2014 Rocky Mountain Section AAPG Annual Meeting

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Abrupt Basinward Changes in the Stratigraphic Expression of a Sequence Boundary in the Ferron Sandstone (Turonian), Southeast Utah, USA


The deltaic Ferron Sandstone is one of the few rock bodies in which a single sequence boundary (SB) can be traced in continuous exposure over long distances. The SB has been traced for ∼70 km along strike, but the present study is the first to document the character of the SB and its enclosing strata basinward along depositional dip on the southern flank of the Henry Mountains syncline. Sixteen sections were measured from ∼20 m of strata bounding the SB from the main outcrop belt eastward for a distance of 8 km. Up-dip, the SB is underlain by low-angle cross bedded to ripple and flat laminated sandstone (mouth bar). The SB lies atop a heavily rooted siltstone and ∼.5 m thick coal or underlies a surface of incision at least 5 m deep. Overlying strata are tabular to trough cross bedded, locally pebbly, medium sandstone (channel deposits). Down-dip, the lower part consists of 2–3 coarsening-upward sandstone/siltstone (delta front) cycles capped by 1–2 m thick, sharp-based cross bedded and hummocky cross stratified sandstones (mouth bar). The SB becomes a cryptic surface atop a thin (<1m), massive to faintly laminated, carbonaceous siltstone, which is overlain by a thoroughly bioturbated transgressive sandstone. The upper part contains heavily bioturbated and carbonaceous mouth bar sandstones transitional into coarsening-upward delta front cycles. The exceptional exposure provides novel insight: Although the SB is clearly defined up-dip, within just 8 km down-dip it is identifiable as a major unconformity only upon very close inspection and detailed correlation.