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Foraminiferal biostratigraphy and paleoecology of the Mancos Shale (Upper Cretaceous), southwestern Black Mesa, Arizona

By
James Olesen
James Olesen
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Published:
January 01, 1991

The study area at Blue Point is the southwesternmost exposure of upper Cretaceous strata in Black Mesa, Arizona. The Mancos Shale at Blue Point was deposited in an embayment along the western margin of the Greenhorn Seaway during late Cenomanian-early Turonian time. Foraminifera from the Mancos Shale were investigated to interpret their biostratigraphic and paleoecological significance.

Most species of foraminifera identified are long-ranging taxa of limited biostratigraphic value. Nevertheless, foraminiferal assemblages reflect the interaction between oceanic circulation in the Greenhorn Seaway and local paleoenvironmental controls.

Foraminifera indicate the existence of a shallow marine environment in the study area during the late Cenomanian and the influence of warm Tethyan waters. Depths were above wave base, substrates were oxygenated, and the water column was periodically hyposaline due to freshwater influx.

During the Turonian, the study area became a deeper, more normal marine, offshore environment. This increase in depth continued into the basal 14 m of the middle Turonian Collignoniceras woollgari ammonite zone, although waters were probably less than 100 m deep during peak transgression. Peak transgression at Blue Point postdates the onset of regression at the axis of the seaway near Pueblo, Colorado. The anomalous depth increase may indicate a period of basin subsidence at Black Mesa or may suggest that eustatic highstand in the Greenhorn Seaway occurred during C. woollgari time.

After peak transgression, the water column was periodically hyposaline, and salinity and depth at Blue Point decreased with regression of the Greenhorn Seaway. Increasing hyposalinity may have been caused by the retreat of Tethyan waters, an increase in freshwater influx associated with strandline progradation, the influence of boreal waters, or some combination of these factors. Before disappearing in upper strata of the Mancos Shale, foraminiferal assemblages indicate a hyposaline, marginal marine environment.

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GSA Special Papers

Stratigraphy, depositional environments; and sedimentary tectonics of the western margin, Cretaceous Western Interior Seaway

J. Dale Nations
J. Dale Nations
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Jeffrey G. Eaton
Jeffrey G. Eaton
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Geological Society of America
Volume
260
ISBN print:
9780813722603
Publication date:
January 01, 1991

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