Mark A. Kirschbaum, Robert D. Hettinger
The Late Campanian Mount Garfield Formation in western Colorado consists of 150-200 m of coal-bearing strata and includes continuous sandstones previously interpreted as wave-dominated deltas. Erosional surfaces within the sandstones were thought to be cut by distributary channels. In contrast, we interpret strata above these laterally extensive surfaces of erosion as tidally influenced valley-fill deposits based on wavy/lenticular bedding, mudstone drapes, Teredolites, and compound crossbedding. The valley-fill deposits represent basinward facies shifts over shoreface strata, and the erosional surfaces are interpreted as sequence boundary unconformities. At least four high-resolution sequences are recognized within the Mount Garfield based on these sequence boun aries.
Westward 150 km, the time-equivalent Neslen Formation contains four extensive tidally influenced units encased in coal-bearing coastal-plain strata. Tidal deposits are characterized by inclined heterolithic strata, compound crossbedding, brackish-water mollusks, and abundant trace fossils. Coal zones in the Neslen can be correlated into the four sequences defined in the Mount Garfield Formation. Based on these correlations, the Neslen can also be divided into four sequences dominated by transgressive-systems-tract deposits.
Coarse-grained fluvial facies and estuarine facies at the top of the Neslen and Mount Garfield Formations represent a major basinward facies shift also recognized at the top of the Castlegate sequence at Price River Canyon in central Utah. These correlations suggest that highstand deposits of the Castlegate sequence are actually comprised of several high resolution sequences and emphasize the necessity to conduct sequence stratigraphic studies on a regional scale.
AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana