Evidence of Basement-Block Movement, Middle Ordovician Shelf Sediments, Western Passive Margin, North America

Ronald R. McDowell

Short-lived, basement-block movements initiated sedimentation in the early Middle Ordovician Kanosh basin. The basin was a north-south, elongate, intrashelf structure covering approximately 90,000 km² of Utah and Nevada, and was subdivided into the northern Utah and Ibex subbasins by the east-west-trending Tooele arch. The Kanosh Shale, an organic-rich (TOC <= 5.6%), graptolitic shale with numerous, thin, interbedded calcarenites, was deposited throughout the basin. Deposition began abruptly and was nearly synchronous, corresponding roughly to the base of macrofossil zone M, early Whiterock Stage. The intertidal and shallow, subtidal carbonate sedimentation that preceded the Kanosh Shale continued in areas surrounding the basin during Kanosh deposition; abru t, local subsidence rather than regional sea level rise initiated Kanosh deposition. Basal sands of the lower Swan Peak quartzite and carbonates of the Lehman Formation encroaching on the basin terminated Kanosh deposition. Recurrent uplift of the Tooele arch during the deposition of these two units (middle and late macrofossil zone N, middle Whiterock) caused them to thin markedly over the axis of the arch; this effect is much less noticeable in the underlying Kanosh Shale.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.