ABSTRACT

A thin, but distinctive red-weathered oolitic ironstone occurs 7272within the Upper Cretaceous Mancos Shale of east-central Utah. It is characterized by a grain-supported, ooid-rich lithology containing iron-clay (berthierine), muscovite, hematite/goethite and carbonate minerals around clastic nuclei. Associated grains of phosphatic material (bone and fish teeth) are common, admixed with fossil hash and wood pieces. Internal structures within the mappable oolitic ironstone bodies include channel scours, cross-bedding, contorted bedding, load structures, mud rip-up clasts, and marine trace fossils.

The oolitic ironstone is encased within the Mancos Shale and cannot be traced laterally. However, four additional facies occur in vertical succession with the ironstone: Inoceramus mounds, siltstone-shale, fine-grained channel sandstone, and coarse-grained lithic sandstone. The sedimentologic features of the oolitic ironstone and its associated facies indicate a prodelta environment, influenced by river-dominated flooding in the delta front environment (providing sand fed through small channels) and by fluctuating or alternating sea-floor bottom conditions due to periodic storm or current action. The oolitic ironstone is inferred to have formed as shoals within the inner shelf-like setting.

A key factor in formation of the oolitic ironstones is eustatic control in the Western Interior Seaway. The ironstone was deposited during a relative sea-level low and then winnowed during a transgressive rise to create a condensed section. Thus, the interval is an important stratigraphic marker in an otherwise "monotonous" section of shale, and it represents a parasequence boundary expression in offshore deposits where eustatic influences are commonly difficult to recognize.