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
Rs. Molinagarza et al., 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, Tectonics, 14(4), 1995, pp. 979-993
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.