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nmive b d t and minor ulttamafic rocks. The
ultramalic rocks are an ollvine and clinapyroxene-rich
differentiate of gabbro. Posibly as much as 80 to 90
percent bf the exposed rock is Igneous, and so the
amount of sedimentary rock present in the Rampart
Group is very small. Pillow basalt in the Rampart
Group ls also rare, althougb weU-developed pillows
were observed at several places (pb, fig. 24). The
relations of the basalt to nearby red argillite and chert
could not be established.
The Rampart Croup, including the voluminous
rnafic intrusive rocks, structurally overlies undated
mica schist, marble, end metachert of the Ruby
terrane, as suggested by Patton and others (1977). The
contact between these two assemblages is interp~ted
as a nearly flat fault, marked by an extensive zone of
mylonite, above which the lower part of the Rampart
Croup is intensely sheered and disrupted. None of the
mafic Intrusive rocks that cut the Rampart Group
occur in the metamorphic rocks of the Ruby terrane
beneath this basal fault, and so faulting clearly postdetes
intrusion of the mafic rocks.
Chert sampled from the Rampart Group occurs
in three Lithic associations (fig. 24; table 6): (1) Red
and black chert and red argillite without volcaniclastic
rockg In which the chert is Late Mississippian to Early
Pennsylvanityd?) (locs. 2, 4, 5, 6, fig, 24); (2) red chert,
red ~gilute, find green to gray tuff and volcaniclastie
graywacke, In which the chert is Pennsylvanian(?) to
Permian (fig. 24, locs. 1, 3, 7); and (3) finewed
gmywacke, argillite, and minor chert, undated (fig. 24,
loc. 8).
In addltfon to these occurrences, loose blocks of
red and green chert, as much as 0.5 m long, occur
along the north bank of the Yukon River near Point No
Point (lot. 12, fig. 24); these blocks contain wdpreserved
Late Triassic radiol&ans. The source of
the blocks has not yet been determined.
MESOZOIC FLYSCH BELT
A complexly deformed assemblage of graywacke,
quartzite, argillite, and minor conglomerate forms an
extensive belt trending more then 200 km northeast
from the Tanana River (fig. 24). Although hte
Mesosoic fossils have long been known from this belt
near Quail Creek in the Rampart distrbt, radiolarim
chert had not previously been reported from these pre-
dominantly clastic rocks. During our reconnalssance
studies, two exposures of radiolarim chert of Late
Triassic age were discovered. Locality 10 (fig. 24)
consbts of several tens of meters of black chert inter-
bedded(?) with black siltstone and graywacke, and
locality 11 (fig. 24) is a small rubbly exposure of chert
In a fault zone separating vitreous quartzite from
graywauke. In addition, chert clasts h conglomerate
at loeallty 9 (fig. 24) yielded Late Triassla radio-
larlans.
CONCLUSIONS
Sedimentary rocks of the Rampart Group are
dominantly deep water chert, argillite, andesitic
volcaniclastlc rocks, and very mlnor tuffaceous bio-
clastic limeslone. Dated rocks range in age from Late
Mksissippian to Perminn, and float blocks suggest the
presence of Upper Triassic strata. The volcaniclastic
rocks appear to be mainly Permien. All these sedi-
mentary rock are Intruded by large sill-like bodles of
rnafic igneous rock, apparently largely sills, of
probable latest 'hiastic age that may be feeders to
scarce piUow flows. None of these rocks appears to be
genetically related to ophiolite, And the presence of
oceanic crust within the Rampart Group in the Rarn-
part district is unsllbstmtiated. The Rampart Group
structurally overlies rnelaroorphosed sedimentary and
igneous rock of the Ruby terrane along a major thrust
Table 6.--Ages of radiolarlans from chert local itlcs in the Rampart district
Chert locality F1
(fig. 241
eld M. Age Remarks
4 81-JTN-37 Late Misstsslppt an to Rampart Group
Lower Pennsylvanl an. (nonvolcdnic
6 HI-JTN-40 do. assemhl age). -
2 81-Ach-5A do.
5 79-Ach-89 do.
7 81-JTN-3 Pennsylvanian to Permian--- Rampart Group
1 81-JTN-23 do. (volcaniclastic
1 $1-JTN-24 Pennl'an -assemblage). .
1 81-JTN-25 do.
3 81-JTH-27 (lo.
8 81-jf&13 Unknown-------------------- Rampart Groap
(graywecke-argill i te
assemblage).
12 81-~~~-10 ate Tritrslc-------------- Float, presunably from
the Rampart Group but
source unknown.
LO 81-JTH-18 Late TrYarslc-------------- Mesozoic tlysch belt
11 131-51-1-19 do. (clasts in conglanerate).
9 81-JTN-26 do.