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Abstract

The Lower Cretaceous (Aptian (?) - Albian) Dakota Group, as exposed along the Skyline Drive, near Canon City, Colorado, and along the Arkansas River, near Pueblo, Colorado, is divided into four distinct lithologically-sitnilar and genetically related formations. In ascending order, these are: (1) the Lytle Formation; (2) Plainview Formation; (3) Glencairn Formation; and (4) Muddy Sandstone.

The Lytle Formation is composed of white to light gray, friable, poorly sorted, cross-stratified, pebbly sandstone, deposited by braided fluvial systems draining the Mogollon Highlands and Sevier Orogenic belt. The Plainview Formation is composed of interbedded brown-weathering fluvial sandstone and carbonaceous shale. The Clencairn Formation is composed of five upward-coarsening, marine shale to sandstone, progradational shoreface sequences, separated by regional transgressive disconformities. The Muddy Sandstone, a massive, brown-weathering, cross-stratified fluvial sandstone, that grades upward into interbedded fluvial and marginal marine sandstone and shale, represents aggradation of a previously eroded paleotopography during base level rise.

The Dakota Group records two major third-order tectonoeustatic sealevel changes: the Kiowa-Skull Creek and Greenhorn transgressive-regressive cycles. The Plainview and Glencairn Formations were deposited during the Kiowa-Skull Creek cycle, representing coastal plain aggradation, followed by transgression of, and small-scale fluctuations of the Kiowa-Skull Creek strandline as the cycle approached peak transgression. The fourth progradational sequence represents peak transgression. The Kiowa-Skull Creek regression is recorded as a widespread, regional erosional unconformity, which developed on top of Glencairn-Kiowa-Skull Creek deposits. The initial transgression of the Greenhorn seaway into the Canon City - Pueblo areas is recorded by the Muddy Sandstone, which aggraded this erosional paleotopography.

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