REPETITIVE PATTERNS OF SEDIMENTATION DURING THE SAUK SEQUENCE IN WESTERN UTAH

The Sauk Sequence comprises more than five kilometers of mixed carbonate and siliciclastic strata in the eastern Great Basin. Paleogeography and geologic structures, such as the House Range Embayment, and Tooele and Wah Wah arches, influenced sedimentation, but subsidence and sea-level fluctuations were dominant controls. Repetitive stratigraphic patterns are apparent at several different scales with this interval, particularly in the range of meters to tens of meters. Two are especially conspicuous: (1) meter-scale carbonate cycles tended to accumulate during highstands of sea level, and (2) shaly successions, which generally accumulated during sea-level lowstands, commonly have middle carbonate units.

Thick successions of meter-scale cycles are present in the Middle Cambrian Pierson Cove, Trippe, and Wah Wah Summit formations, the upper Cambrian Sneakover Limestone Member of the Orr Formation, Hellnmaria and lower Lava Dam members of the Notch Peak Formation, and the lower Ordovician Wah Wah and Juab limestones. These meter-scale cycles are dominantly carbonate, generally are characterized by biologic and climatic stasis, and are the sedimentological response to astronomical forcing. High-frequency, low-amplitude sea-level changes appear to have resulted from thermal expansion and contraction of well-mixed oceans.

In contrast, shale-rich intervals commonly have fewer meter-scale cycles and tend to be associated with sea-level falls or lowstands. In western Utah, these intervals include the Chisholm, Whirlwind, Candland, Corset Spring, and Steamboat Pass shales. A relatively thick, wedge-shaped middle carbonate unit is found in each of these intervals or between shales. Lowstand carbonate wedges represent short-lived recovery of the carbonate platform prior to a second stage of siliciclastic progradation and burial.

Important exceptions to these general patterns include the Pioche Shale, a highly radioactive shale that accumulated during the initial transgression of the Sauk Sequence, the combined Wheeler Shale-lower Marjum Formation, which accumulated in response to fault movement along the House Range Embayment, and the Fillmore Formation, the enigmatic and complex culmination of mixed shale and carbonate sedimentation during the Sauk Sequence on the Lower Paleozoic miogeocline.