Paleomagnetism of the Front Range (Colorado) Morrison Formation and an alternative model of Late Jurassic North American apparent polar wander

A paleomagnetic study of the Upper Jurassic Morrison Formation in the Front Range of central Colorado yields high-unblocking-temperature, dual-polarity magnetizations. With respect to known paleohorizontal, the inclinations (absolute mean = 57.3°, 95% confidence interval = 52.3° to 63.1°, N = 8 sites) pass tilt and reversal tests, whereas the dispersion in declinations can be attributed to apparent or real tectonic rotations and sedimentary processes. The site-centered colatitudinal locus of possible Front Range Morrison poles partially overlaps the "upper" pole, but it excludes the "lower" pole from the Morrison Formation on the Colorado Plateau as well as the 151 Ma Glance conglomerate pole from the Basin and Range province of southeastern Arizona. We offer various explanations for these disparities and suggest an alternative model of Late Jurassic North American apparent polar wander through ~70°N which is supported by Late Jurassic European poles (with positive stability tests) transferred to North American coordinates

Paleomagnetism of 122 Ma Plutons in New England and the Mid-Cretaceous Paleomagnetic Field in North America: True Polar Wander or Large-Scale Differential Mantle Motion?

A paleomagnetic study of Cretaceous White Mountains plutonic complexes in New Hampshire and Vermont yields high unblocking temperature, dual polarity magnetizations in different types of igneous rocks. The resulting pole position for three plutons (71.9° N, 187.4° E, A_95 = 6.9°, age = 122.5 Ma) agrees with previously published mid-Cretaceous poles for North America, which together give a mid-Cretaceous standstill reference pole slightly revised from Globerman and Irving [1988] at 71.2° N, 194.1° E (A_95 = 3.7°, N = 5 studies). We argue on the basis of the wide geographic distribution of these studies, the variety in tectonic settings and rock types, positive reversal tests, and an overall reversal pattern consistent with geomagnetic polarity time scales, that this mean pole represents the North American mid-Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). The standstill pole limits to within ±4°, the motion of the North American plate relative to the Earth's spin axis. During the same mid-Cretaceous interval, the New England hotspot track (124 Ma Monteregian Hills, 122.5-Ma Cretaceous White Mountains, and 103- to 84-Ma New England seamounts) requires 11°±4° of north-poleward motion of North America, in direct conflict with the paleomagnetic standstill. A similar (~13°) discrepancy is independently demonstrated between the spin axis and the Tristan da Cunha hotspot track on the African plate during the mid-Cretaceous interval. The hotspot/spin axis discrepancies ended by ~90 Ma when it is shown that both Atlantic hotspots agree with North American and African dipole paleolatitudes and present-day locations. Nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis, leaving as possible interpretations true polar wander and large-scale (but differential) mantle motion. The southerly motion of the mid-Cretaceous Louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics. The motions of the three widely separated mid-Cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid-Cretaceous "superplume.

High-Latitude Paleomagnetic Poles from Middle Jurassic Plutons and Moat Volcanics in New England and the Controversy Regarding Jurassic Apparent Polar Wander for North America

A paleomagnetic study of Middle Jurassic plutonic and volcanic rocks in New England (White Mountains Magma Series) yields high-latitude pole positions for North America. High unblocking temperature, moderate to high coercivity magnetizations of normal polarity have been isolated in three plutons (White Mountains batholith, Mount Monadnock, and the Belknap Mountains; mean age ~169 Ma), but the mean pole (88.4°N, 82.1°E, A_95 = 6.1°) is not distinguishable from the geographic axis and therefore the hypothesis that the plutons have been contaminated by recent field overprints can not be rejected. However, a dual polarity, high unblocking temperature, and high coercivity magnetization isolated from the Moat volcanics (169 Ma, Rb-Sr age) was apparently acquired soon after caldera collapse and tilting, at about the time of intrusion and cooling of the Conway granite (reported ages K-Ar biotite, 168 Ma; zircon fission track, 163 Ma). The Moat volcanics pole position (78.7°N, 90.3°E, dp = 7.1°, dm = 10.2°) calculated using the mean magnetization direction of reversed polarity (the Cr component) falls at high latitude but is distinguishable from the spin axis. Moreover, published Middle Jurassic paleomagnetic poles from Gondwana (Africa, Australia, and East Antarctica) transferred to the North American reference frame also suggest a high-latitude Middle Jurassic pole position for North America, in agreement with the Moat volcanics pole. The new evidence for a Middle Jurassic loop to high latitudes in the North American apparent polar wander path conflicts by 15°-20° with some key published Jurassic reference poles (e.g., the Newark Trend N2 and the Corral Canyon poles) used to constrain current paleomagnetic Euler pole (PEP) apparent polar wander paths for the Jurassic. We suggest that a plausible explanation for the discrepancy is that the N2 and Corral Canyon magnetizations are in fact secondary and were acquired after tilting. The hypothesis that the North American apparent polar wander path ventured to high latitude in the Middle Jurassic requires further testing, however the results of this study already suggest that the path may be more complicated than that proposed by recently published PEP studies

Miocene stable isotopic stratigraphy and magnetostratigraphy of Buff Bay, Jamaica

Previously reported biostratigraphic relationships from middle-upper Miocene sections exposed near Buff Bay, Jamaica (18°N, tropical bioprovince), differ from the subtropical North Atlantic (Sites 563 and 558). Time scales for this interval rely on correlations established at these subtropical sites, and the differences with the tropical section have implications to global correlations. Planktonic foraminiferal Zones N13 and N15 are thick at Buff Bay but are virtually absent at Sites 563 and 558; nannofossil Zone NN9 is associated with Zone N15 and uppermost Zone N14 at Buff Bay but is associated with Zone N16 at the other sites. Magnetostratigraphic data presented here further complicate the interpretation: Zone NN9 is associated with a thick normal magnetozone at Sites 563 and 558; at Buff Bay, it is associated with a thick reversed magnetozone. Although a secondary magnetization at Buff Bay makes it difficult to identify confidently Miocene normal magnetozones, the thick reversed magnetozone most likely represents the paleomagnetic field and correlates with Chron C5r. The magnetobiostratigraphic relationships require either diachrony of taxa or two mutually exclusive hiatuses in Jamaica and the North Atlantic. We address this problem by analyzing benthic foraminiferal δ^18O and δ^13C from the Buff Bay section. These isotopic data allow us to evaluate three hypotheses that reconcile the magneto-, bio-, and isotopic stratigraphic data and conclude that the first and last occurrences of five taxa were diachronous by ~0.3-0.5 m.y. between tropical and subtropical locations. This requires revised age estimates for late middle to early late Miocene biostratigraphic datum levels. We suggest that the ranges of several taxa are useful for endemic tropical or subtropical zonations, but correlations between the low and midlatitudes were affected by an increase in latitudinal thermal gradients during the late middle Miocene. However, we admit that further studies are needed before this issue is resolved

Integrated Paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy: DSDP Site 384 (NW Atlantic Ocean)

At Deep Sea Drilling Site 384 (J-Anomaly Ridge, Grand Banks Continental Rise, NW Atlantic Ocean) Paleocene nannofossil chalks and oozes (∼70 m thick) are unconformably/disconformably underlain (∼168 m; upper Maastrichtian) and overlain (∼98.7 m; upper lower Eocene) by sediments of comparable lithologies. The chalks are more indurated in stratigraphically higher levels of the Paleocene reflecting increasing amounts of biosiliceous (radiolarians and diatoms) components. This site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy. We report the results of a magnetostratigraphic study which, when incorporated with published magnetostratigraphic results, reveals an essentially complete magnetostratigraphic record spanning the interval from Magnetochron C31n (late Maastrichtian) to C25n (partim) (late Paleocene, Thanetian). Integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of, and constrain the estimated duration of, a short hiatus (∼0.9 my) within the younger part of Chron C29r (including the K/P boundary) and an ∼6 my hiatus separating upper Paleocene (Magnetozone C25n) and upper lower Eocene (Magnetozone C22r) sediments. Some 30 planktonic foraminiferal datum levels [including the criteria used to denote the Paleocene planktonic foraminiferal (sub)tropical zonal scheme of Berggren and Miller, Micropaleontology 34 (4) (1988) 362–380 and Berggren et al., SEPM Spec. Publ. 54 (1995) 129–212, Geol. Soc. Am. Bull. 107 (11) (1995) 1272–1287], and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology. Planktonic foraminiferal Subzones P4a and P4b of (upper Paleocene) Zone P4 are emended/redefined based on the discovery of a longer stratigraphic extension of Acarinina subsphaerica (into at last Magnetozone C25n). Stable isotope stratigraphies from benthic foraminifera and fine fraction (<38 μm) carbonate have been calibrated to the biochronology and magnetostratigraphy. A minimum in benthic foraminifer δ13C was reached near the Danian/Selandian boundary (within Chron C26r, planktonic foraminiferal Zone P3a and calcareous nannoplankton Zone NP4) and is followed by the rise to maximum δ13C values in the late Thanetian (near the base of C25n, in Zone P4c and NP9a, respectively) that can be used for global correlation in the Paleocene

Pass-through core measurements of magnetic susceptibility and natural gamma ray, New Jersey Coastal Plain

We measured magnetic susceptibility (MS) and core gamma radiation (CGR) on 3162 ft (963.9 m) of core recovered by the New Jersey Coastal Plain Drilling Project (Ocean Drilling Program Leg 150X) at Island Beach, Atlantic City, and Cape May, New Jersey. Integration of core lithology, core/log (MS and CGR), and downhole gamma-ray (DGR) log studies have (1) documented the core/log expression of previously determined unconformities; (2) shown that MS is a proxy for glauconite percent in the New Jersey Coastal Plain; (3) illustrated a major change in sedimentation from shelfal glauconite evidenced by very high MS values to deltaic deposition with low MS values in the earliest Miocene (ca. 22 Ma); (4) shown that comparison of MS and CGR with DGR can be used to resolve discrepancies in depth between downhole logs and cores; and (5) shown that the CGR detects some zones noted in the lithology (e.g., phosphate rich zones) that are not resolved in the DGR

Reply to Comment by Robert F. Butler, Steven R. May, and David R. Bazard on "High-Latitude Paleomagnetic Poles From Middle Jurassic Plutons and Moat Volcanics in New England . . ."

Butler et al. [this issue] question the reliability of our 166 Ma Moat volcanics pole (82øN, 090øE, A95 = 5.6 ø) and high-latitude model for Middle Jurassic North American apparent polar wander (APW) [Van Fossen and Kent, 1990]. They review arguments in support of the 172 Ma Corral Canyon result (62øN, 116øE, A95 = 6.2 ø [May et al., 1986]) as a key Middle Jurassic reference pole for their lower-latitude palcomagnetic Euler pole model of Jurassic North American APW [May and Butler, 1986]. In this reply we address their concerns and maintain that the Corral Canyon pole and the lower-latitude model of APW it contributes to are suspect in light of global Jurassic palcomagnetic data and especially recent palcomagnetic studies that document tectonic rotations in the Basin and Range province of southern Arizona where the Corral Canyon pole was obtained

Paleomagnetic study of 143 Ma kimberlite dikes in central New York State

A palaeomagnetic study of 143 Ma serpentinized kimberlite dikes near Ithaca, New York, yields a dual-polarity, high unblocking temperature, and high coercivity magnetization which passes the reversal test and two baked contact tests. The mean pole position (58"N, 203"E; A95 = 3.8", N = 7) differs from published late Jurassic-early Cretaceous North American poles currently used to define the apparent polar wander path. The angular dispersion in mean directions (863 =3.5") is low but the presence of reversals argues that the Ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation. Similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite, supporting the suggestion that thermo-chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike. A consistent but weak foliation in anisotropy of ARM parallels the N-S and vertical orientation of the Ithaca dikes, but ips aparently unrelated to the northwest-down or southeast-up remanence. The Ithaca kimberlites pole may therefore record a previously undocumented sharp bend or 'cusp' at -143Ma, and the initiation of a Cretaceous and Cenozoic interval of apparent polar wander that generally follows a great circle along the 200"E meridian to geographic north. A coeval (-145 Ma) kimberlite pole from southern Africa transferred to North America agrees with the Ithaca kimberlites pole position whereas reported poles from the Berriasian stratotype (southern France) and 144Ma Svalbard dolerites provide less diagnostic tests of the Ithaca kimberlites pole due in part to uncertainties in Europe-North America reconstructions

Glass From The Cretaceous Tertiary Boundary In Haiti

Tektite-like glasses preserved at the Cretaceous/Tertiary boundary at Beloc in Haiti provide clear evidence of an impact event. The glass composition suggests that the impact occurred on a continental shelf region, generating a silica-rich glass with chemical composition that reflects the melting of continental crustal rocks, and a calcium-rich glass produced by the fusion of marl sediments. These findings indicate that catastrophic release to the atmosphere of 10(15) moles of CO2 from vaporized marl occurred during the impact.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62562/1/349482a0.pd