SEISMITES IN THE MESOPROTEROZOIC ALTYN FORMATION (BELT SUPERGROUP), MONTANA - A TEST FOR TECTONIC CONTROL OF PERITIDAL CARBONATE CYCLICITY

The Mesoproterozoic Altyn Formation of western Montana contains abunda nt seismites that record the physical state of the sediment at the tim e of earthquake-induced ground motion. Earthquakes were likely generat ed by normal faulting during subsidence. Because part of the Altyn is composed of cyclic, upward-shallowing peritidal carbonates, it may aff ord an opportunity to test whether spasmodic subsidence provides a via ble mechanism for the episodic generation of the requisite accommodati on space at cycle tops. No stratigraphic relation between the seismite s and peritidal cycles is perceived, however, so it appears that this process may not be a control of cyclicity.