Title

Trace fossil assemblages and significant surfaces in Upper Cretaceous sediments in central Montana as indicators of environmental conditions and depositional setting

Presentation Type

Poster

Abstract

Trace fossil assemblages and sedimentary features were studied to establish environmental conditions and depositional setting using a 122 meter continuous core through the lower half of the Eagle Formation, the entire Telegraph Creek Formation, and the upper most part of the Niobrara Formation from the Billings area. In central Montana the Late Cretaceous Niobrara and overlying Telegraph Creek formations consist primarily of poorly exposed mudstones. Above the Telegraph Creek Formation is the Eagle Formation, which is dominated by cliff-forming sandstones. The environmental conditions such as salinity and water depth are poorly constrained, particularly so in the Telegraph Creek Formation. The 122 meters of core was described in detail, with features including lithology, primary sedimentary structures, trace fossils, and bioturbation intensity and frequency. Trace fossil assemblages indicate that the upper part of the Niobrara Formation was deposited in a deeper water marine setting while the overlying Telegraph Creek Formation was deposited in shallower water with decreased marine influence. High bioturbation intensity in these formations indicates relatively low sedimentation rates. The contact between the Telegraph Creek Formation and the overlying lower part of the Eagle Formation is gradational, indicating that these formations are part of the same genetically related regressive depositional package. In addition, it was possible to identify in the core two stratigraphically significant surfaces (a transgressive ravinement surface and a sequence boundary) that had been identified in previous outcrop studies of the Eagle Formation. By examining core it was possible to determine environmental and depositional conditions of these formations, particularly in the mudrocks, that could not be deduced from outcrop exposures alone. In addition, this is the first time that stratigraphically significant surfaces have been identified in core that can be directly tied to Eagle Formation outcrop exposures in central Montana.

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Apr 12th, 3:00 PM Apr 12th, 4:00 PM

Trace fossil assemblages and significant surfaces in Upper Cretaceous sediments in central Montana as indicators of environmental conditions and depositional setting

UC Ballroom

Trace fossil assemblages and sedimentary features were studied to establish environmental conditions and depositional setting using a 122 meter continuous core through the lower half of the Eagle Formation, the entire Telegraph Creek Formation, and the upper most part of the Niobrara Formation from the Billings area. In central Montana the Late Cretaceous Niobrara and overlying Telegraph Creek formations consist primarily of poorly exposed mudstones. Above the Telegraph Creek Formation is the Eagle Formation, which is dominated by cliff-forming sandstones. The environmental conditions such as salinity and water depth are poorly constrained, particularly so in the Telegraph Creek Formation. The 122 meters of core was described in detail, with features including lithology, primary sedimentary structures, trace fossils, and bioturbation intensity and frequency. Trace fossil assemblages indicate that the upper part of the Niobrara Formation was deposited in a deeper water marine setting while the overlying Telegraph Creek Formation was deposited in shallower water with decreased marine influence. High bioturbation intensity in these formations indicates relatively low sedimentation rates. The contact between the Telegraph Creek Formation and the overlying lower part of the Eagle Formation is gradational, indicating that these formations are part of the same genetically related regressive depositional package. In addition, it was possible to identify in the core two stratigraphically significant surfaces (a transgressive ravinement surface and a sequence boundary) that had been identified in previous outcrop studies of the Eagle Formation. By examining core it was possible to determine environmental and depositional conditions of these formations, particularly in the mudrocks, that could not be deduced from outcrop exposures alone. In addition, this is the first time that stratigraphically significant surfaces have been identified in core that can be directly tied to Eagle Formation outcrop exposures in central Montana.