Depositional Remanent Magnetization Research Articles

Timing of remanent magnetization acquisition in red beds: A case study from a syn-folding sedimentary basin

Red beds have been recognized as a useful source of ancient geomagnetic field. However, the timing of remanence acquisition in red beds is still incompletely understood. Fold tests may be used for an estimation of the timing of red bed remanence acquisition, but an essential ability of this kind of test is to conclude whether magnetizations are acquired before or after folding. Even a positive fold test does not necessarily represent a primary nature of magnetization. Because the relationship between the timing of fold growth and sedimentation is well understood for a small, syn-folding sedimentary basin, we select such a sedimentary basin as a focus of study. A case study example examining the timing of red bed remanence acquisition based on fold tests is described in this paper. It shows that the observed remanence directions carried by hematite are given a syn-folding nature. This syn-folding nature is more observable in the lower part of the sedimentary sequence that was under the more intense folding during its sedimentation than the upper part. A pure depositional remanent magnetization when beds were horizontal is not applicable to this case, but a post-depositional remanent magnetization, with possible existence of an initial slope of the deposition surface, is considered to have been imparted prior to complete lithification. Microscopic observations showing the presence of hematite as sedimentary particles can be used for an independent estimation supporting the detrital origin. Chemical compositions of the hematite grains agree well with the maximum laboratory unblocking temperatures of the samples, and we draw the conclusion that these grains are the carrier of the detrital magnetization.

Magnetic Anisotropy as an Aid to Identifying CRM and DRM in Red Sedimentary Rocks

To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal. In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock’s remanence may have been affected by strain and could have originated as a DRM or a CRM. Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.

Does lithology influence relative paleointensity records? a statistical analysis on South Atlantic pelagic sediments

The relative paleointensity (RPI) method assumes that the intensity of post depositional remanent magnetization (PDRM) depends exclusively on the magnetic field strength and the concentration of the magnetic carriers. Sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces. Just how strong these mechanical and electrostatic forces are, depends on many petrophysical factors related to mineralogy, particle size and shape of the matrix constituents. We therefore test the hypothesis that variations in sediment lithology modulate RPI records. For 90 selected Late Quaternary sediment samples from the subtropical and subantarctic South Atlantic Ocean a combined paleomagnetic and sedimentological dataset was established. Misleading alterations of the magnetic mineral fraction were detected by a routine Fe/κ test (Funk, J., von Dobeneck, T., Reitz, A., 2004. Integrated rock magnetic and geochemical quantification of redoxomorphic iron mineral diagenesis in Late Quaternary sediments from the Equatorial Atlantic. In: Wefer, G., Mulitza, S., Ratmeyer, V. (Eds.), The South Atlantic in the Late Quaternary: reconstruction of material budgets and current systems. Springer-Verlag, Berlin/Heidelberg/New York/Tokyo, pp. 239–262). Samples with any indication of suboxic magnetite dissolution were excluded from the dataset. The parameters under study include carbonate, opal and terrigenous content, grain size distribution and clay mineral composition. Their bi- and multivariate correlations with the RPI signal were statistically investigated using standard techniques and criteria. While several of the parameters did not yield significant results, clay grain size and chlorite correlate weakly and opal, illite and kaolinite correlate moderately to the NRM/ARM signal used here as a RPI measure. The most influential single sedimentological factor is the kaolinite/illite ratio with a Pearson's coefficient of 0.51 and 99.9% significance. A three-member regression model suggests that matrix effects can make up over 50% of the observed RPI dynamics.

Magnetostratigraphic Study of the Continental Sedimentary Sequence of the Chiang Muan Basin, Northern Thailand: Implications for the Age of the First Miocene Hominoids from Thailand

We report results of a magnetostratigraphic study of the Chiang Muan Basin sequence (northern Thailand) that contains the recently discovered Middle Miocene hominoid, cf. Lufengpithecus chiangmuanensis (Chaimanee et al., 2003). Rock magnetic investigations indicate the presence of both high- and low-coercivity minerals. Specimens subjected to progressive thermal demagnetization procedures show that nearly all of them exhibit a low-temperature magnetization component, and a high-temperature one, characterized by either normal or reverse polarity, considered as the characteristic Miocene magnetization. The two directions (D = 9.8° I = 32.8°, n = 15, and D = 175.9°, I = -37.5, n = 5) are not exactly antipodal, but pass the analytical reversal test with "C" quality classifications. The mean direction is close to the expected direction derived from the apparent polar wander (APW) path of Eurasia for the Middle Miocene. The inclination value is lower than expected at the site latitude, consistent with a sedimentary inclination error for the original depositional remanent magnetization. The section studied was tentatively correlated with the geomagnetic polarity time scale, using biostratigraphic data previously proposed for the fauna found in the same basin. These correlations have been achieved by comparing variations in the sedimentation rate derived from alternative correlations. The proposed correlations suggest an age between 13.5 and 10 Ma for the new hominoid fossil cf. Lufengpithecus chiangmuanensis.

Fidelity and time resolution of the magnetostratigraphic record in Siwalik sediments: high-resolution study of a complete polarity transition and evidence for cryptochrons in a Miocene fluviatile section

SUMMARY The Surai Khola section in southwest Nepal, a 5000 m continuously exposed record of fluvial sedimentation since Middle Miocene, was revisited for high-resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field. Polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions. Polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column. Almost antiparallel palaeoremanence directions, residing in diVerent haematite phases in the same specimens, could generally not be separated properly by thermal demagnetization. DiVering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions, but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization. Accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization (DRM) with considerable inclination errors and scattered directions for individual specimens.

Remanence acquisition and its alteration on sediments

Previous palaeomagnetic work shows that the mechanism of remanence acquisition on sediments is very complicated. Different magnetizing processes under different conditions may produce some anomalous magnetic signals which have nothing to do with the changes of the geomagnetic field and are difficult to explain. The depositional remanent magnetization (DRM) may produce an inclination error and a bedding error, while postdepositional remanent magnetization (PDRM) may result in a time delay between the magnetic and sedimentary ages. When there are some different magnetic phases with different lock-in depths in the sediments, it is difficult to judge the depth of the reversal or excursion, or even the artificial magnetic signal may be produced. Even if there is only one magnetic phase in the sediment, the recorded magnetic signals may also be different if the lock-in depth is changed comparative to the lasting period of one reversal or excursion. Demagnetization may also change the primary magnetic records and the changes of water content may result in the displacement of magnetic boundary. The so-called “unstable core”, representing subtle changes in mineralogy into and out of a self-reversal region or a viscous magnetization region, gives a complicated palaeornagnetic record with many apparent changes in the palaeomagnetic direction. Moreover, some physical disturbances, such as biodisturbance, deformation, drilling processes and drying effects, may result in the remanence alteration. Some palaeomagnetic investigations demonstrate that good magnetic signals are invariably achieved on homogenous mud or clay sediments, but the worse signals on inhomogeneous profiles, especially on sandy layers.

Facies and early diagenetic influence on the depositional magnetization of carbonates

Research Article| September 01, 1997 Facies and early diagenetic influence on the depositional magnetization of carbonates Donald F. McNeill Donald F. McNeill 1Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Donald F. McNeill 1Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (9): 799–802. https://doi.org/10.1130/0091-7613(1997)025<0799:FAEDIO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Donald F. McNeill; Facies and early diagenetic influence on the depositional magnetization of carbonates. Geology 1997;; 25 (9): 799–802. doi: https://doi.org/10.1130/0091-7613(1997)025<0799:FAEDIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rock-magnetic data from a core (Clino) on the margin of Great Bahama Bank show that both depositional facies and early diagenesis contribute to preservation of depositional remanent magnetization. This primary magnetization, carried by single-domain magnetite, and resultant directional data recorded in carbonates are influenced by diagenetic alteration that is largely controlled by facies and to a lesser degree relative sea level. Although polarity does not change, there is usually inclination shallowing relative to the axial dipole value, and the inclination values are more dispersed as a result of cementation. Four magneto-diagenetic zones were identified in Clino: from top down, these are (I) a low-Mg calcite, a cemented, shallow-water facies with dispersed inclinations, relatively high remanence coercivity, and magnetostatic interaction; (II) an upper-slope facies, moderately cemented, that has original aragonite mineralogy, produced more statistically consistent inclination angles, lower magnetostatic interactions and coercivity, and higher remanence intensity; (III) a cemented, lower-slope facies having characteristics of both the overlying zones, but containing some dolomite and abundant fractures; and (IV) a partially dolomitized, basal lower-slope interval with magnetic characteristics (dispersed inclination, weaker intensity, higher coercivity, higher magnetostatic interaction) similar to the shallow-water facies. The relatively uncomplicated diagenetic history, excellent age control, and depositional facies provide a spectrum of magnetization destructive processes and characterization of polarity data from platform carbonates. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-Unblocking-Temperature Haematite Magnetizations of Late Palaeozoic Red Beds From the Okcheon Zone, Southern Part of the Korean Peninsula

The Late Carboniferous and Permo-Triassic red beds of the Pyeongan Supergroup in the Okcheon zone, southern part of the Korean Peninsula, were found to contain three components of remanence. These components, which are resolved by thermal demagnetization, are characterized by high (Hhc for Late Carboniferous and Hhpc for Permo-Triassic), intermediate (Hm) and low (HI) unblocking temperatures. As the maximum unblocking temperature spectra of the Hhc (Hhpt) component often overlapped the Hm spectra above 680° C, 3° to 5° C steps were needed above 670° C to resolve the Hhc-(Hhpt-) component magnetization. The tilt-corrected mean direction of the Hhc component red beds is D=255.5°, 1=-2.4° (α95=20.8°), and that of the Hhpt component is D=271.9°, 1=-33.4° (α95=16.5°). The precisions of mean directions of Hhc and Hhpt components improve after tilt correction and allow the directions to be distinguished from each another. This indicates that both Hhc and Hhpt component magnetizations were acquired during or shortly after the deposition of the red beds. According to micropetrographic observations, the haematite in the red beds can be divided into three types: detrital specularite (A), non-detrital specularite (B), and pigmental haematite (C). Type A grains are tens of micrometres in size, and many of them are martite. The remanent magnetization of type A is possibly depositional remanent magnetization (DRM), post-DRM or chemical remanent magnetization (CRM) acquired close to the deposition time, whereas that of types B and C is of CRM origin acquired during and after folding, respectively. The time of folding is well established as a Triassic to Jurassic event. We believe that the remanent magnetization carried by type A grains is primary, and is correlated to Hhc-(or Hhpt-) component magnetization.

A geomagnetic event recorded in loess deposits of the Tönchesberg (Germany): identification of the Blake magnetic polarity episode

SUMMARY High-resolution palaeomagnetic studies on a collection of nearly 350 specimens. taken from a 1.30m thick section of an Upper Pleistocene loess profile from the Tonchesberg (Germany), are described. The Blake magnetic polarity episode is recorded in a layer above the partly eroded palaeosol of the Eemian interglacial. It is identified by its stratigraphic position and thermoluminescence data (=90.0110.0 kyr: Zoller, Conard & Hahn 1991; Frechen 1991, 1992). Magnetite and haematite contribute to the natural remanent magnetization (NRM). Directional behaviour during thermal and alternating-field cleaning suggests that magnetite carries a primary, (post-)depositional remanent magnetization (pDRM) and haematite a secondary, chemical remanent magnetization (CRM) of early diagenetic origin.

Magnetic recording in nature: the medium, the mechanism and the message

The capacity of rocks to magnetically record Earth history has played a crucial part in transforming the way we look at our planet. It has also provided much stimulus for the study of the medium which carries the record, and the mechanisms by which the record has been implanted. The medium consists of compounds of iron (oxides and sulphides) often with high concentrations of titanium. Oxides with spinel structure, which may be highly nonstoichiometric, are abundant. Particle sizes are often too large to be ideal recording materials but seem to carry the palaeomagnetic record effectively. This has led to a concentration of effort on understanding the magnetic state and the magnetization process in particles with few domains. Several recording mechanisms operate in nature: thermoremanent magnetization may be acquired as hot rocks cool to ambient temperature; chemical remanent magnetization can be acquired as particles grow in size or change intrinsic magnetic properties due to a change in composition; and a sedimentary rock may acquire a depositional remanent magnetization as the magnetic particles settle in the forming sediment. The requirements on the medium and mechanism are somewhat stringent: the message must be accessible after exposure of the recording to an environment which may be physically and chemically hostile for times up to hundreds of millions of years. The success of palaeomagnetism testifies to the robustness of nature's magnetic recordings.

The influence of salinity and flocculation upon the acquisition of remanent magnetization in some artificial sediments

SUMMARY A range of redeposition experiments with both magnetite and maghemite grains embedded in kaolinite or quartz has revealed a significant, non-linear relationship between salinity and the intensity and magnitude of the inclination error of the depositional remanent magnetization (DRM). Increasing DRM intensities and inclination errors were observed for salinities between 0–1 g/l. A dramatic reduction in intensity as well as in inclination error, appears for salinities above ca 2 g/l. Although there is a negligible effect on intensity of salinity for concentrations above 3 g/l, a significant inclination error persists over this range of salinities, approaching less than 5° at concentrations larger than 30 g/l. The observed relationships are suggested to reflect mechanisms related to flocculation. The results are probably not generally applicable to natural sediments, since floc formation in nature usually involves organic compounds.

Effects of clay mineralogy and the electrical conductivity of water on the acquisition of depositional remanent magnetization in sediments

New experiments on the interrelationship of clay minerals, solution properties, and postdepositional remanent magnetization (pDRM) provide interesting insights into pDRM acquisition. We have found that clay mineral content, composition, and solution conductivity affect pDRM strongly. With increasing clay concentration or electrolyte concentration, pDRM intensity decreases. Clay minerals may also be responsible for the inclination shallowing of pDRM. Different clay minerals behave differently: for example, montmorillonite diminishes pDRM intensity more rapidly than kaolin. Anson and Kodama (1987) have suggested an electrostatic model to explain their results of a similar study. However, we cannot explain all of our results by using their electrostatic model. To us, it seems that colloid coagulation is the key to explaining the results. Two kinds of forces exist in a colloidal solution: electric repulsion and van der Waals attraction. In low electrolyte concentration solutions, repulsion is larger than attraction. Strong Brownian motion is, therefore, required to enable the particles to cross over the repulsive energy barrier in order to stick to each other. But in high electrolyte concentration solutions, repulsion becomes relatively small, and particles stick together much more easily. Thus the effective mass of the grain becomes larger (clay + magnetite), but the magnetic moment remains the same, and hence the magnetic torque is relatively less effective. As a result, the magnetic carriers do not align effectively in the direction of the ambient field, making the pDRM intensity weaker.

Magnetic glasses in the Azuki and AT volcanic ashes in Japan

Magnetic properties of volcanic glass shards have been described on the widespread Azuki and AT ashes in Japan. Thermomagnetic analysis and isothermal remanent magnetization (IRM) acquisition experiments were performed on both bulk samples and separated glass samples of the Azuki volcanic ash. Together with progressive thermal demagnetization of natural remanent magnetization (NRM), the results demonstrate that titanomagnetite contained in volcanic glass shards is the sole contributor to the stable NRM of the Azuki ash. The results of a Lowrie-Fuller test suggest that the remanence carrier of the ash layer is single-domain grains of titanomagnetite, which can be seen as submicron particles using electron microscopy. On the basis of progressive IRM acquisition in samples with various contents of AT volcanic glass, it is also suggested that titanomagnetite in glass shards is a significant magnetic carrier of NRM for the AT ash. Since most of the glass shards are tens of microns in size, they are expected to acquire depositional remanent magnetization (DRM) without experience of succeeding post-depositional process under aqueous conditions. A redeposition experiment with the Azuki ash shows that stable DRM, whose intensity is comparable to that of the NRM of the water-laid Azuki ash, is acquired during several hours.

High-resolution palaeomagnetic study of Late Pleistocene sediments from Baffin Bay:first results

The detailed palaeomagnetic study of core 85027016, collected in central Baffin Bay, confirms the occurrence of anomalous directions in the depositional remanent magnetization, previously observed in the upper part of the Pleistocene sequence. Chronology and sedimentation rates are discussed on the basis of new 14C ages, 18O isotopic stratigraphy, and palaeoclimatic arguments provided by other works. Two successions of anomalous inclinations are recorded between 15 000 and 30 000 years BP, preceded by long-period secular variations. The possible occurrence of geomagnetic excursions in this region is discussed in considering contemporaneous high-resolution palaeomagnetic records of the Northern Hemisphere.

Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya

The Koobi Fora Formation, a Pliocene and Pleistocene sequence of sedimentary deposits northeast of Lake Turkana, has yielded numerous fossils and stone artifacts of early hominids. Stratigraphic correlation of the hominid‐bearing deposits throughout the Turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies. We have reinterpreted previously published magnetostratigraphy from the upper part of the Koobi Fora Formation because the original stratigraphy and dating of tuffs have been revised. In our reinterpretation we include previously unpublished data from the uppermost part of the formation. The upper magnetozones correlate with parts of the Brunhes Normal‐Polarity Chron and Matuyama Reversed‐Polarity Chron (about 0.6–0.85 Ma) and are separated from the magnetozones of the upper part of the Matuyama (2.0–1.25 Ma) by a disconformity. The Olduvai Normal‐Polarity Subchron is represented within the Matuyama, but the lower part of the Matuyama (2.4–2.0 Ma) is missing due to an erosional disconformity. We have also determined magnetozones in the lower part of the Koobi Fora Formation, which had not been sampled for paleomagnetism during the earlier studies. Our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of Koobi Fora tuffs with dated tuffs in the Shungura Formation of southern Ethiopia. The tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the Koobi Fora and Shungura formations. The lower part of the Koobi Fora spans the interval from about 4 Ma to 2.4 Ma, within the Gilbert Reversed‐Polarity and Gauss Normal‐Polarity chrons. Rock magnetic studies indicate that detrital magnetite carries most of the stable remanence, although hematite contributes to the remanence as indicated by thermal demagnetization. The hematite, which presumably formed by postdepositional oxidation of original iron oxide grains, carries chemical remanent magnetization (CRM) that diminishes the sharpness of polarity zone boundaries. The CRM accumulated continuously within the uppermost 10 m of sediment below the land surface, so that the CRM reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70,000 years in sections where the sedimentation rate was approximately 15 cm/1000 years.

The magnetic remanence and anisotropy of susceptibility of cave sediments from Agen Allwedd, South Wales

Summary This paper describes the palaeomagnetism and magnetic fabric of 40 samples of a laminated silt from Agen Allwedd, Powys. The sediments contain a very stable depositional remanent magnetization due to the presence of magnetite. The similarity of remanence directions at several locations suggests that deposition was synchronous throughout the cave. A method is used to correct the remanence for the effects of water currents, taking into account the information provided by the susceptibility anisotropy. Evidence provided by these sediments suggests that a model which describes bedding errors observed in the laboratory can be successfully applied to these natural materials on slopes as high as 29|Mo.

On the theory of depositional remanent magnetization in sedimentary rocks

The effects of various factors such as thermal agitation, coagulation, anisotropy of susceptibility, and shape irregularity on the alignment of magnetic carrier grains during the process of acquisition of depositional remanent magnetization in sedimentary rocks is discussed.

On the possibility of using lunar fines to determine the intensity of the ancient lunar magnetic field

The remanent magnetization of iron grains in lunar fines can be studied by allowing a sample of fines to acquire a depositional remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a natural remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a palaeointensity of ca. 1.0 Oe was determined by this method, several factors complicate the interpretation of the result. These include the origin and nature of the iron metal in lunar fines, and the time and acquisition process of its magnetization.

Depositional magnetization of Carboniferous Limestones from the Craven Basin of northern England

ABSTRACTCarboniferous Limestones in the Craven Basin of northern England carry a stable natural remanent magnetization (NRM) the intensity of which is facies dependent. Dark argillaceous limestones are most strongly magnetized and pure, pale coloured limestones most weakly magnetized. Partial thermal and alternating field demagnetization suggest that magnetite is the principal carrier of the remanence although some haematite is present in the limestone. The presence of magnetite is confirmed by the low temperature transition, isothermal remanent magnetization (IRM build‐up curves) and microprobe analysis.Partial demagnetization of IRM and ARM suggest that the magnetite is relatively coarse grained and in the multidomain state. There are no indications of pseudo‐single domain behaviour but magnetite of this type cannot be excluded as a possible remanence carrier. A grain size estimate of 10–20 μm based on coercive force and remanent coercive force is compatible with the theoretical consideration of grain size.The limestones show a weak but marked magnetic susceptibility anisotropy. This anisotropy defines a depositional fabric which indicates that the magnetization is a depositional remanent magnetization (DRM). A DRM was acquired by each specimen before compaction and cementation and was preserved because of the reducing conditions which prevailed in the early diagenetic environments of the limestones.

Correlation of a volcanic ash bed in pleistocene deposits near Mount Blanco, Texas, with the Guaje Pumice Bed of the Jemez Mountains, New Mexico

A rhyolitic volcanic ash bed about 0.3 m thick is exposed in a roadcut along Texas Highway 193 near Mount Blanco in the upper part of a sequence of Pleistocene sedimentary deposits at the type locality of the Blanco Formation, about 59 km northeast of Lubbock, Texas. This ash, here named informally the Guaje ash bed, has chemical and petrographic characteristics closely resembling those of the rhyolitic air-fall tephra (Guaje Pumice Bed) that directly underlies ash flows of Pleistocene age in the Jemez Mountains of northern New Mexico. The Guaje Pumice Bed and the ash flows belong to the Otowi Member of the Bandelier Tuff. Properties common to the Guaje ash bed and the Guaje Pumice Bed include: refractive index of glass, 1.497–1.498; microphenocrysts of quartz, sanidine (Or 42–44), ferrohedenbergite (Fe 51Ca 42Mg 7), chevkinite, allanite, zircon, and magnetite. Chemical composition of the glass of the Guaje ash bed matches that of the Guaje Pumice Bed for all major elements except K and Na and for trace elements determined by standard chemical analyses, atomic absorption, and neutron activation. Paleomagnetic measurements indicate that the ash has reverse depositional remanent magnetization. Glass shards of the ash have a fission-track age of about 1.4 ± 0.2 m. y. Sanidine from the Guaje Pumice Bed and its genetically related ash-flow sheet in the Jemez Mountains was KAr dated at about 1.4 m. y. by R. R. Doell and his colleagues in 1968. Correlation of the Guaje ash bed with the radiometrically dated Guaje Pumice Bed establishes a minimum age of about 1.4 m. y. for the Blanco Formation.