A Comparative Study of the Diagenesis in Diapir- Influenced Reef Atolls and a Fault Block Reef Platform in the Late Albian of the Vasco-Cantabrian Basin (N orthern Spain) J. REITNER 1 1 Introduction The Mesozoie Vasco-Cantabrian Basin is situated in the Spanish Basque-Lands of northern Spain, dose to the French bord er (Fig. 1). To the west the basin is delineated by the Paleozoic massifs of the Cantabrian Mountains, to the east by the Pyrenees, and to the south by the Late Tertiary Ebro Basin. The tectonic history of the Vasco-Cantabrian ranges from the Triassie to the Tertiary but the major part of its fill (8000 m) is attributed to Late Mesozoie deposition. During the Late Albian due to rapid subsidence, salt diapirs (Keuper age) developed between basement fault blocks. Shallow marine carbonate reefs (Urgonian Facies) developed on these highs . Two types of reef can be distinguished: (1) larger fault block reef platforms on tilted basement segments and (2) smaller reef atolls on top of rising diapirs. The diapir reefs can be differentiated from the fault block reefs through sedimentological, paleobiological, diagenetic, and geochemical criteria. The purpose of this paper is to present the different diagenetic phases in diapir reefs and fault block reef development. To achieve this, the respective paleobiological and sedimentological frameworks will be described first and the petrographie and geochemical aspects of the diagenetic sequences will be treated subsequently. 2 Previous Studies The first detailed work on the "Keuper" diapirs of northern Spain by Lotze (1953) did not indude facies analysis. Brinkmann et al. (1967) studied some of these diapirs in greater detail using facies and structure analysis. Von Stackelberg (1967) first suggested that the small-sized carbonate reefs of the Late Campanian Oro Limestone situated above the Murguia diapir may have been indirectly link- ed to the diapir . Other similar smaller carbonate reefs also exist near the Villa- sana de Mena diapir of the Vasco-Cantabrian Basin. These latter reefs are es- pecially interesting since the presence of large quantities of reworked Keuper ma- terial indicates a collapse event in the Villasana de Mena diapir during the Late Albian (Vraconian) (Schroeder 1980, Schroeder and Willems 1983, Reitner 1 Institut für Paleontologie, Freie Universität, Schwendener Straße 8, 1000 Berlin 33, FRGermany Reef Diagenesis Edited by J. H. Schroeder and B. H. Purser © Springer-Verlag Berlin Heidelberg 1986 - - - _. Astur ia':;- - Transcurrent +-+ 1 Fault +- . 2 -l---I- 3 1- -1-4 "TTr 5 ---->. 6 ~ :..-=-~ 7 ~---- • • 8 -- ; 9 '50km' KI Fig. 1. Location map and geological structure of the Vasco-Cantabrian Basin in northern Spain (Reitner 1985). 1 Anticline; 2 assumed anticline; 3 syncline; 4 assumed syncline; 5 thrust fault; 6 strike slip fault; 7 assumed strike slip fault; 8 salt diapirs; 9 salt pillows ;I> (') 0 3 'Cl I>l ..., ~ :;:- '" (J) <= Cl. '< 0 ...., ;. '" 0 jii' O ~ 188 J . Reitner 1980, 1982). According to Reitner (1985), this mayaiso be the case in the Murguia diapir. This presentation deals with the carbonate reefs associated with these two diapirs. 3 Methods The Villasana de Mena diapir and the Murguia diapir were mapped in detail. Sec- tions across each diapir were measured and the lithological changes were es tab- lished according to biologie, sedimentologic, and petrographic criteria. A similar procedure was followed in the fault block constituting the Albeniz-Eguino reef platform of the same area. Diagenetic features were studied by petrographie microscope, scanning elec- tron microscope (SEM) with energy dispersive X-ray analyzer, atomie absorption spectrometry (AAS), and X-ray fluorescence. Selected thin sections were stained with potassium ferricyanide in order to distinguish between Fe and Fe-free cal- cites. 4 Diapir Reef Atolls 4.1 The Late Albian Villasana de Mena Reef Atoll (Caniego Limestone) 4.1.1 Reef Structure Due to outcrop conditions, the original reef structure cannot be determined with precision. The size of the reef, however, may correspond to the diameter of the diapir roof which may have measured 2 to 5 km. Sedimentation in the surround- ing basins consisted of pro delta muds (Blank 1983). The ascending diapir formed a submarine dome which lifted the diapir roof out of the terrigenous influence into the photic zone and allowed the development of a reef. In an advanced stage, the diapir roof collapsed due to salt subrosion. In this process the reef car- bonates were fractured and dissected by fissures and creviees through which diapiric material, such as Keuper pelites, hypersaline brines, and Keuper ophites, were extruded. A small diapir basin may have developed within the reef with a surrounding barrier of grains tone shoals and coralgal framework. With this atoll-like structure, water circulation between the open sea and the diapir lagoon was probably restrieted. 4.1.2 Facies Belts of the Caniego Limestone (Fig. 2) There are five distinctive facies belts within the Caniego Limestone, but only the key facies for diapir reef identification are summarized here. A more detailed profile, which includes 16 facies zones, is presented in Reitner (1985). The facies belts, ranging from the basin and slope to lagoonal, have sharp contacts. A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls 189 The basin and slope facies belt is distinguished by reef debris and planktonic foraminifera while the reef core belt contains the coralgal framework. The reef flat belt is characterized by rudists and grainstone shoals while the lagoonal facies contains a hypersaline fauna (miliolid foraminifera and thin-shelled ostra- cods) and tempestites. The intertidal facies belt resembles the typical Loferite mudstone facies of the Late Triassie Dachstein Limestone of the Northern Calcareous Alps (Fischer 1964). 4.2 The Late Albian Murguia Diapir Another example of an Albian diapir reef system is found 50 km east of the Vil- lasana de Mena diapir. The Murguia diapir exhibits further facies and diagenetic peculiarities as a result of the influence of salt brines. The first major feature of the Murguia diapir is the ooid facies which was aragonitic. This is in sharp contrast to other Cretaceous ooid facies which were exclusively calcitic (Sandberg 1975,1983). The Mg/Ca ratios in the Cretaceous open sea was 2: 1, while in the area of the diapirs the Mg/Ca ratio may have reached 5: 1 due to the Mg-rich salt brines. This may have promoted the production of aragonitic ooids in the diapir reef lagoon (Lippmann 1973) (Fig. 7 e, f). The second major feature of the Murguia diapir is the presence of sulfide ores in the fractures and fissures of the reef. The circulation of diapiric waters through the reef complex caused the formation of sphalerite, galena, pyrite, etc. (Lietz 1951) (Fig. 8d). The Vasco-Cantabrian Basin contains a large resource potential in sulfides and sulphates. 5 Fault Block Reef Platform 5.1 The Late Albian Albeniz-Eguino Reef Platform (Figs. 1 and 3) This reef platform is situated about 70 km east of the Villasana de Mena diapir and is about 150 square km in area. The reef beds range from 200 to 500 m in thickness and consist of eight separate reef bodies, each separated by deep grabens filled by siliciclastic pelites and carbonate turbidites. The Albeniz- Eguino reef platform is situated on tilted fault blocks (the Aitzgorri high), this type of reef platform being a typical shelf-margin island platform. The sedimentary cycles comprising the platform carbonates are transgfessive. They range in age from the uppermost Middle Albian to the lower Late Albian (R otalipora ticinensis zone) (Reitner 1985). Five facies belts are distinguished in this reef platform. The basin and slope facies belt contains common planktonic foraminifera and debris flows of reef material containing coralline sponges while the reef core facies belt is composed of a coralgal framework and the reef flat facies has caprinid rudists . The carbonate intertidal facies belt contains mud cracks and stromatolites. The hyper- saline lagoon facies is characterized by miliolid foraminifera while the littoral intertidal facies consists of siliciclastics. Facies belts Sequences of Diagenesis '" ~. Marine phreatic Marine vadose Fresh water vadose Marine phreatic Marine vadose Fresh water phreatic Fresh water vadose Fresh water phreatic Deep burial Major Lithology Fauna Flora Tide Basin/slope Wackestones: Packstones Planktonic foram Planktonic crinoids Cephalopods Reworking of reef sediments Stromatolitic hardgrounds with phosphorites Partial replace- ment of the phos- phates by pyrite Part ial rework- ing of the stro- matolitic crusts Dissolution of aragonite Neomorphic calcite Bladed ferroan calcite Stylolites Bitumen Reef core Reef tlat Framestones Floatstones Microsolenid- corals Coralline algae Coralline sponges Dissolution of biogenics Red micrite filling Dog tooth 11- dripstones Caprinal Plagloptychus belt Wacke-, Float-, Packstones Caprinid rudists Rhodoliths of coralline algae Orbitolinid forams Micrite envelopes Aragonite dis- solution; molds Dog tooth 11 dripstones Vadose slit Reworking; lithoclasts Micrite fill Radiaxial fi brous calcitic cement Red micrite fil!- Syntaxial calcitic ing; calcitic overgrowth dog tooth llI -cements Equant calcite cement Partial dolomiti- sation of cements Stylolites Bitumen Stylolites Bi tumen Grainstone shoal Cross-bedded grain- stones, rudstones Trocholinid forams Micrite envelopes. coated grains Beachrock; meniscus and dripstone cements Syntaxial ca1citic overgrowth Equant calcite Bitumen Lagoon lntertidal Polyconites/ Mudstone bell Toucasia bell Wackestones Floatstones Requienid rudists Carpotinid rudists Miliolid forarns Dissolution of aragonite Dog tooth 11- dripstone Vadose silts Fibrous isopachus cement Dog tooth 1lI- calcite cernent Brecciation Rotation of cornponents Fissures Micrite fil! Dog tooth llI-cernent Equant ferroan calcite cement Stylolites Bitumen Mudstones Wackestones Packstones Ostracods Stromatactis Sulphates, pro to- dolomites Replacement of sulfates by quartz Micrite fill Microkarst Dog tooth l-drip- stone in strorna- tactis Equant ferroan calcite Replacernent of ferraan calcite by dolomite Calcedony Pyrite Gypsurn cements Sulfur cernents Stylolites Bitumen Mudstones Wackestones Bindstones (Stom.tolites) Ostracods Storrn layers Fibraus dripstone- cement Sheet cracks, tepees Intraformational breccia Equant ferraan calcite cernent Dolomite/ ankerite Authigenic quartz cement Stylolites Bitumen Fig. 2. Facies model and diagenetic environments of the Villasana de Mena diapir reef platform (Caniego Limestone) 192 Fades beils Sequences of Diagenesis -! ~. Marine phreatic Marine vadose Fresh water vadose Fresh water phreatic Deep burial Major Lithology Fauna Flora Basin/slope Grey marls Turbidites Debris flows Planktonic rarams Ammonites Sponges Reworking of reef sediments Dissolution of the siliceous spange scleres Fibrous cement Neomorphic calcite high Mg-calcite to low Mg-calcite pre- serving the original crystal size Equant calcite Bitumen Sponge Mud Mound Bindstones Packstones Siliceous sponges Coralline sponges Bluegreen algae Stromatactis Sediment fill ShoTt fibrous calcite cement Sediment fill Dissolution of the sponge scleres Radiaxial fibrous calcite cement Sediment fill Neomorphic calcite Equant ferroan calcite Bitumen Reef core Framestones Floatstones Microsolenid-corals Coralline algae Coralline sponges Micrite fill with- in the coralgal frame Short fibrous calcite cernent Multiphased sediment fill Neornorphic calcite, dissolution of aragonite Molds Microkarst Dog tooth 11· dripstone Sediment fill Dog tooth 111· cement Equant calcite cernent Syntaxial calcitic overgrowth Stylolites Bitumen J. Reitner Reef f1at Caprina bell Floatstones Packstones Graintones Caprinid rudists Radiolitid rudists Articulate coralline algae Calcareous green algae Orbitolinid forams Reworking of (arge elasts Micrite envelopes coated grains Dissolution of sponge scleres Short fibrous calcite cement Brownish radiaxial fibrous meniscus cement Neornorphic calcite micritisation of the Mg-calcite Equant ferroan calcitic cement Syntaxial calcitic overgrowth Stylolites Bitumen Fig. 3. Facies model and diagenetic environmens of the Albeniz-Eguino fault block reef platform A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls Reef flat Grainstone belt Grainstones Rudstones Trocholinid forams Coralline algae Reworking of elasts Micrite envelopes coated grains ShoTt fibrous calcite cement Neomorphic calcite Dog tooth lll- cement Equant ferroan calcite Syntaxial calcitic overgrowth Stylolite, Bitumen Intertidal mud-pank Mudstones Wackestones Bindstones (Stromatoljte,) Ostracods Short fibrous isopachous cemeot Shrinkage Brownish radiaxial fibrous calcitic cernent Dog tooth 1- dripstone calcite cement Dissolution of aragonite Mold, Dog tooth 111- calcitic cement Equant ferroan calcite cement Bitumen Lagoon Siliciclastic litto ral Wen bedded wacke- WeIl sorted medium stolles, mudstones and fine grained Miliolid and beach and inter- arenaceous f orams tidal sands tones Glauconite in forams Lithoclasts Reworking of elasts Micrite envelopes with micrite envelopes Neomorphic calcite molds Dog tooth lll- calcitic cements Chert replaced calcite Equant ferroan calcite cement Pyrite Dissolution of Bio- genies Mold, Dog tooth lll - calcite cement Inter granular granular calcite cement Limonitic cemen t relacing the calcitic granular cement Syntaxial calcitic overgrowth Sparry dolomites replacing the granular cemenls 193 Table 1. Characteristics and interpretations of observed cements -\0 ""'" Cement Fig. Composition Habitlfabric Dimensions Interpretation Reference B1aded scalenohedral type I 4a Neomorphic Dripstone with relic 40-80 Ilm Marine vadose Purser (1969) low-Mg calcite structures of a fibrous aragonite cement Hanor (1978) cement type II 4b Low-Mg Dripstone without inclu- 50-60 Ilm Freshwater vadose Schneider calcite sions cement (1977) type III 4e, 7a, b Low-Mg Isopachous crusts 80-330 Ilm Freshwater phreatic Pierson and calcite cement Shinn (1985) Radiaxial 4d,5d,6b Neomorphic Dripstone, meniscus, 20 Ilm-5 mm Marine phreatic-marine Bechstaedt fibrous cement low-Mg calcite multiphased isopachous vadose aragonite (1974) crusts cements Short fibrous Neomorphic Isopachous crusts 60-100 Ilm Marine phreatic high- Longman calcite cement low-Mg calcite Mg calcite cement (1980) (-20/0 MgCo3 , EDAX) Micro-granular Neomorphic Isopachous crusts 15 -30 Ilm Marine phreatic high- Longman calcite cement low-Mg calcite Mg calcite cement (1980) (-1.5% MgCo3 , EDAX) Micritic cement Neomorphic Dripstone 8-10 Ilm Marine vadose high- low-Mg calcite Mg calcite cement (-4-5% MgCo3 , EDAX) Fibrous calcite 5b Neomorphic Tannish colored iso- 80-100 Ilm Marine phreatic cement low-Mg calcite pachous crust aragonite or high-Mg calcite cement !-' :00 Equant calcite 4c Low-Mg B10cky crystals with 100 Ilm Freshwater vadose Longman !!. cement calcite enfacial junctions cement (1980) Ei Cl> ... Equant ferroan 5d Fe calcite B10cky crystals with 100 Ilm Freshwater phreatic Oldershaw and calcite cement .... enfacial junctions cement (shallow burial) Scoffin (1967) Bladed cements Low-Mg Bladed crystals 100 - 120 11m length Meteoric cement Longman calcite 30 - 45 11m width (1980) Syntaxial Low-Mg Syntaxial overgrowth on 200-500 11m Freshwater phreatic Neugebauer cements calcite echinoderm and ino- cement and Ruhrmann ;.. ceramid skeletal remains (1978) n 0 Sulfate cements 7d Gypsum, Limpid idiomorphic 50- 800 11m Burial cement 3 '0 I» anhydrite elongated crystals ..., ~ Sulfur cements 8a,b, c Elementary Blocky crystals 100-530 11m Burial cement Feely and ~t (1) sulfur Kulp (1957) Vl 8' Ellison (1971) Po '< Sparry dolomite 7c Dolomite with Idiomorphic crystals 120-300 11m Burial Mattes and 0 -, Fe-rich zones replacing earlier cements Mountjoy g. (1) (X-ray (1980) t:) analysis) rö" OQ (1) Anhedral Dolomite Dolomicrite with brown 4- 1O llm Intertidal, early dia- Fischer (1964) ::; dolomite coloured anhedral crystals genetic formation Folk and Land ~. (1975) 5" t:) Ankerite Ankerite (X- Idiomorphic crystals 100-350 11m Deep burial cement Oldershaw and rö" '0 cement ray analysis) Scoffin (1967) =r Early diagenetic 8a, f Quartz with Idiomorphic crystals 10-50 11m Early diagenetic , prior Grimm (1962) 5' ~ authigenic indusions of commonly pseudomorphic to lithification ~ (1) sulfates, after anhydrite ::; quartz pH 9) favor the precipitation of sulfates whereby quartz is dissolved, while under acidic conditions ( < pH 5) quartz is precipitated. The formation of in-situ authigenic quartz in the Villasana de Mena diapir reef was probably influenced by both pH and hypersaline condi- tions. The hypersalinity was caused by the diapir waters, while the acidity prob- ably was related to the presence of vadose freshwater. This type of quartz is not observed in the fault block reef platform. 200 J. Reitner A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls 201 7.2 Allochthonous Idiomorphic Quartz In addition to the in-situ authigenic quartz, allochthonous idiomorphic quartz occurs in increased quantities at the base of the Caniego Limestone where a debris flow of diapir material (Keuper clays and ophites) is present (Schroeder 1980, Reitner 1982). This idiomorphic quartz is partly fractured and is identical to authigenic quartz from the Early Jurassie Carniolas Formation of this area. 8 Microstalagmitic Dripstone (Figs. 4 b, 5 b) A new type of vadose cement which is reminiscent of typical cave dripstones is observed in both types of reef. According to Purser (1969), microstalactitic cement crusts (Fig. 5 e) can be found in large open pores, especially in beach- rocks. In some fissures and molds microstalagmitic cements are present. This type of microcave dripstone indicates a long period during which pores remained open. There is no preferred cement type within these dripstones, but dog tooth (11) and radiaxial fibrous cements are common. The cement crystallength varies between 100 and 280 !-lm. 9 Microkarst In both diapir and fault block reefs 0.5 -10 cm cavities with irregular shapes cut sedimentary and biogenie structures. Such cavities are characteristic of microkarst and are morphologically distinct from stromatactis fabrics, lacking the irregularly shaped roofs and flat bottom surfaces (see Sect. 10). 10 Stromatactis (Figs. 4b, 6a) Stromatactis-like voids occur in carbonate-rich (more than 950/0) mudstones, wackestones, and bindstones of both reef types. These el on gate voids measure 1 - 5 cm in length with a vertical width 5 -10 mm. The roof of these voids is irregular while the bottom is flat and partly filled with sediment. A halo of very Fig. 6. a Tempestite in lagoonal facies of the VMDRP. The tempestite contains a basal packstone unit (p), a central grainstone unit (g), and an upper wackestone unit (w). 1 Stromatactis fabric; 2 synsedi- mentary fissures with multiple micrite fillings; 3 internal sediment of the stromatactis fabric; 3 ' internal sediment of the stromatactis fabric rota ted during a probable collapse event. b Large cavity in the reef core of the AEFBP. 1 Short fibrous marine cement; 2 brownish radi axial fibrous cement; 3 vadose silt; 4 late diagenetic nonferroan calcite cement. c Intertidal muds tone with sheet and prism cracks filled by dripstone cements from the VMDRP. d Hard-ground of the basin and slope belt of the VMDRP. 1 Forereef debris (packstone); 2 phosphorite layer; 3 pyrite layer; 4 hard- ground surface with encrusting organisms; 5 pelagic mudstone. Arrows indicate upper surface J . Reitner Fig.7 A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls 203 dense micrite surrounds the stromatactis voids, which is in sharp contrast to the less dense micrite of the matrix rock. The grain size of both types of micrite is similar (0.5 -1 ~m, SEM). These stromatactis voids presumably are the product of a very early compac- tion of the micritic sediment (HeckeI1972, Bechsteadt 1974). Tsien (1985), on the other hand, believes that stromatactis fabrics are biogenic in nature, as described from the type locality at St. Remi in Belgium. The origin and nature of st~omatactis is unelear and Tsien gives an overview of the problem. The forma- tion of stromatactic fabrics may weIl be due to a combination of both physical and biogenic processes. In the Albian reef complexes, no regularity of structures or ecological zonation is observed and the sizes of the voids are not compatible with the size of organisms such as sponges, corals, etc. 11 Interpretation of the Hard-Grounds and Stromatolitic ernsts of the Basin and Slope Belt of the Diapir Reefs (Fig. 6 d) The formation of hard-grounds and stromatolitic crusts occurred contemporane- ously with sedimentation. The crusts are colonized with encrusting foraminifera, such as Placopsilina and Coscinophragma. Further evidence for synsedimentary formation is the presence of hard-ground elasts in the tempestites of the lagoon facies. The hard-grounds are cut by fine pyritized strings resembling fungi mycels, suggesting participation of fungi and bacteria in the formation of the stromatolitic crusts. The presence of phosphatic crusts (apatite) may be explained by upwelling along the diapir slope. The same process also may be responsible for reduced sedimentation rates in this area favoring the establishment of reefs. This partic- ular hydrodynamic situation was the consequence of a change in the tectonic pat- terns du ring the Late Albian (Vraconian) (Reitner 1985). Cool waters from the deeper parts of the Bay of Biscay may have found their way into the Vasco- Cantabrian Basin and caused upwelling around the tectonic highs and diapirs. This change in hydrodynamic situation during the Late Albian resulted in the dis- appearance of Albian coralgal reefs, which were partly replaced by crinoid/algae bioherms (Reitner 1985). Fig. 7. a Diagenetically altered rudists which are now filled with multiple generations of micrite and dog tooth (III) cements. Lagoon facies with Polyconites in the VMDRP. b Details of a illustrating the laminar fill of alternating micrite and cement. Arrow indicates upper surface. c Zoned sparry dolomite of the intertidal facies belt of the VM.DRP. Gray zones are Fe-rich. d SEM photo of gypsum cement from the lagoon facies of the VMDRP. e Ooid from the Murguia diapir reef platform (MDRP) showing neomorphic calcite (crossed nicols). 1 Bioclast. f Radiaxial fibrous ooid from the MDRP 204 J. Reitner Fig.8 A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls 205 12 Comparison of the Two Reefs Types (Table 2) In the reef atoll of the Villasana de Mena diapir there occur certain cement types which are not found or are rare in the Albeniz-Eguino fault block reef. These include scalenohedral cements, certain fibrous cements, sulfur cements, gypsum cements, dolomite, and ankerite. Conversely, marine cements are rare in the diapir reef and common in the fault block reef. Scalenohedral low Mg-calcite cements are found in molds, microkarst cavities, and primary voids as first generation cement in the diapir reefs. All de- scribed types of scalenohedrals occur; dog tooth (11) and (111) are common while dog tooth (I) is rare. The abundant occurrence of dog tooth (11) as dripstone cements within the molds and microkarst cavities indicates freshwater vadose conditions. Dripstone formation in a marine vadose environment is observed in only a few cases (dog tooth I). Within the Po/yconites facies of the diapir reef atoll scalenohedral cements also are present in large vugs as multiple isopachous cements (dog tooth 111), where cements and sediment fills alternate with the latter consisting of graded packstone, mudstones with ostracods, and calcareous silt ("vadose silt"). According to Schneider (1977) and Pierson and Shinn (1985), all scalenohedral cements are formed under meteoric conditions. Closely linked with these freshwater cements is extreme aragonite dissolution, mold formation, and microkarst, which is present in all the facies of the diapir reef atolls. Scaleno- hedral cements are rare in the fault block reef; they are present as neomorphic cement only in the reef flat facies . Early marine fibrous cements are very rarely observed in the diapir reef atolls but occur infrequently in the basin and slope and reef flat facies . Radiaxial fibrous cements commonly follow the scalenohedral cements as the next cement generation in the diapir reefs. Certain burial cements of the diapir reef atolls appear to be closely related to the circulating diapir waters of the diapir body, these include the sulfur and gypsum cements, dolomite, and ankerite. Sulfur and gypsum cements are found in cavities in the lagoonal facies of the diapir reefs. Gypsum cement is found in the fault block reef associated only with corroded pyrite. The presence of sulfide ores in the Murguia diapir reef also tends to confirm the unique diagenetic influence of diapirs on reef carbonates since these phenomena are not observed in the fault block reef. Dolomites and ankerites, the last cementation phase, destroy former cements and crystallize in tectonic fissures. The cement filling of the large fissures (width of several decimeters) are dark drusy and fibrous calcite and are presumably the youngest of the late cements, since the fissure systems show a radial pattern linked to the last diapir collapse in the Late Tertiary. Fig. 8. a Diagenetic sulfur cement in a large dissolution cavity of the lagoonal mudstone facies of the VMDRP (1) (SEM photo). b Calcium distribution pattern from the sulfur cement of a; scale as in a. c Sulfur distribution pattern from the sulfur cement of a (1000 counts S- I; 20 kV); scale in a. d Ore paragenesis in a fissure of the MDRP (SEM photo). e Large early diagenetic authigenic quartz with Na-rich inclusions in the lagoonal facies of the VMDRP (crossed nicols). f Authigenic quartz from the lagoonal mudstone of the VMDRP (SEM photo) 206 J. Reitner Table 2. The differences between fault block reefs and diapir reefs Fault block reefs Reef Structure Island platforms with iso la ted reefs, ca. 15 x 20 km Organism diversity of selected groups Corals: 36 species Sponges: 11 species Mg-calcite algae: 6 species Aragonite algae: 15 species Stromatolites: rare Mollucs: 18 species Diapir reefs Atolls and small pinnacle ca. 5 x 5 km Corals: 5 species Sponges: 8 species Mg-calcite algae: 4 species Aragonite algae: 2 species Stromatolites: abundant Mollucs: 8 species Microfacies (fjeld observation, quantitative analysis of profiles) Micrite - sparite 70 : 30 Mudstone rare Geochemistry Sr 300 - 1300 ppm Mn 100 - 500 ppm Fe 100-20000 ppm Na 110-400 ppm Mg 120-4000 ppm Diagenesis Early marine cements in all facies zones, except in the intertidal zones; Neomorphism of aragonite Calcite ooids Micrite - sparite 90 : 10 Mudstone with authigenic quartz common Sr 100- 250 ppm Mn 100-1600 ppm Fe 300 - 30000 ppm Na 60-3300 ppm Mg 140 - 5400 ppm Early marine cements rare; Meteoric cements abundant; Early diagenetic authigenic quartz formed by hypersaline solutions; Late diagenetic gypsum and sulfur cements; Burial dolomite and ankerite; Multiphased sedimentation and cementation in cavities; Sulfide ores; Aragonitic ooids The fault block reef exhibits a totally different cementation history. The abundance of marine cements, especially radi axial fibrous and short fibrous cements, is in direct contrast to the diapir reefs. An intense freshwater diagenesis is not observed except in the intertidal and reef core fades of the Albeniz-Eguino fault block reef platform. The cementation sequence is controlled by subsidence of the fault block. It appears, therefore, that the principle diagenetic difference between the two types of reefs concerns the timing of the freshwater diagenesis which is related to differing fault block and diapir movements. 13 Discussion and Conclusions The key difference between diapir reef atolls and fault block reef platform dia- genesis appears to be the early introduction of meteoric water into the diapir reef A Comparative Study of the Diagenesis in Diapir-Influenced Reef Atolls 207 complex. This is demonstrated by the presence of certain non-marine cements, dissolution of fossils, microkarst, extremely low Sr (100 - 250 ppm), and Mg values (500 -1600 ppm) (results from bulk analyses of limes tones in the diapirs). Also interesting is a comparison of high-Mg calcite sponges (Acanthochaetetes) between the two reef types. The sponges of the fault block reef contain more than twice the amount of Mg than the diapir reef sponges. Within the Villasana de Mena diapir reef geochemical anomalies were also noted for the trace elements Na, Fe, and Mn. The Na values of up to 3300 ppm, the Fe content of up to 30000 ppm, and the Mn values of up to 1500 ppm were measured in the soluble portion of the limes tone from the reef facies. The anomalies are linked both with large amounts of reworked Keuper sediments from the diapir and with early dia- genetic quartz caused by the extruding hypersaline brines associated with the syn- sedimentary collapse event of the diapir and the corroded Keuper ophites (con- tinental tholeiites, Meschede 1985). It is evident that diapiric movement was active during reef development. Fossil constituents also confirm both the diagenetic and the sedimentary dif- ferences between diapir and fault block reefs. In all facies within the diapir reef complex organism preservation is significantly poorer than in the fault block reef platforms. The reduction mainly affects organisms with aragonitic skeletons, the intense early meteoric diagenesis causing the selective dissolution of aragonite. Low animal diversity mayaiso be due partly to unfavorable ecological conditions reflecting the hypersalinity in these diapir-influenced reefs. All peculiarities of the diapir reef platform may therefore related to the move- ment of the diapir. Thus, meteoric diagenesis, the multiple sediment filling, and the cementation may have been conditioned by the repeated, but relatively brief, elevations of the diapir. These periodic movements brought the reef complex repeatedly into the vadose environment. Each emergence of the entire reef complex resulted in the multiple filling of the reef cavities and cementation of the sediments with a meteoric cement, as weIl as the death of the reef fauna. The mobility of the diapirs may have resulted from the complicated interplay of general subsidence of the Vasco-Cantabrian Basin, ascent of the diapir, sea level changes, and dissolution processes in the diapir subsequent to collapse events. This oscillating vertical movement is not found in the Albeniz-Eguino fault block reef platform. The absense of certain cements, such as sulfur and sulfate cements, and of diapir-linked sulfur ores, as in the Murguia diapir reef, also cleinonstrates the lack of diapir influence in the burial conditions of the fault block reef. The geochemical data from the Albeniz-Eguino fault block reef plat- form exhibits high Sr values of 200 -1500 ppm (mean values appro'x. 400 ppm), ' which are not observed in the diapir reef atolls. Modern examples of diapir reef atolls can be found in the Gulf of Mexico as pinnacle reefs (West Flower Garden Bank). The faunal and floral diversity of these diapir pinnacle reefs are significantly less than in the fault block reef platforms in the Caribbean (Reitner 1982). However, the direct influence of diapiric brines and diapir material on the reef complex has not yet been observed. Ancient examples of diapir-influenced reefs outside the Vasco-Cantabrian Basin are known from the Albian of Tunisia, which exhibit typical collapse structures. 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