Comparison of the Cretaceous-Tertiary 

Boundary Impact Events and the 

0.77-Ma Australasian Tektite Event: 

Relevance to Mass Extinction 

U.S. GEOLOGICAL SURVEY BULLETIN 2050 
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY 

Instructions on ordering publications of the U.S. Geological Survey, along with prices of 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 may be listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price 

and Availability List" may be 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 ESIC-Open-File Report Sales, Box 25286, Denver, CO 

80225. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. 

Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. 

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 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, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY from 

the 

Superintendent of Documents 

Government Printing Office 

Washington, D.C. 20402 

(Check or money order must be payable to Superintendent of 

Documents.) 

Maps 

For maps, address mail orders to 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Residents of Alaska may order maps from 

U.S. Geological Survey, Earth Science Information Center 

101 Twelfth Ave. - Box 12 

Fairbanks, AK 99701 

OVER THE COUNTER 

Books and Maps 

Books and maps 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-Rm. 101, 4230 University Dr. 

• LAKEWOOD, Colorado-Federal Center, Bldg. 810 

• MENLO PARK, California-Bldg. 3, Rm. 3128, 345 Middlefield Rd. 

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

Sunrise Valley Dr. 

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

State St. 

• SPOKANE, Washington-U.S. Post Office Bldg., Rm. 135, 

West 904 Riverside Ave. 

• WASHINGTON, D.C.-Main Interior Bldg., Rm. 2650, 18th 

and C Sts., NW. 

Maps Only 

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

Geological Survey offices: 

• FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave. 

• ROLLA, Missouri-1400 Independence Rd. 

• STENNIS SPACE CENTER, Mississippi-Bldg. 3101 
Comparison of the Cretaceous-Tertiary 

Boundary Impact Events and the 

0.77-Ma Australasian Tektite Event: 

Relevance to Mass Extinction 

By E.C.T. CHAO 

Reinterpretation of iridium anomalies, shocked quartz, and 

microtektites attributed to cratering events in CretaceousTertiary boundary sections. Evidence from the Ries crater of 

Germany and Australasian tektites is essential to understanding 

giant craters and mass extinction 

U.S. GEOLOGICAL SURVEY BULLETIN 2050 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, 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: 1993 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Chao, E.C.T. 

Comparison of the Cretaceous-Tertiary boundary impact events and the 

0.77-Ma Australasian tektite event : Relevance to mass extinction I by 

E.C.T. Chao. 

p. em. - (U.S. Geological Survey bulletin ; 2050) 

Includes bibliographical references. 

Supt. of Docs. no.:l 19.3:2050 

1. Cretaceous-Tertiary boundary. 2. Tektite. 3. Extinction (Biology) 

I. Title. II. Series. 

QE75.B9 no. 2050 

[QE689] 

557.3 s-dc20 

[551.7'7] 93-9330 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Representative Cretaceous-Tertiary Boundary Sections of Western U.S. Interior Basins 

and Haiti: Indication of Separate Events 2 

Sources of Ir Anomalies 5 

Layer with Shocked Quartz Attributed to the Manson Crater 7 

Pseudomorph-Microtektites and Microtektites 9 

Abundance, Grain Size, and Ejecta Transport 10 

The Ries and the Chicxulub Craters 11 

The Ries Crater of Germany 11 

The Chicxulub Crater of the Yucatan 14 

Models of Giant Terrestrial Impact Craters and Geologic and Petrographic Constraints 15 

The Place of Origin of the Australasian Tektites and Microtektites 16 

Comparison of the Manson and Chicxulub Craters of the Cretaceous-Tertiary Boundary 

Events with the Australasian Tektite Event 18 

Summary 18 

Acknowledgments 18 

References Cited 19 

FIGURES 

1. Photograph of Dean R. Chapman, Howard K. Larson, and E.C.T. Chao 

inspecting tektites from various localities in Thailand 2 

2. Aerial photograph showing the landscape of the Ries meteorite crater of 

Germany 4 

3. A, Photograph of a mounted Cretaceous-Tertiary boundary section from Clear 

Creek North, Colo. B, Photograph of a whole, doubly polished ultrathin thin 

section of a sample from a portion of the Cretaceous-Tertiary boundary section 

from Clear Creek North, Colo. 6 

4. Photographs of two Cretaceous-Tertiary boundary sections from Dogie Creek, 

Wyo. 7 

5. Photograph of the lower portion of the Cretaceous-Tertiary boundary section near 

Beloc, Haiti 8 

6. Diagram of maximum launched range of projectile versus optimum angle of 

launch at various velocities through stationary atmosphere 10 

7. Map showing orientation of striae and gouges on bedrock and distribution of 

ejecta types, Ries crater, Germany 12 

8. Photograph of limestone bedrock surface with parallel striae and gouges produced 

by unshocked sedimentary ejecta containing huge blocks that moved over the 

bedrock during the impact event (Gundelsheim quarry, Germany) 13 

9. Photograph of limestone bedrock with parallel striae, grooves, and gouges 

overlain by an ejecta blanket of sedimentary rocks (Gundelsheim quarry, 

Germany) 14 

Contents Ill 
10. Map showing the huge strewnfield of land Australasian tektites 17 

11. Photographs of ablated minitektites from Serpentine Lake, Central Australia 18 

TABlE 

1. Sources of iridium concentrations 8 

IV Contents 
Comparison of the Cretaceous-Tertiary Boundary 

Impact Events and the 0.77-Ma Australasian Tektite 

Event: Relevance to Mass Extinction 

By E.C.T. Chao 

Abstract 

Cretaceous-Tertiary (K-T) boundary sections provide 

records of events of the K-T transition: retreat of the 

Cretaceous marine sea that left continents dotted with 

peat swamps, and impacts that left craters on exposed 

land and the ocean floor. Interpretation of the occurrence 

of iridium anomalies, layers containing weakly shocked 

quartz, laminae of coaly materials, and the clay layer that 

separates a gorceixite-goyazite layer from the layers above 

in representative K-T boundary sections can put constraints on and raise key questions regarding the types of 

events that occurred and their effects. 

Data are presented to support the premise that the 

principal source for the iridium anomalies along major 

stratigraphic hiatuses, including the K-T boundary, may be 

ablation products of stony meteorites instead of being 

from dispersed, probably highly diluted, vaporized 

asteroids of impact events. In order to best understand 

the K-T boundary record, geologic, geochemical, and 

petrographic data obtained from the study of tektites and 

three impact craters are reviewed and compared in this 

study. The three craters are the 14.8-million-year-old, 

25-kilometer-diameter crater of Ries, Germany, and two 

candidate K-T boundary craters, the 35-kilometerdiameter Manson crater of Iowa and the 180-kilometerdiameter Chicxulub crater of Yucatan, Mexico. Data from 

the well-studied Ries crater are also recommended as 

constraints for proposed models of giant impacts. 

The Haitian microtektites, possibly from the Chicxulub crater event, are compared to the tektites and 

microtektites associated with the larger Australasian 

event. Those who favor the terrestrial origin of all tektites 

and microtektites must explain how there could have 

been mass extinction caused by the Chicxulub cratering 

event if there was no mass extinction associated with the 

0.77-million-year-old Australasian tektite event. 

Manuscript approved for publication, December 17, 1992. 

This bulletin is based on the address presented by the author on the 

occasion of his receipt of the Barringer Medal of the Meteoritical Society, 

July 27, 1992, Copenhagen, Denmark. 

INTRODUCTION 

I fondly remember first becoming acquainted with the 

Barringer Meteor Crater of Arizona in June 1960, when Dr. 

John O'Keefe (NASA, Greenbelt, Md.) brought me for 

study a small piece of shocked Coconino Sandstone from 

the U.S. National Museum, collected by George Merrill in 

1911. It was from this sample that I first discovered the 

natural occurrence of coesite. 

When I was first assigned a project to study tektites in 

the summer of 1960, I had no idea what a tektite looked 

like, so I learned from studying tektite collections in the 

British Museum, the private collection of von Konigswald 

in Holland, the collection of tektites from the Museum of 

Natural History in Paris, and other collections. Later, in 

September 1962, it was a rewarding coincidence that I 

visited various museums and field occurrences of tektites of 

Southeast Asia and Australia at the same time that Dean 

Chapman and Howard Larson of NASA's Ames Research 

Center near San Francisco planned a similar tour to 

Southeast Asia, the Philippines, Java, and Australia. 

Without knowing each other's schedules, we met in 

Bangkok, Manila, Sangiran in Java, and in Australia. 

Figure 1 is a photograph taken in Bangkok at the Thailand 

Museum, where Dean Chapman, Howard Larson, and I met 

to study the museum collection of Thailand tektites. Later I 

visited Professor Virgil Barnes of the University of Texas 

on my visits to the bediasites, I visited Dr. Vladimir Bouska 

in Prague regarding the sites of moldavites of Czechoslovakia, and I made field studies of tektites of the Ivory 

Coast in Africa. The discovery of Ni-Fe metallic spherules 

(some containing schreibersite; Chao and others, 1964) in 

some of the Australasian tektites and the distinctions that 

can be made between tektites and terrestrial impactites may 

help us to better interpret the place of origin of tektites 

(Chao and others, 1961; Chao 1963a, 1963b, 1964a, 1964b; 

Chao and others, 1964; Chao and Xie, 1990). 

Observations accumulated from study of the 25-km 

Ries meteorite crater of Germany (fig. 2), the focus of my 

terrestrial crater study from about 1964 to 1977, may be 

Introduction 
Figure 1. Dean R. Chapman, Howard K. Larson, and E.C.T. 

Chao (left to right) inspecting tektites from various 

localities in Thailand. (Thailand Museum, 1962.) 

helpful in understanding the Chicxulub crater of the 

Cretaceous-Tertiary (K-T) boundary period. The major 

features of the Ries as a model shallow-basin-type meteorite 

crater include (1) evidence of the mechanics of ejecta 

transport; (2) the distribution and superposition of an 

unshocked sedimentary ejecta blanket and strongly shocked 

glass-bearing suevite; and (3) the location of the metallic 

component of the impacting body in the shatter-coned 

amphibolite of the crystalline bottom of the crater. 

Izett (1990, 1991a) provided detailed and accurate 

descriptions and key identifications of shock products in 

K-T boundary sections from the Western United States and 

from the Haiti section near Beloc. Examination of data from 

these sections, and comparison of data from the Chicxulub 

K-T boundary crater (Hilderbrand and others, 1991) and the 

Ries crater, provides critical geologic and petrographic 

constraints regarding models of terrestrial impacts from 

giant asteroids. My background as a coal petrologist from 

1977 to 1986 is helpful in the interpretation of the K-T 

boundary sections involving coaly materials and clays. 

This paper discusses the interpretation of impactshock evidence found in the K-T boundary sections and 

discusses the constraints imposed by the Australasian tektites and microtektites on the role of the 180-km Chicxulub 

crater in mass extinction. 

REPRESENTATIVE CRETACEOUS-TERTIARY 

BOUNDARY SECTIONS OF WESTERN U.S. 

INTERIOR BASINS AND HAITI: INDICATION 

OF SEPARATE EVENTS 

The K-T boundary sections represent the records of 

an exposed land surface on a continent such as North 

America during the end of a time period when the marine 

sea withdrew and left a continental landscape dotted with 

peat swamps. The change from a marine to a continental 

ecology and climate was environmentally drastic in itself 

and may have been a significant contributor to the K-T 

extinction event. 

Some K-T boundary sections that have been studied 

occur at Clear Creek North, Colo., Dogie Creek, Wyo., 

Teapot Dome, Wyo., and Brownie Butte, Mont. In all, 

about 20 sites occur in the Raton basin of Colorado and 

New Mexico, and at least 10 sites occur in Wyoming, 

Montana, and western Canada (lzett, 1990). The two types 

of K-T boundary sections are ( 1) sections that contain 

evidence of only one cratering event, such as one of the 

sections from Clear Creek North in Colorado (such sections 

are less common and may not be typical), and ( 2) sections 

that contain evidence of two impact events, such as Dogie 

Creek of Wyoming and most K-T boundary sections in the 

Raton basin. 

The importance of the detailed record of the K-T 

boundary sections cannot be overemphasized, because the 

sections ( 1) record events across the K-T boundary 

transition, including the retreat of the Cretaceous sea and 

transition to a continental environment, and (2) record 

cratering impact events. What is present in and what is 

absent from this record are equally important. If we can 

identify, read, and interpret this K-T boundary record 

correctly, we can put constraints on the types of events 

responsible for the transition or mass extinction of fauna 

and flora at this time. 

Figure 3A is a photograph of the K-T boundary 

section from Clear Creek North, Colo., taken from Izett 

(1990). Figure 3B is a photomicrograph of a doubly 

polished ultrathin thin section (about 10 J.Lm thick) made 

from a sample provided by Izett that covers most of a K-T 

boundary section from the same location. A doubly 

polished ultrathin thin section is necessary for optical stu.dy 

of the clay minerals and coaly materials. 

As shown in figure 3B, this K-T boundary section can 

be divided into 9 units that are based on mineral contents. Ir 

anomalies, which usually occur above the shocked quartz, 

extend through units 1-5. The top layer, unit 1, is a finely 

laminated band consisting of scattered detrital clastic grains 

separated by thin, discontinuous laminae of coaly vitrinitic 

materials, and some jarosite (ferrous Fe sulfate). The clastic 

grains consist mostly of quartz in an illite matrix. Weakly 

shocked quartz grains with closely spaced microfractures 

are present. Most of the grains are larger than 100 J.Lm in 

size. Nearly all the illite grains in the matrix are larger than 

10 J.Lm, and none are less than 1 J.Lm. 

Unit 2 consists of a nearly solid band of yellow 

jarosite with a few grains of quartz, and rare minute pyrite 

grains. Units 5 and 6 are jarosite bands similar to unit 2. 

The presence of jarosite bands is atypical in K-T boundary 

2 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
sections of the Raton basin (Glen Izett, written commun., 

1992). 

Unit 3 is similar to unit 1, with scattered detrital 

grains in a laminated layer consisting of alternating laminae 

of discontinuous coaly materials and illite. Detrital clastic 

grains are more abundant in unit 3 than in unit 1. They 

consist dominantly of quartz and of spindle-shaped graupen 

(aggregates of kaolinite). Small amounts of lithic quartzite 

fragments and chert are also present, as are coarsely 

crystallized kaolinite grains. Many of the spindle-shaped 

grains have a brownish iron-stained coating. Weakly 

shocked quartz grains with closely spaced microfractures 

are also present in this unit. 

Unit 4 is a dominantly kaolinitic layer with scattered 

quartz and lithic fragments. One of the quartzite fragments 

shows weakly shocked parallel micro fractures. This unit 

contains many discontinuous laminae of coaly materials. 

Unit 7 consists mostly of smectitic clays that have 

grain sizes a few micrometers across. Spherulitic textures in 

the smectitic clays are often present. Unit 8 underlies unit 7 

and consists mostly of kaolinitic clays generally without 

spherulitic textures. Some of the kaolinitic clays on the left 

of figure 3B show signs of reworking. The kaolinitic layer 

lies directly above unit 9. Clastic grains are generally absent 

in both units 7 and 8. Unit 9 consists of Cretaceous 

well-layered illite-dominated carbonaceous shale with very 

fine grained quartz. Gorceixite-goyazite spherules are not 

observed in this K-T section. 

Most of the material above the smectitic and 

kaolinitic clays consists of layers containing detrital clastic 

grains in a clayey matrix that alternate with laminae of coaly 

materials and bands or lenses of jarosite. I concur with Izett 

(1990) that the dominant clays (units 7 and 8) are neither 

ejecta material nor volcanic ash. They resemble the underclay of peaty materials of a poorly drained swamp. This 

clay layer was deposited probably from stagnant water, 

which may have taken years to accumulate and provided a 

substrate for plant precursors of peat to grow. This clay also 

underwent leaching under low pH and reducing anaerobic 

conditions. 

Figure 4 shows two polished slabs of a K-T boundary 

section from Dogie Creek, Wyo. Shocked quartz and 

anomalous Ir occur above the underclay. Gorceixitegoyazite (Ba- and Sr-Al phosphate) spherules occur under 

the smectite-kaolinite underclay. Recently, Izett (1991a) 

has shown evidence of teardrop and dumbbell morphology 

and surface sculptures on gorceixite-goyazite spherules that 

resemble microtektites, although the assumed previous 

glassy cores or hollow spheres had been completely 

replaced by gorceixite-goyazite. Some of the teardrop- and 

dumbbell-shaped and spherical particles may be pseudomorphs of microtektites. However, many of the hollow 

particles without such teardrop and dumbbell shape and 

sculptures may not be pseudomorphs, because hollow 

microtektites are very rare. They are spherules of unknown 

origin that have been replaced by aluminum phosphate. The 

Teapot Dome, Wyo., K-T boundary sections also contain 

gorceixite-goyazite spherules below the claystone or underclay layer. 

Because the gorceixite-goyazite spherule-bearing 

layer is generally separated from the shocked-quartzbearing layer above the underclay, I agree with Izett 

(1991b) and Shoemaker and Izett (1992) that this layer 

below the underclay probably represents products from 

another event. However, the gorceixite-goyazite spherulebearing layer or horizon is unique. So far, the principal Ir 

anomaly is associated solely with the shocked-quartzbearing layer above the underclay. The spherule layer 

below the underclay does not contain shocked quartz and 

other impact ejecta materials (Glen Izett, oral commun., 

1992). If the gorceixite-goyazite spherule layer were the 

product of Chicxulub, then it would be reasonable to expect 

the presence of a strong Ir anomaly severalfold that of the Ir 

anomaly produced by the Manson crater of Iowa, and more 

shocked ejecta as well. As will be shown, an Ir anomaly is 

not necessarily attributable to an impact event. 

Vickery and others (1992) suggested that the layer 

containing shocked quartz and the gorceixite-goyazite layer 

could have been produced by a single event. These authors 

are probably not aware of (1) the nature of the separating 

underclay and (2) the fact that, if they were from the same 

event, the layer of shocked quartz representing materials of 

low shock should underlie and not overlie the gorceixitegoyazite layer, which represents materials of high shock. 

Figure 5 is a photograph of a polished slab of the 

lower part of a K-T boundary section from Haiti. This slab 

was described by Izett (1991a) particularly with reference 

to the finding of smectite-coated microtektites with glassy 

cores. The 0.5-m K-T boundary section is described as a 

turbidite with shocked quartz mixed in. According to Izett 

(1991a, fig. 3) there are similarities in the size distributions of shocked quartz grains from Clear Creek North, 

Colo., and from near Beloc, Haiti. Izett showed 

photographs of teardrop- and dumbbell-shaped smectite 

particles (Izett, 1991a, fig. 4) and flow structures on 

smectite pseudomorphs and on glass cores (Izett, 1991a, 

figs. 8 and 18, respectively). These are convincing evidence 

of microtektites. The 40 Ar/39 Ar ages of these microtektites 

are 64.5±0.1 Ma (Izett and others, 1991) or 65.01 ±0.08 

Ma (Swisher and others, 1992). The numbers differ due 

only to methods of data reduction, so the age is the same in 

both cases. The Haitian microtektites are probably the 

oldest tektites known, although glassy particles yet to be 

firmly identified as microtektites have been reported from 

Devonian sections recently by Wang (1992) and Claeys and 

others (1992). Izett interprets the Haitian microtektites as 

having been derived from the Chicxulub impact structure of 

the Yucatan. 

On the basis of comparisons with studies to determine 

the place of origin of the Australasian land tektites and 

K-T Boundary Sections, Western U.S. Interior Basins and Haiti 3 
Figure 2. Aerial photograph showing the landscape of the Ries meteorite crater of Germany. The clusters of cumulus 

clouds and forests (dark areas) in the foreground mark the rim of the crater. The interior of the crater is covered by 

microtektites, in my view the Haitian microtektites, which 

should be compared with the nature and age of the Chicxulub impactites, may not be related to Chicxulub and may 

have a much wider distribution than the impactites from 

Chicxulub. 

Before trying to further understand and interpret the 

K-T boundary sections, it is important that we examine the 

sources of the Ir anomalies, the possible places of origin of 

weakly shocked quartz grains and the method of their 

transport, and the possible places of origin of microtektites 

4 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
cultivated fields and scattered towns, such as Nordlingen, and villages. The photograph is a gift to the author from 

Professor Ekkehard Preuss of Munchen, Germany. Reprinted with permission from Mayor Paul Kling of Nordlingen. 

and pseudomorphs of microtektites. In order to evaluate 

what is present in the K-T boundary sections, we need to 

know more about events such as that which produced the 

Australasian tektites and microtektites, and we need to 

know more about the Chicxulub crater. 

SOURCES OF lr ANOMALIES 

Most K-T boundary investigators have attributed the 

source of theIr anomalies to cratering events, in spite of the 

fact that cosmic spherules and other sources equally will 

Sources of I r Anomalies 5 
1 2 

I 1 CM 

Figure 3A. Mounted Cretaceous-Tertiary boundary section from Clear Creek North, Colo. (From lzett, 1990.) 

have produced theIr anomalies. They assumed that there is 

a constant rate of sedimentation in which the cosmic 

contribution has been included. Hence, the K-T boundary Ir 

anomalies are caused by the addition from the K-T 

boundary cratering event contributed by the impact fusionvaporization of the impacting body. This interpretation, in 

my view, is probably incorrect, because the K-T boundary 

is a stratigraphic hiatus, meaning that there is a break in the 

normal terrestrial sedimentation cycle. During this break, 

an erosional surface existed with a time interval of tens of 

thousands of years, where there was no normal terrestrial 

sedimentation but only deposition and accumulation of 

extraterrestrial materials such as ablated meteorite spherules 

and other cosmic particles. I favor this latter interpretation 

. because it is a normal geological phenomenon. Hence, it is 

possible that the most important source of the Ir anomalies 

is ablation products from chondritic stony meteorites. In 

order to demonstrate the importance of this source, the 

fusion crusts of the Pultusk chondrite from Poland and the 

Nuevo Mercurio chondrite from Mexico were analyzed by 

Hugh Millard (this study) using the INAA (instrumental 

neutron activation analysis) method. Table 1 lists the 

findings of Ir concentrations from three sources: impactites 

of meteorite crater ejecta, cosmic particles, and fusion 

crusts of stony meteorites. 

Glassy impactites from small craters produced by iron 

meteorites such as Barringer (Meteor) Crater of Arizona and 

the Wabar Crater from Saudi Arabia contain 350-500 and 

87-740 ppb Ir, respectively (table 1). However, the average 

from 8 of 23 impactite samples enriched in Ir from the Ries 

crater of Germany contain only 0.015 ppb Ir. Impactite 

glass samples from other craters such as Lake Mistastin, 

Canada; Lonar Lake, India; and Strangways crater, 

Australia, are not particularly high in Ir either. Substantial 

metallic components of cratering events such as the Ries 

crater are actually found in the bottom of the crater (El 

Goresy and Chao, 1976; Chao and El Goresy, 1977). 

Hence, metal concentrations, including Ir, are enriched in 

the bottom of terrestrial craters and not necessarily in their 

ejected impactites. 

6 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
Figure 38. A whole, doubly polished ultrathin thin section 

of a sample from a portion of the Cretaceous-Tertiary 

boundary section from Clear Creek North, Colo. The 

sample is from Glen lzett. The units, which are based on 

variation of mineral components, are described in the 

text. Size of section is 2 x 3 em. 

The Ir concentrations from cosmic particles are 

generally much higher, ranging from less than 5 to more 

than 2,000 ppb (table 1). The magnetic fusion crusts of 

stony meteorites, about 100-300 f..Lm thick, are typical 

ablation products of chondrites such as those of the Pultusk 

and the Nuevo Mercurio. They contain 860± 15 ppb Ir. The 

fusion crusts of these two chondrites also contain 177 ± 9 

and 166±5 ppb Au, respectively. The ablation products of 

shooting stars and stony meteorites are extremely high in Ir, 

and these events occur frequently! When stony meteorites 

enter the Earth's atmosphere at high velocities, they often 

break up into small pieces, scatter, and ablate. The ablated 

Ir-rich particles are decelerated and further spread by wind 

drifts over land surfaces and oceans. Hence, Ir anomalies in 

the K-T boundary sections, which are geologic hiatuses, 

have little value as a definitive indicator of an impact 

cratering event. Ir anomalies should be widespread and can 

be found on any land surface that represents a hiatus where 

the normal terrestrial sedimentation cycle was interrupted 

and ceased. They certainly should be more widespread in 

occurrence than shocked quartz, for example. 

0 I 2 3 4 Scm 

I I I I f 11111 lt·tt I II f I It It I 1111111111111111 f I t IIIII ! It 

Figure 4. Two Cretaceous-Tertiary boundary sections from 

Dogie Creek, Wyo. Note the gorceixite-goyazite spherule 

layer below the underclay. Arrows point to fine-grained 

sand filling burrows. (From lzett, 1990.) 

An independent observation also supports the 

presence of ablation products found in the K-T boundary 

clay. Magnesioferrite spinels, which have never been 

reported from impact ejecta, were found in K-T boundary 

sections from Caravaca, Spain, and sites in Italy (Izett, 

1991 a). Recently, Robin and others (1992) showed that 

such spinels are ablation products from cosmic objects, 

probably stony meteorites, entering the Earth's atmosphere. 

Far too few large terrestrial craters (35 km in diameter 

or larger) have been identified to have produced the many Ir 

anomalies that occur in association with widespread stratigraphic hiatuses. Similarly, microtektites have been found 

in different stratigraphic horizons, but not enough large 

terrestrial craters have been identified that could have 

produced them. 

LAYER WITH SHOCKED QUARTZ ATTRIBUTED 

TO THE MANSON CRATER 

The origin of weakly shocked quartz and lithic fragments in the illitic clay layer of K-T boundary samples (see 

fig. 3B) just below the Ir anomalies was attributed by Izett 

(1990, 1991b) and Hartung and others (1990) as being the 

Manson crater in Iowa. As described in the doubly polished 

Layer with Shocked Quartz Attributed to the Manson Crater 7 
Figure 5. The lower portion of the Cretaceous-Tertiary boundary section near Beloc, Haiti. Smectite pseudomorphs 

with glassy cores were identified by lzett as microtektites, concentrated in the lower centimeter of the section. 

Dashed line marks the contact between the Cretaceous-Tertiary marker bed (above) and the Cretaceous marl 

(below). (See lzett, 1991a.) Scale is in millimeters. 

Table 1. Sources of iridium concentrations 

[ppb, parts per billion] 

Sample lr (ppb) Reference 

lmpactites of Meteorite Crater Ejecta 

8 of 23 Ries, Germany, crater ejecta, enriched in Ir ............... 0.015 ............. Morgan and others (1979). 

1 of 5 impact melts from Lake Mistastin, Canada ................. 0.055 ............. Morgan and others (1979). 

6 Aouelloul glasses, Africa ...................................... 0.115 ............. Morgan and others (1979). 

Lonar Lake, India, vesicular glass ............................... 0.004-0.026 ...... Morgan (1978). 

Strangways crater, Australia: 

6 granite melts ............................................. 0.059-2.86 ....... Morgan and Wandless (1983). 

1 shale melt ............................................... 0.025 ............. Morgan and Wandless (1983). 

Meteor Crater, Arizona, Fe-meteorite impactite glass .............. 350-500 .......... Morgan and others (1979). 

Wabar crater, Saudi Arabia, Fe-meteorite impactite glass .......... 87-740 .......... Morgan and others (1979). 

Cosmic Particles 

Type A shiny spherules from marine sediment .................... 300-1,500 ........ Millard and Finkelman (1970). 

8 stony spherules from marine sediment. ......................... <5-2,900 ........ Millard and others (1985). 

Fusion Crusts of Stony Meteorites 

Pultusk, Poland, USNM 3003 fusion crust. ....................... 860 .............. Millard, this study. 

Nuevo Mercurio, Mexico, USNM 6091 fusion crust. .............. 860 .............. Millard, this study. 

Nuevo Mercurio, Mexico, USNM 6091 metal cone ................ 2,160 ............. Millard, this study. 

ultrathin thin section of Clear Creek North (fig. 3B), weakly 

shocked quartz and shocked lithic fragments occur in the 

K-T boundary sections in more than one layer alternating 

with laminae of vitrinitic coaly materials and jarosite, 

above the underclay. Regardless of the number of parallel 

sets of closely spaced microfractures in the shocked quartz, 

there is so far no evidence of vitrification along the 

microfractures. There is also a lack of strongly shocked, 

8 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
completely vitrified thetomorphic quartz and feldspars 

(solid-state vitrification, Chao, 1968) and an absence of 

impactite glass laden with shocked mineral and lithic 

inclusions in the layer containing weakly shocked quartz. 

The 35-km-diameter, 65.7±1-Ma Manson crater 

(Kunk and others, 1989) of Iowa is similar to the 25-

km-diameter, 14.8-Ma Ries crater of Germany with respect 

to the target rock formations in which they are located. The 

Manson crater is excavated in a sequence of Middle 

Proterozoic, Paleozoic, and Cretaceous sedimentary rocks 

(including marine carbonate rocks) that overlie a basement 

of Proterozoic metamorphic gneisses intruded by granitic 

rocks and diabase dikes (Hartung and others, 1990). The 

Ries crater, which will be described in more detail in 

comparison with the Chicxulub crater, is also excavated in 

a sequence of Mesozoic rocks that overlies pre-Hercynian 

basement crystalline gneisses and amphibolite rocks 

intruded by granitic rocks. 

According to Horz (1982), quoting Cohen's (1963) 

suggestion that moldavites in Czechoslovakia were derived 

from the Ries when the impacting body first contacted the 

Earth's surface, a Miocene surface soil upon impact was 

fused by a fireball into melt and was ejected while molten 

and cooling, 300 to 400 km from the Ries, to form the 

moldavite strewnfield in Czechoslovakia. 

The major obstacle for deriving moldavites, including 

an aerodynamically spalled teardrop (Chao, 1964), from the 

Ries crater is the effect of aerodynamic drag. The Ries 

crater is on land, is only 25 km in diameter, and its 

ballistically transported ejecta must go through the 

atmosphere. According to figure 6, moldavites, several 

grams in weight and up to several centimeters in size, could 

not have traveled more than a few kilometers from the Ries. 

In order to overcome this aerodynamic obstacle, Lin (1966) 

outlined two mechanisms by which he suggested that it is 

dynamically possible for Australasian tektites to have 

originated from a gigantic comet impact on Earth. He 

calculated that, in order to remove the Earth's atmosphere, 

the impact would have to excavate a crater over 300 km in 

diameter. Chapman and Gault (1967), in rejecting Lin's 

suggestion, commented that the energies involved in the 

Ries, as well as the Bosumtwi, Africa, event, were entirely 

inadequate, less than about 10- 3 and 10-4 , respectively, of 

the required energy for atmospheric blowoff. The Ries and 

Bosumtwi events cannot be regarded as dynamically 

plausible sources of the moldavites or Ivory Coast tektites. 

In addition, recently, Blum and Chamberlain (1992) 

showed that the moldavite tektites have a 8180 value 4.5 per 

mil lower than values for the Miocene surficial soil from the 

Ries. This discrepancy in 8180 is difficult to explain if 

moldavites originated from surface soil of the Ries. The 

moldavite tektites must have formed under reducing 

conditions with extremely low ferric Fe and volatiles. 

Clearly, there is no evidence that tektites similar to 

moldavites were produced from the Manson crater. 

Otherwise, such tektites or their pseudomorphs with or 

without glass cores would have been found in the layer 

containing weakly shocked quartz in the K-T boundary 

section of, for example, Clear Creek North. 

If both Ir and shocked quartz of the K-T boundary 

section of Clear Creek North came from the Manson crater, 

and if the source of Ir was from the vaporized impacting 

body upon contact with the surface soil, then theIr anomaly 

layer should underlie instead of overlie the layer with 

shocked quartz, which presumably originated from the 

crystalline basement rocks of the Manson structure. The 

absence of strongly shocked impactite-bearing ejecta above 

the layer bearing shocked quartz at Clear Creek North 

suggests that the source of the Ir anomaly layer at Clear 

Creek North may not have come from the Manson structure. 

Geologic evidence from the Manson crater and the 

Ries crater suggests to me that, upon contact with the 

Earth's surface in a 1-km-sized meteorite or asteroid impact 

event, the moment of penetration through the soil was 

perhaps of less than microsecond or nanosecond in duration 

(that is, instantaneous), so that the surface material was 

displaced rather than strongly shocked or fused. This 

scenario is perhaps realistic because the not-yet-vaporized 

impacting body continued to penetrate the sedimentary 

sequence without producing highly shocked ejecta. The 

strongly shocked ejecta we observed both at Manson and 

the Ries were produced when the impacting body hit the 

resisting bottom. The impact caused a sudden increase in 

pressure and heating such that the major fraction of the 

strongly shocked ejecta is glassy impactites laden with 

debris of shocked basement crystalline rocks. These 

geologic and petrographic observations are of critical 

importance and have the following implication: regardless 

of the nature of the overlying formations, be they 

limestones or evaporite, if they were penetrated by the 

impacting body before the crater bottom was reached, in 

less than a fraction of a microsecond, they probably would 

not have been strongly shocked or fused. 

PSEUDOMORPH-MICROTEKTITES AND 

MICROTEKTITES 

Assuming that some of the teardrop-, dumbbell-, and 

spherical-shaped gorceixite-goyazite particles with flow 

structures are microtektite pseudomorphs, then we are faced 

with the controversial consideration of the source crater of 

microtektites. There is consensus and good evidence that 

microtektites are of impact origin (Chao and others, 1961, 

1964). The controversy, if tektites are of terrestrial origin, 

lies in correlating microtektites of a certain age with 

particular giant cratering events of the same age. Izett 

(1991a) linked the Haitian microtektites to the Chicxulub 

crater of Yucatan Peninsula, Mexico. Proving this linkage 

will be very difficult even if both show similar ages. Where 

Pseudomorph-Microtektites and Microtektites 9 
MASS FOR TEKTITE SPHERE 

LlJ 

CJ 

z 

<( 10 km 

a: 

~ 

::) 

~ 

x 1km 

<( 

~ 

100m 

10m 

.01 .1 10 

M= mass 

A = area of object 

normal to fl1ght 

c0 =drag coefficient 

+ 

M!C0 A (g/cm2) 

Figure 6. Diagram of maximum launched range of projectile versus optimum angle of launch at various velocities 

through stationary atmosphere. Note the limited distance, less than 100m, of travel for minitektites, which is due 

to aerodynamic drag. (Courtesy of Dean R. Chapman.) 

is the giant crater that should have been the source of the 

North American microtektites of late Eocene age (35.9± 

0.4 Ma, time scale of Montanari and others, 1985) found in 

deep-sea sediments of the west equatorial Pacific and 

offshore of New Jersey (Keller and others, 1987)? I suggest 

that, in order to understand the place of origin of any 

microtektites, we must first understand the place of origin 

of the largest and most important Australasian land tektite 

and microtektite strewnfield. 

ABUNDANCE, GRAIN SIZE, AND EJECTA 

TRANSPORT 

Typical K-T boundary sections of the interior basins 

of the Western United States are less than 3 em in total 

thickness. More than two-thirds of this thickness is 

accounted for by the underclay. Hence, the total thickness 

of ejecta from the two candidate crater events (Manson and 

Chicxulub) was about 1 em. This amount of ejecta is too 

little to be dispersed in the atmosphere to obliterate 

sunlight. Furthermore, a substantial source of< 1 1-1.m crater 

ejecta particles is needed so that such particles may reside in 

the atmosphere for weeks before settling, in order to block 

out the sun and change the climatic conditions to cause mass 

extinction. 

According to Toon and others (1982), in order to 

obtain worldwide distribution of ejecta, without removal of 

the atmosphere, the residence time in the atmosphere has to 

be greater than several weeks. This restricts ejecta particle 

size to 1 1-1.m or less. As described from the doubly polished 

ultrathin thin section of the Clear Creek K-T section and the 

10 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
grain size histogram of shocked quartz from the Clear Creek 

North locality (lzett, 1991a), none of the shocked quartz 

grains are less than 10 J.Lm in size, and no particles 

identifiable as ejecta are 1 J.Lm or less. Hence, there is no 

evidence at all of ejecta of such small particle size. The 

author's study of suevite (strongly shocked glass-bearing 

ejecta) from the Ries also found no trapped ejecta particle 

less than 1 J.Lm. Could it be that regardless of crater size, the 

impact process is not an efficient process of producing 

extremely fine particulate materials? 

Unless the impact crater is about 300 km in diameter 

(Lin, 1966), large enough to remove the atmosphere, all 

impact ejecta transported ballistically must encounter 

aerodynamic drag. Figure 6 shows the limited launch range 

of particulate ejecta, and it shows that most ejecta of any 

terrestrial meteorite or asteroid impact are deposited within 

a distance of a few diameters of the crater. Those that were 

transported farther than several diameters of the crater size 

must rely on ejection to heights of several kilometers into 

the stratosphere and rely on atmospheric movements or 

wind drifts in order to achieve distribution throughout part 

of the world or worldwide. 

THE RIES AND THE CHICXULUB CRATERS 

The 25-km-diameter, 14.8-Ma Ries meteorite crater 

in Germany is well preserved and exposed. It is one of the 

best studied craters, although because of its unusually 

shallow basin geometry (Chao, 1977; Pohl and others, 

1977), some crater investigators either did not accept its 

shallow depth geometry or, accepting the shallow depth, 

considered the crater atypical. The 180-km-diameter Chicxulub crater on the Yucatan Peninsula, Mexico, is apparently 

also a shallow-basin-type crater (Hilderbrand and Boynton, 

1990; Hilderbrand and others, 1991; Smit and others, 

1992). In addition to the general geometry of the crater, the 

stratigraphic section of the Ries is also similar to that of the 

Chicxulub. The Ries crater was excavated in a series of 

some 500 to 650 m of gently dipping sedimentary Triassic 

sandstones and shales and Jurassic limestones, sandstones, 

and shales that unconformably overlie a basement of preHercynian metamorphic crystalline rocks that are intruded 

by Hercynian granitic rocks. On the basis of preliminary 

data, the Chicxulub crater was excavated in nearly 

horizontal Cretaceous limestone, marl, shale, bentonite, 

and evaporite that overlie a poorly known crystalline 

basement of probable Paleozoic age (Hilderbrand and 

others, 1991). Igneous clasts in breccias, and andesitic 

rocks (380 m thick) occur in drill cores (Hilderbrand and 

others, 1991). It is probable that the Chicxulub crater 

reached bottom in the poorly known Paleozoic crystalline 

basement rocks, since the most strongly shocked fragments 

are crystalline rocks or are of crystalline rock composition. 

Because of their similarities, comparison between the Ries 

and Chicxulub craters may provide interesting and useful 

information. 

The Ries Crater of Germany 

The Ries crater of Germany was a subject of my 

research between 1964 and 1977 (Chao and Minkin, 1977; 

Chao, 1977; Chao and others, 1978; Chao and Xie, 1990). 

The major mappable ejecta units of the Ries, based on 

complete geologic map coverage at the scale of 1:25,000 

(fig. 7), are: 

1. The essentially· unshocked multicolored sedimentary 

breccias (bunte Breccie), which contain huge blocks of 

limestones, shales, and sandstones, some more than 100 

m across. The matrix consists of fragments that range 

from fine to tens of centimeters in size and also consists 

of sedimentary rocks of the overlying formation. Some 

of the limestone blocks are finely crushed into small 

chips that are referred to as "Gries." Some investigators 

referred to those ejecta containing many huge blocks, 

such as those at Harburg, as megabreccias. 

2. Weakly shocked to unshocked crystalline rock ejecta 

consisting mostly of basement crystalline rocks. 

3. Suevite, a fallout and fallback unsorted finer grained 

type of ejecta containing numerous weakly to strongly 

shocked crystalline basement rocks, small amounts of 

unshocked to weakly shocked sedimentary rocks, and 

scattered glassy impactites in the form of pancakes 

(fUiddler) and highly deformed and irregular-shaped 

glassy fragments, a few millimeters to several 

centimeters in size, laden with shocked and unshocked 

mineral and lithic fragments. The glassy inclusion-laden 

impactites contain numerous flattened vesicles coated 

with montmorillonite clays. Glassy impactite or melt 

rock within the suevite, larger than 50 em across, is 

extremely rare. 

Most of the crystalline rock and suevite ejecta lie 

within the crater rim. The crystalline rock breccias occur as 

displaced and uplifted ejecta blocks forming a concentric 

ring of about 12 km in diameter. Only small patches of 

suevite and crystalline rock ejecta occur outside of the 

crater, usually within one crater radius from the rim. 

Generally, the strongly shocked suevite ejecta overlie the 

essentially unshocked, multicolored sedimentary ejecta. 

The sedimentary ejecta blanket extends beyond a crater 

diameter and is the most abundant and extensive ejecta of 

the Ries. 

Evidence for nonballistic transport of ejecta blanket 

of sedimentary breccias with huge blocks under confining 

pressure is as follows: radially distributed striations, 

grooves, and gouges (striated surface, or schliffflache) 

occur on the limestone bedrock surfaces outside the crater 

below the ejecta blanket in the eastern, southern, and 

southwestern parts of the crater (fig. 7; Chao, 1977). Such 

The Ries and the Chicxulub Craters 11 
EXPLANATION 

___. I Orientation of striae gouges. Arrows indicate the 

· direction of movement of ejecta over the bedrock. 

Multicolored sedimentary ejecta and ejecta blanket 

(bunte Breccie) 

~I Crystalline rock ejecta 

J .,.. I Suevite 

---- Crater rim, dashed where inferred 

----- Inferred crystalline wall 

N 

l 

.,...,.------....... , ,...- ' / ' // \ 

I \ 

u \ 

I \ 

I \ 

/ ~ Center of crater \ 

I Ell I 

I I 

I I 

\ NORDLINGEN / ~~~ '® I cP[( 

0-~ // 1\ .... _.... ({} .......... ____ -:---a\1 • f4ll ~ Crystal\lfle ~ 

50° 

48° 

LOCATION OF RIES CRATER 

I 

I 

,; 

I 

I 

I 

I 

I 

0 10 15 KILOMETERS 

EC:::E:::Ic=JB=;=======E======~::=:c=======:::J 1 o MILES 

12 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
<1111111 Figure 7. The orientation of striae and gouges on 

bedrock and the distribution of ejecta types, Ries 

crater, Germany. In some cases, the direction of the 

striations (arrows) does not appear to be exactly 

radiating from the center of the crater. Those striations 

were probably influenced by the preexisting valleys 

along which the ejecta were diverted. 

striations are absent or have not been preserved on 

sandstone and shale bedrock surfaces forming the crater rim 

to the north and northwest of the crater. Abundant cobbles 

and boulders within the sedimentary breccia blanket 

containing huge blocks show gouges and striations. More 

specifically, freshly broken angular limestone fragments 

within the clayey matrix of the sedimentary breccia show 

either deeply incised mineral-produced striae or a high 

degree of clay polish (Chao, 1976). Limestone concretions 

within the clayey matrix are sheared and ruptured with 

offset fractures. These observations are used to conclude 

that the multicolored sedimentary breccias with huge 

crushed but not-disrupted blocks were nonballistically 

transported out of the crater under confining pressure as 

required by the production of mineral striae on angular 

limestone fragments, by radially rolling and gliding over the 

limestone bedrock surface, to distances more than 14 km 

away from the crater rim. The sedimentary breccias form 

the basal portion of the ejecta blanket. At least one-half of 

the total ejecta, consisting of those outside of the 12-km 

central crystalline wall, was estimated to have been nonballistically displaced out of the crater. 

Figure 8 is a photograph of the striated limestone 

bedrock surface under the sedimentary ejecta blanket 

containing huge limestone blocks, at Gundelsheim quarry, 

7 km east-northeast beyond the crater rim. Figure 9, also 

from the Gundelsheim quarry, is a photograph of an 

exposure from July 1992 showing the extensive striated 

bedrock surface without a trace of secondary craters. Note 

the huge block of limestone in the unshocked sedimentary 

ejecta blanket that sits above the bedrocks surface and is 

separated from it by about 0. 3 m of fine-grained clayey 

ejecta matrix. Such huge limestone blocks would have 

definitely produced secondary craters if they had been 

ballistically ejected and transported. In reality, they are 

completely embedded in a clayey matrix containing 

centimeter-sized angular limestone fragments with clay 

Figure 8. The limestone bedrock surface with parallel striae and gouges that were produced by unshocked sedimentary 

ejecta containing huge blocks that moved over the bedrock during the impact event. Hammer indicates direction of the 

striae. Location of photograph is Gundelsheim quarry, 7 km east-northeast from the Ries crater rim. 

The Ries and the Chicxulub Craters 13 
Figure 9. The limestone bedrock with parallel striae, 

grooves, and gouges (not resolved in this photo) in the 

foreground, overlain by an ejecta blanket of sedimentary 

rocks. Note the large limestone block surrounded by a 

fine-grained sedimentary debris consisting of angular 

centimeter-sized limestone fragments embedded in a 

clayey shaly matrix. Note that the huge limestone block 

polish and (or) mineral striae produced under confining 

pressure. 

The following observational data from the Ries are 

also included as reference for comparison with pending 

studies of impactites or fusion products from the Chicxulub 

crater. These data indicate features that are either not 

observed or play a limited role. 

1. The general absence of ejecta 1 JJ.m or less in grain size. 

2. The general absence of strongly shocked limestone from 

the overlying sedimentary formations above the 

crystalline basement rocks and the absence of impact 

glasses in suevite that originated from limestone. This 

situation is similar to the one in Barringer Crater of 

Arizona, where strongly shocked and fused dolomite is 

absent from the Kaibab Formation's dolomite beds that 

overlie the strongly shocked Coconino Sandstone. The 

only glassy impactites produced in the Ries are from the 

basement crystalline rocks from the bottom of the crater. 

sits about 0.3 m above the striated bedrock surface. This 

exposure is an excellent example of an occurrence of 

huge limestone blocks in the sedimentary ejecta blanket 

transported nonballistically. If the huge limestone block 

were transported ballistically, then surely secondary 

craters would have occurred on the bedrock surface. 

Location of this photograph is also Gundelsheim quarry. 

3. The lack of evidence of extensive mixing by impact 

fusion, such as mixing of fused cover sedimentary rocks 

with fused basement crystalline rocks. Impact fusion and 

melting is an instant process, so the glassy impactite 

usually reflects the parent rocks fused in situ, such as 

fused amphibolite versus fused granite. 

4. Very small amounts of glassy impactites (roughly 

estimated to be a small fraction of 1 percent of the total 

ejecta). It is suggested that meteorite impact is not a very 

efficient process for producing large amounts of glass or 

melt. The inefficiency lies in the short duration (in terms 

of seconds) of the high-temperature, high-pressure 

impact process. 

The Chicxulub Crater of the Yucatan 

Descriptions of ejecta from the Chicxulub crater in 

published reports are restricted to samples from drill cores 

14 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
and thus are very scarce. In order to account for negative 

anomalies, an estimated 200- to 450-m thickness of breccia 

inside the crater was assumed (Hilderbrand and others, 

1991). These authors also described a 90-m thickness of 

breccia 50 km outside of the crater's edge that probably 

represents the crater's ejecta blanket, and they reported that 

wells penetrated coarse breccias at depth. There are, 

however, no details concerning the coarse breccias. 

Sedimentary limestone and bentonite breccias as well as 

glass-bearing breccias were also mentioned. It certainly is 

premature to draw conclusions from such preliminary 

descriptions except that there is some shock evidence in 

some of the breccias. 

It is important to know what the coarse breccias are 

composed of. We need to know the rock types of the 

blocks, their size, and whether such blocks are unshocked 

or weakly shocked. Because the Chicxulub crater may be 

the largest or next to the largest terrestrial crater known so 

far, and is shallow, megabreccias may be present. On the 

basis of the fragmentary data on breccias described so far, 

the impression is that such a huge structure did not produce 

a very thick ejecta blanket. If megabreccias do exist, then 

such megabreccias probably would have been transported 

nonballistically rather than ballistically. 

Because inclusion-laden impactite glass of andesitic 

composition and Haitian microtektites have been linked to 

the same structure, it may be the only case where microtektites can be shown to have a terrestrial source crater. 

However, even if the ages of the impactite and melt rocks of 

Chicxulub are the same (Swisher and others, 1992) and 

cannot be resolved from that of the Haitian microtektites, it 

remains possible that the Haitian microtektites described by 

Izett (1991a) and the impactite glass in the ejecta blanket of 

Chicxulub may not be products of the same event. The main 

reason for this seemingly outrageous suggestion is that 

tektites must have formed under reducing conditions, 

whereas impactites and impact melts are formed under high 

temperatures and pressures and dominantly highly oxidizing 

conditions. How these two environments can be combined 

into one event is the key question. 

MODELS OF GIANT TERRESTRIAL IMPACT 

CRATERS AND GEOLOGIC AND 

PETROGRAPHIC CONSTRAINTS 

To be realistic, any hypothetical model for terrestrial 

giant asteroid impact, whether on land, in a shallow sea, or 

in the ocean, should conform with existing geological 

observations of well-studied craters and with experimental 

data. Hence, data from the Chicxulub crater will be 

extremely important. It is unfortunate that all of the Chicxulub crater is buried and inaccessible to study except from 

drill cores. Understanding ejecta relationships through drill 

cores is difficult because of extreme limitations. 

Nevertheless, on the basis of the nature and relationship of 

the well-preserved ejecta deposits of the Ries crater and 

drill-core data from the Manson crater, we may derive the 

necessary geologic and petrographic constraints for giant 

terrestrial craters in excess of 100 km in diameter. 

Previously published theoretical models for giant 

craters in excess of 100 km, either in the ocean or on land 

(O'Keefe and Ahrens, 1982; Melosh, 1982, 1989), that 

have not taken geological constraints into consideration 

may be seriously flawed. First, O'Keefe and Ahrens 

recognized that the atmosphere and ocean are not expected 

to significantly impede the penetration of a 10-km bolide. 

The hypothetical model, however, did not take into 

consideration the instantaneous penetration (in terms of 

nanoseconds) of a bolide 10 km in diameter and an entry 

velocity of greater than 15 km/s into a soft surface and cover 

sedimentary rocks. Because I have found no evidence of 

fused or strongly shocked surface rocks at Barringer Crater 

of Arizona, and at the Ries crater, I suggest that we examine 

more closely the displacement phenomenon of the cover 

beds during the penetration phase of the meteorite-asteroid 

bolide and perhaps pay less attention to the jetting of fused 

and vaporized target and bolide at the initial contact and 

compression stage (Melosh, 1989). I also suggest that, 

because of the instantaneous (submicrosecond) initial stage 

of penetration, the energy transfer was probably not very 

efficient from the impacting body to the target rock, so the 

target surface material was displaced without being fused or 

highly shocked. Studies of Barringer Crater and the Ries 

crater also indicate that the maximum peak pressures and 

temperatures were reached when the impacting body ceased 

penetrating as it reached the more resistant basement 

material. For a giant impact, the bolide very likely has 

excavated and bottomed in crystalline basaltic rocks in the 

ocean or in crystalline volcanic, metamorphic, or plutonic 

rocks on the continent under sedimentary beds before 

reaching peak temperature and pressures at the bottom of 

the crater. Much of the energy of the asteroid impact would 

be dissipated in excavating the crater and expended in the 

basement crystalline rocks. Because asteroids of stony 

meteorite composition have a density of about 2.93 to 3.5 

g/cm3

, the resultant crater would very likely be of a 

shallow-basin type. Such may be the case with Chicxulub. 

Hence, nonballistic transport of the ejecta blanket should be 

considered as an important part of the impact cratering 

model. 

Craters such as the Ries, Manson, and Chicxulub, as 

well as many other large craters (larger than 25 km in 

diameter), all bottomed in crystalline basement rocks. The 

high-temperature ejecta, suevite of the Ries and glassy 

impactites from Manson, do not contain strongly shocked 

limestone from the sedimentary cover beds. Preliminary 

data from the Chicxulub crater (Hilderbrand and others, 

1991) indicated that the crater also bottomed in basement 

crystalline rocks because the 380 m of "melt rocks" 

Models of Giant Terrestrial Impact Craters and Geologic and Petrographic Constraints 15 
penetrated by drill holes are siliceous, with about 62 percent 

silica. In addition, on the basis of oxygen isotopic data, 

Blum and Chamberlain ( 1992) excluded the evaporite as a 

possible source of the calcium-rich glassy impactite from 

Chicxulub. Therefore, target cover rocks such as limestone 

and evaporite do not contribute as agents of mass 

extinction. It is important to keep in mind that the strongly 

shocked target rocks of Chicxulub are primarily crystalline 

rocks of andesitic composition. 

In addition, in applying the model proposed by 

O'Keefe and Ahrens (1982), it would be much more 

convincing if there were data to support the premise that 

approximately 0.1 percent of the bolide mass is ejecta 1 IJ-m 

or less in size. Such fine ejecta must eventually be deposited 

on the land surface. The burden of proof in the K-T 

boundary section lies in verifying the existence of 

significant amounts of ejecta particles 1 IJ-m or less in size. 

I have singled out the importance of the need to 

understand the penetration phenomenon when an asteroid 

first contacts the ground surface. We also need to 

understand how giant terrestrial impacts produce microtektites and tektites. As will be described in the next section, 

there is a geochemical constraint that, unlike glassy impactites embedded within highly shocked crater ejecta 

produced from the basement of the crater, tektites are 

characteristically low in volatiles and, more importantly, 

are dominated by divalent instead of trivalent Fe. Shiny 

metallic Ni-Fe spherules have been found in many tektites 

(Chao and others, 1961 , 1964). Their presence here means 

that tektites were formed in a high-temperature, reducing 

environment and not in a highly oxidizing environment, 

engulfed in hot gas or steam. It is difficult to understand 

how hot jets of steam or heated air, an incompatible 

environment for tektites, could help to propel molten tektite 

glass particles to great distances, but such would be 

required in order to overcome in some way the obstacle of 

aerodynamic drag. 

Shoemaker suggested that terrestrial craters larger 

than 50 km are more likely to show evidence of impacts by 

comets (Shoemaker, 1991). There are very few observed 

geologic data for use as a guide to hypothesizing the 

specifics of low-density (1-1.3 g/cm3

) and very low 

strength or cohesion cometary impact. Comets without 

asteroid nuclei would perhaps have difficulties penetrating 

the atmosphere and the solid earth without breaking up, 

scattering, and dispersing. The only data available are with 

regard to the Tunguska comet encounter on June 30, 1908 

(Krinov, 1963; Rocchia and others, 1990). The Tunguska 

event had thunder upon atmospheric entry and travel and a 

spectacular fireball, but it only felled trees in an area 35 km 

in diameter and charred and seared trees in an area 15 to 18 

km across on the ground. Nothing, except for some spherules, was recovered that could be attributed to the comet. If 

the Tunguska event were a meteoroid (Melosh, 1989), some 

evidence of its remains should be found. Because no crater 

was excavated by this low-density cometary body on 

contact with the ground surface, the Tunguska event was 

not even included in Grieve's ( 1991) list of terrestrial 

craters. 

Because both Ir anomalies (result of vaporization of 

the impacting body) and microtektites have been linked to 

giant terrestrial craters such as the Chicxulub, any 

hypothetical giant terrestrial cratering model should explain 

why Ir anomalies and microtektites and tektites do not seem 

to coexist. 

THE PLACE OF ORIGIN OF THE 

AUSTRALASIAN TEKTITES AND 

MICROTEKTITES 

We have already mentioned moldavite tektites, 

pseudomorphs of microtektites, and microtektites. The 

most abundant and best preserved tektites are the 0.77-Ma 

(Izett and Obradovich, 1992) Australasian tektites and 

microtektites. In contrast to ferric-iron-rich, debris-laden 

glassy impactites from highly shocked ejecta, tektites are, 

in general, microlite free, water free (very low volatile), 

ferrous iron dominated, and well represented in primary 

forms (or forms of revolution) such as spheres, oblate 

spheroids, ellipsoids, discoids, teardrops, rods, and 

dumbbells. Of great importance is the fact that numerous 

Australasian tektites show well-preserved, ablated flanged 

buttons and features of spalling. The chemical compositions 

of tektites vary widely. They are usually high in silica, but 

different chemical groups are represented, such as the 

high-magnesian group and the high-calcium group (Chapman and Scheiber, 1969). Most tektites are also high in U 

and Th, similar to granophyres and some stony meteorites 

(Morgan, 1969, 1970). Other investigators who favor a 

terrestrial origin for tektites are convinced that they 

resemble sedimentary rocks because of the high silica 

content. Hence, trying to reach a conclusion on tektites on 

compositional grounds seems futile. 

Figure 10 shows the strewnfield distribution of the 

Australasian land tektites (Chapman, 1964). If the land 

tektite strewnfield and the Australasian microtektites from 

ocean floors are combined, the Australasian tektites and 

microtektites represent the areally largest tektite event on 

Earth. In terms of strewnfield size, recovered weight, and 

the sizes of tektites produced, the event responsible for 

producing the Australasian tektites and microtektites is also 

the largest (Chapman, 1971; Glass and others, 1979). 

Events producing the North American tektites, the late 

Eocene microtektites, and the Haitian microtektites are 

apparently smaller. 

Two places of origin of the Australasian tektites and 

microtektites have been proposed. The majority of 

investigators, who have a geological background, are in 

favor of a terrestrial origin, on the basis of geochemical 

16 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
o100 orless 

• 1000 or more 

Figure 10. The huge strewnfield of land Australasian tektites. (From Chapman, 1964, 1971.) 

data. A minority of investigators, represented by Chapman 

and his colleagues, who utilized extensive aerodynamical 

and chemical compositional evidence, favor a lunar origin. 

I consider the experimental and observational aerodynamic 

evidence more specific and precise than the geochemical 

evidence. 

For those who favor a terrestrial origin for tektites and 

microtektites, a critical reading and review of the 

contributions of Chapman and his colleagues (Chapman and 

Larson, 1963; Chapman, 1964; Chapman and others, 1964; 

Chapman and Scheiber, 1969; Chapman, 1971) might make 

the importance and difficulty of resolving the question 

concerning the place of origin of tektites clearer. 

According to Chapman and Larson (1963), 

aerodynamic drag is the major obstacle to the acceptance of 

a terrestrial origin for tektites. Figure 6 includes a plot of the 

launch range, or distance ejected, through a stationary 

atmosphere, with respect to the ratio of mass over the drag 

coefficient (which equals 1 for a sphere) times the crosssectional area normal to flight. Figure 11 shows 

photographs of ablated mini tektites from Serpentine Lake in 

central Australia. In an atmospheric condition, the minitektites that range in weight from 0. 007 to 0. 2 g cannot have 

traveled more than 100 m (fig. 6) through the Earth's 

atmosphere because aerodynamic drag prevents travel over 

longer distances. Also, according to Chapman (oral commun., 1992), if the ablated 2- to 3-mm-sized flanged 

australite buttons from Serpentine Lake had been produced 

by terrestrial impact, then more than 99.9 percent of the 

atmosphere must have been removed so that such small 

glass particles could overcome aerodynamic drag and 

escape through the atmosphere. This scenario would require 

a crater several hundred kilometers in diameter. 

To account for the amount of ablation and the 

flattening of the flanges of ablated buttons, Chapman (oral 

commun., 1992) is absolutely certain of the following, 

based on aerodynamic experiments performed in the 

laboratory. 

1. In order to account for the observed ring waves and the 

degree of flange flattening, the entry angle of australites 

and javanites into the atmosphere could not have been 

grazing or shallow but was relatively steep. 

2. In order to account for the spalled ablated anterior shells 

of Australasian tektite cores of 4- to 6-cm size from 

Australia, Indonesia, and northern Luzon, these 

relatively large tektites must have been cooled well 

below the glass strain temperature (650°C), thus 

becoming rigid before entry into the atmosphere. Hence, 

smaller tektites (for example, 2 em in size) were 

certainly rigid and relatively cool prior to entry. 

3. In order to account for the observed amount of ablation, 

the entry velocity of australites and javanites must have 

been considerably greater than 5 km/s, the limit of 

velocity below which ablation of rigid tektite glass 

would not occur. 

A second problem, in addition to the obstacle of 

aerodynamic drag, is the identification of a 0. 77-Ma 

terrestrial crater perhaps 300 km in diameter that should be 

somewhere in Scandinavia, on the basis of the observed 

partial strewnfield distribution. A terrestrial crater of this 

size and age cannot be hidden easily or obliterated by 

subduction in this time span. 

A third problem is explaining the absence of more 

than 95 percent of the huge amounts of nontektite ejecta 

from this same source crater. Some of the Australasian 

tektites were found embedded in unconsolidated sediments 

or cemented in laterite in Thailand (Chao, unpub. data, 

1964). There were no nontektite ejecta present in the same 

site. 

The general objection to the lunar origin of the 

Australasian tektites and microtektites is that none of the 

returned lunar samples (except perhaps sample no. 12013) 

resembles tektites in composition. Samples from the Apollo 

15 and 16 geochemical data-collecting orbiter along the 

lunar equator also did not reveal high-silica tektite 

composition. Nevertheless, Chapman (1971) proposed that 

The Place of Origin of the Australasian Tektites and Microtektites 1 7 
Australasian tektites were ejected specifically from Tycho, 

along the Rosse ray from that crater. Until areas such as the 

Rosse ray have been sampled, a lunar origin should not be 

dismissed for the Australasian tektites and microtektites. 

COMPARISON OF THE MANSON AND 

CHICXULUB CRATERS OF THE CRETACEOUSTERTIARY BOUNDARY EVENTS WITH THE 

AUSTRALASIAN TEKTITE EVENT 

Most investigators would agree that the 35-kmdiameter Manson crater of Iowa, a possible source of 

shocked quartz in the K-T boundary sections, was too small 

and insignificant to have caused mass extinction. 

If the coarse breccias that were present from the drill 

holes from the 180-km-diameter Chicxulub crater tum out 

to be significant, then by analogy with that of the Ries, 

many of these coarse breccias may not have been airborne. 

The extent of the Chicxulub crater and its ejecta 

(beyond Haiti) fits nicely inside the Australasian strewnfield 

(fig. 10), without taking into account the size and location 

of the source crater of the Australasian tektites. The 

Chicxulub cratering event was therefore much smaller than 

the Australasian cratering event. Those who favor the 

terrestrial origin of tektites and microtektites must explain 

how the smaller Chicxulub cratering event could have 

caused mass extinction if there was no mass extinction 

caused by the Australasian tektite source crater at 0. 77 Ma. 

SUMMARY 

K-T boundary sections are records of events during 

the transition from Cretaceous to Tertiary time. What is 

present in or absent from this record is critical to our 

understanding and reconstruction of the K-T events. 

1. The principal source of Ir anomaly above the layer 

containing shocked quartz may be ablation products of 

stony meteorites and not vaporized impacting asteroids. 

If this is true, occurrences of Ir anomalies along stratigraphic hiatuses are likely to be more widespread and 

not correlated with any cratering events or occurrences 

of shocked quartz. 

2. Two events are tentatively identified from records of the 

K-T boundary sections of Western U.S. interior basins: 

(1) the 35-km-diameter Manson crater, a probable 

source of the shocked quartz, but too small a crater to 

cause mass extinction and (2) the 180-km-diameter 

Chicxulub crater of the Yucatan Peninsula, Mexico, the 

probable source of the gorceixite-goyazite spherules. 

The two events were separated by the underclay, which 

may have required many years of accumulation as 

substrate of the peat in a peat swamp. 

It would be nice to be able to explain why the Ir 

anomaly and ejecta with shocked quartz are absent in the 

1 em 

Figure 11. Ablated minitektites from Serpentine Lake, 

Central Australia. (Photographs courtesy of Dean R. Chapman.) These ablated minitektites provide strong aerodynamic evidence against a terrestrial origin for the Australasian tektites and microtektites. (See Chapman and 

Larson, 1963.) A, Minitektites with teardrop, oval, oblate, 

spherical, and disc shapes. A few show clear ablation 

flanges. 

gorceixite- goyazite spherule layer if the source were the 

Chicxulub crater. 

3. For those who favor a terrestrial origin for the 0. 77-Ma 

Australasian tektites and microtektites, the following 

problems remain: 

• To overcome the principal obstacle of aerodynamic 

drag, a source crater several hundred kilometers in 

diameter is required to remove the atmosphere. This 

gigantic crater must be found and identified. Is it 

somewhere in Scandinavia? 

• In addition, the total absence of more than 95 percent 

of nonglass ejecta from the same source crater must be 

accounted for. 

• There was no mass extinction caused by the source 

crater for the Australasian tektites, so it is very 

unlikely that Chicxulub, which is much smaller, 

could cause mass extinction. 

4. The Chicxulub crater and its various types of ejecta 

should provide the geologic and petrographic constraints 

for models of giant terrestrial craters. Is the gorceixitegoyazite spherule layer the only record Chicxulub left in 

the K-T boundary sections in North America? 

ACKNOWLEDGMENTS 

This paper could not have been written without 

generous help and provision of data from my colleagues and 

18 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
1 em 

Figure 11.-Continued. B, Flattened minitektites, 0.007 to 0.043 g. A few are 

the same as in A. 

1 em 

Figure 11.-Continued. C, Ablated flanged minitektite buttons, 0.08 to 7.9 g. 

friends Glen Izett and Dean R. Chapman and the 

discussions I had with them. Some of the illustrations 

included here were provided by Glen and Dean. Glen 

showed me polished sections of various Cretaceous-Tertiary 

boundary sections and the contained impact-shock evidence 

from the interior basins of the Western United States and 

Haiti. Dean provided quotations and discussions of his 

current views regarding the major objection to terrestrial 

origin for the Australasian tektites, minitektites, and 

microtektites, and he provided photographs of ablated 

minitektites from Serpentine Lake, Australia. I am also 

grateful to my colleague Hugh Millard for the analytical 

data on Au and Ir concentrations in fusion crusts of the 

Pultusk and the Nuevo Mercurio HIV chondrites. I thank 

Roy Clarke, curator of the meteorite collection at the U.S. 

National Museum, for choosing and providing the fusion 

crust specimens of Pultusk and Nuevo Mercurio for the Ir 

analyses. I am also grateful for reviews by Chapman, Izett, 

and my colleagues Jean A. Minkin and Judith M. Back, and 

careful editing by John Watson. 

REFERENCES CITED 

Blum, J.D., and Chamberlain, C.P., 1992, Oxygen isotope 

constraints on the origin of impact glasses from the 

Cretaceous-Tertiary boundary: Science, v. 257, no. 5073, 

p. 1104-1107. 

Chao, E.C.T., 1963a, Geological occurrences of some southeast 

Asian and Australian tektites, in U.S. Geological Survey, 

Astrogeologic studies, annual progress report, August 25, 

1961, to August 24, 1962, part C, Cosmochemistry and 

petrography: U.S. Geological Survey Open-File Report 685, 

p. 101-102. 

References Cited 19 
---1963b, The petrographic and chemical characteristics of 

tektites, in O'Keefe, J .A., ed., Tektites: University of Chicago Press, p. 51-93. 

---1964a, A spalled aerodynamically modified moldavite 

from Slavice, Moravia, Czechoslovakia: Science, v. 146, 

no. 3645, p. 790-791. 

---1964b, Some geologic occurrences of Australasian tektites, in U.S. Geological Survey, Astrogeologic studies, 

annual progress report, July 1, 1963, to July 1, 1964, part C, 

Cosmic chemistry and petrology: U.S. Geological Survey 

Open-File Report 779, p. 10-64. 

---1968, Pressure and temperature histories of impact metamorphosed rocks based on petrographic observations, in 

French, B.M., and Short, N.M., eds., Shock metamorphism 

of natural materials: Baltimore, Md., Mono Book Corp., 

p. 135-158. 

---1976, Mineral-produced high pressure striae from clay 

polish: Key evidence for non-ballistic ejecta transport from 

the Ries Crater: Science, v. 194, no. 4265, p. 615-620. 

---1977, The Ries crater of southern Germany-A model for 

large basins on planetary surfaces: Geologisches Jahrbuch, 

no. 43, p. 3-81. 

Chao, E.C.T., Adler, 1., Dwornik, E.J., and Littler, J., 1961, 

Metallic spherules in tektites from Isabela, Philippine 

Islands: Science, v. 135, no. 3498, p. 97-98. 

Chao, E.C.T., Dwornik, E.J., and Littler, J., 1964, New data on 

the nickel-iron spherules from southeast Asian tektites and 

their implications: Geochirnica et Cosmochimica Acta, v. 

28, no. 6, p. 971-980. 

Chao, E.C.T., and El Goresy, Ahmed, 1977, Shock attenuation 

and the implantation of Fe-Cr-Ni veinlets in the compressed 

zone of the 1973 Ries research deep drill core: Geologica 

Bavarica, v. 75, p. 289-304. 

Chao, E.C.T., HUttner, R., and Schmidt-Kaler, H., 1978, Principal exposures of the Ries Meteorite Crater in southern 

Germany: Munich, Germany, Bayerisches Geologisches 

Landesamt, 84 p. 

Chao, E.C.T., and Minkin, J.A., 1977, Impact cratering phenomenon for the Ries multiring structure based on constraints of geological, geophysical, and petrological studies 

and the nature of the impacting body, in Roddy, D. J. , Pepin, 

R.O., and Merrill, R.B., eds., Impact and explosion cratering: Planetary and terrestrial implications: New York, Pergamon Press, p. 405-424. 

Chao, E.C.T., and Xiande, Xie, 1990, Mineralogical approaches 

to geological investigations: Beijing, Science Press, 380 p. 

Chapman, D.R., 1964, On the unity and origin of the Australasian tektites: Geochimica et Cosmochimica Acta, v. 28, no. 

6, p. 841-880. 

---1971, Australasian tektite geographic pattern, crater and 

ray of origin, and theory of tektite events: Journal of Geophysical Research, v. 76, no. 26, p. 6309-6338. 

Chapman, D.R., and Gault, D.E., 1967, Critique of "Cometary 

impact and the origin of tektites": Journal of Geophysical 

Research, v. 72, no. 10, p. 2695-2699. 

Chapman, D.R., and Larson, H.K., 1963, On the lunar origin 

of tektites: Journal of Geophysical Research, v. 68, no. 14, 

p. 4305-4358. 

Chapman, D.R., Larson, H.K., and Scheiber, L.C., 1964, Population polygons of tektite specific gravity for various localities in Australasia: Geochirnica et Cosmochimica Acta, v. 

28, no. 6, p. 821-830. 

Chapman, D.R., and Scheiber, L.C., 1969, Chemical investigation of Australasian tektites: Journal of Geophysical 

Research, v. 74, p. 6737-6776. 

Claeys, Philippe, Casier, J.G., and Margolis, S.V., 1992, 

Microtektites and mass extinctions: Evidence for a Late 

Devonian asteroid impact: Science, v. 257, no. 5073, 

p. 1102-1104. 

Cohen, A.J., 1963, Asteroid or comet impact hypothesis of tektite origin: The moldavite strewnfield, in O'Keefe, J.A., ed., 

Tektites: University of Chicago Press, p. 189-211. 

El Goresy, Ahmed, and Chao, E.C.T., 1976, Evidence of the 

impacting body of the Ries crater-The discovery of 

Fe-Cr-Ni veinlets below the crater bottom: Earth and Planetary Science Letters, v. 31, no. 3, p. 330-340. 

Glass, B.P., Swincki, M.B., and Zwart, P.A., 1979, Australasian, Ivory Coast, and North American tektite strewn fields: 

Size, mass, and correlation with geomagnetic reversals and 

other earth events, in Proceedings of the Tenth Lunar and 

Planetary Science Conference, v. 3, Planetary interiors and 

surfaces, March 19-23, 1979: Pergamon, Geochimica et 

Cosmochimica Acta Supplement 11, p. 2535-2543. 

Grieve, R.A.F., 1991, Terrestrial impact: The record in the rocks: 

Meteoritics, v. 26, no. 3, p. 175-194. 

Hartung, J.B., Kunk, M.J., and Anderson, R.R., 1990, Geology, 

geophysics, and geochronology of the Manson impact structure: Geological Society of America Special Paper 247, p. 

207-221. 

Hilderbrand, A.R., and Boynton, W.V., 1990, Proximal 

Cretaceous-Tertiary boundary impact deposits in the Caribbean: Science, v. 248, p. 843-847. 

Hilderbrand, A.R., Penfield, G. T., Kring, D .A., Pilkington, 

Mark, Camargo, Z.A., and Jacobsen, S.B., 1991 Chicxulub 

crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico: Geology, v. 19, no. 

9, p. 867-871. 

Horz, F., 1982, Ejecta of the Ries Crater, Germany: Geological 

Society of America Special Paper 190, p. 39-56. 

Izett, G.A., 1990, The Cretaceous/Tertiary boundary interval, 

Raton basin, Colorado and New Mexico, and its content of 

shock-metamorphosed minerals; evidence relevant to the KIT 

boundary impact-extinction theory: Geological Society of 

America Special Paper 249, 100 p. 

---1991a, Tektites in Cretaceous-Tertiary boundary rocks on 

Haiti and their bearing on the Alvarez impact extinction 

hypothesis: Journal of Geophysical Research, v. 96, no. E4, 

p. 20879-20905. 

---1991b, Are two comet or asteroid impacts recorded at 

western North American K-T boundary site?: Eos supplement, v. 72, no. 44, Oct. 29, 1991, AGU 1991 Fall Meeting, p. 278. 

Izett, G.A., Dalrymple, G.B., and Snee, L.W., 1991, 40ArP9

Ar 

age of Cretaceous-Tertiary boundary tektites from Haiti: Science, v. 252, no. 5012, p. 1539-1542. 

Izett, G.A., and Obradovich, J.D., 1992, Laser-fusion 40ArP9

Ar 

ages of Australasian tektites, in Lunar and Planetary Science 

20 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 
XXIII, Abstracts of Papers Submitted to the Twenty-Third 

Lunar and Planetary Science Conference, March 16-20, 

1992, part 2: Houston, Tex., Lunar and Planetary Institute, 

p. 593-594. 

Keller, G., D'Hondt, S.L., Orth, C.J., Gilmore, J.S., Oliver, 

P.Q., Shoemaker, E.M., and Molina, E., 1987, Late Eocene 

impact microspherules: Stratigraphy, age and geochemistry: 

Meteoritics, v. 22, no. 1, p. 25--60. 

Krinov, E.L., 1963, The Tunguska and Sikhote-Alin meteorites, 

in Middlehurst, B., and Kuiper, G.P., eds., The solar system, v. IV, Moon, meteorites and craters: University of 

Chicago Press, p. 208-234. 

Kunk, M.J., Izett, G.A., Haugerud, R.A., and Sutter, J.F., 

1989, 40 ArP9 Ar dating of the Manson impact structure: A 

Cretaceous-Tertiary boundary crater candidate: Science, v. 

244, no. 4912, p. 1565-1568. 

Lin, S.C., 1966, Cometary impact and the origin of tektites: 

Journal of Geophysical Research, v. 71, no. 10, 

p. 2427-2437. 

Melosh, H.J., 1982, The mechanics oflarge meteoroid impacts in 

the Earth's oceans: Geological Society of America Special 

Paper 190, p. 121-128. 

---1989, Impact cratering: A geologic process: Oxford, Clarendon Press, 245 p. 

Millard, H. T. , Jr. , Englert, P. , and Herperes, W. , 1985, Instrumental neutron activation analysis of small spheres of various origins, in Sansoni, Bruno, ed., Instrumentelle multielementanalyse: Weinheim, Germany, Verlagsgesellschaft, p. 

573-576. 

Millard, H.T., Jr., and Finkelman, R.B., 1970, Chemical and 

mineralogical composition of cosmic and terrestrial spherules 

from a marine sediment: Journal of Geophysical Research, v. 

75, no. 11, p. 2125-2134. 

Montanari, A., Drake, R., Bice, D.M., Alvarez, Walter, Curtis, 

G.H., Turrin, B.D., and DePaolo, D.J., 1985, Radiometric 

time scale for the upper Eocene and Oligocene based on 

KJ Ar and Rb/Sr dating of volcanic biotites from the pelagic 

sequence of Gubbio, Italy: Geology, v. 13, no. 9, 

p. 596-599. 

Morgan, J. W., 1969, Uranium and thorium in tektites: Earth and 

Planetary Science Letters, v. 7, no. 1, p. 53-63. 

---1970, Uranium and thorium in tektites: An additional 

comment: Earth and Planetary Science Letters, v. 8, no. 2, 

p. 141-168. 

---1978, Lonar crater glasses and high-magnesium australites: Trace element volatilization and meteoritic contamination, in Proceedings of the Ninth Lunar and Planetary 

Science Conference, v. 2, Lunar and planetary surfaces, 

Houston, Tex., March 13-17, 1978: Pergamon Press, 

Geochimica et Cosmochimica Acta Supplement 10, p. 

2713-2730. 

Morgan, J.W., Higauchi, H., Ganapathy, R., and Anders, 

Edward, 1975, Meteoritic material in four terrestrial meteorite craters, in Proceedings of the Sixth Lunar Science Conference, v. 2, Chemical and isotopic studies, Houston, Tex., 

March 17-21, 1975: Pergamon Press, Geochimica et Cosmochimica Acta Supplement 6, p. 1609-1623. 

Morgan, J.W., Janssens, M.J., Hertogen, Jan, Gros, Jacques, 

and Takahashi, Hiroshi, 1979, Ries impact crater, southern 

Germany: Search for meteoritic material: Geochimica et 

Cosmochimica Acta, v. 43, no. 6, p. 803-815. 

Morgan, J.W., and Wandless, G.A., 1983, Strangways Crater, 

Northern Territory, Australia: Siderophile element enrichment and lithophile element fractionation, in Proceedings of 

the Thirteenth Lunar and Planetary Science Conference, 

Houston, Tex., March 1982: Washington, D.C., American 

Geophysical Union, Journal of Geophysical Research v. 87 

Supplement, p. A819-A829. 

O'Keefe, J.D., and Ahrens, T.J., 1982, The interaction of the 

Cretaceous/Tertiary extinction bolide with the atmosphere, 

ocean, and solid earth: Geological Society of America Special Paper 190, p. 103-120. 

Pohl, J., Stoffler, D., Gall, H., and Ernstson, K., 1977, The Ries 

impact crater, in Roddy, D.J., Pepin, R.O., and Merrill, 

R.B., eds., Impact and explosion cratering: Planetary 

and terrestrial implications: New York, Pergamon Press, 

p. 343-404. 

Robin, E., Bonte, P., Froget, L., Jehanno, C., and Rocchia, R., 

1992, Formation of spinels in cosmic objects during atmospheric entry: A clue to the Cretaceous-Tertiary boundary 

event: Earth and Planetary Science Letters, v. 108, no. 4, p. 

181-190. 

Rocchia, R., Bonte, P., Jehanno, C., Robin, E., de Angelis, M., 

and Boclet, D. , 1990, Search for the Tunguska event relics 

in the Antarctic snow and new estimation of the cosmic 

iridium accretion rate: Geological Society of America Special Paper 247, p. 189-194. 

Shoemaker, E.M., 1991, Geological and astronomical evidence 

for comet impact and comet showers during the last 100 

million years, in Abstracts for the International Conference 

on Asteroids, Comets, and Meteors 1991, Flagstaff, Ariz., 

June 24--28, 1991: Houston, Tex., Lunar and Planetary Institute, p. 199. 

Shoemaker, E.M., and Izett, G.A., 1992, Stratigraphic evidence 

from western North America for multiple impact at the KIT 

boundary, in Lunar and Planetary Science XXIII, Abstracts 

of Papers Submitted to the Twenty-Third Lunar and Planetary Science Conference, March 16-20, 1992, part 3: Houston, Tex., Lunar and Planetary Institute, p. 1293-1294. 

Smit, Jan, Montanari, Alessandro, Swinburne, N .H.M., Alvarez, 

Walter, Hilderbrand, A.R., Margolis S.V., Claeys, Philippe, Lowrie, William, and Asaro, Frank, 1992, Tektitebearing, deep-water clastic unit at the Cretaceous-Tertiary 

boundary in northeastern Mexico: Geology, v. 20, no. 2, p. 

99-103. 

Swisher, C.C., Grajales-Nishimura, J.M., Montanari, Alessandro, Margolis, S.V., Claeys, Philippe, Alvarez, Walter, 

Renne, Paul, Cedillo-Pardo, Esteban, Maurrasse, F.J.M.R., 

Curtis, G.H., Smit, Jan, and McWilliams, M.O., 1992, 

Coeval 40 ArP9 Ar ages of 65.0 million years ago from Chicxulub crater melt rock and Cretaceous-Tertiary boundary tektites: Science v. 257, no. 5072, p. 954--958. 

Toon, O.B., Pollack, J.B., Ackerman, T.P., Turco, R.P., 

McKay, C.P., and Liu, M.S., 1982, Evolution of an impactReferences Cited 21 
generated dust cloud and its effects on the atmosphere: Geological Society of America Special Paper 190, 

p. 187--200. 

Vickery, A.M., Kring, D.A., and Melosh, H.J., 1992, Ejecta 

associated with large terrestrial impacts: Implications for the 

Chicxulub impact and Kff boundary stratigraphy, in Lunar 

and Planetary Science XXIII, Abstracts of Papers Submitted 

to the Twenty-Third Lunar and Planetary Science Conference, March 16--20, 1992: Houston, Tex., Lunar and Planetary Institute, p. 1473-1474. 

Wang, Kun, 1992, Glassy microspherules (microtektites) from an 

Upper Devonian limestone: Science, v. 256, p. 1547-1550. 

22 Comparison of K-T Boundary Impact Events and the Australasian Tektite Event 


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 7.5- 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 7 .5-minute quadrangle photo geologic maps on planimetric 

bases that 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 for quadrangle or 

irregular areas at various scales. Pre:.l971 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 blackand-white 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, Map Distribution, Box 25286, Bldg. 810, Federal Center, 

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

"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" are 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, 5 82 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.