The Mesoproterozoic Altyn Formation of western Montana contains abundant seismites that record the physical state of the sediment at the time of earthquake-induced ground motion. Earthquakes were likely generated by normal faulting during subsidence. Because part of the Altyn is composed of cyclic, upward-shallowing peritidal carbonates, it may afford an opportunity to test whether spasmodic subsidence provides a viable mechanism for the episodic generation of the requisite accommodation space at cycle tops. No stratigraphic relation between the seismites and peritidal cycles is perceived, however, so it appears that this process may not be a control of cyclicity.

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