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Inner Shelf Mudstones in the Cretaceous Seaway - Processes and Products


Laminated shelf mudstones have been interpreted as reflecting primarily suspension settling with periods of anoxia. However, recent models suggest that Upper Cretaceous mudstone units may have a greater degree of bedload transport, in which waves and tides play a role in moving sediment on slopes as low as 0.03°. We focus on bed-by-bed analysis of mudstones of the Mancos Shale Formation, and Juana Lopez Member, in Utah, Colorado and New Mexico, correlated over a distance of 115 km. Paleogeographic reconstructions suggest deposition around 100km from coeval shorelines to the west, in water depths of <100m. Measured section are at the mm-cm scale. Upward-coarsening facies successions, in the Carlile and Juana Lopez members are typically a few meters thick, and are interpreted as parasequences. Bioturbation index is higher in clay-rich intervals and decreases with sand content. Sandier facies show a predominance of wave to combined flow ripples, interpreted as tempestites and wave enhanced sediment gravity flows. The Upper Mancos shows abundant bidirectional current ripples and structureless clay drapes interpreted to reflect an increase in tidal influence. Sand-grade never exceeds very fine (125microns), but the dominance of cross lamination indicates ubiquitous bedload transport. Low-levels of burrowing, and bedload transport, indicate a fully turbulent water column and suggest relatively high sedimentation rates. The transport of sand, in <100m of water within 100km of the shoreline, suggests a predominance of geostrophic storm-flows, at times aided by tides. There is very little evidence of direct delivery from deltas via hyperpycnal plumes. Hyperpycnites and turbidites are found in prodelta facies, directly below sandy parasequences, but appear to limited to water depths <40m, and probably within 20 km of shorelines, versus the more distal facies considered here. The bioturbated claystones are more difficult to interpret, owing to the high levels of bioturbation. This does indicate lower sedimentation rates, possibly indicating slower settling from suspension, and/or less frequent deposition in deeper water. Biogenic, carbonate debris is also more common in the distal facies. In the Juana Lopez, 520 paleocurrents show a predominate SE unidirectional component of flow, that is likely geostrophic in nature, whereas wave ripple crests are oriented NW-SE indicating SW directed winds. It is likely that sediments in the San Juan Basin in NM were derived from deltaic systems to the north west, such as the Ferron Sandstone, and transported along the shelf by a variety of storm and tide-aided processes. Systematic lateral variations may also suggest proximity to deltaic point sources.