ABSTRACT

Thin sandstone units that comprise the Ferron Sandstone Member of the Mancos Shale (Upper Cretaceous) in the northern part of Castle Valley, Utah, were deposited along a low-energy epeiric sea margin. Interpretation of these units by uniformitarian comparison with modern low-energy coastal zones, in particular the Sapelo Island coast of Georgia, reveals an interplay between sediment-distributive processes that accompany storms and the sediment-disruptive activities of burrowing organisms in fairweather periods between storms. During storms, nearshore sediment is thrown into suspension and moved seaward where, upon storm abatement, it settles out. A return to fair weather brings renewed biogenic activity, completely homogenizing thinner storm-deposited beds, but reworking only the uppe parts of thicker beds. Storms also accentuate sedimentation processes in tidal inlets and on offshore bars seaward of the shoreface.

Sediment now in these thin Ferron units was derived from the large Vernal Delta, located north and west of the Castle Valley outcrops, and was transported generally southwestward, parallel with the coast. Specific interpretations of depositional environment have been made for four informally named sandstone units. The Clawson unit was deposited in the offshore environment; it consists of bioturbated silty sandstone with numerous large concretions. The Washboard unit has, in addition to bioturbated, concretion-rich sandstone, intercalated thin sandstone beds that are even parallel laminated and have burrowed upper parts; this unit was deposited in the lower shoreface environment. In a limited part of the outcrop belt the Washboard unit is replaced by a tidal inlet deposit termed the Farnham unit, that comprises shelly, little-burrowed sandstone with bipolar, coast-normal, trough cross lamination. And the Woodside unit is a sequence of thin, medium- to coarse-grained sandstone beds that have bipolar, coast-parallel, trough cross lamination; these beds accumulated as submerged offshore sand bars in a shallow shelf sea.