Abstract

The foraminifera of a problematic 1.5-m-thick silt-stone/shale unit within the Aberdeen Member siltstone of the Upper Cretaceous (lower Campanian) Blackhawk Formation of Utah were used to define the environment of deposition of this unit. Paleoenvironmental interpretation was based on comparison with modern marginal and shallow marine foraminiferal distributions (at the generic level) and with foraminiferal assemblages (at the species level) from the marginal marine Spring Canyon Member of the Blackhawk Formation and the coeval marine deposits of the Mancos Shale. Facies interpretations of these latter two units had been established in previous sedimentologic/stratigraphic works.

The Mancos Shale assemblage is comprised of calcareous and agglutinated taxa (e.g., Bathysiphon, Dentalina, Lenticulina and Nodosaria), that are usually found in fully marine environments today, together with the extinct Gavelinella, also typical of open marine settings. The Spring Canyon Member assemblage is characterized by an entirely agglutinated fauna dominated by Textularia, Ammobaculites, Haplophragmoides and Trochammina. The co-occurrence and dominance of these genera in low-diversity assemblages confirms marginal marine conditions.

The 1.5-m-thick Aberdeen Member siltstone/shale assemblage contained taxa found in both the Mancos Shale and Spring Canyon Member. This assemblage is interpreted as marginal marine in origin but with influence from open marine waters. Uniformitarian comparisons suggest deposition of the Aberdeen Member siltstone/ shale behind a barrier island in a lagoonal environment adjacent, in places, to a tidal inlet(s). This interpretation indicates that deposition of the siltstone must have occurred immediately following a relative drop in sea level.

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