetic stratigraphy of Pliocene Centerville Beach section, Geol. Soc. Am. Ahstr. Progr. 7 (1975) 1054. 18 J.C. Ingle, Jr., Summary contents on Neogene biostratigraphy, physical stratigraphy, and paleooceanography in the marginal northeastern Pacific Ocean, in: Initial Reports DSDP 18 (1973) 949. 19 H.P. Johnson, H. Kinoshita and R.T. Merrill, Rock magnetism and paleomagnetism of some North Pacific deepsea sediments, Geol. Soc. Am. Bull. 86 (1974) 412. 20 E.E. Larson and T.R. Walker, Development of chemical remanent magnetization during early stages of red-bed formation in late Cenozoic sediments, Baja California, Geol. Soc. Am. Bull. 86 (1975) 639.
381
I5]
PALEOMAGNETIC STRATIGRAPHY OF PLIOCENE CENTERVILLE BEACH SECTION, NORTHERN C A L I F O R N I A J. ROBERT DODD, JUDSON MEAD Department of Geology, Indiana University, Bloomington, Indiana (USA) and ROBERT J. STANTON, Jr. Department o f Geology, Texas A & M University, College Station, Texas (USA) Received October 4, 1976 Revised version received January 20, 1977 Samples from the upper two-thirds of the approximately 1900 m thick Neogene section exposed south of Centerville Beach on the northern California coast have predominantly reversed detrital remanent magnetism. Fossil evidence suggests a lower Plioeene through lower Pleistocene age for the section. The combined paleomagnetic and fossil data indicate that a large part of the section was deposited during the Matayama reversed epoch (2.4-0.7 million years ago). Samples from correlative sections exposed a few kilometers inland from the Centerville Beach section have a predominantly normal polarity and appear to have been remagnetized. The Centerville Beach section is important because it may serve as a standard with which to compare both other on-land Pliocene sections from western North America and nearby Deep Sea Drilling Project cores.
1. Introduction Although magnetostratigraphy has mainly been used for correlating deep-sea cores [1,2] a number o f onland Neogene sections have also been studied. For example, strata in New Zealand have been correlated with deep-sea sections with a high degree o f certainty [3,4]. Similar studies have also been conducted in Japan [5], Italy [6,7], and the United States [8]. Magnetostratigraphy is potentially more useful for correlation of shallow marine and continental sections exposed on land than it is for correlation o f deep-sea cores. The relative uniformity o f the deep-sea environment makes correlation o f oceanic cores b y standard paleontologic techniques easier than is usually the case with sections o f shallow-water sediments. Paleomagnetic correlation o f on-land sections may in some cases be preferable to correlation b y fossils because, ideally, it is independent o f environmental factors which
strongly influence the distribution o f fossils. However, environmental factors may be important in determining whether the polarity record is preserved or complicated by diagenesis [7]. Two of the greatest difficulties in paleomagnetic correlation of shallow marine and non-marine sections are finding sections with the original polarity record adequately preserved and correlating polarity changes in sections which contain discontinuities in sedimentation. This second problem can be resolved by combining paleontologic data with the incomplete paleomagnetic sequence in order to identify specific polarity changes. For example, if paleontologic data indicate that a section is lower Pliocene in age, the polarity reversals in that section can then be matched with lower Pliocene reversals in the standard magnetostratigraphic section for more precise correlation. The problems of paleontologic correlation and the potentials o f paleomagnetic correlation are well il-
382
lustrated in the Neogene of California. Thick sections of Neogene sedimentary rocks were deposited in several basins and in a broad range of environments [9]. Because of the marked geographical provincial differences and because of the great thicknesses of strata deposited during the relatively short time interval in which lineages might have evolved, biostratigraphic zonation is coarse, although the fossils have' been studied extensively. Thus magnetostratigraphy would appear to be a valuable means of improving correlations within the California Neogene. We have studied the Centerville Beach section because it is an especially good starting point in establishing the magnetostratigraphy of the outcropping Pliocene section and in tying it to that of the adjacent Pacific, where the magnetostratigraphy has been well established from deep-sea cores [2,10].
2. Location and stratigraphy The 1900 m of terrigenous sedimentary rocks which crop out at Centerville Beach (Figs. 1 and 2) were deposited in the Humboldt (or Eel River) Basin, one of several basins of Neogene age on the Pacific Coast. The section has been studied by several previous workers [11-15]. It consists of a series of northwarddipping beds continuously exposed in sea cliffs. The outcrops are nearly continuous and essentially unweathered because the cliffs are being actively eroded by the surf. The rocks, which range from mudstone to coarse sandstone, are predominantly siltstone and very fine sandstone. Recent work by Piper et al. [15] and Ingle [13] indicates that the sediments were deposited in shoaling basin environments grading from a 2000 m deep basin plain during deposition of the Pullen Formation to a shelf less than 100 m deep during deposition of the upper part of the upper Rio Dell Formation. Much of the lower part of the section consists of turbidite deposits. Most earlier workers determined an upper Miocene through Pliocene age for the Pullen, Eel River, and Rio Dell Formations. However, Ingle [13], on the basis of the planktonic foraminifera, has shown that the upper part of the Rio Dell Formation, comprising about 40% of the section, is Pleistocene in age so that the section is generally younger than previously recognized.
LOCATION • MAP
HUMBOLDT BAY
eFortuno
N/
Ferndale
A
B Rio Dell
N l
0 I
5 I
0
I
5
I
10 Miles I I0
1
15Kin
I
Fig. 1. Index map showing the location of the Centerville Beach section and the approximate location of sections at Wildcat Ridge (A), Price Creek (B), and Scotia (C),
3. Method Oriented rock specimens were collected at approximately 25-m stratigraphic intervals through the Centerville Beach section (Fig. 2). A cube approximately 3 cm on a side was cut from each sample and the paleomagnetic field direction measured with a spinner magnetometer with a capability of measuring samples with a minimum intensity of 1 X 10 -6 emu. Four randomly selected samples were subjected to stepwise, tumbler-type demagnetization in an alternating magnetic field of increasing peak intensity to determine the optimum demagnetization field. All other samples were magnetically cleaned in a lO0-Oe field to remove viscous magnetization. 4. Results The intensity and orientation of magnetization of the samples subjected to stepwise demagnetization
383 120
I AO
I
I
i
200
300
400
I00
Centerville Beech
8O
~ 6o 40 2O
LEGEND
O
100
Peak Alternating F i e l d (Oe)
Hookston Fm.
Fig. 3. Percent of the natural remanent magnetization (NRM) remaining in four samples from Centerville Beach after demagnetization in an alternating magnetic field of increasing peak intensity. The sample numbers refer to sites shown in Fig. 2.
Rio Dell Fm. lupper, middle, and lower member)
Eel River Fm.
Pullen Fro.
False Cape shear zone Formation contact 20 .>-Bedding attitude
Trdm
Sample site
O I
I
2000 [
I
I
1
I
Feet
I
I
15--i 10--
Trdl
,5~_~ j~34
Ter
1OOO M e t e r s i
Tp
fc
~ .
Fig. 2. Geologic map of the Centerville Beach, California area showing location of samples collected for this study. The geology is taken from Ogle [ I 1].
did not change markedly with increasing peak fields suggesting that the magnetization is quite stable (Fig. 3); however, the scatter of the polarity direction of reversed samples is reduced somewhat by the demagnetization process. The intensity of magnetization of many of the samples was less than 1 × 10 - 6 emu and could not be measured. As a result, spacing of usable samples was greater than the 25-m spacing of collected samples. In fact, all of the samples from the Pullen Formation and the lower part of the Eel River Formation were too weakly magnetized for paleomag. netic measurement. Three reversed and three normal zones are present in the Centerville Beach section (Fig. 4) with the number of reversed samples exceeding the number of normal samples by a factor of more than two to one. The stratigraphic position of magnetic reversals is accurate within 100 m, the maximum thickness between bounding samples. Spacing between samples is greater in normal intervals than reversed intervals, so the possibility of undetected short reversed intervals within normal intervals isgreater than for undetected short normal intervals within the reversed intervals. Correlation with the standard magnetostratigraphic time scale of the Centerville Beach section requires additional, paleontologic data because the section is not continuous to the present. The upper portion of the section with samples that could be measured, is
384 Sample NO,
DegreesIn¢linotion
_
9C~
60"
30"
30 ~
60" E
Intensity emu X Io -6
+ 90" ~
0 J
8243 41-4059 ----
3O-28--
- t200 -
Middqe
R
~2
~N~
* - 900
24-21-2O--
8-7--
GAuss ~
- 150G
Upper
36-34--
151613--
d o.
• 1800
48~:~0 _--
~
....
-
- 600
:x \ \ \ x
EEL ?
RWER
- 300
~
R
<
~N~ R BRUNHES~.~"
PULLEN
I 500
i
L
L
Thickness
Fig. 4. Inclination of the magnetic vector and magnetic intensity of samples from the Centerville Beach section. Sample numbers correspond to sites shown in Fig. 2. The formation names are shown with the intensity curve. The interpreted polarity epochs are printed vertically next to the polarity column. Polarity events are shown with the inclination curve. The asterisk beside the thickness line marks the PliocenePleistocene boundary as recognized by Ingle [ 13 ].
upper Pliocene to lower Pleistocene [13], the interval of geologic time including the Matayama reversed epoch in the standard polarity section. As samples at Centerville Beach are predominantly reversed, we suggest that these rocks were largely deposited during the Matayama reversed epoch. The normal polarity interval in the middle Rio Dell Formation apparently represents the Olduvai event. This correlation agrees very well with the location of the Pliocene-Pleistocene boundary on the basis of foraminifera as determined by Ingle [13] at about 1000 m (Fig. 4) because the base of the Pleistocene is generally defined as occurring at the base of the Olduvai event [16]. The upper normal interval in the Centerville Beach section is in the correct stratigraphic position to correspond to the Jaramillo event, and the lower normal interval correlates with the upper portion of the Gauss normal epoch. A plot of the thickness of the section against the age of the interpreted polarity changes (Fig. 5) suggests a nearly constant sedimentation rate. This would seem to support our interpretation of the magnetostratigraphy. The relatively high sedimentation rate of 70 cm per year is comparable to that found in similar Neogene sections in New Zealand [3] and accords well with the inferred turbidite origin of much of the section [15].
~
i 1000
J
,
1
1500
(m)
Fig. 5. Diagram showing sedimentation rate for the Centerville Beach section. The paleomagnetic polarity boundaries from Fig. 4 are plotted against the ages of the interpreted polarity changes taken from Opdyke [21. The slope of the line indicates a sedimentation rate of about 70 cm per 1000 years.
Based on earlier paleontologic interpretations which placed the lower Pullen Formation in the upper Miocene and the remainder of the section entirely in the Pliocene [11,18], we had previously concluded that the sediments of the Centerville Beach section had been deposited largely during the Gilbert reversed epoch [17]. The subsequent change in our interpretation, necessitated by Ingle's [13] recent paleontologic work, emphasizes the difficulty in matching reversals in an unknown section with those in the standard section, and the importance of combining magnetostratigraphic and biostratigraphic data.
5. Magnetostratigraphy of other sections in the Humboldt Basin Sections exposed a few kilometers inland from Centerville Beach on Wildcat Ridge, along Price Creek, and along the Eel River at Scotia and Rio Dell (Fig. 1) were also studied. These sections, which are up to 2400 m thick, include additional lower and middle Pleistocene strata above the Rio Dell Formation. At least the upper parts of these sections were probably deposited in a somewhat shallower environment than the strata of the beach section [11 ]. Although the sections at Wildcat Ridge, Price Creek, and Scotia are correlative with that at Centerville
385 N
•
•
0 0 0 0
•@
0
•
0
0
0 °
000
0
0
+
\
°
Before correction for structure
/
Fig. 6. Equal area stereographic projection of the orientation of magnetic polarity of samples from the upper part of the Scotia section. The open circles show the orientation of the samples after a correction has been made to indicate what the orientation would be if the strata were horizontal (i.e., undeformed). All samples plot in the lower hemisphere.
Beach, the polarity of samples from these sections is predominantly normal. These samples probably have been remagnetized or possibly the original remanent magnetization, although still present, is obscured by later viscous magnetic overprint which we have not been able to remove. In order to determine if the samples have been diagenetically altered, we compared the measured sample polarity of the steeply dipping strata before and after correction was made for the post-depositional deformation (Fig. 6). If the rock had been diagenetically remagnetized after deformation, the uncorrected magnetic polarity should correspond to the magnetic field which existed at the time of alteration. That field should have had approximately the same orientation as the modern field or the reverse of the modern field. The results strongly suggest post-deformation alteration. The apparent magnetic alteration of the samples from the inland sections is difficult to explain. The samples from the Scotia section were taken along rapidly eroding river banks and appear as fresh as those from Centerville Beach. Erosion at Price Creek
is probably not quite so rapid, but outcrops and samples appear to be fresh. The samples from Wildcat Ridge were from roadcuts, at positions up to 15 ft below the original land surface, well below the depth of any normal surface sampling method. We have also determined the polarity of Pliocene samples from the Coalinga region of central California and they also seem to be magnetically altered. They mostly have normal polarity, show a rapid drop in magnetic intensity with increased demagnetization field intensity compared to the slight drop for the Centerville Beach samples, and show an orientation parallel to the modern field even when collected from dipping beds. Our results contrast with reports in the literature of other sections from similar settings which appear to contain the original paleomagnetic orientation [3,4,8]. Subtle chemical alteration of the paleomagnetic orientation of rocks from both deep-sea cores and sections exposed on land has been described [7,8,19]. Alteration may result from post-depositional changes in Eh and pH, resulting in formation of new magnetic minerals oriented in the magnetic field existing at the time of the alteration [7]. Johnson et al. [19] and l_arson and Walker [20] suggest post-depositional low-temperature oxidation of the magnetic mineral may also result in loss of detrital remanent magnetization. Detailed mineralogic and petrologic study of the apparent ly altered samples from the inland sections as compared to the Centerville Beach section might provide information about the diagenetic process. In any event, the Centerville Beach section should make an excellent standard with which to compare future resuits from on-land marine Pliocene sections in the Pacific Coast area.
6. Conclusions The remanent magnetic stratigraphy of the Pliocene section exposed along Centerville Beach is preserved and can be correlated with the standard magnetostratigraphic time scale by concurrent use of biostratigraphic data. This section provides a reference section for comparison with the existing biostratigraphic framework for on-land sections in the California Pliocene and with the sequence established in nearby Deep Sea Drilling Project cores. Hopefully, in the future the magnetostratigraphy can be deter-
386 mined for other on-land sections in the California Neogene improving our understanding o f the time relationships b e t w e e n these sections.
Acknowledgements The authors wish to thank Edward L. Crisp, Diane Suchomel, and Steven R. Y o u n g for helping in the preparation and running o f samples, Charles Helsley for his suggestions and Ken K o d a m a for c o m m e n t i n g on the manuscript. This w o r k was supported by N S F research grants G A - 3 0 8 7 8 ( D o d d ) and G A - 3 0 7 4 6 (Stanton).
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