A FLUVIAL DEPOSITIONAL SYSTEM AS AN INDICATOR OF PALEOCLIMATE: THE LOWER CRETACEOUS KOOTENAI FORMATION IN SOUTHWESTERN MONTANA

The Lower Cretaceous (Aptian) Kootenai Formation in southwestern Montana contains successions of fluvial sandstone, mudstone, and lacustrine limestone reflecting terrestrial deposition in arid environs, punctuated by periodic, perhaps seasonal flooding, which produced ephemeral, high-discharge and variable-flow stream conditions. The base of the Kootenai, a pervasive quartzite and chert cobble conglomerate ranging to 10 m, unconformably overlies the Jurassic Morrison Formation. Subsequent to the conglomerate are mudstone and sandstone units, referred informally to the Kootenai “lower clastics,” that range to 150 m in thickness and comprise >90% of the lower Kootenai. Lower clastics mudstones are characterized by deeply oxidized red beds interbedded with caliche bearing paleosols. Caliche from the paleosols display evidence of shrink-swell fracturing and increase in abundance upward within the paleosols, culminating in laminated calcretes. Dominated by mudstones, (approximately 80% of the interval), the lower clastics reflect small-scale, inter-channel, pluvial lake and over-bank deposits typically associated with sediment burdened, ephemeral streams common to arid environments. Lower clastic sandstones, chiefly chert arenites, occur as sporadic, 1-4 m thick bodies, which filled anastomosing channels, or represent overbank splays that extend individually from 1-30 m along strike. Paleoflow within the fluvial system was generally eastward; consistent with a source in the rising Sevier orogenic belt to the west. The lower portions of the channel fills contain conglomerates, rich in caliche and mudstone clasts, ripped-up during high-flow events; deposited as currents waned. Depositional architecture in the lower clastics reflects rapid aggradation and bifurcation indicative of high discharge, ephemeral streams in arid landscapes, with rapid burial by episodic flooding preserving the paleosols. Depositional architecture, oxidized red beds, ephemeral stream deposits and caliche in the lower Kootenai infer arid to sub-humid climates, with rain-shadow effects caused by the Sevier uplift to the west, and occasional heavy, perhaps seasonal, stream-flow at time of deposition.