CONTRASTING STYLES OF LOWSTAND SEDIMENTATION WITHIN THE SAUK SEQUENCE ON THE MIOGEOCLINE, IBEX AREA, WEST-CENTRAL UTAH

Rapid subsidence of the miogeocline and the associated relative rise of sea level during deposition of the Sauk Sequence accommodated up to 13,000 ft (4.0 km) of Cambrian and Ordovician strata in western Utah. Included in this thick succession are rocks deposited during sea-level lowstands. We use "lowstand strata" here without reference to system tract models. Middle to early Late Cambrian (Steptoean) lowstand strata are shale-dominated successions, and they differ from late Late Cambrian (late Sunwaptan) to Early Ordovician (Skullrockian) lowstand strata, which are mostly carbonates. Shale-dominated lowstand deposition returned later in the Ordovician during the Stairsian to Whiterockian.

We interpret that the transition from shale-dominated to carbonate-dominated lowstand deposition was related to mantling of siliciclastic source areas by deposition of the 1500 ft (460 m) of clean carbonates of the Hellnmaria Member of the Notch Peak Formation and its correlates during a sea-level highstand. Above the Hellnmaria, six lowstand intervals occur within ~1100 ft (335 m) of carbonates (Red Tops, Lava Dam, House Limestone, and lower Fillmore Formation). Quartz sand is rare in the lowest of these six intervals but increases upward, reaching 10% of some limestone intervals. Shale again became a significant lithologic component in most of the Fillmore Formation.

Features of our carbonate-dominated lowstand strata include: 1) red to rusty-brown color on weathered surfaces; 2) high-energy lithologies, including trilobite, intraclast, peloid, and ooid grainstones, flat-pebble conglomerate, and laminated to cross-laminated (tidally influenced) fine grainstone; 3) high quartz sand content in some carbonate grainstones; 4) thin stromatolite and thrombolite boundstones; 5) paleokarst; 6) thin fine siltstone and sand pockets that filled paleokarst; 7) white to brown laminated chert [probably recrystallized from siliciclastic sediment], locally up to 50% of rock; 8) chert-pebble conglomerate. Strata deposited during sea-level lowstands are of different thicknesses (25-145 ft, 8-44 m), vary significantly along strike, and do not contain all of these features.