Sedimentation in a tectonically partitioned, nonmarine foreland basin: The Lower Cretaceous Kootenai Formation, southwestern Montana

Abstract

The Lower Cretaceous Kootenai Formation in southwestern Montana was deposited in the nonmarine, Cordilleran foreland basin in the United States during a period of intensified uplift in the westward adjacent, but increasingly impingent, Sevier fold-thrust belt. Concurrently, the foreland basin was partitioned by uplift of intra-foreland structural elements and incipient plutonism. Kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture. The Kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin. The following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the Kootenai: (1) complex, internalized drainage patterns which locally diverge from the over-all regional paleoslope direction and a typical longitudinal pattern; (2) intraformational and interformational unconformities, both proximal to the fold-thrust belt and within the foreland basin; (3) evidence for cannibalization and redistribution of foreland-basin sediments; (4) abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts; and (5) dramatic base-level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin. Kootenai sedimentation can be divided into six phases. 1. A tectonic pulse in the Sevier fold-thrust belt produced a thin but widespread sheet of gravel alluvium, referred to as the “Basal Conglomerate Member,” which was deposited by a shallow, perhaps ephemeral, gravel-bed, braided-stream system. Drainage was predominantly eastward, across the axis of the foreland basin. 2. Subsequently, initial activation of intra-foreland structures over the present Tobacco Root, Madison, Gravelly, and Beartooth Archean blocks and the Boulder batholith caused reworking of the Basal Conglomerate in all but the westernmost foreland basin. Although the topographic manifestations of these uplifts were subtle, the foreland basin was effectively partitioned into at least four distinct drainage basins. The resulting deposit, commonly called the “Pryor Member,” but here referred to as the “First Sandstone Member,” was deposited by sand- and gravel-dominant, braided-stream systems in the west and east, respectively. Paleodrainage directions in the western foreland basin were southward, but over-all drainage was northward. 3. A period of mud-dominant sedimentation followed deposition of the First Sandstone, probably resulting from a combination of decreased tectonism, influx of siliceous volcanic ash, and change in source lithology. 4. Renewed tectonic activity in the fold-thrust belt generated a second episode of coarse-grained alluvial sedimentation, producing the Kootenai Second Sandstone Member. Basin partitioning was accentuated, but drainage directions remained essentially unchanged. Initial activation of the Blacktail-Snowcrest uplift dammed the southern portion of the Second Sandstone fluvial system. Base level was elevated, and a major anastomosed-stream system developed upstream from the structural dam. Downstream from the axis of the uplift, a transport-efficient, probably entrenched, meandering system developed. 5. The remainder of Kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems. The foreland-basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued. An extensive carbonate lacustrine interval punctuated this period of sedimentation. 6. Gradual southward transgression of the Cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost Kootenai and overlying Blackleaf Formation. Rejuvenation of orogenesis in the fold-thrust belt, augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources, renewed the supply of coarse-grained detritus to the foreland basin during deposition of the Blackleaf.