NO. 196.
SUBMARINE CEMENTATION OF GRAINSTONE FABRIC, ST. CROIX, U.S. VIRGIN ISLANDS
by
Lee C. Gerhard
Issued
by
THE SMITASONIAN INSTITUTION Washington, D.C., U.S.A.
February
1977SUBMARINE CEMEN'$'KfION 611'
B:I<,!i;BNS'~BBiYICFABRIC,
ST.CRBBBX,
il1.S. VIltGlMT liSB,iSNlDSSubmarine cemented carrbonate sand i?oGu:I.i.i; occur o:ff t h e n o r t h e a s t c o a s t of S t . C r o i x ,
U.
S. V i r g i n 1sla.n.ds. Cemeniation o c c u r s a s a 50 micron t h i c k r i m of a r a g o r ~ i t e ncc:iles ovcr: ;I. i:.l?:i.:? roi.cri.tc envelope s u r - roundi.ng t h e cemented g r a i n s , Tile l o c a i i t y ill. \ri?icii tlie nodules a r e found i s c h a r a c t e r i - z e d by l a r g e r i p p l e s w i t h t h e s a n d :[in n e a r l y c o n s t a n t motion. :Che sand i.s medi.urn.-gra:i.ried w i ~ t l ? pract:ica:!.ly no very f i n e sandand no m i c r i t e ; ' n e i t h e r a r e pell.ets p r e s e n t , Cci~ieirtat:i.on i s t h e r e s u l t of physicochemical p r o c e s s e s , form:i.ng a b a s i c ::i.a.ii?si:orie f a b r i c .
Nodules o:E cemented c a r b o n a t e r;ke%.etal. saild occur s e v e r a l c e n t i - meters I>el.org and a t t h e sedi.ment.-watcr i n t e i i c c r i.11 35 f e e t of w a t e r o f f
t h e n o r t h e a s t c o a s t o:E S t . C r o i x , 11, S . Virg:in i s l a n d s ( F i g . I ) , j u s t west of Cottongarden P o i n t . These nodules l i e i n l o o s e sand i n a chan- n e l between and p a r a l l e l t o t h e m a i i i harr:i.er r c c f and n second, o u t e r , r e e f which does n o t r e a c h t h e s u r f a c e , Aci:%vely moving la.rge r i p p l e s i n t h e sand bottom ( P i g . 2 ) c h a r a c t e r i z c t h e i?nirj.-i:c bottom of t h i s a r e a , i n - cludi.ng b o t h a r e a s o:f cemented nodulcs ar7.d a r e a s whecc no cementation o c c u r s .
I n d i v i d u a l nodul-es r a n g c from a Ccw ccnieini-ed gr:ni.i?s t o masses 8 c e n t i m e t e r s in maximum d i a m e t e r (1':i.g. 3 )
.
(;r;r i.i~.i: i n ceme1?ted nodules, nodule-bearing s a n d s > and non,-n.odu:!~c:,-bcxi.xig s;ir!ds a r e mediun-grained, w e l l - s o r t e d , s1ce:leta.l. sands ( F i g , 4 ) . i'c?:cc::i:in;:e of indi\ij.dilal s l c e l e t a lc o n s t i t u e n t s a r e approximately t h c same :in the cemented m a t e r i a l , nodule- b e a r i n g s a n d s , and now-nodulc-bearing sand:; ('Cab1.e I.), and no c l a s t i c r o c k g r a i n s a r e p r e s e n t i.n eiirller ilodii:i.cs 01: sands;, I?racti.call.y no m i c r i t e o r p e l l e t s a c e p r e s e n t i.n any sa1npi.c.
The cement i s e x c l u s i v e l y ara.gon.ii:e, dcizerinined hy SEN and e l e c t r o n microprobe (Fred T . Macl[:en.zie perr,onaI. cornrmunication). Aragonite
n e e d l e s r i m i n d i v i d u a l g r a i n s o r sma:i.L c l u s t e r s of seve1:a.l. g r a i n s . These rims average about 50 microns i n t h i . c k n e s s , w:i.l:-i ind:i.~:i.diial c1:ystals
I West 1 n d i . e ~ T,aboratory, P. 0. Annex Rox 4O:ii)
C h r i s t i a n s t e d , S t , C r o i x ,
U.
S. Virgin. T.sI.ands 00820 C u r r e n t a d d r e s s : U n i v e r s i t y S t a t i o nNorth Dalcota Geologica.l. Survey Grand F o r k s , North Dakota 58201
(Manuscript recei-ved August 1974--I?ds.)
oriented normal or subnormal to grain boundaries (Fig. 5). Nearly every grain coated with the aragonite rim cement also has a thin micrite en- velope below the aragonite. Cementation appears to be more complete in the centers of nodules than towards the exterior. Coarse particles are not cemented as well as those of near-mode size,
Sand ripples are s p e t r i c a l approximately 10 centimeters high from trough to crest, and the sand is in relatively constant motion during times of normal ocean swell (Fig.
2).Although the site of cementation
;-as
not been visited during s.Ecnm swe:LJ. times, logically the motion is
even stronger then.
DISCUSSION
Many examples of modern carbci?.ate cementation are lcnown from a variety of geographic locations and environments. Nearly all of these examples are from "special" environments, that is, those with subaerial exposure, high salinity, or other non-normal marine characteristics.
Most of these examples also involve either pellets in the sediment, micrite, or both. (A short note is not the place to give an extensive literature summary, but the papers and references of Macintyre et al, 1968; Bricker, 1971; and Sibley and Murray, 1972 are pertinent). Many of these previously described types of cementation occur on St. Croix
in addition to the cement described here.
The major unique characteristics of the cementation described here are the occurrence of the cement in open, normal marine water, lack of pellets, lack of fine-grained sediment, and lack of a physical stabi- lizer. Cementation under these circumstances appears to be rare
(Chilingar, et al, 1967, p. 1.86), although this cement is similar to that reported by Shinn (1969) in the Persian Gulf.
Granulometric analy:.is of nodule-bearing and non-nodule-bearing sands from the cement area provides little insight into the cause of cementat ion (Fig. 4.). Dotli samples axe well-sorted. Graphic standard deviation of the host sand is -4.0 phi, a well sorted sand (Folk, 1961) and the same parameter in the non-host sand is .50 phi, still a well- sorted sand. In both samples less than 5% of the total sample is less than 1.5 phi size, demonstrating that any fine sand or smaller particles either have been winowed out or have not been deposited. There is a sharp inflection in the cumulati.ve curves of these samples between the most frequently occurring particle size and the next smaller 112 phi
interval, although a much less marked inflection occurs to larger phi sizes. There is nearly no insoluble residue in these samples. The two samples are virtually identical in sike and sorting except for the broader total size spread of the nodule-bearing area sample compared with the non-nodule-bearing area sample (-2.0 to 3 phi compared to -1.5
to 2.5 phi).
Constituent particle analysis of the two sand samples and the cemented sand itself sheds little additional information (Table 1).
Coral content of each sample is about the same, no sample contains any
pellets, and foraminifera, molluscs, and coralline algae form the
remainder of the samples. The category "other" contains unidentified
g r a i n s , Halimeda, echinoderms, and o t h e r m i s c e l l a n e o u s s k e l e t a l m a t e r i a l s . I n d i v i d u a l g r a i n s k e l e t a l f a b r i c s a r e u n a l t e r e d . No d i s - c o l o r a t i o n i s p r e s e n t n o r a r e any of t h e cemented g r a i n s p r e v i o u s l y m i c r i t i z e d .
Radiometric d a t i n g h a s n o t been done, b u t t h e c e m e n t a t i o n i s i n t e r - p r e t e d t o b e modern because t h e cemented sands a r e i d e n t i c a l t o t h e s a n d s c u r r e n t l y being g e n e r a t e d and t r a n s p o r t e d i n t h a t environment, no p r e e x i s t i n g cemented c a r b o n a t e s of t h i s k i n d have been d i s c o v e r e d i n t h i s g e n e r a l a r e a ( o r a n y p l a c e e l s e on t h e i s l a n d ) , no t e r r i g e n o u s ( c l a s t i c ) g r a i n s e x i s t i n t h e cemented o r uncemented s a n d s , a l t h o u g h s u c h g r a i n s a r e u b i q u i t o u s i n n e a r s h o r e o r beach d e p o s i t s ( i f t h e cemented nodules were remnants of e a r l i e r Holocene beachrock, t h e y would c o n t a i n l a r g e numbers of c l a s t i c g r a i n s ) , and g r a i n s a r e n o t per- v a s i v e l y m i c r i t i z e d a s i s corrunon i n most of t h e S t . C r o i x examples of cemented c a r b o n a t e sands.
Cementation c a u s e i s probably p u r e l y physicochemical. Cement mor- phology i s t h a t p r e d i c t e d f o r normal marine s u b t i d a l cement by Badio- zamani, Maclcenzie and T h o r s t e n s o n (1973) from l a b o r a t o r y s t u d i e s
(Mackenzie, p e r s o n a l communication). Sands a r e n o t s t a b i l i z e d a t t h e s u r f a c e , b u t about 4 c e n t i m e t e r s below t h e troughs of r i p p l e s t h e sand i s n o t moved e x c e p t d u r i n g uncommonly h i g h s e a s . Cementation a p p a r e n t l y t a k e s p l a c e i n t h e t e m p o r a r i l y s t a b i l i z e d sand forming n o d u l e s , which a r e p e r i o d i c a l l y moved by s t o r m waves. No cemented l a y e r h a s been found, s o t h e nodules do n o t l i e a t r e s t f o r r e a l l y extended p e r i o d s of time, b u t a r e moved s e v e r a l times e a c h y e a r . N u c l e a t i o n of cement a p p e a r s t o b e p r e f e r e n t i a l t o g r a i n s w i t h m i c r i t e envelopes. The
r e s u l t i n g f a b r i c (Fig. 5) i s t h e p r e c u r s o r of t y p i c a l g r a i n s t o n e f a b r i c such a s t h a t of t h e Lower Ordovician Manitou Limestone (Gerhard, 1972, p. 1 8 ) .
CONCLUSIONS
Cementation i n b a s i c a l l y u n s t a b i l i z e d c a r b o n a t e s a n d s which have no a p p r e c i a b l e m i c r i t e component and which a r e d e p o s i t e d w e l l below low t i d e o c c u r s . There a p p e a r s t o he no g r a n u l o m e t r i c o r c o n s t i t u e n t s k e l e t a l p a r t i c l e b a s i s f o r c e m e n t a t i o n c o n t r o l , a s n o d u l e - b e a r i n g and non-nodule-hearing sands a r e v i r t u a l l y i d e n t i c a l i n s i z e , s o r t i n g , and c o n s t i t u e n t p a r t i c l e s . Cementation a p p e a r s t o be t h e r e s u l t of p u r e l y physicochernical p r o c e s s e s i n t e m p o r a r i l y s t a b i l i z e d c a r b o n a t e s a n d s . T h i s example of submarine c e m e n t a t i o n i s s i g n i f i c a n t because it o c c u r s under normal marine c o n d i t i o n s , forms a g r a i n s t o n e f a b r i c , and can be t h e s o u r c e of g r a i n s t o n e f a b r i c i n t r a c l a s t s i n o t h e r t e x t u r a l t y p e c a r b o n a t e r o c k s .
ACKNOWLEDGEMENTS
I wish t o thank Fred T. Maclcenzie f o r a n a l y s i s of t h e cement comp- o s i t i o n and f o r h i s d i s c u s s i o n s of t h e problem, and John Milliman who r e a d t h e manuscript and made s u g g e s t i o n s f o r i t s improvement. T h i s i s West I n d i e s L a b o r a t o r y C o n t r i b u t i o n No. 20.
REFERENCES CITED
Badiozamani, Khosrow, Mackenzie, F. T., and Thorstenson, D.C., 1973, Diagnosis of diagenetic environments of carbonate rocks by the fabric of their cement (Abstract).
Bricker,
0 .P., editor, 1971, Carbonate cements: The Johns Hopkins Univ. Studies in Geology, No. 19, The Johns Hopkins Press, Balti- more, Md., 376 p.
Chilingar, G o V., Bissell, H. J., and Wolf, K. H., 1967, Diagenesis of carbonate rocks, in Diagenesis in sediments, Chilingar, G. V. and Larsen, G., eds.; Developments in Sedimentology, v. 8, Elsevier Publishing Co., New York,
p.179-322.
Folk, R., 1961, Petrology of sedimentary rocks: Hemphill's, The Uni- versity of Texas, 154 p.
Gerhard, Lee C., 1972, Canadian depositional environments and paleo- tectonics, central Colorado: Quarterly Colo. School Mines, v. 67, n. 4 ,
p.1-36.
Macintyre, I. G., Mountjoy, E. W., and D'Anglejan, B. F., 1968. An occurrence of submarine cementation of carbonate sediments off the west coast of Barbados, W.I.: Jour. Sed. Petrology, v. 38,
p. 660-664.
Sibley, D. F., and Murray, R. C., 1972, Marine diagenesis of carbonate sediment, Bonaire, Netherland Antilles: Jour. Sed. Petrology, v.42, p. 168-178.
Shinn, E. A., 1969, Submarine lithification of Holocene carbonate
cements in the Persian Gulf: Sedimentology, v. 12,
p.109-144.
T a b l e 1. C o n s t i t u e n t p a r t i c l e a n a l y s i s of cemented nodules, nodule- b e a r i n g sand and non-nodule b e a r i n g s a n d .
C o r a l
F o r a m i n i f e r a C o r a l l i n e Algae Mollusc
Other
Cemented Nodule Nodule-bearing Non-Nodule-
Sand Bearing Sand
50.6% 5 1 -7% 51.1%
20.6 16.5 26.5
14.5 18.9 12.9
11.6 8 . 2 5.7
CEMENTED NODULES
64O45'
I
TTONGARDEN-
5 MilesST. CWOlX
Fig. 1 Location map showing St. Croix and enlargement of area where cementation occurs.
Fig. 2 Sand ripples in cemented area, 35 feet deep, showing crestal sand in motion. Ripples are about 10 cm. high.
Fig.
3 Nodule of cemented s a n d f1:om n o r t h w e s t o f C o t t o n g a r d e n Poin?.,
St:.
C:~:oi.x,F i g .
4.
H%sto[:rams o f s a n d C r a m non-cemnented a r e a (non-nodule b e a r i n g sane!.) (I\) a n d cemented a.rfa (nodu1.e-bearing s a n d )(13). Nore t:he a h r u p t c u t o f bel.ow % , 5 P h i s i . z e . 1.1i.sto- gram dl:;l'$7n on :I/% R1:i :i.n.terval.s,
F i g . 5 A r a g o n i t e r i m s on m i c r i t e cnvelopes ( d a r k g r a i n c o a t i q g s ) c e m e n t i n g m o l l u s c and b o r e d c o r a l s a n d g r a i n s . F i e l d view i s 1 nun. Crossed n i c h o l s .
