Contr. Mineral. and Petrol. 13, 89--96 (1966)

Recent Dolomitization of Quaternary Bioealearenites from Fuerteventura (Canary Islands) GERMAN MiJLLER and GERD TIETZ

Received 5{ai 17, 1966

Abstract. Quaternary marine and eolian biocalcarenites in the supratidal breaker and spray zone along the Barlovento coast of the peninsula Jandla, Fuerteventura (Canary Islands) are dolomitized by percolating brines with a high Mg/Ca ratio resulting from evaporation of seawater on the sediment's surface. Only fragments of calcareous algae primarily consisting of high-magnesian calcite are replaced by a cryptocrystalline variety of dolomite. Dolomite also occurs in large euhedral crystals in intraparticle and interpartiele pore spaces. In the marine biocalearenite dolomite has a composition of Cas~Mg4~(COa) 2. I t is well ordered.

A. Introduction; Occurrence of Calcarenites Quaternary carbonate sediments, composed mainly of skeletal material, occur at several places along the coast of the island of Fuerteventura (Canary Islands). These biocalcarenites are especially common along the Barlovento coast of the peninsula Jandia in the south of Fuerteventura (Fig. 1). A terrace-forming layer made up of a strongly indurated marine biocalcarenite about 0.5--1.0 m thick extends for about 10 km between Punta Paloma and Playa de la Pared (Figs. 2, 3). I ts basal layer consists of a littoral conglomerate with rounded pebbles and boul- ders of olivine basalt. This calcarenite overlies a nearly planar abrasion surface cut into olivine basalt.

Numerous shallow depressions on a centimeter to meter scale (Fig. 4) occur on the surface of the calcarenite-platform. During occasional heavy seas they become filled with seawater. This evaporates after the retreat of the sea, leaving crusts of gypsum and halite after complete drying. The platform is overlain by calcarenites as much as 20 m thick, which exhibit large- scale cross-bedding. Generally, they are much less strongly indurated than the platform-ealcarenites. The top-layers are completely friable; they are likely to have been deposited very recently.

H a v s n v (1958) tentat ively dated the strongly indurated platform-calcarenites as Pleistocene, Ultimate Interglacial. According to HAVsE~, the shore-line migrated from a high level at the Ultimate Interglacial towards a low level of the Last Glaciation and returned to approximately the same line of the present. After sub- aerial exposure of the marine carbonate sand, mineralogical and textural changes led to lithification.

The younger bioealcarenites overlying the marine biocalcarenites are interpreted as eoliarrites; such sediments are still formed today.

B. Composition of Biocalcarenites; Occurrence of Dolomite The elastic components of the biocalearenites are identical to those of Recent beach- sands on the island (MOLLEe, 1964). The individual grains are rounded fragments

7 Contr. Mineral. and Petrol., u 13

90 G. ~/[i#LLER and G. TIETZ:

of chiefly calcareous algae, mollusks, foraminifera, echinoderms, as well as frag- ments of volcanic material. The sorting is good. The degree of diagenetic alteration differs between the individual calcarenite layers (TI~TZ, in preparation).

The occurrence of dolomite is especially common in the coastal stretch between Punta Paloma and Playa de la Pared (Fig. 1). Other occurrences on the island will be described elsewhere.

q ~, ~ Playa de ia Par tO . J : . . , Istmo de l a ~ G r a n Tarajal

.o

~ ~ a of dolomit izal ion

Jandia Morro Jable

-29 o

-28 o

I 117 o I I 1B ~ W.GREENWICH 16 ~ 15 ~

A T L A N T I C O C E A N

~ kA PALMA

~ / ~ E N ERIFE

� 9 LA GOMERA ~ j ~

~ I E R R 0 GRAN CANARIA

I I 1

10 20km i i

i

14Q

L A N Z A R O T ~

FUERTEVENTURA

~_ . . . . . . . J

AFRICA/ I I t

Fig. 1. Area of dolomitization along the Barlovento coast of 5andia peninsula, Fuerteventura (Canary Islands)

X-Ray, chemical, and mineralogical examinations show that dolomite predomi- nantly occurs in the strongly lithificd marine calcarenite-platform, immediately above the basalt. Dolomite also occurs in the overlying eolianites up to a height of 6 m above the basalt (8--9 m above sea level), although to a much lesser extent. Samples were taken in a section at the Barlovento coast of Jandia at 1 m (upper- most layer of the marine calearenite), 4 m, 6 m, and 8 in (eolianites) above the basalt, corresponding to 3 m, 7 m, 9 m, and I1 m above medium high tide sea level (Fig. 3). MinerMogie composition of the uppermost sample (4) is identical to that of recent unlithified carbonate sands of the island (M~LL~R, 1964), as shown by X - g a y analyses (Fig. 5): Calcite and high-magnesian calcite dominate whereas aragonite is less abundant. High-magnesian calcite is always restricted to the algal parts.

Recent Dolomitiz~tion of Quagernary Bioeulearenites 91

Fig. 2. :NE view of Barlovento coast, Yandda, with marine hiocalearenite-pla~form and overlying younger eolianit

NW �9 I~ SE

13

M a i n wind d i r e c t i o n ~, 12

~ s I 10 younger, prayl / ... ~.:~___ ~- -- g

] EOLIANITE$ ~ 3 . . . . . . . LE]iOCALCAREN,TES~ ,~ | 8 -o ~

/ ~ . ~ otpe~e!r, t io, I 7 : - Jotder ~ y ~ i t h evaporating I r .~

~ s e a - w a t e r ( s p r a y ) ~ I ~ "~

back H o w of s e a - w a t e r ~ ~ I 5 E XA^D ~l~ = nr^l~_ ] ' ~ \ .... b reakers atheavy sea ARENITE with I ~ . ~ 1 ~ ~' -I- "

i BASAL CONGLOMERATEI ~ ' ~ ..... ~ - ~ = .... I ~ 3 E s l a t h e I I ~.

. . ~downward percolation and ; ~ 7 . . . . . . . . . . . . . . . . r e f l ux o f b r i n e s w i th high

. . . . . . . . . . . . . . . . . . . M g / C a - r a t i e s

Fig. 3. Schematic section through the coastal stretch of Barlovento coast, 5andia. 1 - - 4 sample positions (surf should be replaced by swash)

Aragonite and high-magnesian calcite arc completely absent in the sample imme- diately below (3), they are completely replaced by calcite and a small amount of dolomite. The dolomite content increases in the next-lower sample (2) and is highest in the lowermost (1). The chemical analysis confirms these results. The MgCQ-eontent increases corre- sponding to the change in dolomite cont.ent downwards in the section (Table). Based on the position of the (211)-reflection the dolomite of the marine bioeal- carenite contains a CaCOa-exeess of 6 - -7 mol percent. It is well ordered. The low- magnesian cMeite shows an excess of MgCO:3 of 3.0--3.5 mol percent.

7*

92 G. ~idLLE~ and G. TIETZ:

Tuble. Carbonate mineralogy and percentage o/CaCOe, MgCOs, and Sr in bioealcarenites 1--~, Barlovento coast, Jandla, Fuerteventura

Sample- m above ra above Itigh-Mg Arago- Low-Ng Dolomite % % % Nr. basalt high tide calcite nitc calcite CaCO~ MgCOa Sr

sea level

4 8 10 q-q-+ q-q- q--}- -~ -- 75.46 11 .48 0.16 3 6 8 -- -- q- q- q- q- q- -~ 67.06 8.30 0.01 2 4 6 -- -- + + + + - - + 87.08 12.60 0.09 1 1 3 - - -- - -~ -+ - - + + 73.86 24.96 0.08

The Sr-eontent is highest in the uppermost youngest sample (4). I t is doubtlessly incorporated in the aragonite lattice as SrC03 in solid solution. The Sr-content of the samples (1) and (2) exceeds the average Sr-content of limestones (0.06 percent, TUR~KIA~ and KuLP, 1956). Microscopic examination of the marine bioealcarenite shows tha t small amounts of celestite (S rSQ) are responsible for the relatively high St-content of the sediment. Celestite is to be expected because the Sr of the

Fig. 4. Surface of the marine biocalcarenite, full of "pans" which are partly filled with gypsum and halite

former aragonite could not completely be incorporated into the calcite-lattice. I n the presence of sulfate ions the Sr must be precipitated as poorly soluble SrSO~ in the sediment (Mi~LL~R, 1962). Dolomite occurs in 3 different manners in the marine biocalcarenite, as shown in thin-sections stained with Alizarin Red: a) cryptoerystall ine var ie ty as replacement of the grains of calcareous algae (Fig. 6), formerly composed of high-magnesian calcite. The original organic skeletal struc- ture is largely preserved. b) in intrapart iele and interparticle pore spaces of the sediment as large euhedral crystals (up to 1 m m in diameter).

Recent Dolomitization of Quaternary BiocMcarenites 93

Calcite~ Low -Mg High- Mg

Calcite

Aragonite

Low-t,4 g Calcite

Do[omite~

Low -lg

A

I - i I I [ I F- I I 33 32 31 30 29 28 27 26 25 ~

Fig. 5, X-l~ay diffraction diagrams of samples 1--4. Scale in degrees of 2 v% eeppez ~'udi~tion

c) in large single crystals (maximum size 2 ram) in which only traces of former organic structures e~n be recognized, probably because o~ a recrystMlization of

94 G. M~)LLER and G. TIETZ:

Fig. 6. Fragments of calcareous algae altered into cryptoerystalline dolomite. Some former cell-walls are still preserved as calcite (stained with Alizarin Red)

Fig. 7. Euhedral dolomite in foraminifera chamber partly filled with drusy calcite

the cryptocrystMlhle dolomite. These crystals are always definitely larger than the individuM skeletal fragments, around which the erystals have grown. I t is evident, that only the calcareous algae, formerly consisting of high-magnesian calcite, have been dolomitized, but never those skeletal fragments primarily consisting oi low-magnesian calcite or aragonite.

Recent Dolomitization of Quaternary Bioealearenites 95

The dolomitization of the algal parts is almost always complete. Only in very rare eases (Fig. 6) do the carbonates, representing former cellwalls, sonsist of calcite. The spatial arrangements shows that the euhedrM, coarsely crystalline dolomite (b) was only formed after drusy calcite had grown in empty interpar title or intraparticle pores. This drnsy calcite according to F~IEDIeAN (1964) is produced on subaerial exposure by leaching aragonite to form moldie porosity which follows the trans- formation of high-magnesian calcite into low-magnesian calcite. So, the dolomite growth only began after the main stages of early diagenesis of skeletal limestone

Fig. 8. Large dolomite crystal with "inclusions" of completely destroyed organic (?) structures and well preserved algae structures

(disappearance of high-magnesian calcite and aragonite) had already terminated. Tile time of formation of the eryptocrystalline dolomite in the algal parts cannot be unequivocally fixed. I t could have occurred either by direct transformation of the high-magnesian calcite into dolomite, or after the transformation of the high- magnesian calcite into low-magnesian calcite was completed. I t is planned to study this problem by 14C-analyses.

C. Formation of Dolomite

The marine biocMcarenites which are most extensively dolomitized lie above high tide sea level, but can easily be inundated by breakers at heavy sea (Fig. 3). The bulk of the seawater directly flows back over the surface of the calcarenite- platform. A small amount, however, can percolate into the pore-spaces of the sediment and be retained in the surface pits. After the retreat of the heavy sea part of the surface water evaporates in the arid climate of the island. In the resulting brine the Mg/Ca ratio is strongly increased by the deposition of carbonates, gypsum, and, finally, halite. Small amounts of the brine can percolate into the pore-space of the calcarenite during the evaporation process. They flow back into the sea

96 G. MiTLLER and G. TIETZ: Recent Dolomitization of Quaternary Biocalcarenites

along the ca lcareni te /basa l t interface. The Mg-rich solutions cause a t rans forma- t ion of a lgal calcite in to dolomite and the growth of dolomi te crystals in in t ra- par t ic le or in te rpar t ic le pores dur ing percolat ion. The dolomite forms in a similar manner in the eolianates. I t is, however, less a b u n d a n t because wate r for evapora t ion is suppl ied only b y the sea spray. Deta i led inves t igat ions mus t show, how far the do lomi t iza t ion has l a te ra l ly progressed into the rock. Aut igenic dolomite is res t r i c ted to the supra t ida l b reaker and sp ray zone along the coast. I t is formed dur ing downward percola t ion of brines wi th a high Mg/Ca rat io, which are p roduced b y evapora t ion of seawater on the surface of the sediment . This process of do lomi t iza t ion b y downward percola t ion of brines in the supra t ida l zone ("seepage ref luxion", ADAMS and RHODES, 1960) is s imilar to the dolomit iza- t ion on the Sugarloaf K e y (Florida) and on the Bahamas (S~I~-~ and GINSBURG, 1964; S m ~ , GI~SBV~G and LLOYD, 1965). The only difference is, t h a t in F lo r ida and on the Bahamas unconsol ida ted l%ecent carbonate sediments are dolomit ized, whereas on F u e r t e v e n t u r a this process occurs in a l ready consol idated sediments.

Acknowledgements. The authors want to express their sincere thanks to the Deutsche For- schungsgemeinschaft for its kind financial assistance.

References ADAMS, J. E., and M. L. RI~ODES: Dolomitization by seepage refluxion. Bull. Am. Assoc.

Petrol. Geologists, 44, 1912--1920 (1960). FRIEDM-A-~, G. M. : Early diagenesis and ]ithification in carbonate sediments. J. Sediment.

Petrol. 34, 777--813 (1964). H•USEN, H. : On the geology of Fuerteventura (Canary Islands). Soc. Sci. Fennica, Commenta-

tiones Phys.-Math. 22, 211 p. 1958. MffLLE~, G. : Zur Geochemie des Strontiums in ozeanen Evaporiten unter besonderer Beriick-

sichtigung der sediment~ren Coelestinlagerst~tte yon Hemme]te-West. Geologie (Berlin) 11, Beih. 35, 90 S. 1962.

- - Friihdiagenetische allochthone Zementation mariner Kfisten-Sande durch evaporitische Ca]cit-Ausscheidung im Gebiet der Kanarischen Inseln. Beitr. Mineral. Petrog. 10, 125--131 (1964).

S ~ I ~ , E. A., and R. N. GINSBV~G : Formation of Recent Dolomite in Florida and the Bahamas. Abstract. Bull. Am. Assoc. Petroleum Geologists. 48, 547 (1964).

- - , R. 1~. GI~SBU~G, and R. M. LLOYD: Recent Supratidal Dolomite from Andros Island, Bahamas. In: L. C. PRAY, and R.C. Mu~RAY, (editors) Dolomitization and Limestone Diagenesis, a Symposium. Soc. Econ. Paleont. Mineralog., Special Pub]. ~To 13, 112--123 (1965).

Prof. Dr. GE~M~ Mi~LLEt~ and cand. min. I~E~D TIETZ Mineralogiseh-Petrographisches Institut der Universit~t Heidelberg Laboratorium fiir Sedimentforschung 69 Heidelberg, Kurfiirstenanlage 59