Marine rocks of the Santa Rosalía basin, Baja California Sur, were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on Gulf of California rifting. Progressive demagnetization of samples from the Boleo Formation, the earliest marine sequence overlying the deeply eroded basement, reveals a high-coercivity characteristic remanent magnetization (ChRM) in addition to a low-coercivity overprint. The ChRM appears to be a primary magnetic remanence with stratigraphically bound normal- and reversed-polarity directions. A single 40Ar/39Ar isotopic age of 6.76 ± 0.90 Ma (2σ) was obtained for the cinta colorada, a tephra deposit of reversed paleomagnetic polarity within the Boleo Formation. The age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed-polarity intervals of the geomagnetic polarity time scale (GPTS) in the ±2σ range 5.86–7.66 Ma. The interval with the highest probability is 6.57–6.94 Ma. In conjunction with the isotopic age, preliminary magnetostratigraphy of the Boleo Formation is correlated with the GPTS in order to further delineate the onset of marine sedimentation. The most likely correlation yields an age of 6.93–7.09 Ma (GPTS subchron C3Bn) for the base of the Boleo Formation and 6.14–6.27 Ma for the top. This correlation, indicating an average sedimentation rate of 28 ± 4 cm/k.y., could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the Boleo Formation. However, even if the isotopic age of the cinta colorada is used as the only age constraint, the result is consistent with data from the northern Gulf of California and shows that rifting started much earlier than the ca. 3.6 Ma commencement of sea-floor spreading at the mouth of the Gulf of California. The deposition of the Boleo Formation was probably related to an early phase of gulf rifting caused by a change in Pacific–North American plate motions.

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