The Erieau Excursion revisited

Paleomagnetic results from two cores collected from the central and eastern basins of Lake Erie contain the same remanence direction shift from positive to negative inclination at a depth of approximately 9 m. Confirmation of the existence of this boundary over a wider extent of Lake Erie may initially appear to support the interpretation of this feature as the record of a geomagnetic excursion: the Erieau Excursion. Many details in the paleomagnetic record of these cores indicate that this feature is not of geomagnetic origin: the lack of synchroneity of this event across the lake; the association of the reversed directions with deformed sediments; the lack of any record of the transitional magnetic field; and the absence of any correlation between the reversed directions from the two cores. In core EK, the apparent magnetic transition is coincident with the stratigraphic boundary between the lacustrine and glaciogenic sediments. The model most compatible with this new paleomagnetic data involves regional deformation of the substrate of Lake Erie by some mechanism prior to 9500 years ago. The most likely mechanism to have produced this regional deformation was subglacial deformation of the underlying sediments.

Northeast-trending Early Proterozoic dykes of southern Superior Province: multiple episodes of emplacement recognized from integrated paleomagnetism and U–Pb geochronology

Integrated paleomagnetic and U–Pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of Early Proterozoic tholeiitic dykes of northeast trend in the southern Superior Province, previously referred to collectively as Preissac dykes. It is demonstrated that they are readily separated on the basis of paleomagnetism into subsets, referred to as the Biscotasing and Senneterre swarms. In addition a pair of unnamed dykes may be associated with the north-and northwest-trending Matachewan swarm farther west.Biscotasing dykes have a down-west magnetization of single polarity with a corresponding paleopole at 27.8°N, 136.7°W (dm = 12.3° and dp = 9.4°). Senneterre dykes carry an up-north (or occasionally down-south) direction with corresponding paleopole at 15.3°S, 75.7°W (dm = 7.0°, dp = 4.4°). The Senneterre direction is indistinguishable from the primary N1 remanence direction that dominates the magnetization of Nipissing sills of the Southern Province. Paleomagnetic field tests described herein or in earlier studies indicate that Biscotasing and Senneterre directions are primary and, hence, that two ages of intrusion are involved, with the age of Senneterre dykes coinciding with the intrusion of most Nipissing sills. U–Pb dating of baddeleyite conducted at a paleomagnetic sampling site yields an age of 2214.3 ± 12.4 Ma for the Senneterre swarm, indistinguishable from the age of 2217.2 ± 4 Ma reported from an N1 Nipissing sill site in another study. A U–Pb age on baddeleyite and zircon of 2166.7 ± 1.4 Ma was obtained from a paleomagnetic site in the Biscotasing swarm. The primary paleopoles for the Senneterre, Nipissing, and Biscotasing rocks define a direction of polar wander opposite to that of the most widely used polar wander paths for North America for this period, suggesting that these paths should no longer be used.

Paleomagnetic dating of the transformation of oolitic goethite to hematite in iron ore

We show that the oolitic hematite ores of Birmingham, Alabama, carry a remanence direction stable enough to have survived major folding. Nevertheless, this remanence was very likely acquired in the Pennsylvanian, about 130 Ma after the ore's Early Silurian deposition. This long delay is understandable if we accept the common hypothesis that the ore originated as oolitic goethite. We would then not expect acquisition of a stable remanence until deep burial in the Pennsylvanian raised the temperature to the ~80 °C probably needed to transform coarse goethite to hematite in water. Although the paleomagnetism of oolitic hematite may be of limited value in defining primary paleopoles, it may allow dating of when the ore's temperature was first raised to ~80 °C, perhaps aiding studies of sedimentary basin evolution and oil formation. Our results also support using caution in interpreting the paleomagnetism of more common red beds, emphasizing that a positive fold test is not necessarily evidence of primary remanence and that burial history may control the timing of remanence acquisition.

Paleomagnetism of the Little Dal lavas, Mackenzie Mountains, Northwest Territories, Canada

The Proterozoic stratigraphic column of the Mackenzie Mountains is dominated by two main successions, the platformal "Mackenzie Mountains Supergroup" beneath, and the Windermere-equivalent Rapitan Group and younger strata, of rift-depression and slope origin, above. The former succession is at least partly older than 770 Ma, the latter younger than 770 Ma. These two main successions are locally separated by an unconformity-bounded succession, the "copper cycle." An important question is whether the copper cycle is more closely related in time and in origin to the older or the younger main succession. Determination of the paleomagnetism of the basaltic lavas locally preserved at the top of the Little Dal Group (top of the Mackenzie Mountains Supergroup) and comparison of their remanence directions with those published for other rocks bearing on the question were thought to be one way of shedding light on this question. Accordingly, paleomagnetic investigation of 10 sites in the Keele River area and six sites in the Thundercloud Range area was undertaken to obtain the remanence direction related to the initial extrusion of the lavas. Coherent directional groupings were only obtained from Little Dal lavas in the Keele River area. Of the three magnetizations found, LD-L (D = 304°, I = 20°, α95 = 7°) is assumed to represent the magnetization acquired on crystallization of the lavas. If this assumption is correct, the significant difference from the direction LD-A (D = 265°, I = 26°, α95 = 4°) reported elsewhere for strata low in the Little Dal Group suggests either that the lavas significantly postdate the group or that significant movement of the North American plate occurred during accumulation of the 2 km or so of platformal strata between the lower Little Dal beds and the lavas. The new results presented here also admit the conclusion that the Little Dal lavas do not represent the same igneous events as diabase intrusions dated at about 770 Ma that cut the lower Little Dal.

Defining a paleomagnetic polarity pattern in the Monteregian intrusives

Oka and nearby small plutons on the western end of the Monteregian Hills were sampled for paleomagnetic study at 43 sites (569 specimens). Every specimen was AF step demagnetized in 4 kA/m increments to 20 or 24 kA/m. Consistent remanence directions were found for 36 sites (452 specimens). Use of a stability index to select only those specimens with the best defined end points does not improve the site statistics. The Oka, Brilund, Carillon, and Ile Cadieux plutons have statistically similar mean remanence direction populations which are different from the Ste. Dorothée sill direction. Except for one Carillon site, all site mean directions are normally polarized, whereas all nine plutons, except for Mt. Johnson, from the middle and eastern end of the Monteregian Hills are reversely polarized. Normally and reversely polarized plutons give statistically similar but antiparallel pole positions, giving a combined pole position of 169.0°W, 72.4°N (δp = 2.8°, δm = 3.7°), which is consistent with the 120 ± 4 Ma radiometric age. The polarity pattern evidence suggests that Oka and adjacent plutons were emplaced rapidly during one normal polarity interval, and that the Monteregian Hills plutons were emplaced progressively from west to east during two normal and two reversed polarity intervals lasting ~ 2 Ma. This leads to some speculations on the plume and rift modes of emplacement.

Paleomagnetism of Igneous Rock Units from the Coast of Labrador

Paleomagnetic results are reported for the first time from the coast of Labrador. The rocks are from (1) 20 sites of gently folded basalts of the Mugford Series, dated 948 ± 90 m.y., near Cape Mugford, and (2) 12 gabbroic dikes in a swarm dated 2080 ± 42 m.y., near Indian Harbour. Both dates are from preliminary, published K–Ar determinations. After partial alternating-field (AF) or thermal demagnetization, nearly all samples from both rock units revealed a stable thermoremanence that is most probably of primary origin. For the Mugford basalts, a mean remanence direction relative to bedding planes was calculated, after AF treatment, from 16 stably magnetized sites ([Formula: see text] flows). The corresponding pole is 49 °N, 143 °W, with dp = 9°, dm = 11°, which is northeastward of most North American poles from rocks dated 1200-1000 m.y. The pole calculated from the 12 Indian Harbour dikes, after AF treatment, is 6 °S, 117 °W, with dp = 6°, dm = 12°.