CHARACTERIZING THE DETAILED ARCHITECTURE OF CHANNELIZED STRATA IN THE CRETACEOUS EAGLE FORMATION, SOUTH-CENTRAL MONTANA

The Santonian-Campanian Eagle Sandstone in south-central Montana is a succession of fluvial, tidal, deltaic, and shallow marine deposits. Based on distribution of these facies, the Eagle Formation can be subdivided into four members, all bounded by regionally significant stratal surfaces. Within this framework, fluvial and tidal sediments are best developed in the middle two members of the Eagle Formation, which were deposited during a major regression of the Eagle system to the east.

Some of the best exposures of the fluvial/tidal strata and associated facies can be found at Horse Butte, an approximately 2.6 sq kilometer butte with near vertical cliff exposure on all sides. Especially striking in these outcrops are a series of inclined accretion sets best preserved in the northwestern- and southeastern-facing cliffs. At these locations accretion sets up to five meter high can be traced over lateral distances of ~500 m before terminating into mud-filled channel forms. The average width of individual accretion sets as well as the average width of the channel forms is 60 to 80 meters. These measurements enabled us to reconstruct channel morphologies, using published empirical relationships. Channels with similar depth to width ratios commonly have a sinuosity of approximately 1.7 and meander wavelength ranging from 1200 to 1600 m. Using these calculated parameters, our reconstruction suggests that all accretion sets and mud-filled channels exposed on Horse Butte are related and represent a series of meanders deposited by a single channel.

Most accretion sets at Horse Butte are dominated by fine to medium grained sandstone, but some sets have a higher percentage of fine-grained sediments and organic material. Associated with this change in grain size are distinct sedimentary structures, namely sigmoidal cross beds and other re-activation surfaces that reflect the episodic influence of tidal currents on the primarily fluvial deposition in the Horse Butte system.