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

Helsley, Charles E.

doi:10.1130/0016-7606(1969)80[2431:mrsotl]2.0.co;2

Cited by 58 publications

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“…In certain cases they are strongly magnetized and have closely grouped, directions stable to AF and thermal demagnetization, (Collinson & Runcorn, 1960;Irving, 1964;Baag & Helsley, 1974;Helsley & Steiner, 1974). In certain cases they are strongly magnetized and have closely grouped, directions stable to AF and thermal demagnetization, (Collinson & Runcorn, 1960;Irving, 1964;Baag & Helsley, 1974;Helsley & Steiner, 1974).…”

Section: The Directional Properties Of Chemical Remanent Magnetizatiomentioning

confidence: 99%

The palaeomagnetic evolution of continental red beds

Turner

1979

Geol. Mag.

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The palaeomagnetism of continental red beds is considered in the light of evidence concerning the diagenetic origin of various textural phases of hematite. The relative roles of pigmentary hematite and specularite as magnetization carriers is discussed and it is concluded that all magnetizations are post-depositional: the magnetization of continental red beds is therefore of diagenetic origin.Three types of non-viscous magnetization, A, B and C, are recognized in continental red beds. Type-A magnetizations retain some features of the original magnetization and are most closely coincident with the depositional age of the rocks. Type-B are composite magnetizations acquired over a long period (more than 10 6 yr) of time and after substantial changes in the ambient geomagnetic field. Type-C magnetizations bear no relationship to the depositional age of the rocks and represent the ultimate end-product of the diagenetic processes which affect continental red beds. Thus, these magnetizations represent stages in the diagenetic evolution of red beds. The rate of diagenetic evolution, and hence of the modification of the magnetization, is variable and depends on a variety of local geological variables.The study enables some assessment of the reliability of palaeomagnetic data from red beds. Type-A magnetizations, although of diagenetic origin, are considered to be palaeomagnetically reliable, but only within certain limits. Red beds with Type-B and Type-C magnetizations are not considered to be generally suitable for palaeomagnetic study. Palaeomagnetic studies are, however, considered to have great potential value for dating specific diagenetic processes in continental red beds.

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Section: The Directional Properties Of Chemical Remanent Magnetizatiomentioning

confidence: 99%

The palaeomagnetic evolution of continental red beds

Turner

1979

Geol. Mag.

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“…Magnetostratigraphic data have been available for nonmarine Permian and Triassic rocks for some time (Picard 1964;McMahon & Strangway 1968;Helsley 1969;Helsley & Steiner 1974;Reeve & Helsley 1972). Usually, temporal resolution of non-marine sequences is low, and correlation with the geological time-scale is difficult.…”

Section: Introductionmentioning

confidence: 99%

Palaeomagnetism and magnetostiatigraphy of Triassic strata in the Sangre de Cristo Mountains and Tucumcari Basin, New Mexico, USA

Molina-Garza

Geissman

Lucas

et al. 1996

Geophysical Journal International

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SUMMARY We report palaeomagnetic data and a composite magnetic polarity sequence for Middle and Upper Triassic rocks assigned to the Anton Chico Member of the Moenkopi Formation and Chinle Group, respectively, exposed along the eastern flank of the Sangre de Cristo Mountains and in the Tucumcari Basin of eastern and northeastern New Mexico. Thermal demagnetization isolates a well‐defined, dual polarity, characteristic magnetization, carried in most cases by haematite and interpreted as an early acquired chemical remanent magnetization (CRM). Characteristic magnetizations from 74 palaeomagnetic sites (one site = one bed) are used to define a magnetic polarity sequence, which we correlate with previously published Triassic data obtained from both marine and non‐marine rocks. Preliminary correlation suggests that the resolution of magnetostratigraphic data derived from continental strata is not necessarily of lesser quality than that from marine rocks. On the basis of the magnetostratigraphic data, a profound unconformity is believed to separate lower‐middle Norian and upper Norian‐Rhaetian strata of the Chinle Group. Palaeomagnetic poles derived from selected sites in steeply dipping (> 85°) strata for the Middle Triassic (Anisian, ∼240 Ma: 50°N 121°E; N= 8), late Carman‐early Norian (∼225 Ma: 53°N 104°E; N= 16), and late Norian‐Rhaetian (∼208 Ma: 59°N 77°E; N= 8) are in relatively good agreement with previously published data for the Moenkopi Formation and Chinle Group and related strata in southwest North America. None the less, comparison with palaeomagnetic poles obtained from gently dipping or flat‐lying Triassic strata from this study (Anisian, 46°N 112°E; N= 13; late Carnian, 54°N 87°E; N=12) and previously published Triassic poles in southwest North America suggest that a modest ‘apparent rotation’ not greater than about 5° affects declinations from steeply dipping rocks. The distribution of palaeomagnetic poles indicates ∼25° (angular distance) of apparent polar wander between about 240 and 208 Ma.

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“…There have been a number of studies of the magnetic stratigraphy of the Permian and Triassic Systems but in NW Europe and N America interpretation is complicated by the fact that these Systems comprise very largely of continental red beds and carbonate-evaporite sequences which are 61 strongly affected by diagenesis. As a result there has been much controversy as to the validity of published Permo-Triassic polarity-reversal sequences (Helsley 1969;Elston & Purucker 1979;Walker et al 1981), even though reversal sequences are known in the Triassic and can be correlated over some distance (Lienart & Helsley 1980). For example, Herrero-Bervera & Helsley (1983) described an apparent polarity transition in a 1 m interval between normally (below) and reversely magnetized rocks (above) and Larson & Walker (1985) subsequently contended that the intermediate directions in this zone were produced by the superimposition of antiparallel components at least one of which was produced during diagenesis.…”

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confidence: 97%

Palaeomagnetism of Permo-Triassic rocks in the Iberian Cordillera, Spain: acquisition of Secondary and characteristic remanence

Turner

Ramos

et al. 1989

JGS

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Magnetostratigraphy is widely used as a tool for stratigraphical correlation but there are problems in its application to rocks which have suffered complex diagenesis, particularly sandy, continental red beds. In such cases the complete magnetic record should be studied in relation to its depositional, diagenetic and structural history. To this end a detailed palaeomagnetic study has been made of Permian and Triassic rocks in the northern Iberian Cordillera, Spain. Four stratigraphical units comprising the Autunian, 'Saxonian', Buntsandstein and Muschelkalk have been studied in a single section near Molina de Aragon which is well-documented sedimentologically. Autunian rocks have a characteristic remanence which is exclusively reversed and corresponds to the Kiaman Interval of Lower Permian age.A pole position (49N, 167 W) is consistent with previous studies which indicate the rotation of Iberia in post-Triassic times relative to the stable European plate. The Saxonian facies show complex, multicomponent magnetizations; no polarity zonation can be resolved but individual components were acquired prior to the rotation of Iberia.The Buntsandstein is remarkable in that much of it is completely remagnetized in a direction similar to the present local geomagnetic direction. This secondary remanence is carried by fine grained hematite which has been formed as a result of carbonate dissolution and other diagenetic reactions. Initially, these iron oxides were probably superparamagnetic and remanence was only blocked-in by structural inversion of the Iberian Cordillera in Tertiary times and after rotation of the Iberian plate. Some of the primary remanence is preserved in some fine grained lithologies of the Buntsandstein, indicating that normal and reversed zones of magnetization were originally present. The magnetization of the Muschelkalk carbonates is also complex; secondary components similar to those of the Buntsandstein are present but there is evidence that primary components were mainly normal during Karnian times.

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

Cited by 58 publications

References 0 publications

The palaeomagnetic evolution of continental red beds

The palaeomagnetic evolution of continental red beds

Palaeomagnetism and magnetostiatigraphy of Triassic strata in the Sangre de Cristo Mountains and Tucumcari Basin, New Mexico, USA

Palaeomagnetism of Permo-Triassic rocks in the Iberian Cordillera, Spain: acquisition of Secondary and characteristic remanence

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