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Abstract

The Cretaceous Ferron Sandstone is a fluvial-deltaic system that is superbly exposed along the western flank of the San Rafael Swell in east-central Utah. The Ferron consists of fluvial, near shorezone, and shallow-marine strata that were deposited along the active margin of an evolving foreland basin. The 180-m-thick (590-ft) Ferron Sandstone forms an east- to northeast-thinning clastic wedge that is bounded by marine strata and pinches out over the distance of 40 km (25 mi). Numerous local transgressive intervals further subdivide the Ferron into 10-20-m (30-60-ft) thick successions of fluvial and shallow-marine strata that are similar to the commonly used ‘parasequence.’

Along the Molen Reef escarpment, cliffs of 100 m (300 ft) in height and 20 km (12 mi) in length provide a nearly complete dip view through the entire thickness of the Ferron Sandstone. A detailed stratigraphic framework is constructed for the exposure by tracing beds between closely spaced measured sections in the field and from photomosaics.

The stratigraphic framework documents abrupt lateral facies changes across five surfaces interpreted as unconformities. The surfaces are characterized by incision (up to 30 m [90 ft]) of fluvial and estuarine strata into shallow-marine strata and an abrupt basinward shift of coastal-plain and nearshorezone strata. The unconformities are onlapped by coastal-plain and near-shorezone strata associated with sets of aggradational-to-retrogradational parasequences. Accompanying the change in parasequence stacking pattern are distinct changes in the facies of near-shorezone strata. Those associated with the abrupt basinward shifts are relatively mud-rich, display rapid lateral changes in facies, and are dominated by sediment-gravity-flow processes. In contrast, near-shorezone strata associated with the aggradational and retrogradational parasequences are relatively sand-rich, display minimal changes in facies laterally, and are dominated by wave and storm processes.

The incision of fluvial strata and abrupt basinward shifts in environments are interpreted to reflect falls in relative sea level related to minor tectonic and/or eustatic events. Variations in near-shorezone deposits, between those dominated by sediment gravity flows and those dominated by wave or storm reworking, are interpreted to reflect changes in the ratio of sediment supply and accommodation.

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