Charles E. Helsley

Evidence for long intervals of normal polarity during the cretaceous period

Paleomagnetic studies of Cretaceous rocks from more than 35 widely separated sites in North America show that the polarity at all but two of the sites is consistently normal, suggesting that the Cretaccous period may be characterized by a magnetic field dominantly of one polarity. The published literature concerning Cretaceous rocks substantiates this hypothesis and indicates that reversals are present primarily in the Lower Cretaceous and the upper portion of the Upper Cretaceous period. The longest period of constant normal polarity consistent with the data presently available is about 25 m.y. and is present in rocks from Upper Albian to middle Santonian age. Magnetic anomaly patterns at sea show a smooth interval older than anomaly 32 (80 m.y. in age) which may correspond to this long period of normal polarity observed in Cretaceous rocks from North America.

Paleomagnetism of the Lower Triassic Moenkopi Formation

Paleomagnetic studies on the upper half of the Moenkopi Formation have identified three reversed and three normal polarity zones; this brings the total number of polarity zones observed in this formation to ten. The variable thickness and presumed duration of these polarity zones when combined with smaller variations internal to individual polarity zones suggest that this reversal sequence should be very useful as a regional correlation tool. Several zones of anomalous magnetization have been recognized within the formation. These anomalous zones constitute less than 5 percent of the total sample population but are very important, because they may assist in the recognition of individual polarity zones. The pole position derived from this study is at 103° E., 55° N. (dp 1.6°, dm 3.2°) and, due to the large number of samples and the length of time represented, probably represents an excellent average for the Lower Triassic of North America.

Reversal pattern and apparent polar wander for the Late Jurassic

A paleomagnetic study of the Upper Jurassic (Kimmeridgian-Tithonian) Morrison Formation near Norwood, Colorado, indicates the existence of thirteen polarity intervals. The seven reversed intervals occupy much more of the section than the six normal intervals, suggesting that during this time the dominant polarity was reversed. Pole positions were computed for each portion of the section where directions were tightly grouped. The pole positions form two separate groups directly related to the stratigraphic position in the section of the samples from which they were computed. The two mean pole positions for the Morrison Formation, 142.2°E, 61.4°N (dp = 4.0°, dm = 6.5°) and 161.8°E, 67.5°N (dp = 3.5°, dm = 5.0°), define a path which includes the Cretaceous pole positions for North America. The data indicate that the Jurassic apparent polar wander curve for North America is approximately a line of latitude (present-day coordinates) connecting published Triassic and Cretaceous pole positions. The data disagree with the commonly held view that the Jurassic portion of the North American apparent polar wander curve includes the present axial dipole.

Magnetic Reversal Sequence in the Upper Portion of the Chinle Formation, Montoya, New Mexico

An investigation of the magnetic reversal stratigraphy in the Redonda Member and the underlying shale member of the Chinle Formation at Montoya, New Mexico, reveals the presence of at least eight separate polarity intervals in the upper 133 m of the Chinle Formation. At Lucianna Mesa south of Montoya the contact between the Redonda Member and the underlying unnamed shale member occurs 3 m above a transition from normal (below) to reversed polarity (above), the reversal sequence within the Redonda having eight polarity intervals. Measurement of samples collected from the same stratigraphic interval on Redonda Mesa 30 km to the east serves as a check on a portion of this reversal sequence. The Chinle Formation in eastern New Mexico is considered to have been contemporaneous with the Newark group in the eastern United States on the basis of vertebrate paleontology. Published investigations of the Newark group have revealed only normal magnetic polarities. Because the magnetic field reversed for the entire earth the Newark cannot exhibit different polarities from those in the Chinle if the two units acquired their magnetization at the same time. Thus, there are inconsistencies between the paleontologic and paleomagnetic data for the two rock units. These inconsistencies are best answered by assuming the Chinle in New Mexico is partially or completely older than the Newark. The latter explanation is improbable when carried to the extent of assigning the Chinle a Middle Triassic age as proposed by McElhinny and Burek (1971). Until the temporal relation between the Chinle and Newark is clarified, the Newark group should not be used to define a reversal chronology. A mean paleomagnetic pole position at 79° E. long, 58° N. lat was calculated for approximately the upper half of the Chinle Formation at Montoya. Pole positions for two well-demagnetized polarity intervals (one from each member studied) have been combined with other published Upper Triassic pole positions (demagnetized) to obtain a mean Upper Triassic paleomagnetic pole position at 89° E. long, 62.5° N. lat. The similarity of this mean and of the individual Chinle poles to the published Lower Triassic pole determinations based on demagnetized samples indicates little, if any, latitudinal polar wander-continental drift relative to North America during most it not all of the Triassic.

Magnetic Reversal Stratigraphy of the Lower Triassic Moenkopi Formation of Western Colorado

Oriented cores, taken at stratigraphic intervals of 9 to 12 inches, have been collected from the basal 500 feet of the Moenkopi Formation along the Dolores River in western Colorado. The NRM and the demagnetized data from these samples indicate that the Lower Triassic magnetic field is characterized by at least eleven reversals. Samples from the normal polarity intervals are initially well clustered and remain so after AC or thermal demagnetization. Samples with reversed polarity are initially scattered and remain so after AC demagnetization. However, after thermal demagnetization to 500° C, they become well clustered and have a direction approximately 180° from that of the normal intervals. All samples belong to either the normal or reversed groups, and no samples transitional between normal and reverse were observed. A tentative correlation of the resulting polarity reversal sequence with that from the Chugwater Formation of Wyoming is proposed. A magnetic anomaly pattern similar to this polarity reversal sequence is observed in magnetic profiles at sea, indicating that polarity reversal sequences may be a useful means of both regional correlation and calibration of the sea-floor spreading rates postulated for the ocean basins.

Heat Flow and Continuous Seismic Profiles in the Cayman Trough and Yucatan Basin

The average of 8 heat-flow measurements in the western half of the Cayman Trough is 2.07 ± 0.25 HFU, markedly higher than the averages of 1.46 ± 0.19 HFU (n = 9) and 1.38 ± 0.19 HFU (n = 3) obtained on the Yucatan Basin and Cayman Ridge, respectively. There is a tendency for the highest heat-flow values to be situated in the deepest areas of the trough. No systematic variation of heat flow with distance along the trough was observed. The existence of a long, narrow zone of uniformly high heat flow along the floor of the Cayman Trough, along with other geophysical data, suggest a tectonic origin for the trough by extension normal to the axis of the trough and/or by strike-slip faulting related to the eastward movement of the Caribbean lithospheric plate relative to the Atlantic plate. The mean heat flows through the Yucatan Basin and Cayman Ridge are nearly equal to the average world heat flow. Seismic profiler data and piston cores from the Cayman Trough and Ridge and in the Yucatan Basin show that the Cayman Trough is a geologically young feature, probably having originated since the early Tertiary, when the trough and adjacent ridge developed simultaneously. Subsequently, the western end of the trough has received terrigenous sediment from a source located near the Gulf of Honduras, rather than from the Yucatan Basin or Honduras. The central and eastern parts of the trough have been and remain isolated from any major sediment sources. Tectonic activity in the trough has been largely restricted to the margins, as evidenced by the location of the deepest basins, fault structures, and seismicity along the southern margin of the trough west of 83° W. and along the base of the Cayman Ridge east of 81° W.

Late Jurassic magnetic polarity sequence

Abstract Polarity reversal sequences have been observed in the Brushy Basin Member of the Morrison Formation (Upper Jurassic, Kimmeridgian-Tithonian age) at two sites in western Colorado separated by 80 km. The two polarity reversal sequences display a good correlation of relative lengths and number of polarity intervals and thus may provide a significant correlation tool in this laterally variable formation. The whole polarity sequence observed in the Morrison Formation correlates very well with the oldest portion of lineated anomalies of the sea floor. The sedimentary record on land and the anomaly pattern of the oceanic crust both show the same relative lengths and approximate number of polarity intervals, as well as ratio of reversed to normal polarity.

Paleomagnetism of tertiary and recent lavas of Israel

Abstract Magnetizations in 24 flows of Tertiary age in Israel indicate two stable directions, each of which has both normal and reversed polarities. AF demagnetization decreases the scatter of the NRM results. Typical Tertiary poles are near 70°N 110°W and another set of anomalous poles are near 34°N 50°W. These are similar to other reported Tertiary and Cretaceous poles.

The paleomagnetism of cretaceous rocks from Israel

Abstract The original directions of magnetization of oriented samples from two extensive lava fields in Israel, one of Lower and one of Upper Cretaceous age, show considerable scatter (K⋍ 2) that is reduced appreciably upon AC magnetic cleaning (K⋍ 50). The resulting pole positions are: 105.8E, 59.5N for Lower Cretaceous and 95.8E, 41.64N for Upper Cretaceous. Since azimuthal errors in the field orientations were suspected, a statistical model is proposed in which the declination and inclination have independent Gaussian distributions. Correlation coefficients regressions, and confidence intervals computed from the above data show that the proposed statistical model is reasonable for one volcanic unit, but does not seem appreciably different from Fisherian statistics for the second. The statistical results indicate a marked change in confidence limits by use of the bivariate model thus indicating the inadequacy of the Fisher statistics for some paleomagnetic purposes.

Magnetic properties of lunar dust and rock samples.

Determinations on 20-to 80-milligram portions of the rock samples and the -150 mesh fraction of the lunar dust show pronounced Curie points between 680� and 780�C. Remanent intensities of five rock fragments vary from 8.4 x 10-5 to 0.30 X J0-5 emu/gram. Upon demagnetization, two of the samples had only viscous magnetization and two other samples had stable magnetizations with remanent coercivities in excess of 50 oersteds. Partial thermal demagnetization suggests that these apparently stable moments may have been acquired in a magnetic field in excess of 1500 gammas. xsxs