ABSTRACT
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.
Kozinski, J., 1985. Sedimentology and tectonic significance of the
Nutzotin
Mountains Sequence, Alaska. Unpublished MSc. thesis, State University
of
New York at Albany. 132 pp., +xii; 2 folded plates (maps)
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1985 K68 x
Kozinski MS thesis (scanned
text
pdf - 8 MB)
Plate 1 - Generalized
geologic map of
the Eastern Alaska and St. Elias Ranges
(coloured geological map; scale 1:500,000) - pdf
file 6.7MB
Plate 2 - Paleocurrent
data and provenance
sample localities, Bonanza Creek section, Nutzotin Mountains sequence
(stratigraphic columns and paleocurrent rose
diagrams from measured sections; scale ~1:1,970)
Return to MS Theses completed in the
Geological
Sciences Program, University at Albany