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

We report paleomagnetic data for 26 accepted sites collected in two se ctions of flat-lying strata of the upper Carnian-lower Norian (similar to 225 Ma) Dockum Group, northwest Texas. Six additional sites in coa rse-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 the ir mean is inconsistent with the accepted upper Carnian-lower Norian r eference pole. In detail, 12 sites in grayish white (nonhematitic) san dstones have weak magnetizations (less than about 1 mA/m) carried by m agnetite or maghemite that give a paleopole at 56.4 degrees N-96.3 deg rees E (N=12 dual-polarity VGPs; K=44.2; A95=6.6 degrees) in close agr eement with other results for upper Carnian-lower Norian rocks in Nort h America. The 14 remaining sites in tan and red-colored (hematitic) s andstones, siltstones, and claystones give high unblocking temperature characteristic magnetizations carried by hematite, with paleopoles at 59.0 degrees N-53.8 degrees E (normal; N=7 VGPs, K-62.4, A95=7.6 degr ees) and 59.3 degrees N-77.8 degrees E (reverse; N=7 VGPs, K=204.2, A9 5=4.2 degrees). These poles fall along the younger track of poles and near the J-1 cusp of the North American APWP as defined by (unrotated) poles derived from Colorado plateau rocks. We suggest that the charac teristic 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 obser vation of conflicting magnetostratigraphies. The extreme westward posi tion of the poles derived from red bed sites, particularly those with normal polarity, confirms the general position of the J-1 cusp indicat ed by poles in the Piedmont province, the Newark basin, and the Colora do plateau; it also suggests that the magnitude of rotation of the Col orado plateau is no greater than about 5 degrees. We compile an appare nt polar wander path for North America, including Colorado plateau dat a, which suggests a fast rate of apparent polar wander throughout the Triassic period (about 0.8 degrees/m.y.) with a gradual increase that preceded the opening of the Atlantic.