Research - Geologic History
Moore, T.E., Wallace, W.K., Bird, K. J., Karl, S.M., Mull, C.G., and
Dillon, J.T., 1994, Geology of northern Alaska, in Plafkler, G., and Berg,
H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society
of America, The Geology of North America, v. G-1.
BIRD, K.J. (1999). Geographic and Geologic Setting. In The Oil and Gas
Resource Potential of the 1002 Area, Arctic National Wildlife Refuge, Alaska.
By ANWR Assessment Team, U.S. Geological Survey Open-File Report 98-34.
Geographic Placement
The 1002 area of the Arctic National Wildlife Refuge is a 1.5-million-acre
area in northeastern Alaska. It is bounded on the east by the Canning and
Staines Rivers, on the north by the Beaufort Sea, on the east by the Aichilik
River and the Canadian border, and to the south by Brooks Range.
Most of the 1002 area lies within the Arctic Coastal Plain physiographic
province, a marshy area that slopes gradually towards the Arctic Ocean on
the north. A small part along the southern margin that constitutes less
than 5% of the total area lies within the Arctic Foothills physiographic
province, a series of hills and ridges that descend from more than 500 m
in the Brooks Range to 300 m in elevation to the northern foreland.
Beaches, low steep cliffs, barrier islands, shallow lagoons, and river
deltas form the coast of the 1002 area, with hills rising to more than 300
m in the south. Many rivers and stream flow between these hills towards
the Arctic Ocean.
Origin
The North Slope and its continental shelves, Brooks Range and the northeaster
part of Siberia are considered to form the Arctic Alaska microplate, a small
lithospheric plate with boundaries that are not clearly known. One hypothesis
for the plate tectonic history of northern Alaska suggests that the region
was originally next to the Canadian Arctic Islands. Creation of oceanic
crust in the Canada basin during the Jurassic and Early Cretaceous caused
the microplate to rotate 60 degrees counter clockwise, placing it in its
current position.
Surface Geology
Nearly all of the surface of the 1002 area is covered by a thin layer,
less than 30 m thick, of unconsolidated, frozen silt- to gravel-sized sediments
of nonmarine origin. They originated from the erosion of the Brooks Range
during the late Cenozoic.
The surface exposures of rock formations inside the 1002 area are mostly
restricted to deposits of the Sagavanirktok, Jago River, and Canning Formations
of Tertiary age. There are some smaller outcrops of Hue Shale, pebble shale
unit, and Kingak Shale from the Cretaceous and the Jurassic. To the east
of the Sadlerochit Mountains, in the southern border of the 1002 area, some
of the oldest exposures, of limestones from the Mississippian Lisburne Group,
are found.
Subsurface Structure
Tectonics:
During the rifting that separated northern Alaska from the Canadian Arctic
Island, the Barrow arch, a structural high, was formed. The northern side
of the Barrow arch formed the continental margin, while the southern flank
received the thrust of the formation of the Brooks Range orogen.
The part of the 1002 area located to the south and east of the Marsh
Creek anticline and north of 69-degrees is the Brooks Range orogen. The
Brooks Range is more than 1000 km long and up to 300 km wide, and the distribution
and character of its major structures is not symmetrical. Deformations in
this region occurred during the Cenozoic era, forming both east-northeastward
and eastward structures. Parts of Jurassic to Cretaceous shales, Mississippian
shales, and of a horizon in the pre-Mississippian basement rocks were separated
from their source due to extreme folding and compression.
The structure of the Brooks Range is formed by a series of broad anticlines
with a core of pre-Mississippian rocks and younger rocks deformed on the
borders. To the north of the Brooks Range, the structure, called thin-skinned
deformation, is composed of numerous folds and faults developed in rocks
of Cenozoic and Mesozoic age. Several broad domes are present in pre-Mississippian
rocks in that area.
Stratigraphy
The stratigraphic record of the North Slope is divided into three sequences:
Franklinian, Ellesmerian and Brookian.
The Franklinian sequence comprises a thick succession of mainly sedimentary
rocks with a minor amount of igneous rocks of Cambrian to Devonian age that
lie beneath the pre-Mississippian unconformity, where a gap in the geologic
record exists.
Two separate layers of rock are present in northern Alaska. The shallow
marine carbonates are composed of rocks of Proterozoic age and rocks of
Lower Devonian age, separated by unconformities where Silurian strata are
missing. They are composed of quartzite (metamorphic rock consisting of
quartz grains, formed by recrystalization of sandstone), argillite compact
rock derived from mudstone or shale, product of weak metamorphism), and
basalt (dark-colored igneous rock), with intrusions of sandstone (the consolidate
equivalent of sand, with 85-90% quartz), and shale (fine-grained sedimentary
rock, formed by clay, silt or mud). The deeper marine layers include quartzite,
conglomerate, phyllite (metamorphic rock, finer than schist), argilite,
limestone (sedimentary rock formed by calcium carbonates), and granite (light-colored,
coarse-grained igneous rock). The compression, uplift and erosion of this
sequence during the Ellesmerian orogeny formed the pre-Mississippian unconformity.
The Ellesmerian sequence is hundreds of meters thick, and is composed
of layers of marine and nonmarine sedimentary rocks of Middle Devonian to
Triassic age, that rests on top of the pre-Mississippian unconformity. The
lower unit of this sequence is the Endicott Group, which, in the 1002 area,
consists of Mississippian coal-bearing sandstone, conglomerate, and shale
of the Kekiktuk Conglomerate and Kayak Shale, of Devonian and Mississippian
age. The Kekiktuk Conglomerate is a proven oil-bearing reservoir. Large
amounts of limestone and dolomine of the Lisburne Group were deposited in
the North Slope up to a thickness of 500 to 1000 meters during the Mississippian
and Pennsylvanian. It is separated from the overlying Sadlerochit Group
by an unconformity formed during the Middle Pennsylvanian to Early Permian
in when sea level fell.
Water resources
Snow: The North Slope is covered in snow from October to May of
every year, and while snow might fall during any month, the precipitation
during the summer is mainly rain. In spring, melting snow is the main source
of water for lakes and streams in that region, and water stored in snowdrifts
is released slowly and provides water for them during the summer. In the
1002 area, the depth of the snow increases considerably away from the coast.
Ice: Around mid-September, water bodies freeze across the North
Slope, with an average maximum thickness of 1.80 m at the end of winter.
By the middle of January, the ice is thick enough to support a loaded C-130
Hercules or a D-8 tractor.
Ground water: There is very little liquid ground water in the
coastal plain, but there is ground ice underlying the surface, which can't
be used as a water supply. |