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

Steiner M.

Volumina Jurassica

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2014

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Vol. 12, no. 2

13--22

EN

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.

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Magnetostratigraphy and paleopoles of the Kayenta Formation and the Tenney Canyon Tongue

Steiner M., Tanner L.

Volumina Jurassica

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2014

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Vol. 12, no. 2

31--38

EN

The Kayenta Formation is the third in a series of stratigraphic units making up the Glen Canyon Group that were sampled along US Hwy 89 in southern Utah. The Kayenta is dominantly reversed polarity with a number of very short normal polarity intervals. Above the Kayenta and interbedded in the Navajo Sandstone is the Tenney Canyon Tongue of the Kayenta Formation. The lower half of the Tenney Canyon Tongue was also sampled and is dominantly normal polarity with three short reversed polarity intervals. The dominantly reversed magnetostratigraphy of the Kayenta appears to match that of Early Pliensbachian polarity interval “e-Pli R.” The dominance of normal polarity of the Tenney Canyon Tongue suggests that the Tenney Canyon may have been deposited in the upper half of the Pliensbachian polarity interval “ePli-N.” The suggested polarity matches indicate that the Kayenta and Tenney Canyon Tongue strata are 187–190 Ma in age. The paleopoles of the two units are statistically identical. The combined data of the Kayenta-Springdale-Whitmore Point show that the J-1 cusp terminated before the deposition of the Kayenta Formation. The North American continent/pole returned to its Late Triassic position during/after Springdale time, apparently along the same path used to reach the apex of the J-1 cusp.

3

Age of Lower Jurassic Springdale Sandstone of Southwestern Utah from its Magnetic Polarity Sequence

Steiner M.

Volumina Jurassica

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2014

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Vol. 12, no. 2

23--30

EN

The Springdale Sandstone records a large number of reversals of the geomagnetic field. Forty-three percent of the 60 m section investigated showed 25 polarity intervals. These frequent reversals follow the nearly constant normal polarity of the underlying Whitmore Point strata. Comparison of the reversal sequence of the Springdale Sandstone with a core from the Paris Basin suggests that the Springdale Sandstone spans an interval extending possibly from as early as late Hettangian to early-mid Sinemurian time. The paleomagnetic pole calculated from Springdale paleomagnetic directions is statistically identical to that of the underlying Whitmore Point Member of the Moenave Formation, and considerably different from that of the overlying Kayenta Formation. This similarity suggests that the Springdale Sandstone should be retained as a member of the Moenave Formation, not considered part of the Kayenta Formation.