Sedimentology and tectonic significance of the Nutzotin Mountains Sequence, Alaska

• Main Author: Kozinski, Jane
• Format: Text
• Language: unknown
• Published: Scholars Archive 1985
• Subjects: sedimentology; tectonics; Nutzotin Mountains Sequence; Geology; Tectonics and Structure; Yukon; Bonanza; Dezadeash; Nutzotin Mountains; alaska range; Alaska
• Online Access: https://scholarsarchive.library.albany.edu/cas_daes_geology_etd/47; https://scholarsarchive.library.albany.edu/cgi/viewcontent.cgi?article=1046&context=cas_daes_geology_etd

The Nutzotin Mountains Sequence, a Mesozoic flysch sequence in the eastern Alaska Range, was studied along the southern border and in the central portions of the outcrop belt. Three lithologic associations are recognized in the Bonanza Creek section (southern margin) that together indicate a coarsening-upward trend, suggestive of a prograding fan system. These associations are (from bottom to top): 1) debris flow conglomerates overlain by 500 m of intercalated mudstone and base-missing turbiditic siltstone, and mass movement features such as slump folds and slump horizons, 2) 195 m of thicker, coarser turbidites intercalated with mudstones; turbidites are graded but lack one or more of the Bouma C-E divisions, and, 3) 1075 m of massive mudstones alternating with thinly bedded sandstone, overlain by silty turbidites; mollusc fossil fragments are common in both sandstone and mudstone beds. Facies associations 1 and 2 are interpreted to represent deposition on the mid-fan portion of a submarine fan system. Facies association 3 represents either inner fan over-bank and channel margin deposition, or deposition in the slope environment. Paleocurrent indicators from the Bonanza Creek section indicate an overall northward-directed current. The Sheep Creek section (middle of outcrop belt) consists of very thinly bedded silty turbidites and mudstones. Flaser and lenticular bedding suggests reworking of the sediment by bottom currents. The rocks are similar to channel over-bank deposits reported in other turbidite studies. Thick, massive coarse sandstone beds are also found in the Sheep Creek section; these may represent channel-fill deposits. The thinly bedded turbidites and the massive sandstones were most likely deposited in channel and over-bank environments, either in the inner- or mid-fan portions of the fan system. Paleocurrent data from this section also demonstrate a northward-directed current. Sandstones from the Bonanza Creek and Sheep Creek sections plot on or slightly above the feldspar-lithic fragment join of the QFL diagram. Mafic/intermediate volcanic clasts are the most abundant framework grain variety, and zoned, euhedral plagioclase is the most common feldspar. Grains of euhedral monocrystalline quartz with resorption cavities are present in some of the Bonanza Creek suite sandstones. The sandstones lack continentally-derived detritus. The composition of the sandstones indicates an active volcanic arc as the main sediment source, and the Wrangellia Terrane as a minor source. The composition of the sandstones and the facies associations and paleocurrent directions observed in the Nutzotin Mountains Sequence are compatible with or similar to those features of the Dezadeash Group in the Yukon. This work supports the notion that the two flysch sequences were once a continuous belt, disrupted by 300-400 km of dextral strike-slip on the Denali Fault. The Nutzotin Mountains Sequence was most likely deposited in a backarc or intra-arc basin. Blocks of Triassic(?) limestone (Wrangellian basement) within the flysch may be evidence of normal, reverse, or strike-slip faulting contemporaneous with deposition. This may suggest that the depositional basin was in part extensional.