Analysis of the Paleoclimate and Depositional Environment of the Kootenai 2 Formation, 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 subdivded into four correlative units Kootenai 1-4 (KK1-KK4). KK1 and KK3 are siliclastic 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 sequence. Measured sections from Kootenai 2 Formation outcrops at Block Mountain and Shepherd Mountain near Dillon, Montana, indicate this unit thins approximately 30 meters to the north and roughly 80 meters to the south and east. All 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. KK2 carbonate hand samples were collected to better define the environmental settings of this unit. Thin sections were analyzed petrographically with plane polarized light and cathodoluminescence. Initial observations indicate that when lake levels were high, carbonate containing a biota of gastropods, bivalves, algae, and charophytes was deposited. Evidence of suberial expossure surfaces resulting from rootlets, fenestral pore spaces, circum-granular fractures and in situ brecciated rock fragments indicate that the lake dried up episodically. These observations record the fluctuations in water level throughout the history of the lake. Analysis of δ18O and δ13C data indicate the paleotemperature of the lake was relatively warm with prolonged exposure prior to the deposition of KK3. Additional isotope and petrographic data from measured sections at Frying Pan Gulch and Block Mountain will be added to this initial dataset. Kootenai 2 Formation 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. Interpretations from this study may expand the information known on lacustrine deposits and can be used to compare other lakes deposited during the same period as the Kootenai 2 Formation.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017