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Over 120 m (400 ft) of terrigenous sediment within the Miocene Chalk Hills Formation was deposited in proximal to distal fluvial to lacustrine settings during extensional tectonism along the southwestern Snake River Plain. spectacular exposures of basin-fill facies along deep tributary gorges allow for detailed reconstruction of major sedimentary environments along this extensional basin margin.
Vertically, Chalk Hills sediments comprise a transgressive sequence of fluvial-floodplain, marginal lacustrine, and deep lake systems which progressively onlap basin-margin silicic volcanics. Fluvial-floodplain facies, deposited in and along large, slightly sinuous rivers, consist of trough-cross-bedded boulder gravel to coarse sand paleochannels which incise floodplain fine sands and muddy silts. These pass basinward and vertically into coarse, marginal lake facies, commonly exhibiting tabular cross-sets in excess of 18 m (60 ft) in thickness, with individual inclined units reaching 1 m (3.3 ft) in thickness. These constructional units are characterized by dips to the northeast of 15 to 22°. Closely spaced sections demonstrate that most well-developed foreset-topset couplets hav great extent along the basin margin, and were probably deposited as lateral benches which repeatedly developed along interfluvial headlands. Coarse sediment supplied from one or more fluvial sources along the lake margin was winnowed by waves on shallow bench platforms prior to deposition on steep basinward-dipping bench slopes. In addition, localized Gilbert-type deltas may have been responsible for lobate cross-set sequences which are laterally restricted in comparison to the tabular cross-sets which characterize bench sequences. Marginal lake facies in turn grade basinward and upward vertically into deep lake silts and muds which were deposited during continued lake transgression over steep lake margin volcanics. These units are commonly horizontally bedded or massive.
Unlike lacustrine systems deposited in broad compressional intermontaine basins, facies within the Chalk Hills Formation of the southwestern Snake River Plain exhibit abrupt lateral and vertical changes, recording both spatial narrowness and temporal instability of fluvial-lacustrine transitional environments in extensional rift basin settings. The unique relationships exhibited by these sediments, resulting from tectonic instability, may be characteristic of many rift-valley lacustrine systems.
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