TRE PACIFIC SCrnCE BOARD

ATOLL

R E S F a B BULLETIN

---

No. 104

Carbonate sediments

of

Half

Moon Cay, British Honduras by

D.

R. Stoddart

Issued, by

TRE PACIFIC

SCrnCE

BOARD

National

Academy

of Sciences--National Research Council Washington,

D.

C.

September 30,

1964

Carbonate s e d i m e n t s o f H21f Moon Cay, B r i t i s h Zonduras

D. 2. S t o d d a r t

The r e e f i s l a n d s o f t h e B r i t i s h Honduras b h r r i e r r e e f and a t o l l s have been t h e s u b j e c t o f a s e r i e s o f r e p o r t s ( S t o d d a r t 1962, 1 9 6 3 ) , t o which t h e p r e s e n t paper i s e s s e n t i a l l y supplementary. Z a r l i e r p a p e r s were concerned w i t h t h e d i s t r i b u t i o n and a r e a l v a r i c t i o n o f t h e r e e f i s l m d s , a b o u t which l i t t l e w a s p r e v i o u s l y 'known. Napping o f topogra- phy, sediment d i s t r i b u t i o n , and g r o s s v e g e t z t i o n p a t t e r n s i n v o l v e d t h e zpproximate d e l i m i t s t i o n of s ' e c o l o g i c f i e l d u n i t s i i (Cloud 1952) f o r u s e i n c o n s t r u c t i n g f i e l d maps. A s i n o t h e r r e e f s t u d i e s , a primary d i s t i r c t i o n was made i n sediment d i s t r i b u t i o n between f i n e ("sand") and c o a r s e ( " s l i i n g l e " j s e d i m e n t s , which w i t h q u a l i f y i n g ud j e c t i v e s ( " f i n e " ,

" m e d i ~ r n ' ~ , "coarse") became t h e b a s i s of sediment mapping.

The d a t a r e p o r t e d h e r e r e s u l t from a n rlttempt t o d e f i n e t h e l i m i t s o f t h e s e sediment u n i t s i n q u c m t i t a t i v e t e r m s , and t h e r e s u l t s a r e o f i n t e r e s t p r i m a r i l y I n t e r m s o f t h e o r i g i n a l f i e l d re,or.ts.

They a r e n o t i n t e n d e d a s a c o n t r i b u t i o n t o t h e s t u d y o f c a r b o n a t e s e d i - ments as such. Because o f t h e l i m i t e d t i m e av::.ilable no s p e c i f i c sampling d e s i g n was s e t up, and s a n p l e s were simply c o l l e c t e d a l o n g t h e c o u r s e o f l e v e l l i n g t r u v e r s e s . G e n e r a l l y t h e s e were o r i e n t e d

normal t o t h e beach, and hence p a r a l l e l t o t h e g r e a t e s t sedunent g r a d i - e n t s . !The g r e a t v a r i a b i l i t y i n sediment p a t t e r n on most f o r e s h o r e a r e a s p r e s e n t s a mujor problem, which i n any subsequent d e s i g n would r e q u i r e s t r a t i f i e d random sainpling (Krunbein 1954, firurnbein and S l a c k 1 9 5 6 ) . The d e n s i t y o f szmpling p o i n t s a l s o v a r i e d i n v e r s e l y w i t h v e g e t z . t i o n cover: a t H a l f Moon Cay few samples were t a k e n from t h e d e n s e Cordia-Bursera bush a t t h e west end o f t h e i s l a n d . Because of t h i s v a r i a b i l i t y and t h e absence o f d e s i g n , no a t t e m p t i s made t o draw any c o n c l u s i o n s from t h e s e d a t a o n , f o r example, t h e r e l a t i v e i m p o r t a n c e o f c o n t r i b u t i n g organisms i n above-water sebiments. The a i m i s t o d e m o n s t r a t e t h e q u m t i t a t i v e s i g n i f i c a c c e o f t h e terms "sand"

and " s h i n g l e " as a p p l i e d t o B r i t i s h Honduras c a y s , and t o e x p l o r e t h e r e l a t i o n s h i p between sediment c a l i b r e m d o r g m i c c o n s t i t u e n t s i n t h e c o l l e c t e d s a n p l e s .

When t h i s work w a s i n p r o g r e s s , few p a p e r s had been p u b l i s h e d on r e e f - i s l a n d s e d i m e n t s , w i t h t h e n o t a b l e e x c e p t i o n o f t h e B i k i n i r e p o r t s

(Emery, Trncey and Ladd 1954, pp. 35-43). Tracey

,

Abbott and . irnow (1961, pp. 58-67) have rei:orted on t h e s e d i m e n t s o f I f a l u k ' i t o l l . . The p a u c i t y of t h e l i t e r a t u r e i s i n s t r o n g c o n t r a s t t o t h ~ t on underwater c a r b o n a t e s e d i m e n t s . ;5 major advance i n t h e s t u d y o f above-water c s r b o n a t e s e d i m e n t s h a s r e s u l t e d from t h e work o f Robert L. F o l k and a s s o c i a t e s i n t h e s o u t h e r n Gulf o f Ilexico. F o l k , Bayes and S h o j i (1962) have d e s c r i b e d c a r b o n z t e s e d i m e n t s froin Isla Mujeres, e a s t c o a s t o f Yucatan; C h a r l e s Hoskin (1963) h a s i n c l u d e d beach s e d i m e n t s i n h i s comprehensive s t u d y o f i i l a c r a n Reef s e d i m e n t s ; and F o l k h i m s e l f (1962) h a s t r e a t e d . t h e r e l z t i o n s o f s i z e and s o r t i n g i n c a r b o n a t e beach s e d i - ments a t s e v e r a l Gulf o f I.iexico beach l o c a t i o n s . T!ae fundamental s t u d y

of Caribbean r e e f i s l a n d s e d i m e n t s i s t h a t o f F o l k and 2 o b l e s

(1964)

on t h e s e d i m e n t s o f I s l a F e r e z , .':lacran d e e f ; 1;rofessor F o l k k i n d l y allowed me t o r e a d t h e m n u s c r i p t of t h i s pailer w h i l e i n t h e f i e l d i n

1961

and some o f t h e d a t a r e p o r t e d h e r e supplement h i s c o n c l u s i o n s . F o l k a d h i s a s s o c i a t e s a r e working on f u ~ t h e r s t u d i e s o f o t h e r A l a c r a n i s l a d s e d i m e n t s .

P r o c e d u r e

Most o f t h e samples d e s c r i b e d were c o l l e c t e d

tn

1961 by t?.e w r i t e r and Stephen P. Murray a t Half FIoon Cay, L i g h t h o u s e Leef. I Z d f Floon Cny l i e s on t h e e a s t e r n (windward) r e e f f l a t of t h e L i g h t h o u s e Reef a t o l l , a t ' a p o i n t wliere t h e r e e f f r o n t c u r v e s s h a r p l y r r e s t v a r d s ( l i L a r e 1 ) . Tic?al r a n g e i s s m a l l , and t h e dominant waves Lire e ~ s t e r l y a n d n o r t h - e a s t e r l y , r e f r a c t i ~ g round t h e r e e f a n g l e t o appro:!ch t h e s o u t h s i d e o f t h e i s l : m d from t h e s o u t h e x t . .Tne i s l a n d i t s e l f i s 1100 y z r d s l o n g . I t i s h i g h e s t d o n s t h e s o u t h and s o u t h e a s t s h o r e s , where i n 1961 i t r e s c h e d a maximum h e i g h t o f 10 f e e t above s e a l e v e l . :he i s l z n d f a l l s n a t u r a l l y i ~ t o two ?arts. The e a s t e r n s e c t o r , b u i l t o f sand. e x c e p t f o r s h i n g l e r i d z e s a t t h e e x t e r n p o i n t , i s a r c u c t e i n shajx?, w i t h a s t e e p r i d g e 1 0 f e e t h i s h f a c i n g t h e s o u t h e a s t bag, t h e s u r f e x e f a l l i n g q u i t e s t e e 2 l y t o t h e n o r t h e r n s h o r e . T h i s p a r t o f t h e cey i s covered w i t h c o c c n u t s , w i t h l i t t l e :round v e g e t a t i o n . The w e s t e r n s e c t o r o f t h e cay i s still covered w i t h a d e n s e t h i c k e t o f v e g e L t i o n , mainly C o r d i a s e b e s t e n a and I3ursera simaruba. The s o u t h s h o r e i s formed by a s h i n g l e

---

r i d g e , d e c r e a s i n g i n h'eight--(in 1961) from ? f e c t n t t h e e a s t end t o

3

^{f e e t} a t t h e west. The cay s u r f z c e f a l l s g e n t l y from t h i s r i d g e c r e s t toward t h e n c r t h s h o r e , and i s b u i l t of sand and r o t t e n c o r d f r q m e n t s . Tne n o r t h s h o r e is sandy and s u b j e c t t o m v e a c t i o n o n l y d u r i n g w i n t e r u n o r l h e r s t ' ; t h e o f f s h o r e x e a i s covered w i t h T h e l a s s i a , v i t h o n l y a few s c a t t e r e d c o r a l p a t c h e s . Off t h e s o u t h s h o r e s , s t e e p waves e n t e r t h e s o u t h e a s t bay, which i s f l o o r e d w i t h s t r o n g l y ri:>..;leci sand w i t h l i t t l e l i v i n g r e e f . F u r t h e r w e s t , o f f t h e s o u t h s h i n g l e s h o r e , l i v i n g r e 2 f

~ p p r o a c h e s v i t h i n a few f e e t of t h e s h o r e i t s e l f . Cemented ber:.ch s a n d s a r e widespread i n t h e i n t e r t i d a l a r e z , b u t a r e n o t t r e a t e d i n t h i s p?..per; f o r a f u l l a c c o u n t o f t h e i s l e n d , w i t h c o n t o u r e d topo;;raphic maps, s e e S t o d d a r t 1962, pp. 64-77, F i k x r e s 3 - 3 4 , and S t o d d a r d

1963,

pp. 94-98, F i g u r e s

58-61.

I n a d d i t i o n t o t h e Half Eoon Cay samples, s a m l ~ l e s were a l s o t a k e n from s h o a l w a t e r a r e a s ( l e s s th3.n 2 fathoms) on t h e a t o l l s and b z r r i e r r e e f s , and i n t h e T u r n e f f e l a g o o n , a n d from a number of o t h e r i s l c n d s , c h i e f l y Hendezvous Cay and Cay Glory ( b a r r i e r r e e f ) , Northern Cay ( L i g h t h m s e ~ e e f )

,

Southwest Cays (Glover' s & e f )

,

some o f t h e c e n t r z l b a r r i e r r e e f l a g o o n cr?.ys, and a f e u mainland and b a r r i e r Beach s t a t i o n s . A l l t h e s e d i m e n t s c o n s i s t e d of c a r b o n a t e s , e x c e p t f o r q u a r t z s a n d s from t h e ma.in1-ad and b a r r i e r beach s t a t i o n s . The s a m s l e s t a k e n i n

1961

a t R e ~ d e z v o u s and H a l f Foon C2.y~ were r e p e a t e d a s f a r as i ~ o s s i b l e i n 1962, f o l l o w i n g major changes t o cay s e d i m e n t s m d t o p o p a p h y d u r i n g H u r r i c a n e H a t t i e i n 3 c t o b e r 1961. I n a l l 202 samples were s t u d i e d , o f which

155

a r e from Half Moon Cay.

D i f f e r e n t t e c h n i q u e s were n e c e s m r i l y adopted f o r s m d -and s h i n g l e s i z e m a t e r i a l . F o r S ~ T L : s i z e s e d i a e n t s , s u r f a c e scoop samples of a ~ ~ p r o x i - mately 750 grams were c o l l e c t e d , a n d t a k e n t o C a m b r i d ~ e f o r s i e v i n g w i t h a f i e s t of 13ril;ish 5l;andard s i e v e s , 2nd f o r microscopic i n s p e c t i o n t o d e t e r m i n e a p p r o x i n a t e o r g m i c c o c s t i t u t i o n . l.ppo::imate org;mic c o n s t i - t u t i o n was noted d u r i n g t h i s i n s p e c t i o i l , b u t no more r e f i n e d a n a l y s i s was c a r r i e d o u t . C u r m l ~ . t i v e fr e s u e n c ; ~ c u r v e s were c o n s . t r u c t e d from t h e s i e v i n g d a t a , and mean s i z e m d s o r t i n g v a l u e s comi.uted, u s i n g t h e methods recommended by McCammon (1962) :

$16 + @50 +

a84

16

Mean s i z e

-

-

3

-

1685

+

$95 - ^@5 - ^$15

@

S o r t i n g

-

5.4

S i z e v a l u e s a r e g i v e n i n p h i

(12;)

units (16 ⁼ - l o g D , where D i s p a r t i c l e d i a m e t e r i n r n i l l i a e t e r s ) f o r e a s e of h m d l i n g (Xrunbein and P e t t i j o h n 2

1938,

p. 8 4 ) . Zero

16

i s e q u i v a l e n t t o a > a r t i c l e s i z e of 1 m m ; s m a l l e r p a r t i c l e s i z e s a r e i n d i c a t e d by p o s i t i v e and l z r g e r by n e g a t i v e

16

v a l u e s .

16

p e r c e n t i l e s were r e a d f r m t k e culnulative c u r v e s .

Only sand s i z e s e d i m e n t s and small s h i n g l e up t o

-6jZi

could be t a k e n t o C m b r i d g e ; l a r g e r s h i n g l e and b o u l d c r s i z e s e d i n e n t s evade s t a n d z r d i s e d s i e v i n g t r e ~ t r g e n t . For c o a r s e s e d i m e n t s , t h e t e c h n i q u e i2.doptzd was t o m e a s w e t h e l o n g a x i s o f n o t l e s s t h a n 50 s a r t i c l e s i n t h e f i e l d , t o con- s t r u c t cumul.ative frequency c u r v e s from t h i s d e t a , a d t o c a l c u l z t e mean s i z e and s o r t i n g v a l u e s . S i n c e t h e c u m u l a t i v e f r e q u e n c y curves of sand sam2les e x p r e s s t h e q u a n t i t y of sand by weight i n each c l c s s i n t n r v a i r a t h e r t h a n t h e number of p a r t i c l e s r e t a i n e d by e a c h s i e v e , i t was d e c i d e d i n t h e c a s e of measured l o n g a x e s t c sum t h e l e n z t h s o f p a r t i c l e s w i t h i n e a c h cl;:ss i n t e r v a l ? a t h e r t h a n t h e numbers o f p w t i c l e s , a s do Folk a d Robles f o r I s l ~ l e r e z s h i n g l e . Folk and 2 o b l . e ~ c o n s t r u c t e d frequency c u r v e s by nu.mbers, and c o n v e r t e d t h e s e t o w e i g h t f r e q u e n c y d i s t r i b u t i o n s by addifig a.pproximately

0.516

t o t h e number val.ue ( i o l k 1362, p. 232).

I n t h i s c a s e F o l k was d e n l i n z mainly w i t h Acropora c e r v i c o r n i s s h i ~ ~ l e , i n which p m t i c l ? voluine and shape b e a r some s t a b l e r e l a t i o n s h i p t o l o n g a x i s meacurement

.

D i f f e r e n t r e l a t i o n s h i p s o b t a i n , however, when d e a l i n g w i t h o t h e r o r g a i s m s , such as g l o b u l a r o r hei;iis;>herical c o r a l s . For t h i s r e a s o n t h e sums of l e n g t h s fiiethod i s u s c d , s i n c e it i s probably more d i r e c t l y compc.rable o v e r t h e whole s h i n g l e r a g e t o t h e weight measurenents o f s a n d s . T h i s t e c h n i q u e f o r h a n d l i n g c o a r s e m a t e r i a l i s e x t r e m e l y u n s o p h i s t i c a t e d : i t s c h i e f m e r i t s a r e speed and e a s e of e x e c u t i o n under c o n d i t i o n s of d i s c o m f o r t . Owins t o t h e i n h e r e n t q u a l i - t i e s o f r e e f - d e r i v e d o r g a n i c c a r b o n a t e m a t e r i a l , t h e r e i s u s u a l l y no q u e s t i o n o f d e r i v i n , s i m p l e shape i2ararneters such a s t h o s e of Zingg (1935) o r n e e d and Folk (1958)

,

u s i n g t h r e e - d i m e n s i o n a l and o t h e r me&sureinents. The problem o f t h e q u a n t i t a t i v e d e s c r i p t i o n o f h i g h l y i r r e g u l a r p a . r t i c i e s such as r e e f - d e r i v e d s e d i m e n t s has y e t t o be i n - v e s t i g a t e d i n d e t z i l , and i s c o t d e a l t w i t h h e r e .

S i z e C h a r a c t e r i s t i c s 2nd Oriffir:

~ - - - - - - - - - - - - - - - -

The dominant c h a r a c t e r i s t i c o f r e e f i s l a n d s e d i m e n t s , talcen a s a s i n g l e p o p u l a t i o n , i s b i m o d a l i t y . F i g u r e 2 i n c l u d e s cunulc.tive f r e - quency c u r v e s f o r

115

sarnlJes from Half I'ioon Cay, c o l l e c t e d i n 1961.

The sampleo d i v i d e t h e m s e l v e s q u i t e c l e a r l y i n t o two f n m i l i e s : one w i t h a maan s i z e r a n g i n g from a b o u t -0.3g t o +l.O$, f a i r l y t i g h t l y grouped; t h e o t h e r , x i t h a r a t h e r wider ranp:e, f r o u a b o u t

-5

t o -10@, b u t s t r o n g l y c l u s t e r e d round

-8@.

These corres:;cnd, r c s p c c t i v e l y , w i t h t h e "sand" and " s h i n g l e and b o u l d e r s " o f th.e cay d e s c r i p t i o n s . There i s no a p l x r e n t g r a d a t i o n i n s i z e between t h e t u o . Soae sand s a p l e s c a n t a i n l a r g e r s h i n g l e f r a g m e n t s , a c c o u n t i n g f o r t h e opsn ended c u r v e s on t h e r i g h t of F i g u r e 2 , b u t t h e d i s t i n c t i o n between t h e two f a m i l i e s i s o t h e r w i s e abrui:t. To arr u n d e t e r n i n e d c:rtenL t h i s d i s t i n c - L i o n nay be due t o t h e two methods o f measuring sedi.rilents u s e d , b u t t h i s i s unli!cely t o be v e r y s i g n i f i c a n t . The c u r v e s sho1.d t h a t s e d i x e n t s l a r g e r

t h a n

-lo$,

^between

-5

a n d

-1g,

and f i n e r t h m +20 a r e uncomon on s a n d and s h i n g l e c a y s . F i n e s e d i m e n t s a r e probably i m p o r t a n t on mangrove i s l a n d s , b u t no samples were tslcen. T h i s s h a r p d i s t i n c t i o n between two t y p e s o f sedinient i s prim..:ry e v i d e x e o f ( a ) tine d i v e r s e o r i g i n o f cay s e d i m e n t s , and ( b ) t h e p a r t i c u l z r breakdown p a t t e r n s o f s e d i m e n t s o f o r g a n i c o r i g i n on cays. k t t h e same t i m e , t h e f l i s t i n c t i o n h i d e s o t h e r f i n e r v a r i a t i o n s w i t h i n e a c h of t h e major sedimentfamilies

,

v a r i a - t i o n s whi.ch, though e x p r e s s e d t o p o g r a p h i c a l l y and i n sediment c a l i b r e , can be u n d e r s t o o d o n l y by r e f e r e n c e t o t h e organisms from which t h e s e d i m e n t s d e r i v e . I n t h i s , cay s e d i m e n t s d i f f e r fundamentally from most beach m a t e r i a l s , such as t h e P l a c e n c i a q u a r t z s a n d s on t h e mainlaqd c o a s t .

Composition

Cay s e d i m e n t s a r e d e r i v e d from t h e wide aqd complex range of organisms o f many s h a p e s and s i z e s forming and. i n h a b i t i n g t h e l i v i n g r e e f and n e a r - r e e f environments, and s u b j e c t t o s o r t i n g and modifica- t i o n by marine a c t i o n . T h e o r e t i c a l l y , t h 2 s i z e and shape o f sediment p a r t i c l e s on c a y s nay be i n i t i a l l y as v a r i e d as t h o s e o f my r e e f organism, w i t h i n t h e range o f t h e t r a n s p o r t i n g c a p a c i t y o f t h e waves.

A t t r i t i o n by wzvee, s u b a e r i a l w e ~ ~ t i i e r i n g , o r b i o l o g i c a l means may pro- duce f w t h e r changes i n b o t h s i z e and s h a p e , t h e p r o d u c t s of which mzy ,

be s e l e c t i v e l y t r a n s p o r t e d and d e p o s i t e d by waves and c u r r e n t s . While g e n e r a l i z a t i o n i s , t h e r e f o r e , d i f f i c u l t , i t i s soon r e c o g n i z e d i n t h e f i e l d t h a t s e d i m e n t s of c e r t a i n s i z e s t e n d t o c o n s i s t l a r g e l y o r evec e x c l u s i v e l y o f s i n g l e organisms. Thus b o u l d e r m a t e r i a l ( l a r g e r t h a n

-s@)

c o n s i s t s of whole hemispberi.ca1. c o r a l c o l o n i e s , o f t e n d e p o s i t e d by storm wclvzs beyond t h e r e a c h o f normal wave a c t i o n and exposed t o chemical w e a t h e r i n g o n l y ; i n t e r m e d i a t e s h m g l e

(-8$)

c o n s i s t s o f fragmented Acropora c o l o n i e s , s m a l l e r c o r a l s and s h e l l s ; c o a r s e s a n d (C$) o f a l g a l f r a g m e n t s ; and medium sand (+0.5g) o f fragmented c o r a l p a r t i c l e s hnd c o 1 6 n i a l f o r a m i n i f e r a .

I n most o f t h e s e c a s e s , t h e c h a r a c t e r i s t i c s o f t h e sediment r e f l e c t d i r e c t l y t h e c h a r a c t e r i , s t . i c s of t h e dominant c o n s t i t u e n t orgmism. S i n c e each o r ~ a n i s m u s u a l l y h a s f a i r l y sliarply defined s i z e and shape charac- t e r i s t i c s , sediments which c o n s i s t l a r g e l y o f a s i n g l e organism have similar s i z e and shape c h a r a c t e r i s t i c s . Thus r e e f - f l a t c o l o n i e s of S i d e r e s t r e a and D i p l o r i a , growing t o d i a n e t e r s o f

2-3

f e e t , when p i l e d up by waves form a sediment i n t h e s i z e range

-9

t o

-lo$,

with rounded and hemispherical p a r t i c l e s . Acropora c e r v i c o r n i s , which when growing forms t a n g l e d t h i c k e t s with narrow c y l i n d r i c a l stems and branches, may be brcken down by waves i n t o c y l i n d r i c a l fragments up t o -7g

(5

i n c h e s ) l o n g and

-3.5

t o -4.5g d . i m e t e r , c l o s e l y following t h e o r i g i n d growth dimensions. Halimeda i s a bushy green a l g a with abundant p l a t e - l i k e segments up tm-ji n d i a m e t e r , b u t extremely t h i n ; when t h e $) segments a r e detached by wave a c t i o n and accun:ulcte t h e y form a sediment with a mean s i z e of 0 t o -2$. These groups of o r g m i s m s ;ire among t h e most abundant; o t h e r o r g a i s m s a l s o have c h a r a c t e r i s t i c sha2es and s i z e s , b u t a r e n o t u s u a l l y ? r e s e n t i n s u f f i c i e n t q u a n t i t y t o dominate t h e charac- t e r i s t i c s of a sediment population.

C o n s t i t u e n t o r g a n i c m a t e r ~ a l s c o n t r o l n o t only t h e s i z e c h a r a c t e r - i s t i c s of a sediment, b u t a l s o t h e s o r t i n g , o r measure o f t h e spread of g r a i n s i z e s p r e s e n t . I n sediments d e r i v e d from a s i n g l e o r g a n i c con- s t i t u e n t , t h e mean s i z e w i l l c l o s e l y r e f l e c t t h e s i z e o f t h e o r i g i n a l organism, and t h e s o r t i n g ~ d l l r e f l e c t t h e range of s i z e s p r e s e n t i n t h e l i v i n g population. I n g e n e r a l , s i z e s o f l i v i n g r e e f organisms tend t o

c l u s t e r f a i r l y t i g h t l y round a mem v a l u e , and t h e d e r i v e d s c d i n e n t shows small s c s t t e r about t h e mem s i z e , o r i s w e l l s o r t e d . T h i s a p p l i e s t o most sediments c o ~ p o s e d of a s i n g l e t y p e of o r g a n i c c o n s t i t u e n t . Selec- t i v e t r a n s : ~ o r t a t i o n by waves imposes a s ~ c o n d a r y s o r t i n g e f f e c t , b u t t h i s is a 7 9 a r e n t l y l e s s important t h a n t h e primary corngosition of t h e s e d i - ment. idhen a sediment i s composed o f more than one o r g a n i c c o n s t i t u e n t , each w i l l i n f i u l n c e t h e sediment c h a r a c t e r i s t i c s i n p r o p o r t i o n t o i t s abundance. Xence sarrples composed of c o r a l g r i t a i d Halimeda, o r Acropora s t i c k s and s m a l l e r c o r a l s , w i l l have a bimodal frequency d i s t r i b u t i o n , with poorer s o r t i n g as t h e dominance of a s i n g l e organism decreases. I n some samples, p a r t i c u l a r l y underwater samples, a s on t h e Turneffe lagoon f l o o r , many organisms a r e p r e s e n t and t h e s o r t i n g i s extremely poor. On c a y s , however, most sediments a r e formed from one o r two main c o n s t i t u e n t s , a r e moderately o r w e l l s o r t e d , and c l o s e l y r e f l e c t t h e s i z e and shape of t h e o r i g i n a l c o n t r i b u t i n ~ organisms.

I n B r i t i s h Honduras, t h e main sediiilent s o u r c e s and sediment-con- t r i b u t i n g organisms, i n o r d e r of d e c r e a s i n g s i z e , a r e as follows:

1. Conglomerate beachrock b l o c k s , dislodged from beachrock pavements by wave a c t i o n . Sxcept f o r some massive c o r a l s t h e s e form the l a r g e s t p a r t i c l e s , up t o -10g i n l o n g e s t dimension. T h e i r shape is ? l a t e - l i k e , and probably because f r a c t u r e i s conditioned by i n h e r e n t j o i n t weaknesses i n t h e beachrock, t h e fragments tend t o be of c l o s e l y s i m i l a r s i z e : t h e sediment i s t h u s extremely w e l l s o r t e d .

2. Large hemispheric c o r a l s of t h e genera Montastrea, D i p l o r i a , Sider- a s t r e a , e t c . These vary from

-8

t o

-lo$,

but t h e l a r g e r specimens a r e v e r y r a r e .

3.

Acropora palmata: a massive branched c o r a l ( e l k h o r n ) , i n which t h e b r a n c h e s b r e a k down i n t o s l a b s which e x c e p t i o n a l l y a r e up t o

-99

⁽²⁰

i n c h e s ) l o u g , b u t a r e n o r e u s u a l l y -7 t o

-89

(5-10 i n c h e s ) i n l e n g t h and up t o 2 i n c h e s t h i c k . T h i s i s more widespread i n rough w a t e r n i - e x , p a r t i c u l a r l y as a s h i n g l e r i d g e c o n s t i t u e n t on t h e a t o l l i s l a n d s .

4.

Gastro2od s h e l l s , 2 a r t i c u l z . r l y massive specimens of Strombus g i g a s , which a r e l o c a l l y v e r y abundant. They a . r s g e n e r a l l y found whole, with g r e a t e s t dimensions of a b o u t -7#, b u t mag b r e a k down under s e v e r e wive a c t i o n i n t o curved a d d i s c - s h a ~ e c l fragments.

5.

kc::opora c e r v i c o r n i s , one of t h e most im;:ortmt sedi:.ien+, p r o d u c e r s ar.6 perhaps t h e nos5 widespread c o n s t i t u e n t of s h i n g l e r i d g e s . Dimen- s i o n s of t h e o r i g i n a l colcny v a r y c o n s i d e r a b l y w i t h d e p t h and d e g r e c of exlmsure t o wive a c t i o n . I n esposed 1.ocations t h e b r a n c h e s a r e s t u b b y , massive and t i g h t l y c l u s t e r e d ; i n c z h \.later t h e c o l o n i e s ;*re open- Srailched and t a l l e r , The d i a m e t e r of b r a n c h e s i s normally g r e a t e r i n d e e i x r and r o u g h e r v a t e r , and i s l e e s t i n protectecl b a r r i e r r e e f a r e a s . For d a t a on growth form a t i:iacran, s e e K o m i c k e r and ijoyd 1962, pip. 667- 668. I n n o s t s h i n g l e r i d g e s

8.

c e r v i c o r n i s b r a n c h e s v,vy from

-5

t o -79 i n l e n g t h , b u t on p r o t e c t e d b a r r i e r r e e f c a y s such a s Robinson P o i n t and Wild Cane Cay t h e y r a r e l y exceed

-59.

6.

Pfedium h e m i s p h e r i c c o r a l s , a group wkich i n c l u d e s such s m a l l c o r a l s as S i d e r a s t r e a

-

rnclians, Dichocoenia, I s o p h y l l a s t r e a , P o r i t e s a s t r e o i d e s , small " o n t a s t r e a , e t c . , v a r y i n g i n s i z e Eron

-6.5

t o

-3@.

'2hese c o l o n i e s a r e g e n e r a l l y mixed with Acropora fragments i n s h i n s l e riclges.

7. Small c o r e l s such as P o r i t e s , F a v i a 2nd S i d e r a s t r e a , i n t h e s i z e r m g e

-5

t o

-69:

t h e s e a r e n o t i n d i v i d u a l l y i t n l ~ o r t a n - t sediment s o u r c e s .

8.

IYodular a l g a e , p a r t i c u l a r l y --... Gonioli.thon

-

b o e r g e s e n i i . G o n i o l i t h o n forms l o o s e ilodules, c y l i n c k i c a l i n shape w i t h a srxoo-bh o r maminillate s u r l a c e , a p t o

-6..5$

I n l e n g t h . G o n i o l i t h o n i s found l i v i n g on t h e e a s t r e e f s of Lighthouse a n d G l o v e r ' s Reef c a y s , where i t forms a prominent minor c o n s t i t u e n t of s h i n g l e r i d g e s . It i s p o s s i b l e t h a t f r a g e n t e d G o n i o l i t h o n i s i n p o r t a n t i n t h e sand f r a c t i o n , where i t h a s n o t y e t been i d e n t i f i e d .

9 .

Branching a l g a e , i n c l u d i n g t h e m a s s i v e l j ~ branched G o n i o l i t h o n s t r i c t u r n and t h e nore d e l i c a t e l i t h o t h a m n i u n ca1careu.g. These a r e foun6 i n a l l r e e f p r o v i n c e s , b u t a r e q u a n t i t a t i v e l y unimportant as sedii.;ent s o u r c e s . 1 0 . Halimeda f r o n d s , which a r e t h e main c o n s t i t u e n t of c o z r s e sand-s.

The s e g a e n t s a r e e a s i l y d i s t i n g u i s h e d by t h e i r shape a d dead w h i t e , chalky appearance. They a r e mainly retai:?ed by t h e Nos.

5

and 1 0 s i e v e s , have a maximum d i a m e t e r of a b o u t -29, and form s e d i m e n t s w i t h nedian d i a m e t e r ap;.,roximately 09. I n d i v i d u a l s e g n e n t s brealc i n t o two o r t h r e e fragments which a r e e a s i l y i d e n t i f i e d by shape and s t r u c t u r e . i n t h e P a c i f i c H a l i n e d a i s more i m p o r t z n t a s a s o u r c s of lagoon f l o o r s e d i m e n t s t h a n as a beach c o n s t i t u e l ? ; t ( s e e , f o r example, T r a c q , Abbott and ilrnow

1951,

p. 6.5).

11. Coral g r i t , fragments of c o r a l s no l o n z e r i d e n t i f i a b l e , c o n s i s t i n g of equant, g r e y i s h p a r t i c l e s , h i g h l y ~ o l i s h e d i n a r e a z of h i & wave a c t i o n b u t d ~ ~ l l on p r o t e c t e d beaches, .md i n v a r i a b l y nixed u l t h small amounts oI" r e d Homotrema a s a s e c o n d z y c o a s t i t u e n t . Coral g r i t is l a r g e l y r e t a i n e d by t n e lios. 22 and 30 s i e v e s , with nean s i z e s of

0.5

t o +I$, e x t e n d i n s down t o +2$. _{C o r d grit i s} t h e dominznt c o n s t i t u e n t o f medium and f i n e sanC.s.

1 2 . Small o r g a n i c d e b r i s found i n most smd s ~ ~ m y l e s , b u t i n d i v i d u a l l y and c o l l e c t i v e l y of sniall iri!ydx.nce. The g0u.p i n c l u d e s echinoderm s p i n e s and t e s t s , peneroplid foraminifer?., g a s t r o r o d and pelecgpod t e s t s , sponge spicul-es md f i s h bones. Foraminifera a s a group a r e n o t i m -

p o r t a n t i n cay s a n d s , by c o n t r z s t t o most P a c i f i c a t o l l s .

l Very f i n e sediments, +2@ and f i n e r , of u n i d e n t i f i e d o r i g i n and q u a n t i t a t i v e l y i r s i g n i f i c a n t , founc? i n many samples of f i n e snnd.

14. F i n a l l y , t3.e remains of many o t h e r reef-dwelling organisms which a r e thrown up on beacb.es and c o n t r i b u t e i n soae way t o cay sediments.

These i n c l u d e t h e remains of numerous gorgonians, sgonges and c r u s t a - ceans.

Breakdown

Fie16 o b s e r v a t i o n and sediment samples show t h a t o f t h e s e groups, t h r e e a r e dominant i n t h e formation of cay sediments:

1. Acropora c e r v i c o r n i s s t i c k s and similar s i z e c o r a l s i n t h e s h i n g l e c l a s s ;

2. Halineda segments i n t h e c o z r s c send c l a s s ; and

3.

Coral g r i t wi-th Ilomotrema i n t h e sand c l a s s .

The corresp:;ndence between organism and sediment c l l a r a c t e r i s t i c s is b e s t seen i n t h e c o a r s e r m a t e r i a l s : i n sedilvents f i n e r than Haliineda sand whole organisms become r e l a t i v e l y s c a r c e r and d e t r i t a l fragments more important. However, organisms d i r e c t l y i n f l u e n c e t h e c h a r a c t e r i s t i c s n o t only of ffprimaryif sediments ( t h o s e formed by simple s k e l e t a l accurnil- l a t i o n ) , but a l s o o f lfsecondaryll s e d i a e n t s f o ~ m e d by t h e breakdown o f whole s k e l e t o n s and t h e accumulation of d e t r i t a l fragments.

Many r e e f organisms do n o t break down g r a d u a l l y , g i v i n g a con- t i n u o u s sequence o f s i z e s , b u t s h a r p l y and suddenly, i n a manner determined l e s s by t h e rnechm-isms of e r o s i o n t h a n by t h e i n h e r e n t s t r u c t u r e and wealaiesses of t h e s k e l e t o n (h'entworth and Ladd 1931, pp. 9-12). Thus, l a r g e c o r a l heads do n o t become s m a l l e r and s m a l l e r under t h e i n f l u e n c e of mechanical o r chemical weathering, but a f t e r s u b a e r i a l exposure a r e observed t o f a l l a p a r t by r a d i a l f r a c t ~ l r e i n t o a number of ~ s y r m i d a l b l o c k s , which i n turn, d i s i n t e p a t e i n t o cuboidal

fragments one inch a c r o s s . These then break down d i r e c t l y i n t o gri't.

The p r o c e s s h-.s been s e e n i n Montastrea nnnulixi-G,

&.

c a v e r n o s a , L i p l o r i a c l i v o s a , and on one o c c a s i o n i n Colpophyl&i~ n z t m s . I n t h e c a s e o f Acro-,;or& c e r v i c o r n i s t h e i n d i v i d u a l b r a n c h e s f i r s t l o s e t h e i r c n l i c e s by mechanj.ca1 w e a t h e r i n g , t h e n b r e a k down d i r e c t l y i n t o g r i t . Iia1imed.a segments a 2 p a r e n t l y b r e a k i n t o two o r t h r e z f r ~ g m e n t s , and t h e n d i r e c t l y i n t o fine-sand o r s i l t - s i z e a r a g o n i t e p a r t i c l e s ( t h o u & a t I f a l u k i n l a g o o n s e d i m e n t s T r a c e y m d o t h e r s found evidence of g r a d u a l breakdown:

T r a c e y , ASbott and krnow 1 9 6 1 , p .

65).

2ach of t h e s e b r e d d o w n s t a g e s i s c o n t r o l l e d by f a c t o r s i n h e r e n t i n each p a r t i c u l a r o r g x i s m , and t h i s con- t r o l s t h e r e s u l t i n g sediment c h a r a c t e r i s t i c s t o a d e g r e e second o n l y t o t h a t of t h e o r i g i n a l whole s k e l e t o n s . The p r o c e s s of bre&down i s a l s o i m ~ ~ o r t a n t , a d i s l a r g e l y i n f l u e n c e d by t h e p o i n t o f lodgment of t h e p a r t i c l e s i n q u e s t i o n : l a r g e p a r t i c l e s lodged nea? tile t o p of t h e beach w i l l o n l y be a f f e c t e d bx c h e a i c a l breakdown, whereas t h o s e of t h e f o r e - s h o r e wil!. be a f f e c t e d by wave a b r a s i o n a l s o . Discontinuous breakdown i n cay s e d i m e n t s is e x p l o r e d i n g r e a t e r d e t a i l f o r

k .

c e r v i c o r n i s znd Halimeda by F o l k m d R o b l e s , b u t t h e r e i s much scope f o r d e t z i l e d i n - v e s z i g z t i o n i n t h e p r e c i s e node of b r e d c d o m i n o t h e r r e e f organisms on

Ma.ior Sediment T y ~ e s

T h i s s e c t i o n d 2 a l s with t h e s i z e , s o r t i n g and composition charac- t e r i s t i c s of s i x slain t y p e s of s e d i m e n t s found on t h e B r i t i s h Iionduras c a y s , p a r t i c u l a r l y on Iialf Pioon and Rendezvous Cays.

Bench s a n d s

Beach s a n d s a r e of two main t y p e s : c o r d g r i t sand, and Halimeda sand. These a r e composed. of d i f f e r e g t organisms, have d i f f e r e r - ; ; s i z e c h m a ~ f - e r i s ' i i c s , and a r e found i n d i f f e r e n t environ:;ients. C o r a l g i t sand c c l n s ~ s t s of e q u m t

,

rounded, s o ~ x e t i a i e s iio!.isheci p a r t i c l e s of c c r a l , no l o n g e r s p e c i f i c c i l l y i d e n t i f i n b l e

,

m d i ~ i o l l u ~ c m t e r i a l , and sv.bordinate amounts of r e d Hornetrema

----

1i;liich g i v e n p i n k i s h c a s t t o t h e send. Fioskin h a s s t r e s s e d t h e u s e f u l n e s s of Homotrema a s a n e n v i l - ~ m e n t a l i n d i c a t o r , s i n c e i t o c c u r s i n l i v i n g - r e e f a r e a s ( ~ i o s l i i n 1953). Echinoid q 3 i n e s and o t h e r m a t e r i a l s a r e p r e s e n t i n small a u a n t i t y , w i t h v e r y srnall amounts of f o r a m i n i f e r a . T h i s sand c o n c e n t r a t e s i n t h e ifo. 22 s i e v e (0.558) ; a t Half Iioon Cay ( ~ i g u r e

3 ) 16

c o r a l g r i t beacl: s a n d s had a mean s i z e of 0.09@, and nean s o r t i n g of

0.77g.

C o r a l g r i t s a n d s a r e found on more e ~ ~ ~ o s e d c c ~ y s , p a r t i c u l a r l y on t h e a t o l l s and s o u t h e r n S a r r i e r r e e f . The g r i t seems t o r e i r e s e n t a s t a b l e s t a g e i n t h e breskdoun of l a r g e r c o r d c o l o n i e s . Halirneda s a n d s a r e of s m a l l importance i n t h e s e ,:nore ex;:osed cays.

Haiimer?a --.-.- s a n d s , which illay c o n s i s t of up t o 100% Halimeda segments,

---

a r e c h i e f l y found on more p r o t e c t e d c a y s , p a r t i c u l a r l y on t h e n o r t 3 - _- c e n t r a l b a r r i e r r e e f between Erqlish and South ! l a t e r Cays, where s x d c o n s i s t s of b o t h c o r a l g r i t and B a . l i m . ~ d a , and c u r e i i ~ d i f i ~ e i k beaches a l s o o c c u r . P a t c h e s o f p u r e Halirnedn, o f t c n i n unsegnented ciurnps, a r e u s u a l l y

found h i g h on t h e f o r e s h o r e , where t h e y a r e lodged by a s e l e c t i v e s o r t i n g

p r o c e s s . F i g u r e

4

shows c u m u l a t i v e f r e q u e n c y c u r v e s f o r e i g h t Halimeda s a n d s a t Cay G l o r y , and F i g u r e

5a

c u r v e s f o r 11 Ealimeda s a n d s f o r Rendezvous Cay. I n F i g u r e

6 ,

s i z e i s p l o t t e d a p i n s t s o r t i n g f o r b o t h groups. The s a n d s t e n d t o c l u s t e r round a nzm s i z e of Ojd, w i t h a s o r t i n g v a l u e o f 0.6@ ( m o d e r a t e l y good t o good). Piem v a l u e s (unwei@ted a r i t h - m e t i c means) f o r each group o f s e d i m e n t s a r e :

Cay Glory

No. o f samples Mean s i z e Mean s o r t i n g

6

-O.12@ 0.64%

Rendezvous Cay

1961

11

Rendezvous Cay 1962 7

Halimeda segmeilts ay2ea;. more abundant i n n e a ~ s i l o r e s ~ n d s t h a n i n i n t e r i o r s a n d s , p o s s i b l y as t h e r e s u l t o f t h e g r e a t e r m o b i l i t y of flnt, ? l a t e y segments under wave a c t i o n , and t h e i r selective concentra-';ion i n t h e n e a r s h o r e zone. The importance of Balimeda h a s been n o t e d e l s e w h e r e on Caribbean beaches ( P u e r t o Bico: Van Overbeek and S r i s t 19Lt7; Jamaica:

S t e e r s and o t h e r s lg!iO), a d 170lk and Robles (

1 9 6 4

) e s t i m a t e t h a t t h i s a l g a forms 50-70;: of t h e s-mds of Is12 P e r e z , Hlacran. I n t h e P a c i f i c , however, Ealimeda i s riiost i r q o r t a n t as a s e d i n ~ e n t c o n t r i b u t o r on lagoon f l o o r s and i s n o t i m p o r t a n t on b e a c h e s ( ~ u n a f u t i : David and

Sweet 190Lt ; P i a r s h a l l I s l ~ m d s : Emery, Ti-acey and 1,add 1954 ; h p i n g a m a r a n g i : PicICee

,

Chronic and Leopold 1359).

Kcmy s a n d - s i z e c o n s t i t u e n t s o f cay s a n d s a r e i l l - a d z p t e d t o s i e v e a n a l y s i s because of t h e i r d e i ~ a r t u r e from s p h e r i c a l o r e q u z n t form. J e l l - rounded c o r a l g r i t a 5 p e a r s t c apyroach c1oses.L t o "normal:' q u a r t z s a n d s i n s h a p e , b u t o t h e r c o n s t i t u e n t s , s u c h as H a l i ~ e d a p l a t e s , e c h i n o i d s p i n e s , s p i n d l e - s h a g e d s p i c u l e s , rnollilsc f r a g m e n t s m d f o r a ! a i n i f e r a 1 t e s t s d-epart c o n s i d e r a b l y from q u m c form. G l a t e - l i k e a l g a l segments, f o r esLmi:le, b e a r no r e l a t i o n i n weight o r s e t t l i n g v e l o c i t y t o q u a r t z s p h e r e s o f com- p a r z b l e "diameter", and a r e l i g h t e r t l l a m a y " f i n e r " c o r a l g r i t s . T h i s i s e x p l o r e d i n some d e t a i l by F o l k and i i c b l e s : t h e l i m i t a t i o n s of t h e u s e of s i m ~ l e c u m u l a t i v e f r e q u e n c y c u r v e s d e r i v e d from s i e v e a n a l y s i s a r e t h u s 2;:parent.

Beach s h i n g l e

A l l t h e . i d a t a on beach s h i n g l e come frc:;~ R a l f 1-loon Cay, where s e d i - ments u p t o

-10s

d i a m e t e r a r e found, t h e c o a r s e s t formed by broken

conglomerate-pavement b l o c k s . The term 7 : s l ~ i n ~ l e " , n o t used i n t h e ilent- worth s c a l e , i s conmonlg used. by S n g l i s h w r i t e r s f o r c o h b l e s and g r a v e l and i s i m p r e c i s e l g d e f i n e d ; i t h e r e r e f e r s t o c o a r s e , h i g h l y ixmneable, i n - t e r l o c l c i n g c o r a l f r a g e n t s , d e r i v e d w i t h a n ~ i n i c ~ u n ~ o f breakage from c o l o n i e s o f l i v i n ~ c o r a l and o t h e r oi-gmisms, which a r e s t i l l s p e c i f i c a l l y r e c o g n i z a b l e .

Of

53

samples f o r which c u m u l a t i v e f r e q u e n c y c u r v e s a r e g i v e n i n F i s v r e 2 , t h e nlean s i z e i s -7.46g and mem s o r t i n g 0.53g. Such f i g u r e s c o v e r much v a r i a t i o n , and i n ; ; a r t i c u l : . r t h e d i f f e r e n l i a t i o n a t k l f I.= lovn - Cay o f beach s h i n g l e

into

zones d i s t i n ~ u i s h e d by s i z e and c o l o r v a r i a t i o n . The f o o t o f a s h i n g l e beach i s o f t e n formed by a discon- t i n u o u s zone o f c o a r s e , r o t t e d , y e l l o w c o r a l b l o c k s ; t h e f o r e s h o r e

i t s e l f by a low but continuous r i d g e of f r e s h , white, small c a l i b r e s h i n g l e ; a d t h e b e x h c r e s t by a zone of l a r g e , blackened c o r a l blocks.

The following t a b l e g i v e s mean s i z e of sediments i n esch zone a t four t r m s e c t s a c r o s s t h e beach on t h e souti1 shore o f X a l f i'loon Cay; cumula- t i v e frequency curves f o r t h e same s t a t i o n s a r e given i n Figure 7, For l o c a t i o n of scampling s t a t i o n s , s e e Figure

14.

Black zone (91) 7.42p( (85)

7 . 8 1 ($01

8.10pl ( 7 3 7.96g

White zone i90) 6,37 (84)

6.46

(79) 6,75 (74)

6.93

Yellow zone (89)

8.01 (83) 8.34.

(78) 8.05 (73)

8.40

F i g u r e s i n b r a c k e t s a r e sample numbers

The f r e s h c o r a l s of t h e h t e zone a r e c l e a r l y being c o n t i n u a l l y deposited by day-to-day wave a c t i o n : wave competence i s small, and t h e r e f o r e only small m a t e r i a l i s trans:;orted beachward. Necl~anical abrasion i s important i n t h i s zone. The l z r g e r bloclrs of t h e r i d g e c r e s t must dnte from inajor storms: mostly they c o n s i s t of little-damaged whole c o r a l s picked from t h e bottom and de2osited on the beach c r e s t : h e r e they a r e exliosed only t o chemical and b i o l o g i c a l breakdown, and n o t t o mechanical a c t i o n . The ridge-foot blocks probably accumulate by t r a n s p o r t a c r o s s t h e r e e f f l a t during bad weather periods. Between t h e easternmost most exposed s t a t i o n s (73, 74, 75) and t h e more protected western s t a t i o n s (89, 90, g l ) , t h e r e i s l i t t l e l a t e r a l v a r i a t i o n

yellow zones, i n d i c a t i n g approximately whole shore during storms; but c a l i b r e t i n u o u s l y from e a s t t o west, away from t o decreasing wave energy a s waves a r e and t r a v e l westwards,

i n c a l i b r e i n e i t h e r black o r equal wave competence along t h e

i n

t h e white zone decreases con- t h e seaward r e e f s , i n response r e f r a c t e d round the e a s t e r n p o i n t

I n a d d i t i o n t o c o r a l s , l a r g e gastropods such a s Strombus g i g a s a r e l o c a l l y important. A t two s t a t i o n s ,

HXC

21 a d HFiS

54,

Strombus s h e l l s were s e p a r z t e l y measured.

S t a t i o n Number Length Sr ead t h

HNC 21

40

6.25

3.9

i n c h e s

HMC 54

^{25 7 - 2 4.7 inches}

A t HNC

54,

of 100 measured pebbles and cobbles, 37 were whole Strombus s h e l l s . Pumice occurs i n smaller q u a n t i t i e s i n Balf Noon Cay s h i n g l e r i d g e s , and i s of f i n e r c a l i b r e :

a t HLiC

g l a , t h e mean diameter of 100 pumice pebbles was 0.96 i n c h e s , and the l a r g e s t cobble seen had maximum dimensions of

4

inches. l$o d a t a a r e a v a i l a b l e on t h e break- down of pumice pebbles, which probably proceeds by a t t r i t i o n of the pebbles and production of g r i t ; but c o l l e c t i o n of Strombus fragments i n an a r e a of s t r o n g wave a c t i v i t y suggests t h a t breakdown i s gradual, producing a wide range of p a r t i c l e s i z e s and shapes.

I n t e r i o r s c a d s

Away from t h e b e x h a s t h e c h a r a c t e r of cay sediments changes, c o l o r changing with humus f o r n a t i o n , and c a l i b r e with t h e breakdol~n of p a r t i c l e s and accusiulztion of f i n e s Curing t h e soil-forming p-ocess. A t Half Noon Cay soil-forination h a s proceeded f u r t h e s t where t h e v z g e t e t i o n cover is d e n s e s t . Almost a l l t h e beach sediments contain a c e g l i g i b l e s i l t

f r a c t i o n ( i . e . , f i n e r than +3.75@). Under t h e e a s t e r n coconut p l a n t a - t i o n a t Half i:ioon, s i l t content varies from 0.1 t o 0.65; while under t h e Cordia-Bursern woodland t h i s r i s e s t o 1

.O-3,.:.

The maximum s i l t f r a c t i o n of

4.95%

i s found a t HFIC

113

(Figure 14). The silt f r a c t i o n c o n s i s t s of brown m a t e r i a l which does nc$ d i s s o l v e i n d i l u t e H C 1 ; i t g e n e r a l l y a p p e m s i n s i e v e No. 22 and i n c r e a s e s i n r e l a t i v e q u a n t i t y tlmough No. 200; a t a l l s t e g e s i t i s q u i t e d i s t i n c t from t h e white c a r - bonate g r a i n s . The l a r g e r c o r a l grit g r a i n s can be seen t o be p i t t e d and i n f i l l e d . with t n i s m a t e r i a l .

Figure

3

compares camulative freauency curves f o r 1 6 beach and 26 i n t e r i o r sands a t Half Moon Cay, and Figure

8

p l o t s s i z e a g a i n s t s o r t i n g f o r both t h e s e s e t s of data. S i z e m d s o r t i n g means f o r t h e two groups a r e as fcllows:

Beach s a r ~ d s

Number Mean s i z e Mean s o r t i n g

16 -'XE@-

0 77@

I n t e r i o r sands 26 +O .44 0.78

There i s no d i f f e r e n c e i n s o r t i n g between t h e two groups, but t h e i n t e r i o r sands a r e considerably f i n e r than t h e beach sands. T h i s i s probably t h e r e s u l t of breakd.obm of s a d p a r t i c l e s by s o i l forming processes under a v e g e t a t i o n cover. On small cays without a v e g e t a t i o n cover, such as Cay Glory, t h e s e d i f f e r e n c e s between beach and i n t e r i o r sands a r e n o t found

.

Dune s a n 2

Dune sands n r e a s p e c i a l type of i n t e r i o r s a n d s , c h a r a c t e r i s e d by f i n e n e s s , but c o n s i s t i n g , l i k e beach and i n t e r i o r s a n d s , of c o r a l g r i t , Homotrema and mollusc fragments. Figure

9

g i v e s cumul~.tive frequency

curves f o r dune sands from Half Moon Cay (BPIC 5 5 ) , Ambergris Cay ( t h r e e samples)

,

and Northern Cay (Lighthouse ~ e e f )

.

These sands a r e all f i n e r than normal i n t e r i o r o r beach sands (compare Figure 3 ) , but t h e samples a t Northern Cay, where the dunes a r e most e x t e n s i v e , i s a l s o e x c e p t i o n a l l y w e l l s o r t e d (mean s i z e +1.59@; mean s o r t i n g 0.35@). The p o s i t i o n of t h e cumulative frequency curves compared with normal beach and i n t e r i o r s a d c u r v e s suggest t h ~ t t h e r e i s a r e a l d i f f e r e n c e between t h e two groups, which may be d i a g n o s t i c of o r i g i n i n c a s e s where t h i s i s u n c e r t a i n , as a t Ambergris Cay. However, t h e number of szmples c o l l e c t e d i s too small t o reach any conclusion: dune a r e a s a r e uncomxon on t h e B r i t i s h Honduras r e e f i s l a n d s .

H u r r i c m e d e p o s i t S

! & a l i t a t i v e d a t z on h u r r i c m e d e y o s i t s have a l r e z d y been ix-esented f o r a l a r g e number of i s l a n d s ( S t o d d a r t 1963). I n 1962,

follow in^

H u r r i c a n e H a t t i e , sand samples were taken on 2endezvous and k l f hoon Cays a t l o c a t i o n s sampled i n

1961

b e f o r e t h e h u r r i c a n e . It was n o t p o s s i b l e i n 1962 t o c a r r y o u t any m e a s u r e ~ i ~ e n t s of s h i n @ d e ~ o s i t s , and w h e r e , as a r e s u l t of t h e s t o r m , sand was o v e r l n i d with s h i n ~ l e , and v i c e v e r s a , no cotnpsrison i s p o s s i b l e . S m d - s i z e h u r r i c a n e de:;osits were l e s s w e l l s o r t e d t h a n normal beach d e p o s i t s , m d microscopic excmi- n a t i o n showed t h e presence o f many organisms, m i n l y g a s t r o ~ o d s ,

pelecypods and f o r a m i n i f e r a , n o t normally s e e n i n beach s a n d s . These f a i r l y uniform s a n d s were s p r e a d over l a r g e e r e a s of cay s u r f a c e s , and were n o t confined t o f o r e s h o r e a r e a s . They proba'olg o r i g i n s t e d i n r e e f - f l a t m n d de;:osi ts

,

which were trans;,orted landward and roughly s o r t e d by t h e storm waves. The f o l l o w i n g t a b l e summarizes changes a t Rendezvous and Half LOOP Cays 1961-2, and t h e Lendezvous Cay frequency c u r v e s f o r

1961

and 1362 a r e g i v e n i n F i g u r e

4.

F i g u r e 10 p l o t s mean s i z e and s o r t i n g of ?re- and p o s t - h u r r i c a n e sands a t E a l f Moon Cay.

Year Number liean s i z e Mean s o r t i n 4

Rendezvous Cay

1961

11 +O .04@ 0 74$

1962

7

-0 2 5

0.60

Half Moon Cay

1961 16

+0.09

0.77

1962 28 +0.69 0.92

The a p p a r e n t m o n a l y , t h a t Renclezvous s a n d s i n a p r o t e c t e d s i t u a t i o n are c o a r s e r t h a n t h e Half Moon s a n d s on a n exposed c a y , i s e x p l a i n e d by t h e f a c t t h a t t h e former a r e Halimeda s a n d s and t h e l a t t e r c o r a l g r i t s .

Unllerwa.ter s a n d s

By c o n t r a s t t o beach s a n d s , underwater s&m!s c o n s i s t o f a l a r g e range of g r a i n s i z e s corresponding t o l o c a l o r g a n i c p r o d u c t i o n , and i n t h e absence o f wave a c t i o n z r e p o o r l y s o r t e d . I n g e n e r a l , s o r t i n g w i l l be poorer i n more p r o t e c t e d l o c a t i o n s . F i g u r e 11 g i v e s cumulative frequency c u r v e s f o r sediments from t h e leeward r e e f f l a t , G l o v e r ' s Reef, from s o u t h e r n l a g o o n , T u r n e f f e I s l a n d s , and from t h e anchorage a t Rendezvous Cay, f o r which s i z e axd s o r t i n g a r e sum::larized i n t h e follow- i n g t a b l e :

Mean s i z e S o r t i n g

Glovert s Reef (Bushy S p o t ) -1-0

. ^%@

^{0 92@}

Turne f f e ( s o u t h e r n lagoon) +O

.55

2.13

Rendezvous Cay A 1 -0.80 0.70

Rendezvous Cay A2 +O .29 1

.03

S a r q l e s a r e q u i t e i n s u f f i c i e n t i n number t o make any d e t a i l e d comparison o f above and underwater s e d i m e n t a t i o n on t h e r e e f s ; f o r a v e r y d e t a i l e d s t u d y of mainly underwater s e d i m e n t a t i o n at >.lacran, s e e Hoskin (1963).

Sediments o f Half Moon Cay

Dcta f o r

155

samples a t Half Moon Cay a r e s u f f i c i e n t t o map t h e d i s t r i b u t i o n of sediment c l ~ a r a c t e r i s t i c s ( F i g u r e L!); t h e s n n p l e s are w e l l d i s t r i b n t a d e x c e p t i n t h e Cordia- 3 u r s e r a woodland ( F i L u r e 1 3 ) . A t t h e e a s t end of tlie c a y , c l o s e t o t h e s e -..ward r e e f s , an?! a z a i n a l o n g t h e s o u t h ' s h o r e , t h e beach i s b u i l t of s h i n g l e c o a r s e r t h a n

-6j8

b u t

g e n e i - i l l y l e s s c o a r s e than

-:@.

'These a r e both a r e a s of h i g h wc;ve e n e r g y , and h i & c o r a l r r o d u c t i v i t y c l o s e i n s h o r e . I n b o t h a r e a s t h e r e i s a

well-marked z o n a t i o n i n t o y e l l o w , w h i t e accl b l a c k z o n e s , w i t h d i s t i n c t i v e c a l i b r e . I n t h e ycllow and b l a c k zones c c l i b r e t e n d s t o be uniform

irrespective of l o c a t i o n a l o n g t h e beach, b u t g r a d l ~ t i o n i s found i n t h e white zone i n r e s p o n s e t o ch,anges i n wave energy. Two p a t c h e s of

-9

t o -10% sediment on t h e s o u t h e a s t beach a r e formed by accumulations of broken beach conglomerate s l a b s .

Beach sands a r e g e n e r a l l y f a i r l y c o a r s e (lj8 o r 0 . 5

m d ,

though f i n e r t h a n t h e Halimeda s a n d s of 2endezvous Cay. Over most of t h e cay s u r f a c e , s a n d s under coconuts a r e c o a r s e r (0.5 t o 1@) t h a n t h o s e under dense woodland ( 0 t o 0.5g). T h i s ijrobably r e s u l t s from wind-winnowing o f f i n e s i n t h e former a r e a , l e a v i n g a f a i r l y c o a r s e s u r f a c e l a g , and from i n c r e a s e d bre&.down by chemical and biomechanical means i n t h e

woodland. ' d i t h i n t h e woodland t h e s u r f a c e i s l i t t e r e d with o l d , blackened and r o t t e d c o r a l b l o c k s i n t h e s i z e r a n g e

-5

t o

-8%.

S o r t i n g v a r i e s l e s s r e g u l r l y . The c o a r s e r m a t e r i a l on t h e beaches i s i n ~ e n c r a l b e s t s o r t e d , w i t h v a l u e s a l o n g t h e s o u t h s h o r e and a t t h e e a s t p o i n t of l e s s t h a n 0.5%. E x c e l l e n t s o r t i n g i s found i n some s h i n g l e a l o n g t h e s o u t h s h o r e (0.29 t o 0.3$), i n dune s a n d s a t t h e s o u t h p o i n t

(0.26@), and i n conglomerate block eccumulations (0.21@). Bare sand under coconuts i s moderately s o r t e d , with v a l u e s r a n g i n g from 0.7 t o

C.85$,

i n c o n t r a s t t o beach s a n d s , which have a f a i r l y uniform s o r t i n g v a l u e of 0.5 t o

o.@.

Under t h e dense woodland, where t h e sand i s o f t e n mixed w i t h c o r a l b l o c k s and t h e sediments a r e bimodal, s o r t i n g i s p o o r , with v a l u e s i n e x c e s s of 1.0@. The s m d a l o n e i n t h e s e m e a s i s as w e l l s o r t e d as under t h e coconuts.

The v a r i a t i o n i n silt c o n t e n t h a s a l r e a d y been noted. S i l t ( f i n e r t h a n +3.75@) i s only a p p r e c i a b l e (more t h a n 1;;) under t h e Cordia-Bursera woodland, r e a c h i n g a maximm of n e n r l y

5;;:

^elsewhere i t i s n e g l i g i b l e .

For n o t e s on such e r r a t i c s e d i m e n t s a s brown l i m e s t o n e a d pumice, s e e S t o d d a r t (1362, pp. 105-106); t h e same paper a l s o d e s c r i b e s c o n s o l i - d a t e d sediments on Half Xoon Cay.

S i z e and S o r t i n q

blhen a l l v a l u e s of s i z e and s o r t i n g a t Half Ploon Cay a r e p l o t t e d a g a i n s t each o t h e r i n F i g u r e

15,

c e r t a i n broad r e l a t i o n s h i p s a r e seen.

The b e s t s o r t e d sediments a r e t h e c o a r s e s t a.nd t h e f i n e s t . T h i s i s v e r y w e l l s e e n i n t h e s h i n g l e s e c t o r , where s o r t i n g g e t s p r o g r e s s i v e l y b e t t e r

a s s i z e i n c r e a s e s frcm

-5

t o

-lo@.

The b e s t s o r t e d sediments ( s o r t i n g l e s s t h z n 0.5@ a r e t h o s e g r m p e d n e a r a mean s i z e o f -;(if. T h i s t r e n d i s p r o b a b l y p r r t l y due t o t h e narrowing r m g e o f organisms as s i z e i n -

c r e a s e s : t h e -8pl s e d i m e n t s c o n s i s t l a r g e l y o f mall. b r a i n c o r t l l s and Acropora palmata s l a b s . I n t h e s a n d - s i z e s e c t o r t h e s c a t t e r o f s i z e and s o r t i n g i s g r e a t e r , b u t t h e r e i s a tendency f o r s o r t m g t o i ~ i l p o v e with d e c r e a s i n g c a l i b r e from -I@ t o +2@. It h a s a l r e - d y been s e e n t h a t Iinlimeda s a n d s a t Zendezvous

Czy,

w i t h a m e a s i z e of O@, have a s o r t i n g v a l u e of O.@ ( F ~ g i r e 5 ) .

I I

A L Isla P e r e z , A l a c r a n , F o l k (1962) h a s demonotrs.tcc! a s l n u s o i d a l r e 1 a t i o n s : i i p b e t r ~ e z n s i z a a d s o r t i n g i n t h e s i z e r a n g e -3@ t o +2.5g, 1wiCL b e s t - s o r t ~ n g ;.:eaks a t mean s i z e s of

-3@,

0@ tin2 +2@. hch pe&

c o r r e s 2 o n d s w i t h t h e modal s i z e of a p r t i c u l ; n o r g a n i c constituent:

t h e

-3

t o

-4.g

pe& w i t h Acropora cervicor- b r ~ n c h e s , th e 0@ pea!; w i t h I i a l i n ~ c d a s a n d , arid t h e 2$ penli w i t h c o r a l g r i t . A t I s l ~ 1 - e r e z , s e d i - ments w i t h mean s i z e s between t h o s e which c o r r e s ~ c n d t o peaks f o r each p a r - t i c d a r o r g a n i c zroul) have Seen shown .Lo c o n t a i n rnorc t h a n on?

t y p e o f s e d . i w n t , w i t h c o r r e s ~ ~ o n d i n g l y s o o r e r s o r t l n g . I n g e n e r a l ,

Foliis s h y p o - t h e s i s i s f a l l ? conf irined by tlie B r i t i s h fIonduras d a t a , though d e t s i l s v a r y . me absence i n Honduras of sedi.:ents i n t h e -1 t o

-5@

r a n g e r u l e s o u t d i r e c t c o : ~ q a r i s o n w i t h F o l k s s cGrves, and i n t112 Honduras d a t a s o r t i n g v a l u e s a r e l e s s good. f o r b o t h Hnlirneda sand and c o r d g r i t t h a n a t Alacraii. ,The H a l f Xoon c o r a l g r i t ( m e m s i z e + l @ ) i o c o n s i d e r a b l y c o a r s e r t h a n t h e dominantly +2@ g r i t a t Isla

S o r t i n g v a l u e s f o r b o t h sand and s h i n g l e a t I k l f i;oon Cay have been p l o t t e d as cumulative f r e ~ u e n c y c u r v e s i n F i g u r e 1 6 , which may be compared w i t n F o l k ' s silai1z.r f i g u r e f o r Gulf of ~ . ; e x i c o c z r b c n a t e s a i d s (Foil<, 1962, p. 241). S h i n g l e i s c o n s i s t e n t l y b e t t e r s o r t e d t h a n sand (0.53@ a g a i n s t 0.74@) and t h e mean s o r t i n g o f a l l s e d i n e n t s i s C.Gl(if.

T h i s c o l i i p r e s w i t h 0.47@ f o r F o l k ' s c a r b o n a t e beacl: s a n d s . It i s i n t e r - e s t i n g t h a t s o r t i n g i s s o c l o s e l y comparable on a d j n c e n t b e a c h e s where c a l i b r e may v a r y o v e r t h e r a n g e from +2@ t o

-lo@

(% t o 1024 am). Sub- marFne d e p o s i t s a p p e z r t o be v e r y much more p o o r l y s o r t e d .

Conclusion

The d a t a r e p o r t e d h e r e a r e s u f f i c i e n t t o add some d e f i n i t i o n t o t h e c o n c e p t of e c o l c g i c a l f i e l d u n i t s s u g g e s t e d by Cloud, i n s o f a r as t h e s e have been used as a f i e l d s u r v e y t e c h n i q u e i n B r i t i s h Honduras.

S e d i r i ~ e n t s of d i f f e n t c a l i b r e s a r e shown t o have s h a r p l y d e f i n e d s i z e and s o r t i n g c h a r a c t e r i s t i c s , and a l s o t o be c h a r a c t e r i z e d by r e g u l x oi-ganic d e r i v a t i o c . Hence, tlie f i e l d u n i t s >can be r e a d i l y d e f i n e d i n t e r m s b o t h o f s i z e and c o m p o s i t i o n , t h e two a - t t r i b u t e s t o g e t h e r bein; d i r e c t l y r c - l a t e d . F u t u r e f i e l d mapping can t h u s u s e t e r m s such as c e r v i c o r n i s s h i n g l e , Ha1imed.a s a n d , and c o r a l g r i t s a d , w i t h t h e kEowlcdge t h a t t h e a t t r i b u t e s o f each group a r e f z i r l y s t a b l e i n t e r m s of s i z e , s o r t - i n g and corn;<osition, and a r e m u t u a l l y d i s t i n c t . Becsuse t h i s s u r v e y w a s n o t d e s i g n e d s p e c i f i c a l l y t o s t u d y t h e s e d i m e n t s , t h i s a n a l y s i s is c a r r i e d no f u r t h e r : a sampling p l a n could r e a d i l y be d e v i s e d t o t e s t t h e s e p o l n t s and e s t a b l i s h t h e i r s i g n i f i c a n c e .

The r e s u l t s a l s o i n l a r g e measure confirm t h e o r i g i n a l work by F o l k a t Xlacran and Isla Mujercs, where h e showed t h a t s i z e and s o r t i n g depend l e s s on m v e energy t h a n on o r g m i c composition. Vave energy

c o n t r o l s n e a i s i z e ( i .e. t h e t y p e of organism p r e s e n t ) , and t h e t y p e of orgm.isrn t h e n c o n t r o l s s o r t i n g .

m e r e i s c l e a r l y f u - r t h e r scope f o r ( a ) s p e c i f i c d e s i g n f o r f u r t h e r sediment s t u d y both above and helow water on t h e B r i t i s h Honduras c a y s , t o t e s t t h e r e l a t i o n s h i p s of s i z e , s o r t i n g and o t h e r sediment charac- t e r i s t i c s t o o r g m i c composition and wave energy, x i d t h e n a t u r e of t h e i n t e r l o c k s between them; ( b ) f o r e x p e r i m e n t a l s t u d i e s on breakdown under wave a c t i o ~ o f d i f f e r e n t t y p e s of o r g m i c c a r b o n a t e s k e l e t o n s ; a r d ( c ) f o r s t u d i e s of t h e e f f e c t o f d i v e r s e s i z e and shape c h a r s c t e r i s t i c s on n o b i l i t y under x a t c r ziid on tlic f o r e s h o r e . To a l a r g e e x t e n t , t h e s o l u t i o n of t h e s e problems i n v o l v e s a f o u r t h , ( d ) t h e d e r i v a t i o n of rnemingful measures of s i z e a.nd shape f o r r e e f - d e r i v e d c a r b o n a t e p a r t i c l e s .

References

Cloud, P.E., Jr. 1952. P r e l i m i n a r y r e i ~ o r t on geolozy and marine environ- ments of Onotoa A t o l l , G i l b e r t I s l a n d s . A t o l l d e s e a r c h B~i11.

2: 1-73.

David, T.II.E., and G. Sweet. 1904. The geology of F u n a f u t i . F u n a f u t i Report, Zoyal S o c i e t y , 61-124.

I h e r y , K.O., J.I. Tracey and H . S . Ladd. 1954. Geology of B i k i n i and nearby a t o l l s . U.S. Geol. Surv. P r o f . Paper 260-A.

F o l k , B.L. 1962. S o r t i c g i n some c a r b o n a t e beaches of Fiexico. Trans.

New York Acad. S c i . S e r . 11,

2:

222-244.

F o l k , H.L., K.O. :]ayes and i?. S h o j i . 1962. Carbonate sediments of I s l a ivlu j e r e s , Q u i n t a n a Roo, Elexico

,

and v i c i n i t y . New O r l e a n s G e o l o g i c a l S o c i e t y , Yucatan P e n i n s u l a F i e l d T r i p Guidebook, 85-100.

F o l k , R.L., and R. Robles. 1964. Carbonate s a n d s of I s l a P e r e z , Alacran Reef, Pucatan. J o u r . Geol.

72

(3) : 255-292.

Hoskin, C .ivi. 1963. Ilecent c a r b o n a t e s e d i ~ i ~ e n t c t i o n on k l a c r a n Reef, Yucatan, iblexico

.

N a t i o n a l i;.caderny of Sciences-National Aesearch Council, F o r e i g n F i e l d ilesearch Proli;ram, Eeport No.

2:

i - x i i , 1-160.

Kornicker, L.S., and D.!d. 3oyd. 1962. Shallow water ~ e o l o g y a i d environ- ments of Alacran r e e f complex, Campeche Bank, Mexico. B u l l . kmer.

Assoc. P e t r o l . Geol. 46: 640-673.

Krumbein, W.C. and F.J. P e t t i j o h n .

1938.

Nanual of sedimentary p e t r o - graphy. New Yorlc, i-xiv, 1-549.

Krumbein, (J.C. and 3.A. S l a c k .

1956.

R e l a t i v e e f f i c i e n c y of beach sampling methods. Beach 2 r o s i o n Board, Tech. Mem. 90: 1-43,

-

A1-A9.

McCammon, R.B. 1962. E f f i c i e n c i e s o f p m c e n t i l e measures f o r d e s c r i b i n g t h e mean s i z e md s o r t i n g o f s e d i m e n t a r y p a r t i c l e s . J. Geol.

70:

453-465.

McKee, E.D., J. Chronic, .and

i.B.

Leopold.

1959.

Sedimentary b e l t s i n lagoon of Kapingamarangi a t o l l . S u l l . Arner. Assoc. P e t r o l . Geol.

43:

501-562.

Sneed, E.D. and R.L. Folk.

1958.

P e b b l e s i n t h e lower Colorado R i v e r , Texas. A s t u d y i n p a r t i c l e rnorphogenesis

.

^J

.

^Geol

. ⁶⁶

^{: 114-150.}

S t e e r s , J . A . , V . J . Chapman, J. Colman and J.A. Lofthouse. 1940. Sand c a y s and rnmgroves i n Jamaica. Geog. J.

36:

3 5 - 3 2 8 .

S t o d d a r t , D.2. 1962. !Diree Czribbean a t o l l s : T u r n e f f e I s l a n d s , Light- house Reef and G l o v e r ' s Reef, B r i t i s h Konduras. A t o l l Aesearch

BUL

87: 1-151-

S t o d d a r t ,

D.B.

1963. E f f e c t s of Huri-icme H a t t i e on t h e 3 r i t i s h Iionduras r e e f s and c a y s , October 30-31,

1961.

A t o l l Research B u l l .

95:

1-142.

T r a c e y , J.I., D.P. Abbott, and

T.

Arnow.

1961.

N a t u r a l h i s t o r y of I f a l u k a t o l l : p h y s i c a l environment. B e r n i c e P. Bishop Nuseum B u l l . 222:

1-75.

Van Overbeek, J . , and R.Z. C r i s t . 1947. 'The r o l e of a t r o p i c s 1 green a l g a i n beach sand formation. Amer. J . Botany, 34: 299-300.

Wentwortli, C.K., and I-I.S. Ladd.

1931.

P a c i f i c I s l a n d sedirnexts.

U n i v e r s i t y of Iowa, S t u d i e s i n N a t u r a l X s t o r y ,

13

( 2 ) : 1-47.

- .

.

Zingg,

T.

1935. B e i t r a g z u r S c h o t t e r a n a l y s e . Schweiz. m n . u. P e t . b ? i t t

. ¹⁵

^{: 39-140.}

SEDIMEM SAMPLE L M S A T ~ N

--- THREE FATHOM LINE

Figure 1. B r i t i s h Honduras: l o c a t i o n of a r e a s sampled

Figure 2. Cumulative frequency curves f o r

115

Half Moon Cay sediment samples