Edwin E. Larson

Development of Chemical Remanent Magnetization during Early Stages of Red-Bed Formation in Late Cenozoic Sediments, Baja California

Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits. The chemical remanence in the sediments is carried predominantly by three authigenic minerals — hematite, goethite, and a Mn · Ba compound, herein called “hydropsilomelane” — that occur as pigments and as concretions. Remanence directions generally associated with each mineral are goethite, reverse; hydropsilomelane, normal; and hematite, normal or reverse. Some samples possess a remanence that is strong and normal or strong and reverse; these generally contain only one of the authigenic minerals in abundance. Many samples, however, are weak in intensity and random in direction. When such samples are split into parts and measured, it is generally found that each part is strongly magnetized but that some parts are normal and others are reverse in direction. In such cases, the magnetization of the whole sample is resultant of the multiple components that are generally carried by two or more of the authigenic minerals. The following conclusions can be drawn concerning the acquisition of CRM in the Baja California deposits: (1) The sediments contain chemically unstable iron- and manganese-bearing minerals, such as hornblende and biotite, that are susceptible to postdepositional alteration, and they have provided the parent material for the authigenic magnetic minerals. (2) Authigenic magnetic minerals, growing from crystallites, generally acquired a remanence that was parallel to the Earth9s field when they surpassed the critical grain size. (3) The rate of CRM acquisition has not been uniform in these sediments, probably because the processes of alteration and formation of authigenic magnetic minerals depend on an interplay of highly variable factors such as the chemistry of the interstitial water, hydraulic gradients, and mineralogy of the sediments. (4) Complex variability in acquisition of CRM has led to a remanence stratigraphy that bears little discernible correlation with the geomagnetic field at the time of deposition.

Anomalous Paleomagnetic Pole from Isotopically Dated Cambro-Ordovician Intrusives in Colorado

Paleomagnetic data from isotopically dated Cambrian and Ordovician plutons in central Colorado indicate a Cambrian-Ordovician paleopole close to the late Paleozoic pole position. Samples were partially demagnetized by both ac and thermal techniques. Magnetic directions were essentially the same regardless of treatment. Polished section and Curie temperature analysis indicate that the principal magnetic phase is low-titanium titanomagnetite. In many samples, titanomagnetite is the only magnetic iron oxide; in some samples, hematite occurs additionally. However, those samples containing hematite show the same remanent directions as those containing only titanomagnetite. All of our studies indicate that the magnetization is thermoremanent magnetization (TRM) acquired during the Cambrian and Ordovician. Our results suggest that either little polar wandering occurred between the early and late Paleozoic or, if substantial wandering did occur, by late Paleozoic time the paleopole has returned to a position that was nearly coincident with that of the early Paleozoic.

Petrologic, paleomagnetic, and structural evidence of a Paleozoic rift system in Oklahoma, New Mexico, Colorado, and Utah

A west-northwest–trending dike swarm consisting of at least 50 mafic dikes cuts Precambrian basement rocks in the area in and south of the Black Canyon of the Gunnison, Colorado, and in Unaweap Canyon near the Colorado-Utah border. The dikes have been studied petrographically, petrochemically, and paleomagnetically, and they have been radiometrically dated. Lithologically, the dike rocks are very similar, consisting of tholeiitic diabase composed primarily of labradorite, augite, and granophyric intergrowths of quartz and alkali feldspar. A Rb-Sr isochron based on mineral separates from one dike yields an age of 495 ±15 m.y. which is statistically identical with a previously reported Rb-Sr date. Pooling the age data for these two dikes gives an age of 497 ± 16 m.y. Whole-rock K-Ar dates on four of the dikes are scattered around the Rb-Sr date and indicate that the dikes have been relatively unaffected by reheating or hydrothermal activity since emplacement. Paleomagnetic directional data from nine of the dikes are well grouped and consistent after both alternating-field (AF) and thermal demagnetization. Several lines of evidence indicate that the remanence reflects the dipolar geomagnetic field direction at the time the dikes were emplaced. Paleopoles corresponding to the mean directions are located at 37.0°N, 101.0°E, δp = 8.6°, δm = 16.4° (AF demagnetization) and 37.0°N, 102.3°E, δp = 4.9°, δm = 9.4° (thermal demagnetization). The consistency of the paleomagnetic data corroborates the idea that the dike swarm is the result of one short pulse of mantle-derived magmatic activity. The dike swarm and other Cambro-Ordovician igneous rocks in Colorado (Powderhorn alkalic complex, Wet Mountains alkalic complexes and dikes) are aligned along a linear west-northwest trend. Cambrian diabase dikes in northeastern New Mexico and a bimodal suite (basalt–gabbro-rhyolite–granite) in southern Oklahoma (Wichita and Arbuckle Mountains, Anadarko Basin) are on line with this trend. Petrologically, all of these Cambrian and Cambro-Ordovician igneous rocks represent types that are typically associated with extensional tectonic regimes. From these data and consideration of documented Paleozoic tectonic activity along the trend of the plutons, it is concluded that throughout the Paleozoic, a west-northwest–trending tectonic zone extended from southeastern Oklahoma, through northeastern New Mexico and Colorado, and into southeastern Utah. Bimodal (basalt-rhyolite) and alkalic igneous activity was restricted to the Cambrian and Cambro-Ordovician, but tectonic activity occurred intermittently along the zone throughout the Paleozoic.

Regional comparison of a Miocene geomagnetic transition in Oregon and Nevada

Abstract A reverse-to-normal geomagnetic transition has been recorded in a sequence of Late Miocene volcanics dated at about 15 my in the Santa Rosa Range, northcentral Nevada. This transition, probably the same as that previously described in southern Oregon, exhibits large-scale east-west oscillations of the local field directions. A composite transition path, made by combining those of Oregon and Nevada, is sufficiently complete to permit analysis of the relative contributions of the dipole and non-dipole components during the reversal through mathematical modeling. Conclusions are that the non-dipole components predominated for most of the transition period and that the best estimate of the minimum time required for the reversal is 2000 yr.

Pliocene and Pleistocene geologic and climatic evolution in the San Luis Valley of south-central Colorado

Rogers, K.L., Larson, E.E., Smith, G., Katzman, D., Smith, G.R., Cerling, T., Wang, Y., Baker, R.G., Lohmann, K.C., Repenning, C.A., Patterson P., and Mackie, G., 1992. Pliocene and Pleistocene geologic and climatic evolution in the San Luis Valley of south-central Colorado. Palaeogeogr., Palaeoclimatol., Palaeoecol., 94:55 86. Sediments of the Alamosa Formation spanning the upper part of the Gauss and most of the Matuyama Chrons were recovered by coring in the high (2300 m) San Luis Valley of south-central Colorado. The study site is located at the northern end of the Rio Grande rift. Lithologic changes in the core sediments provide evidence of events leading to integration of the San Luis drainage basin into the Rio Grande. The section, which includes the Huckleberry Ridge Ash (2.02 Ma) and spans the entire Matuyama Chron, contains pollen, and invertebrate and vertebrate fossils. Stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the San Luis Valley. At the end of the Olduvai Subchron, summer precipitation decreased, summer pan evaporation increased, and temperatures increased slightly compared to the earlier climate represented in the core. By the end of the Jaramillo Subchron, however, cold/wet and warm/dry cycles become evident and continue into the cold/wet regime associated with the deep-sea oxygenisotope Stage 22 glaciation previously determined from outcrops at the same locality. Correspondence between the Hansen Bluff climatic record and the deep-sea oxygen- isotope record (oxygen-isotope stages from about 110-18) is apparent, indicating that climate at Hansen Bluff was responding to global climatic changes.

Suitability of nonwelded pyroclastic-flow deposits for studies of magnetic secular variation: A test based on deposits emplaced at Mount St. Helens, Washington, in 1980

Paleomagnetic directions obtained from nonwelded pyroclastic-flow deposits that were emplaced at Mount St. Helens, Washington, in 1980 have a precision and accuracy similar to data obtainable from lava flows. It is concluded that nonwelded pyroclastic-flow deposits, like lava flows, are suitable for studies of magnetic secular variation. Although clast rotations apparently caused an “inclination error” in the paleomagnetic directions, the error is a few degrees at most.

Paleomagnetism of a miocene transition zone in southeastern Oregon

Abstract A transition in the Earths magnetic field, recorded in a sequence of basaltic lavas in southeastern Oregon (15.1 ± 0.3m.y.) , has been investigated. The reversal pattern is similar at the various sample sites (up to 55 miles apart) and the apparent paleo-intensity ranges from 0.5 Oe outside the transition to a minimum of 0.025 Oe inside the transition. This range is somewhat greater than that reported in other studies of transition zones. The similarity in transition pattern at the widely separated locations indicates that crustal magnetic anomalies could not have given rise to the recorded field. It is probable that the main dipole field and/or non-dipole centers were the primary features influencing the acquisition of TRM during the transition.

Lithology, chemistry, age, and origin of the Proterozoic Cardenas Basalt, Grand Canyon, Arizona

Abstract The Cardenas Basalt, a 300-m-thick flow sequence, occurs in the middle of the 4000-m-thick Proterozoic sedimentary sequence in the Grand Canyon. It consists of a 100-m-thick lower member composed of about six, poorly exposed, coarsely ophitic pahoehoe flows of olivine basalt. In contrast, the upper member, 200 m thick, comprises four to six, resistant, aphyric, intersertal to intergranular flows that range from quartz tholeiite to tholeiitic andesite (icelandite). Petrographically, mafic intrusions exposed throughout the Inner Gorge of the Grand Canyon are similar to the lower member. Newly acquired RbSr isotopic data, in combination with those previously available, define a well-constrained isochron (10 points) at 1103±66 Ma. This date is concordant with a RbSr date from one of the mafic intrusions, supporting the conclusion that the Grand Canyon mafic magmatic episode occurred ∼ 1.1 Ga. The magma that fed both the intrusions and the lower member appears to have been principally derived from a slightly enriched mantle source. Subsequently, this magma underwent ∼ 20% fractionation of olivine and, possibly, minor assimilation of continental crust. Differences in SiO2, A12O3, TiO2, and P2O5 contents between the lower member and intrusions, apparently reflect heterogeneities in the mantle source, or different degrees of melting and/or crustal assimilation. The quartz-tholeiite magma of the upper member apparently resulted from more-extensive fractionation and crustal contamination of a mantle-derived magma. After its formation, this magma underwent further fractionation to produce, tholeiitic andesite (icelandite).

New Virtual and Paleomagnetic Pole Positions from Isotopically Dated Precambrian Rocks in Wyoming, Montana, and Arizona: Their Significance in Establishing a North American Apparent Polar Wandering Path

Paleomagnetic directions have been determined from well-dated Precambrian rocks, in large part mafic dikes, in Wyoming, Montana, and Arizona. Ages, determined radiometrically, range from 700 to 2,550 m.y. When these data, together with the compilation of acceptable well-dated paleomagnetic poles from North America, are treated by a running-mean method, a generalized belt of apparent polar wandering is suggested. This belt is simple in form, and the rate of polar wandering is nearly uniform. The existence of large-scale loops of relatively rapid wander velocity is largely unsupported.

Thermomagnetic analysis of meteorites, 3. C3 and C4 chondrites

Abstract Thermomagnetic analysis was made on samples of all known C3 and C4 chondrites in a controlled oxygen atmosphere. Considerable variation was noted in the occurrence of magnetic minerals, comparable to the variation observed earlier in the C2 chondrites. Magnetite was found as the only major magnetic phase in samples of only three C3 chondrites (2–4 wt.%) and the Karoonda C4 chondrite (7.7 wt.%). The magnetite content of these three C3 chondrites is only about one-third that observed in the C1 and C2 chondrites which were found to contain magnetite as the only magnetic phase. Five C3 chondrites were observed to undergo chemical change during heating, producing magnetite: this behavior is characteristic of troilite oxidation. Upper limits on initial magnetite content of about 1–9% were established for these meteorites. Samples of the remaining five C3 chondrites and the Coolidge C4 chondrite were found to contain both magnetite and metallic iron. In two samples, iron containing ≤2% Ni was observed, while in the other four, the iron contained 6–8 wt.% Ni. In addition to containing both magnetite and iron metal, three of these samples reacted during heating to form additional magnetite. Variations in the magnetic mineralogy and, hence by inference bulk mineralogy, of C3 and C4 chondrites indicate a more complex genesis than is evident from whole-rock elemental abundance patterns.