SEDIMENTOLOGY, STRATIGRAPHY, AND DIAGENESIS OF LATE CRETACEOUS KOOTENAI FORMATION LACUSTRINE DEPOSITS, SOUTHWESTERN MONTANA

The Upper Cretaceous Kootenai Formation in southwestern Montana was deposited as part of the North American Cordilleran foreland basin system. The Kootenai Formation is regionally subdivided into four correlative units Kootenai 1-4 (KK1- KK4). KK1 and KK3 are siliciclastic units deposited in fluvial, overbank, and floodplain settings during active thrusting. Conversely, KK2 and KK4 are carbonate dominated units interpreted to record lacustrine deposition during tectonic quiescence. Outcrops of KK2 and KK4 (up to 80 m thick) at Block Mountain, Frying Pan Gulch and Shepherd Mountain were studied near Dillon, Montana. All KK2 measured sections are predominantly gray skeletal wackestone or lime mudstone interbedded with red or green shale, and contain a medial interval of yellow dolomite that is correlated lithostratigraphically. When KK2 lake levels were high, carbonate containing a biota of gastropods, phosphatic bivalves, algae, sharks teeth and charophytes was deposited. Evidence of subaerial exposure surfaces resulting from rootlets, fenestral pore spaces, circum-granular fractures and in situ brecciated rock fragments indicate that the lake dried up episodically. Analysis of δ18O and δ13C data indicate the paleotemperature of the lake was relatively warm with prolonged exposure prior to the deposition of KK3. KK2 was deposited in a low-energy, low-gradient, warm water 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, root traces and carbonate nodules indicate more humid environmental conditions during deposition. KK4, commonly known as the "Gastropod Limestone" consists predominantly of pelloidal gastropod wackestone/packstone with few other skeletal grains, save rare phosphatic bivalves or algae. There is little or no evidence KK4 was exposed during deposition or prior to deposition of the overlying Colorado Group.