--> Ichnological Evidence for Temporal and Spatial Variations in Paleo-oxygenation in the Western Interior Seaway: An Example from the Lower Bridge Creek Limestone, by C. E. Savrda and B. B. Sageman; #90986 (1994).
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Abstract: Ichnological Evidence for Temporal and Spatial Variations in Paleo-oxygenation in the Western Interior Seaway: An Example from the Lower Bridge Creek Limestone

Charles E. Savrda, Bradley B. Sageman

The lower Bridge Creek Limestone Member (Cenomanian-Turonian) of the Greenhorn Formation is characterized by decimeter-scale alternation of pelagic limestones and marly shales. Investigations of this unit in two cores--USGS #1 Portland from central Colorado and Previous HitRebeccaNext Hit K. Bounds #1 from western Kansas--demonstrate the utility of ichnological studies for assessing, in detail, both temporal and spatial changes in benthic oxygenation within the Western Interior seaway.

The vertical disposition of laminated fabrics and four recurring oxygen-related ichnocoenoses in the Portland core record the following: (1) a broad trend towards decreased benthic oxygenation, which continues into the middle and upper Bridge Creek; (2) a long-term, high-magnitude redox cyclicity corresponding to limestone/marly shale couplets; and (3) a higher-frequency, lower-magnitude redox cyclicity expressed within the marly shale intervals. Trends (2) and (3) define a bundling that is reminiscent of cycles mediated by eccentricity-precession.

Ichnological trends (1) and (2) are similarly well-expressed in the Previous HitRebeccaTop Bounds core. However, bioturbated horizons in marly shale intervals are less common, thinner, or absent altogether, suggesting that paleo-oxygenation levels were lower towards the east, at least during the clastic-dominated phases of depositional cycles. This trend, which is supported by organic geochemical data, may reflect higher productivity and oxygen demand in the eastern part of the seaway.

Further integration of ichnological observations with geochemical, sedimentologic, and paleontologic data from this interval, as well as other selected lithochronozones, will provide a clearer understanding of paleoceanography, cyclic sedimentation, and organic matter accumulation in the Western Interior seaway.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994