Nature and origin of hematite in the Moenkopi Formation (Triassic), Colorado Plateau: A contribution to the origin of magnetism in red beds

Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite-hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron-bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite-magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post-depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite-hematite intergrowths (‘tiger striped’ grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high-temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magnetism in the rocks.

[1]  R. Howie,et al.  An Introduction to the Rock-Forming Minerals , 1966 .

[2]  C. Helsley.  Magnetic Reversal Stratigraphy of the Lower Triassic Moenkopi Formation of Western Colorado , 1969 .

[3]  T. R. Walker,et al.  Diagenetic Origin of Continental Red Beds , 1976 .

[4]  S. A. Vincenz,et al.  Magnetization of the Cambrian Lamotte Formation in Missouri , 1971 .

[5]  D. Collinson.  The Role of Pigment and Specularite in the Remanent Magnetism of Red Sandstones , 1974 .

[6]  T. R. Walker,et al.  Development of Chemical Remanent Magnetization during Early Stages of Red-Bed Formation in Late Cenozoic Sediments, Baja California , 1975 .

[7]  T. R. Walker,et al.  Formation of Red Beds in Modern and Ancient Deserts , 1967 .

[8]  S. Haggerty.  Chapter 8. OPAQUE MINERAL OXIDES in TERRESTRIAL IGNEOUS ROCKS , 1976 .

[9]  A. H. Johnson.  Palaeomagnetism of the Jurassic Navajo Sandstone from South-western Utah , 1976 .

[10]  E. D. Mckee.  STRATIGRAPHY AND HISTORY OF THE MOENKOPI FORMATION OF TRIASSIC AGE , 1954 .

[11]  C. Helsley,et al.  Evidence for penecontemporaneous magnetization of the Moenkopi Formation , 1974 .

[12]  P. Turner,et al.  Magnetisation history of lower old red sandstones from the Gamrie Outlier, Scotland , 1975 .

[13]  F. B. Houten.  Iron Oxides in Red Beds , 1968 .

[14]  T. R. Walker.  Formation of Red Beds in Moist Tropical Climates: A Hypothesis , 1974 .

[15]  M. Steiner,et al.  Jurassic polar movement relative to North America , 1972 .

[16]  G. Hanson,et al.  Paleomagnetism and Age of Nonacho Group Sandstones and Associated Sparrow Dikes, District of Mackenzie , 1974 .

[17]  P. Ramdohr.  The ore minerals and their intergrowths , 1969 .

[18]  T. R. Walker,et al.  Iron Content of Modern Deposits in the Sonoran Desert: A Contribution to the Origin of Red Beds , 1969 .

[19]  J. L. Roy,et al.  Red beds: DRM or CRM? , 1972 .

[20]  D. P. Elston,et al.  Detrital magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona , 1979 .

[21]  C. Keen,et al.  Seismic Refraction Studies in Baffin Bay: An Example of a Developing Ocean Basin , 1972 .

[22]  M. Steiner,et al.  Paleomagnetism of the Lower Triassic Moenkopi Formation , 1974 .

[23]  M. Steiner,et al.  Magenetic Polarity Sequence of the Upper Triassic Kayenta Formation , 1974 .

[24]  Subir K. Banerjee.  New Grain Size Limits for Palaeomagnetic Stability in Haematite , 1971 .

[25]  K. Creer,et al.  Thermal demagnetization studies on the old red sandstone of the Anglo‐Welsh Cuvette , 1964 .

[26]  P. Turner.  The palaeomagnetic evolution of continental red beds , 1979, Geological Magazine.

[27]  D. Nota.  Sediments of the western Guiana shelf , 1958 .

[28]  T. R. Walker,et al.  GEOCHEMISTRY OF HORNBLENDE ALTERATION IN PLIOCENE RED BEDS, BAJA CALIFORNIA, MEXICO , 1967 .

[29]  D. Carroll.  Role of clay minerals in the transportation of iron , 1958 .

[30]  P. Turner,et al.  Diagenetic development of unstable and stable magnetization in the St. Bees Sandstone (Triassic) of northern England , 1977 .

[31]  A. Buddington,et al.  Iron-Titanium Oxide Minerals and Synthetic Equivalents , 1964 .

[32]  M. Steiner,et al.  Reproducible Anomalous Upper Triassic Magnetization , 1974 .

[33]  D. Lindsley.  Chapter 2. EXPERIMENTAL STUDIES of OXIDE MINERALS , 1976 .

[34]  T. R. Walker,et al.  Diagenesis in first-cycle desert alluvium of Cenozoic age, southwestern United States and northwestern Mexico , 1978 .