Geologic Studies in Alaska by the U.S. Geological Survey, 1992

72 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
of the authors of that paper agreed that the Arctic Alaska
terrane and the Angayucham terrane were well-defined,
fault-bounded geologic entities with entirely different
stratigraphies and geologic histories with respect to each
other and to adjacent geologic entities. The Arctic Alaska
terrane is interpreted to have stratigraphic affinities with
North America, but is not located in its original orientation
with respect to North America (Mayfield and others, 1988;
Moore and others, 1992). The Arctic Alaska terrane is a
continental margin lithologic succession with continental
basement; the Angayucham terrane is oceanic. Each ter-
rane has a long independent history prior to their juxtapo-
sition in the Mesozoic.
2. Subterranes are inappropriate.-Application of the
term "subterrane" (Silberling and Jones, 1984) added an
unnecessary layer of complication to the discussion of the
geology of Arctic Alaska Some of the subterrane bound-
aries of Jones and others (1987) were drawn along the
same (controversial) boundaries as the allochthons defined
by Mayfield and others (1983), with major local inconsis-
tencies that Moore and others (1992) tried to correct.
Mayfield and others (1983) pointed out that these particu-
lar geologic entities have long been recognized as
tectonostratigraphic units. They were first named "thrust
tectonic units" (Snelson and Tailleur, 1968; Tailleur and
Brosge, 1970), in which specific stratigraphic sequences
were recognized (Martin, 1970). Consequently they were
referred to as "thrust sequences" by Mayfield and others
(1978), and repackaged as allochthons and parautochthons
by Mayfield and others (1983, 1988), Mull and others
(1987), and Mull (1989). "Modernization" of the termi-
nology by again repackaging these thrust sheets,
allochthons, and parautochthons as subterranes (Jones and
others, 1987)(tabIe 1) added no new information, insights,
or understanding to the geology of the area. In fact,
subterrane nomenclature obscures the fact that some of the
subterranes are thrust sheets, some are single allochthons,
some include several allochthons, some are metamorphic
belts, some are structural belts, and some are belts of au-
tochthonous or parautochthonous rocks. In other words,
the single term (subterrane) represents apples, oranges, and
watermelons. Moreover, (1) in many places in the Brooks
Range, these "subterranes" are stacked; (2) in many
places these "subterranes" are fault slivers only a few
meters thick; and (3) in many places different
"subterranes" are intimately imbricated, rendering their use
locally impractical. To our way of thinking, thrust se-
quences, allochthons, and metamorphic belts convey a lot
more useful (and still objective) information to other ge-
ologists than do "subterranes."
Figure 1 is a tectonic map of northern Alaska showing
the various structural belts within the Arctic Alaska ter-
rane. These structural belts are stratigraphically related,
but have distinct structural character and styles of defor-
mation. These belts are tectonic components of a larger
tectonostratigraphic entity. Allochthons and thrust sheets
are tectonostratigraphic components of some of these belts,
but are too smaIl to be mapped at a regional scale. We
emphasize that figure 1 is a tectonic map and not a substi-
tute for a geologic map; it shows the application of the
tectonostratigraphic terminology we propose.
3. Superterranes are inappropriate.-Recently
Moore, (1992), recognizing "problems of the subterrane
nomenclature as applied in northern Alaska," proposed that
these thrust sheets and allochthons, or subterranes, be el-
evated to terrane status, which requires elevation of the
Arctic Alaska terrane to superterrane status. This results,
for example, in one terrane, the Endicott Mountains "ter-
rane," which is a single imbricated rootless thrust sheet, or
allochthon, and another terrane, the DeLong Mountains
"terrane," which includes four previously identified
allochthons. Such terrane designation gives faults with
lesser amounts of displacement (such as the Endicott-Pic-
nic Creek allochthon boundary) the same weight as major
sutures (such as the boundary between the Arctic Alaska
and Angayucham terranes). In addition, four, and possibly
five of these new "terranes" in the Arctic Alsaka
"superterrane" (Moore, 1992) contain the same strati-
graphic units. The same formations within the Endicott,
Lisburne, and Etivluk Groups occur on several adjacent
thrust sheets, or "terranes" of Moore (1992). For example,
the Endicott Group is found in the "Endicott Mountains
terrane" and the "DeLong Mountains terrane," and the
Nakolik River unit has been identified as a depositional
link between the Baird Group of the "Hammond terrane"
and the Endicott Group of the "Endicott Mountains ter-
rane" (Dumoulin and Harris, 1987).
If the Endicott, Lisburne, and Etivluk Groups have a
different basement in each "terrane," then they are techni-
cally part of an "overlap assemblage." Since the pre-De-
vonian or pre-Mississippian basement of a large number of
thrust sheets is missing due to faulting, it will be necessary
to erect a large number of "suspect terranes," one for each
of these thrust sheets, because the nature of their basement
may never be known for certain. The implications of pur-
suing terranes to this level of detail are staggering because
at present terrane boundaries are drawn along Mesozoic
structures, and it would be necessary to identify all the
pre-Mississippian structures instead.
If the Endicott, Lisburne, and Etivluk Groups do not
have different basements in different terranes, terrane des-
ignation in this case subordinates the recognition of facies
changes and a formal stratigraphic relationship between
Endicott, Lisburne, and Etivluk rocks that link different
thrust sheets or allochthons or "terranes." Does the
Lisburne Group indeed have a different geologic history in
one place from the Lisburne Group in another place? Ad-
mittedly this may be a semantic problem--every grain of
sand on this earth probably has a unique history with respect
to every other grain of sand-but where do we draw the line?