Stable Single-Domain to Superparamagnetic Transition During Low-Temperature Oxidation of Oceanic Basalts

Recent experimental data on synthetic titanomaghemites indicate that oxidation is accompanied by a decrease in magnetocrystalline anisotropy K1 as well as a decrease in the saturation magnetization σs. Theory of relaxation times predicts that the decrease of σs and K1with oxidation will cause single-domain titanomagnetite grains with initially low blocking temperatures to become superparamagnetic as they are oxidized to titanomaghemite. For titanomagnetites (Fe3−x TixO4) of composition x = 0.4 and x = 0.5 the affected blocking temperature range is approximately 200° and 125°K below the Curie point, respectively. However, for x = 0.6 titanomagnetites the blocking temperature range affected is within 80°K of the Curie point and spans a temperature interval from 325° to as high as 380°K, depending on the degree of oxidation. The rapid quenching of oceanic pillow lavas (average composition x ≅ 0.6) produces efficient thermoremanence carriers but will also result in a significant grain size and blocking temperature distribution. Thus low-temperature oxidation of oceanic basalts could cause a significant portion of the originally stable carriers of natural remanence to become superparamagnetic. This mechanism could aid in explaining the large natural remanent magnetization intensity decrease of dredged basalts with distance from the mid-Atlantic ridge

Magnetic Mineralogy of Continental Deposits, San Juan Basin, New Mexico, and Clark\u27s Fork Basin, Wyoming

Magnetic concentrates were obtained from nine bulk samples from the Late Cretaceous through middle Paleocene continental sedimentary section in the San Juan Basin, New Mexico, and from two bulk samples from the late Paleocene and early Eocene section in the Clark\u27s Fork Basin, Wyoming. Strong-field thermomagnetic (JS-T) curves of almost all the San Juan Basin concentrates show only a single Curie temperature in the 175°–195°C range, indicating that the dominant ferrimagnetic mineral is detrital titanomagnetite (Fe3−xTixO4) of composition 0.51 ≤ x ≤ 0.54. The Clark\u27s Fork Basin concentrates exhibited Curie temperatures of 200°C and 580°C, indicating a mixture of x = 0.5 titanomagnetite and magnetite, respectively. No evidence of hematite (or other ferric oxides or oxyhydroxides) was observed in the JS-T data. Isothermal remanent magnetization (IRM) acquisition curves were determined for 56 samples from the Clark\u27s Fork Basin and 114 samples from the San Juan Basin. Although dominated by IRM acquired in magnetizing fields of ≤300 mT, additional IRM was acquired in magnetizing fields from 300 to 700 mT. The IRM acquired above 300 mT is attributed to hematite (or other ferric oxides or oxyhydroxides) and is thought to be the result of minor oxidation, probably during recent weathering. These results indicate that IRM acquisition behavior may be used to monitor the hematite content in continental sedimentary rocks and may indicate stratigraphic intervals within which the natural remanent magnetization (NRM) could contain significant chemical remanence overprints. Although minor hematite content is indicated, the detailed sampling of IRM acquisition behavior for the San Juan Basin and Clark\u27s Fork Basin sedimentary sequences did not reveal any significant correlation of IRM behavior with polarity of the characteristic NRM. These data thus support the previous conclusion, based on paleomagnetic data, that the characteristic NRM is a depositional remanence which provides a valid recording of the geomagnetic polarity sequence during deposition of these continental sedimentary sequences

A Paleocene Paleomagnetic Pole from the Gringo Gulch Volcanics, Santa Cruz County, Arizona

Paleomagnetic data from 25 sites (5 samples per site) in andesite flows of the Gringo Gulch Volcanics in Santa Cruz County, Arizona, were analyzed to determine a lower Paleocene paleomagnetic pole. Alternating - field demagnetization to 500 oe peak field was sufficient to erase secondary viscous components. The mean direction of magnetization (inclination = -58.80, declination = 167.5 °) was obtained by averaging the site mean directions of the 25 sites, which are all reversed. The resultant lower Paleocene pole position is at lat. 77.0 °N, lon. 201.0 °E (dp = 1.2 °, dm = 1.7 °)

The Effects of Noise Due to Random Undetected Tilts and Paleosecular Variation on Regional Paleomagnetic Directions

Random tilting of a single paleomagnetic vector produces a distribution of vectors which is not rotationally symmetric about the original vector and therefore not Fisherian. Monte Carlo simulations were performed on two types of vector distributions: (1) distributions of vectors formed by perturbing a single original vector with a Fisher distribution of bedding poles (each defining a tilt correction) and (2) standard Fisher distributions. These simulations demonstrate that inclinations of vectors drawn from both distributions are biased toward shallow inclinations. There is a greater likelihood of statistically “drawing” a vector shallower than the true mean vector than of drawing one that is steeper. The estimated probability increases as a function of angular dispersion and inclination of the true mean vector. Consequently, the interpretation of inclination-only data from either type of distribution is not straightforward, especially when the expected paleolatitude is greater than about 50°. Because of the symmetry of the two distributions, declinations of vectors in each distribution are unbiased. The Fisher mean direction of the distribution of vectors formed by perturbing a single vector with random undetected tilts is biased toward shallow inclinations, but this bias is insignificant for angular dispersions of bedding poles less than 20°. This observation implies that the mean pole calculated from a large set of paleomagnetic directions obtained for coeval rocks over a region will be effectively unbiased by random undetected tilts of those rocks provided the angular dispersion of the undetected tilts is less than about 20°. However, the bias of the mean can be significant for large (\u3e20°) angular dispersion of tilts. The amount of bias of the mean direction maximizes at about 10°–12° in mid-latitude regions but is usually less than 8°. Consequently, large (\u3e12°) inclination discordances are probably not the result of random undetected tilts, even if the angular dispersion of the tilts exceeds 20°

North American Jurassic Apparent Polar Wander: Implications for Plate Motion, Paleogeography, and Cordilleran Tectonics

Eight paleomagnetic poles are considered to be reliable Jurassic reference poles for cratonic North America. These poles form a consistent chronological progression defining two arcuate tracks of apparent polar wander (APW) from Sinemurian through Tithonian time (203-145 Ma). Combined with reliable Triassic and Cretaceous reference poles, the resulting path is well modeled by paleomagnetic Euler pole (PEP) analysis and is significantly different from previous APW compilations. These differences reflect differences in original data sets, modes of analysis, and geologic time scales and translate into substantial and important differences in paleolatitude estimates for cratonic North America. PEP analysis reveals two cusps, or changes in the direction of APW: one in the Late Triassic to Early Jurassic (Jl) and one in the Late Jurassic (J2). The J1 cusp represents the change in North American absolute plate motion associated with rifting of the central Atlantic and Gulf of Mexico, while the J2 cusp correlates temporally with the marine magnetic anomaly M21 plate reorganization and to various North American intraplate tectonomagmatic events (e.g., Nevadan Orogeny). Analysis of pole progression along the J1 to J2 and J2 to Cretaceous APW tracks indicates constant angular plate velocity of 0.6°-0.7°/m.y. from 203 to 150 Ma followed by significantly higher velocity from 150 to 130? Ma. Late Triassic-Jurassic reference poles indicate more southerly paleolatitudes for cratonic North America than have previous compilations requiring modification of displacement scenarios for suspect terranes along the western Cordillera

Paleomagnetism of the Chinle and Kayenta Formations, New Mexico and Arizona

Paleomagnetic data were obtained from 22 sites (6–10 samples/site) in the Upper Shale Member of the Chinle Formation, 43 sites in the Owl Rock Member of the Chinle Formation, and 35 sites in the Kayenta Formation. Thermal demagnetization and data analyses indicate that within-site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization (ChRM). Site-mean directions define at least four antipodal polarity zones within each member/formation, suggesting the ChRM was acquired soon after deposition. Fifteen site-mean virtual geomagnetic poles (VGPs) from the Upper Shale Member of the Chinle Formation yield an early Norian paleomagnetic pole position of 57.4°N, 87.8°E (K = 60, A95 = 5.0°). Eighteen site-mean VGPs from the Owl Rock Member of the Chinle Formation yield a middle Norian paleomagnetic pole position of 56.5°N, 66.4°E (K = 183, A95 = 2.6°). Twenty-three site-mean VGPs from the Kayenta Formation yield a Pliensbachian pole position of 59.0°N, 66.6°E (K = 155, A95 = 2.4°). Combined with paleomagnetic poles from the Moenave Formation and the Shinarump Member of the Chinle Formation, these data record ∼30 m.y. of North American apparent polar wander (APW) within a regional stratigraphic succession. During the Camian and Norian stages of the Late Triassic, Chinle poles progress westward. During the Hettangian through Pliensbachian stages of the Early Jurassic, the pattern of APW changed to an eastward progression. Even after correction for 4° clockwise rotation of the Colorado Plateau, a sharp comer in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (∼200 Ma) Moenave Formation (59.4°N, 59.2°E). Amongst other implications, the sharp change in the APW path at the J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter

Paleomagnetism of the Middle Jurassic Summerville Formation, East Central Utah

The paleomagnetism of the late Callovian(?) Summerville Formation was analyzed to obtain a late Middle Jurassic paleomagnetic pole for North America. A total of 281 samples were collected from 35 sedimentary horizons (sites) in a single locality in the San Rafael Swell area of east central Utah. Fifteen site-mean characteristic remanent magnetization (ChRM) directions pass the reversals test and define at least five polarity zones within 52 m of stratigraphic section, suggesting that the ChRM was acquired upon, or soon after, deposition. Magnetizations of some specimens are complex, and several horizons yield anomalous site-mean directions. Data analysis included filtering to provide different combinations of virtual geomagnetic poles for calculation of the paleomagnetic pole. However, editing the data did not change the pole position by more than 5°. The preferred paleomagnetic pole position is 56.3°N, 133.4°E (A95 = 7.2°; N = 11 sites). The Summerville Formation paleomagnetic pole is located near the ∼172 Ma Corral Canyon pole and is statistically indistinguishable from the ∼151 Ma Glance Conglomerate and ∼149 Ma Lower Morrison poles. The paleomagnetic pole from the Summerville Formation is located at a much lower latitude and more easterly longitude than the paleomagnetic pole obtained from the ∼165 Ma Moat Volcanics of New England. We propose that the Jurassic North American apparent polar wander path is an age-progressive band at 55°N to 65°N latitude extending from ∼11°E longitude at ∼172 Ma to ∼150°E longitude at ∼149 Ma

An Archaeomagnetic Paleointensity Study of Some Hohokam Potsherds from Snaketown, Arizona

A paleointensity study on nine potsherds from the Hohokam Indian site of Snaketown, Arizona is described. The sherds range in age from A.D. 200-1400. Examination of different temperature subintervals from the Thellier-Thellier double heating experiment reveals that conventional statistical measures sometimes can unambiguously determine the best data subset for paleointensity calculations. However, it is often necessary to visually inspect the data and utilize physical insight in determining this data subset. Results suggest that the paleointensity was about 0.94 FO (FO, present intensity ≃ 0.506 oe) at A.D. 200, 0.72 FO at A.D. 600, and 1.2 FO at A.D. 1400. The shape of our curve of paleointensity vs. age is congruent with a curve previously derived from other Snaketown artifacts, but our paleointensities are systematically lower by about 0.15 oe

Mineralogy of Magnetic Minerals and Revised Magnetic Polarity Stratigraphy of Continental Sediments, San Juan Basin, New Mexico

Detailed paleomagnetic and rock-magnetic analyses have been performed on samples from multiple sections through the Cretaceous/Tertiary (K/T) boundary and throughout a 750 m thick sequence of late Cretaceous through middle Paleocene continental deposits in the San Juan Basin, New Mexico. Curie temperatures have been determined for magnetic separates from 52 levels and, with only two exceptions, they range from 180 to 300°C. Along with microprobe and X-ray analyses these data indicate that the detrital ferrimagnetic mineral is titanohematite with composition 0.45 \u3c x \u3c 0.60. This magnetic mineralogy indicates derivation of the continental San Juan Basin sediments from a volcanic (probably dacitic or andesitic) source. These mineralogical data, along with other geological data and the pattern of magnetic polarity zonation in multiple sections across the basin, argue strongly for deposition of the late Cretaceous and Paleocene continental deposits in the San Juan Basin as a clastic wedge derived from a source to the north or northwest. Demagnetization experiments, coupled with the mineralogy of the magnetic minerals, indicate that revision of our previous correlation of the San Juan Basin stratigraphic sequence with the magnetic polarity time scale is required. This revision indicates that the K/T boundary (recognized above the highest stratigraphic occurrence of dinosaur fossils) occurs within a reversed polarity zone correlative with magnetic polarity chron 29R. This correlation is consistent with the K/T boundary in the marine sedimentary sequence at Gubbio, Italy. Puercan (early Paleocene) fossil mammals occur within a normal polarity zone correlative with chron 29N and Torrejonian (middle Paleocene) fossil mammals occur within polarity zones correlative with chrons 27N, 27R, and 28N. With this revision, consistent and sequential placements within the magnetic polarity time scale have been accomplished for all North American land mammal ages in the Paleocene through early Eocene interval

Paleomagnetism of the Brushy Basin Member of the Morrison Formation: Implications for Jurassic apparent polar wander

The paleomagnetism of the ∼147 Ma (Tithonian) Brushy Basin Member of the Morrison Formation was analyzed to obtain a Late Jurassic paleomagnetic pole for North America. A total of 200 samples were collected from 25 sedimentary horizons (sites) at Norwood Hill in southwest Colorado. At Montezuma Creek in southeast Utah, 184 samples were collected from 26 sites. Detailed thermal demagnetization (up to nine temperature steps between 600°C and 680°C) and principal component analysis were required to confidently isolate characteristic remanent magnetization (ChRM) directions carried by hematite. Demagnetization behavior for many horizons is erratic and does not allow isolation of a high unblocking-temperature ChRM. Data selection criteria required sample ChRM directions to be defined by three or more thennal demagnetization steps and maximum angular deviations of sample ChRM directions to be ≤20°. Eight sites from the Norwood Hill location and 10 sites from the Montezuma Creek location passed these criteria. The 18 site-mean virtual geomagnetic poles yield a paleomagnetic pole position from the Brushy Basin Member of 68.3°N, 156.2°E (A95 = 4.8°, K = 53). This pole position is within 2° of the paleomagnetic pole which Steiner and Helsley (1975a) reported for the “upper” Morrison Formation at Norwood Hill, Colorado. A second paleomagnetic pole was calculated after excluding sites with site-mean α95 \u3e 20° and sites with fewer than three samples that passed the above selection criteria. This additional editing did not significantly change the paleomagnetic pole position at the 95% confidence level. Along with other paleomagnetic poles from the continental interior the paleomagnetic data from the Brushy Basin Member of the Morrison Formation are interpreted to indicate that the Late Jurassic part of the North American apparent polar wander path progresses from a late Middle Jurassic (∼160 Ma) position at ∼60°N, 135°E toward the mid-Cretaceous pole position at 72°N, 191°E