Abstract

Dolostone of the St. Charles Formation (Upper Cambrian and Lower Ordovician) and limestone of the Garden City Limestone (Lower Ordovician) both yield low-inclination characteristic magnetizations upon progressive thermal demagnetization. Susceptibility and alternating-field demagnetization data indicate that the magnetizations reside in magnetite. The polarity of nearly all samples from the St. Charles is reversed, whereas the polarity of the Garden City is normal. The two directions are not significantly different from antipodal; together they yield a north paleomagnetic pole at 63° N., 52° E., alpha −95 = 6° (six sites averaged), near a Late Triassic pole.

Remagnetization during Triassic time is the most likely origin of these characteristic magnetizations. The low inclination of both magnetizations makes an age younger than Triassic unlikely and, in any case, both antedate Late Jurassic-Early Tertiary folding of the Paris allochthon. The mechanisms of remagnetization are not established, but at least one may result from authigenic magnetite precipitated from through-flowing brines as has been recently demonstrated to occur in other early Paleozoic carbonates in North America. Development of a sedimentary basin to the west in the Late Triassic, in association with uplift farther to the west, may have resulted in over-pressures that caused an influx of deep basinal brines.

One magnetization may have resulted from the setting of a viscous partial thermoremanence (VPTRM); observed conodont color alteration index (CAI) of 4 indicates that burial heating was about 190° C. If one of the characteristic magnetizations is a Triassic VPTRM, maximum heating in these strata was reached before its imposition. In any case, the rocks were never hot enough to obliterate the observed ancient magnetizations.

Minimum temperatures reached by Upper Cambrian and Lower Ordovician rocks, measured by observed CAI indices, suggest that either the geothermal gradient was depressed during the early Mesozoic or the Paris allochthon was not buried as deeply as indicated by palinspastically restored stratigraphic thicknesses in the Bear River Range area.