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Paleoclimate Inferred from Fluvial Sediments: The Lower Cretaceous (Aptian) Kootenai Formation, Southwestern Montana

Howard, Christopher S.1; Dupree, Ryan T.1; Zachry, Doy L.1
1 Geosciences, University of Arkansas, Fayetteville, AR.

The Lower Cretaceous (Aptian) Kootenai Formation in southwestern Montana is a sequence of fluvial sandstone, mudstone, and lacustrine limestone reflecting deposition in arid to sub-humid conditions, punctuated by episodic flooding, which produced short-lived, high-discharge and variable-flow stream conditions. The basal Kootenai, marked by a widespread, quartzite and chert cobble conglomerate ranging to 10 m, rests unconformably on the Jurassic Morrison Formation. Succeeding the conglomerate are mudstone and sandstone units, referred informally to the Kootenai “lower clastics,” that range to 150 m in thickness and comprise >90 percent of the lower Kootenai. Lower clastics mudstones are characterized by deeply oxidized red beds interbedded and capped with caliche and calcrete-bearing paleosols. Caliche nodules from the paleosols display evidence of shrink-swell fracturing. They increase in abundance upward within the paleosols, culminating in laminated calcretes. Dominated by mudstones, (approximately 80 percent of the interval), the majority of the lower clastics reflect small-scale, inter-channel, pluvial lake and overbank deposits typically associated with sediment laden, ephemeral streams suggesting arid environments. Lower clastic sandstones, chiefly chert arenites, occur as discontinuous, 1 to 4 m thick bodies, which filled anastomosing channel complexes, or represent overbank splays that extend individually from 1 to 30 m along strike. Paleoflow within the channel systems 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 clasts that were ripped-up during high-flow events and deposited as currents waned. Depositional architecture in the lower clastics facies reflects rapid aggradation and bifurcation indicative of high discharge, ephemeral streams in arid landscapes. Rapid burial by episodic flooding preserved the lower clastics paleosols. Depositional architecture, deeply oxidized red beds, ephemeral stream deposits and caliche/calcrete 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.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009