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New magnetostratigraphy and paleopole from the Whitmore Point Member of the Moenave Formation at Kanab, Utah

Steiner M.

Volumina Jurassica

2014

Vol. 12, no. 2

13--22

The entire Whitmore Point Member of the Moenave Formation was sampled in close stratigraphic sequence (+0.3 m) from a vertical exposure in southwestern Utah. The polarity sequence in the Whitmore Point Member is essentially normal polarity, with five or more very short reversed intervals interspersed and a <1 m reversed interval at the top of the sequence. This polarity pattern dates the Whitmore Point Member as Hettangian. In the earliest Jurassic, the North American plate rotated even further westward from its Late Triassic position, and the movement appears to have been accompanied by an abrupt increase in plate motion because of the similarity in position of many Late Triassic paleopoles. The Moenave pole forms the ‘J-1 cusp’ of the North American apparent polar wander curve. The paleopole obtained by this study is somewhat further westward than those of previous studies. Within the 27 m of a mostly normal polarity sequence, the data show multiple, exceedingly short polarity intervals. The magnetization carrier is a maghemite-magnetite mineral, with the magnetization of an additional hematite carrier superposed. The lithostratigraphic sequence of the Moenave Formation is terminated by an unconformable surface, overlain by the Springdale Sandstone. Paleomagnetic directions of the Whitmore Point Member are exceedingly similar to those of the overlying Springdale Sandstone. Even though the two lithologic bodies are separated by a clear disconformity, the similarity in pole positions suggests that the two are closely related in time. It is possible that this disconformity represents the termination of the westward excursion of North America in earliest Jurassic time.The entire Whitmore Point Member of the Moenave Formation was sampled in close stratigraphic sequence (+0.3 m) from a vertical exposure in southwestern Utah. The polarity sequence in the Whitmore Point Member is essentially normal polarity, with five or more very short reversed intervals interspersed and a <1 m reversed interval at the top of the sequence. This polarity pattern dates the Whitmore Point Member as Hettangian. In the earliest Jurassic, the North American plate rotated even further westward from its Late Triassic position, and the movement appears to have been accompanied by an abrupt increase in plate motion because of the similarity in position of many Late Triassic paleopoles. The Moenave pole forms the ‘J-1 cusp’ of the North American apparent polar wander curve. The paleopole obtained by this study is somewhat further westward than those of previous studies. Within the 27 m of a mostly normal polarity sequence, the data show multiple, exceedingly short polarity intervals. The magnetization carrier is a maghemite-magnetite mineral, with the magnetization of an additional hematite carrier superposed. The lithostratigraphic sequence of the Moenave Formation is terminated by an unconformable surface, overlain by the Springdale Sandstone. Paleomagnetic directions of the Whitmore Point Member are exceedingly similar to those of the overlying Springdale Sandstone. Even though the two lithologic bodies are separated by a clear disconformity, the similarity in pole positions suggests that the two are closely related in time. It is possible that this disconformity represents the termination of the westward excursion of North America in earliest Jurassic time.

The Whitmore Point Member of the Moenave Formation: Early Jurassic Dryland Lakes on the Colorado Plateau, Southwestern USA

Tanner L., Lucas S.

Volumina Jurassica

2008

Vol. 6, no. 1

11-21

The Lower Jurassic Whitmore Point Member of the Moenave Formation in Arizona-Utah, USA, comprises fish- and coprolite-bearing shales, siltstones, sandstones, and minor limestones. These facies were deposited in ephemeral and perennial lakes subject to episodic desiccation and incursions of coarse clastics during floods. Meromictic conditions developed during perennial episodes, probably due to salinity stratification, which enhanced preservation of organic matter in gray to black shales. These lakes formed on the floodout of a north-northwest oriented (relative to modern geography) system of mainly ephemeral streams on a broad and open floodplain. The Whitmore Point Member both overlies and interfingers laterally with alluvial red-bed facies of the Dinosaur Canyon Member of the Moenave Formation. The vertical transition from alluvial to lacustrine sedimentation recorded by the Dinosaur Canyon and Whitmore Point members of the Moenave Formation most probably resulted from a eustatically-controlled rise in base level during the Early Jurassic (Hettangian). The Dinosaur Canyon Member also interfingers laterally with eolian dune deposits of the Wingate Sandstone, which was deposited by winds that reworked coastal plain sediments to the north of the study area. Thus, on this part of the Colorado Plateau, fluvial, lacustrine and eolian sedimentary facies were deposited contemporaneously in laterally adjacent paleoenvironments.