Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia

摘要

The Pahrump Group in the Death Valley region of eastern California records a rich history of Mesoproterozoic to Neoproterozoic tectonic, climatic, and biotic events. These include the formation, stability, and onset of rifting of the Rodinia superconti-nent, two potentially low-latitude glaciations correlative with global "snowball Earth" glacial intervals, and the onset of complex microbiota (e.g., testate amoebae). Poor direct age control, however, has significantly hindered the progress of understanding of these important stratigraphic units. New LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) detrital zircon data from clastic units directly overlying a major unconformity within the Mesoproterozoic Crystal Spring Formation provide a maximum depositional age of 787 ± 11 Ma for the upper member of the Crystal Spring Formation. This unconformity, representing a duration of ≥300 Ma, is now recognized in sedimentary successions across southwestern Laurentia. These new age data, in addition to the distinct stratigraphic style above and below the unconformity, result in the proposed formal stratigraphic revision to elevate the upper member of the Crystal Spring Formation to the Neoproterozoic Horse Thief Springs Formation and separate it from the remainder of the underlying Mesoproterozoic Crystal Spring Formation (ca. 1100 Ma). New age relations and revised stratigraphic nomenclature significantly clarify stratigraphic and tectonic correlations and imply ca. 1250-1070 Ma assembly, 1070-780 Ma stability, and 780-600 Ma breakup of the supercontinent Rodinia along the southwestern Laurentian margin.