Petrology and petrography of beachrock (Pleistocene?),Sonoran coast, northern Gulf of California

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Authors Jones, Peggy Louise, 1951-

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PETROLOGY AND PETROGRAPHY OF BEACHROCK

(PLEISTOCENE?), SONORAN COAST,

NORTHERN GULF OF CALIFORNIA

by

Peggy Louise Jones

A T hesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In P artia l Fulfillm ent of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 5

STATEMENT BY AUTHOR

This th e s is has been subm itted in partia l fulfillm ent of r e ­quirem ents for an advanced degree at The U niversity of Arizona and is deposited in the U niversity Library to be made availab le to borrowers under ru les of the Library.

Brief quotations from th is th e s is are allow able w ithout sp ec ia l perm ission , provided that accurate acknowledgm ent of source is m ade. R equests for perm ission for extended quotation from or reproduction of th is m anuscript in whole or in part may be granted by the head of the major departm ent or the Dean of the Graduate C ollege when in h is judg­ment the proposed use of the m aterial is in the in te re s ts of sch o lar­sh ip . In a ll other in s ta n c e s , how ever, perm ission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This th e s is has been approved on the date shown below:

ACKNOWLEDGMENTS

I would like to exp ress my thanks to the members of my th e s is

com m ittee. P rofs. D ietm ar Schum acher and Terah Smiley and , e sp e c ia lly .

Prof. Joseph F . Schreiber, J r . , th e s is d irec to r, for th e ir help and guid­

ance during the preparation of th is th e s is . P ro fessors Schreiber and

Sm iley, D epartm ent of G eo sc ien ces , The U niversity of A rizona, provided

information referenced in th is p ap er. M ichael W . R ose, a graduate s tu ­

dent in the Departm ent of G eo sc ien ces , allow ed me to u se inform ation

from h is th e s is re se a rc h . Union Oil Company of C alifornia provided

a c c e ss to the scanning e lec tron m icroscope. D r. James M ille r, Union

Oil Company of C alifo rn ia , and D r. Norman Hodgkin, M icrograph ics,

I n c . , Newport B each, C alifo rn ia , provided valuable a ss is ta n c e in ob­

ta in ing the SEM m icrographs. The oblique aeria l photographs and some

fie ld photographs are from the co llec tion of P rofessor Schreiber.

This th e s is is ded ica ted to M rs. M aggie Swafford, whose loving

concern and financ ia l support made p o ss ib le the com pletion of my deg ree .

i ll

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS...................................................................................... v

LIST OF TABLES................................................................... v ii

ABSTRACT................................................................................................................... v iii

INTRODUCTION...................................................................................................... 1

P u rpose ............................................................................................................... 1Location and Physica l Featu res of Area of S tudy ............................... 1

LITERATURE SURVEY................................................................................................ 5

FIELD AND LABORATORY METHODS................................................................. 9

F ield M ethods ................................................................................................ 9Laboratory M e th o d s ....................................................................................... 10

PHYSICAL DESCRIPTION OF THE BEACHROCK........................................ ... . 12

G eographic E x te n t ......................................................................................... 12T h ic k n e s s ................................. 14C om position, S iz e , and S o r t in g ....................................... 16Relation to P resen t C o asta l C onfiguration ............................................ 19E ro s io n ............................................................................................................... 23

CEMENTATION............................................................. 26

A cicular C e m e n t............................................................................................. 26Blocky C e m e n t................................................................................................ 31M icritic C e m e n t............................................................................................. 35D istribu tion of Cement F ab ric s . . ............................................................... 37

CONCLUSIONS............................................................................................................. 41

A g e ........................................... 41O rig in ...................................................................................................................... 41D ia g e n e s i s .......................................................................................................... 42Influence of S ea -lev e l C hanges and T e c to n is m ................................... 43

SUMMARY................................................................................ 44

REFERENCES. ................................................................................................................ 45

P age

iv

LIST OF ILLUSTRATIONS

1. Location map ........................................... 2

2. Aerial photograph of beachrock e a s t of PuertoP en asco , looking n o rth .................................................................... 13

3 . Aerial photograph of the Puerto Penasco a re a ,looking w e s t ....................................................................................... 13

4 . Aerial photograph of Playa Hermosa B each,looking s o u th e a s t ............................................................................. 15

5 . G ently dipping beachrock at Norse Beach ju s te a s t of P elican P o in t ....................................................................... 20

6 . D iscordant beachrock body ju s t w est of entranceto E stero M a n i a ................................................................................ 20

7 . Beachrock c li f f , Adair B ay.................................................................... 22

8 . W ave-cu t c liff , C holla B a y ................................................................. 22

9 . Karst surface developed on b e a c h ro c k ................................................ 24

10. Chem ical erosion of beachrock , entrance toE stero M a n ia ....................................................................................... 24

11. Scanning e lectron m icrograph of ac icu lararagonite cem en t................................................................................. 27

12. Scanning e lectron m icrograph of b locky ,equid im ensional c e m e n t ................................................................. 27

13. Scanning e lec tron micrograph of randomlyoriented aragonite c e m e n t.................................................................. 28

14. Petrographic th in sec tio n photom icrograph ofaragonite f r i n g e ................................................................................. 29

15. Scanning e lec tron m icrograph of eroded aragonite cem ent. . . 32

16. Scanning e lec tron m icrograph of high-m agnesium c a lc ite . . . 33

F igu re P age

v

LIST OF ILLUSTRATIONS— Continued

Figure Page

17. Petrographic th in sec tion photomicrograph of h ig h -magnesium c a lc ite cem ent and m icritic c o a t i n g s ........................34

18. Scanning electron micrograph of chem ical d isso lu tio nof c a lc ite c e m e n t ............................................................................... 36

19. Algal s ta b iliz a tio n of in te rtida l s e d im e n ts ..........................................38

v i

LIST OF TABLES

1. L ithologic descrip tion of beachrock m easured a t thePlaya Hermosa B each, April 1974 .................................................. 17

2 . Scanning electron microprobe r e s u l ts .............................................. 30

3. D istribu tion of cem ent fa b r ic s , Playa HermosaBeach s e c tio n ........................................................................................ 39

T ab le P ag e

v i i

ABSTRACT

Beachrock is common along the co ast of the no rtheastern Gulf of

C alifornia in the v ic in ity of Puerto P enasco , Sonora, M exico. The rocks

are found in the p resen t in te rtida l zone and may extend below low -tide

le v e l. Beach, tid a l f la t , and barrie r-bar sedim ents are rep resen ted in

the beachrock and can be recognized on the b a s is of grain c h a ra c te ris tic s ,

fo s s il co n ten t, and geomorphic re la tion of beachrock bodies to the c o a s t­

al configuration .

Cem enting m aterial is aragonite and high-m agnesium c a lc i te .

The c a lc ite is more c h a rac te ris tic of low -in te rtida l b each ro ck s, and

aragonite is more common in h ig h -in te rtid a l b e ac h ro c k s . Both cem ents

rep lace organic carbonate of fo s s il fragm ents. A m icrite coating is p re s ­

ent on carbonate and noncarbonate c l a s t s . P resence of the m icrite is

a ttribu ted to a lgal a c tiv ity . Algae may a lso be im portant in promoting

cem entation p ro c e s s e s .

It is proposed th a t the beachrock formed during la te P le is tocene

by evaporation of marine w aters and p rec ip ita tion of calcium carbonate

during periods of low t id e . The p resen t in te rtida l p o sitio n of the b each ­

rock is believed to be by co incidence on ly .

v i i i

INTRODUCTION

Beachrock is a friab le to w ell-indu ra ted rock com posed of sh e ll

fragm ents w ith or w ithout terrigenous m aterial cem ented w ith some form

of calcium carbonate . B eachrock, commonly re s tr ic te d to the in te rtid a l

zone, has been described as occurring ch iefly along c o a s tlin e s in la t i ­

tudes of coral growth (Bricker, 1971; G insburg, 1953). How ever, recen t

fie ld work along the arid Sonora, M exico co ast of the northern Gulf of

C alifornia has shown beachrock to be common at a la titude of 31°30 ' N .

in a clim ate th a t may be c la sse d as a sub trop ical d e se rt (BWh in the

Koppen c la s s if ic a tio n , T rew artha, 1954, Appendix A).

Purpose

This study is a fie ld and laboratory exam ination of the b each ­

rock occurring in the v ic in ity of Puerto P enasco , Sonora, M ex ico . Its

purpose is to deduce the origin and d iagenetic h isto ry of the beachrock

by determ ining the m ineralogic ty p e , tim e, and m echanism of cem enta­

tion and to determ ine th ic k n e s s , geographic e x te n t, and re la tio n to the

p resen t co as ta l configuration .

Location and P hysica l Features of Area of Study

The area of study is cen tered around Puerto P en asco , Sonora,

M exico, some 96 km (60 m iles) so u th -so u th w est of L ukev ille , Arizona

(fig. 1). Sampling lo c a litie s range from the mouth of E stero M arua, 9 .6

km (6 m iles) e a s t of Puerto Penasco to a point in Bahia de Adair a t la t

31030' N . and very c lo se to Black M ountain or Cerro P rie to .

1

PINACATE REGION

BAHIA DE ADAIR

BLACKMOUNTAIN

GULF OF CALIFORNIA

\\ 9

CHOLLA BAY ̂P E L I C A N / ^

POINT/7̂ 7 ^ 7x /

PUERTO^PENASCO

ESTEROMARUA

KILOMETERS /////STUDY AREA

Figure 1. L ocation map

Beachrock stud ied is ind ica ted by the d iagonal p a tte rn .

3

Puerto Penasco is s itu a ted a t la t 310 18' N . and lie s a t the edge

of the Sonoran D esert th a t borders the no rtheastern side of the Gulf of

C alifo rn ia . The area is warm and very a rid . Rainfall averages only 74

mm per year (Green, 1969). Land surface tem peratures range from an

average of 11 .4°C in January to 2 9 .7°C in August (Green, 1969).

Runoff in the nearby Sonoita River is ra re . M ost of the year the

river is dry and , except for an occasiona l storm , con tribu tes very lit tle

sedim ent or m eteoric w ater to the Puerto Penasco a re a . Sea surface s a ­

lin itie s average 36 .0 parts per thousand during the w inter to as high as

36 .7 parts per thousand during the sp ring . These va lues are only av er­

a g e s , and much higher sa lin it ie s have been reco rded . Sea surface tem ­

pera tu res are low est in January with an average of 14 .9°C and h ighest

in August w ith an average of 3 1 .2°C (Rosenberg, 1969).

The main sources of energy and means of sedim ent transport in

the area are the t id e s , w ind, and w a v es . Irregu lar, sem idiurnal tid e s

charac te rize the a re a , and a t Puerto Penasco a maximum tid a l range of

about 6 .7 m eters (22 feet) is a tta ined 9 months of the y e a r . The months

of January through April have the m ost days when th is maximum tid a l

range o c c u rs . An annual tid a l calendar for the Puerto Penasco area is

published by the Departm ent of B iological S c ien ces , The U niversity of

A rizona.

S trongest w inds for the Puerto Penasco area occur from la te

w inter to early spring (January to April) from the so u th -so u th w est and

from O ctober through Decem ber from the northw est. The w eak est w inds

for a ll months of the year and a ll hours of the day are from the no rtheast

(Green, 1969). M ost w aves in the northern Gulf of C alifornia are due to

4

local w inds w ith w aves generated in the open P acific to the south rarely

reaching the northern g u lf. Sea b reezes generate the w aves seen m ost

commonly in the a re a . S ea-b reeze w inds occur most frequently in the

spring and summer and are most in tense during the a fte rn o o n s . W ave

refraction e ffec ts are alm ost n il in the Puerto Penasco a re a . S ea-b reeze

b reakers may reach heigh ts of 4 to 6 fee t (1.2 to 1 .8 m) (Gayman, 1969).

Under se a -b re e z e c o n d itio n s , w aves move northward toward the Puerto

Penasco shoreline and are stronger during the day ligh t h o u rs . The t id e s ,

w ind, and w aves combine to make th is co astlin e a h igh-energy one .

There is no quan tita tive data for the amount of m aterial t ra n s ­

ported by longshore drift and breaking w aves in the Puerto Penasco a re a .

However, large amounts of sand are moved,with the co arse r m ateria ls

(chiefly sh e ll debris and quartz) being deposited nearshore in a m ultiple

ridge and runnel system and on the b e a c h e s . The finer m ateria ls are d e ­

posited p rincipally on the tid a l f la ts . Gayman (1969) has ca lcu la ted the

onshore tran spo rt of sh e ll m aterial to be as much as hundreds of cubic

yards per n au tica l mile per y e a r .

The northern Gulf of C aliforn ia has had a very active tec to n ic

h isto ry (Henyey and B ischoff, 1973). The course of even ts appears to

be extrem ely com plex, and no one to date has tack led the problem of

unraveling the sequence of even ts for the Puerto Penasco a re a . That

up lift h as occurred in the a rea is ev idenced by w ave-cu t benches eroded

into the g ran ite of P e lican Point 32 fee t (9 .8 m) above p resen t mean low

w a te r . According to H ertle in and Emerson (1956), sh e lls from th is te r ­

race belong to a P le is tocene fa u n a .

LITERATURE SURVEY

O ccurrences of beachrock have been recognized in trop ica l

a reas for many years (G insburg, 1953). Increased in te re s t in carbonate

cem entation and d iag en es is since the la te 1950's has re su lted in many

s tu d ies of modern and P le is tocene carbonate sed im en ts . The Bermuda

B iological S tation for Research held an in ternational conference in

Septem ber 1969 to study carbonate cem ents and th e ir m echanism s of

form ation (Bricker, 1971). One part of th is conference w as devoted to

beachrock and in te rtid a l cem ent. It is sign ifican t th a t th is part of the

conference is the f irs t part of B ricker's (1971) book. The following

paragraph in troduces the se c tio n , "Beachrock and In te rtida l Cem ent"

(Bricker, 1971, p . 1):

The term "beachrock" refers to sedim ent lith ified in the in te rtid a l p lus -sea spray z o n es , w hether on h ig h - or low - energy b each es; or even on broad tid a l f la ts and tid a l chan ­n e ls . Inasm uch as th is type of lith ifica tio n is not re s tr ic te d to true b each es in the geomorphic s e n s e , a p referable term might be " in te rtid a l ro c k ." Beachrock g rad e s , below low tide le v e l, into subm arine cem ent and, aw ay from sea w ater in ­flu en ce , into subaeria l cem ent. Like the beach i ts e l f , it rep re sen ts a tra n s itio n a l s itu a tio n , som etim es laved in marine w a te r, and in o ther lo c a litie s a t tim es affected by fresh w a te r. Much of the lith ified m aterial extending up c reeks and into the defin ite ly fresh w ater realm which w as formerly ca lled "beachrock" by ce rta in authors ( e .g . , R u sse ll) , is genera lly now excluded; it should be c a lle d "cay rock" or some other pertinen t term .

B ricker's d isc u ss io n further d e sc rib e s beachrock in g e n e ra l.

Beachrock may have a v a rie ty of com positions, from to ta lly vo lcan ic

fragm ents to quartz and fe ld sp ars to to ta lly carbonate fragm ents. C e­

m en ts, e ith e r aragonite or h igh-m agnesium c a lc i te , in itia lly p rec ip ita te

5

6

as sim ple pore f illin g s . Beachrocks may form rap id ly and are u su a lly

re s tric te d to the tro p ic s . C em entation may take p lace as a re su lt of s e a ­

w ater ev apo ra tion , or it may p rec ip ita te a t the fresh w ater—s a lt w ater

in te rfac e . Other papers in the symposium point out severa l of the com ­

p lex itie s a sso c ia te d with beachrock form ation.

Emery and Cox (1956) stud ied beachrock in the H aw aiian I s ­

lan d s . No m echanism of form ation w as proposed, but they re la te the

occurrence of beachrock to a reas where the in te rs ti tia l w ater is sea

w ater or where the ground w ater has been in con tact w ith calcium car­

bonate .

Ginsburg (1953) gave ev idence that evaporation of sea w ater

during periods of low tid e w as responsib le for p rec ip ita tion of calcium

carbonate to cem ent beach sa n d s . Cement in Ginsburg*s F lorida b each ­

rock is ac icu la r a ra g o n ite , a s iden tified by sta in ing of th in s e c tio n s .

Temperature and ra te and degree of beach drainage apparen tly influence

the p rec ip ita tion of a ra g o n ite . .

Other authors (M abesoone, 1964; C ooray, 1968) c ite occu r­

rences of beachrock of H olocene age and attribu te cem entation to a

m echanism proposed by R ussell (1959, 1962, 1963, 1971) and R ussell

and M clntire (1965) th a t c a lls upon ground-w ater p rec ip ita tio n of cem ent.

A study of beachrock on the is lan d of Kauai by Inm an, Caym an, and Cox

(1963) found beachrock a sso c ia te d with fresh -w a te r lake seepage and

cem ented d u n e s . C a lc ite p rec ip ita ted from the lake w ater presum ably

is the cem enting ag en t.

Bathurst (1974) and Moore (1973) attribu ted cem entation of

beachrock to mixing of fresh and sea w a te rs , resu lting in p rec ip ita tion

7

of calcium carb o n a te . Moore reported the p resence of a c icu la r c ru st c e ­

m ent, m icrite c ru s t cem ent, c lea r b laded -equan t crust cem ent, and mix­

tu res of the three v a r ie tie s , but he did not d ifferen tia te betw een c a lc ite

and a rag o n ite . Moore found th a t the po re-w ater chem istry corresponded

w ell w ith cem ent chem istry . Bathurst a lso reported varia tions in the

c ry s ta l form of cem ents and s ta ted th a t the need le like cem ent w as a rag ­

onite and the blocky cem ent w as c a lc i te . Factors controlling the m iner­

alogy of the cem ent were sub tle b ecause both kinds of cem ent were found

in one p la c e . The in te rtid a l mixing p rocess probably accounted for the

extrem ely rapid ra te of cem entation as compared to cem entation tak ing

p lace below the tid a l zo n e . Bathurst w as not sure of the ex ac t m echa­

nism of cem entation b ecause there w as such g rea t v a riab ility in the in ­

te rtid a l zo n e . Factors like sa tu ration by w a te r, turbulence in the surf

zone, tem pera tu re , and location of the fresh w ater—sa lt w ater in terface

may vary g rea tly .

Stoddart and Gann (1965) questioned R u sse ll 's theory of b e a c h -

rock form ation. They found abundant aragonite cem ent and opted for a

marine o rig in . They found beachrock in arid and sem iarid lo c a litie s

where it would be d ifficu lt to find enough ground w ater to cau se p rec ip i­

ta tio n of calcium ca rb o n a te .

In a paper on M editerranean beachrock , A lexanders son (1972a)

s ta ted th a t high-m agnesium c a lc ite may be a re su lt of a com bination of

fac to rs , such as evaporation and mixing of w a te r. The p resence of

aragonite w as a ttribu ted to evaporation of sea w ater.

In a study of P le is tocene ree f rock cem entation , Schroeder (1973)

d ea lt with marine a s w ell as vadose cem ent. The marine cem ent

8

co n sis ted of aragonite and high-m agnesium c a lc i te , w hile the vadose

cem ent w as w holly low -m agnesium c a lc i te . His c rite ria for determ ining

type of cem ent were fourfold: (1) c ry s ta l hab its and fa b r ic s , which are

not alw ays d iagnostic alone; (2) magnesium co n ten t, which im plies in ­

creased sa lin ity w ith in creased magnesium; (3) c ircum stan tia l e v id e n c e ,

such as location with re sp ec t to sea level; and (4) cem ent assem blages

u sed w ith h igher re lia b ility than iso la ted cem ent ty p e s . W inland (1969)

u sed free -energy ca lcu la tio n s to support form ation of aragonite and h ig h -

magnesium c a lc ite under the influence of sea w a te r . If the carbonates

rem ained in con tac t w ith marine w a ter, d iag en es is to low -m agnesium .

c a lc ite w as not e x p e c te d . S ie sse r (1974) c ited d iagenetic a lte ra tio n to

low -m agnesium c a lc ite in African beachrock as ev idence of su b aeria l

e x p o su re .

The role of organism s in beachrock formation is a m atter of

some d e b a te . W iedem ann (1969) a ttribu ted m icritic coatings around

grains to organic a c tiv ity . A cid-inso lub le m ucilaginous su b stan ce w as

found by Shearman and Skip with (1965) to coat grains in carbonate rocks

of various a g e s . D avies and Kinsey (1973) thought tha t a lgae are not

im portant in ac tu a l contribution of cem ent but th a t mucus may help to

trap m icritic p a rtic le s th a t la te r serve a s nuc leation s i te s for aragonite

cem ent. The algae may ac t as a binding agent in beach sed im ents and

c rea te a somewhat s ta b iliz e d environm ent in w hich to p rec ip ita te cem ent

from in te rs titia l so lu tio n s . Taylor and Illing (1969) a lso a ttribu ted s ta ­

b iliza tio n of loose sedim ents to a lg a e .

FIELD AND LABORATORY METHODS

Field M ethods

Beachrock outcrop lo c a litie s were se lec te d from aeria l photo­

graphs taken at low t id e . W idths of the outcrops were m easured d irec tly

on the a e ria l photographs where they could not be m easured on the

ground a t low t id e . The photographs were a lso u sed to determ ine the

geographic ex ten t of beachrock in the v ic in ity of Puerto Pefiasco.

The area of study is en tire ly in te rtid a l, thus periods of extrem e­

ly low tid e s were n e ce ssa ry for doing field work. T h icknesses were

m easured using a Brunton com pass. W ell-ex p o sed , a c c e ss ib le lo c a litie s

were se le c te d for ex tensive sam pling: the entrance of Estero M arua e a s t

of Puerto Penasco; the Playa Hermosa Beach in the town of Puerto Pefias­

co; Norse Beach (Sandy Beach) ju s t e a s t of P elican Point; beachrock e x ­

posed in Cholla Bay; and a beachrock promontory near Black M ountain.

Sam ples from a sa lt-w ater w ell at the U niversity of Sonora—U niversity of

Arizona D esa lin iza tio n Plant and from a w ell in the P elican Point v ic in ity

w ere a lso u se d .

Sampling in the beachrock proved to be som ewhat ch a llen g in g .

The top few cen tim eters are v irtu a lly teem ing with organic a c tiv ity —

boring m o llu scs , w orm s, b a rn a c le s , and a lg a e . Care w as taken to c o l­

le c t sam ples w ith as few living organism s as p o ss ib le , but th is fea t w as

rare ly a tta in a b le .

M easurem ent and sam pling of lo c a litie s w ith c liff ex p o su res ,

such a s the promontory near Black M ounta in , were sim p lified . Sam ples

9

were co llec ted from v e rtica l ex p o su re s , and th ic k n e sse s of exposures

were m easured w ith a ta p e . All sam ples co llec ted were sto red in po ly­

ethylene bags for tran spo rta tion to the labo ra to ry .

Laboratory M ethods

W hen the sam ples reached the labo ra to ry , they were immedi­

a te ly removed from the sam ple bags and soaked in a one percen t so lu tion

of hydrogen peroxide in d is til le d w ater for a minimum of 48 h o u rs . This

procedure k illed any organism s living in the beach rock . Two 24-hour

soakings in d is tille d w ater were then made to insure rem oval of any

h a lite p re sen t. The sam ples were removed from the ba ths and allow ed

to a ir dry. Specim ens were view ed with the b inocular m icroscope to note

fea tu res of d isso lu tio n and cem entation .

A h a lf-in ch th ick s lic e of each specim en w as im pregnated under

vaccuum with a p o ly este r c astin g re s in to which had been added sev era l

drops of an opaque blue d y e . The im pregnated specim ens w ere cured

overnight in a warm (45°C) oven . T h in -sec tion chips were cut from the

s l ic e s . M ost specim ens had to be im pregnated a second time to fill

voids tha t were not reached by the re s in during the f irs t trea tm en t. A

surface of each chip w as ground, using 350-m esh g r it, and cem ented to

a frosted petrographic g la s s slide w ith L akeside 70 therm oplastic cem ent.

The Lakeside fa iled to form a bond w ith the p o ly este r re s in , and the

chips were removed from the s l id e s , reground , and mounted w ith an

epoxy cem ent. H alf of each th in sec tio n w as p laced for 30 seconds in a

1% H C l-A lazarin Red-S so lu tion for sta in ing of calcium ca rb o n a te .

10

11

Broken fragm ents were mounted on m etal s tubs and coated with

gold for scanning in a Cambridge ste reoscann ing e lec tron m icroscope

equipped with p ro b e .

Two sh e ll sam ples were prepared for radiocarbon age determ ina­

tion by soaking in hydrochloric ac id baths to d isso lv e the ou ter sh e ll

m aterial and to avoid any contam inating ca rb o n a te . The sam ples were

subm itted to the U niversity of Arizona Laboratory of Isotope Geochem ­

istry for age d e te rm in a tio n s.

PHYSICAL DESCRIPTION OF THE BEACHROCK

G eographic Extent

Beachrock deposits are numerous and w idespread in the Puerto

Pefiasco a re a . In ae ria l pho tog raphs, the beachrock appears a s dark

bands along the shore tha t a re , for the most p a rt, under w ater at high

tide (fig. 2). The dark color is due to organic a c tiv ity , predom inantly

a lg a e . These bands can be recognized everywhere along the beaches at

low t id e , excep t where absen t along the s teep ly c liffed c o a s ts of Punta

Pefiasco and P elican Point (fig. 3). There is some ind ication th a t the

beachrock ex tends inland from the p resen t c o a s t . Samples from a s a l t ­

w ater w ell a t the U niversity of Sonora—U niversity of Arizona D e sa lin iz a ­

tion P lant are litho log ica lly sim ilar to beachrock exposed on the beach

in front of the p lan t (J. F . Schreiber, J r . , personal com m un., 1975).

Samples from a deep p it near P elican Point and loca ted north of the beach

and 8 fee t (2 .4 m) below the surface contain whole Chione sh e lls and

sh e ll debris th a t show in itia l cem entation .

Beachrock occurs e lsew here along the c o a s tlin e s of the northern

Gulf of C a lifo rn ia . W alker and Thompson (1968) have described b each ­

rock outcrops near San Felipe on the Baja side of the gu lf. These ou t­

crops are on te rraces th a t are now w ell above sea le v e l. Radiocarbon

age determ inations show th a t th e se d ep o sits are probably o lder than

30,000 y e a r s .

12

Figure 2. Aerial photograph of beachrock e a s t of Puerto Pefiasco , looking north .

Note uniformity in s trik es of beachrock and c o a s tlin e . Building complex is the U niversity of Sonora—U niversity of Arizona D esa lin iza tion P lan t. Low-tide photograph by J . F . Schreiber, J r . , April 6 , 1974.

Figure 3. Aerial photograph of the Puerto Pehasco a re a , lookingw est

Paralle l bands along the shore are beachrock . The low h ill in the left is Punt a P ehasco . P elican Point is in the d is ta n c e , w ith the Cholla Bay tid a l fla t im m ediately north and to the right of P elican P o in t. Low -tide photograph by J . F . Schreiber, J r . , April 6 , 1974.

13

■- - i

' - tv ULX

Figure 2. Aerial photograph of beachrock e a s t of Puerto P ehasco , looking north

w estFigure 3. Aerial photograph of the Puerto Pehasco a rea , looking

14

T hickness

Total th ick n ess of beachrock v a rie s from p lace to p lace in the

a re a . Individual beds range from a few cen tim eters to as much as 1 .2

m (4 feet) th ic k . True th ick n ess of the beachrock on any one beach is

d ifficu lt to determ ine accu ra te ly due to the p resence of a sand cover,

the d istribu tion of which may be ex ten siv e ly a ltered by wave a c tio n .

Any storm may resu lt in the exposure or b lanketing of wide ex p an ses of

beachrock . The PI ay a Hermosa Beach area (fig. 4) is an ex ce llen t ex ­

ample of the v ariab ility in amount of sand cover. In early April 1974,

a th ick n ess of 52 fee t (15.9 m) of beachrock w as m easured . At tha t

tim e, le s s than a th ird of the beachrock w as covered , but subsequent

dredging of a harbor channel and construction of a seaw all ju s t e a s t of

the area of trav e rse a lte red loca l c ircu la tion p a tte rn s . By M arch 1975,

over h a lf of the beachrock w as b lanketed by san d .

A th ic k n ess of 56 fee t (17.0 m) has been m easured a t Norse

Beach (Sandy Beach) im m ediately w est of P elican Point and sam ples from

a sa lt-w a te r w ell a t the d e sa lin iz a tio n p lant ind ica te as much as 29 .5

fee t (9 m) of cem ented m aterial (J. F . Schreiber, J r . , pe rsonal com m un.,

1975). To d a te , 17 m eters is the g re a te s t th ic k n ess of beachrock known

in the Puerto Penasco a re a . What underlies the beachrock is unknow n.

D ips in a seaw ard d irec tion of the above-m entioned sec tio n s

of beachrock are a ll in the range of 2 to 3 deg rees or approxim ately th a t

of the p resen t beach su rfa c e . B ecause of such d ip s , w idths of b e ac h ­

rock exposed a t low tid e are g re a t . For exam ple, 500 fee t (184 m) of

beach in front of the d e sa lin iz a tio n p lan t are covered w ith beach rock .

On the PI ay a Hermosa B each, the beachrock and in term itten t sand

15

Figure 4. Aerial photograph of Playa Hermosa B each, looking sou theast

Note ex tensive sand cover on beachrock in lower left of photo­graph. Beachrock is absen t on the point formed by Punt a Pehasco but can be seen as a dark brown strip along the shore e a s t of Punta P ehasco . Low-tide photograph by J. F . Schreiber, J r . , April 6, 1974.

16

blanket cover a w idth of about 1,980 fee t (600 m ). The w idth of b each -

rock exposed at low tide a t the mouth of E stero M am a is about three

tim es th a t exposed at the d e sa lin iza tio n p lan t b each .

There is more cem ented m aterial below the low est low -tide

lev e l. C onversations with lo ca l fisherm en ind ica te th a t hard ground ex ­

tends for some d is tan ce o ffshore . This m aterial could be beachrock , but

sam pling d ifficu lties lim ited th is study to the in te rtid a l zone .

C om position, S ize , and Sorting

Beachrock has been described as being com posed of w e ll-so rted

sand or sh e ll fragm ents th a t correspond c lo se ly to the com position of

the ex is ting beach sand in the a r e a . Such beachrock outcrops are mo­

notonously uniform . M abesoone (1964) described ree fs in Brazil th a t are

composed of a w e ll-so rte d quartz sand cem ented w ith c a rb o n a te . Another

study (Allen and o th e rs , 1969) em phasized th a t the ac id -in so lu b le m ate­

ria l w as w ell so rted w ith litt le or no fine-g ra ined m atter. Moore (1971)

lis te d percen tages of various com ponents of beachrock on Grand Cayman

Is la n d , B ritish W est In d ie s , and noted the sim ila rity to p resen t beach

sed im ent. T ietz and M uller (1971) s ta te d that the g rains of the b each ­

rock are of the same m ateria l and grain s ize a s the surrounding sed im ent.

The beachrock in the Puerto Pehasco area is variab le in com ­

p o sitio n , grain s iz e , and so rtin g . There are d ifferences betw een v e r­

tic a lly consecu tive un its as w ell a s la te ra lly w ithin the same u n it.

Table 1 show s the litho log ic changes th a t occur w ith in the beachrock

exposed a t the Playa Hermosa B each. As a ru le , m ost of the beachrock

along s tra ig h t, h igh-energy b each es is w ell so rted , but there are

17

Table 1. L ithologic d escrip tio n of beachrock m easured a t the Playa H er- mosa B each, April 1974

U nit T h ick n ess ,N o. D escrip tion fee t m eters

11 C a lc iru d ite , fo s s il ife ro u s , poorly sorted ; c la s ts are predom inantly broken m olluscan fragm ents (espec ia lly pelecypods) with le s se r am ounts of rock fragm ents, quartz g ra in s , co ra l frag ­m ents ................................................................................ 4 .5 1 .4

10 C a lc iru d ite , very poorly sorted; numerous whole pelecypod sh e lls and large sh e ll fragm ents p resen t; abundant s a n d -s iz e , w ell-rounded quartz g rains with a few rock fra g m e n ts ............ 10 .0 3 .1

9 C a lc a re n ite , m oderately sorted ; abundant sh e ll hash w ith no obvious whole sh e lls ; abundant fragm ents of m o llu scs , c o ra l, and worm tu b e s . 7 .0 2 .25

8 C a lc a re n ite , w ell sorted; m olluscan she ll hashand abundant w ell-rounded quartz g rains . . . . 12 .0 3 .7

7 Sand c o v e r............................................................................. 10 .0 3 .2

6 C a lc iru d ite , poorly so rted , bimodal d istribu tion ; abundant whole T urritella sh e lls and other whole m olluscan sh e lls ; abundant sh e ll hash and quartz g r a i n s ........................................................ 0 .5 0 .2

5 C a lc iru d ite , m oderately sorted; abundant iron - rich pumice fragm ents and rock fragm en ts, sh e ll h ash and quartz g ra ins; no whole sh e lls o b v io u s ................................................................................. 0 .5 0 .2

4 C a lc iru d ite , poorly sorted; whole pelecypodsh e lls and large sh e ll fragm ents; poorly sorted quartz g ra in s , sh e ll fragm en ts, and rockf r a g m e n ts ........................................................................... 1 .0 0 .3

3 C a lc a re n ite , m oderately sorted; abundant sh e llh a sh , quartz g ra in s , a few rock fragm ents . . . 1.0 0 . 3

2 Sand co v e r.............................................................. ... . . . . 5 .0 1 .5

1 C a lc a re n ite , w e ll-so rte d ; abundant w ell-rounded quartz g ra ins; minor am ounts of sh e ll hash and rock f ra g m e n ts .................................................................. 0 . 5 0 . 2

Total th ic k n ess 52 .0 15.9

18

exception to the ru le . The P lays Hermosa Beach is p ro tec ted from full

wave energy by Punta P en asco . Because the supply of terrigenous m ate­

r ia l is low , m ost of the beachrock c o n s is ts of predom inantly m olluscan

she ll fragm ents w ith minor amounts of coral d e b ris , quartz g ra in s , and

rock fragm ents.

A predom inance of terrigenous m ateria l, prim arily w e ll-so rted

quartz grains of sand s iz e , ch a rac te rize s the beachrock exposed a t the

mouth of Estero M am a e a s t of Puerto P ehasco . Based on v isu a l e s t i ­

m ates of th in se c tio n s , w e ll-so rte d quartz grains com prise as much as

66 percen t of the c la s t ic m ate ria l. The large amount of quartz over sh e ll

m aterial is due to the proxim ity of the Sonoita River th a t in term itten tly

em pties i ts supply of sedim ent into E stero M am a. The riv er flows

through a ra ther long (30 m ile s , 48 km) dune field before em ptying into

the e s tu a ry . Although flow is in te rm itten t, the supply of terrigenous

m aterial is enough to m ask the supply of carbonate sedim ent from off­

shore .

The beachrock of C holla Bay and Black M ountain promontory is

very poorly so rted . The ca lc im d ite is charac te rized by an abundance of

whole sh e lls of C h ione , G lvcvm eris, and Trachvcardium ; o ther p e le c y -

pods and gastropods are found a sso c ia te d w ith much finer sand and sh e ll

d e b ris . Rounded g ran ite pebb les are not uncommon, and many large

(about 1 phi) quartz g ra ins are p re se n t. The com position and sorting of

th is beachrock do not correspond to th o se of the p resen t b e a c h e s . To

the con tra ry , the beachrock resem bles m ost c lo se ly sed im ents found on

the ex tensive tid a l f la ts and in the lagoon of C holla Bay (M. W . R ose,

personal com m un., 1975).

V ariability is a lso no ticeab le in a d irec tion th a t is normal to

the c o a s t . In the Playa Hermosa se c tio n , d is tin c t un its can be reco g ­

nized on the b a s is of sorting or by the p resence of unusual c la s ts

(table 1). How ever, the d is tin c t un its cannot be traced la te ra lly for any

sign ifican t d is ta n c e . None of the un its of the Playa Hermosa sec tio n

can be recognized unequivocally a t the beach a t the d e sa lin iza tio n p lan t.

Relation to P resen t C oasta l Configuration

W ith a few notable e x c e p tio n s , the beachrock of the Puerto

Penasco area occurs as linear bod ies strik ing p a ra lle l to the loca l c o ast

and dipping 2 to 3 degrees seaw ard (fig. 5). This fac t suggests th a t the

co astlin e in the area has had approxim ately the same shape since the

time of form ation of the b each ro ck . The beachrock is a t p resen t in the

in te rtida l zone—the same environm ent hypo thesized for its form ation

la te r in th is th e s is . The location of the beachrock can be a ttrib u ted to

one of two p o ss ib le h is to r ie s : (1) th a t the beachrock formed prior to su b ­

sequent tecton ism and se a - le v e l changes and is p resen tly in the in te r­

tid a l zone by co incidence only or (2) th a t the beachrock has formed

re la tiv e ly recen tly and has alw ays been in the in te rtid a l zone .

In the volume on carbonate cem entation , Bricker (1971) in d ica te s

th a t sedim ents lith ified in the in te rtida l zone are not alw ays re s tr ic te d to

beaches in the s tr ic t geom orphic sen se and su g g ests the u se of the term

"in te rtida l ro c k ." This term might be more app licab le to some of the

lith ified sedim ents in the Puerto Penasco area than the term "b each ro ck ."

For exam ple, some of the beachrock near the mouth of E stero M an ia , lo ­

cated 6 m iles (9 .6 km) e a s t of Puerto Pefiasco , s tr ik es p a ra lle l to the

19

Figure 5. Gently dipping beachrock a t N orse Beach ju s t e a s t of Pelican Point

Strike of the beachrock is same as th a t of the c o a s t . Low -tide photograph taken by J . F. Schreiber, Jr.

Figure 6 . D iscordant beachrock body ju s t w est of entrance to Estero M ama

Strike of body in foreground is N . 45° E . , w hile strike of loca l co ast is N . 60° E. Photograph by J . F . Schreiber, Jr.

20

Figure 5. G ently dipping beachrock at Norse Beach ju s t e a s t of Pelican Point

Figure 6. D iscordant beachrock body ju s t w est of entrance to Estero Mama

21

co ast and d ips gen tly seaw ard . H ow ever, some beds have s tr ik e s th a t

show no apparent re la tio n to the p resen t c o as tlin e and dips are a s high

a s 30 degrees (fig. 6) . The ob liquely strik ing rock lie s ju s t to the w est

of the p resen t en trance of E stero M arua . Com position of the rock is

predom inantly w ell-rounded and w e ll-so rted quartz sand and sh e ll hash

cem ented by ca rb o n a te . Shape and strike of the ob liquely strik ing rock

bodies are sim ilar to sand bars a sso c ia te d w ith the p resen t mouth of the

e s tu a ry . The beachrock may rep re sen t the loca tion of an ea rlie r en trance

to Estero M arua . Lateral drift to the e a s t along the c o a s t has resu lted

in m igration of the mouth of the e s tu a ry .

The lith ified sedim ents of C holla Bay are another excep tion to

beachrock formed on beaches in the s tr ic t geomorphic s e n s e . These

rocks are very poorly sorted and con ta in whole gastropod and la rg e ,

a rticu la ted pelecypod s h e l ls . These sh e lls are not abraided and show

no signs of tran sp o rta tio n . The o ther p a rtic le s are much finer g rained

and are sim ilar to sedim ents found on the Cholla Bay tid a l f la t (M. W .

R ose, personal commun. , 1975). The lith ified sed im ents are not exposed

everywhere in C holla Bay, but they form iso la ted outcrops of very sim ilar

litho logy . One such outcrop forms a promontory th a t is about 2 m eters

above the surrounding sand cover. Large boulders of the rock lie a t the

b ase of the c liff and are being eroded by w aves (fig s. 7 and 8) . One

ge ts the im pression th a t the iso la ted exposures are the rem nants of a

cem ented expanse of t id a l- f la t sedim ents subsequen tly eroded by w a v es .

Figure 7 . Beachrock c liff , Adair Bay

C liff is 1 .75 m eters h igh . Black M ountain is the b a sa lt h ill in the background. Photograph by J . F . S ch re iber, Jr.

Figure 8 . W ave-cu t c lif f , Cholla Bay

Note ex tensive undercutting of the beachrock and boulders of beachrock in the foreground.

22

Figure 7. Beachrock c lif f , Adair Bay

Figure 8. W ave-cu t c liff , Cholla Bay

23

Erosion

Much of the beachrock in the Puerto Penasco area exh ib its p ro ­

nounced erosional fe a tu re s . Numerous tide pools occur in the expanse

of beachrock near the d e sa lin iz a tio n p lan t. M any su rfaces of beachrock

in the v ic in ity of C holla Bay (fig . 9) show a w ell-d ev e lo p ed karst topog­

raphy . The m ost strik ing ev idence for erosion may be se en near the

mouth of Estero M am a. W idespread d isso lu tio n h as taken p lace re s u l t­

ing in a h ighly irregular surface of cav itie s separa ted by kn ife-edge

w alls (fig. 10).

Revelle and Emery (1957) proposed a m echanism for in te rtida l

d isso lu tio n of beachrock resu lting in features sim ilar to those seen

around Puerto P en asco . There are very marked diurnal changes in the

a lk a lin ity -ch lo rin ity ra tio s of the w ater left in d ep ress io n s in the b each ­

rock a t low t id e . If some of the calcium d isso lved in se a w ater is o r­

g an ica lly com plexed or h yd ra ted , d isso lu tio n of calcium carbonate may

be favored under c e rta in co n d itio n s. Revelle and Emery (1957, p . 699)

s ta te d th a t "the tim e involved in the formation and d isso c ia tio n of com­

p lexes containing calcium is long re la tiv e to the time required to p re ­

c ip ita te free calcium ions or to obtain them from so lu tion of calcium

c a rb o n a te ." D isso lu tion is favored during a low tide occurring a t night

b ecause a drop in tem perature in c reases so lu b ility and re sp ira tio n by

anim als cau se s a drop in pH and an in c rease in carbon dioxide concen­

tra tio n . During a daytim e low tid e , the re s id u a l w ater is warmed and

p lan t pho to syn thesis re s u lts in a d ecrease in CO2 concen tra tion and an

in crease in pH favoring p rec ip ita tio n . The new c ry s ta ls of calcium

Figure 9 . Karst surface developed on beachrock , Cholla Bay

Dark color of the beachrock is due to an a lga l co a tin g . Photo­graph by J . F . Schreiber, Jr.

Figure 10. Chem ical erosion of beachrock , en trance to EsteroMania

Solution b a s in s are separa ted by knife-edged w a lls . Photograph b y j . F . Schreiber, Jr.

24

MaruaFigure 10. Chem ical erosion of beach rock , entrance to Estero

carbonate a re , for the m ost p a rt, flu shed out by the next high t id e ,

suiting in a net lo ss of m ateria l (Revelle and Emery, 1957).

CEMENTATION

Three d is tin c t cem ent fab rics can be recognized in the b each -

rock of the Puerto Pehasco area on the b a s is of g ro ss morphology of the

cem ent. An ac icu la r fabric c o n s is ts of elongate n eed les u su a lly more

than 20 p long and le s s than 5 p wide (fig. 11). A blocky fabric c o n s is ts

of equidim ensional c ry s ta ls ranging in size from about 5 p to over 200 p

(fig. 12). The third fa b ric , a m icritic c o a tin g , can be seen encrusting

both carbonate and noncarbonate g rains in a ll specim ens s tu d ied .

A cicular Cement

The ac icu la r cem ent w as determ ined to be aragonite on the

b a s is of c ry s ta l structure observed under the scanning e lec tron m icro­

scope (fig . 13). M icroprobe a n a ly s is of cem ents (table 2) y ie lded no

de tec tab le m agnesium or strontium in the a rag o n ite . D etec tion lim its

for the instrum ent used are on the order of one p e rcen t, thus strontium

may be p resen t in the aragonite in concentra tions of le s s than one p e r­

c e n t. Aragonite cem ent in beachrocks has been reported in many lo c a l­

itie s (Stoddart and C ann, 1965; A lexanders so n , 1972c, 1972d; M oore,

1973; D avies and Kinsey , 1973).

The aragonite cem ent is com posed of long need lelike c ry s ta ls

th a t are 1 to 5 p wide and up to 25 p long. The c ry s ta ls often form a

th ick fringe of cem ent tha t is u su a lly seen on sk e le ta l fragm ents or in

the in ternal c av itie s of gastropod sh e lls (fig . 14). O rientation of the

a c icu la r c ry s ta ls is uniformly rad ia l to the surface of the sh e ll. C av ities

26

Figure 11. Scanning e lectron m icrograph of acicu lar aragonitecement

Note rad ia l o rien ta tion of cem ent c ry s ta ls ; 500X.

Figure 12. Scanning e lec tron micrograph of b locky , equid im en- sional cem ent— 5,000X

27

Figure 11. Scanning e lectron micrograph of ac icu lar aragonitecement

Figure 12. Scanning electron micrograph of b locky , equidim en- sional cement

28

Figure 13. Scanning electron micrograph of randomly oriented aragonite cement

29

Figure 14. Petrographic thin section photomicrograph of arago­nite fringe

Radially oriented aragonite forms as a fringe around fo ss il frag­ments . The fringe is about 20 ju long. Aragonite occasiona lly fills the internal cav ities of fo ss ils as is illu stra ted in the gastropod shell in the upper right corner of the photograph. C ross n ic o ls , SOX.

30

Table 2. Scanning e lectron microprobe re su lts

Sample No. a Polymorph M agnesium

EM-1 aragonite (b)

EM-2 aragonite (b)

NB-1 ca lc ite p resen t

PH-2 c a lc ite p resen t

PH-3 c a lc ite p resen t

CB-9 c a lc ite p resen t

a . EM—Estero M am a; NB—Norse Beach; PH—Playa Hermosa;CB— C holla Bay.

b . Not d e te c te d . D etection lim its of instrum ent u sed are about one percen t concen tra tion .

are com pletely filled w ith the rad ia lly oriented aragonite o c c a s io n a lly ,

but it most often occurs a s a fringe lining c a v i t ie s .

F in e , random ly orien ted aragonite need les occur in in tergranular

pore spaces in sev era l specim ens. The need les form a mat of cem ent,

and reso lu tion of sing le c ry s ta ls is nearly im possib le w ith the petrograph­

ic polarizing m icroscope. H ow ever, w ith the scanning e lec tron m icro­

scope , the mat is seen a s an aggregate of a c icu la r c ry s ta ls of aragonite

th a t are randomly oriented (fig . 13). The su rfaces of early c ry s ta ls a c t

as nucleation s ite s for new er c ry s ta ls which are formed at the expense

of further growth of the in itia l c ry s ta ls . The resu ltin g mat of m aterial is

very sim ilar to aragonite produced by bubbling a ir through warmed sea

w ater (32oc) for severa l w eeks (Taylor and I llin g , 1969, p . 84).

31

Aragonite cem ent in the Puerto Pefiasco beachrock forms as the

d is tin c tly term inated c ry s ta ls seen in figure 13. However, in some lo ­

c a l i t ie s , such as the en trance to Estero M ania, e rosion is ram pant (fig.

10), and aragonite cem ent from th is loca lity show s evidence of chem ical

d isso lu tio n (fig. 15). The c ry s ta l faces are corroded and term inations

are in d is tin c t. Appearance of such d isso lu tio n supports the idea th a t

the erosion in the Estero M ania is predom inantly chem ical rather than

m echan ica l. A p lausib le exp lanation of in tertida l d isso lu tio n is d is ­

cu ssed by Revelle and Emery (1957) and in the ea rlie r sec tio n on erosion

in th is t h e s i s .

Blockv Cement

Abundant blocky cem ent is ch a rac te ris tic of many sam ples of

the beachrock in the Puerto Pefiasco area (fig. 16). On the b a s is of

sta in ing with A lizarin R ed-S , the blocky cement w as found to be c a lc i te .

D etec tab le amounts of magnesium were m easured w ith the scanning e le c ­

tron microprobe (table 2). The lower detection lim it of the microprobe is

above one percent e lem en tal concen tra tion . If m agnesium is p resen t in

concentra tions of over 1 .15 p e rc e n t, then m agnesium carbonate com­

p rise s a t le a s t 4 percen t of the to ta l c a rb o n a te . H igh-m agnesium c a l­

c ite is defined to contain over 4 percen t MgCOg. H igh-m agnesium

ca lc ite is a common cem ent in many beachrocks (Taylor and I llin g , 1969;

A lexandersson, 1972a; S ie s se r , 1974).

In the Puerto Penasco a re a , high-m agnesium c a lc ite commonly

lines in tergranular c av itie s and som etim es com pletely f i lls the c a v itie s

in the beachrock (fig. 17). The tendency is for c ry s ta l s ize to in crease

Figure 15. Scanning electron micrograph of eroded aragonitecement

The w ell-form ed c ry s ta ls seen in figures 11 and 13 have been destroyed by chem ical d isso lu tion ; 2,000X.

33

The ca lc ite cem ent forms as equidim ensional c ry s ta ls tha t tend to increase in size toward the cen ters of c a v itie s . M agnesium ca lc ite is the most abundant type of cement present in the Puerto Pehasco beachrock; 1 ,0 0 OX.

34

Figure 17. P e tro g rap h ic th in section photomicrograph of h igh- magnesium ca lc ite cem ent and m icritic coatings

The dark line in the center of the picture is a m icritic calcium carbonate coating that is found on most c la s ts . The orig inal shell m ate­r ia l has been d isso lved and ca lc ite is filling the c a v ity . The micrite p reserves the original outline of the sh e ll. H igh-m agnesium ca lc ite c e ­ment surrounds both she ll fragm ents and the fe ld spar in the left of the photograph. Cross n ico ls; 150X.

35

toward the cen ter of the cav ity , and s iz e s over 200 are a tta in e d . How­

ever, the ca lc ite tha t in itia lly grows from the m icritic borders of sh e ll

fragm ents is sm aller, u su a lly 5 to 10 ji.

H igh-m agnesium c a lc ite not only forms as cem ent betw een

grains but a lso rep laces orig inal sh e ll m ateria l. Figure 17 illu s tra te s

that blocky ca lc ite does not d irec tly replace the organic carb o n ate . Par­

tia l cem entation of the fragm ents is followed by p refe ren tia l d isso lu tio n

of sh e ll m ateria l. M icritic coatings on fo ss il fragm ents are left u n d is­

tu rbed . Blocky c a lc ite is then p rec ip ita ted in the new v o id .

The c a lc ite is a lso sub jec t to chem ical d isso lu tio n resu ltin g in

ex tensive p itting of the su rfaces of c ry s ta ls (fig. 18).

M icritic Cement

F ine-g rained calcium carbonate cem ent coa ts most of the c a r­

bonate and noncarbonate g rains (fig. 17). The carbonate is le s s than 4 p

in s iz e and is recognized in th in sec tio n as a dark ou tline around g ra in s .

P resence of th is coating is a ttribu ted to the m ucilaginous su b s tan ce s e ­

cre ted by algae (Shearman and Skip w ith , 1965). Shearman and Skipwith

d isso lved carbonate g ra ins in 10% HC1 and found tha t m icroscopic out­

lin e s of the grains rem ained afte r the g rains them selves had com pletely

d isso lv e d . The outline m ateria l w as iden tified a s ac id -in so lu b le a lg a l

m ucus. Similar ghost o u tlines rem ained afte r carbonate grains in a th in

sec tio n of the Puerto Penasco beachrock were d is so lv e d .

The role of algae in cem entation is an unsolved q u estio n .

A lexanders son (1972b) say s th a t such an organic coating might help to

explain the absence of su b s tra te dependence in beachrock c e m e n ts .

36

Figure 18. Scanning electron micrograph of chem ical d is so lu ­tion of ca lc ite cement

D isso lu tion has resu lted in destruction of c ry s ta l faces and pitting of a ll su rfaces; 1,000X.

37

D avies and Kinsey (1973) do not think tha t a lgae p lay an im portant part

in ac tual cem entation but th a t mucus may act a s a trap for m icritic pa r­

tic le s tha t may la te r serve as nuc lea tion s ite s for ino rgan ica lly p rec ip ­

ita ted cem en ts. Taylor and Tiling (1969) believe tha t a lgae may help to

s tab ilize the su rfaces of in te rtid a l sedim ents and c rea te a favorable

situa tion for cem entation . Algal s ta b iliz a tio n of in te rtida l sand su rfa ce s ,

which vary from horizontal to a lm ost v e rtic a l, have been observed in the

Puerto Penasco area (J. F . S chreiber, J r . , personal commun. , 1975).

Figure 19 illu s tra te s th is phenomenon in the beachrock of Cholla Bay.

D istribu tion of Cement Fabrics

M icritic coatings are u b iq u ito u s. Carbonate grains and noncar­

bonate grains are c o a ted , and a ll sam ples stud ied con tain th e se c o a tin g s .

D istribu tion of aragonite and high-m agnesium c a lc ite is com­

p lex , as illu s tra te d by sam ples from the PI ay a Hermosa sec tio n (table 3).

The tendency is for h igh-m agnesium c a lc ite dom ination in low in te rtid a l

a reas ; tha t i s , a reas th a t are bathed w ith sea w ater tw ice a day every

day . Aragonite as the major cem ent type is re s tr ic te d more often to the

high in te rtida l a re a s— areas th a t are covered w ith sea w ater only a few

tim es each month and rece iv e sea w ater in the form of wave sp ra y . C e­

ment type does not seem to be dependent on su b stra te m ineralogy. Both

aragonite and high-m agnesium c a lc ite can grow on the edges of the same

sh e ll fragm ent. H igh-m agnesium ca lc ite is often the f irs t cem ent to form,

but any attem pt to e s ta b lish a defin ite sequence of even ts is te n u o u s . In

a few sam p les , aragonite c ry s ta ls are growing from a c a lc ite su b s tra te ,

but the occurrences are r a r e . In sam ples where aragonite is the dom inant

38

Figure 19. Algal s tab iliza tio n of in tertida l sedim ents

Note that the surface layer ac ts as a single unit even after b e ­ing cut by a kn ife . The top few centim eters are a light green due to the a lg a e . Photograph by J. F . Schreiber, Jr.

39

Table 3. D istribu tion of cem ent fa b r ic s , Playa Hermosa Beach sec tion

Sample Location Aragonite M agnesium C alc ite

PH-1 low in te r tid a la p re se n t, minor p re sen t. dom inant

PH-2 low in te rtida l p re se n t, minor p re se n t. dom inant

PH-3 low in te rtid a l p re se n t, minor p re sen t. dom inant

PH-4 low in te rtid a l p re se n t, minor p re se n t, dom inant

PH-5 low in te rtid a l p re se n t, minor p re se n t. dom inant

PH-6 tran s itio n a l p re se n t, minor p re se n t. dom inant

PH-7 tran s itio n a l p resen t p resen t

PH-8 tran s itio n a l p resen t p resen t

PH-9 high in te rtida l b p re se n t, dom inant p re sen t, minor

PH-10 high in te rtid a l p re se n t, dominant p re se n t. minor

PH-11 high in te rtid a l p re se n t, dominant p re se n t. minor

PH-12 high in te rtid a l p re se n t, only type iden tified

PH-13 high in te rtid a l p re se n t, only type iden tified

PH-14 high in te rtid a l p re se n t, only type iden tified

a . Low in te rtid a l— covered with sea w ater tw ice a day every d ay .

b . High in te rtid a l— covered only a few tim es each month for short periods of tim e .

cem ent ty p e , c a lc ite cem ent may be absen t or p resen t in only minor

am ounts. Aragonite is u su a lly p resen t in sam ples th a t are cem ented

predom inantly w ith c a lc i te .

A lexandersson (1972c) d is c u s se s the chem ical fac to rs important

in marine p rec ip ita tio n of aragonite and magnesium c a lc i te . He d e te r­

mined th a t the chem istry of the m icroenvironm ent— sm all carbonate

n ich e s— may be som ewhat d ifferen t from the chem istry of the open sea

w a te r. P recip ita tion of calcium carbonate is favored in sm all v o id s . He

a lso s ta te s th a t aragonite and c a lc ite may form sim ultaneously under

uniform general conditions and th a t the major physicochem ical factors

operative in open sea w ater do not determ ine the m ineralogy of the p re­

c ip ita te . He com pares carbonates p rec ip ita ted in in tragranular pore

sp aces favorably to cem ents formed in beachrocks and im plies th a t con­

d itions n e ce ssa ry for form ation of each are s im ila r.

40

CONCLUSIONS

Age

The a n a ly sis of two sam ples subm itted for radiocarbon age d e ­

term ination w as delayed due to m echanical d ifficu ltie s in the radiocarbon

laboratory . No concrete ages may be a ssigned to the Puerto Penasco

beachrock a t th is tim e. No modern a r t i f a c ts , such as b o ttle s and c a n s ,

were found cem ented in the beachrock to ind ica te th a t cem entation is

taking p lace a t p re se n t. To the co n tra ry , the dom inant p rocess a t the

p resen t seem s to be destruction of the beachrock .

There is evidence for an older time of form ation, p o ss ib ly la te

P le is to cen e . The fo ss ils iden tified by H ertlein and Emerson (1956) as

belonging to a la te P le is tocene fauna occur in a sso c ia tio n w ith beach ­

rock cropping out near the northw est tip of Pelican Point (J. F . Schreiber,

J r . , personal commun. , 1975). Beachrock a lso underlies unconso lida ted

sedim ents in the Cholla Bay lagoon th a t have been radiocarbon dated at

g rea ter than 30 ,000 B .P . (M . W . R ose , personal commun. , 1975). This

information agrees w ith the s e a - le v e l fluctuation curve computed by

M illiman and Emery (1968) th a t p laces sea leve l near i ts p resen t lo ca ­

tion about 35 ,000 B .P . If the beachrock w as formed during th is tim e,

then one would expect it to be in the in te rtida l zone to d ay .

Origin

The cem ent of the Puerto Penasco beachrock is aragonite and

high-m agnesium c a lc i te . The p resence of th e se cem ents a s opposed to

41

42

low-m agnesium c a lc ite strongly su g g ests a marine origin for the cem ent.

Fresh w ater is alm ost nonex isten t in the a re a . The clim ate is hot and

very aird and has probably been the same for a t le a s t 15,000 years and

p o ss ib ly much longer (Terah Sm iley, personal com m un., 1975). S ign ifi­

cant fresh -w a te r influence in the cem entation p ro cess would not have

been lik e ly . T herefore , the cem ent w as most like ly p rec ip ita ted from

sea w ater.

The location" of the environm ent of form ation cannot be e s ta b ­

lished unequ ivocally . The cem ent could have p rec ip ita ted from a sa line

w ater tab le some d is tan ce beneath the surface of the ground as sea level

fe ll from its high of 35,000 years ago (M illiman and Emery, 1968). This

origin might help exp lain why no observab le cem entation is occurring at

p re se n t. An a lterna te origin in which I concur is advocated by many con­

tem porary in v es tig a to rs (Bricker, 1971; G insburg, 1953; Stoddart and

G ann, 1965; A lexanders so n , 1972a; Schroeder, 1973). The a lterna te

origin c a lls for in te rtid a l evaporation of marine w aters and p rec ip ita tion

of aragonite and high-m agnesium c a lc ite during periods of low tide when

tem perature , C 02 concen tra tion , and pH favor p rec ip ita tio n . Algae con­

tribute to the cem entation by stab iliz in g surface sedim ents and by s e ­

creting a mucus th a t trap s fine-g ra ined carbonate th a t may promote

growth of inorgan ica lly p rec ip ita ted cem ent.

D iaqenesls

In itia l cem entation of beach sedim ents is favored by a mucous

coating th a t may a ttrac t very fine grained carbonate m ateria l which

serves as nu c le a t ion s ite s for growth of aragonite or m agnesium ca lc ite

43

cem ent. The polymorph p rec ip ita ted ev iden tly depends only on the con­

d itions of the m icroenvironm ent as both forms apparently may occur s i ­

m ultaneously . How ever, the magnesium ca lc ite occup ies m ost

cem en t-filled void sp aces and may p o ssib ly rep lace a rag o n ite . No

concrete ev idence for rep lacem ent of inorganic aragonite by c a lc ite w as

found, but th is fac t is not su rp ris in g , since replacem ent could destroy

orig inal s tru c tu re . Replacem ent of organic calcium carbonate is fairly

w idesp read . D isso lu tion of the o rig inal fo ss il m aterial p recedes in fill­

ing of the new void s p a c e . Aragonite and magnesium c a lc ite rep lace

organic ca rb o n a te , with c a lc ite rep lacem ent the more common. D isso lu ­

tion of cem ent is a lso common.

Influence of S ea -lev e l C hanges and Tectonism

If the Puerto Pefiasco beachrock is indeed P le is to c e n e , one

must ask why the beachrock is in te rtid a l a t p re se n t. Sea leve l has flu c ­

tuated over a w e ll-e s ta b lish e d curve since about 35 ,000 y ears ago

(Milliman and Emery, 1968). The Pehasco area is a te c to n ic a lly ac tive

province, and changes in e levation of the area cannot be ruled o u t. It

may be tha t the beachrock is in te rtida l today only b ecause of a se rie s

of c o in c id e n c e s . If the beachrock w as above sea leve l a t one tim e , the

clim ate w as arid enough to prevent any s ign ifican t a lte ra tio n to low -

magnesium c a lc i te .

If the Pehasco beachrock is of Holocene a g e , one may assum e

th a t it is in the same in te rtid a l zone in w hich it formed and has under­

gone no d ra s tic changes w ith re sp e c t to sea le v e l. R esults of the rad io ­

carbon age determ inations should support one view point or the o ther.

SUMMARY

The beachrock in the v ic in ity of Puerto P ehasco , Sonora,

M exico , is charac terized by aragonite and high-m agnesium c a lc ite

cem ent and is hypo thesized to have formed by in te rtida l evaporation of

sea w ater. The age of formation cannot be accu ra te ly assig n ed at

p re sen t.

Environments rep resen ted in the beachrock include geomorphic

b e ac h es , tid a l f l a t s , and barrier b a rs .

D iagenetic e ffec ts observed include rep lacem ent of fo ss il

m aterial by both aragonite and high-m agnesium c a lc ite and chem ical

d isso lu tio n of the cem ent i ts e l f . Algae were im portant in cem entation

by acting as a s ta b iliz a tio n medium for su rfaces and by contributing a

mucous coating th a t helped to promote cem en tation .

44

REFERENCES

A lexandersson, Torbj'orn, 1972a, M editerranean beachrock cem entation: marine p rec ip ita tion of M g -c a lc ite , _in S tan ley , D . J . (e d .) .The M editerranean Sea: a natural sedim entation laboratory: S troudsburg, P a . , Dowden, H utch inson , and R oss, p . 203- 223.

_________ 1972b, M icritiza tion of carbonate p a rtic le s : p ro ce sse s of p re ­c ip ita tio n and d isso lu tio n in modern shallow -m arine sedim ents: G eol. In s t . U niversity of U ppsala B u ll . , new s e r ie s , v . 3 , p . 201-236.

_________ 1972c, In tragranular growth of marine aragonite and M g-ca lc iteby p rec ip ita tion from supersa tu ra ted seaw ater: Jour. Sed. Petrology, v . 42, p . 441-460 .

_________ 1972d, Shallow -m arine carbonate d iag en es is as re la ted to thecarbonate sa tu ra tion level in seaw ater: P a leo n t. In s t . U niver­s ity of U ppsala Pub. 126, 10 p .

A llen, R. C . , G av ish , E liezer, Friedm an, G. M . , and S anders , J . E . , 1969, A ragonite-cem ented sandstone from outer con tinen ta l sh e lf off Delaw are Bay: subm arine lith ifica tio n m echanism y ie ld s product resem bling beachrock: Jour. S ed . Petrology, . v . 39, p . 136-149.

B athurst, R. G . C . , 1974, Marine d iag en es is of shallow w ater calcium carbonate sed im ents: Ann. Rev. Earth P lanetary S c i . , v . 2, p . 257-274 .

Bricker, Owen P . (e d .) , 1971, Carbonate cem ent: Baltim ore, M d .,Johns Hopkins P re s s , 376 p .

C ooray, P . G . , 1968, A note on the occurrence of beachrock along thew est co as t of Ceylon: Jour. Sed. Petro logy, v . 38, p . 650-654.

D av ies , Peter J . , and K insey, D . W . , 1973, Organic and inorganic fa c ­tors in Recent beachrock form ation. Heron Is la n d , G reat Barrier Reef: Jour. S ed . Petrology, v . 43, p . 59 -81 .

Emery, K. O . , and Cox, D . C . , 1956, Beach rock in the H aw aiian I s ­lands: P acific S c i . , v . 10, p . 382-402.

45

46Gayman, W . , 1969, W aves and wave ac tio n , in Thomson, D . A .,

M ead, A. R . , and Schreiber, J . F . , Jr. (e d s .) , Environm ental im pact of brine e ffluen ts on Gulf of C aliforn ia: U .S . D e p t. of In terior Research and Developm ent Progress Rept. 387, p . 56 -67 .

G insburg, Robert N . , 1953, Beachrock in south Florida: Jour. Sed. Petrology, v . 23, p . 8 5 -92 .

G reen, C . R. , 1969, M eteoro log ical co n d itio n s , in Thomson, D . A ., M ead, A. R . , and S chreiber, J . F . , Jr. ( e d s .) . Environm ental im pact of brine effluen ts on Gulf of C aliforn ia: U .S . D ep t, of In terior R esearch and Developm ent Progress Rept. 387, p . 7 -1 5 .

H enyey, Thomas L . , and B ischoff, Jam es L . , 1973, T ectonic elem ents of the northern part of the Gulf of C alifornia: G eol. Soc. Amer­ica B u ll . , v . 84, p . 315-330 .

H ertle in , L. G . , and Em erson, W . K ., 1956, Marine P le is tocene inver­teb ra te s from near Puerto P ehasco , Sonora, M exico: San Diego Soc. N atu ra l H istory T ra n s . , v . 12, p . 154-176.

Inm an, D . L . , Gayman, W . R . , and C ox, D . C . , 1963, L ittoral s e d i­mentary p ro ce sse s on Kauai, a sub trop ica l high island : P acific S c i . , v . 17, p . 106-130.

M abesoone, J . B. , 1964, Origin and age of the sandstone ree fs of Per­nambuco (northeastern Brazil): Jour. Sed . Petrology, v . 34, p . 715-726 .

M illim an, John D . , and Emery, K. O . , 1968, Sea level changes during the p a s t 35,000 years: S c ien ce , v . 162, p . 1121-1123.

M oore, C . H . , J r . , 1971, Beachrock cem en ts. Grand Cayman Is la n d , B .W .I . , hi B ricker, Owen P . ( e d .) , Carbonate cem ent: Baltim ore, M d ., Johns Hopkins P re ss , p . 9 -1 2 .

_________ 1973, In tertida l carbonate cem entation . Grand Caym an, W estInd ies : Jour. S ed . Petrology, v . 43, p . 591-602.

R evelle , Roger, and Emery, K. O . , 1957, Chem ical erosion of beach rock and exposed ree f rock: U .S . G eo l. Survey Prof. Paper 260-T , p . 699-709 .

R osenberg, D . H . , 1969, D istribu tion of tem p era tu re , s a lin ity , and oxygen, in Thomson, D . A ., M ead, A. R . , and Schreiber,J . F . , Jr. (e d s .) . Environm ental im pact of brine e ffluen ts on Gulf of C alifornia: U .S . D ep t, of In terio r R esearch and D e­velopm ent Progress Rept. 387, p . 16-38 .

R u sse ll, Richard J . , 1959, C aribbean beach rock o b se rv a tio n s: Annals of Geom orphology, v . 3, p . 227-236.

47

R u sse ll, Richard J . , 1962, Origin of beach rock: Annals of Geomorph­ology, v . 6, p . 1 -16 .

_________ 1963, Recent re c e ss io n of trop ical c liffy c o a s ts : S c ien ce ,v . 139, p . 9 -1 5 .

_________ 1971, W ater-tab le e ffec ts on se a c o a s ts : G eo l. S oc. AmericaB u ll., v . 82, p . 2343-2348.

_________ , and M cln tire , W . , 1965, Southern hem isphere beach rock:G eog. R ev ., January, p . 17 -45 .

Schroeder, Johannes H . , 1973, Submarine and vadose cem ents in P le is ­tocene Bermuda reef rock: S ed . G eology, v . 10, p . 179-204.

Shearm an, D . J . , and Skip w ith , Patrick A. d 'E . , 1965, Organic m atter in Recent and ancien t lim estones and its role in the ir d iagene­s is : N atu re , v . 208, p . 1310-1311.

S ie s se r , W illiam G . , 1974, Relict and Recent beachrock from southern Africa: G eo l. Soc. America B u ll., v . 85, p . 1849-1854.

Stoddart, D . R . , and G ann, J. R . , 1965, N ature and origin of beach rock: Jour. Sed . Petro logy, v . 35, p . 243-247 .

Taylor, J . C . M ., and Illin g , L. V ., 1969, H olocene in te rtid a l calcium carbonate cem entation , Q a tar, Persian Gulf: Sedim entology, v . 12, p . 69-107 .

T ie tz , G erd, and M uller, German, 1971, H igh-m agnesian c a lc ite and aragonite cem entation in Recent b each ro ck s , F uerteven tu ra , Canary I s la n d s , Spain , in Bricker, Owen P. (e d .) . Carbonate cem ents: Baltim ore, M d ., Johns H opkins P re ss , p . 4 -8 .

Trew artha, Glenn T . , 1954, An in troduction to c lim ate: New York, M cG raw -H ill Book Company, Inc .

W alker, Theodore R . , and Thompson, Robert W . , 1968, Late Q uaternary geology of the San Felipe a re a , Baja C alifo rn ia , M exico: Jour. G eology, v . 76, p . 479-485 .

W iedem ann, Hartmut U . , 1969, Solutions in in te rtid a l and sup ra tida lenvironm ents of modern carbonate sedim entation: th e ir im plica­tio n on d iag e n es is : Chem . G eology, v . 4 , p . 393-409 .

W inland, H . D a le , 1969, S tab ility of calcium carbonate polymorphs in warm, shallow seaw ater: Jour. Sed. Petro logy, v . 39, p . 1579-1587.

0 5 5? >0t