© Copyright by SARAH HUSON, 2009 All Rights Reserved

QUANTITATIVE ANALYSIS OF THE DEFORMATIONAL HISTORY AND TIMING OF

THE SIERRA MADERA IMPACT STRUCTURE, WEST TEXAS

SARAH ANN HUSON

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY School of Earth and Environmental Science

MAY 2009

To the Faculty of Washington State University:

The members of the Committee appointed to examine the dissertation of SARAH ANN HUSON find it satisfactory and recommend that it be accepted.

______Michael C. Pope, Ph.D., Chair

______F. F. Foit, Ph.D.

______D. R. Gaylord, Ph.D.

______A. J. Watkinson, Ph.D.

ACKNOWLEDGMENTS

I would like to thank my advisor, Mike Pope, for giving me the chance to work on

this exciting project. Mike is a great advisor and I appreciate the opportunity I had to work with him. I also want to thank the members of my committee, John Watkinson,

Nick Foit, and David Gaylord. They did their fair share of carrying rocks while in Texas as well as offering advice especially on how to cover up dents in field vehicles.

I also owe thanks to Scotty Cornelius for helping me with microprobe work and

Charles Knaack for hiring me on as a tech assistant when my funding from NASA was delayed. Also, I would also like to thank Ray Donelick for the apatite fission track dating, James Donnelly from the core facility at UT Austin for help with collecting unshocked core samples for XRD analysis, Dave Katz for sending carbonate samples from the Mission Canyon Formation for XRD analysis, and Bruce Arey from the Pacific

Northwest National Lab for his assistance with SEM analyses. Paul Olin and Zsuzsanna

Balogh-Brunstad were extremely helpful office-mates and provided needed distractions from time to time and for that, I am extremely appreciative.

I would like to thank the members of the Lyda family who live on the property where I conducted my field work. Also, I am extremely grateful to Glenn Lang who was very instrumental in getting me access to the property and without whose efforts this research could not have taken place.

Finally, and most importantly, I would like to thank my mother, Donna, and sister, Rachael, who have been unwavering in their support all these years. Rick

Laughlin, your support and advice are unmatched and I thoroughly thank you.

iii

QUANTITATIVE ANALYSIS OF THE DEFORMATIONAL HISTORY AND

TIMING OF THE SIERRA MADERA IMPACT STRUCTURE, WEST TEXAS

Abstract

by Sarah Ann Huson, Ph.D. Washington State University May 2009 Chair: Michael C. Pope

The Sierra Madera impact structure is a 12.9 km diameter, well-eroded remnant of a complex impact crater located in West Texas. The crater was last studied in the early

1970’s and is an excellent site for deformation and timing studies using modern research techniques.

In this dissertation, Chapter 1 provides an introduction to the Sierra Madera structure including previous geologic research regarding the crater, stratigraphy and structure/crater formation. Chapter 2 lists deformational features documented within the central uplift of the crater and assigns temperature and shock pressure values to the conditions of crater formation. Chapter 3 details the use of X-ray diffraction as a possible method to determine shocked rocks from terrestrially deformed rocks. Chapter 4 documents the use of the fission-track method to assign a radiometric age to the impact

event. Chapter 5 is a field guide for the impact crater.

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS……………………………………………………..…………iii

ABSTRACT…………………………………………………………..………………….iv

LIST OF FIGURES………………………………………………..………...………….viii

LIST OF TABLES……………………………………………………………...…...……xi

CHAPTER 1: Introduction …………………..…………………………………………1

Introduction………………………………………………………………..2

Previous research at Sierra Madera………………………………………..2

Geologic history of area…………….……………………………………..6

Stratigraphy…...…………………………………………………………...8

Structure………….………………………………………………………12

Chapter summaries……………………………………………………….16

References...... 19

CHAPTER 2: Deformational feature and impact generated breccia from the Sierra

Madera impact structure, West Texas………………………...... ……23

Introduction...... 24

Geologic setting...... 26

Methods……………………...……………………………………...……29

Deformational features…..………………………………………...……..30

Target rocks……………………………………………………...30

Impact generated breccia………………………..……………….38

Discussion.………………………………………………...……….…….45

Conclusions………………………………………………………...…….50

Acknowledgements………………………………………………...…….50

References……………………………………………………...………...51

CHAPTER 3: Rietveld analysis of X-ray powder diffraction patterns as a potential

tool for the identification of impact deformed carbonate rocks...... 58

Introduction………………………………………………………...….…59

Geologic setting…..………………………………………………...……61

Experimental…………………………….………………………….....…63

Results……………...……………………………………………….....…70

Discussion……………………………………………………………..…75

Conclusions……………………………………………………..……...... 79

Acknowledgements………………………………………………..…...... 80

References………………………………………………………..………81

CHAPTER 4: Fission track age for the Sierra Madera impact structure……..……88

Introduction………………………………………………………..……..89

Geologic setting….………………………………………………..……..90

Methods….………………………………………………………..….…..92

Sample description………………………………………………..….…..92

Results…………………..…………………………………………...…...94

Discussion………………………………………………………….….…94

Conclusions……………………………………………………….…….100

Acknowledgements……………………………………………….…….100

References………………………………………………………….…...101

CHAPTER 5: Field guide to the Sierra Madera impact structure………….……..104

Introduction…………………………………………………………..…109

Location and background……….………………………………….…...110

Stratigraphy……..………………………………………………….…...113

Brief regional geologic history…...………………………………....….117

Past Studies…………………………...…………………………….…..118

Current and future research…………………………………………..…119

Field trip stops and descriptions……………………………………..…122

References……………………………………………………………....137

APPENDICES

A. X-ray analysis data for Sierra Madera and Mission Canyon Fm calcite

samples……………………………………………………………….143

B. X-ray analysis data for Sierra Madera and Mission Canyon Fm

dolomite samples…………………………………………………….276

C. Single peak FWHM values for calcite-rich Sierra Madera and Mission

Canyon Formation samples…………………………………………..403

D. Single peak FWHM values for dolomite-rich Sierra Madera and

Mission Canyon Formation samples………………………………...408

E. Rietveld parameters for all X-ray diffraction samples………………414

F. Rietveld data for Sierra Madera and Mission Canyon Fm calcite

samples……………………………………………………………….416

G. Rietveld data for Sierra Madera and Mission Canyon Fm dolomite

vii

samples……………………………………………………………….420

H. Electron microprobe data for calcite-rich and dolomite-rich X-ray

diffraction samples…………………………………………………...424

viii

LIST OF FIGURES

Page

1.1. Location of the Sierra Madera impact crater………………………………….….…..3

1.2. Geologic map of Sierra Madera...…………………………………………….………4

1.3. Stratigraphic column of rocks exposed at Sierra Madera……...……………………..9

1.4. Stages of crater development for a complex crater…………………………..……...13

1.5. Cross section of Sierra Madera………………………………………………...……15

2.1. Geologic map of Sierra Madera impact crater……………..…………………..……27

2.2. Photomicrographs of sandstone deformation from the Sierra Madera impact

structure…………………………………………………………………………….31

2.3. Possible surface deformation in zircon……………………………………...………33

2.4. Shatter cone deformation…………...…………………………………………..…...35

2.5. Geologic map of the central uplift of the Sierra Madera impact crater……………..37

2.6. Geologic map of the central uplift of the Sierra Madera impact crater showing

location of breccia types………………….………………………….…...………..40

2.7. Monomict impact generated breccia from Sierra Madera……………...…...………42

2.8. Polymict breccia outcrop and hand sample photographs……………………...……44

2.9. Complex impact crater showing impactites locations………………………………49

3.1. Geologic map of Sierra Madera and stratigraphic column for rocks exposed at Sierra

Madera ……………………………………...………………………...……………62

3.2. Location of Mission Canyon Formation samples and General stratigraphic column of

the Mission Canyon Formation.…………………………………....………..………64

3.3. X-ray powder diffraction patterns of calcite and dolomite samples from the Sierra

Madera impact structure and Lower Mississippian Mission Canyon

Formation.……………………………………………………...………………...... 69

3.4. Single peak profiling using FWHM values for Sierra Madera and Mission Canyon

Formation samples……………….…………..………….……………………..……73

3.5. Rietveld refinement analysis for Sierra Madera and Mission Canyon Formation

samples.………………………...………………………………...………….………74

4.1. Geologic map of the Sierra Madera impact structure…………………………….....91

4.2. Shatter cone from the Gilliam Limestone at the Sierra Madera impact crater…...…93

4.3. Age comparison diagram……………………………………………………………96

4.4. Complex impact crater showing impactites locations………………………………98

5.1. Location of impact craters in Texas…………….....…………………………….…111

5.2. Location of the Sierra Madera structure near Fort Stockton, Texas…………....….112

5.3. Stratigraphic column of rocks that outcrop in the central uplift and crater rim of the

Sierra Madera impact structure.……………..…………………..….……..…….…114

5.4. Scanning electron micrographs of shocked zircon surface textures.………………121

5.5. World’s largest roadrunner, Paisano Pete, Fort Stockton, TX ………………..…...123

5.6. Geologic map of the Sierra Madera impact structure and location map for the field

trip………………………………….…...………………………………………….125

5.7. Sierra Madera impact crater...………………………………………………….…..126

5.8. Site locations for Stop 2……...…………………………………………………….126

5.9. Overlapping shatter cones from the Gilliam Formation and a shatter cone from the

Upper Hess Formation.………………………. …………..……...………………..128

5.10. Weathered out block of Gilliam Limestone with a fractured appearance.…...... 129

5.11. Solution collapse breccia in the Tessey Limestone …………………...…………129

5.12. Monolithic breccia in the Gilliam Limestone.…...………………………...……..131

5.13. Slickensides in a sandstone unit of the Gilliam Limestone.……………………...131

5.14. Polymict breccia outcrop …………………………………….……………...…...133

5.15. Site location for Stop 3………………………………………….………………..135

5.16. Gently dipping Lower Cretaceous rocks exposed on the north crater rim of Sierra

Madera …...……………...………….…………………………………...……….135

LIST OF TABLES

Page

2.1. Stratigraphic units exposed within and surrounding Sierra Madera impact crater….28

2.2. Comparison of Breccia Types at Sierra Madera………………...…………….…….39

3.1. X-ray Powder Diffraction Samples……………………………………………...….65

3.2. Electron microprobe analytical data…………………………………………….…..68

4.1. Fission track data……….……………………………………………………….…..95

xii CHAPTER ONE

INTRODUCTION

Abstract

This first chapter serves as an introduction to the Sierra Madera impact structure and the topics presented in this dissertation. The geographic location of the field area is followed by a brief discussion of the previous geologic research. A geologic history of the region covers the major events that have shaped the Sierra Madera region from Precambrian to Recent. The stratigraphy section provides a detailed account of the type and depositional history of rocks exposed within the Sierra Madera impact structure. The structure section focuses on basic impact crater formation processes and the structural components found within the Sierra Madera impact crater.

1 Introduction

The Sierra Madera impact crater is a 12.9 km diameter, well-eroded, well-exposed complex impact crater located ~30 kilometers south of Fort Stockton, Texas (Figure 1.1). Of the

>200 impact craters positively identified on the Earth, it is one of ~60 craters hosted in carbonate-rich sedimentary rocks (Grieve and Robertson 1979; Grieve and Pesonen, 1996) and one of four impact structures located in Texas. In addition to carbonates, Sierra Madera also includes interbedded siliciclastic target rocks (Figure 1.2) making it an ideal place to comparatively study quartz-carbonate deformation produced during an impact event. Similarly, the size of the crater is between other craters hosted in carbonate rocks, namely, Haughton (24 km), Ries (24 km), Steinheim (3.8 km), and Barringer (Meteor) Crater (1.2 km). Therefore, study of the Sierra Madera crater provides important information for understanding the continuum of carbonate deformation produced by meteorite craters.

The objectives of this research were to document shock deformation and determine the timing of the impact event. Modern research techniques were used to identify pressure and temperature conditions during the event as well as attempt to assign a radiometric age to the structure.

Previous Research at Sierra Madera

Sierra Madera was first mapped by King (1930) who noted a circular zone of intensely deformed, uplifted Permian strata near the northern end of the Glass Mountains. Boon and

Albritton in 1937 suggested a meteorite impact origin due to the damped wave form of the structure and lack of volcanic material. However, an impact origin was not firmly established until 1959 with the discovery of shatter cones at the site making Sierra Madera the fifth structure

Figure 1.1. Location map of the Sierra Madera impact crater. Also shown are the major structural features of the region, including the Delaware Basin, and extent of the Capitan

Reef Formation exposed in mountain ranges surrounding the basin. The solid line represents the mapped extent of the reef. The dashed line represents the approximate location of the reef. Modified from Hill (1996).

Figure 1.2. Geologic map of Sierra Madera. Modified from Wilshire et al. (1972). Dashed line

represents location for cross section in Figure 1.5.

4 in the world where shatter cones were found (Dietz, 1960; Eggleton and Shoemaker, 1961). A gravity-magnetic study revealed negative gravity and positive magnetic anomalies (Van Lopik and Geyer, 1963). While indicating the absence of a buried meteorite the results still support the impact origin of Sierra Madera since negative gravity anomalies are typical in impact craters due to the intensely shattered rock produced during cratering events (Van Lopik and Geyer, 1963).

The structure was first considered to be the 3.2 km diameter well-eroded remnant of a simple crater (Boon and Albritton, 1937). Polymict breccia outcrops were thought to occur as a single lens-shaped mass within the center of the central uplift and the structure was tentatively compared to Meteor Crater in Arizona (Eggleton and Shoemaker, 1961). The theory was revised with later fieldwork that concluded Sierra Madera was the eroded remnant of a complex crater, currently containing a ~6 km diameter central uplift surrounded by a ~12 km diameter rim and ring depression. These studies documented shock deformational features such as impact generated breccias, shatter cones, planar deformation features (PDFs) in quartz, grain fracturing, and deformed quartz and carbonate minerals and also produced a detailed geologic map of the structure (Howard and Offield, 1968; Wilshire and Howard, 1968; Wilshire et al., 1971; Howard et al., 1972, Wilshire et al., 1972).

One hypothesis postulates the impact event occurred when the region was covered by 1-2 miles of water (Kelly, 1966). A water-impact event would deform rocks immediately below the point of impact while leaving the rim moderately undisturbed thus explaining the highly deformed rocks of the central uplift and relatively flat-lying rocks of the crater rim.

The Sierra Madera structure was proposed along with the Manson structure in Iowa as a possible crater for the K-T boundary event before Chicxulub was discovered (French, 1984).

However, Sierra Madera was considered unlikely due to its relatively small size and lack of

5 characteristic K/T boundary clastic material (French, 1984). Recently a modeling study was

conducted to reproduce the current crater diameter and associated deformational features (Goldin

et al., 2006). Not all of the features of the crater were reproducible with the diameters chosen,

the most prominent being the lack of overturned beds on the crater rim, however, a water-impact

model was not tested.

Currently, Sierra Madera is located within an actively producing gas field. Two

exploration wells were drilled in the central uplift in 1921, the Stanley Thomas Elsinore No. 1

(total depth 2,238 m) and Phillips’ Elsinore No. 1 (total depth 3,687 m; Wilshire et al., 1972;

Constant, 1978). The Stanley Thomas Elsinore No. 1 well blew out and was abandoned in 1940.

Drilling in other locations on the crater resumed in 1977 and continues to the present (Constant,

1978; Donofrio, 1998).

Geologic History of Area

The geologic history of the Delaware Basin region is divided into eight phases (Hill,

1996). During the Precambrian (1300-850 Ma) the Delaware Basin developed as an

intracratonic basin. Granitic basement rock was broken by high-angle faulting during the

Grenville orogeny. These faults became areas of regional weakness and were reactivated during

later deformational events. The Passive Margin Phase (850 – 310 Ma) represents a time when the region experienced little tectonic activity. Deposition occurred in a slowly subsiding area known as the Tobosa Basin and was characterized by epicontinental seas with periods of subaerial exposure and karstification (Hill, 1996). During the Late Devonian the Tobosa Basin was uplifted, tilted, and eroded. Deposition continued until the Collision Phase (310-265 Ma).

At this time, Precambrian zones of weakness were reactivated during the Marathon-Ouachita

6 orogeny and the Tobosa Basin was divided into the Midland and Delaware Basins separated by the Central Basin Platform (Hill, 1996). The Permian Basin Phase (265-230 Ma) is characterized by basin subsidence and reef growth. During the Permian the Delaware Basin rapidly subsided and filled with carbonate material and clastics shed off highland areas including the Central

Basin Platform. By the middle of the Guadalupian the Midland Basin was isolated from normal marine circulation and filled in while extensive reefs developed at the shelf margin in the

Delaware Basin as exemplified by the Capitan Formation in Guadalupe Mountains National Park

(Hill, 1996). During the Early Ochoan the Delaware Basin connection to the open ocean was closed resulting in a decline of reef building and the end of normal marine carbonate deposition.

Ochoan age deposits are characterized by red beds, evaporites, and fossil poor limestone. The

Stable Platform Phase (230-80 Ma) was a period of tectonic stability. From the Triassic to Early

Cretaceous the area remained exposed with deposition in terrestrial environments and subsequent erosion. During the Early Cretaceous an epicontinental sea transgressed the area depositing shallow marine sediments for the last time in the region. The sea remained until the end of the

Cretaceous at which time the area was affected by the Laramide orogeny. The Laramide Phase

(80-40 Ma) resulted in uplift, tilting and arching in the Delaware Basin (Hill, 1996). Volcanic

Phase (40-30 Ma) marked a transition from Laramide compression to Basin and Range extension and crustal thinning. Intrusive and extrusive volcanism occurred in the Glass Mountains and

Davis-Barilla Mountains and throughout Mexico and western New Mexico. During the Basin and Range Phase (30-0 Ma) north-northwest trending high-angle normal block faulting resulted in the uplift of the Guadalupe, Apache, Delaware and Glass Mountains (Hill, 1996) and created the present landscape we see today.

7 Stratigraphy

The Delaware Basin of west Texas and southern New Mexico was a major marine basin throughout the Permian (Hill, 1996). The rocks at Sierra Madera and other parts of the basin record a gradual filling in of the basin capped by widespread evaporitic conditions by the end of the Permian. The Triassic and Jurassic were marked by uplift and erosion and rocks of these ages outcrop discontinuously. Triassic and Jurassic rocks have not been identified at Sierra

Madera. The Cretaceous is marked by a transgression represented by fluvial to shallow water marine deposits.

Below is a description and stratigraphic column (Figure 1.3) of rocks outcropping within the central uplift and along the crater rim of the Sierra Madera impact structure. Descriptions are compiled from Wilshire et al. (1972), Hill (1996), and the author.

Hess Formation – The Permian (Leonardian) Hess Formation is a 730 - 850 meters thick, fine- grained, medium to thick bedded limestone and dolostone with interfingering sandstone, siltstone, and shale. Sediments were deposited in a shallow water lagoon. Common fossils include fusulinids, brachiopods, and bryozoa. There is a gradual up section decrease in clastic material and increase in dolomite in this unit. At Sierra Madera 365 - 430 meters of Hess

Formation outcrop at the surface and the unit gradually increases in thickness to the north

(Wilshire et al., 1972).

Cathedral Mountain Formation – The Permian (Leonardian) Cathedral Mountain Formation is a

0-24 meter thick conglomerate containing chert and quartz pebbles in a dolomite matrix. Beds of cross-bedded coarse, quartz sandstone occur locally within the Cathedral Mountain Formation

Figure 1.3. Stratigraphic column of rocks exposed at Sierra Madera.

9 (Wilshire et al., 1972). This unit grades up conformably into the lower part of the Word

Formation. The depositional environment for the Cathedral Mountain Formation is debated as

being a shallow water deltaic complex (Wardlaw et al., 1990) or basinal where clastics are

derived from debris flows or turbidity currents (Ross, 1986).

Word Formation – The Permian (Early to Middle Guadalupian) Word Formation is divided into

two members: the lower Limestone Member and the upper Sandstone Member. The lowermost

member of the Word Formation is an unnamed limestone unit 0-76 meters thick (Wilshire et al.,

1972). It contains thin to thick bedded limestone, with crinoid and fusulinind fossils, and some

interbedded finely crystalline dolomite. The upper Sandstone Member of the Word Formation

(0-10 meters thick) consists of interbedded coarse grained sandstone with silicified crinoid stems

and silty to fine-grained sandstone with calcite cement. Locally this unit grades laterally to a

sandy dolomite. The depositional setting for the Word Formation is controversial and is

proposed to be basinal with turbidity currents moving clastics off the shelf (Ross, 1986) or a

shallow water lagoon (Rohr et al., 1982; Wardlaw et al., 1990).

Vidrio Formation - The Permian (Middle Guadalupian) Vidrio Formation (60-230 meters thick) consists of thick to massively bedded dark dolomite with sparse fossils. Dolomitization has destroyed the original fabric of the rock and many fossils. Chert nodules are common. The

Vidrio represents initial reef development in the Delaware Basin. Reefs of the Vidrio were early

precursors to major reef building in the Late Guadalupian (Hill, 1996).

10 Gilliam Limestone – The Permian (Late Guadalupian) Gilliam Limestone is a 213-290 meters

thick, dolomite and calcite cemented fine-grained sandstone deposited in a lagoonal and back

reef setting (Hill, 1996). Sandstone layers in this unit pinch out laterally and are discontinuous.

This unit grades conformably into the Vidrio Limestone Member of the Word Formation.

Tessey Limestone – The Permian (Ochoan) Tessey Limestone is a 0-120 meters thick, gray brown dolomite with sparse fossils. In some locations the unit is identified by areas of solution

collapse breccia composed of laminated small (cm-size) to large (several meter-size) fragments of Tessey Limestone cemented mosaic-like with calcite and/or dolomite cement. Deposition

occurred in a hypersaline basin that underwent periods of subtidal to supratidal deposition and subaerial exposure (Haneef and Wardlaw, 2000). Meteoric dissolution created karst features and collapse breccias (Hill, 1996). These breccias occur mostly on the east side of the central uplift

(Wilshire et al., 1972).

Fredericksburg and Lower Washita Groups – The Fredericksburg Group rocks are Early

Cretaceous (Middle Comanchean) in age. They represent the youngest rocks exposed at Sierra

Madera on the crater rim. The Maxon Sandstone occurs at the base of the Fredericksburg Group

(Hill, 1996). It is a 15-30 meter thick red-brown, fine- to coarse-grained sandstone cemented with silica cement. The Maxon Sandstone can be differentiated from other sandstone units at

Sierra Madera because of its silica cement and a thin quartz pebble layer at the base of the unit

(Wilshire et al., 1972). At Sierra Madera the Maxon Sandstone unconformably overlies the

Tessey Limestone. The Edwards Limestone (40-60 meters thick) is a platy light gray and brown limestone. The Kiamichi Formation (~15 meters thick) is a thin bedded light gray limestone.

11 The Cretaceous (Late Comanchean) Washita Group consists of platy to massive, medium-

grained, shaly limestone. The Fredericksburg and Washita Groups represent deposition in a

fluvial/deltaic to shallow marine setting (Hill, 1996) and are not differentiated at Sierra Madera

due to poor exposures and lack of stratigraphic control.

Structure

Complex Crater Formation

The formation of a complex impact crater is divided into three separate stages (Figure

1.4). During the Contact and Compression Stage the projectile contacts the target surface and a

shock wave (compressional) begins to radiate spherically outward from a central point (French,

1998). As the shock wave travels away from the contact point, it is followed by a rarefaction

(tensional) wave which unloads intense shock pressure by fracturing and brecciating the

underlying target rock. Simultaneously, a rarefaction wave is reflected back into the projectile

which will cause it to break up and vaporize (Melosh, 1989).

The Excavation Stage begins when material is ejected from the expanding crater

(Melosh, 1989). The shock wave continues to move material outward and down. Material

ejected from the crater blankets the surface of the surrounding target rock. This stage ends when

target material ceases to be ejected from the crater resulting in a maximum-sized open bowl shaped depression, called the transient cavity (French, 1998; Melosh, 1989).

The Modification Stage occurs when the target material in the center of the transient

cavity rises to form a central uplift, similar to the rebounded central peak of a depression formed

by a rain drop hitting a liquid surface (Melosh, 1989). Additionally, the transient crater rim

collapses in towards the central uplift under the influence of gravity forming a series of ring

Figure 1.4. Stages of crater development for a complex crater. Modified from French (1998).

13 grabens (French, 1998).

Sierra Madera Structure

Sierra Madera is divided into three structurally different regions; the uplifted crater rim encloses the ring depression and peaks of the central uplift (Figure 1.2). In general the crater is a bowl-shaped structure with deformation gradually lessening ~2500 meters beneath the central uplift (Figure 1.5; Wilshire, 1972).

Rim – The crater rim at Sierra Madera is a ~0.8 km wide well-eroded feature. Strata exposed on the rim include the Permian Gilliam and Tessey Formations and the Cretaceous

Maxon Sandstone and lower most Fredericksburg Formation. These units are generally flat lying or dip between 5-20° however they can increase to 80° near faults (Wilshire et al., 1972). The crater rim is truncated by normal faults with the down-thrown side towards the central uplift.

Mild folding of the lower Cretaceous Fredericksburg Group occurs on the northwest and eastern sides of the rim. Fold axes parallel the crater rim boundaries.

Ring depression – The ring depression is a ~1.5 km wide feature encircling the central uplift. Oil and gas drilling records indicate lower Cretaceous rocks are covered by Quaternary alluvium (Wilshire et al., 1972). No breccia or melt rocks have been reported in the drilling data however rocks are intensely deformed with dips near vertical (Wilshire et al., 1972). On the eastern side of the depression, lower Cretaceous rocks are exposed gradually merging with strata from the central uplift. Strata generally dip 5-20° and are deformed by small discontinuous folds with axes parallel to the boundaries of the depression. Ring normal faults with the down-thrown side towards the center of the crater separate the depression from the crater rim (Wilshire et al.,

1972).

Figure 1.5. Cross section of Sierra Madera. Modified from Wilshire et al. (1972).

15 Central Uplift – The central uplift of Sierra Madera is a ~7 km wide, circular zone of

intensely folded and faulted peaks rising ~250 meters above the ring depression. Units exposed

within the central uplift include the Permian Hess Formation, Cathedral Mountain Formation,

Word Formation, Gilliam Formation, Tessey Limestone and the Cretaceous Maxon Sandstone

and lower Fredericksburg and Washita Groups. Stratigraphic uplift of the centrally located Hess

Formation is ~1200 meters (Wilshire et al., 1972). Strata dip from 15° to vertical and overturned with no apparent relationship between angle of dip and proximity to the center of the structure.

Most faults within the central uplift are high-angle normal or reverse however low-angle thrusts and at least one strike-slip fault have been documented (Wilshire et al., 1972).

Chapter Summaries

Chapter Two: Deformational features and impact generated breccia from the Sierra Madera impact structure, west Texas, will be submitted to GSA Bulletin.

In this study, impact deformed rocks from the Sierra Madera impact crater were re- examined applying modern research techniques and the current knowledge of cratering products to better understand deformational products associated with a smaller to medium-sized diameter carbonate crater and to provide data in a critical intermediate size/energy range for which information currently is lacking. This study presents new descriptions of impact-generated breccia, the possible identification of zircon deformation, and descriptions of a shatter cone weathering texture in host limestone. Additionally, information regarding the temperature and pressure conditions during the impact event is presented with a discussion relating the above deformational features to the modern crater erosional level for the Sierra Madera impact structure.

16 Chapter Three: X-ray powder diffraction Rietveld analysis as a tool for the identification of impact deformed rocks, submitted to Meteoritics and Planetary Sciences.

In this study XRD patterns of calcite and dolomite from the Sierra Madera impact crater were compared to those of unshocked carbonate samples from the Mississippian Mission Canyon

Formation of the northern Rockies, United States and to unshocked/undeformed mineral standards. The objective was to determine if XRD patterns of the shocked carbonates can be distinguished from XRD patterns of carbonates that were deformed solely by terrestrial processes

(e.g. burial or tectonism). This study has implications for using XRD pattern analysis as a tool for the identification of shock metamorphic processes where conventional impact characteristics

(i.e., shatter cones and PDFs in minerals) are lacking or poorly developed.

Chapter Four: Fission track age for the Sierra Madera Impact Structure, may be submitted to

Geology.

The object of this study was to determine the age of the Sierra Madera impact event using radiometric age-dating techniques. The relative age of the Sierra Madera impact crater is known from stratigraphic and structural relationships; however, an absolute date for the impact is not known. Deformational features (i.e. shattercones and PDFs in quartz grains) within Sierra

Madera indicate post-impact temperatures ranged from 100°C to at least 150°C and the structure was not deformed by a thermal event subsequent to its formation. Therefore, low-temperature

(<125°C) dating techniques are ideal for studying the timing of this impact event.

17 Chapter Five: Field Guide to the Sierra Madera Impact Structure.

This document was written as a guide for participating members of the Impact in the

Field Studies Group annual field trip in 2006. The trip was held in conjunction with the 37th

Annual Lunar and Planetary Science Conference in Houston, Texas.

18 References

Boon, J. D., and Albritton, C. C., Jr., 1937, Meteorite scars in ancient rocks: Field and

Laboratory, v. 5, no. 2, p. 53-64.

Constant, F., 1978, The Elsinore; Pikes Peak, East; and Sierra Madera fields of Pecos County,

Texas, in Gilbertson, R., ed., Energy quest for the Southwest: West Texas Geological

Society, v. 78-69 SWS, p. 61-70.

Dietz, R. S., 1960, Meteorite impact suggested by shatter cones in rock: Science, v. 131, no.

3416, p. 1781-1784.

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geologic and hydrologic sciences: U.S. Geol. Survey Prof. Paper 424-D, p. D151-D153.

French, B. M., 1998, Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in

Terrestrial Meteorite Impact Structures: LPI Contributions No. 954, Lunar and Planetary

Institute, Houston, 120 p.

French, B. M., 1984, Impact event at the Cretaceous-Tertiary boundary: A possible site: Science,

v. 226, no. 4672, p. 353.

19 Goldin, T. J., Wünnermann, K., Melosh, H. J., and Collins, G. S., 2006, Hydrocode modeling of

the Sierra Madera impact structure: Meteoritics and Planetary Science, v. 41, no. 12,

p.1947-1958.

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no. 3850, p. 261-265.

20 Howard, K. A., Offield, T. W., and Wilshire, H. G., 1972, Structure of Sierra Madera, Texas, as

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2795-2808.

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245 p.

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21 Wardlaw, B. R., Davis, R. A., Rohr, D. M., and Grant, R. E., 1990, Leonardian-Wordian

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Bull., 1881, pp. A1-14.

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Madera Crytoexplosion Structure, Pecos County, Texas: U.S. Geol. Survey Prof. Paper

599-H, p. 1-49.

22 CHAPTER TWO

DEFORMATIONAL FEATURES AND IMPACT GENERATED BRECCIA FROM THE

SIERRA MADERA IMPACT STRUCTURE, WEST TEXAS

Abstract

Deformational features from the Sierra Madera impact structure, west Texas were studied

to determine shock temperature and shock pressure values during the formation of this crater.

Sandstone, limestone and polymict impact generated breccia samples occur within the central

uplift. Sandstone samples here contain several deformational features including toasted quartz grains and planar deformation features (PDFs). Zircon mineral grains separated from the sandstone samples may show evidence of shock deformation. Both sandstone and carbonate within the central uplift contain shatter cones whereas polymict impact generated breccia samples contain shatter cone fragments. Most high temperature features associated with less eroded structures were lacking; however, some small (millimeter-size) possible devitrified glass clasts within polymict breccias were found at several locations. Lower pressure and temperature features such as shatter cones, toasted quartz, a jigsaw texture between quartz grains, and quartz grains with multiple PDFs pressure conditions during the impact were 5.5 to 20 GPa with a post- shock temperature range of 350 to >1000° C. Deformational conditions during the impact event were almost certainly higher; however, erosion presumably removed most of the overlying evidence subsequent to crater formation.

23 Introduction

A significant amount of information regarding shock deformation in meteorite impacted target rocks and impact generated deposits has been collected from impact cratering studies over the last 40 to 50 years. We know, for example, craters can contain a variety of shocked and unshocked clasts derived from underlying basement rocks and surface cover (Metzler et al.,

1988; Stöffler et al., 1988). These clasts may be included in multiple, complex breccia units

associated with impact events. For instance, the Devonian Alamo breccia contains nine different types of breccias formed during, and immediately subsequent to, the impact event (Warme,

2004). Mechanisms forming these breccias include: the impact event, debris falling into the excavated crater, repeated tsunamis, and the failure of the pre-existing carbonate platform margin

due to the meteorite impact (Warme and Kuehner, 1998; Warme, 2004).

Additionally, recent studies have identified planar deformational features (PDFs) in

zircon mineral grains (Bohor et al., 1993; Glass and Liu, 2001). PDFs are closed, narrow,

parallel features of highly amorphous material generally oriented parallel to crystallographic

planes within the host mineral (French, 1998). They commonly occur in quartz but also occur in

other silicate minerals such as zircon. In addition to PDFs, granular surface textures in zircon

mineral grains were identified from larger (>~90 km diameter) impact structures (i.e. Sudbury,

Chicxulub, Manicouagan, Chesapeake Bay; Bohor et al., 1993; Glass and Liu, 2001). However,

these features are not yet documented in smaller diameter craters. Finally, the study of shatter

cone fracturing and orientations within impact craters indicates the formation of these features is

more complicated than once thought (Wieland et al., 2006).

Many impact cratering studies have concentrated on understanding deformation in

siliciclastic minerals because of the relative abundance of quartz at the Earth’s surface (French,

24 1998). Yet, not all impact craters contain quartz-bearing rocks. Therefore, pressure and

temperature deformation results based on quartz are not applicable to all impact structures since

~30% of all terrestrial impact structures formed primarily in carbonate rocks (Grieve and

Robertson 1979; Grieve and Pesonen, 1996). Consequently, in structures containing little

siliciclastic material it is often more difficult to determine the deformational history of an impact.

However, impact craters containing both siliciclastic and carbonate rocks can provide a bridge to

understand deformation to both rock types. Well-studied carbonate and mixed siliciclastic and

carbonate craters include Meteor Crater (diameter < 2 km) in Arizona (Kieffer et al., 1976; Burt

et al., 2005) the Steinheim and Ries structures in Germany (diameters of 3.8 km and 24 km,

respectively) and the Haughton structure in Canada (diameter of 24 km; Graup, 1999; Skála and

Jakeš, 1999; Osinski and Spray, 2001; Osinski, 2007).

Studies of carbonate-rich craters have also focused on cratering products. For example, carbonate rocks in impact craters can contain high-temperature melts (Graup, 1999; Osinski and

Spray, 2001; Tuchscherer et al., 2004). For instance, the Ries and Haughton craters, both containing carbonate sedimentary rocks as part of the target rock succession and having similar diameters of ~24 km, demonstrate textural evidence of carbonate-silicate liquid immiscibility

(Graup, 1999; Osinski and Spray, 2001; Tuchscherer et al., 2004).

The Sierra Madera impact structure is a well-exposed complex crater with a diameter of

12.9 km, last studied in the late 1960s (Wilshire et al., 1972). Target rocks at Sierra Madera are predominantly carbonate with a few interbedded siliciclastic units. Impact deformed rocks from the Sierra Madera impact crater were re-examined to better understand deformational products associated with a smaller to medium-sized diameter carbonate crater by providing data in a critical intermediate size/energy range for which information currently is lacking. We present

25 new descriptions of impact generated breccia, the possible identification of zircon deformation,

and descriptions of shatter cones. Using these features we determined the temperature and

pressure conditions during the impact event. Additionally, we relate the occurrence and

distribution of the deformational features to the current crater erosional level for this impact

structure.

Geologic Setting

The Sierra Madera impact structure near Fort Stockton, Texas, is a well-exposed, eroded

remnant of a complex impact crater (Figure 2.1; Eggleton and Shoemaker, 1961; Wilshire and

Howard, 1968; Howard et al., 1972; Wilshire et al., 1972). The central uplift at Sierra Madera is

composed of steeply dipping, folded, and faulted Upper Paleozoic to Cretaceous carbonate and siliciclastic rocks. Upper Permian carbonate and siliciclastic strata include the Hess, Cathedral

Mountain, Word and Vidrio formations, and the Gilliam and Tessey Limestones (Table 2.1).

Lower Cretaceous rocks of the Fredericksburg and Lower Washita Groups are exposed along the eastern margin of the central uplift. The Permian Hess Formation, the oldest stratigraphic unit exposed in the central uplift, was displaced vertically at least 790 meters during the excavation and modification stage of crater development (Wilshire et al., 1972). Impact generated polymict and monolithic breccias occur within the central uplift of the crater. The crater rim at Sierra

Madera is composed of Lower Cretaceous carbonate and siliciclastic rocks of the Fredericksburg and Lower Washita Groups. On the southwest side of the rim, rocks from the Upper Permian

Gilliam and Tessey Limestones are exposed. Cretaceous strata occur ~30-60 meters above the alluvium-filled ring depression. Strata on the rim generally dip 0-5° radially, though at some locations dip can be as great as 30°. Post-impact erosion removed an estimated 610 meters of

Figure 2.1. A) Geologic map of Sierra Madera impact crater (modified from Wilshire et al.

1972). Dashed box in center indicates region shown in Figures 5 and 6. B) Stratigraphic column for rocks exposed at Sierra Madera.

27 Strat. Unit Description Alluvium 0-15 meters of unconsolidated boulder to silt size, angular to rounded clasts of dolomite, limestone Quaternary limestone, chert, and sandstone, locally well-cemented as caliche (Wilshire et al., 1972).

Fredericksburg The lower part of the Fredericksburg consists of the Maxon sandstone, a 15-30 meter thick red- and Lower brown, fine- to coarse-grained silica cemented sandstone. The Edwards Limestone (40-60 meters Washita thick) is a platy light gray and brown limestone. The Kiamichi Formation (~15 meters thick) is a Groups thin bedded light gray limestone. The lower Washita Group consists of platy to massive, medium- Lower grained, shaly limestone. The Fredericksburg and Washita Groups represent deposition in a Cretaceous fluvial/deltaic to shallow marine setting (Hill, 1996) and are not differentiated at Sierra Madera due to poor exposures and lack of stratigraphic control.

Tessey 0-120 meters thick, gray brown dolomite with sparse fossils. In some locations this unit contains Limestone areas of solution-collapse breccia composed of laminated small (cm-size) to large (several meter- Upper Permian size) fragments of limestone cemented mosaic-like with calcite and/or dolomite cement. This unit is unconformable with the underlying Gilliam Limestone. Deposition occurred in a hypersaline basin that underwent periods of subtidal to supratidal deposition and subaerial exposure (Haneef and Wardlaw, 2000). Meteoric dissolution created karst features and collapse breccias (Hill, 1996). These breccias occur on the east side of the central uplift (Wilshire et al., 1972).

Gilliam 213-290 meters thick, light-colored dolomite interbedded with calcite cemented fine-grained Limestone sandstone; sandstone layers pinch our laterally and are discontinuous. This unit is conformable Upper Permian with the Vidrio Formation. The Gilliam Limestone was deposited in a lagoonal and back reef setting (Hill, 1996).

Vidrio 60-230 meters thick massive to thick-bedded dark dolomite with sparse fossils. Rounded chert Formation nodules are common often and gray-colored with an orange weathering rind. The original rock Lower Permian fabric and fossils were of this unit are difficult to characterize and interpret thus the depositional environment at Sierra Madera is unknown. However, in other parts of the Delaware Basin the Vidrio Formation records initial reef development, though the reef facies is not seen at Sierra Madera (Hill, 1996).

Word 0-86 meter thick dolomitized limestone divided into two members. The lowermost member Formation is an unnamed limestone unit 0-76 meters thick (Wilshire et al., 1972). It contains thin to thick Lower Permian bedded limestone, with crinoid and fusulinind fossils, and some interbedded finely crystalline dolomite. The upper Sandstone Member (0-10 meters thick) consists of interbedded coarse grained sandstone with silicified crinoid stems and silty to fine-grained sandstone with calcite cement. Locally this unit grades laterally to a sandy dolomite. The depositional setting for the Word Formation is controversial being either basinal with turbidity currents moving clastics off the shelf (Ross, 1986) or a shallow water lagoon (Rohr et al., 1982; Wardlaw et al., 1990).

Cathedral 0-24 meter thick conglomerate containing chert and quartz pebbles in a dolomite matrix (Wilshire Mountain et al., 1972). Beds of cross-bedded coarse, quartz sandstone occur locally as lenses. This unit Formation grades conformably upward into the lower part of the Word Formation. The depositional Lower Permian environment was a shallow water deltaic complex (Wardlaw et al., 1990) or basinal where clastics are derived from debris flows or turbidity currents (Ross, 1986).

Hess 730 - 850 meters thick, fine-grained, medium- to thick-bedded limestone and dolostone inter- Formation bedded with sandstone, siltstone, and shale. Sediments were deposited in a shallow water lagoon Lower Permian Common fossils include fusulinids, brachiopods, and bryozoa. There is a gradual up section decrease in clastic material and increase in dolomite in this unit. At Sierra Madera 365-430 meters occur at the surface and the unit gradually increases in thickness to the north (Wilshire et al., 1972). Table 2.1. Stratigraphic units exposed within and surrounding Sierra Madera impact crater.

28 material on the crater rim eliminating the overturned strata characteristic on rims of younger

craters such as at Meteor Crater, AZ (Wilshire et al., 1972).

Methods

We mapped, sampled, and carefully described impact generated breccias from Sierra

Madera to determine their distribution and formation processes. Characteristics such as textural differences, clast composition, possible melt products or glasses, deformational features in minerals, and flow features within breccias are described. Additionally, deformational studies of the target rock include shatter cone outcrop characteristics and fracture patterns as well as deformation in quartz and other minerals. Thin section samples of shatter cones, target rocks and impact generated breccias from the central uplift of Sierra Madera were analyzed using a petrographic microscope. Selected thin sections were analyzed for evidence of melt features using plane light and scanning electron microscopy (SEM). Carbon coated, polished thin section

slides were analyzed using SEM with backscatter electron (BSE) imagery and energy dispersive

spectrometry (EDS) to identify silica-rich phases from carbonate material. The SEM used in this

study is housed at the Pacific Northwest National Laboratory in Richland, WA.

We also document possible shock-induced surface textures on zircons from Permian

sandstones within the central uplift of the Sierra Madera crater. Zircon grains separated from

sandstone located in the central uplift of the Sierra Madera impact structure were studied for

shock deformation features and to determine the age of shock deformation (e.g. with fission track

analyses). Sandstone samples were collected from the Maxon Sandstone, sandstone beds of the

Permian Gilliam Limestone and the sandstone member of the Permian Word Formation. Zircon

grains (<100 µm) were separated using a Gemini table, magnetic separator and heavy liquid

29 (methelyene iodide) after the mineral separation techniques of Donelick et al. (2005). Fractions

for light microscopy were mounted in epoxy, polished, and studied under high magnification

using both transmitted and reflected light. Zircon fractions for SEM analysis were etched with a

saturated solution of NaOH at 70°C for 1.5 hours to reveal shock features after Bohor et al.

(1993). These grains were mounted on SEM stubs and carbon-coated prior to SEM analysis.

Deformational features

Target Rocks

Quartz deformation in carbonate cemented sandstones at Sierra Madera

There are four sandstone units exposed the at Sierra Madera impact crater, from oldest to youngest they are; the Permian Cathedral Mountain Formation, a sandstone member of the

Permian Word Formation, sandstone within the Permian Gilliam Limestone and the Cretaceous

Maxon Sandstone. All units contain shock-induced deformation features within quartz grains

(Figure 2.2).

The Cathedral Mountain Formation is characterized by 0.1-0.2 mm quartz grains within a carbonate matrix. Typically, quartz grains are not in contact with one another. Deformation in these quartz grains include grain fracturing, grain partitioning and quartz grains with undulose extinction. Planar deformation features (PDFs) and decorated PDFs (PDFs with fluid inclusions along the PDF planes) occur in 5-10% of the quartz grains. Less than 5 % of quartz grains with

PDFs are “toasted” quartz having a grainy brown appearance due to an increase in fluid inclusions in some shock metamorphosed quartz grains (e.g. Short and Gold, 1996; Whitehead et al., 2002).

Figure 2.2. Photomicrographs of sandstone deformation from the Sierra Madera impact structure. A) Toasted quartz from the Word Formation. See arrows. Note darkened, blotchy appearance and PDFs in the grain (see lower half of grain). B) Symplectic texture between quartz grains from the sand-rich unit of the Gilliam Limestone. Note areas of high birefringence. C) Jigsaw texture between quartz grains from the Maxon Sandstone. D)

Decorated PDFs in quartz from the Maxon Sandstone. Photomicrograph a is plane polarized light. Photomicrographs b, c, and d are cross polarized light.

31 Similar to the Cathedral Mountain Formation, the sandstone member of the Word

Formation is a carbonate matrix supported rock with 0.1-1.6 mm size quartz grains. Grain

fracturing and suturing along long contacts is common as well as undulose extinction and grain

partitioning. PDFs occur in less than 10% of quartz grains. Between 5-10% of those grains

containing PDFs also have a toasted appearance (Figure 2.2A).

The sandstone of the Gilliam Limestone contains the most shock features within

sandstone at Sierra Madera. Silty sand units within Gilliam are characterized by 0.04-0.1 mm

size quartz grains and are grain supported. The grains are closely packed and carbonate material

is reduced to a microbreccia. This texture was termed “jigsaw” by Kieffer (1971) indicating an

interlocking of grains. Quartz grains are highly fractured, commonly have undulose extinction,

and contain grain partitioning. Symplectic regions between quartz grains were identified by high birefringence and opaque regions in plane light (Figure 2.2B). Decorated PDFs and PDFs occur in 15% of quartz grains; 5% of quartz grains with PDFs are toasted.

The Cretaceous Maxon Sandstone of the Lower Fredericksburg Group is characterized by

0.04-1.2 mm size inclusion-rich quartz grains with carbonate cement. The sand grains in this unit have a jigsaw texture of interlocking quartz grains similar to those in the Gilliam Limestone sandstone bed (Figure 2.2C). Quartz grains typically also have undulose extinction, fractures, and grain partitioning. 5% of quartz grains have decorated PDFs (Figure 2.2D) with >5 % of these being toasted.

Zircon deformation

Zircon mineral grains separated from sandstone with the central uplift of Sierra Madera also display evidence of shock damage (Figure 2.3). Parallel features on polished grain mounts

Figure 2.3. Possible surface deformation in zircon. A) Light microscopic image of planar deformation feature-like (PDF) lineations from a polished zircon embedded in epoxy. b and c) SEM images of zircon surface textures from Sierra Madera. B) Possible surface expression of planar deformation features. C) Granular surface texture with remnant patches of the original smooth surface.

33 of zircon grains were documented Permian sandstones from Sierra Madera using transmitted

light microscopy (Figure 2.3A; Huson et al., 2005). The lineations occur in the interior of the

zircon grains and were tentatively called planar elements since they were not measured and

compared to the crystallographic planes in zircon. These features were not abundant and occur

in less than 5% of the samples studied.

Zircon mineral surfaces also were studied using SEM to document shock-induced

deformation. Three types of surface textures were documented on rounded and euhedral zircon

grains; 1) unshocked, 2) PDF-like lineations, and 3) a granular texture. Most zircon mineral

surfaces were unshocked and showed no signs of shock deformation. Less than 5% of the zircon

surfaces studied showed either PDF-like lineations or a granular texture (Figure 2.3B, C). These surface features are similar to those reported for the K/T boundary ejecta layer and Chesapeake

Bay impact structure (Bohor et al., 1993; Glass and Liu, 2001).

Shatter cones

Shatter cones are unique impact produced fracture surfaces characterized by diverging striations from cone apex to base. They have two basic morphologies, a flat striated surface of cones that overlap like shingles and complete to partial 3-dimensional cones (Dietz, 1968;

French, 1998; Wieland et al., 2006;).

Shatter cones were discovered at Sierra Madera in 1959 (Dietz, 1960; Kelly, 1966) and

they occur in abundance within a 2 km radius from the center of the central uplift (Howard and

Offield, 1968; Wilshire et al, 1972). Shatter cones occur in all Permian carbonates but are most

common in the Gilliam Limestone, Vidrio, Word, and Hess Formations (Figure 2.4). They also

occur within the sandstone member of the Word Formation, sand units within the Gilliam

Figure 2.4. Shatter cone deformation. A) Shatter cone in sandstone from the Gilliam Limestone. Scale bar is in cm. B) Photograph and line-drawing of curved, irregular shatter cone from the Gilliam Limestone. Sample is 8.5 cm in width. C) Distinct weathering fracture pattern of Gilliam Limestone sample containing cm-scale shatter cones. Lens cap is 5 cm in width. D) Photograph and line-drawing of shattercones oriented in opposite directions within the Hess Formation. Arrows point in direction of cone apex. Sample is 4 cm in length. E)

Photomicrograph showing a toasted quartz grain (outlined) adjacent to non-deformed quartz grain (right) from the Word Formation. Note three directions of PDFs within grain (arrows) and over all darkening of grain.

Plane polarized light. F) Photomicrographs of diaplectic glass in quartz (arrows) from the Word Formation.

Left panel viewed under plane polarized light. Right panel viewed under cross polarized light. Note inclusions in grain when in plane polarized light. Additional quartz grains with inclusions (non-glass) are located on the right side of the frame.

35 Limestone and are weakly developed in the Cathedral Mountain Formation (Figure 2.4A). Cone fragments are also reported at a depth of ~3657 m beneath the central uplift (Howard and

Offield, 1968; Wilshire et al., 1972). Cone sizes range from 1 cm to ~ 45 cm in height and 0.5 cm to ~20 cm in diameter (Wilshire et al., 1972). Both cone morphologies, flat surfaces and 3- dimensional cones, occur at Sierra Madera (Wilshire et al., 1972) and while straight striations and typical shatter cone morphologies are most common at Sierra Madera, curved striations and irregular cones and also present (Figure 2.4B). Shatter cones are not uniformly developed in all units of the central uplift and may occur at one location but not occur in the same formation at a different location.

Small centimeter-sized shatter cones within the Gilliam Limestone have a distinct weathering fracture pattern (Figure 2.4C). When broken, blocks of this unit contain many small complete shatter cones. The fractured appearance results from weathering along shatter cone planes although the exact relationship is not well understood.

Reorientation of beds containing shatter cones to their original position at Sierra Madera indicates shatter cones generally point towards the center of this impact crater. However; it is not uncommon for cone apices to be oriented in directions other than toward the crater center

(Figure 2.5). Additionally, some shatter cones are oriented in different, sometimes opposite directions within the same sample (Figure 2.4D).

Thin sections of shatter cones are composed of fine-grained carbonate and silt to sand sized quartz grains. Quartz deformation from shatter cones samples include PDFs, grain fracturing, and toasting (Figure 2.4E). Additionally, diaplectic quartz glass may also occur in quartz-rich shatter cones (Figure 2.4F). Diaplectic glass forms when a shock wave passes through a mineral grain and shatters the bonds between the atoms. Deformational features such

Figure 2.5. Geology map of the central uplift of the Sierra Madera impact crater. Numbers next to arrows indicate the cone axis orientation of shatter cones in beds restored to horizontal position. Includes data from Wilshire et al. (1972) and this study.

37 as twinning or brecciation are not obvious within the carbonate material.

Carbonate Rocks

Deformation in carbonate-rich rocks is not apparent in all rocks found within the central uplift of Sierra Madera. In thin section, most carbonate rocks are fine-grained and contain deformed fossil and microfossil fragments. Solution features such as stylolites are common in the Word Formation. All carbonate formations at Sierra Madera contain shatter cones.

Deformation in carbonate rocks was studied using X-ray diffraction (XRD; see Chapter 3).

Shocked carbonate-rich rocks from the central uplift were compared with unshocked rocks from outside the crater using XRD peak patterns and Rietveld refinement of full-width at half- maximum (FWHM) peak values. It was possible to distinguish impact shocked rocks from comparable unshocked rocks since shocked samples have broader peak patterns and greater

Rietveld FWHM values for higher 2θ values (see Chapter 3).

Impact Generated Breccia

Four breccias units occur within the Sierra Madera impact structure; impact generated monomict and polymict breccias, an evaporite collapse breccia within the Tessey Limestone, and modern day caliche (Table 2.2). Impact generated breccias were identified at the Sierra Madera impact structure (Figure 2.6) by Wilshire et al. (1972). The first, called a “monolithic breccia”, is characterized by an in situ shattering of rock of a single lithology. The second breccia, termed

“mixed breccia”, contains clasts of two or more lithologies in a fine-grained carbonate matrix.

This study uses the terms “monolithic impact breccia” and “polymict impact breccia” to refer to

Wilshire et al’s (1972) breccia types after the terminology of Stöffler and Grieve (1994).

regionally

crust 3 fracturing with fracturing 1999).

desert and arid regions. regions. arid desert and slope wash. Occurs in Occurs wash. slope outside of crater of crater outside Madera. Madera. (Hill, in situ to Sierra clasts. Local little orrotationno of

cator Interpretation Interpretation cator melt Madera. Sierra Local cones, cones, of breccia composed possible possible from types rock all material Madera. to Sierra

Occurs

east side of crater within solution collapse cavities. streams streams

s- r Mostly None Cementing of clasts and and wash, Slope None CaCO cros

Shape of Outcrops Indi Outcrops of Shape In situ

Outcrop Distribution Shock Shock Distribution Outcrop

central Clast Clast uplift uplift in central cutting uplift uplift (dikes)

at Sierra Sierra Clast

Range Range

Size

Millimeter Every rock Rounded 3 as drusy drusy

Dolomite Millimeter Tessey Angular Irregula

and angular carbonate of angular, regions meter dolomite dolomite calcite Mosaic-like cementing matrix and to Limestone lithologies matrix clasts of differing layered cement to centimeter exposed type to angular irregular ephemeral cementing material in Intensely Fine- Millimeter Very or two Rounded Irregular Random grained to meter None localized Shock wave induced and , tabular

laminated with laminated Caliche Rounded CaCO Collapse cement clasts

cemented Madera compositions the surface cemented together lithology same lithology Type Description Cement of Clasts Types Shape Shape Types Clasts of Cement Breccia Matrix/ Description Type Monomict single. material, material, clasts shattered single. in grained to clasts of a Polymict localized and Clasts of matrix, centimeter Fine- Millimeter Very angular Rounded Irregular, Random Shatter Shock wave induced

Table 2.2. Comparison of Breccia Types at Sierra Madera.

Figure 2.6. Geologic map of the central uplift of Sierra Madera showing location of breccia types. Modified from Wilshire et al. (1972).

40 Evaporite collapse breccia in the Tessey Limestone is characterized by mosaic-like

cementing of angular, laminated dolomite clasts. Deposition occurred in a hypersaline basin that

underwent periods of subtidal to supratidal deposition and subaerial exposure (Haneef and

Wardlaw, 2000). Meteoric dissolution created karst features and collapse breccias (Hill, 1996).

The evaporite collapse breccia in the Tessey Limestone is distinguishable from monomict impact

breccia by the presence of detrital quartz within the matrix and widespread occurrence outside of

the crater (Wilshire et al., 1972; Hill, 1996).

Caliche, common in arid regions of the southwest United States, is a carbonate crust that

cements material in slope wash and ephemeral stream channels. At Sierra Madera, it can be

easily confused with polymict impact generated breccia because both breccia types contain clasts

of several compositions. However in thin section, the cement in caliche is layered, reflecting

numerous, slow cementation events within the matrix and surrounding clasts over time whereas

the cement/matrix in polymict impact breccia is massive.

Monomict

Monomict impact generated breccias occur in all Permian carbonate formations and the

Lower Cretaceous Maxon Sandstone within the central uplift of Sierra Madera. Irregular zones

of brecciation are randomly distributed throughout the central uplift (Figure 2.6). At some

locations breccias show little clast rotation and appear as fractured bedrock. In other locations,

the breccia contains a large amount of matrix material producing rough and irregular outcrops

(Figure 2.7A and B). Monomict impact generated breccia thin sections are characterized by

intensely shattered clasts within a dark fine-grained matrix (Figure 2.7C-F). Shattered clasts are

Figure 2.7. Monomict impact generated breccia from Sierra Madera. Monomict impact breccia is characterized by fracturing and internal shattering of clasts with fine-grained material between larger coherent clasts. A) Monomict breccia with little clast rotation and small amounts of matrix between clasts. Scale bar is in centimeters. B) Monomict breccia with a large amount of matrix between clasts. Note irregular rough appearance of outcrop. Field book 15 cms long. C-F) Photomicrographs of monomict impact breccia. Samples include the Vidrio Formation (C), Word Formation (D and E), and Tessey Limestone (F). Samples c and e show evidence of flow in the fine-grained material. All photomicrographs imaged under plane polarized light.

42 either broken in place with matrix-filled cracks between well-defined fragments or clasts show some movement with less distinct edges and the surrounding matrix contains internal flow foliation (Figure 2.7C and E). The dark fine-grained material within monomict impact breccia was previously identified as mylonite (Wilshire et al., 1972).

Polymict

Impact generated polymict breccia occurs in all formations in the central uplift of Sierra

Madera (Figure 2.8A). Outcrops may or may not be associated with faults and occur parallel or perpendicular to bedding (Wilshire et al., 1972). The three-dimensional configuration of breccia in the subsurface is not well understood. For example, it is not known if the outcrops are connected by a network of pipes or if each outcrop is a single occurrence of breccia. Polymict breccia occurs only within the central uplift of Sierra Madera. The largest outcrop by area occurs on the western flank of one of the northern ridges of the central uplift (Figure 2.6).

Small (5-15 cm in diameter) injection dikes of polymict breccia intrude into host limestone (Figure 2.8B). Clasts within the dikes are rounded to sub-angular and smaller in size than clasts within tabular breccia sheets. Dike material contains less clastic material and the carbonate matrix has a finer texture than the host rock (Figure 2.8C).

Polymict breccia at Sierra Madera contains clasts derived from all rocks exposed at the surface of the structure in a tan to reddish fine-grained carbonate matrix. Clast size ranges from millimeter to meter-sized. Depending on the location of the breccia, one outcrop may be enriched with clasts from a particular formation when compared to other breccia outcrops.

Carbonate clasts within the polymict breccia contain small fractures filled with a fine-grained dark material. Sandstone clasts within the polymict breccia contain toasted quartz and quartz

Figure 2.8. Polymict breccia outcrop and hand sample photographs. A) Typical polymict breccia outcrop. Note hammer for scale. B) Injection dike of polymict breccia into the Hess Fm outlined in white. Field book (right side of photo) is 15 cm in length. C) Photomicrograph of injection dike-host rock boundary. Plane light. D) Polymict breccia with shatter cones in chert clast. Scale bar is 5 cm. E) Photomicrograph of melted sandstone clast within a polymict breccia. Note contorted layering and chaotic nature of clast material. Plane light. F) Photomicrograph of melted sandstone clast within a polymict breccia. Note flow within clast. Plane light. G) Photomicrograph of a dark brown, opaque possibly devitrified silica glass clast within a polymict breccia sample. Plane light. H) Fine-grained carbonate melt or “quenched” material. Lens cap is 5 cm in diameter.

44 grains have multiple PDFs. Shatter cones occur in some limestone and chert clasts within polymict breccia (Figure 2.8D). Quartz grains within shatter cones contain multiple PDFs and are highly toasted. Polymict breccias also contain sandstone clasts that are melted and the resulting clasts contain flow features (Figure 2.8E and F). Possible devitrified silica glass within polymict breccia occurs as clear or light brown to partially opaque irregular blebs (Figure 2.8G).

These clasts do not occur in all polymict breccia samples/locations. When possible devitrified silica glass is present as fragments within polymict breccia, it occurs as less than 5% of the clasts. Carbonate melt or “quenched” material was possibly identified at one location within the central uplift of Sierra Madera (Figure 2.8H). This material is a fine-grained dark orange carbonate. In thin section, the material contains small clasts that are unmelted and partially melted. Partially melted clasts are identified by having a coarse crystalline texture that gradually becomes more aggregate-like. When rapidly quenched, carbonate material forms either a holocrystalline material or an aggregate of fine-grained carbonate (Graup, 1999).

Discussion

Zircon Data

Preliminary SEM work on zircon mineral grains from the Sierra Madera impact structure led to the possible identification of two different impact related surface textures. Less than 5% of the zircon surfaces studied showed either PDF-like lineations or a granular texture. Similar surface textures were reported from the ejecta layer of the Chesapeake Bay structure and the distal ejecta of the Chicxulub structure (Bohor et al., 1993: Glass and Liu, 2001).

Documentation of shock-induced surface textures on zircon grains from the Sierra Madera crater

45 is first report of these features from a <90 km diameter impact structure and may provide more constraints on the pressures and temperatures in which these features form.

Shatter cone Data

The distinct weathering pattern of shatter cones within the Gilliam Limestone may be similar to the multiply-striated joint surfaces (MSJS) at the Vredefort structure, where the authors associated striated surfaces and jointing to the occurrence of shatter cone formation

(Wieland et al., 2006, Nicolaysen and Reimold, 1999). If the weathering pattern in the Gilliam

Limestone from Sierra Madera is similar to MSJS from Vredefort, these features correspond to a cm-size joint or fracture pattern within the rock. While Sierra Madera has a post-impact regional joint pattern (Wilshire et al., 1972) a joint or fracture pattern related to the impact event within the crater has not been determined.

Previous researchers have favored a relatively late formation of shatter cones with regards to MSJS (Nicolaysen and Reimold, 1999). At Sierra Madera, shatter cones also occur as fragments within polymict breccia indicating these features form early, during the contact/compression stage of impact crater development. Additionally, multiple directions of shatter cones and irregular cone shapes indicate the method of formation for these features is complex. Both full and partial 3-D cones as well as the overlapping, shingle morphologies are present within the central uplift at Sierra Madera. It was not within the scope of this study to determine the timing of shatter cone formation however shatter cone observations at Sierra

Madera indicate it would be an interesting location for such research.

46 What do the deformational features reveal about temperature and pressure conditions during the impact event?

When rapidly quenched, carbonate material forms either a holocrystalline material or an aggregate of fine-grained carbonate (Graup, 1999). It is therefore difficult to use carbonate material as the sole means for determining pressure and temperature conditions during an impact event. However, using the shock classification scheme for impact metamorphosed sandstones developed by Kieffer (1971) and modified by Osinski (2007), we can assign shock pressure values to CaCO3 cemented sandstones at Sierra Madera and apply those values to the adjacent carbonate units. Impact metamorphosed sandstones at Sierra Madera give a shock pressure range from 5.5-20 GPa based on the presence of “toasted” quartz, a “jigsaw” interlocking texture between quartz grains, and multiple sets of planar deformation features (PDFs) within quartz grains. This shock pressure range corresponds to a post-shock temperature range of 350-

>1000°C (Osinski, 2007). Most likely the host rocks at the current erosional level Sierra Madera were not exposed to shock pressures as great as 20 GPa based on the low amount of diaplectic glass within sandstone units.

Sandstone units at Sierra Madera have a range of deformational features that do not increase towards the center of the central uplift or towards the region of expected highest shock.

For example, the Cathedral Mountain Fm contains less deformational features than the sandstone from the Gilliam Limestone even though it is more centrally located in the central uplift. This may indicate shock pressure and temperature conditions were not distributed evenly during the impact event.

47 What do these features tell us about the amount of deformation observed to the final crater/erosional level within the crater?

The presence of cm-size injection dikes at Sierra Madera may indicate this structure is deeply eroded since breccia dikes are common in deeply eroded craters (Lambert, 1981). An additional indicator of a deeply eroded structure is the absence of an impact melt sheet within the ring depression. At Sierra Madera, evidence of an impact melt sheet was not found during this study or previous studies (Wilshire et al., 1972). The melt sheet was eroded or is buried under sediment within the ring depression in the crater. However, the eroded crater rim, the presence of injection dikes, suevite-like breccia and monolithic breccia within the central uplift and lack of abundant high-temperature deformational features indicates the current surface at Sierra

Madera is below the level at which the impact melt sheet would occur (Figure 2.9; French,

1998). Additionally, past research suggests erosion may have removed an estimated 610 m of overlying material at Sierra Madera (Wilshire et al., 1972). Therefore, pressure and temperature estimates reported above (5.5-20 GPa pressure range and post-shock temperature range of 350-

>1000° C) should be considered low estimates of deformational conditions.

If the crater was not deeply eroded, an alternate theory is that the Sierra Madera structure may have formed underwater (Kelly, 1966). A water-impact event would deform rocks immediately below the point of impact while leaving the crater rim moderately undisturbed thus explaining the highly deformed rock of the central uplift and relatively flat-lying rocks of the crater rim (Kelly, 1966).

Figure 2.9. Complex impact crater showing impactite locations. Modified from French

(1998). Dashed line in final complex crater indicates current erosional surface at Sierra

Madera.

49 Conclusions

Deformational features within the host rocks and impact generated breccias were used to determine pressure and temperature conditions during the Sierra Madera impact event.

According to the presence of toasted quartz, a jigsaw texture between quartz grains, and quartz grains with multiple PDFs pressure conditions during the impact were 5.5-20 GPa with a post- shock temperature range of 350->1000° C. Based on the projected lower erosional level within the Sierra Madera impact structure, it is possible deformational conditions during the time of impact were greater. However, a water-impact may also explain the lack of high temperature deformational features at Sierra Madera.

Acknowledgments

The authors would like to thank the Lyda and Lang Families for granting access to the property for sample collection. Also, the SEM used for the zircon work was made available by the Pacific Northwest National Laboratory in Richland, WA. This research was supported by

NASA Grant NNX06AE69G. Additional funding was provided by the Barringer Crater

Company.

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57 CHAPTER THREE

RIETVELD ANALYSIS OF X-RAY POWDER DIFFRACTION PATTERNS AS A

POTENTIAL TOOL FOR THE IDENTIFICATION OF IMPACT DEFORMED

CARBONATE ROCKS.

Abstract

Previous X-ray powder diffraction (XRD) studies revealed that shock deformed carbonates and quartz have broader XRD patterns than those of unshocked samples. Entire XRD patterns, single peak profiles and Rietveld refined parameters of carbonate samples from the

Sierra Madera impact crater, west Texas, unshocked equivalent samples from 95 miles north of the crater, and the Mission Canyon Formation of southwest Montana and western Wyoming were used to evaluate the use of X-ray powder diffraction as a tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks. At Sierra Madera dolostone and limestone samples were collected from the crater rim (lower shock intensity) and the central uplift (higher shock intensity). Unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater. Carbonate rocks of the Mission Canyon

Formation were sampled along a transect across the tectonic front of the Sevier and Laramide orogenic belts.

Whereas calcite subjected to significant shock intensities at the Sierra Madera impact crater can be differentiated from tectonically deformed calcite from the Mission Canyon

Formation using Rietveld refined peak profiles, weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite. In contrast, Rietveld analysis readily distinguishes shocked Sierra Madera dolomite from unshocked equivalent

58 dolostone samples from outside the crater and tectonically deformed Mission Canyon Formation dolomite.

Introduction

The physical record of impact craters on the surface of the Earth dates from 3.5 Ga

(Grieve 1998) and it is thought that impact cratering was an essential process even earlier in

Earth’s history (Ryder 2002). Putative impact craters commonly are identified by the presence of meteorite fragments, siderophile element anomalies, and specific shock deformational features including shatter cones, planar deformation features (PDFs) in minerals, diaplectic glass, and melting features (French 1998). Circular or nearly circular geomorphic structures lacking these distinctive features of suspected impact origin often are the subject of controversy (i.e. the

Silverpit structure; Stewart and Allen 2002; Underhill 2004; Reimold 2007).

Currently, there are ~170 confirmed terrestrial impact craters listed the Earth Impact

Database (www.unb.ca/passc/ImpactDatabase). Methods used to confirm circular structures on the Earth’s surface as meteorite craters vary with target rock type, age, bolide size, and amount of material removed by erosion. However, exposure to short-lived shock pressure is a unique characteristic of the target rocks at all impact craters. Therefore, a method of identifying minerals/rocks subjected to shock metamorphism, but lacking the usual shock deformational features, would be extremely useful for distinguishing suspected impact craters from non-impact generated structures. Past studies, X-ray diffraction (XRD) and petrographic, often focused on both experimentally and naturally shocked silicate minerals due to the abundance and robust nature of these minerals at the Earth’s surface (Hörz, 1968; Short, 1968; Hörz and Quaide 1973;

Grieve et al. 1996). An X-ray powder diffraction method for evaluating shock deformation

59 levels within impact craters formed in carbonate host rocks was developed by Skála and Jakeš

(1999). A broadening (i.e. increase in peak width) and reduction in intensity of CuKα X-ray

diffraction peaks in the 2θ° range 56-62° was observed in impact-shocked carbonate from the

Kara Crater (~65 km diameter) in Russia and the Steinheim Basin (3.8 km diameter) in Germany

when compared to those of unshocked standards (Skála and Jakeš 1999). Furthermore, peak

broadening and loss of peak intensity increased with increasing shock pressure (Skála and Jakeš

1999). However, carbonate samples from the Meteor Crater (1.2 km diameter), Arizona and

nearby Canyon Diablo did not show XRD peak broadening with expected increasing shock

effects from crater rim to crater center. These results were attributed to a non-uniform

distribution of shock pressure or smaller sized crater (Burt and Pope 2001; Burt et al. 2005). In

an effort to resolve this discrepancy, carbonate and siliciclastic samples from the Sierra Madera

impact structure (12 km diameter) in Texas were analyzed using XRD techniques adopted from

Skála and Jakeš (1999). Preliminary studies of the Sierra Madera carbonates indicated increased

peak broadening in shatter cone samples (Huson et al. 2006a; Huson et al. 2006b).

In the present study XRD patterns of calcite and dolomite from the Sierra Madera impact

crater are compared to those of unshocked equivalent carbonate samples from outside the crater,

unshocked carbonate samples from the Mississippian Mission Canyon Formation of the northern

Rockies, United States and to unshocked/undeformed mineral standards. The objective is to determine if XRD patterns of the shocked carbonates can be distinguished from XRD patterns of

carbonates that were deformed solely by terrestrial processes (e.g. burial or tectonism). This

study has implications for using XRD pattern analysis as a tool for the identification of shock

metamorphic processes where conventional impact characteristics (i.e., shatter cones and PDFs

60 in minerals) are lacking or poorly developed, especially since ~30% of all terrestrial impact structures formed primarily in carbonate rocks (Grieve and Robertson 1979).

Geologic Settings

Sierra Madera impact structure

The Sierra Madera impact structure, located near Fort Stockton, Texas, is a well-exposed, eroded remnant of a complex, 12 km diameter impact crater (Eggleton and Shoemaker 1961;

Wilshire and Howard 1968; Howard et al. 1972; Wilshire et al. 1972). The central uplift is composed of steeply dipping, folded, and faulted Upper Permian strata (Fig. 3.1). Lower

Cretaceous rocks occur along the eastern margin of the central uplift. The Permian Hess

Formation, the oldest stratigraphic unit exposed in the central uplift, was displaced vertically at least 790 m during the excavation and modification stage of crater development (Wilshire et al.

1972). The crater rim at Sierra Madera is composed of eroded Lower Cretaceous carbonate and siliciclastic rocks with exception of the southwest side where stream incision has exposed Upper

Permian strata. Rim strata are exposed ~30-60 m above the alluvium-filled ring depression and generally dip 0-5° radially, though at some locations the dip can be as great as 30°. Post-impact erosion has removed an estimated 600 m of material from the crater rim (Wilshire et al. 1972).

Past studies at Sierra Madera documented shock deformation features including shatter cones in siltstone and fine-grained sandstone, PDFs in quartz, grain fracturing, impact-induced brecciation, and deformed carbonate minerals (Dietz 1960; Eggleton and Shoemaker 1961;

Howard and Offield 1968; Wilshire et al. 1972; Huson et al. 2005).

Fig. 3.1. A) Geologic map of Sierra Madera (modified from Wilshire et al. 1972). 1 = Kwf CR, 2 = Kwf CC, 3

= Kwf AA, 4 = Kwf ZZ, 5 = Kwf NN, 6 = PtUU, 7 = Pg B1, 8 = Mlb 22-2, 9 = Pwls 22-1, 10 = Pwv O1, 11 =

Ph C1. B) Stratigraphic column for rocks exposed at Sierra Madera.

62 Unshocked Equivalent Dolostone Samples

Ninety-five miles north of the Sierra Madera impact crater, the equivalent dolomitic

Queen and Yates Formations were deposited on the northern edge of the Delaware Basin during the late Permian. These formations were deposited in a shallow shelf and later evaporitic environment as the Delaware Basin gradually became restricted (Hill 1996).

Mission Canyon Formation

The Lower Mississippian (Late Osagean-Middle Meramecian) Mission Canyon

Formation in western Wyoming and southwestern Montana records shallow water carbonate deposition along a broad, carbonate shelf that deepened westward, forming during the Antler

Orogeny ([Late Devonian to Early Mississippian]; Fig. 3.2; Sando 1976; Smith and Dorobek

1993). This unit was subsequently altered by Late Mississippian karstification and dolomitization and documents deep to shallow burial from west to east (Sando 1988; Smith and

Dorobek 1993). Additional deformation of Paleozoic-Mesozoic strata of southwestern Montana and western Wyoming occurred during the Cretaceous-Paleogene Sevier and Laramide orogenies

(Snoke 1993).

Experimental

Sample Collection and Preparation

Fine-grained dolostone and limestone samples were collected from the central uplift and eroded crater rim of the Sierra Madera impact crater (Fig. 3.1; Table 3.1). Unshocked samples of the dolomitic Queen and Yates Formations, equivalent to the Upper Word and Gilliam

Limestone at Sierra Madera, were collected from the Austin Core Facility in Austin, Texas. The

Fig. 3.2. A) Location of Mission Canyon Formation samples. 1 = Shoshone Canyon (SH), 2

= Sheep Mtn (SM), 3 = Benbow Mine (BM), 4 = UPR, 5 = Church Buttes (CB). Gray arrow indicates increasing burial depth and deformation for all samples sites from east to west. B)

General stratigraphic column of the Mission Canyon Formation.

64 Sedimentary Deformational Sample Composition Characteristics Features 1 Lithology 2

Sierra Madera Samples Crater Rim Kwf AA Calcite Rare fossil fragments, fine-grained --- MS Kwf CC Calcite Fossils, fine-grained matrix --- WS Kwf CR Calcite Fine-grained F MS

Central Uplift Kwf ZZ Calcite Fossil fragments, fine-grained matrix --- WS Kwf NN Calcite Fine-grained --- MS Pt UU Dolomite Matrix supported breccia B Mono Pg B1 Dolomite Fine-grained F, SC MS Mlb 22-1 Dolomite Matrix supported breccia B Mono Pwv O1 Dolomite Fine-grained SC MS Pwls 22-1 Dolomite Fossils, fine-grained matrix --- WS-PS Ph C1 Dolomite Fine-grained SC MS

Unshocked Equivalent Samples Queen/Word Dolomite Fine-grained --- MS Yates/Gilliam Dolomite Fine-grained --- MS

Mission Canyon Samples Shoshone Canyon SH 202 Dolomite Recrystallized Fc DS SH 234 Calcite Fine-grained --- LS

Sheep Mountain SM 307 Dolomite Fine-grained --- DS SM 421 Calcite Fine-grained --- LS

Benbow Mine BM 454 Dolomite Fine-grained --- DS BM 579 Calcite Fine-grained --- LS

UPR UPR 77183 Dolomite Fine-grained Fc DS

Church Buttes CB 725 Calcite Medium-fine grained --- LS CB 751 Dolomite Fine-grained --- DS CB 770 Dolomite Fine-grained --- DS 1 F= fractures; Fc = fractures filled with coarse calcite; B = brecciation; SC = shatter cones. 2 MS = mudstone; WS = wackestone; WS-PS = wackestone-packstone; Mono = monomict breccia; DS = dolostone; LS = limestone. 3 Contains visible sulfur.

Table 3.1. X-ray Powder Diffraction Samples.

65 cores were located in the North Ward-Estes Oil Field in Ward County, Texas, 95 miles north of

the Sierra Madera impact crater. The authors were not able to obtain unshocked calcite-rich

samples from outside the crater.

Fine-grained samples of dolostone and limestone also were collected from the Mission

Canyon Formation of the Madison Limestone Group of southwest Montana and western

Wyoming (Fig. 3.2; Table 3.1). Three optically clear samples of quartz, Iceland spar, and

dolomite from Washington State University’s School of Earth and Environmental Sciences

(WSU SEES) mineral collection served as unshocked standards for comparison. All samples were ground under isopropyl alcohol using a mortar and pestle and the powders were passed through a 27 μm mesh sieve.

Sample Compositional Characterization

Since compositional differences can lead to XRD peak position and breadth variability

(Fiquet et al. 1994), the magnesium, iron, and calcium content of all calcite and dolomite samples were analyzed using a Cameca electron microprobe in the WSU SEES GeoAnalytical

Laboratory. No other elements were observed in more than trace amounts.

All samples used in this study were also examined petrographically to determine if there were noticeable compositional differences between sample sets that may lead to preferential groupings. In calcite-rich samples, both Sierra Madera and Mission Canyon Formation samples contain micritic limestone with bioclasts, peloids and intraclasts. Abundances of allochems vary within each sample. Dolomite-rich samples have less variability, with both Sierra Madera and

Mission Canyon Formation samples containing fine-grained dolomite and no allochems. The

unshocked Queen and Yates Formation samples contain fine-grain dolomite with allochems.

66 There is no indication of an enrichment of allochems or changes in grain size for either calcite- rich or dolomite-rich samples that would group samples preferentially.

Polished grain mounts of the coarser size (45-63 μm) fraction of each sample used for

XRD analysis were carbon coated and analyzed; sample compositions are presented in Table 3.2.

The experimental parameters were; 15 kV acceleration voltage, 15 nA beam current, 4 μ beam

diameter and 10 s dwell time on peak and background. The calibration standards (analytical line

and spectrometer crystal) were; calcite (CaKα, PET), Obendorf dolomite (MgKα, TAP), and

Irugtut siderite (FeKα, LiF).

X-ray Powder Diffraction Analysis

XRD patterns were collected using CuKα radiation and a Siemens Kristalloflex

Diffractometer equipped with a graphite diffracted beam monochromator and a scintillation

detector. A combination of beam divergence and receiving slits yielding an instrument profile

breadth of 0.083° 2θ for the 311 reflection of silicon powder were used (Foit 1992). The <27 µm powders were sifted onto a vaseline-coated, quartz zero background plate to reduce preferred orientation of the grains (Foit 1992). The diffractometer was operated at 35 kV and 30 mA and the samples were step-scanned over the range 15°-120° 2θ using a 0.02° 2θ step and dwell time of 10 sec. Although the samples were scanned from 15°-120° 2θ, only the portion of the scan covering 55-69° 2θ is shown in Fig. 3.3 to highlight peak broadening differences. XRD powder patterns for all samples were compared to dolomite or calcite standard patterns.

67 Oxide wt % 1 Sample Name MgO FeO CaO CO2 Total Oxide wt %

CALSTD 0.00 0.02 56.88 43.76 100.66 Sierra Madera Samples Kwf ZZ 0.56 0.09 53.66 44.46 98.76 Kwf NN 0.50 0.01 55.49 44.03 100.03 Kwf CR 0.39 0.00 55.89 43.96 100.24 Kwf AA 0.40 0.06 54.60 44.25 99.30 Kwf CC 0.27 0.02 55.81 43.98 100.08

Mission Canyon Samples SH 234 0.39 0.01 56.97 43.76 101.14 BM579 0.30 0.00 55.86 43.98 100.14 CB 725 0.20 0.01 53.84 44.53 98.57 SM 421 0.30 0.01 55.08 44.18 99.56

DOLSTD 21.72 0.08 30.61 47.67 100.08 Sierra Madera Samples Ph C1 20.60 0.01 31.72 47.55 99.88 Mlb 22-1 20.22 0.03 32.02 47.51 99.79 Pwls 22-1 20.58 0.04 29.94 48.02 98.58 Pwv O1 20.64 0.06 30.28 47.92 98.90 Pg B1 21.43 0.39 30.45 47.51 99.91 Pt UU 21.35 0.87 30.40 47.51 100.13

Mission Canyon Samples SM 307 20.95 0.05 31.48 47.55 100.04 BM 454 21.24 0.01 31.22 47.60 100.07 CB 770 21.22 0.02 31.27 47.58 100.09 CB 751 21.31 0.02 31.22 47.58 100.14 SH 202 21.02 0.07 31.24 47.60 99.93 UP 7718 21.51 0.02 31.03 47.61 100.16

Unshocked Equivalent Samples Yates 21.47 0.31 29.72 NA 99.38 Queen 21.36 0.37 30.14 NA 99.64 1 Calculated from stoichiometry.

Table 3.2. Electron microprobe analytical data.

Fig. 3.3. X-ray powder diffraction patterns of calcite and dolomite samples from the Sierra Madera impact structure and Lower Mississippian Mission Canyon Formation. A) Calcite-rich samples compared with the calcite standard (CALSTD). B) Dolomite-rich samples compared with the dolomite standard (DOLSTD). All scans were arranged from least (bottom of chart) to most (top of chart) broadened. See Table 1 for sample descriptions.

69 Diffraction Peak Profile Analysis

XRD Peak broadening due to shock deformation was analyzed using both single peak profiling and Rietveld refinement of peak shape parameters, both of which were carried out using Jade 8.0 software (Materials Data Corporation). With single peak profiling, the peak full width half maximum (FWHM) values are not appreciably affected by small variations in sample composition, therefore they are potentially useful for distinguishing weakly from more intensely shocked samples. Peak broadening can also be analyzed by Rietveld crystal structure refinement

(Young, 1993) of the peak shape parameters (Skála and Jakeš, 1999). XRD peak FWHM values

are a function of the diffraction angle theta and three least squares refineable parameters U, V,

and W (Caglioti, 1958):

(FWHM)2 = UTan2 theta + Vtan theta + W

The values of FWHM, which are related to the amount of shock-induced deformation, were calculated over the range 15°-120° 2θ. Single peak and Rietveld refined FWHMs were then fitted with a fourth order polynomial.

Results

Diffraction Patterns: Limestone

XRD patterns of calcite-rich Mission Canyon Formation samples are compared with

calcite-rich Sierra Madera samples (Fig. 3.3A). Samples obtained from the central uplift of

Sierra Madera display more peak broadening than those from the crater rim with an almost

complete disappearance of peaks at 58° and 63° in sample Kwf NN. Although XRD peaks of

70 Mission Canyon Formation sample SH 234 (Fig. 3.3A) are broader than crater rim samples from

Sierra Madera (Kwf CR, Kwf AA, and Kwf CC), in general, calcite-rich Sierra Madera samples show more peak broadening than calcite-rich Mission Canyon Formation samples. Some Sierra

Madera and Mission Canyon samples contain extra peaks. Samples Kwf NN, Kwf ZZ, Kwf AA and SM 421 contain minor quartz peaks while samples SH 234 and CB 725 contain minor anhydrite peaks (Fig. 3.3).

Diffraction Patterns: Dolomite

Of the fourteen unshocked samples collected from the Austin Core Facility only two, one from the Queen Formation and one from the Yates Formation, are dolomite. The remaining 12

samples contained dolomite with significant amounts of quartz and anhydrite and are therefore

not compared to the shocked samples from Sierra Madera. XRD patterns of dolomite-rich Sierra

Madera samples are compared with two unshocked equivalent samples of the Queen and Yates

Formations (Fig. 3.3B). Shocked samples from Sierra Madera have more peak broadening than

the unshocked Queen and Yates samples. For example, note broadening between sample

Yates/Gil and Pwls 22-1 in the 60° and 67.5° regions.

Samples from Sierra Madera show an increase in peak broadening towards the central

uplift of the impact structure. For some samples (Pwls 22-1, Ph C1, and Pwv O1) peak

broadening is great enough to result in almost a complete disappearance of the three peaks in the

64° to 66° 2θ range.

For Mission Canyon Formation samples, patterns of samples from locations close to the

Antler Orogenic front have less peak broadening than those from farther away. For example, the

peaks for sample UP 7718 (location 4; Fig. 3.2A) are sharper than those for sample SM 307

71 (location 2); especially note the three peaks in the 64° to 66° region. In general, dolomitic Sierra

Madera samples have considerably more peak broadening than Mission Canyon Formation and unshocked Queen and Yates samples.

Single Peak Profiling – Full Width Half Maximum (FWHM)

The FWHM curves (Fig. 3.4A, B) that deviate most from the standard curves (CALSTD and DOLSTD) are those that experienced higher shock (shatter cones, monomict impact breccias and samples from the crater rim). Unshocked Queen and Yates samples plot close to the

standard curves for lower (below ~105) 2θ values. Also the majority of the Mission Canyon

Formation samples tend to plot between the scans of the standards and those of the Sierra

Madera samples (with the exception of BM 454 and SH 234; Fig. 3.4A, B). However, some samples (CB 725 and Mlb 22-1, for example) have a convex curve instead of the expected concave curve as seen in the calcite and dolomite standards.

Rietveld Refinement of Peak Shape Parameters

FWHM curves (Fig. 3.5A) for calcite-rich Mission Canyon Formation samples SH 234 and BM 579 overlap calcite-rich Sierra Madera crater rim samples Kwf CR and Kwf AA/Kwf

CC, respectively. FWHM values obtained from Rietveld crystal structure refinements increase with shock intensity for all calcite-rich Sierra Madera samples. FWHM values of shocked dolomite from the central uplift of Sierra Madera are distinctly higher than tectonically deformed dolomite from unshocked Queen and Yates samples as well as samples from the Mission Canyon

Formation (Fig. 3.5B). Additionally, FWHM values for shocked Sierra Madera samples follow a pattern of progressive broadening from crater rim to central uplift. Sierra Madera samples with

Fig. 3.4. Single peak profiling using FWHM values for Sierra Madera and Mission Canyon

Formation samples. A) Calcite values compared with a calcite standard, CALSTD. B)

Dolomite values compared with a dolomite standard, DOLSTD.

Fig. 3.5. Rietveld refinement analysis for Sierra Madera and Mission Canyon Formation samples. A) Calcite values compared with a calcite standard, CALSTD. B) Dolomite values compared with a dolomite standard, DOLSTD.

74 shock features (shatter cones, monomict breccia) have uniformly higher FWHM values than

those without shock features.

Discussion

All XRD peak patterns deviate from those of the calcite and dolomite standards. This is

particularly true for calcite and dolomite from the central uplift of Sierra Madera. Several factors

contribute to XRD peak profiling variability including Mg content (Gavish and Friedman 1973),

grinding time of powder (Burns and Bredig 1956; Gavish and Friedman 1973), grain size of

powder (Langford et al. 2000; Klug and Alexander 1974), and phase transformations (Burns and

Bredig 1956; Fiquet et al. 1994; Merrill and Bassett 1975). The above mentioned factors were considered when preparing samples for this study. The Mg, Fe, and Ca contents of all carbonate samples were measured (Table 3.2) and while all samples deviate slightly from the calcite and dolomite stoichiometric values, there is no correlation between composition and variability in

XRD peak profiles or Rietveld refined FWHM curves. The grain size of the powder used was

<27 μm since a larger crystallite size can lead to inaccurate and imprecise intensity measurements (Bish and Reynolds 1989). Phase transformations (e.g., calcite to aragonite due to grinding; Burns and Bredig 1956) were not a factor since XRD powder patterns for samples from

Sierra Madera show no peaks other than those of dolomite, calcite, and in few cases, quartz and anhydrite (Fig. 3.3). Quartz is a common mineral in nearshore carbonate rocks. The Mission

Canyon Formation contains anhydrite and other evaporitic minerals due to evaporitic tidal flat depositional environments (Dorobek et al. 1993).

Single peak profile patterns of calcite and dolomite samples from all samples, while useful at lower 2θ values, are complex and indicate little relationship to shock intensity at higher

75 2θ values. This is because weaker high angle reflections are poorly estimated by the profile

shape function. High angle reflections from shocked material may disappear from the pattern

since these reflections are often low intensity and shocking decreases the degree of the

crystallinity (Skála and Jakeš 1999). While single peak profiling may be useful at lower 2θ

values, at higher values, especially above ~80° 2θ, the method is unreliable.

More useful for this study was the Rietveld refinement of peak shape parameters.

Rietveld refinement of peak shape parameters yields better resolution of overlapping diffraction

peaks and a more precise measure of the 2θ angular dependence of peak FWHM, thus, providing a better measure of the degree of shock deformation (Skála and Jakeš 1999).

Rietveld results for the mildly shocked calcite samples from the crater rim overlap with those of Mission Canyon Formation calcite samples whereas, those of the central uplift are distinguishable from MCF samples. Dolomite samples from Sierra Madera also show increasing

Rietveld refined FWHM values as the central uplift is approached reflecting increasing deformation of the dolomite crystal structure with increasing shock pressure (Skála and Jakeš

1999). Noticeably, Sierra Madera samples plot well above unshocked equivalent dolomite-rich samples from outside of the crater. Therefore, XRD with Rietveld refinement analysis indicates that it may not always be possible to distinguish calcite-rich samples subjected to terrestrial tectonic processes from those mildly shocked in an impact event. However, with exposure to increasing shock pressures within the central uplift of the crater, impact shocked rocks are readily distinguishable from unshocked rocks from outside of the crater and other terrestrially deformed rocks.

Interestingly, Mission Canyon Formation samples do not indicate a systematic variation

in Rietveld-refined FWHMs with distance from the Sevier and Laramide orogenic fronts. With

76 Mission Canyon Formation samples, due to deformation associated with increasing compression

and burial depth, peak broadening was expected to increase as sample locations approach the

tectonic front of the Sevier and Laramide Orogenies (Fig. 3.2). The ideal and expected order for

sample locations, from least to most peak broadening, is Sheep Mountain (SM), Benbow Mine

(BM), Shoshone Canyon (SH) to Church Buttes (CB). However, observed XRD patterns for

Mission Canyon Formation samples indicate that peak breadth increases in the order SM 421 to

CB 725 to BM 579 to SH 234. The fronts of these orogens may be more complicated, with deformation unequally distributed along thrust fronts or out of phase thrusting may have occurred. Similarly, tectonic forebulge vs. foreland basin events overprinted burial deformation

of the Antler orogeny, increasing the complexity of the deformational history in the area.

Additionally, secondary alteration of the Mission Canyon Formation associated with burial and

recrystallization (Smith and Dorobek 1993) may also have affected XRD parameters, however,

precipitation of secondary dolomite, one potential cause of the unexpected order, can be ruled

out since it should result in peak profiles more similar to those of the standard.

Hydrothermal and secondary alteration in Sierra Madera

Hydrothermal and secondary alteration in rocks may affect XRD peak patterns due to

modification of the original rock material. Evidence of fluorite crystals occurring in some

polymict breccia outcrops (Wilshire et al. 1972) indicate hydrothermal alteration occurred as a

post-impact process at Sierra Madera. However, it is not uncommon for impact craters to

interact with the hydrosphere either as a disruption of the local water table or on a larger scale

such as an ocean basin (Osinski et al. 2001). Rocks at Sierra Madera were not deformed after the

impact event (Wilshire et al. 1972). Therefore, hydrothermal alteration that occurred at Sierra

77 Madera was directly related to the impact event and not due to a subsequent deformational

process.

Shock pressures and XRD broadening

Using the shock classification scheme for impact metamorphosed sandstones developed

by Kieffer (1971) and modified by Osinski (2007), we can assign shock pressure values to

CaCO3 cemented sandstones at Sierra Madera and apply those values to the adjacent carbonate

units.

Impact metamorphosed sandstones at Sierra Madera give a shock pressure range from

5.5-20 GPa based on the presence of “toasted” quartz, a “jigsaw”, interlocking texture between

quartz grains, and multiple sets of planar deformation features (PDFs) within quartz grains. The

presence of shatter cones in porous sandstone indicates a shock pressure range of 3-10 GPa

(Osinski 2007). Since well developed shatter cones in porous sandstone will not form above

shock pressures of 10 GPa, most likely the rocks at Sierra Madera were not exposed to shock

pressures as great as 20 GPa. This shock pressure range corresponds to a post-shock temperature

range of 350->1000° C (Osinski 2007).

Sandstone beds within the Gilliam Limestone have the greatest amount of deformational

features and therefore, one would assume XRD peak patterns from the adjacent limestone would be the broadest of all Sierra Madera samples. However, the broadest XRD peak patterns come

from the Hess Formation located within the center of the central uplift (Fig. 3). While the adjacent quartz-rich Cathedral Mountain Formation and sandstone beds within the Word

Formation contain shock features (multiple sets of PDFs and the generation of quartz microbreccia), these units are matrix-supported, not grain-supported like the sandstone beds

78 within the Gilliam Limestone. Without the interaction between quartz grains, higher shock

pressure indicators, such as symplectic regions between grains, are not present within the

Cathedral Mountain Formation and Word Formation. Therefore, shock features within these two

quartz-rich units do not accurately indicate the highest shock pressures to which these rocks or

the adjacent limestones were exposed. Nevertheless, the presence of shatter cones within the

Hess, Cathedral Mountain, and Word Formations indicate rocks from the central uplift of Sierra

Madera were exposed to shock pressures in the range of 3-10 GPa, however the actual pressure

range of formation is probably narrower (Osinski 2007; French 1998; Baratoux and Melosh

2003; Wieland et al 2006).

Conclusions

Rietveld refined parameters (especially XRD peak FWHM) potentially can be used to distinguish impact-shocked rocks from unshocked equivalent rocks from outside the crater.

However, this study suggests that when comparing shocked rocks to those exposed to mild

tectonic deformation, shock pressures must reach at least 5.5 to 20 GPa to make this distinction

since only samples from the central uplift of Sierra Madera have significantly higher FWHM

values (peak broadening) than the tectonically deformed Mission Canyon Formation samples.

Similarly, it is not known how a sample exposed to intense tectonic deformation compares to a

shocked sample. The Mission Canyon Formation was never exposed to intense forces associated

with direct convergent boundary contact. Instead the formation was only associated with the

distal edge of two orogenies, the Sevier and Laramide. If the XRD patterns of less intensely

deformed Mission Canyon Formation calcites and dolomites are indistinguishable from those exposed to lower shock levels along the crater rim of Sierra Madera, it is conceivable that

79 minerals severely deformed by tectonic processes will also be indistinguishable from higher shocked rocks from the central uplift of an impact crater. Therefore, a study of more highly shocked rocks from a different impact structure and similar rock types from a more intense tectonic setting should be carried out to further determine the usefulness of Rietveld analysis and x-ray diffraction as a tool for the characterization of impact deformed rocks.

Acknowledgements:

The authors would like to thank David Katz of the University of Miami for providing

Mission Canyon Formation samples and James Donnelly at the Austin Core Facility in Austin

Texas for allowing SAH access to collect unshocked carbonate samples from cores located north of Sierra Madera. Additional thanks goes to Glenn Lang and the Lyda Family who provided access to the Sierra Madera impact structure. This manuscript benefited from constructive reviews by E. Buchner, R. Skála, P. Buchanan and W.U. Riemold. This work was supported by

NASA Grant NNX06AE69G and funding provided by the Barringer Crater Company.

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87 CHAPTER FOUR

FISSION TRACK AGE FOR THE SIERRA MADERA IMPACT STRUCTURE

Abstract

Samples for fission track analysis were collected from the central uplift of the Sierra

Madera impact crater. Apatite mineral grains were separated from three sets of samples: 1)

weakly deformed sandstone adjacent to polymict, impact-generated breccia, 2) sand-rich

polymict breccia and sandstone clasts from polymict breccia, and 3) shatter cones formed in

sandstone. Including poorly constrained samples, those with less than 10 apatite grains analyzed

in the separate, the fission track age ranges from 157 +/- 8 to 26.3 +/- 8.9 Ma. Well constrained

apatite fission track ages, those with 10 or more apatite grains in the separate, range from 147 +/-

11 to 106 +/- 10 Ma. Most of the youngest well constrained ages are older than the estimated

maximum crater age based on regional stratigraphy (Late Cretaceous to Early Tertiary for the

deposition and partial consolidation of the Lower Washita Group) indicating these grains were

not completely reset during the impact event. One age might match the maximum crater age

(sample PgssG3 with an age of 106 +/- 10 Ma), however, it does not correspond to recent

paleomagnetic ages determined for Sierra Madera of 70 to 50 Ma (Dennie et al., 2007). Due to

the eroded nature of the crater, it is possible that most of the mineral grains with completely reset

isotopic values were removed by erosion. However, the presence of possible devitrified silica glass as clasts within polymict impact generated breccia indicates some breccia outcrops likely reached the conditions necessary for resetting isotopic values within apatite mineral grains.

Sampling of glass-rich polymict breccia outcrops for apatite may yet yield the correct fission track age for the Sierra Madera impact structure.

88 Introduction

Age determination of impact cratering events is essential for understanding impact flux

over time, extinction events, and effects on global climate (Rampino and Haggerty, 1994). One

technique, the fission track method, is well-known for dating impact craters (Wagner and Van

Den Haute, 1992). Fission track dating requires temperatures of at least 80°C in apatite for track

annealing and the resetting of isotopic ratios (Deutsch and Schärer, 1994). Additionally, shock

pressures of ~2 to 10 GPa erased most fission tracks in apatite from the Hardhat and Sedan

nuclear detention localities and pressures over 40 GPa erase all tracks (Fleischer et al., 1974).

These temperatures and pressures are easily produced during an impact event. At Marquez

Dome impact structure, Texas, fission track analysis of apatite from sand-rich breccia dikes

produced an age of 58 +/- 2 Ma (McHone and Sorkhabi, 1994). Likewise, apatite fission track

analyses of fallback suevite at the Ries crater produced an age of 14.7 +/- 0.4 Ma (Wagner and

Van den haute, 1992). As a low temperature technique, apatite fission track dating is an

advantageous dating method for crater studies where smaller impact structures lack evidence of

high temperature deformational features such as melt glass.

The relative age of the Sierra Madera impact crater from stratigraphic and structural

relationships is between 100-2 Ma (Wilshire et al., 1972). Recent paleomagnetic studies narrow

the age of this impact event to ~70-50 Ma (Dennie et al., 2007). Deformational features (i.e.

shattercones and PDFs in quartz grains) at the Sierra Madera crater indicate post-impact

temperatures ranged from 100°C to at least 150°C (Wilshire et al., 1972). Additionally, the

structure was not deformed by a thermal event subsequent to its formation (Wilshire et al., 1972).

Therefore, low-temperature (<125°C) dating techniques are ideal for studying the timing of this

89 impact event. This study uses fission track analysis of apatite mineral grains to determine a radiometric age for the Sierra Madera impact crater.

Geologic Setting

The Sierra Madera impact structure, 12 km diameter in west Texas is a well-exposed, eroded remnant of a complex impact crater (Eggleton and Shoemaker, 1961; Wilshire and

Howard, 1968; Howard et al., 1972; Wilshire et al., 1972). The central uplift of Sierra Madera is composed of steeply dipping and folded Upper Permian strata (Figure 4.1). Lower Cretaceous rocks outcrop along the eastern margin of the central uplift. The Permian Hess Formation, the oldest stratigraphic unit exposed in the central uplift, was displaced vertically at least 790 m during the excavation and modification stage of crater development (Wilshire et al., 1972). The crater rim at Sierra Madera is composed of Lower Cretaceous carbonate and siliciclastic rocks that commonly are eroded. On the southwest side of the rim, Upper Permian rocks are exposed.

Cretaceous strata are ~30-60 m above the alluvium-filled ring depression. Strata on the rim generally dip 0-5° radially though at some locations dip can be as great as 30°. Erosion removed an estimated 610 m of material on the crater rim eliminating the overturned flap of strata characteristic in younger crater rims (i.e., Meteor Crater, AZ; Wilshire et al., 1972). Past studies at Sierra Madera documented shock deformation features such as shattercones in siltstones and fine-grained sandstones, planar deformation features (PDFs) in quartz, grain fracturing, breccias, and deformed quartz and carbonate minerals (Dietz, 1960; Eggleton and Shoemaker, 1961;

Howard and Offield, 1968; Wilshire et al., 1972; Chapter 2 of this thesis).

Figure 4.1. Geologic map of the central uplift of the Sierra Madera impact structure (modified from Wilshire et al. (1972)). 1 = PgA, 2 =PcmF, 3 = PgssK, 4 = PwssG, 5 = KbB, 6 = KbC, 7 = KbE, 8 = Pg W1, 9 = Hp mb A3, 10 = Mb E3, 11 = Pwss D3, 12 = Ph dike E1, 13 = MbRR, 14 = Pgss K, 15 = PgssG3. Circles indicate weakly deformed sandstone samples. Squares indicate sandstone clasts sampled from within polymict breccia. Triangles indicate shatter cones in sandstone.

91 Methods

Apatite grains for fission track analysis were separated by crushing, sieving, magnetic

and heavy liquid methods using techniques described in Appendix 1 of Donelick et al. (2005).

Samples were irradiated at Washington State University’s Triga Reactor. Fission track analyses

were measured at Apatite to Zircon, Inc. in Potlatch, Idaho. Samples were described using

petrographic thin sections.

Sample description

The central uplift of the Sierra Madera impact crater contains the most intensely

deformed rocks in this structure and therefore apatite with the greatest probability of having reset

isotopic values in this area (Wilshire et al., 1972; Huson et al., 2007). Samples for apatite fission

track analysis were collected from: 1) sandstone with weakly developed deformational features,

2) sandstone clasts in polymict breccia, and 3) shatter cones in sandstone.

The first group of samples was collected from sandstone with weakly developed

deformational textures such as long contacts between grains and sutured quartz grains in thin section. Planar deformation features, distinctive parallel planar regions, occur in two of the

seven sandstone samples (PcmF and PwssG).

The second group of samples consist of sand-rich polymict breccia and sandstone clasts

within polymict breccia. In thin section, the breccia contained evidence of possible silica

glass clasts (Huson et al., 2007) and planar deformation features (PDFs) in quartz grains.

The third group consisted of shatter cones in the Gilliam Sandstone. Shatter cones are

unique impact-produced fracture surfaces characterized by diverging striations from cone apex to

base (Figure 4.2). Shock intensities needed to create well developed shatter cones in a porous

Figure 4.2. Shatter cone from the Gilliam Limestone at the Sierra Madera impact crater.

93 sandstone range from 3- ~10 GPa (Osinski, 2007). Quartz grains within the sandstone are less

than 0.1 mm in size, highly fractured and closely packed with a jigsaw texture. Planar

deformational features (PDFs) were identified in less than 15% of the quartz grains.

Results

Of the fifteen samples analyzed, eight are poorly constrained with samples bearing less than 10 apatite grains (Table 4.1). Seven remaining apatite separates collected from sandstone

adjacent to polymict breccia, sand-rich polymict breccia, and shatter cones in sandstone yield

ages ranging from 147 +/- 11 to 106 +/- 10 Ma. Sand-rich polymict breccia samples yielded

ages of 26.3 +/- 8.9 to 157 +/- 8 Ma. The youngest ages in this sample set are poorly constrained

with samples providing less than 10 grains per sample. Sandstone adjacent to polymict breccia

samples yielded ages of 34.0 +/- 19.8 to 147 +/- 11 Ma with the youngest ages poorly

constrained. Apatite separates from shatter cones in sandstone contain the youngest well-

constrained ages, 106 +/- 10 and 123 +/- 9 Ma.

Discussion

All samples from the Sierra Madera impact structure analyzed in this study indicate

fission tracks within apatite mineral grains were partially reset. All Permian age target rocks

yield Late Jurassic to Early Cretaceous fission track ages. The oldest ages come from samples of

sandstone adjacent to polymict breccia and sand-rich polymict breccia, whereas shatter cone

samples yielded the youngest ages. The range for all well constrained samples is 147 +/- 11 to

106 +/- 10 Ma. The youngest well constrained ages agree with the Late Cretaceous estimated

maximum age of the crater (Figure 4.3, sample PgssG3; Wilshire et al., 1972). Noticeably, well

94 Sample Description Original Age of Apatite Apatite Fission Name Host Rocks Grains Track Age (Ma) Sandstone adjacent to Polymict Breccia PgA Gilliam SS Permian 37 139.0 +/- 7 PcmF Cathedral Mtn Permian 6 128.0 +/- 25 PgssK Gilliam SS Permian 15 147.0 +/- 11 PwssG Word SS Permian 4 83.0 +/- 13.9 KbB Maxon SS Cretaceous 1 71.0 +/- 20.7 KbC Maxon SS Cretaceous 1 34.0 +/- 19.8 KbE Maxon SS Cretaceous NA NA

Sand-rich Polymict Breccia PgW1 Gilliam SS Permian 36 157 +/- 8 HP MB A3 Polymict breccia with Permian 3 26.3 +/- 8.9 Cath. Mtn clasts MBE3 Sand-rich polymict Permian 9 56.7 +/- 11.3 Breccia PwssD3 Sand-rich polymict Permian 12 132 +/- 12 Breccia Ph dike E1 Hess injection dike Permian 2 68.6 +/- 26.0 MBRR Polymict breccia Permian 25 140.0 +/- 9

Shatter cones in Sandstone PgssF3 Gilliam SS Permian 31 123 +/- 9 PgssG3 Gilliam SS Permian 17 106 +/- 10

Table 4.1. Fission track data.

Figure 4.3. Age comparison diagram. Comparison of fission track ages from this study with the paleomagnetic age of Dennie (2007) and the stratigraphic and structural ages of Wilshire et al. (1972). Fission track ages are represented by a small filled circle with error bars. Gray circles with error bars represent samples with less than 10 apatite grains. An “X” indicates the depositional ages for each sample. Question marks indicate an unknown depositional age for a sample.

96 constrained fission track results from this study are not comparable with the recent

paleomagnetic age range of 50 to 70 Ma (Dennie et al., 2007). One conclusion is that the majority of high pressure and high temperature deformational features and samples ideal for apatite fission track dating were removed from Sierra Madera due to the deep erosional level of the structure (Figure 4.4). Wilshire et al (1972) estimated 610 m of overlying material was removed from Sierra Madera by comparing deformational features within the crater to a range of eroded natural craters and experimental nuclear craters. Therefore, fission dating of mineral grains from such a well-eroded impact structure may not yield the correct age for the impact event.

Alternatively, the shock pressure and temperature conditions may not be evenly distributed at Sierra Madera (see Chapter 2). Sandstone units at Sierra Madera have a range of deformational features that do not increase towards the center of the central uplift or towards the region of expected highest shock. For example, the Cathedral Mountain Formation contains fewer deformational features than the sandstone from the Gilliam Limestone even though it is more centrally located in the central uplift. Uneven shock distribution can lead to the sampling of units that were not exposed to the highest shock conditions, resulting in incorrect fission track ages for the impact event.

Temperatures of at least 80°C are required for fission-track annealing in apatite (Deutsch and Schärer, 1994). At Sierra Madera some polymict breccia samples contain possible devitrified silica melt glass. Quartz is stable at atmospheric pressures up to 870°C (Deer et al.,

1999). The presence of melted sandstone clasts and possible devitrified silica melt glass within some polymict breccia outcrops may indicate temperatures within the breccias were locally great enough to reset isotopic abundances within apatite mineral grains. These breccias were injected

Figure 4.4. Ideal complex impact crater showing impactite locations. Modified from French

(1998). Dashed line indicates current erosional surface at Sierra Madera.

98 downward into the target rocks during the impact event and might be the only remaining sites for high temperature deformational features since the crater was eroded to a lower level. Better sampling of sandstone clasts or sand-rich zones from known polymict breccia outcrop containing devitrified silica melt glass may lead to the determination of a radiometric age for this structure.

Shock pressure can also erase fission tracks within apatite mineral grains (Fleischer et al.,

1974). Pressures needed to reset most apatite mineral grains of a sample range from 2-10 GPa grains leading to partially reset ages. At 40 GPa all fission tracks in an apatite sample are erased

(Fleischer et al., 1974). Shatter cone samples from the Sierra Madera impact structure were expected to yield ages correlating to timing of the impact event. However, shatter cones form in a range of 3 to ~10 GPa in porous sandstone (Osinski, 2007). Therefore, apatite mineral grains within shatter cones from Sierra Madera were likely not completely reset and samples contain a mixture of partially reset and unaffected ages which do not reflect the age of the impact event.

The usefulness of fission track dating for smaller diameter impact craters is varied. At the 12.7 km diameter Marquez Dome, Texas, fission track analyses of sandy breccia dikes produced an age of 58.3 +/- 3.1 Ma and agreed well with the known stratigraphic age of 58 +/- 2

Ma (McHone and Sorkhabi, 1994). At the 13 km diameter Kentland impact crater, Indiana, fission track analyses yielded an age of 184 +/- 13 Ma and were not comparable to the <97 +/- 10

Ma paleomagnetic age (Weber et al., 2005). Like the Sierra Madera crater both structures have similar diameters (12-13 km) and are hosted in sedimentary rocks. Marquez Dome is hosted in

Cretaceous marls and shale whereas the Kentland structure is hosted in deformed Ordovician to

Mississippian carbonates and shale with one prominent sandstone unit (the St. Peters Sandstone;

McHone and Sorkhabi, 1994; Weber et al., 2005). In all three studies, apatites were collected from sand-rich units. At this time it is unclear why fission track age dating methods worked at

99 Marquez Dome and not at Kentland or Sierra Madera. It is possible that both the Kentland crater

and Sierra Madera are simply too eroded to obtain accurate fission tracks ages.

Conclusions

Apatite fission track age dates analyses of samples from the central uplift of the Sierra

Madera impact crater do not document a short-lived thermal event within the known

stratigraphic age of this crater. Erosion most likely removed the majority of high pressure and

high temperature deformational features required for fission track analysis. However, the presence of possible devitrified silica glass within some polymict breccia outcrops indicates

temperatures associated with these samples should have reset isotopic abundances within apatite

mineral grains. Re-sampling of sand-rich polymict breccias from locations known to contain

possible devitrified silica glass may provide a fission track age for Sierra Madera.

Acknowledgements

The authors would like to thank the Lyda and Lang Families for granting access to the property for sample collection. This research was supported by NASA Grant NNX06AE69G.

Additional funding was provided by the Barringer Crater Company.

100 References

Deer, W. A., Howie, R. A., and Zussman, J., 1999, An Introduction to the Rock Forming

Minerals, 2nd ed.: John Wiley and Sons, New York, 696 p.

Dennie, D., Elmore, R. D., Huson, S. A., and Madden, M. E., 2007, Paleomagnetism of the

Sierra Madera impact structure, Texas: Geological Society of America Abstracts with

Programs, v. 39, no. 6, p. 372.

Deutsch, A. and Schärer, U., 1994, Dating terrestrial impact events: Meteoritics, v. 29, p. 301-

322.

Dietz, R. S., 1960, Meteorite impact suggested by shatter cones in rock: Science, v. 131, no.

3416, p. 1781-1784.

Donelick R. A., O’Sullivan, P. B., and Ketcham, R. A., 2005, Apatite fission track analysis, in

Low temperature thermochronology: Techniques, Interpretations, and Applications:

Reviews in Mineralogy and Geochemistry, vol. 58, p 49-94.

Eggleton, R. E. and Shoemaker, E. M., 1961, Breccia at Sierra Madera, in Short papers in the

geologic and hydrologic sciences: U.S. Geol. Survey Prof. Paper 424-D, p. D151-D153.

101 Fleischer, R. L., Woods, R. T., Hart Jr., H. R., Price, P. B., and Short N. M., 1974, Effect of

shock on fission track dating of apatite and sphene crystals from the Hardhat and Sedan

underground nuclear explosions: Journal of Geophysical Research, v. 79, no. 2, p. 339-

342.

Howard, K. A. and Offield, T. W., 1968, Shatter cones at Sierra Madera, Texas: Science, v. 162,

no. 3850, p. 261-265.

Howard, K. A., Offield, T. W., and Wilshire, H. G., 1972, Structure of Sierra Madera, Texas, as

a guide to central peaks of lunar craters: Geological Society of America Bulletin, v. 83, p.

2795-2808.

Huson S. A., Pope M. C, Foit F. F. Jr., and Watkinson A. J., 2007, Immiscibility features

between silica-rich and carbonate-rich material in breccia from the Sierra Madera Impact

Structure, 38th Lunar and Planetary Science Conference, Abstract, no. 1817.

McHone, J. F. and Sorkhabi, R. B., 1994, Apatite fission-track age of Marquez Dome impact

structure, Texas: 25th Lunar and Planetary Science Conference, Abstract, p. 881-882.

Osinski, G. R., 2007, Impact metamorphism of CaCO3-bearing sandstones at the Haughton

structure, Canada: Meteoritics and Planetary Science, v. 42, p. 1945-1960.

102 Rampino M. R. and Haggerty, B. M., 1994, Extraterrestrial impact and mass extinctions of life,

in Gehrels, T., ed., Hazards due to comets and asteroids: University of Arizona Press,

Tucson, p. 827-857.

Wagner, G. and Van Den Haute, P., 1992, Fission Track Dating: Dordrecht, Kluwer Academic

Publishers, p. 217-225.

Weber, J. C., Poulos, C., Donelick, R. A., Pope, M. C., and Heller, N., 2005, The Kentland

Impact Crater, Indiana (USA): An apatite fission-track age determination attempt, in

Koeberl, C. and Henkel, H., eds, Impact Tectonics: Berlin and Heidelberg, Springer, p.

447-466.

Wilshire, H. G. and Howard, K. A., 1968, Structural pattern in central uplifts of cryptoexplosion

structures as typified by Sierra Madera: Science, v. 162, no. 3850, p. 258-261.

Wilshire, H. G., Offield, T. W., Howard, K. A., and Cummings D., 1972, Geology of the Sierra

Madera Crytoexplosion Structure, Pecos County, Texas: U.S. Geol. Survey Prof. Paper

599-H, p. 1-49.

103 CHAPTER FIVE

FIELD GUIDE TO THE SIERRA MADERA IMPACT STRUCTURE

Abstract

This document was provided as a handout for the participants of the Impact in the Field

Studies Group annual field trip in association with the Lunar and Planetary Science Conference in 2005. The trip took two days and left from San Antonio, Texas. The trip consisted of an overview of the crater and a detailed look of shatter cones and impact generated breccia from within the central uplift of the structure.

104

105

Field Guide to the Sierra Madera Impact Structure

Edited by S. A. Huson1 1 M. C. Pope

Written by S. A. Huson1

in association with the 37th Annual Lunar and Planetary Science Conference Houston, Texas

March 11-12, 2006

1 Department of Geology Washington State University Pullman, Washington 99164-2812

106

107 Contents

Introduction 109

Location 110

Stratigraphy 113

Brief Geologic History 117

Past Studies 118

Current and Future Studies 119

Trip Stops and Descriptions

Saturday: San Antonio to Fort Stockton to Sierra Madera 122

Stop 1: Crater Rim near Gas Plant 124

Stop 2: East side of Central Uplift 124

Shatter cones from the Central Uplift 127

Stop 2a: Solution Collapse Breccia in the Tessey Limestone 127

Stop 2b: Monomict Breccia in the Gilliam Limestone 130

Stop 2c: Deformation in sandstone of the Gilliam Limestone 130

Stop 2d: Stanley Thomas Elsinore No. 1 well derrick 130

Stop 2e: Polymict Breccia at Phillips’ Elsinore No. 1 well 132

Stop 2f: Madera Peak (time and weather permitting) 132

Sunday: Sierra Madera to Fort Stockton to San Antonio 134

Stop 3: North side of Central Uplift 134

Stop 4: North side of Crater Rim 134

References 137 Cover: Oblique angle digital aerial photograph of the Sierra Madera impact structure. View is to the northwest. Photo credit: Tom Jones NASA Astronaut Corps.

108 Introduction

Welcome to the 2006 Impact Field Studies Group field trip to the Sierra Madera impact

structure! Upon arrival to the Fort Stockton area we will spend the next 24 hours exploring the

structure. Saturday we’ll explore the east side of the central uplift with a trip to the top of

Madera Peak (elev. 1400 m). Sunday will be devoted to the north side of both the central uplift

and the crater rim.

Sierra Madera is Spanish for “wooded hills” and as you will see the central uplift of the

structure is covered with juniper trees. Sierra Madera is located in west Texas and contains

Chihuahuan Desert flora and fauna. Local vegetation consists primarily of oak, juniper, sparse

grasses, some mesquite, scrub brush, creosote bushes and several varieties of cactus. Local

fauna include rattlesnake, deer, mountain lion, javelina, jack rabbit, road runner, tarantulas, and

many bird species.

Please take care when hiking over slopes with loose rock. Make sure to carry an ample

supply of water (two or more liters each day) as the day-time high temperatures may reach 90°F

(32°C).

Thank you for your interest in Sierra Madera! Your participation will be greatly appreciated. If you have any comments or questions, please let us know!

Sarah A. Huson, Field Trip Leader

Mark F. Thompson, IFSG Field Trip Coordinator

109 Location and Background

The Sierra Madera impact structure is one of four impact structures located in Texas

(Figure 5.1). Sierra Madera is located ~20 miles south of Fort Stockton and ~30 miles north of Marathon, Texas (Figure 5.2). Sierra Madera is 12 km in diameter and is a well-exposed remnant of a complex impact crater. It was last studied during the late 1960’s and early 1970’s

by Wilshire et al. (1972). After the first lunar landings Sierra Madera was used as a field training

site for astronauts to see impact generated breccias and shattercones (Margolin, 2000). The

structure is one of ~60 impact craters that formed in predominately carbonate host rocks (Grieve

and Shoemaker, 1994). In addition to carbonates, Sierra Madera also includes interbedded

siliciclastic target rocks making it an ideal place to comparatively study quartz-carbonate

deformation produced during an impact event. Similarly, the size of the crater is between other

craters hosted in carbonate rocks, namely, Haughton (24 km), Ries (24 km), Steinheim (3.8 km),

and Barringer (Meteor) Crater (1.2 km). Therefore, study of the Sierra Madera crater may

provide important information for understanding the continuum of carbonate deformation

produced by meteorite impacts. Currently, the Sierra Madera structure has not been

radiometrically age dated. This is due to the lack of obvious high-temperature deformation

features such as melt rocks and glass formation. However, recent work on carbonate craters

reveals melt features from these structures may be subtle (Osinski and Spray, 2001). Based on

stratigraphic and structural relationships, the Sierra Madera impact occurred sometime during the

Late Cretaceous to Early Tertiary (Wilshire et al., 1972). The objective of this field trip is to

introduce you to the stratigraphy, structure and deformation of the Sierra Madera impact crater.

110

Figure 5.1. Location of impact craters in Texas, Bee Bluff (BB), Marquez Dome (MD),

Odessa (O), and Sierra Madera (SM). The Sierra Madera structure occurs near Fort Stockton,

Texas. Dashed box in Texas map is outline of Figure 2.

111

Figure 5.2. Location of the Sierra Madera structure near Fort Stockton, Texas.

112 Stratigraphy

Madera. The Cretaceous is marked by a transgression represented by fluvial to shallow water marine deposits.

Below is a description and stratigraphic column (Figure 5.3) of rocks outcropping within the central uplift and along the crater rim of the Sierra Madera impact structure. Descriptions are compiled from Wilshire et al. (1972), Hill (1996), and the author. The entire Permian section will be seen on the first afternoon of the trip whereas Cretaceous units will be examined the following morning.

Formation outcrop at the surface and the unit gradually increases in thickness to the north

(Wilshire et al., 1972).

113

Figure 5.3. Stratigraphic column of rocks that outcrop in the central uplift and crater rim of the Sierra Madera impact structure. Column was drawn using the maximum thickness of the unit (thickness in parentheses.)

114 Cathedral Mountain Formation – The Permian (Leonardian) Cathedral Mountain Formation is a

0-24 meter thick conglomerate containing chert and quartz pebbles in a dolomite matrix. Beds of

cross-bedded coarse, quartz sandstone occur locally within the Cathedral Mountain Formation

(Wilshire et al., 1972). This unit grades up conformably into the lower part of the Word