Michael T. Lewchuk

Sedimentologic-magnetic record of western Pangean climate in upper Paleozoic loessite (lower Cutler beds, Utah)

Sedimentologic, pedologic, and magnetic data within the upper Paleozoic lower Cutler beds of the southwestern Paradox basin (Utah) record high- and low-frequency climate changes that operated at equatorial latitudes of western Pangea. The lower Cutler beds consist of ∼250 m of lithified eolian silt (loessite) and marine-reworked and fluvially reworked loessite, with abundant intercalated paleosols comprising Protosols, Argillisols, and Calcisols. The evolution from loessite and marine-reworked loessite with abundant Calcisols in the lower section to loessite and fluvially reworked loessite with abundant Argillisols in the upper section records a long-term transition from semiarid conditions in western equatorial Pangea in latest Pennsylvanian time to seasonally wet conditions in earliest Permian time. This shift could record intensification of the Pangean megamonsoon and associated seasonal incursions of moisture-laden westerlies. Paleosols record relatively high-frequency fluctuations between drier, dustier glacials and wetter interglacials of the late Paleozoic. Bulk magnetic-susceptibility values in paleosols exhibit variations that track paleosol type and are significantly elevated relative to parent loessite, attributable to the occurrence of both ultrafine-grained (superparamagnetic) and coarser-grained (remanence-carrying) magnetite. This signature reflects in situ pedogenic production of ferrimagnetic phases and a subordinate component of allochthonous, magnetic dust influx during pedogenesis, analogous to processes inferred for the magnetic signature in the Pliocene–Pleistocene loess-paleosol sequences of, e.g., the Chinese Loess Plateau. Integration of sedimentologic, geochemical, and magnetic data further suggests that enhancement of magnetic susceptibility in loessitic paleosols of this section relates primarily to climatic conditions and secondarily to durations of pedogenesis. Whereas peak susceptibility values in mature paleosols (Argillisols and Calcisols) do not vary significantly through the study section, peak values for Protosols track facies evidence for wetter conditions through time. Accordingly, relative changes in paleosol susceptibility values can provide paleoclimatic information, but should be integrated with other data to fully assess the origin of the signature. Overall, our data document the applicability of analytical approaches used on recent loess to very ancient loessite; this result is significant, because loess commonly records high- resolution evidence of terrestrial climate and climate change.

Correlating paleomagnetic, geochemical and petrographic evidence to date diagenetic and fluid flow events in the Mississippian Turner Valley Formation, Moose Field, Alberta, Canada

Abstract Petrographic, geochemical and paleomagnetic analyses of the Mississippian Turner Valley Formation provide constraints on diagenesis and fluid flow events in the Western Canada Sedimentary Basin. Paleomagnetic plugs and companion geochemical samples were taken from two drillcores, with Fullbore MicroImage log orientations. Dolomite from both wells yielded two magnetization directions. The low-temperature, low-coercivity direction is a drilling-induced remanence rather than a viscous remanent magnetization. The high-temperature, high-coercivity remanence direction is Cretaceous, and there is no sign of a primary Mississippian direction. Geochemical analyses of matrix dolomite yield δ18O values ranging from 0.65 to −3.34‰ (VPDB standard) and δ13C values ranging from 1.77 to 3.05‰ VPDB. The least depleted samples have stable isotope values consistent with, or only slightly depleted from, postulated Mississippian dolomite values. The remaining sample values exhibit a negative covariant trend consistent with either mixing with another diagenetic fluid or recrystallization during burial. Petrographic analysis reveals the presence of a recrystallization event that caused zoning and a gradual increase size of the dolomite crystals. This event is thought to have caused both the Cretaceous paleomagnetic remanence and the altered geochemical values. The minor enrichment in Sr radiogenic isotopes, relative to coeval seawater values, suggests that both an extrabasinal source for any fluid and large-scale fluid flow are unlikely. The results also indicate that magnetic remanences are very sensitive to visually minor changes in carbonate recrystallization from heat or pressure, so that great care must be taken in correlating paleomagnetic and geochemical data.

Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada

Abstract The Coldwell Complex is a large multiphase alkaline intrusion on the north shore of Lake Superior. It was emplaced into Archean metavolcanics of the Superior Province in at least three distinct magmatic episodes during the Late Precambrian, giving isotopic ages ranging from 1010 to 1188 Ma. Detailed AF and thermal demagnetization was done on 416 specimens from 33 sites representing all petrologic phases. Additional data are included from an unpublished 1972 study by W.A. Robertson. The oldest magnetic component, PCA, was found in 19 episode I sites with a reversed mean direction of D = 121.0°, I = − 70.9° (κ = 104, α95 = 3.3°). Its pole position falls on the Keweenawan APWP at an age of 1109 ± 5 Ma. The second component, PCB, was found in 11 sites with a normal mean of D = 301.8°, I = 60.1° (κ = 49, α95 = 5.8°) that indicates an age of 1103 ± 5 Ma. The youngest component, PCC, was found in nine episode III sites and gives a reversed mean of D = 119.1°, I = − 54.3° (κ = 101, α95 = 5.1°) that indicates an age of 1095 ± 5 Ma. The assigned ages agree closely with recent U/Pb zircon ages but are distinctly older than the K/Ar and Rb/Sr ages. These paleomagnetic results require some changes in existing models for the petrologic evolution of the complex and indicate a much more rapid three-stage cooling history than previously postulated. They also provide strong evidence against the proposed concept of an asymmetrical geomagnetic field in Middle Keweenawan time.

Paleomagnetism of the Clay-Howells Carbonatite Complex: Constraints on proterozoic motion in the Kapuskasing Structural Zone, Superior Province, Canada

Abstract The Clay-Howells Complex is located approximately 130 km east of Hearst, Ontario, at 49°50′N, 82°05′W near the north end of the Kapuskasing Structural Zone (KSZ) of the Superior Province in the Canadian Shield. The pluton is a large oviform pluton of about 16 km2 that is composed dominantly of syenite with minor Carbonatite. It was emplaced into an Archean gneissic terrain of amphibolite to granulite facies with Middle Precambrian diabase dikes. The complex is unmetamorphosed and Late Precambrian in age (Rb/Sr 1075 ± 15 Ma). Multistep alternating field (AF) and thermal demagnetization was carried out on 194 specimens from 18 sites in the intrusion and three sites in the host rock dikes and gneisses. The 18 syenite sites give a unit mean direction of 294.2°, 27.1° ( N = 18 , k = 26 , α 95 = 7.0° ) which yields a pole position of 178.8° E, 26.5°N ( δ p = 4.1° , δ m = 7.6° ) for the Clay-Howells Complex. A crude contact test using the three host rock sites proved inconclusive. The pole for the intrusion is concordant, falling at about 1080 ± 10 Ma on the Keweenawan apparent polar wander path which agrees with the Rb/Sr age. Thus the complex has an untilted, primary remanence that indicates that there has been no significant uplift (

Reaimantation regionale eocene, migration de fluides et mineralisations sur la bordure cevenole (France)

Widespread remagnetization has been identified in fold belts and forelands inboard of mountain ranges and has usually been interpreted as resulting from fluid migration related to orogenesis in these mountain ranges. The geochemical properties of these fluids should be compatible with the formation or the transformation of ferrimagnetic minerals, thus allowing acquisition of remanent magnetization during fluid migration. Carbonate hosted lead-zinc (+ or -barite and fluorite) mineralization of the Mississippi Valley-type (MVT) are also generally considered to have formed during the migration of enormous volumes of fluids and are commonly located in foreland fold belts or their forelands. This suggests a similar origin for widespread carbonate remagnetization and MVT mineralization. The paleomagnetic dating of MVT deposits has been successfully applied in MVT districts, mainly of North America. Thus, it was used for the MVT deposits hosted by Mesozoic carbonate rocks of the Cevennes region of southern France. In view of the structurally complexity in the region, and because the results presented here are intended to provide an initial reference direction on which further results can be based, only sites belonging to the most stable parts of this border are considered here. Most samples were collected from three surface sites in the area of the Largentiee mine and from independently oriented cores of the two deep boreholes (Balazuc and Morte-Merie) from the Geologie Profonde de la France program. Several samples were also collected from three sites around the area of the Saint Felix-de-Pallieres mine. All these samples come from different stratigraphic levels: Carboniferous, Permian, Triassic, Liassic and Middle Jurassic.

Remagnetization signature of Paleozoic sedimentary rocks from the Patterson Creek Mountain anticline in West Virginia

Abstract Paleomagnetic analysis of five adjacent Lower Silurian to Lower Devonian sedimentary units (Oriskany, Helderberg, Tonoloway, Williamsport and McKenzie) plus a carbonate vein, within a single large fold in West Virginia, reveals a secondary, reversed, Permian, magnetization in all rocks. Similar unblocking temperatures (∼350–550 °C) were observed for the characteristic magnetization throughout the study implying similar origins for their magnetization. Unfolding estimates, with 95% confidence intervals, were, Helderberg (60±11%), Tonoloway (69±9%), Williamsport (65±25%), Oriskany (78±11%) and McKenzie (80±4%). Since all units were sampled on a single fold, their burial, temperature and stress histories should be identical. Identification of the mechanism for remagnetization and an explanation of the differences in the fold test results involve awkward interpretations. Hypotheses for remagnetization mechanisms such as fluid migration, clay alteration, partial thermoviscous resetting and strain all appear to be flawed for this data set and true differences in the timing of remagnetization relative to folding would require a very complex multi-stage remagnetization event. The actual mechanism for remagnetization may yet be unidentified. An alternative possibility is that these units have been affected by multiple remagnetizing mechanisms simultaneously and then the magnetization has been modified by minor strain to produce the variation in the results of the fold tests.

Age and duration of the Mississippi Valley-type mineralizing fluid flow event in the Viburnum Trend, southeast Missouri, USA, determined from palaeomagnetism

Abstract The Viburnum Trend is a world-class Mississippi Valley-type (MVT) lead-zinc ore deposit in platform carbonates of the Upper Cambrian Bonneterre Dolomite in the midcontinent of the USA. Palaeomagnetic methods have been used to analyse 233 specimens from early octahedral (nine sites) and late-stage cubic (13 sites) galena ore from four mines along the c. 70 km north-south length of the Trend. The characteristic remanence is carried by single to pseudo-single domain pyrrhotite and magnetite. This is the first MVT deposit in which pyrrhotite is shown to be a remanence carrier and present in galena crystals. The remanence directions define an Early Permian mean age of 273 ± 10 Ma for the ore-stage mineralization, a maximum duration for the mineralization event of 12 Ma, and a time difference of 5 Ma between the early octahedral and late cubic galena ore stages. The Early Permian age for the ore is consistent with models of ore genesis that invoke fluid flow from the Ouachita orogen during Ouachitan orogenesis.