Depositional architecture and paleoclimate of a Lower Cretaceous (Aptian) fluviolacustrine system: The Kootenai Formation, southwestern Montana.

摘要

The Lower Cretaceous (Aptian) Kootenai Formation in southwestern Montana is a sequence of fluvial sandstone, mudstone, and lacustrine limestone units 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, orthoquartzite and chert-cobble conglomerate unit reaching 10.7 meters, rests unconformably on the Jurassic Morrison Formation. Succeeding the conglomerate are mudstone and sandstone intervals, together referred informally to the Kootenai "lower clastics," that range to just over 152 meters and comprise greater than 90% 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, presumably from wet-dry cycles. The fractures within the caliche nodules are filled with calcite spar, while the nodules themselves increase in abundance upward within the paleosols, often culminating in laminated calcretes. Dominated by mudstones (approximately 80% 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 4.5 meter thick bodies, which filled anastomosing and braided channel complexes, or represent overbank splays that extend individually from 1 to 30.5 meters along strike. Paleoflow within the channel systems was generally eastward though there is some variance; consistent with a source in the rising Sevier Orogenic belt to the west. The lower portions of the channel fills often contain clast supported conglomerates, rich in caliche and mudstone clasts that were ripped-up during high-flow events, and chaotically deposited as currents waned rapidly. Depositional architecture in the lower clastics facies (following Miall, 1985) 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.