Tectosedimentary setting and chronostratigraphy of the Neogene reefs in the Almanzora Corridor (Betic Cordillera, Spain)

TECTOSEDIMENTARY SETHNG

AND CHRONOSTRATIGRAPHY OF THE NEOGENE REEFS

IN THE ALMANZORA CORRIDOR

(BETIC CORDILLERA, SPAIN)

ANTONIO GUERRA-MERCHAN & FRANCISCO SERRANO Departamento de ~Teolog~a, Facultad de Cienctas, Untversidad de Mt~laga, 29071 Mddaga, Spain.

GUERRA-MERCH/~ A. & SERRANO F. 1993. Tectosedimentary setting and chronostratigraphy of the Neogene reefs in the Almanzora Corridor (Befit Cordillera, Spain). [Cadre tectos6dimentaire et chronostratigraphie des r6cifs n6og6nes du couloir d'Almartzora (Cordill~re Bdtique, Espagne)]. GEOBIOS, 26, 1 : 57-67. 26.02.1993.

Manuscrit d6po~;6 le 21.01.1992. Manuscrit accept6 d6finitivement le 01.06.1992.

ABSTRACT

The Almanzora Corridor is a narrow Neogene depression which formed part of the system of connecting channels between the Atlantic and Mediterranean marine environments during the late Miocene. Its Miocene filling is siliclastic and is arranged in four tectosedimentary units (TSU). During deposition of TSU II and IH, reefs developed in coastal and very shallow environments. In general, these were small, isolated reefs, built up by corals (mainly Porites and Tarbellastraea) and red algae. The reefal structures belonging to TSU-II are patch-reefs and are associated exclusively with fan deltas which developed on both borders of the basin. The basin marls of this unit contain planktonic foraminifera assemblages that can be dated to the younger part of the early Tortonian and the early part of the late Tortonian, subdividing the Tortonian by using the FAD of Neogloboquadrina humerosa (TAKAYANAOI & SAITO). Two groups of reefs can be distinguished within TSU-III : larger coastal reefs developed on a palaeorelief formed after the intra-Tortonian tectonic phase which concluded TSU-II, and patch-reefs associated to fan deltas. The deepest contemporary sediments contain planktonic foraminifera assemblages from the lower and middle parts of the upper Tortonian. In this basin, reefal development was prematurely interrupted by the upper Tortonian marine regression, which left all the basins of the central sector of the Betic Cordillera disconnected from marine waters. Marine sedimentation during the latest Tortonian - early Messinian (TSU-IV) only continued in the easternmost part of the Corridor, but no reefs related to this sedimentary unit have been observed.

Key-words : Upper Miocene Reefs, Betic Cordillera, Chronostratigraphy, Biostratigraphy, Planktonic Foraminifera, Tectosedirnentary units.

RI~UMI~

Le Couloir d'Almanzora est une d6pression n6og6ne 6troite qui faisait partie du syst6me de chenaux de communi- cation entre les milieux matins aflantique et m6diterran6en pendant le Mioc6ne sup6rieur. Son remblaiement mioc6ne est fbndamentalement silidclastique et il se trouve organis6 en quatre unit6s tectos6dimentaires (UTS). Pendant le processus de d6p6t des UTS II et Ill, des r6cifs se sont d6velopp6s darts les milieux fittoraux et tr6s peu profonds. En g6n6ral, ce sont des 6difices petits et isol6s, principalement construits par des coraux (surtout Podtes et Tarbellastraea) et des algues rouges. Les bioconstructions de la UTS-II sont du type patch-reef et elles sont assod6es exclusivement avec des c6nes deltaiques qui se sont d6velopp6s sur les deux bords du bassin. Les marries de bassin de cette unit6 confiennent des assemblages de foraminif6res planctoniques qni datent de la pattie sup6- rieure du Tortonien inf6rieur et de la partie inf6rieure du Tortortien sup6rieur, en subdivisant le Tortonien au moyen du FAD de Neogloboquadrina humerosa (TAKAYANAOI & S~TO). On peut distinguer deux groupes de r6cifs dans la UTS-III. Le premier se compose de r6cifs c6tiers, qui sont g6n6ralement de plus grandes dimensions et qui sont install6s sur tra pal6orelief cr66 apr~s l'6tat d'activit6 tectonique qui conclue la UTS-II. L'autre groupe se compose de patch-reefs associ6s avec les c6nes deltaiques qui se sont d6velopp6s sur les bords du bassin. Les plus profonds s6diments contemporains confiennent des assemblages de foramlnif~res planctoniques de la parfie inf6rieure et

58

moyenne du Tortonien suprrieur. Dans ce bassin, le drveloppment des r6cifs est interrompu prrmaturrment par la r6gression marine du Tortonien sup6rieur, qui coupe la connexion entre les bassins du secteur central de la Cordil- 16re B6tique et les eaux marines. La srdimentation marine pendant le Tortonien terminal - Messinien inf6rieur a continu6 seulement dans la partie la plus orientale du couloir (UTS-IV), mais on n'a observ6 aucun r6cif qui se rapporte/t cet 6tat s6dimentaire.

Mots-elrs : RScifs Miocrne suprr ieur , Cordil lrre b6tique, Chronostratigraphie, Biostratigraphie, Foramini f r res planctoniques, Uni t r s tecto-

s&limentaires.

I N T R O D U C T I O N

During the late Miocene, the basins around the Medi- terranean were favourable areas for the proliferation of reefs, which were associated with a more general development of clastie-carbonate platforms. The often spectacular appearance of these reefs has attracted the attention of many authors over the last few decades

(Chevalier 1961 ; Dabrio 1975 ; Esteban 1979 ; Dabrio et aL 1981 ; Saint-Martin & Rouchy 1990). These reefal constructions are particularly abundant in the basins of the Betic Cordillera (Fig. 1), where they exhibit a remarkable spatial and temporal variability. Thus, the basins most directly related to the Atlantic Ocean (Guadalquivir, Arcos, Ronda, Antequera) do not contain coral reefs, although elastic-carbonate platforms

J,, /

/

2 4Km / , /

IIa

x " .... I I Ia - - " : . - I I

-~ '~ '~H {GLLE RAL. - - ' l l ld ~ . ~ t:~ . . . . . l l ld ~ Ha / t ~

_ . ,--" m ~ t - ~ > " . ( , ~ - . . - - : ~ k ,r/~¢~ ~a

.... ........... - , , / oF,NE . ' . : / i . - T - - 7

I'ISI:RON_; ~ _.L. ~ (.i.,~b..;J;JO L A. "~ I i l b , IIa CANTORIA

D , ...8-

. . . ' " . . . . .

IIh O~-I~

ITALOA .~i~. ~tO S , . . . . MARMO LE~-

IV IIld " ~

Figure 1 - A : Geological scheme of the Betic Cordillera and spatial-temporal distribution of the coral reefs during the Late Miocene (based on Esteban 1979, Martfn el a/. 1989 and the autors ' own data), a : External Z o n e s , b : Internal Zones ; c : Campo de Gibral tar Complex ; d : Neogene basins ; e : Tor tonian reefs ; f : Messinian reefs. 1-13 : Neogene basins. 1 : Granada ; 2 : Guadix-Baza ; 3 : Almanzora Corridor ; 4 : Huereal .Overa ; 5 : Vera ; 6 : Sorbas-Tabernas ; 7 : Las Alpujarras Corridor 8 : Campo de Dallas ; 9 : Almerfa-N/~ar-Carboneras ; 10 : Lorca ; 11 : Campo de Cartagena ; 12 : For tuna ; 13 : Elche-Santa Pola. B : Distribution of Uppe r Miocene stratigraphic units and location of coral reefs in the Almanzora Corridor. 1 : Basin subsba tum ; 2 : U p p e r Miocene units ; 3 : Patch reefs in TSU-I I ; 4 : Patch reefs in TSU-I I I ; 5 : Coastal reefs in TSU-I I I ; 6 : Unconformity between subs t ra tum and Neogene sediments ; 7 : Principal faults ; 8 : Boundar ies between the tectosedimentary units ; 9 : Lateral and/or vertical facies changes within the tectosedimentary units. SaMma #ologtqu# de la Cordill~e Brtiqua at dtnpostaon spatt~temporelle des rdafl de ~rail du Miockne suI~rieur (bas6e mr Bddban 1979 ; Marlin et al. 1989, et des donndes propres).

59

spread over most of the basins. The resulting material, generally k3aown as "caliTa tosca", is made up of accumulations of bivalve fragments (mainly pectinids and ostreids), benthic foraminifera (often Heterostegina), bryozoaus mad red algae ; in some cases, the red algae are the predominant components of the rock (Ronda basin, algal limestone of Las Mesas Formation ; Serrano 1979). Further East, in the basins of the central sector of the Cordillera, coral reefs are found mainly in small, isolated cortstructions developed on fan deltas and coastal area~,; (Granada basin, Alcal,'i et al. 1988 ; Braga et al. 1990). However, these basins were left disconnected from marine waters during the late Tortonian (before the appearance of Globorotalia mediterranea CATALANO & SPROVIERI), and so reefal development ended prematurely. In the more eastern, clearly Medi- terranean basins, Tortonian reefs appear with characteristics similar to the others, but the largest, best developed reefs belong to the Messinian, when they formed banrier reefs, atolls and rather continuous coastal bands. Esteban et al. (1977) described the unusual features of the Messinian reefs, which are characterized by the very clear predominance of Pontes among the coral-reef constructional elements and the abundance of stromatolithic beds in the last stage of reefal development. Dabrio & Martin (1978) and Esteban (19"79) also laid emphasis on the differences between the Tortonian and Messinian reefs.

Biostratigraphically, most of the Messinian reefs are well suited to precise dating, as they normally fie partially on pelagic stratigraphic sequences and show lateral transi- tions to talus facies and basin marls with abundant planktonic foraminifera. Among the latter, the presence of G. medi terr~ea and the clear predominance of sinistral- ly coiled specimens of the N. acostaensis group (cf. Serrano 1992) indicate that the reefs were mainly formed during the early Messinian, before the salinity crisis. Esteban (1979) and Dabrio et al. (1981) have pointed out that the last stages of reefal development took place at the same time as the deposition of evaporites.

On the other hand, the Tortonian reefs are more difficult to date precisely, as they appear in association with shallow and coastal facies, in which the organic assemblages are normally much less biostratigraphically indi- cative. This may explain why the same reefs in the Almanzora Corridor have been attributed both to the late Tortonia:a (Montenat 1977 ; Eateban 1979 ; Voer- marts et o2. 1~r/9 ; Briend 1981 ; Braga & Martin 1988 ; Martin et al. 1989) and to the Messinian II-Pliocene (Dabrio & Polo 1986 ; 1987). Regardless of the age attributed the above authors note that the reefs in the Almanzora Corridor developed during a single stage of the sedimentary history of the basin. However, the tectosedimentary analysis (Guerra-MerchAn 1992) reveals the

existence of reefs related to different units of the stratigraphic record.

The object of this paper is two-fold : first, we try to show the relations between the reefs and the tectosedimentary filling of the basin ; second, we attempt to determine prec'.lsely the chronology of the reefs on the basis of planktonic foraminifera biostratigraphy both from stratigraphic series containing reefal structures and from deeper sequences, corresponding to lateral facies equivalents of the reefs.

GEOLOGICAL CHARACTERISTICS OF THE ALMANZORA CORRIDOR

The Almanzora Corridor (Fig. 1) is a narrow, elongated Neogene depression running East-West, situated in the northern part of the province of Almeria, between the Sierra de las Estancias and the Sierra de los Filabres. It is approximately 30 km long by 4 to 8 lan wide and, during the late Tortonian, was part of the channel system which connected Atlantic and Mediterranean marine waters. Specifically, the Almanzora channel linked the Hurrcal-Overa basin, of clearly Mediterra- nean characteristics, with the Guadix-Baza basin, which was a wide basin in the central sector of the Cordillera with a sedimentary history unrelated to those of other more markedly Atlantic or Mediterranean basins.

Geological setting - The Almanzora Corridor is located entirely in the Internal Zones of the Betic Cordillera (Fig. 1). In the Sierra de los Filabres most outcrops are formed by metamorphic schists belonging to the Nevado-Filabride Complex, which is the deepest tectonic unit in the Internal Zones. However, in the area closest to the Corridor, schists and marbles of the Alpu- jarride Complex appear, the intermediate tectonic unit which is superposed on the Nevado-Filabride Complex as a large overthrust. The Sierra de las Estancias rises to the North of the Corridor and is almost entirely made up of Alpujarride materials ; at some distance from the Corridor, on the northern edge, there is tectonic superposition of the Mgtlaga Betic, the uppermost structural unit of the Internal Zones.

The Almanzora Corridor began as a sedimentary basin after the latest Serravallian tectonic phase. Three fault systems striking N70-90E, N120-150E and N10-30E, with combined vertical and transcurrent movements, comprise the main strucatral features which strongly controlled the deposits and morphology of the basin (Fig. 1).

Stratigraphic synthesis - The Miocene filling of the Almanzora Corridor is characterized by a predominance of silieiclastic facies, which generally, are coarse

60

® . . . . . . © ®

: . . . . . . . *®

. . . .

on the borders and finer towards the centre of the basin. Nonetheless, recurrences of fades in the series, and tectonic events, make the reconstruction of the basin's stratigraphic architecture difficult. This may explain the considerable differences existing between the stratigraphic schemes proposed by different authors (Martin Garcla 1972 ; Dabrio 1974 ; Voermans et al. 1979 ; Braga & Martin 1988 ; Dabrio & Polo 1988).

In our opinion, four units, separated by unconformities caused by tectonic pulses and subsequent periods of erosion, can be distinguished in the Upper Miocene sedimentary filling (Fig. 2). Each deposifional sequence corresponds to a Tectosedimentary Unit (TSU), sensu Mejias (1982) mainly controlled by tectonics and the supply reception areas. In this context, the effects caused by eustatic changes are normally difficult to detect. Very briefly, the stratigraphic and sedimento- logical charactheristics of each unit are as follows :

TSU-I. These are the most ancient materials deposited after the uppermost Serravallian stage of tectonic activity. They appear in small, discontinuous outcrops located on the borders of the basin. In general, the series commence at the base with very shallow marine deposits (these are sometimes absent) changing to continental deposits towards the top.

Figure - Stratigraphic 2 columns representing the sedimentary filling of the Almanzora Corridor and poaition of reefs. TSU-I : Conglomerates and sands with marine fauna at bottom ; TSU-II : Red conglomerates and breccias from alluvial fan (lla), and grey conglomerates and silts with reefal limestones from fan delta (IIb); TSU-III : Conglomerates and bioclastic sands from shallow marine platform (IIIa), grey conglomerates and sands with reefs from fan delta (IIIb), coastal bioconstructions (llIc), and basin marls with turbidites (IUd) ; TSU-IV: Light-coloured pelagic marls ; TSU-V : Plio-Ouaternary continental sediments. CO/OhmS stratigrapbiquas reprdsentatives du remblaiement sedimentaire du Couloir d'Almanzora at emplacement des rdcifi. UIS-1 : Conglomdrats et ar~nes avac do la faune marine d, la base ; UTS-II : Conglomdrats at br~hes alluviaux rougas (lIa), Conglomdrats at limons grts dalta~quas avac des calcatres de rdc,f (lib) ; UTS-1II : Conglomdrats et

[7~Substratum ardnes biodasliques matins ffeu profonds (Ilia), Cong~n&ats et ardms gris deltafques avoc des Marl (Illb), Bioconslructions o~'t~ras (Ilia), Marnes

i ---]Neog-Ouat, do has, in dos ~rbtditds avec gad) U'I$.IV : Mamas pdlagiqu~ daircs ; UI~- V : Matdriatr¢ ¢ontirtcntm~ plio-quaternaires.

TSU-II. The materials of this unit lie unconformably on the basin substratum, overlapping the deposits of TSU-I. It comprises two lithologlcal groups. The lower of these (IIa) consists of red conglomerates with a lutitic-sandy matrix (100 to 200 m thickness), corresponding to debris flow and channel deposits originated in alluvial fan environments. There is a gradual change to the upper lithological group (IIb), consisting of about 50-80 m thickness of grey conglomerates and silts with interca- lations of ostreid beds and reefs developed in fan deltas. Silty and marly sediments of the delta front and basin appear towards distal areas.

The superposition of deltaic sediments on the alluvial deposits indicates a relative sea-level rise in a context of considerable subsidence and reduction of detrital supply, causing important accumulation of sediments arranged in positive megasequences.

TSU-III. This unit is separated from the previous one by an important regional unconformity (Fig. 3a) related to a compressive tectonic event causing deformation of the rocks and considerable palaeogeographic changes in the basins of the central and eastern sectors of the Betic Cordillera (Montenat 1977 ; Gonzgdez Donoso et al. 1980 ; Ott d'Estevou 1980 ; Briend 1981 ; Est6vez et al. 1982 ; Rodriguez Ferngmdez 1982). Its deposits cover

TSU-II and at times even lie directly on the basin substratum.

Several tithological groups, associated by lateral and vertical facies changes, can be distinguished in TSU-III. In the middle part of the basin a very shallow platform became established, which was frequently subjected to

61

strong currents and on which conglomerates and bioclastic sandstones (30-50 m thickness) with abundant cross bedding were deposited (IIIa). Fan deltas developed towards the West, where about 70-100 m thickness of conglomerates and sands were deposited intercalated with ostreids, red algal and coral bioconstructions (IHb). Large reefs developed in the eastern sector (IHc).

b

e d

Figure 3 - a : Unconformity between fan delta deposits of TSU-II and reef slope deposits of TSU-III. b : Patch reef in fan delta of TSU-II, located near Purchena. c : Patch reef in fan delta of TSU-III ; Baza to Hu&cal-Overa road, km 26.3. d : Coastal reef of TSU-III developed on the Alpujarride substratum of the basin, about 3 km towards East of Urr~cal. e : Detail of the patch reef of Fig. 319, showing a succession formed by Porttes at the lower part and Tarbellastraea at the upper part. a : Discordance entre des ddp6ts deltafques de la UTS-II et des ddO6ts du talus recifal de la OTS-III. b : Patch reef dans ddp6ts delta~ques de la UTS-II, localisde dana la voi,*inage de Purchena. c : Patch reef clans d~p6ts deltafques do la UTS-II1 ; route Baza ~ Huerca'~Overa, km 26"3. d : Rddf cblter de la UTS-Ill, ddvdoppd sur le substrat AlpuJarride du basMn, e : Ddtail du patch reef de la Fig. 3b. On montre une succession ~raposde par Porites d a m la pattie infdrieure et Tarbellastraea dans la partie sttp~'eum.

Towards the central areas of the basin, deposits change laterally and vertically to marly sediments with turbidites (IIId)upto 300 m in thickness, although they do not reach any considerable depth.

The deepening of the basin deduced from the lower part of the stratigraphic sequences seems to be related to a distensive tectonics, which followed the compressive phase mentioned above, and is reflected in the existence of numerous synsedimentary normal faults. Nonetheless, the upper part of this unit shows evolution of lithofacies and biofacies which indicate marine regression.

TSU-IV. Owing to the compressive tectonic stage ending TSU-III, the marine connection of the Mediterra-nean with the basins of the central sector of the Betic Cordillera ceased. In the Almanzora Corridor marine sedimentation (TSU-IV) was restricted to the easternmost part (Albox area) and is more widely spread in the Hu6rcal-Overa basin. In these areas, the deposits comprise of light-coloured marls with abundant planktonic microfauna, very similar to the pre-evaporitic pelagic deposits of the other Mediterranean basins. In the western sector, near Tijola, a small outcrop made up of coastal grey sands and silts are also considered to TSU-IV.

An additional lithic unit with tabular morphology (TSU-V) is found unconformably on the Miocene sediments. This is composed of conglomerates, sands, days, travertines and limestones and represents to the Pliocene-Pleistocene sedimentation, which took place entirely in continental environments.

REEFS OF THE ALMANZORA CORRIDOR

According to the brief stratigraphic description above, there are two episodes in the Neogene history of the Almanzora Corridor (TSU-II and HI) in which reefal development took place together with shallow detrital sedimentation (Figs. 2, 5).

In TSU-II, patch-reefs are found associated exclusively with deltaic deposits located on both borders of the basin (Fig. 3b). Within the fan deltas, the reefal patches developed in areas protected from the detrital inflows. The dynamics of these depositional systems, with fre- quent modifications in the progradafion direction of the deltal lobes, interrupted the development of the reefs at some points, and allowed others to grow. In this context, the resulting reefs are small, 1-2 m thickness and several tens of metres wide, although their size is increased wherever several bioconstruetions linked together. The widest build-ups are found in the vicinity of Purchena and Los Mfirmoles (Figs. 1,2). Other smaller build-ups are spread along the northern border between Lficar

62

and Partaloa. The main components of the reefai- structures are red algae and coral, of which Porites and Tarbellastraea predominate and, very locally, Platyg~a occurs (Dabrio & Polo 1986 ; Braga et al. 1988).

Two types of reef can be distinguished in TSU-III. The first (Ser6n and Ohla del Rio reefs) corresponds to patch-reefs (Fig. 3c) similar to those of TSU-II, which are also associated with fan deltas, but are located mainly on the southern border of the basin, which shows more accentuated subsidence than the northern edge. The other type (Fig. 3d) is made up of normally larger coastal reefs (10-25 m thickness and kilometer order length), which came to develop slopes towards the centre of the basin. These constructions are mainly located in areas protected from siliciclastic supply on the palaeorelief formed after the intra-Tortonian tectonic phase separating TSU-II and III. Most coastal reefs are found between Urracal and Los MLrmoles on the northern border and between Fines and Cantoria on the southern border (Figs. 1,2). In both types of reef the main bioconstructors are Potites and Tarbellastraea, with more local occurences of Plafygyra, Siderastraea and Palaeoplesiastraea (Martin et al. 1989). By contrast red algae played an insignificant role in these build-ups.

Martin et al. (1989) have shown that, generally, the reefs present a dominant internal structure made up domi- nantly of cyclical successions. These begin with an association of Podtes/red algae (or only Porites) in a substratum with abundant silt, and end with predominant development of Tarbellastraea without silt (Fig. 3e). This sequence shows a normal ecological evolution of reefal development.

Biostratigraphic control and ehronostratigraphy of the reefs - The biostratigraphic control of the reefs was carried out on the basis of a slightly modified version of the upper Miocene planktonic foraminifera biozonation by Serrano (1979). In Fig. 4, this biozonation is correlated with both the zonation of Blow (1969) and the standard chronostratigraphic scale. In bio-chronostratigraphic correlations, the first appearance datum (FAD) of Globorotalia mediterranea C A T ~ O & SPROVIERI (= G. conomiozea KENNEF) has normally been used to denote the limit between the Tortonian and Messinian stages (D'Onofrio et al. 1975 ; Zacha- riasse 1975 ; Serrano 1979 ; Iaccarino 1985, etc.). The successive appearances of Neogloboquadrina acostaensis (BLOW), Neogloboquadrina humerosa (TAKAYANAGI & SArro) and Globorotalia suterae CATALANO & SPRO- VIERI allow the distinction of three Tortonian zones. BiostratigraphicaUy, we have used the FAD of N. humerosa to subdivide the Tortonian into two intervals (lower and upper Tortonian).

The Messinian can optionally be subdivided by different bioevents, such as the FAD of Turborotalita multiloba

63

MESSIN

, 6.7-

GLOBIGERINOIDES ELONGATUS s h i f t c o i l i n g ill Neogloboquadrina G. elongatus FAD

'I

"ALIA MEDITJ ~ G, mediterranea FAD

I~ K'VP A T T L~ ~;~ T I"1

G. suterae FAD

TORTOb

. . . . 10.4

SERRA~

~UADRINA t-i

zt;,,L

J ~ lO0tam

'UADRINA A COSTAENSIS

p e r s t s t a l l t s l n i s t r a l Neogloboquadrina

G. pIesiotumida FAD

--~ N. humerosa FAD

G. extremus FAD

-J-- N. acostaensis FAD

Figure 4 - Upper Miocene bio-chronostratigraphic correlations. Available bioevents of planktonic foraminifera in the Mediterranean domain. Chronometric age,s by Harland et al. 1990. Corrdlations bio-cbronostratigraphiques du Miocene Supfrieur. Biodvdnmnents des foramini~eres planctoniques uttlisables darts le d~raaine mdditerrande. Ages chronomdtriques selon Harland ot al. 1990.

64

(ROMEO) (cf D'Onofrio et at 1975) or the FAD of Globigerinoides elongatus (D'ORBIONY) (cf. Scrrano 1979). Another easily detectable alternative, which seems to offer better results, is the use of the first Messinian change to dextral coiling in Neogloboqua- dtina, which is one of the clearest signs indicating the proximity of the latest Miocene salinity crisis in the Mediterranean.

NASINI, ex D'ORBIGNY) and G. menardii form 4 TJALSMA.

Finally, somewhat later, the Neogloboquadrina acostaensis (BLOW) group, which includes the N. conti- nuosa-acostaensis-humerosa plexus, persistently acqu- ired a predominantly sinistral coiling (previously the group occurs with variably dextral or sinistral coiling, in many cases in similar percentages).

The short time in which the Almanzora Corridor reefs developed has obliged us in the interest of greater chronological precision to take into account other Tortonian bioevents which can be detected in the Mediterranean domain. We have therefore also considered:

The FAD of Globigerinoides extremus BOLLI & BERMUDEZ, prior to that of N. humerosa. This bioevent has been chosen as a zonal marker mainly by Italian authors (D'Onofrio et al. 1975 ; Iaccarino & Salvatorini 1982).

Above the FAD of N. humerosa, the FAD of Globorotalia plesiotumida BLOW & BANNER is detected, together with morphotypes assignable to G. pseudo- miocenica BOLLI & BERMUDEZ, G. limbata (FOR.

We have situated the reefal development of the Alman- zora Corridor in this bio-chronostrafigraphic model. (Fig. 5). The patch-reefs of TSU-II are found at different levels in the stratigraphic sequence of the IIb lithological group, related to very shallow marine environments. Although the marly sediments of the inner part of the basin still contain benthic foraminifera assemblages typical of shallow environments (abundance ofAmmonia, Elphidium, Flodlus), they normally contain planktonic foraminifera, such as N. acostaensis, G. e.xtremus, G. ex gr. menardii and, in the upper part of the series, N. humerosa and forms close to G. plesiotumida. In addition, Neogloboquadrina displays predomlnafly dextral coiling, although it is sinistral in some levels. According to this, lithological group Wo and

l TECTONIC LOCAL SEA GLOBAL EU~FATIC CUEVES STRATIGRAPII1C SYNTHETIC BIO- EVENerS LEVEL SEQUENCE EVENTS CHANGES HAQ et a/. (! 387)

oob:d/.6o9O Tsu-~ ~ ~ o %

. . . . . . A-G. medtterranea FAD

~- G. suterae FAD

" ~ U "--~------

. = 7 . ~

"-- ~--~-- ~;2.tS:Q ~ ' ° ' ° 0-~

~ _ p e r s i s t a n t s i n i s t r a l N~ogloboquadrina

G. pIeslotumida FAD

~- N. humerosa FAD

~ d i t

AG E

Tr J s - r 7 5 5 0 2 5 0 a - a ~ e g - " k -- - L . - -- a - -- 1..~.1~ . . . . .

3 . 3 . ME5 SIN

\

I

I

p.

Figure 5 - Chronostratigraphic ranges of reefs and Miocene tectosedimentary units (TSU) in the Almanzora Corridor and correlation between local events and global eustatic curve by Haq et al. (1987). 1 : Coastal reefs. 2 : Patch reefs. ~ abrono- stratigraphiqua des rdai)~ et des unitSs tectosddimantairas (UTS) miochnas du Couloir d'Almamora at corrdlation entre dvdnement~ locaux et la courbe austatique gdndrale de ttaq at al. (1987). 1 : rdcid~ c6tters 2 : patch reaJ~.

65

the reefs associated date to the younger part of the early Tortonian (predominantly dextral N. acostaensis and G. extremus assemblage) and the early part of the late Tortonian (assemblage of G. ex gr. menardii and N. hurnerosa wi~h variable coiling).

There are some difficulties involved in the correlation of this stage when the first reefs formed with the cycles of eustatic variations. The top of TSU-II should be close to the boundary between zones N.16 and N.17 (FAD of G. plesiotumida) and therefore correlate with the third-order cycle TB 3.2 of Haq et al. (1987). But the relative sea-level rise deduced from the strafigraphic sequences in this unit is accompanied by strong subsidence, and it is difficult to detect the eustatic effect. It seems probable that the transgression coincided with the eustatic rise of cycle TB 3.2, but it may also be possible that part of the alluvial facies at the base of the trait (together with TSU-I) could have been deposited during cycle TB 3.1. Sequences similar to TSU-II in other basins (e.g. the Atalaya basin in Murcia) commence with marine deposits before the FAD of G. extremus.

The lowest layers of the TSU-III exhibit planktonic foraminifera assemblages similar to those at the top of the TSU-II, corresponding therefore also to the lower part of the upper Tortonian. This means the are unconformity separating both units does not imply a substantial stratigraphic gap in the whole of the basin. Throughout the stratigraphic sequence typical forms of G. plesiotu- rnida appear, and a little higher up the predominant coiling of Neogloboquadrina clearly becomes perma- nently "si~'stral. On the other hand, no specimens of G. suterae and G. rnediterranea appear here, whose FAD are detected iha TSU-IV. Taking into account that there are reefs throughout all of the TSU-III, we may therefore conclude that reefal development continued during the middle part of the late Tortonian.

The regression detected in the upper part of the unit could be correlated with the eustatic fall characterising the upper part of cycle TB 3.2 of Haq et al. (1987).

C O N C L U S I O N S

During the late Miocene, the Almanzora Corridor formed a na=row, subsiding, elongated channel flanked by high, unstable reliefs. These conditions were conducive to the massive input of detrital sediment, which were mianly accumulated in alluvial fans, fan deltas and ~shallow marine environments. Tectonic pulses occur~ring during the Miocene filling allow differentiation of four depositional units separated by slight unconformities (Tectosedimentary Units I to IV).

During the deposition of Tectosedimentary Units II and I11, in coastaLl and very shallow areas reef growths

developed with the formed predominantly by Porites and Tarbellastraea corals and red algae. In fan deltas, the abundant input of detrital sediment gave rise to an unstable environment which inhibited the prolonged growth of the reefs, and simultaneously formed new areas where reef build ups could take place. The result was the proliferation of numerous small organogenic build ups (patch-reefs). Relatively large reefs only developed in coastal environments sheltered from the detrital inputs during the deposition of TSU-III.

Deeper marine sediments related laterally and vertically to the reefs contain planktonic foraminifera which permit dating. The reefs belonong to TSU-II developed during the terminal part of the early Tortonian and the earliest part of the late Tortonlan. The reefal constructions continued during the deposition of TSU-IH, in the interval corresponding to the early and middle part of the late Tortonian.

The termination of the deposition of TSU-III is related to an important marine regression, which caused the definitive continentalization of the basins in the central sector of the Betic Cordillera and interrupted reefal development. The western sector of the Alrnanzora Corridor was affected by these events, while the easternmost sector continued to behave like Mediterranean basins, which were covered by marine waters during the latest Tortonian and the early Messinian (TSU-IV). Unlike most of these other basins, in the Almanzora Corridor no reefs have been observed in relation to the last Miocene marine stage. If any reefs did develop, they have since been eroded or are covered by more recent continental sediments.

Acknowledgements - This paper has been financed by the Spanish Direcci6n General de Investigaci6n Cienfffica y T6cnica (DGICYT) (Project 398-CO2-02) and by the Junta de Andalucfa (Research Groups 4085 and 4039). Our acknowledgment to Dr. R. Anglada, Dr. W.A. Berggren and Dr. J.P. Saint-Martin for their critical reading and observations, which have undoubtedly improved the original text substantially.

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