ACTA GEOLÓGICA HISPÁNICAX, t. XIII (1978), núm. 1, págs. 20-22
Aberrant features of the Messinian coral reefs, Spain*
M. Esteban (1), F. Calvet (2), C. Dabrio (3), A. Barón (4), J. Giner (2), L. Pomar (S), R. Salas (2) and A. Permanyer (2)
( 1 ) Inst. Jaume Almera. Cniv. Barcelona, (2) Department of Petrology, Univ. Barcelona, (3) Dept. of Stratigraphy, Cniv. Granada, (4) Servicio Geol. Obras Publicas, Mallorca, (5) Dept. of Geology, Cniv. Palma de Mallorca.
RESUMEN
Se describen someramente los principales arrecifes del Messinien- se en España, haciendo hincapié en el conjunto de "anomalias" presentan los arrecifes del Neiigeno inferior en comparación conl;: asociaciones arrecifales actuales. Se discute l a importancia y signi- ficado de Porites sp. como coral claramente dominante, y a menudo exclusivo, en la construcción de edificios arrecifales.
During the Messinian event, coral reefs existed scat- tered along the Mediterranean coast. We should be able to obtain accurate information on Messinian envi- ronments by studying its reefs. Although some cf these reefs a r e recognized in the literature, we a r e not aware of detailed studies on them; one exception in Spain is the work of DABRIO (1974,1975) in the a r e a of Pur- chena. The purpose oh this paper is to present a preli- minary report of our work on the Messinian reefs in Spain, pointing out a constant set of L'anornalies" in the community cornposition of these reefs in relation to earlier Neogene and to rnodern reef cornmunities. Alt- hough the precise age of most of these outcrops is not well tnown, we believe that they a r e roughly conternpo- raneous, mainly because (1) these anomalies in the communities a r e a response to very specific and excep- tional environmental conditions, clearly related to the Messinian crisis, (2) it appears that analogous abe- r r a n t communities existed in other Mediterranean lo- calities in analogous stratigraphic position. We do not have a s yet a cumpletely satisfactory explanation for
al1 of these aberrant features; but a tentative interpre- tation is here outlined a s a working hypothesis.
GENERAL DESCRIPTION
Spanish Messinian reefs vary- considerably in size, morphology, substrate and diagenetic processes. Fig. 1
(') Paper preaetited at the Third lnternetional Coral Reef Symposium, Miami (U.S.A.). May
23-27. 1977. ,
Fig. 1 : Known Messinian reefs ín Spain. 1 : Purchena, 2: Nijar and Lucainena, 3 : Carboneras, 4 : Lorca, 5 : Santa Pola, 6 : Mallorca.
shows the main known localities in Spain and their re- lationship to r e ~ i o n a l structural features. MONTENAT
(1 973, i 9 7 5 ) provides an excellent geologic frame- work f'or most of'these reefs. The thickest reef-wall oc- curs in Santa Pola and Mallorca (20-70 ?m); in Pur- chena they a r e rnuch thinner (2-6 m). In the a r e a of Ni- jar, a t least three prograding reef stages can be seen, each one building up on the slope of the previous one. Nijar and Lucainena de las Torres a r e the rnost late- rally continuous barrier and fringing reef outcrops (a- bout 1 5 km), and they can be traced for a similar dis- tance, or more, in the subsurface a t Mallorca. Entire reef slopes a r e exposed in Carboneras and Nijar, with :ontinuous exposure between flat reef-top and the hori- zontal toe of the slope. We can infer minimum water depths of near 1 5 0 m in Carboneras, and probably si- milar features in the subsurface a t Mallorca. In Níjar, the minimum water depth could be a t about 7 0 m. In general, SE facing slopes a r e better developed (or pre- served) than the NW ones.
Reef substrata a r e siliciclastic fan deposits in Pur- chena and Lucainena, volcanics in Carboneras (pro- bably with atoll-like morphologies), and older Miocene sediments in Mallorca. Preservation of wide reef-wall buttresses is particularly good in Santa Pola and Ni- j a r , and to a lesser degree in Carboneras. Evidence of
extensive submarine cementation is found in Mallorca and decreases toward the west. In Purchena there is practically no cementation. The inferred water turbu- lence was very high in the a r e a of Mallorca, and in ge- neral also seems t o be diminishing toward the west. Gravitational displacement of reef blocks and sediment is quite common during the Messinian and Lower Plio- cene. A characteristic vertical fracture system parallel to the reef-front seems to have contributed to slope instability.
Al1 these variations a r e suggestive of a complex pa- leogeography; in general, the open sea should be to- w a r a the east and, apparently, there 1s not much evl- dence of good communication with the Atlantic.
REEF COMMUNITIES
In view of the variability in the Spanish Messinian reefk, the rather constant coral composition is here considered to be quite significant. The reef builder in the Spanish Messinian is Porites sp. This coral is clearly dominant, and often exclusive, in the reef-wall fi.amewor-k. The typical Porites rnorphology is longn vertical, bifurcating o r trifurcating sticks, 2-10 cm thick and up to 4 m high, in bush-like and organlike configuratioris. In more exposed a r e a s Porites appears in successive flat colonies connected by vertical co- lumns. In more protected a r e a s hemispherical s t a r co- ral heads of Sidereastrea sp and Montastrea sp may occur. Although these coral heads may be up to 5 0 cm in diameter they are not conspicuous a s reef builders. In some places s t a r corals adapt a colurnnar colonial morphology similar to Porites. Brain corals a r e less comon except a t some points Puchena, the a r e a with the highest coral diversity in the Spanish Messinian. That high diversity occurs in the less expected locality, with the thinner reefs, poor cementation, and probably less wave action away from the inferred open seas to the east. Significantly, in this locality Porites is sma- ller than farther east, but still is very abundant and h a s the same morphology.
In the a r e a of Santa Pola, and probably in Carbone- r a s and Mallorca, Porites is intergrown o r closely as- sociated with blue-green (?) alga1 stromatolite bio- herms, one to tens of meters high, in the reef-front. This stromatolite-Porites association is also well deve- loped in oolitebar sequences in more shelfward posi- tions. The apparent absence of tidalite sequences, fe- nestral frabrics o r indications of emergence, together with the stromatolite morphology (units up t o 1 5 m thick of perfect hemispheroidal heads up to 1 0 m in diameter), may suggest a rather deep submarine envi- ronment.
Production of Halimeda sp is usually very high. This codiacean alga occurs in thick (2-5 m) packstone beds in the fore-reef slope. Red alga nodules and branches, vermetid gastropods, bryzoans and serpulid worms a r e present in some locations, but do not play a signifi- cant role a s binders in the reef-wall framework. Orga- nic boring (sponge, mollusc, worm, alga, fungi) of the reef is abundant in come places, specially in Mallorca.
ABERRANT FEATURES
In tbe community composition of the Spanish Messi- nian reefs that we have briefly summarized, we find a significant set of'anomalous or aberrant features: (1) A drastic reduction of diversity of reef-building species, in comparison with earlier Miocene, o r older, reefs in the a r e a (CHEVALIER, 1961, PERMANYER and ES- TEBAN 1973); (2) The exclusivity to the Messinian of the large size arid shape of Porites. This group appea- red in the Middle Cretaceous and is well represented in reefs since the begining of the Cenozoic, showing colo- nial morphologies similar to the ones of modern spe- cies. Only during the Messinian do they develop excep- tionally large columns and flat-and-column structures, or appear in e.xclusive Porites-reef; (3) The presence
and abundance of stromatolites in a reef front or in areas associated with corals, Halimeda and bryo- zoans, al1 these supposedly sensitive indicators of nor- mal marine conditions.
l NTERPRETATION
These anomalous o r aberrant features produce a
special archaic aspect in the Messinian reefs in rela- tion to other reef communities in the area. These featu- res reflect conditions of growth that differ considerably from "normal" marine. Porites and Sidereastrea a r e known in modern seas a s the corals more resistant t o salinity and temperature variations o r t o waters high in suspended sediment. However, in these physically stressed modern environments they occur a s small and isolated colonies, in contrast with the ones in optimal marine conditioris. This suggests t h a t another kind of ecoiogical control had to favor the huge Messinian Po- rites colonies, perhaps growth in absence of ecologic competition.
WORKIKG HYPOTHESIS
Our hypothetical interpretation irr dies two stages. First, a Messinian crisis (salinity -) would eliminate most of the normal marine biota, only preserving some of the more resistant species (particularly, Porites a n d Sidereastrea) and favoring the development of stroma- tolites. Second, a rapid re-establishment of normal o r near-normal marine conditions would provide these communities with the opportunity for growth in a non- competitive situation. The problematic p a r t of this hy-
pothesis is t o explain t h e absence o f c o r a l a n d s t r o m a - tolite r o m p e t i t o r s arid, a t t h e s a m e time, t h e presence
of Halimeda, bryozoans, etc. W h a t k e p t browsers, gra-
z e r s a n d s c r a p e r s o n alga1 m a t s a n d c o r a l s f r o m re- establishing themselves in t h e reef environrnent?
Seberal consideratioris m a y help e x p l a i n this pro- blem. I l u r i n g t h e Tortoriian. c o r a l reefs d i s a p p e a r on t h e Atlantic side o f F'rarice a n d yet they were flouris- h i n g ori t h e Mediterraiiean side (CHEVALIEH 1961),
suggestirig t h a t t h e M e d i t e r r a n e a n w a s a "refuge" du- r i n g the L a t e Miocerie. I n the periods o f re- establishnierit of' maririe conditions d u r i n e t h e Messi- 0 n i a n , t h e r e w e r e n o c o r a l s o r c o r a l c o m p e t i t c r s o n t h e Atlaritic side t h a t could e m i g r a t e t o t h e illediterranean.
1.0 cxplairi o t h c r abseiicrs we c a n a l s o consider t h a t duriiig tlic Messiriiari, periods of norrnal m a r i n e condi- tioris w r r e s o s h o r t o r s o rapidily established t h a t t h e r e w a s rio tinir for iri\asiori ofdiversified Atlantic commu- nities. O t h e r possible corisiderations m a y involve the riature of' t h e coniniuriication with t h e Atlantic o r the aniourit of'orgariic n i a t t e r o r suspended sediment i n t h e w a t e r .
Much m o r e iriforniatiori arid field d a t a a r e needed in o r d e r t o d e l i r i e a t ~ a t~asic, paleogeography a n d paleoe- colopy of'the Messiriiari recfs. The i d e a s here expressed orili a t t e m p t t o stiniulate f u r t h e r w o r k o n this exciting arid poorlj. understood subject.
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