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Abstract

The Niobrara Transgressive Hemicyclothem at Pueblo and to the south and west comprises four discrete lithostratigraphic members, separated by four and possibly five regional disconformities. The sequence disappears to the east and north as disconformities expand, anastomose, and become a single regional unconformity, bringing pelagic Fort Hays limestones, representing maximum transgression and eustatic rise, into erosional contact with maximum regressive shoreface sandstones (Codell) of the underlying Greenhorn Cyclothem. Eight molluscan biozones are defined within the complex Niobrara transgressive sequence in the Axial Basin. These biozones progressively disappear eastward within anastomosing disconformities. Near Pueblo only three molluscan biozones are recognized within the pre-Niobrara part of the cyclothem. A sequence of four foraminiferal biofacies are defined at Pueblo and along the Colorado Front Range which reflect rapid development of clear, warm, well circulated waters early in the transgressive history. These biofacies rise stratigraphically to the west, associated with eustatic rise in sealevel, and grade laterally along any time-line from diverse open water assemblages to environmentally stressed, less diverse, assemblages westward. This lateral gradation, and the rise of foraminiferal biofacies westward, is thought to reflect the westward migration of turbid watermasses associated with the active margin of the seaway during eustatic rise of the Niobrara Marine Cycle.

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