High Resolution Facies & Architectural Analysis of a Tributive, Basal Terrace of a Ferron Compound Incised Valley



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The Ferron Sandstone Member of the Cretaceous Mancos Shale is composed of three fluvio-deltaic wedges deposited during the Turonian age along the western margin of the Cretaceous Seaway. The Notom Delta, southernmost of the three complexes, is spectacularly exposed in the washes, road cuts, and cliffs of the Factory Butte region of southern Utah. Deposits associated with Sequence 1, the youngest of 6 Notom sequences, are documented along the northern kilometer of East Neilson Wash. Correlation of 19 detailed measured sections enabled the identification and description of the major valley surfaces and associated deposits of a compound incised valley that forms much of the base of Sequence 1 throughout the area. The oldest valley terrace, of what are thought to be three constituent valleys formed during a stepped relative fall of sea level, is particularly well exposed along the cliffs of East Neilson Wash. This study documents the facies, architecture, paleohydraulic scale, and stratigraphic morphology of Valley 3, the oldest valley member, in detail. Correlation of the shallowing and intermittently exposed basal surface of Valley 3 indicates the field area occupies a position along the eastern margin of a tributive valley system previously documented in Neilson Wash to the west. A transgressive fill-succession, from tidally-influenced fluvial deposition in lower portions of the valley to tidally-dominated estuarine deposition at its top, is documented in Valley 3. High-resolution photomosaics, paired with measured sections, allowed for detailed characterizations of the facies architecture of a point bar in the basal, fluvial unit and a tidal channel/flats complex in the upper, estuarine unit. A marked, vertical increase in tidal influence and an accompanying landward shift in facies between these two units suggest Valley 3 is “flood-capped”. This study fulfills several objectives: 1.) detailed morphological, lithological, and architectural characterization of previously undescribed portions of Valley 3 in East Neilson Wash; 2.) quantification of the scales of relevant depositional systems; 3.) a description of the outcrop’s depositional position within the local fluvio-estuarine, tributive incised valley system; 4.) determine the predictive efficacy of classic incised valley and fluvio-estuarine models using the ancient analog of this study.



Ferron, Cretaceous, Incised valleys, Tide-dominated Estuary, Fluvial architecture, Fluvial systems, Basal Valley Deposits, Tributive Valley, Notom Delta, Cretaceous Seaway, Factory Butte, Sequence Sedimentology, Rhythmites, Tidal Flats, Tidal Architecture