Age of Tephra Beds at the Ocean Point 

Dinosaur Locality, North Slope, Alaska, 

Based on K-Ar and 40Ar/39Ar Analyses

U.S. GEOLOGICAL SURVEY BULLETIN 1990-C
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with the last offerings, are given in the current-year 

issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey 

publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications that are 

listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability 

List" are no longer available.

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, 

which is for sale in microfiche from U.S. Geological Survey Book and Open-File Report Sales, Box 25425, Denver, CO 80225.

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL 

Books

Professional Papers, Bulletins, Water-Supply Papers, 

Techniques of Water-Resources Investigations, Circulars, publications of general interest (such as leaflets, pamphlets, booklets), single copies of periodicals (Earthquakes & Volcanoes, 

Preliminary Determination of Epicenters), and some miscellaneous reports, including some of the foregoing series that have 

gone out of print at the Superintendent of Documents, are 

obtainable by mail from

U.S. Geological Survey, Book and Open-File Report Sales

Box 25425 

Denver, CO 80225

Subscriptions to periodicals (Earthquakes & Volcanoes 

and Preliminary Determination of Epicenters) can be obtained 

ONLY from

Superintendent of Documents

ILS. Government Printing Office

Washington, DC 20402

(Check or money order must be payable to Superintendent 

of Documents.)

Maps

For maps, address mail orders to

U.S. Geological Survey, Map Sales

Box 25286 

Denver, CO 80225

Residents of Alaska may order maps from

U.S. Geological Survey, Map Sales

101 Twelfth Ave. - Box 12

Fairbanks, AK 99701

OVER THE COUNTER 

Books

Books of the U.S. Geological Survey are available over 

the counter at the following U.S. Geological Survey offices, all 

of which are authorized agents of the Superintendent of Documents.

- ANCHORAGE, Alaska-4230 University Dr., Rm. 101

- ANCHORAGE, Alaska-605 West 4th Ave., Rm G-84

 DENVER, Colorado-Federal Bldg., Rm. 169,1961 Stout 

St.

- LAKEWOOD, Colorado- Federal Center, Bldg. 810

- MENLO PARK, California-Bldg. 3, Rm. 3128,345 MiddlefieldRd.

 RESTON, Virginia-National Center, Rm. 1C402,12201 

Sunrise Valley Dr.

- SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105,125 

South State St.

 SAN FRANCISCO, California-Customhouse, Rm. 504, 

555 Battery St.

 SPOKANE, Washington-U.S. Courthouse, Rm. 678, 

West 920 Riverside Ave.

 WASHINGTON, D.C.-U.S. Department of the Interior 

Bldg., Rm. 2650,1849 C St., NW.

Maps

Maps may be purchased over the counter at the U.S. 

Geological Survey offices where books are sold (all addresses 

in above list) and at the following Geological Survey offices:

ROLL A, Missouri-1400 Independence Rd.

- FAIRBANKS, Alaska-New Federal Building, 101 

Twelfth Ave.
Chapter C

Age of Tephra Beds at the Ocean Point 

Dinosaur Locality, North Slope, Alaska, 

Based on K-Ar and 40Ar/39Ar Analyses

By JAMES E. CONRAD, EDWIN H. McKEE, and 

BRENT D.TURRIN

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1990

EVOLUTION OF SEDIMENTARY BASINS NORTH SLOPE BASIN
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1992

For sale by

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Conrad, James E.

Age of tephra beds at the Ocean Point dinosaur locality, North Slope, Alaska, 

based on K-Ar and ^Ar/^Ar analyses/by James E. Conrad, Edwin H. McKee, and 

Brent D. Turrin.

p. cm. - (Evolution of sedimentary basins-North Slope basin; ch. C) 

(U.S. Geological Survey bulletin; 1990-C)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3: 1990-C

1. Volcanic ash, tuff, etc.~Alaska~Ocean Point Region. 2. Geology, Stratigraphic Cretaceous. 3. Geology-Alaska Ocean Point Region. 4. Potassiumargon dating. 5. Prince Creek Formation (Alaska) I. McKee, Edwin H. II. Turrin, 

Brent D. III. Title. IV. Series. V. Series: U.S. Geological Survey bulletin : 1990-C. 

QE75.B9 no. 1990-C 

[QE461] 

557.3 s-dc20

[552'.23] 91-4319

CIP
CONTENTS

Abstract Cl

Introduction Cl

Geologic setting Cl

Methods C2

Results C4

Conventional K-Ar analyses C4 

^Ar/39 Ar analyses C4

Sample M86LPA19 C5 

Sample 84ACrl80Y C7 

Sample M86LPA14 C7 

Sample 86ACr019 CIO 

Sample M86LPA9 CIO

Discussion Cll

References cited C12

FIGURES

1 . Map showing locations of dated tephra beds of Prince Creek Formation, 

northern Alaska C2

2. Composite schematic stratigraphic section of Prince Creek Formation C3

3. Scanning electron micrographs of untreated glass shards from Colville River 

tephra beds C3

4. Diagrams showing results of ^Ar/^Ar analyses C8

5. Graph showing best age estimates for Colville River tephra beds Cll

TABLES

1 . Results of conventional K-Ar analyses of Colville River tephra beds C4

2. Results of ^Ar/^Ar analyses of Colville River tephra beds C6

3. Summary of K-Ar and 40Ar/39Ar ages of Colville River tephra beds C 10

Contents
Cover. Looking upsection (north) along Colville River, Alaska, at 30-m-high bluffs of Cretaceous nonmarine 

siltstone and sandstone of Prince Creek Formation, unconformably overlain by Pliocene and Quaternary 

Gubik Formation. Low bench in foreground marks a dinosaur-bone-bearing bed. Drawing by J.F. Vigil, from 

photograph by E.M. Brouwers.
Age of Tephra Beds at the Ocean Point 

Dinosaur Locality, North Slope, Alaska, 

Based on K-Ar and 40Ar/39Ar Analyses

By James E. Conrad, Edwin H. McKee, and Brent D. Turrin

Abstract

Tephra layers exposed above and below dinosaur-bonebearing beds of the Prince Creek Formation, North Slope, 

Alaska, provide an opportunity to date these beds by radiometric methods. Although the tephra layers contain few or 

no potassium-bearing minerals, they are composed of apparently unaltered glass shards that appear to be suitable for 

K-Ar dating. *Ar/MAr incremental-heating analyses were 

conducted in order to evaluate the accuracy of the K-Ar 

ages, and these analyses indicate that most of the tephra layers have undergone only minor argon loss. Most of the K-Ar 

and ^Ar/^Ar ages are analytically indistinguishable and indicate that the bone-bearing beds are between 68 and 71 Ma. 

The dinosaur-bone-bearing beds of the Prince Creek Formation therefore appear to be of latest Cretaceous age (the 

Cretaceous-Tertiary boundary is currently considered to be 

about 66 Ma).

INTRODUCTION

The age of dinosaur-bone-bearing beds of the Prince 

Creek Formation exposed along the Colville River near 

Ocean Point, Alaska (fig. 1), has aroused much interest in 

the geologic community because of its bearing on the hypothesis that the Cretaceous-Tertiary extinction was caused 

by an asteroid impact (Alvarez and others, 1980). Paleontological studies based on marine faunas have suggested 

that at least part of the section exposed along the Colville 

River is Paleocene in age (Marincovich and others, 1985), 

and a fission-track age on zircon of 50.9±7.7 Ma from a 

tephra bed near the dinosaur-bone-bearing beds (Carter 

and others, 1977) seemed to confirm this age designation. 

If the Paleocene age is valid it would extend the age range 

of dinosaurs in Alaska into the Cenozoic and would cast 

doubt on the impact hypothesis as an explanation for the

Manuscript approved for publication, April 15,1991.

extinction of the dinosaurs. Other studies suggest that at 

least the part of the Prince Creek Formation that contains 

the dinosaur bones is Late Cretaceous in age (McDougall, 

1987; Brouwers and others, 1987).

Abundant tephra beds exposed along the Colville 

River bluffs about 7 km west of Ocean Point (fig. 1) contain material suitable for dating by K-Ar techniques and 

provide an excellent means for establishing the age of 

these sedimentary rocks. This paper presents the results of 

conventional K-Ar and ^Ar/^Ar analyses of glass shards 

from the tephra beds and evaluates the suitability of these 

relatively old rhyolitic glasses for ^Ar/^Ar and K-Ar dating techniques.

GEOLOGIC SETTING

The Prince Creek Formation consists of a series of 

poorly consolidated sandstones, siltstones, and shales that 

dip gently to the northeast (Gryc and others, 1951; Detterman and others, 1975). The Colville River has cut into this 

formation, creating a bluff that stands some 20-30 m high 

and extends many kilometers upriver from Ocean Point.

At least 10 tephra beds are exposed in the section 

northwest of Ocean Point, and volcanic glass is a major 

component in the sedimentary rocks in much of the remainder of the section (fig. 2). Almost all the tephra beds 

are associated with dark beds that are rich in organic matter and probably represent flood-plain deposits (Phillips, 

1988). Most of the dinosaur bones are found in these 

organic-rich horizons, although scattered bones occur in 

some sandstone beds.

The tephra beds stand out as resistant, buff-colored 

strata ranging from about 1 cm to 2 m in thickness. They 

interfinger with the organic-rich beds, pinching out over 

distances of hundreds of meters. The tephra is composed 

almost entirely of rhyolitic glass shards, although clays, 

quartz, and feldspar are minor constituents, and altered biotite grains occur in some of the tephra beds. The glass

Age of Tephra Beds at the Ocean Point Dinosaur Locality, North Slope, Alaska C1
shards appear well preserved with sharp and ragged edges; 

whole bubbles remain (fig. 3). Microscopic examination 

revealed no traces of devitrification or alteration of the 

glass shards, suggesting that the glass is suitable for radiometric dating. The completely unabraided character of the 

fragile shards indicates they are airfall deposits that have 

not been transported by water for any distance. Clay minerals are generally a minor component of most of the tephra beds, although some beds are completely altered to 

clay. The differences in alteration are probably attributable 

to differences in initial composition of the glass and differences in local ground-water effects.

Because there are few or no potassium-bearing mineral phases in the tephra beds, glass shards were used for 

K-Ar and ^Ar/^Ar dating of the beds. Although relatively 

young tephra deposits are routinely dated by the K-Ar 

method, the reliability of glass older than about 15 million 

years for dating by this technique is questionable because 

glass tends to readily devitrify. This devitrification results 

in the redistribution or loss of radiogenic argon, which in 

turn leads to anomalous ages (Fleck and others, 1977). 

Glass shards from the Colville River tephra beds appear 

fresh and well preserved. In order to evaluate the accuracy 

of the ages, multiple K-Ar, ^Ar/^Ar total-fusion, and incremental, or age-spectrum, ^Ar/^Ar analyses were performed and the resulting ages compared. The 

age-spectrum analyses in particular provide a means to 

evaluate the possibility of alteration or diffusion that 

would lead to anomalous ages.

METHODS

Prior to analysis, the tephra samples were disaggregated and sieved to 100 to 140 mesh (149 to 105 micrometers). After ultrasonic cleaning to remove dust and 

clays adhering to the shards, a combination of magnetic 

and heavy-liquid separation techniques was used to produce a pure glass separate. In order to minimize contamination and insure homogeneity, a variable-density 

heavy-liquid column was used to separate the glass shards 

into a relatively narrow specific-gravity range. This technique also helps to eliminate grains with a greaterthan-average area of altered surfaces. For example, it was 

found that bubble-wall shards, with a higher surface-tovolume ratio than the bubble-corner shards, tended to be 

slightly less dense, presumably owing to a greater percentage of surface alteration.

The glass separates were etched in 5-percent hydrofluoric acid for approximately 45 seconds to remove surface 

alteration and reduce atmospheric-argon contamination. 

Conventional K-Ar analyses on shards not treated with 

acid gave ages as much as 10 percent younger than ages 

from the acid-treated samples. Furthermore, the nontreated 

samples had four times the atmospheric-argon contamination displayed by the treated samples; this contamination 

is significant in determination of the precision (± factor) of 

the age.

151°30' 151°20'

Figure 1. Locations of dated tephra beds of Prince Creek Formation, northern Alaska. Sample numbers correspond to tephra samples dated by K-Ar and (or) 40Ar/39Ar techniques, discussed in text. Numbers 1-25 mark locations of measured sections (Phillips, 

1988).

C2 Evolution of Sedimentary Basins North Slope Basin
Conventional K-Ar analyses were performed using 

standard isotope-dilution techniques similar to those described by Dalrymple and Lanphere (1969). A 60°-sector, 

15.2-cm-radius, Nier-type mass spectrometer was used for 

argon analysis. Potassium analyses were performed by a 

lithium metaborate flux fusion-flame photometry technique, 

using lithium as an internal standard (Ingamells, 1970).

Samples used in the ^Ar/^Ar experiments were heated by two different techniques induction coil (Dalrymple 

and Lanphere, 1969) and continuous laser (York and others, 1981). Samples heated by induction coil the more 

conventional technique were irradiated in the U.S. Geological Survey TRIGA1 nuclear reactor (Denver, Colo.) for 

a period of 20 hours at a power of 1 megawatt. Irradiation

THICKNESS,

IN 

METERS

SAMPLE AND

FOSSIL 

LOCATIONS

r^

160-

140-^^

120-

1 M86LPA9

100 -&£

80-

:*t 84ACM80Y

t M86LPA19

1TRIGA (Training Research Isotope General Atomic) is a registered trademark of General Atomics.

EXPLANATION

Marine deposits

Interdistributary-bay deposits

Nonmarine deposits

'$&& Major fluvial distributaries

 Contact

 ? Fault-Queried where uncertain

t Dated tephra bed

v Vertebrate fossil occurrence

Figure 2. Composite schematic stratigraphic 

section of Prince Creek Formation exposed 

along Colville River 7 km west of Ocean 

Point, northern Alaska. Total stratigraphic section is 1 78 m. Horizons from which samples 

were collected and radiometrically dated are 

labeled with sample numbers (see fig. 1 for locations). Modified from Phillips (1988).

20

 1PM

10 Lim

Figure 3. Scanning electron micrographs of untreated glass 

shards from Colville River tephra beds. A, Sample M86LPA9 

shows little surface alteration and produced an undisturbed 

40Ar/39Ar age spectrum. B, Sample M86LPA14 has significantly 

more surface alteration and showed evidence of recoil in 

40Ar/39Ar analysis. See figure 1 for sample locations.

Age of Tephra Beds at the Ocean Point Dinosaur Locality, North Slope, Alaska C3
Table 1. Results of conventional K-Ar analyses of Colville River tephra beds

[Samples listed in probable stratigraphic order, with sample from lowermost tephra bed at bottom of table, rad, radiogenic; a, standard

deviation. Constants: X£+V£ = 0.581 x 10" 10 yr' 1 , Ka = 4.962 x 10'10 yr" 1 = 1.167 x 10'4 mol/mol]

Sample 

number

M86LPA9 

86ACr019 

M86LPA14 

M86LPA19

Experiment 

number

871589 

871535 

871590 

871452

Material

Glass 

Glass 

Glass 

Glass

K20 

(weight 

percent)

5.74 

3.87 

3.83 

5.46

40Arrad 

(mole/g 

x lO'10)

5.811 

3.732 

3.817 

5.647

40Arrad 

(percent)

88.8 

77.0 

70.7 

76.7

Age±la 

(Ma)

69.5±2.1 

65.8±2.0 

68.5±2.0 

70.4±2.1

flux was monitored by use of the SB-3 biotite standard, 

which has an age of 162.9 Ma. Heating steps were onehalf-hour intervals with higher temperatures (greater than 

about 750 °C) monitored by optical pyrometer. Temperatures of lower temperature steps are approximate, based on 

extrapolation from higher temperature steps along known 

curves of RF (radio-frequency) generator power plotted 

against temperature. Isotopic analyses were made at the 

U.S. Geological Survey in Menlo Park, Calif., using a 

multiple-collector mass spectrometer (Stacey and others, 

1981). Sample handling techniques and corrections for 

calcium- and potassium-derived isotopes used in the Menlo Park laboratory are as described by Dalrymple and 

Lanphere (1971,1974).

Samples used for the laser-heating analyses were irradiated in the TRIGA reactor at the University of California, Berkeley. Irradiation flux was monitored by use of 

standards MMhb-1 hornblende, with an age of 520.4 Ma, 

GHC 305 biotite, with an age of 103.76 Ma, and P-207 

muscovite, with an age of 82.1 Ma. The following 

potassium and calcium corrections for the UC Berkeley 

TRIGA reactor were determined using optical grade 

CaF2 and a laboratory potassium glass: 40ArK/39ArK=0.0086; 

39ArCa /37ArCa=0.0003; and 36Arc./37Arc.=0.0003.

The laser-heated ^Ar/^Ar extractions and isotopic 

analyses were conducted at the Geochronology Center at 

the Institute of Human Origins (Berkeley, Calif.) using a 

laser-fusion microextraction system and an online, ultrasensitive mass spectrometer. Argon backgrounds for this 

system are approximately the following: ^AiM.OxlO'12 

cm3 (STP); 39Ar=1.0xlO'13 cm3 (STP); 37Ar=7.6xlO- 14 cm3 

(STP); and ^Ai^^xlO'14 cm3 (STP). Detection limit for 

this mass spectrometer is on the order of l.OxlO" 14 cm3 

(STP). Use of laser-light energy for fusion greatly reduces 

the background argon because there is less nonfocused 

heating and subsequent outgassing of the ultrahigh vacuum system. This system is designed to analyze samples as 

small as single grains (less than 0.1 mg), but its chief advantage for this study is that as many as four individual 

fusions and analyses per hour are possible. For the 

age-spectrum experiments, less than about 0.25 mg (approximately 30 to 40 shards) was used. The temperature

of the heating steps for the laser-heating experiments was 

not determined; progressive heating of the sample was 

achieved by gradually increasing laser output at each heating step to produce roughly equivalent gas fractions as determined by trial and error. Laser output started at 0.5 

watts for the initial heating steps and was increased at 

0.25- to 0.5-watt increments until fusion occurred. The duration of each heating step was approximately 20 seconds. 

The decay constants used in the age calculations are 

those recommended by Steiger and Jager (1977). Reported 

errors are an estimate of the standard deviation of analytical precision and were calculated using standard 

propagation-of-error methods (Taylor, 1982).

RESULTS

Conventional K-Ar Analyses

Conventional K-Ar ages on the Colville River tephra 

beds range from 65.8±2.0 to 70.4±2.1 Ma (table 1). The 

four samples listed in table 1 (see also fig. 1) are in probable stratigraphic order. (Sample 84ACrl80Y is absent 

from table 1 because it was not dated by the K-Ar method.) Faults and covered intervals in the section prohibit a 

direct correlation between some of the dated tephra units, 

but in general correlation is good, and the section becomes 

younger to the northeast judging from the dip of the beds. 

Within the limits of analytical uncertainty, the ages are indistinguishable from one another, so these data are consistent with a general northeasterly younging of the section 

across faults and covered intervals. The age of sample 

86ACr019 is concordant with the other ages, but *°Ar/39Ar 

data (described later) suggest that this sample may have 

undergone some argon loss. The K-Ar data indicate that 

the age of these beds ranges from about 70-71 Ma to 

about 66-68 Ma.

40Ar/39Ar Analyses

^Ar/^Ar analyses were performed to explore the 

possibility that some or all of the conventional K-Ar ages

C4 Evolution of Sedimentary Basins North Slope Basin
may be too young. Anomalously young ages could result 

if the glass has lost argon via diffusion or alteration. The 

^Ar/^Ar incremental, or age-spectrum, technique is used 

to evaluate the possibility of argon loss or redistribution 

caused by reheating or alteration of minerals. Although it 

is rarely used on terrestrial volcanic glasses, the fundamental concepts of the ^Ar/^Ar spectrum technique 

(Lanphere and Dalrymple, 1978) are the same as when applied to crystalline materials. A sample is heated to progressively higher temperatures, and a series of ages, 

termed apparent ages, is calculated from the argon expelled at each temperature step. Typically, a history of alteration or reheating of a mineral sample will result in 

younger ages for the low-temperature steps. Higher temperature steps will produce older ages that may form a 

plateau around a value thought to reflect the true cooling 

age of the rock. In general, this pattern is attributed to a 

higher argon retentiveness of the high-temperature sites in 

the crystal lattice. Although glass is not directly comparable to minerals in that it lacks a well-developed crystal 

structure, 40Ar/39Ar incremental heatings can nonetheless 

give insight into the argon retentiveness of glass. The assumption used for interpreting incremental heatings of 

glass is that the low-temperature steps (about 300-500 °C) 

release loosely held argon from the outside edges of the 

glass shards and from alteration products. Higher temperature steps (about 550 °C and higher) release argon (by volume diffusion) from inner portions of the shards that have 

been less affected by alteration and diffusion over geologic time. Alteration and diffusion of argon will be most pronounced along the edges of the shards; low-temperature 

steps will reveal these effects of argon loss by yielding 

anomalously young apparent ages.

All samples were analyzed by either induction-heating 

or laser-heating 40Ar/39Ar techniques, and one sample 

(M86LPA9) by both methods (table 2). Three of the samples (M86LPA19, 84ACrl80Y, M86LPA9) produced 

well-defined plateaus in the argon-release sequence 

(fig. 4A) and show only minor radiogenic argon loss at 

low-temperature steps. The amount of argon loss can be 

estimated by comparison of plateau ages and recombined 

total-fusion ages. In a total-fusion analysis, a sample is 

completely fused after irradiation and all of the Ar released is analyzed in a single experiment. To simulate this 

technique, the data can be combined from the incremental 

Ar releases to calculate a total-fusion age, known as a recombined total-fusion, or total-gas, age for each sample. 

The recombined total-fusion ages for all samples are about 

1 m.y. younger than the plateau ages (table 3), indicating 

loss of radiogenic argon of less than 2.5 percent. Two of 

the samples (M86LPA14, 86AGO 19) give less welldefined plateaus, and their calculated ages appear 

anomalous on the basis of the inferred stratigraphic se40 A -/36, quence. Ar/ Ar versus Ar/ Ar isochron diagrams and

36Ar/40Ar versus ^Ar/^Ar correlation diagrams for these 

samples also show evidence of disturbance (figs. 4B, 4Q.

Sample M86LPA19

This tephra bed is inferred to be the oldest unit dated 

in this study on the basis of its stratigraphic position. Glass 

from the tephra was dated twice by the age-spectrum 

method and once by the total-fusion method using 

laser heating (table 2). The plateau ages from the two 

age-spectrum experiments are 70.010.9 and 71.4±0.6 Ma 

(table 3). Although temperatures of individual steps were 

not determined, the general plateau shape is similar to that 

of samples for which temperatures are known. Both age 

spectra for this tephra unit show evidence of minor loss of 

^Ar in the low-temperature steps. This loss is expressed 

by young ages in the first step of experiment LI 15-2, 

which involved about 7 percent of the total release of 39Ar 

from the sample, and in the first two steps of experiment 

L115-3, which involved about 32 percent of the total 39Ar 

release (fig. 4A). The difference in release patterns between the two experiments may be due to inhomogeneous 

heating by the laser heat source, leading to incomplete degassing at the lower temperature steps. In spite of this inconsistency, total radiogenic ^Ar loss, estimated by 

comparison of the recombined total-fusion ages with the 

plateau ages, is less than about 2 percent in each analysis.

Ages calculated from the *°Ar/36Ar versus 39Ar/36Ar 

isochron diagrams (fig. 4B) and 36Ar/40Ar versus 39Ar/40Ar 

correlation diagrams (fig. 4Q for these experiments agree 

with the plateau ages according to the criteria of Lanphere 

and Dalrymple (1978). The error estimates (±) derived 

from the isochron and correlation diagrams are slightly 

higher than those calculated from the plateaus because of 

the extremely low percentage of atmospheric 36Ar contamination in the laser-fusion line. 36Ar for these samples was 

typically only slightly above background values, so analytic error is relatively high. In addition, measured ^Ar/^Ar 

ratios, especially for the higher temperature plateau steps, 

are greater than about 3,000 (fig.4fl), so that the isochron 

must be projected relatively far back to the axis from 

which the initial ^Ar/^Ar ratio is derived. The combined 

effect of these factors is that the ages calculated from the 

isochron and correlation diagrams have errors approximately 2 to 3 times those indicated by the plateau calculations, but in all cases agree with the plateau ages within 

the error limits. Initial 40Ar/36Ar ratios also have large errors of as much as 100 percent or more due to the low 

36Ar content, but in both experiments, initial 40Ar/36Ar 

ratios were consistent with the expected ratio of 295.5, the 

ratio of ^Ar to ^Ar in air. The goodness-of-fit index for 

the regression of the lines in the isochron and correlation 

diagrams, given by SUMS/(N-2), ranges from 0.01 to 0.18. 

Values greater than about 2.5 indicate the presence of geologic

Age of Tephra Beds at the Ocean Point Dinosaur Locality, North Slope, Alaska C5
error (Lanphere and Dalrymple, 1978); values less than 

1.0 indicate that the scatter of points on the line is less 

than can be expected due to analytical error. In these experiments, values significantly less than 1.0 for the goodnessof-fit index suggest that the experimental errors were 

overestimated. The goodness-of-fit index nonetheless 

provides strong evidence that the tephra gives an undisturbed and geologically meaningful plateau age.

Table 2. Results of 40Ar/39Ar analyses of Colville River tephra beds

[Samples listed in probable stratigraphic order, with sample from lowermost tephra bed at end of table. Bold italic entries indicate data used in 

calculation of plateau and isochron ages. In third column, temperatures (°C) are reported for samples heated by induction coil, lettered steps for 

samples heated by continuous laser; TFU means total fusion. J, parameter used in age calculations; rad, radiogenic. Decay constants: X£+X'C = 

0.581 x 10'10 yr' 1 , K = 4.962 x 10'10 yr' 1 , ^K/K = 1.167 x 10'4 mol/mol]

Sample Experiment Temp (°C)

number

M86LPA9

1=0.00281

number or step 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar

L121-2 A 22.28 0.08604 0.06051

B 16.41 .003099 .008048

C 14.73 .01691 .003862

D 1436 .01690 .001943

E 14.18 .01330 .001050

F 14.07 .01413 .0004771

G 14.14 .01139 .0004163

H 14.08 .01649 .0008217

39 AT released

(percent)

1.28

3.90

1634

2732

20.67

10.11

5.80

14.68

40Arrad

(percent)

19.7

$5.5

923

95.9

97.7

98.9

99.0

983

Apparent

age (Ma)

22.1±25.8

69.8±6.5

67.6±1.7

68.1±0.8

68.9&.0

693±L.O

69.7±1.0

68.8±13

Reactor corrections: (36Ar/37Ar)ca=0-0003; (39Ar/37Ar)ca=0-0003; (40Ar/39Ar)K=0-0086

M86LPA9

1=0.004745

87A076 350 222.3 0.05515 0.7520

400 27.77 .0111 .07389

475 11.27 .01715 .01176

600 8.803 .01659 .002101

750 9.160 .01430 .003466

865 1738 .003415 .03180

FUSE 338.2 .1253 1.153

0.17

1.97

13.36

6255

19.41

2.46

.08

0.03

21.4

69.1

92.9

88.8

45.9

-.7

0.6±26.7

50.011.8

65.5±0.4

68.7±0.2

68.3±0.3

67.0±L3

-20.9±78.8

Reactor corrections: (36Ar/37Ar)Ca=0.000234; (39Ar/37Ar)Ca=0.000654; (40Ar/39Ar)K=0.0057

86ACr019

1=0.004745

88A003 450 16.67 0.1756 0.003098

550 9.126 .1562 .004647

625 8.788 .1621 .004158

700 10.01 .1809 .008652

800 13.49 .2357 .01977

FUSE 41.95 1.256 .1153

9.35

42.38

29.18

11.89

5.67

1.52

45.0

84.7

85.8

74.3

56.6

18.9

63.011.8

65.010.5

63.410.7

62.611.4

64.212.8

66.712.8

Reactor corrections: (36Ar/37Ar)Ca=0-000278; (39Ar/37Ar)Ca=0-0007; (40Ar/39Ar)K=0.0332

M86LPA14

1=0.004725

88A004 450 17.30 0.1889 0.03943

525 13.01 .1510 .01474

600 10.07 .1439 .004354

700 9.355 .1463 .002891

775 9.517 .1630 .004225

FUSE 64.77 .3460 .1842

4.59

23.36

32.74

26.64

9.73

2.94

32.5

66.3

87.0

90.6

86.7

16.0

47.412.9

72.110.8

73.210.6

70.910.7

69.011.4

86.015.1

Reactor corrections: (36Ar/37Ar)Ca=0-000278; (39Ar/37Ar)Ca=0-(XX)7; (40Ar/39Ar)K=0.0332

84ACrl80Y

1=0.004745

87A075 350 35.37 0.05557 0.1041

450 16.69 .05418 .03560

525 10.31 .04298 .007529

600 8.773 .03772 .001751

700 8.497 .04671 .008265

850 13.02 3258 .01610

FUSE 14.75 1.956 .02166

0.58

6.26

34.06

24J51

25.86

738

1.34

13.0

36.9

78.4

94.1

97.1

63.6

575

39.018.9

52.010.9

68.010.3

693±0.3

69.3±0.3

695±0.8

71.3±3.9

Reactor corrections: (36Ar/37Ar)ca=0.000234; (39Ar/37Ar)Ca=0.000654; (40Ar/39Ar)K=0.0057

C6 Evolution of Sedimentary Basins North Slope Basin
Table 2. Results of 40Ar/39Ar analyses of Colville River tephra beds Continued

[Samples listed in probable stratigraphic order, with sample from lowermost tephra bed at end of table. Bold italic entries indicate data used in

calculation of plateau and isochron ages. In third column, temperatures (°C) are reported for samples heated by induction coil, lettered steps for

samples heated by continuous laser; TFU means total fusion. J, parameter used in age calculations; rad, radiogenic. Decay constants:

0.581 x 10'10 yr' 1 , Xp = 4.962 x 10' 10 yr' 1 , 40K/Ktotal = 1.167 x 10'4 mol/mol]

Sample Experiment Temp (°C) 

number number or step 40Ar/^Ar

M86LPA19

1=0.00281

LI 15-1 TFU

LI 15-2 A

B

C

D

E

F

G

LI 15-3 A

B

C

D

E

F

15.29

18.15

15.36

14.88

14.85

14.87

14.45

14.77

17.34

15.01

14.77

14.65

14.92

15.03

37Ar/39Ar

0.02571

0.07131

.03294

.02771

.02393

.03198

.03979

.02632

0.007356

.02244

.02248

.01742

.01306

.01364

36Ar/39Ar

0.005813

.02405

.004176

.002610

.003381

.003107

.001380

.001768

0.01290

.003973

.001475

.0007919

.001839

.002773

"Ar released 

(percent)

100

6.29

20.85

17.40

20.15

1137

1.83

22.12

2.80

28.68

47.44

15.67

2.76

2.64

Radiogenic Apparent 

40Ar age (Ma)

88.7

60.8

91.9

94.8

933.

93.8

97.1

96.4

78.0

92.1

97.0

98.4

96.3

94J5

67.5±1.5

55.1±7.4

702+2.4

70.1±2.1

68.8+2.0

69.3±2.9

69.8±3.0

70.8+1.4

67.3±4.2

68.8±0.6

713±0.8

71.6+A.7

71.4+3.3

70.6+3.4

Reactor corrections: (36Ar/37Ar)Ca=0.0003; (39Ar/37Ar)ca=0.0003; (40Ar/39Ar)K=0.0086

Sample 84AO1 SOY

Sample 84ACrl80Y was collected from a tephra bed 

inferred to be the second oldest in the area of study. A 

fault of unknown displacement separates it from the 

M86LPA19 tephra bed, the oldest tephra in the study area. 

Conventional induction-heated ^Ar/^Ar incremental-heating 

analysis on glass from this tephra sample gives a plateau 

age of 69.3±0.4 Ma (table 3). Minor radiogenic ^Ar loss 

is apparent in the 350-, 450-, and 525-°C steps (fig. 4A, 

table 2), which total nearly 41 percent of the total 39Ar 

release. Comparison with the recombined total-fusion age 

of 67.610.4 Ma (table 3) indicates that total radiogenic

40, /39

'Ar loss is about 2.5 percent. The wAi/36Ai versus

/36, vAr/ Ar isochron diagram (fig. 4fl) and Ar/*Ar versus 

39Ar/40Ar correlation diagram (fig. 4Q yield identical ages 

of 69.3±0.4 Ma and initial ^Ar/^Ar ratios of 297.9±6.1 

and 298.0±6.1, respectively; these ratios are identical to 

the expected value of 295.5. The values of SUMS/(N-2) 

are about 0.08 and 0.09 for these diagrams, suggesting that 

the age is geologically accurate. Inclusion of the 525-°C 

step on the plateau does not significantly change the plateau age or the ages derived from the isotope and correlation diagrams, but it changes the SUMS/(N-2) value for 

the diagrams to about 5, suggesting geologic error, in this 

case most likely due to minor radiogenic ^Ar loss.

Sample M86LPA14

Sample M86LPA14 was taken from a tephra bed 

about 18 m stratigraphically above the 84ACrl80Y tephra

bed. Ar/ Ar age-spectrum analysis suggests some 

disturbance of argon in this sample. As in the two previous samples, the initial low-temperature (450 °Q step 

shows evidence of radiogenic ^Ar loss; it is followed by 

an apparent plateau defined by the 525-, 600-, 700-, and 

775-°C steps (fig 4A). This plateau, however, shows significantly more scatter than those of the previous samples, 

with apparent ages ranging from 69.0 to 73.2 Ma (table 2). 

The ^Ar/^Ar versus 39Ar/36Ar isochron diagram (fig. 4fl) 

and the 36Ar/40Ar versus 39Ar/40Ar correlation diagram 

(fig. 4Q also suggest that there is some argon redistribution in this sample. No combination of steps from the apparent plateau yields ages in the isochron and correlation 

diagrams that are concordant with the corresponding plateau age and have a SUMS/(N-2) value of less than about 

4; most combinations give higher values. Initial 40Ar/36Ar 

values are highly variable and range from about 120 to 

more than 500 (expected value is 295.5), depending on the 

heating increments used in the regression. One explanation 

for the anomalous data for this sample is that it contains 

extremely fine-grained alteration products, most likely 

clays, that have lost small amounts of 39Ar by recoil during or shortly after irradiation, thus giving slightly old apparent ages. Samples of glass shards from this tephra unit 

viewed under the scanning electron microscope (fig. 3) 

show significantly more surface alteration than shards that 

give apparently undisturbed spectra. This alteration may 

have penetrated the glass from this tephra bed deeply 

enough that the hydrofluoric acid treatment did not remove

Age of Tephra Beds at the Ocean Point Dinosaur Locality, North Slope, Alaska C7
93 uo;)n|OAg jo Xje)uauiipa$ I|)JON suiseg ado|5 mseg 

PRESENT BEFORE YEARS OF MILLIONS IN AGE, APPARENT 

00 <7> *> to -J 

C O O O O O 

+ )S 

ro -i 

s 3

§

T + 

Tl 

+ C 

1 moi'

P T 

2^
100

ore

00

SL

?f 

S?

2

o 

o 

SZ.

12,000

6000 -

0.004

0.002

(

12,000

<t 6000

80

M86LPA19 

LI 15-3

20,000

10,000

20 40 60 80 

RELEASED, IN CUMULATIVE PERCENT

100

500 1000 1500 

400 800

0.004

^ 0.002

£

0.006

0.004

0.002

800 1600 

K 

ST

Figure 4. 40Ar/39Ar analyses (see table 2). In upper left corner of first 

graph in each row are sample number (first) and experiment number 

(second) that apply to graphs in that row. Samples are in probable 

stratigraphic order, with sample from lowermost tephra bed at bottom 

of figure. A, Argon-release spectra from incremental heating. Solid 

bars indicate apparent ages for intervals shown; uncertainties in age 

(1-sigma) indicated by vertical width of bars. Numbers above bars indicate temperatures (°C) of heating steps; letters indicate heating steps 

whose temperatures were not monitored (see text). Horizontal arrows

show range of heating steps used to calcu

chron diagrams. Straight lines are isochron

data used to fit straight lines; crosses indic

(40Ar/36Ar)i js jnitja| *oAr/36Ar va|ue; SUM

index for regression of lines. C, Correlation 

figure 40. For samples that did not yield rel

is reported in figure 4A and no lines are dra

For sample M86LPA9, experiment 87A076, 

"FUSE" are too narrow to show in figure 4A
Table 3. Summary of K-Ar and 40Ar/39Ar ages of Colville River tephra beds

[Samples listed in probable stratigraphic order, with sample from lowermost tephra bed at bottom of table. , not determined]

Sample 

number

M86LPA9- -

86AO019 -

M86LPA14 

84ACrl80Y 

M86LPA19 -

Experiment 

number

L121-2 

87A076 

871589

88A003 

871535

88A004 

871590

87A075

L115-1 

LI 15-2 

LI 15-3 

871452

K-Ar 

age (Ma)

69.5±2.1 

65.812.0

68.5±2.0

70.4±2.1

^Ar/^Ar 

plateau age (Ma)

68.810.5 

68.510.4

No plateau

No plateau

69.3±0.4

70.010.9 

71.410.6

4°Ar/39Ar 

isochron age (Ma)

69.010.8 

68.710.4

69.310.4

69.513.1 

72.011.1

40Ar/39Ar totalfusion age (Ma)

68.010.7* 

67.610.4*

64.110.5*

71.110.5*

67.610.4*

67.511.5 

69.011.0* 

70.410.6*

*Recombined age.

all the alteration products. The conventional K-Ar analysis 

of this sample gave an age of 68.512.0 Ma, analytically 

indistinguishable from the age of the underlying tephra 

bed (see table 2). Thus, alteration of this tephra bed, which 

appears to have caused some argon redistribution, apparently did not result in appreciable loss of radiogenic ^Ar.

Sample 86ACr019

Sample 86AQ019 was taken from a tephra bed several 

meters stratigraphically above two major dinosaur-bonebearing beds and about 3 m below a third bone bed. This 

tephra bed is inferred to be the second youngest in this 

study, although a covered interval separates this part of the 

section from the three tephra beds described in the previous paragraphs, and a fault of unknown displacement separates it from the younger beds to the east (figs. 1, 2). 

^Ar/^Ar age-spectrum analysis of this sample reflects 

some degree of argon loss and (or) redistribution. Although a plateau appears to be present at about 64 Ma 

(table 2), the overall shape of the plateau is similar to that 

of sample M86LPA14, which is interpreted to indicate a 

disturbed age (fig. 4A). The initial 450-°C step gives a 

slightly young age, and it is followed by a series of steadily decreasing ages for the middle-temperature steps. The 

final two steps give slightly higher ages, similar to the final step for M86LPA14. Moreover, the apparent plateau 

age of about 64 Ma is slightly young on the basis of the 

ages of the other samples. The ^Ar/^Ar versus 39Ar/36Ar 

isochron diagram (fig. 45) and ^Ar/^Ar versus 39Ar/40Ar 

correlation diagram (fig. 4Q bear out the conclusion that 

this sample is slightly disturbed. Ages calculated from the 

isochron and correlation diagrams are concordant with the 

age calculated from the apparent plateau, and initial 

^Ar/^Ar ratios are close to 295.5, but the goodness-of-fit

index, SUMS/(N-2), is about 3.5, indicative of geologic 

error. The pattern of steadily decreasing ages of the plateau steps is a characteristic of recoil of 39Ar after irradiation. The young apparent ages, however, are not typical of 

recoil but instead suggest loss of radiogenic ^Ar. Possibly, 

devitrification and recrystallization of very fine grained alteration products that were susceptible to 39Ar recoil in the 

reactor also led to significant loss of radiogenic ^Ar over 

geologic time. This age spectrum is in contrast to that of 

M86LPA14, which apparently was affected by recoil effects but does not show evidence of argon loss.

Sample M86LPA9

Sample M86LPA9 is from the youngest tephra bed 

exposed near Ocean Point. ^Ar/^Ar age-spectrum analysis was performed by both induction-heating techniques 

(experiment 87A076) and laser-heating techniques (experiment L121-2), giving plateau ages of 68.510.4 and 

68.810.5 Ma, respectively (table 3). Both spectra show 

very well defined plateaus and similar release patterns 

(fig. 4A). Only minor loss of radiogenic argon is apparent 

in the low-temperature release steps. Comparison of the 

plateau ages with recombined total-fusion ages (table 3) 

indicates total radiogenic argon loss is less than 1.5 percent for both types of analysis. The ^Ar/^Ar versus 

39Ar/36Ar isochron diagrams and 36Ar/ 40Ar versus 

39Ar/40Ar correlation diagrams for both these analyses also 

indicate that the plateau ages represent undisturbed ages 

(figs. 4B, 4Q. Ages derived from the isochron and correlation diagrams are concordant with the plateau ages, initial 

^Ar/^Ai ratios are not significantly different from 295.5 

at a 95-percent confidence level, and the goodness-of-fit 

indexes (SUMS/(N-2)) are less than 1.

CIO Evolution of Sedimentary Basins North Slope Basin
DISCUSSION

The results indicate that rhyolitic glass at least as old 

as Late Cretaceous can give geologically reasonable K-Ar 

and ^Ar/^Ar radiometric ages. Analysis of age spectra 

and isochron and correlation diagrams using the criteria of 

Lanphere and Dalrymple (1978) readily distinguishes between samples that have undergone some degree of argon 

redistribution and those that are essentially undisturbed 

and therefore give reliable cooling ages. The ^Ar/^Ar age 

spectra for three of the glass samples (M86LPA19, 

84ACrl80Y, M86LPA9) have concordant, well-defined 

plateaus and show only minor evidence of argon loss 

caused by alteration or diffusion. Samples M86LPA14 and 

86AQ019 gave disturbed age spectra and appear to have 

some degree of argon redistribution, but only one of these 

(86ACr019) shows evidence of significant argon loss.

40, / 39 Ar/ AT ages derived from isochron diagrams termed 

isochron ages are in general preferred ages because the 

method by which they are determined minimizes the effects of alteration, and thus argon loss, and takes best account of analytical errors. The plateau and isochron ages 

also have greater precision than conventional K-Ar ages, 

in part because they are an average of several individual 

age determinations.

Although all of the samples show evidence of minor 

argon loss, three of the five glass samples we dated yielded

ages that represent true cooling ages on the basis of normal criteria for analysis of ^Ar/^Ar age spectra. The minor (<2.5 percent) 40Ar loss observed in the 

low-temperature heating steps can be attributed to loss of 

^Ar due to surface alteration and (or) diffusion near shard 

boundaries. Nonetheless, the ^Ar/^Ar ages are indistinguishable from the conventional K-Ar ages within the limits of analytical uncertainty. Of the two samples giving 

apparently disturbed 40Ar/ 39Ar age spectra, one 

(M86LPA14) gave a K-Ar age (68.5±2.0 Ma) that is reasonable on the basis of the other radiometric dates, suggesting that some argon redistribution occurred without 

significant argon loss. The slightly old recombined totalfusion age of 71.1±0.5 Ma obtained on this sample by 

^Ar/^Ar analysis may be an artifact of the irradiation 

process (recoil of 39Ar). Sample 86AQ019 appears to 

have suffered significant ^Ar loss due to alteration or diffusion on the basis of the incremental heating data. The 

K-Ar age of 65.8±2.0 Ma for this sample is therefore considered a minimum age.

The "best" or preferred age for each of the tephra 

beds is shown in figure 5. For samples M86LPA9 and 

M86LPA19, preferred ages are 68.8±0.4 and 70.4±0.8 Ma, 

respectively, and are calculated from weighted means of 

40Ar/39Ar isochron ages, ^Ar/^Ar total-fusion ages, and 

K-Ar ages for each tephra bed. The preferred age for sample 84A&180Y is the ^Ar^Ar isochron age of 69.310.4 Ma.

63

64

66

67

1 69

* 70

UJ

3

71

72

£

M86LPA9 86ACr019 M86LPA14 

SAMPLE NUMBERS

84ACr180Y M86LPA19

Figure 5. Best age estimates for Colville River tephra beds (dots) and associated 1-sigma errors 

(bars). Shaded area shows best estimate of age of bone-bearing beds on basis of all analyses. See 

text for discussion.

Age of Tephra Beds at the Ocean Point Dinosaur Locality, North Slope, Alaska C11
The best age estimate for sample M86LPA14 is the K-Ar 

age of 68.5±2.0 Ma, because of possible irradiationinduced disturbance in the ^Ar/^Ar analysis. The K-Ar 

age of 65.8±2.0 Ma for sample 86ACr019 is considered a 

minimum age because of significant argon redistribution 

and loss apparent in the ^Ar/^Ar analysis. Although there 

appears to be a slight decrease in age upward through the 

section, none of the tephra beds are demonstrably different 

in age at a 95-percent confidence level. Since the ages are 

concordant, a best age estimate of 69.110.3 Ma for the 

dinosaur-bonebearing beds was calculated using the 

weighted mean of all the analyses. Assuming that offsets 

across the faults and covered intervals are small, the location of the Cretaceous-Tertiary boundary, considered to be 

about 66 Ma (Palmer, 1983), appears to be at least 100 m 

above the youngest dated tephra bed (M86LPA9), provided depositional and environmental conditions remained 

constant. This new dating, by establishing a Late Cretaceous 

age for the dinosaur-bone-bearing beds, is compatible with 

the impact theory for the extinction of the dinosaurs. Unfortunately, no tephra beds have been identified near the 

probable location of the Cretaceous-Tertiary boundary to 

allow for K-Ar or ^Ar/^Ar dating. About 40-60 m above 

the youngest tephra bed, the section passes into a marine 

depositional environment (Phillips, 1988). Thus, any tephra beds deposited at this time would likely be reworked 

and altered by diagenetic interaction with sea water.

REFERENCES CITED

Alvarez, L.W., Alvarez, Walter, Asaro, Frank, and Michel, H.V., 

1980, Extraterrestrial cause for the Cretaceous-Tertiary extinction: Science, v. 208, p. 1095-1108.

Brouwers, E.M., Clemens, W.A., Spicer, R.A., Ager, T.A., Carter, 

L.D., and Sliter, W.V., 1987, Dinosaurs on the North Slope, 

Alaska High latitude, latest Cretaceous environments: Science, v. 237, p. 1608-1610.

Carter, L.D., Repenning, C.A., Marincovich, L.N., Hazel, I.E., 

Hopkins, D.M., McDougall, Kristen, and Naeser, C.W., 

1977, Gubik and pre-Gubik Cenozoic deposits along the 

Colville River near Ocean Point, North Slope, Alaska: U.S. 

Geological Survey Circular 751-B, p. B12-B14.

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon 

dating: San Francisco, W.H. Freeman Company, 258 p.

 1971, 40Ar/39Ar technique of K-Ar dating A comparison 

with the conventional technique: Earth and Planetary Science Letters, v. 12, p. 300-308.

 1974, ""Ar/^Ar age spectra of some undisturbed terrestrial samples: Geochimica et Cosmochimica Acta, v. 38, 

p. 715-738.

Detterman, R.L., Reiser, H.N., Brosge, W.P., and Dutro, IT., Jr., 

1975, Post-Carboniferous stratigraphy, northeastern Alaska: 

U.S. Geological Survey Professional Paper 886, 46 p.

Fleck, R.J., Sutler, J.F., Elliot, D.H., 1977, Interpretation of discordant '"Ar/^Ar age-spectra of Mesozoic tholeiites from 

Antarctica: Geochimica et Cosmochimica Acta, v. 41, 

p. 15-32.

Gryc, George, Patton, W.W., Jr., and Payne, T.G., 1951, Present 

Cretaceous stratigraphic nomenclature of northern Alaska: 

Washington Academy of Sciences Journal, v. 41, no. 5, 

p. 159-167.

Ingamells, C.O., 1970, Lithium metaborate flux in silicate analysis: Analytica Chimica Acta, v. 52, p. 323-334.

Lanphere, M.A., and Dalrymple, G.B., 1978, The use of 

""Ar/^Ar data in evaluation of disturbed K-Ar systems, in 

Zartman, R.E., ed., Short papers of the Fourth International 

Conference, geochronology, cosmochronology, isotope 

geology: U.S. Geological Survey Open-File Report 78-701, 

p. 241-243.

Marincovich, Louie, Jr., Brouwers, E.M., and Carter, L.D., 1985, 

Early Tertiary marine fossils from northern Alaska Implications 

for Arctic Ocean paleogeography and faunal evolution: Geology, v. 13, p. 770-773.

McDougall, Kristen, 1987, Maestrichtian benthic foraminifers 

from Ocean Point, North Slope, Alaska: Journal of Foraminiferal Research, v. 17, no. 4, p. 344-366.

Palmer, A.R., 1983, The Decade of North American Geology 

1983 geologic time scale: Geology, v. 11, p. 503-504.

Phillips, R.L., 1988, Measured sections, paleoenvironments, and 

sample locations near Ocean Point, Alaska: U.S. Geological 

Survey Open-File Report 88^0.

Stacey, J.S., Sherrill, N.D., Dalrymple, G.B., Lanphere, M.A., 

and Carpenter, N.V., 1981, A five-collector system for the 

simultaneous measurement of argon isotopic ratios in a static 

mass spectrometer: International Journal of Mass Spectrometry Ion Physics, v. 39, p. 167-180.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology Convention on the use of decay constants in geo- and 

cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359-362.

Taylor, J.R., 1982, An introduction to error analysis, the study of 

uncertainties in physical measurements: Oxford University 

Press, 270 p.

York, Derek, Hall, C.M., Yanase, Yotaro, Hanes, J.A., and 

Kenyon, W.J., 1981, 40Ar/39Ar dating of terrestrial minerals 

with a continuous laser: Geophysical Research Letters, v. 8, 

p. 1136-1138.

C1 2 Evolution of Sedimentary Basins North Slope Basin
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on.various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List.)

11 Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.