DEPOSITIONAL ENVIRONMENTS AND PALEOCLIMATE OF KOOTENAI FORMATION 2, SOUTHWESTERN MONTANA

The Upper Cretaceous Kootenai Formation in southwestern Montana was deposited in an active foreland basin system of the North American Cordilleran. The Kootenai Formation is regionally sub-divided into four correlative units, KK1-KK4. Kootenai 1 and 3 are siliciclastic units deposited in fluvial, overbank, and floodplain settings during active thrusting. Conversely, Kootenai 2 and 4 are carbonate dominated units interpreted to record lacustrine deposition during tectonic quiescence.

Five measured sections of Kootenai 2 outcrops near Dillon, Montana, indicate this unit thins from approximately 30 meters to the north to roughly 80 meters to the south and east. All sections are predominantly gray skeletal wackestone or lime mudstone interbedded with red or green shale. A medial interval of yellow dolomite is present in all sections as well. KK2 carbonate hand samples were collected to better define the environmental setting of this unit. Thin sections were analyzed petrographically with plane polarized light and cathodoluminescence. Initial observations indicate that when lake levels were high, carbonate was deposited containing a biota of gastropods, bivalves, algae, and charophytes. Subsequently, subaerial exposure surfaces indicated by rootlets, fenestral pore spaces, circum-granular fractures and in situ brecciated rock fragments developed when the lake dried up.

Kootenai 2 records deposition in a low-energy, low-gradient lake environment. Abundant soil processes record periods of subaerial exposure indicative of variations in water level, consistent with low-gradient lakes that may desiccate more often. Additionally, the presence of charophytes and carbonate nodules may indicate more humid environmental conditions.