Paleomagnetism of the Dockum Group (Upper Triassic), northwest Texas: Further evidence for the J‐1 cusp in the North America apparent polar wander path and implications for rate of Triassic apparent polar wander and Colorado plateau rotation

Abstract

We report paleomagnetic data for 26 accepted sites collected in two sections of flat‐lying strata of the upper Carnian‐lower Norian (∼225 Ma) Dockum Group, northwest Texas. Six additional sites in coarse‐grained conglomeratic sandstones gave no usable results. The total assemblage of 26 VGPs is streaked along the Late Triassic ‐ earliest Jurassic track of the North America apparent polar wander path and their mean is inconsistent with the accepted upper Carnian‐lower Norian reference pole. In detail, 12 sites in grayish white (nonhematitic) sandstones have weak magnetizations (less than about 1 mA/m) carried by magnetite or maghemite that give a paleopole at 56.4°N–96.3°E (N=12 dual‐polarity VGPs; K=44.2; A95=6.6°) in close agreement with other results for upper Carnian‐lower Norian rocks in North America. The 14 remaining sites in tan and redcolored (hematitic) sandstones, siltstones, and claystones give high unblocking temperature characteristic magnetizations carried by hematite, with paleopoles at 59.0°N–53.8°E (normal; N=7 VGPs, K=62.4, A95=7.6°) and 59.3°N–77.8°E (reverse; N=7 VGPs, K=204.2, A95=4.2°). These poles fall along the younger track of poles and near the J‐I cusp of the North American APWP as defined by (unrotated) poles derived from Colorado plateau rocks. We suggest that the characteristic magnetization of the non‐hematitic sandstones is an “early” magnetization, acquired during or soon after deposition. However, the characteristic magnetization of the red bed sites is interpreted as a secondary magnetization, for which we infer an earliest Jurassic age. The secondary origin for this magnetization is supported by the observation of conflicting magnetostratigraphies. The extreme westward position of the poles derived from red bed sites, particularly those with normal polarity, confirms the general position of the J‐I cusp indicated by poles in the Piedmont province, the Newark basin, and the Colorado plateau; it also suggests that the magnitude of rotation of the Colorado plateau is no greater than about 5°. We compile an apparent polar wander path for North America, including Colorado plateau data, which suggests a fast rate of apparent polar wander throughout the Triassic period (about 0.8°/m.y.) with a gradual increase that preceded the opening of the Atlantic.