Geophysical Characterization 

Pre-Cenozoic Basement for Hydrocarbon 

Yukon Flats, Alaska

By R.W. Saltus

1 (saltus@usgs.gov), J.D. Phillips

1 (jeff@usg

Stanley

2 (rstanley@usgs.gov), A. Till

3 (atill@usgs.gov), and 

(morin@usgs.gov)

Geophysical Characterization of Pre-Cenozoic BasemenHydrocarbon Assessment, Yukon Flats, Alaska

1 Denver, Colorado 

2 Menlo Park, California, 

3 Anchorage, Alaska USGS O

65° -141°

68°

-153°

Topography

0 100 km

100

2200

m

KDe

TRPzd

PzpCh

PzpCb 

PzpCh Pzvu

Pzvu

JDat 

Qu

Qu

PzpCy

Tg

JDat

JDat

JDat

JDat

JDat

JDat

PzZw

KJmu

KJmu

PzZl

PzZs

PzpCb

PzpCb

DZs

PzZl 

Kg

Kg

Kg

Kg

KgKg

Kg

DZs

DZs

DZs

DZs

DZs

Tv

Tv

Kkyu

Kkyu

Kkyu

Kkyu

-170° -160° -150° -140°

50°55°60°65°

0 1000 km

Index map

Abstract

The Cenozoic basins of interior Alaska are poorly understood, but may host undiscovered hydrocarbon resources in sufficient quantities to serve remote villages and for possible export. Purported oil seeps and the regional occurrence of potential hydrocarbon source and reservoir rocks fuel an exploration interest in the 46,000 km2 Yukon Flats basin. Whether 

hydrocarbon source rocks are present in the pre-Cenozoic basement beneath Yukon Flats is difficult to determine because vegetation and surficial deposits obscure the bedrock geology, only limited seismic data are available, and no deep boreholes have been drilled. Analysis of regional potential field data (aeromagnetics and gravity) is valuable, therefore, for preliminary characterization of basement lithology and structure.We present our analysis as a red-green-blue composite spectral map consisting of: (1) reduced-to-the-pole magnetics (red), (2) magnetic potential (green), and (3) basement gravity (blue). The color and texture patterns on this composite map highlight domains with common geophysical characteristics and, by inference, lithology. The observed patterns yield the primary conclusion that much of the basin is underlain by Devonian to Jurassic oceanic rocks related to the Angayucham and Tozitna terranes (JDat). These rocks are part of a lithologically diverse assemblage of brittlely deformed, generally low-grade metamorphic rocks of oceanic affinity; such rocks probably have little or no potential for hydrocarbon generation. The JDat geophysical signature extends from the Tintina fault system northward to the Brooks Range. Along the eastern edge of the basin, JDat appears to overlie moderately dense and non-magnetic Proterozoic(?) and Paleozoic continental margin rocks. The western edge of the JDat in subsurface is difficult to distinguish due to the presence of magnetic granites similar to those exposed in the Ruby geanticline. In the southern portion of the basin, geophysical patterns indicate the possibility of overthrusting of Cenozoic sediments and underlying JDat by Paleozoic and Proterozoic rocks of the Schwatka sequence. These structural hypotheses provide the basis for an overthrust play within the Cenozoic section just south of the basin. 

Yukon Flats basin

Yukon Tananaprovince

Brooks Range province

Ruby geanticline

Porcupine province

Geology 

Yukon Flats Geophysical Domains

Label Description Geological Association Geophysical Interpretation Risk*

A1 Mottled yellow-green, NESW grain Yukon-Tanana terrane 

(Pzp

C

y)

Gravity lows reflect pervasive felsic plutonism, mag highs reflect metamorphic minerals and grain. 

M 

A2 More dark green than A1, 

NE-SW grain 

White Mountain terrane (PzZw) 

Similar to A1 except less pluton volume and more subdued magnetic grain. 

M 

A3 More blotchy than A1, less 

grain 

Yukon-Tanana terrane (off of map area to south)

Similar to A1 except less pluton volume and blotchy magnetic grain. 

M 

A4 Colors more subdued than 

A1, no grain 

Manley and Livengood terranes (KJmu)

Overall felsic character, no magnetic grain. M 

B1 Uniform yellow Crooked Creek (Tintina 

trough) basin (Qu)

Cenozoic basin. M 

B2 Mostly yellow, boundaries 

indistinct 

Mixed, incl. granite, quaternary, Schist belt, (Kg, Qu, Pzp

C

b)

Voluminous felsic plutons. H 

B3 Yellow, gradational boundary 

to green (K1) 

Tatonduk terrane, Kandik, Glen Shale, Tert. Seds, Porcupine (Kk

u, KJg, P

Cta, 

T

RPs)

Thick Kandik sediments – Tatonduk terrane (NA affinity). 

M 

B4 Yellow with green strands Quaternary cover, Tertiary 

volcanics (Qu, Tv, Kkyu)

Thick Cenozic sediments and/or felsic plutons. 

M 

B5 Faded yellow & blue Granite, YTT (Kg, Pzp

C

y) Felsic plutons M 

C1 Mottled orange pattern on 

green background 

Schist belt, granite (Pzp

C

b, 

Kg)

Schist belt (green field) with extensive felsic intrusion (orange blotches). 

M 

C2 Pattern more faded than C1 Schist belt, Quaternary 

cover, (PzpC

b, Qu) 

Thin schist belt over Tozitna. M 

D1 Diffuse yellow-green stain 

over blue sky background 

Entirely covered by Yukon Flats basin Quaternary (Qu) 

Cenozoic sediments or felsic plutons over Tozitna. 

M 

D2 Small yellow stain Ditto (Qu) Felsic pluton intruded into Tozitna? M D3 Yellow stain Quat cover and 

Angayucham-Tozitna (Qu, 

JDat) 

Cenozoic sediments or felsic pluton. M 

E1 Diffuse prune color with a 

few scattered RTP highs 

Quat cover, AngayuchamTozitna, and Porcupine (Qu, JDat, DCpu) 

Thick, low-density, mostly non-magnetic crustal section. No Angayucham-Tozitna. 

M 

F1 Bright pink Hammond terrane (PzpC

b) Mafic belt L 

F2 ditto Ditto (PzpC

b) Mafic belt L 

F3 ditto Schist belt (PzpC

b) Mafic belt L 

F4 ditto Ditto (PzpC

b) Mafic belt L 

F5 ditto Angayucham-Tozitna 

(JDat) 

Mafic belt L 

F6 ditto Ditto (JDat) Mafic belt L F7 ditto Angayucham-Tozitna 

(JDat) 

Mafic belt L 

F8 ditto Ditto (JDat) Mafic belt L F9 ditto Angayucham-Tozitna and 

cover rocks (JDat, Qu) 

Mafic belt L 

F10 ditto Ditto (JDat, Qu) Mafic belt L F11 Faded pink Quaternary cover (Qu) Mafic belt L 

F12 ditto F13 ditto F14 ditto F15 Bright pink F16 ditto F17 Faded pink G1 Blue sky with lig

clouds

G2 Ditto G3 Ditto H1 Uniform light piH2 Mostly light pink

blue fringes

H3 Bright white H4 White with yelloI1 Blue with minor I2 Pure blu

e 

I3 Light blue 

*Risk denotes relative confidence in geophysical interpretation

GEOLOGIC UNITSUNITS PRESENT IN MORE THAN ONE PROVINCE Qu Unconsolidated and poorly consolidated sediments, undivided (Quaternary) Tb Basalt flows and rare cinder cones (Tertiary) Tg Granitic rocks (Tertiary) Ts Clastic sedimentary rocks (Tertiary) TKs Sedimentary rocks (Tertiary and Cretaceous) Kg Granitic rocks (Cretaceous) TRPgt Glenn Shale, lower part, and Tahkandit Limestone, undivided (Triassic and Permian) JDat Angayucham-Tozitna terrane, undivided (Early Jurassic to Devonian) BROOKS RANGE PROVINCE Tv Volcanic rocks (Tertiary) Kkyu Sedimentary rocks of the Yukon-Koyukuk basin, undivided (Cretaceous) KDe Sedimentary rocks of the Endicott Mountains allochthon of Moore and others (1994) (Cretaceous to Devonian) TRPzd Metasedimentary, metavolcanic, and sedimentary rocks of the Doonerak area (Triassic to lower Paleozoic) Mbu Rocks of Brooks Range sequence of Brosgé and Reiser (2000), undivided, (Mississippian) Pzvu Sedimentary rocks of the Venetie terrane of Silberling and others (1994), undivided (Paleozoic) DZs Metamorphosed sedimentary rocks (Devonian to Proterozoic) PzpCb Metasedimentary and metaigneous rocks of the southern Brooks Range and Ruby geanticline, (Paleozoic and (or) Precambrian) PORCUPINE PROVINCE Kku Sedimentary rocks of the Kandik basin, undivided (Cretaceous) KJg Glenn Shale, upper part (Lower Cretaceous and Jurassic?) KJu Sedimentary rocks, undifferentiated (Cretaceous? and Jurassic?) JMsu Strangle Woman Creek sequence of Brosgé and Reiser (1969), undivided (Jurassic to Mississippian)Cg Granite (Carboniferous) Pzcm Metamorphic rocks (Paleozoic?) Pzqs Sedimentary and igneous rocks (Paleozoic) JMpu Younger strata of the Porcupine River sequence of Brosgé and Reiser (1969), undivided (Jurassic to Mississippian) DCpu Older strata of the Porcupine River sequence of Brosgé and Reiser (1969), undivided (Devonian to Cambrian) PCta Sedimentary rocks of the Tatonduk area (Permian to Cambrian) CPt Sedimentary rocks of the Tindir Group, (Cambrian? and Proterozoic) YUKON-TANANA PROVINCE KJmu Sedimentary rocks of the Manley basin, undivided (Cretaceous and Jurassic) –Mzmv Fine-grained sedimentary rocks and tuff (Mesozoic?) TRPs Sedimentary rocks (Triassic to Early Permian) MzPza Low-grade metamorphic rocks (Mesozoic? and (or) Paleozoic?) DSc Metamorphosed sedimentary rocks (Devonian and Silurian) Pzum Ultramafic rocks (Paleozoic?) PzZs Sedimentary and igneous rocks corresponding to older parts of the Schwatka - Rampart area sequence of Weber and others (1992) (Paleozoic to Proterozoic) PzZl Sedimentary and igneous rocks corresponding to the Livengood area sequence of Weber and others (1992) (Paleozoic to Proterozoic) PzZw Sedimentary and igneous rocks corresponding to the older parts of the Fairbanks-White Mountains area sequence of Weber and others (1992) (Paleozoic to Proterozoic) PzpCy Metamorphic rocks of the Yukon-Tanana Upland, undivided (Paleozoic to Precambrian?) 

See Till and others (2006) for complete geologic unit descriptions