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38. CALCITURBDITES IN DRILLING PROJECT HOLE 416A RECOGNITION OF A DROWNED PLATFORM

Wolfgang Schlager, Rosenstiel School of Marine and Atmospheric Science, Division of and Geophysics, University of Miami, Miami,

ABSTRACT The Tithonian-Hauterivian section of Site 416 contains about 10 per cent and marlstone in well-defined beds between shale and sandstone. Depositional structures, grain composition, and dia- genetic fabrics of the limestone beds as well as their association with graded sandstone and brown shale suggest their deposition by turbid- ity currents below -compensation depth. Carbonate material was initially shed from a shallow- platform with ooid shoals and peloidal sands. Soft clasts of deep-water carbonate muds were ripped up during downslope sediment transport. Shallow-water mud of metastable and magnesium calcite was deposited on top of the graded sands, forming the fine tail of the turbidite; it has been subsequently altered to tight micritic limestone that is harder and less porous than Coccolith sediment under similar overburden. Abnor- mally high strontium contents are attributed to high aragonite con- tents in the original sediment. This suggests that the limestone beds behaved virtually as closed systems during . Drowning of the platform during the Valanginian is inferred from the disappearance of the micritic , the increase in abun- dance of phosphorite, of ooids with quartz nuclei, and of the quartz content in the calciturbidites. In Hauterivian time, the carbonate supply disappeared completely. The last limestone layers are litho- clastic which indicate that erosion has cut into well-lithified and dolomitized deeper parts of the platform rather than sweeping off loose sediment from its top. Of all Tithonian sections recovered from the Atlantic, only the one at Site 416 contains abundant shallow-water debris and was de- posited below the calcite-compensation depth. It is probably located on mid-Jurassic or older crust that had already deeply subsided and was close to the continental slope by Tithonian time. The other sites, located on younger crust, stood high on the flank of the mid- ridge, and were consequently above the compensation level and beyond reach of shallow-water detritus shed down the continental slope.

INTRODUCTION markedly from the cherty limestones recovered in the western Atlantic from the Tithonian-Hauterivian inter- DSDP Site 416 is located on the outer val (Ewing, Worzel, et al., 1969; Hollister, Ewing, et off Morocco (Figure 1). During the Tithonian-Hauteriv- al., 1972; Benson, Sheridan, et al., 1978); Site 416 adds ian interval considered here, the site remained below the a new facet to the spectrum of Jurassic ocean-floor de- calcite-compensation depth. The slow sedimentation of posits in the Atlantic; (3) the limestones close a gap in brown clay was frequently punctuated by influx of tur- the spectrum of diagenetic case histories because they bidity currents, depositing over 700 meters of graded consist largely of shallow-water materials that, unlike beds with thin interbeds of ever-present shale. Carbon- many shallow-water limestones, were never exposed to ate material, in individual beds and (or) mixed with ter- fresh water; (4) the carbonate material at Site 416 forms rigenous sediment, accounts for about 10 to 25 per cent hard, very tight limestones that profoundly influence of the total turbidite contribution. the acoustic properties of the formation; our under- This material deserves special attention because: (1) it standing of the origin and distribution of these lime- indicates a shallow-water carbonate source area now stones will improve our understanding of the nature and buried under thick continental-slope deposits; (2) the lateral variation of certain seismic reflectors in the deep- shale-sandstone-limestone sequence of Site 416 differs ly buried sequences of the Atlantic margins.

733 W. SCHLAGER

TABLE 1 Types of Turbidites

Depositional Bouma Lithology Color Structures Interval

Mazagan Brown (Terrigenous), Cores 7-57 escarpment 4) Red zeolitic Brownish red Burrowed D,E Madeira 2.5-5YR 3-4/3-4 3) Brown mudstone Brown Homogenous or D Site 416% 1 5YR 3-5/24 faintly laminated Essaouira 2) Siltstone Brown to grayish Parallel to C,D Site 370 basin green5YR4/l to cross laminated 5/2 1) Sandstone As above Massive; parallel A,B,C, or cross laminated 0 Canary Is. AFRICA Green (Mixed), Cores 7, 10-36 3) Calcareous mudstone Pale green to Massive or faintly D to marlstone grayish green, laminated. 500 km carbonate mainly 5G 4-6/1-2 burrowed I I nannofossils 10G6/2 10GY 5/2 Figure 1. Location of DSDP Sites 416 and 370. Juras- 2) Quartz-siltstone Gray to Parallel or CD some carbonate greenish gray cross laminated sic shallow-water limestones which crop detritus N6-5GY6/1 out along the Mazagan escarpment and onshore in 1) Calcarenite to Similar to (2) Massive, A,B,C, the Essaouira basin are believed to be the source of lithic sandstone parallel or cross rich in phosphorite. laminated the calciturbidites. some

Gray (Carbonate), Cores 36-57 The emphasis of this report is on standard petro- 3) Micritic limestone Gray or pinkish Burrowed D graphic methods with stained thin sections and peels, or marlstone or greenish gray faintly layered carbonate mainly 10YR7/2 supplemented by work with the scanning electron mi- neomorphic (?) micrite 5Y6/l-2,5Y5/3 croscope, X-ray diffraction, and atomic-absorption 5YR7/2 spectrometry. All thin sections were stained with a com- 5G4-5/1-2 2) Calcisiltite, Colors Parallel or CD bination of Alizarin Red S and potassium ferricyanide quartzose, tightly similar to (3) cross laminated to identify ferroan and non-ferroan calcite as well as cemented grainstone ferroan dolomite (Evamy, 1963). A stain of 4 per cent or packstone ammonium molybdate in 25 per cent nitric acid was 1) Calcarenite, Colors mostly Massive, A,B,C, tightly cemented gray, vary with parallel or used to identify phosphorite. grainstone or grain cross laminated packstone with composition TYPES OF TURBIDITES IN THE TITHONIAN matrix of brown clay TO HAUTERIVIAN SECTION On the basis of the core descriptions prepared on- In any particular core, carbonate and mixed turbi- board ship and subsequent onshore examination, the dites differ from the terrigenous beds, not only by com- turbidites are subdivided into three types (Table 1): ter- position but also by their thicker, coarser beds and more rigenous turbidites (mainly brown), composed of silici- complete Bouma cycles, suggesting deposition closer clastic detritus and clay; carbonate turbidites (mainly to the source. This difference in proximity, together gray) consisting of well-cemented carbonate sand-silt with the lack of transitions between terrigenous and car- and mud (Figures 2,3,4A); and mixed turbidites composed bonate cycles below 1280 meters in the section, suggest of quartz-rich calcarenite or carbonate-rich quartz sand different sources for carbonate and terrigenous tur- with a fine tail of clay and Coccolith mud (Figure 4B; see bidites. also Price, this volume). Only the terrigenous turbidites occur throughout the CARBONATE MATERIAL Tithonian-Hauterivian interval (939 to 1624 m). Pure Because there are many graded beds almost entirely carbonate beds are restricted to the lower part of the composed of carbonate material, I conclude that car- section (1430 tp 1624 m); mixed turbidites are common bonate detritus of all sizes from granule to clay size was and well developed between 1435 and 1280 meters and carried in from the source area. Because identification rare between 1280 and 1190 meters (see also section on of grain kind depends on size, I will deal separately with Downhole Variation of the Calciturbidites). the coarse fraction of granule to medium sand, with the Depositional structures are more or less the same in fine sand to silt, and with the clay-grade material. all three types. The beds are size graded, have sharp bases and gradational tops, and commonly show Bou- Coarse Fraction (granule to medium sand) ma's (1962) sequence of depositional structures; most Ooids of two types, translucent radial calcite ("Baha- cycles start with interval B or C. mian" ooid) or concentrically layered micrite ("pelag-

734 MESOZOIC CALCITURBIDITES

1-30

-20

-40

• 30 cm I

Figure 2. Graded sand layer, starting with Bouma inter- val B. Note intensive burrowing in marlstone (inter- val D); nutrients and burrowers were probably swept in with the current. Cycle ends at base of cross-bedded sandstone at 19.5 cm. Sample 416A- 37-3, 18-33 cm. - 50 cm ic" ooid, Jenkyns, 1972), are common to abundant. See Figure 3. Micritic limestone in brown shale with sand- Figure 5 and discussions below. stone. A full-diameter core is recovered from the Peloids are circular, elongate, or irregular-shaped hard limestone, whereas the shale fractures and grains of structureless micrite or microspar. In our sam- "caves." Note gradational top of limestone layer. ple, we believe the peloids are in part hardened fecal pel- Sample 416A-57-1, 30-50 cm.

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Figure 4. (a) Grainstone from base of calciturbidite (gray type) with ooids, peloids, skeletal grains, and lithoclasts of pelagic mudstone. Sample 416A-50-2, 23-26 cm. Thin-section, negative print, (b) Graded interval of quartzose carbonate-phosphorite turbidite (green type); lower half shows grainstone of carbonate grains (light), quartz (dark), phosphorite (medium gray, arrows); upper half consists of flat intraclasts of brown clay (light), quartz, and phosphorite. Sample 416A-12-2, 1-6 cm; thin-section negative print.

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100µm

100 µm Figure 5. Two types ofooids: (a) "Bahamian " ooid of radially oriented calcite with several micrite rims, thought to have formed as aragonite or magnesian calcite in very shallow water. Section 416A-27-4. (b) "Pelagic" micrite ooid with tintinnid, believed to have formed in slightly deeper water by sediment trapping on algal films. Sample 416A-37-2, 43-44 cm. lets, in part micritized ooids, and in part worm micritic Lithoclasts are divided into two categories: (1) rarely lithoclasts. Very abundant. Figures 6, 11. occurring clasts of ooidal, peloidal, and (or) skeletal Grapestones are lumps of peloids, ooids, and (or) grainstones of shallow-water origin as hard, subangular skeletal grains, loosely bound by micrite cement. Rare. to subrounded clasts; (2) very abundant clasts of mud- Figure 7. stone, or skeletal wackestone with tintinnids, sponge

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100 urn Figure 6. Ooids andpeloids cemented by blocky calcite. Some compaction indicated by fitted grain contacts. Sample 416A-50-2, 23-26 cm. spicules, and other pelagic biota. Most of these fine- stones in which the individual grains are difficult to rec- grained clasts are severely deformed by compaction and ognize. Most of the grains appear to be micritic, proba- were probably soft when being deposited. Grainstone bly derived from breakdown of peloids. Besides micrite, clasts are rare, but soft clasts of mudstone or wacke- I noticed a fine shell hash, possibly of pelagic bivalves stone are very abundant. Figure 8. or tintinnids. Skeletal grains are about as abundant as ooids. They include dasycladacean green algae (cf. Thaumatoporel- Mud la); questionable Clypeina jurassica, a pelagic alga; cal- In the "green" turbidites of mixed terrigenous and cispheres; foraminifers with rotaliid shell structure carbonate material, the graded sand-silt interval is over- (e.g., Lenticulina) and with miliolid structure; frag- lain by a marlstone. Its carbonate fraction consists ments of recrystallized, originally aragonitic hexacorals; mainly of nannoplankton, with a small portion of (neo- bivalves with thick layers of neomorphic calcite (after morphic?) micrite. aragonite); thick prismatic layers of Inoceramus (rare); In the gray calciturbidites the graded layer is capped ostreid shells with foliated structure; very thin-shelled by tight limestone with sparse fragments of tiny shells, ("pelagic") bivalves of the Posidonia type; aptychi; probably of tintinnids. The carbonate is micrite with on- echinid spines; crinoid ossicles; including the bizarre ly traces of recognizable nannoplankton (Figure 9). shapes of the pelagic crinoid Saccocoma; and tintinnids. In both cases the carbonate layers are not perennial Non-carbonate components make up between 1 and sediment, but are part of the fine tail of the turbidite. 50 per cent of the layers of carbonate sand (beds with This is indicated by their position above the graded over 50% non-carbonate material are classified as terri- sand-silt interval of a turbidite and their passing upward genous sandstones). The grains are mainly quartz, intra- into brown or greenish claystone. Between the tight clasts of quartz-bearing brown clay, phosphorite, and limestone and the overlying claystone there are occa- glauconite. sionally a few millimeters of gray marlstone, rich in The clasts of brown clay are intensely deformed and arenaceous foraminifers. I interpret this band as an in- act as a secondary matrix between the harder compo- soluble residue, formed when the top of the calciturbi- nents. Clayey tintinnid limestones and clasts of tintin- dite was dissolved by undersaturated sea water. There nid-bearing clay are common and were probably ripped can be little doubt that both marlstone and tight lime- up in the transition zone between carbonate sediments stone were primarily composed of clay-grade carbonate and brown clay. particles, because both rock types contain only negligi- ble amounts of coarser debris and both occur in the tur- Fine Sand and Silt bidite sequence exactly where fine, muddy sediment is to Only part of the grains of this fraction can be identi- be expected. I believe, however, that the carbonate frac- fied, because of their small size and because the grains tion was of different composition. While the green are often deposited together with mud to form pack- marlstone is mainly composed of Coccolith debris, the

738 MESOZOIC CALCITURBIDITES

100 µm

100 µm Figure 7. Various grapestone grains, composed of peloids held together by micrite cement. Between grapestone lumps is blocky-calcite cement. Sample 416A-50-2, 23-26 cm. limestone was probably predominantly aragonitic mud Contemporary ooids form in warm, shallow water of shallow-water origin. This is discussed further under where they are continuously moved by waves and (or) Origin of the Micritic Limestone. tidal currents (Illing, 1954; Newell et al., 1960; Loreau and Purser, 1973; Bathurst, 1971; Lees and Buller, 1972). Jenkyns (1972) pointed out that certain types of Shallow-Water and Deep-Water Grain Assemblages micritic ooids with pelagic fossils may actually form be- Most of the carbonate grains mentioned above are low several tens of meters of water. Ooids of this kind fairly characteristic of a specific depositional environ- do occur in our samples and may indicate a transition to ment. deep-water (see below).

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1 mm Figure 8. Various shallow-water grains surrounding intraclast of deep-water lime- stone with tintinnids and aptychus valve (fractured during compaction). Large size of intraclast and its deformation by shallow-water grains suggest that clast was deposited as soft sediment of low density. Thin-section, Sample 416A-45-1, 24-28 cm.

Peloids are either fecal pellets or micritized ooids and We can therefore recognize a shallow-water assem- micritized, rounded skeletal grains. Note that in the blage (euphotic zone) among the detrital carbonate ma- Hole 416A material the peloids deposited by the turbidi- terial which includes ooids; peloids; grapestones; clasts ty currents were as hard as ooids and much harder than of ooidal, peloidal, or skeletal grainstone; miliolid fora- the clasts of tintinnid limestone. They must have been minifers; thick-shelled bivalves; and green algae. either lithified fecal pellets or initially hard grains such Ooids and peloids were probably derived from the as ooids and skeletal fragments. This in turn argues for agitated margins of a carbonate platform; grapestone their shallow-water origin; contemporary hardened fe- and miliolids represent the protected and more restrict- cal pellets have been found to form only in shallow- ed facies of the bank interior. Lime mud for the fine tail water environments such as the Bahama Bank, where of the turbidites might have been produced in several micrite cement or oolitic coatings harden the soft mud different environments of the platform, as is suggested pellets (Illing, 1954; Bathurst, 1971, p. 299; Ginsburg by comparable modern platforms (Stockman et al., and James, 1974, p. 137). Micritization is usually caused 1967; Neumann and Land, 1975). by photosynthetic algae and therefore is also most com- The remaining components are clearly part of a deep- mon in shallow water, although in exceptional cases it water assemblage. This category includes intraclasts of can be caused by fungi in deep water (Swinchatt, 1969; skeletal wackestones and with pelagic biota Bathurst, 1971, p. 389). The hard peloids are thus in all such as tintinnids, Saccocoma, and calcisphaerulids. likelihood also derived from a shallow-water source, Sponge spicules are also common in some of the clasts. much the same as were the ooids. The clasts are larger than the associated shallow- Grapestones indicate rapid cementation or organic water grains and are severely deformed by early com- binding and are characteristic of shallow and protected paction (Figure 8). I interpret them as pieces of soft sed- carbonate environments in modern seas (Illing, 1954; iment ripped up by the turbidity currents. The clasts Bathurst, 1971, p. 316-319). seem to represent a spectrum of carbonate facies rang- Of the biota, dasycladacean green algae are diagnos- ing from those deposited at to those deposit- tic of the shallow euphotic zone. Miliolid foraminifers ed at the compensation depth. The shallow end of the are also most common in shallow water of shelves and spectrum is marked by the occurrence of micritic ooids platforms and occur in great abundance even in the re- with tintinnid fragments and by superficially coated stricted environments of the platform interior (Want- clasts of tintinnid wackestones ("pelagic ooids" of Jen- land, 1975; Rose and Lidz, 1977). kyns, 1972). The other, deep-water end of the spectrum

740 MESOZOIC CALCITURBIDITES

is represented by the transition from clasts of tintinnid 1200 limestone to clasts of brown clays as mentioned above. DIAGENESIS Alteration of Grains The preservation of skeletal grains clearly reflects their original mineralogy. Aragonitic shells have been replaced by neomorphic calcite, the original layering of- ten indicated by brown (organic-carbon-rich) bands or by layered inclusions. Green algae, originally probably randomly oriented aragonite needles like the recent Hal- imeda, have been altered to calcite micrite, as were mili- 1300 olid foraminifers (originally magnesian calcite). Echino- derms (originally magnesian calcite) show the usual pat- tern of occlusion of the intra-skeletal pore space, fol- lowed by syntaxial overgrowth of low-magnesium cal- cite. Particles composed originally of low-magnesium calcite, such as ostreid shells, do not appear to be al- tered at all. Others, such as aptychi, are heavily over- grown by syntaxial calcite, and their original porous tex- ture is still visible through organic inclusions. Grains of all mineralogies frequently have micrite rims. Ooids are preserved in two ways. Some consist of concentrically layered and radially oriented clear calcite, 1400 with brown luster and with many inclusions of 0.1 µm. Other ooids are concentrically layered calcite micrite. They lack the brown stain and any radial texture. By analogy with present-day Bahamian ooids, I interpret the clear, radially oriented grains as recrystallized ooids from an active shoal. The micritic, concentrically lay- ered grains resemble the "pelagic ooids" described by Jenkyns (1972) from the Tethyan Jurassic. They are thought to have formed similarly to onkoids, by particle trapping on algal coatings, rather than micritization of clear ooids. The occasional occurrence of calpionellids in these micritic ooids (Figure 5) supports Jenkyns's 1500 view of a slightly deeper, more open marine environ- ment of formation. The original composition of the Hole 416A ooids could have been either aragonite or magnesian calcite. There is no evidence that the ooids have been leached se- lectively from the rock. Sorby (1879) and others have used the absence of selective leaching as an argument against aragonitic composition of many ancient ooids. In our case, this observation is of little diagnostic value, because the ooids in the calciturbidites were almost cer- tainly never exposed to fresh water and therefore had Figure 9. Content of nannofossils (black) and micritic little opportunity to be selectively leached. In the ab- calcite of presumably neomorphic origin (dotted); sence of fresh water, however, aragonitic ooids recrys- non-carbonate fraction left blank. Estimates made tallize to radial calcite ooids and preserve a large part of aboard ship from smear slides. Scarcity of nan- their primary texture. A good example of this sort are nofossils in micritic limestones is one of several the Tertiary ooids Hesse (1973) described from a arguments for a shallow-water-carbonate source of drowned Pacific with radial and concentric the micrite in the gray turbidites. Carbonate in fine texture and still some aragonite preserved in the cortex. tails of green turbidites, on the other hand, is mainly The crystal inclusions in the ooids may eventually pro- Coccolith material. vide a clue to the original composition because calcite as

741 W. SCHLAGER a successor of magnesian calcite often betrays its origin have been carried in from shallow-water environments by dolomite inclusions (Lohmann and Meyers, 1977). In and are not part of the normal sequence of cements in the case of the Hole 416 ooids, however, the inclusions the turbidites. are only about 0.1 µm and could not be identified. The peloids were deposited as hard grains of micrite on the sea floor, as can be inferred from their behavior Metasomatism during compaction. Many of them have the shape and Dolomite was identified by staining and X-ray dif- size of ooids and are probably completely micritized fraction. It occurs in scattered rhombs in the grain- ooids. The larger and more elongate peloids were proba- stones and packstones, but not in the micritic lime- bly fecal pellets, hardened at the sea floor prior to trans- stones. (Authigenic dolomite is common in the brown port in the turbidity current. In contrast to the micrite clay surrounding the calciturbidites.) Lithoclasts of a of the fine tail of the turbidites, the calcite micrite of the sucrosic dolomite rock were found in the Hauterivian peloids is not ferroan. It must have stabilized to calcite breccias of Core 7. I believe they are reworked older before the onset of burial diagenesis and did not recrys- parts of the same shallow-water carbonate platform that tallize during later stages of diagenesis. provided the sand-size debris for the lower part of the section. Cementation Phosphorite was found in the classic layers through- Most of the sand and silt layers of the turbidites are out the cores. Single grains were identified microscopi- free of mud ("grainstones" in the classification of Dun- cally, concentrations of phosphorite of over 0.5 per cent ham, 1962). The original pores are filled by a simple se- (volume %) by a yellow stain with ammonium molyb- quence of cement (now all calcite): (1) a thin druse of cal- date. Most of the grains are unidentifiable, angular to cite microspar occasionally forms the first layer around subrounded fragments which form part of the clastic the grains and predates solution-compaction; this is fol- portion of the turbidite. Some grains have the shape and lowed by (2) slightly ferroan, blocky calcite that postdates size of shell fragments; others are round, with a faint or overlaps with solution and completely fills the concentric structure reminiscent of ooids. I assume that residual pore space (Figures 6,7,10,11). Around the ech- at least part of the phosphorite is a replacement of car- inoderm grains, aptychi, and prism layers of Inoceram- bonate grains and was washed in from a drowned car- us, syntaxial-overgrowth cement competes with the bonate platform (see Downhole Variation of the Calci- blocky-calcite. Most of this overgrowth cement is not turbidites). Precipitation of phosphorite, coupled with ferroan; it must therefore predate the blocky cement. metasomatic replacement of carbonate by phosphorite, The micrite cement in the grapestone lumps and relics is common in areas of slow deposition just below the of a fibrous druse in certain lithoclasts are thought to (Ames, 1959; Gulbrandsen, 1969).

10µm Figure 10. Tightly interlocking mosaic of calcite cement between quartz grains and unidentified skeletal fragment (upper right). Coarse layer of calciturbidite, Sam- ple 416A-57-1, 147-14 cm. SEM micrograph of polished and etched surface.

742 MESOZOIC CALCITURBIDITES

10µm Figure 11. Interlocking mosaic ofblocky cement around micritic peloid. Cement is ferroan calcite of burial diagenesis; peloid is non-ferroan calcite of presumably early diagenetic origin. Sample 416A-57-1, 147-149 cm. SEM micrograph of polished and etched surface.

Lithification Of Mud apart their surfaces show slickenside-like striations rath- The fine tails of the calciturbidites are either marl- er than the common pattern of interlocking cones. I stone composed of cocoliths and clay, or micritic lime- conclude that these vertical stylolites were caused by stone, probably largely recrystallized aragonite mud of strike-slip movements with lateral compression rather shallow-water origin (see Origin of the Micritic Lime- than by simple overburden compaction. stone). In both cases, the fine sediment has been lithi- 3. Pervasive pressure solution along crystal bound- fied, but the diagenetic overprint is more severe in the aries (Wanless, 1977; Scholle, 1977, p. 994) must have limestone than in the marlstone. The limestone has a re- affected the micritic limestone as well as the grainstone sidual porosity of 1 to 7 per cent and consists almost ex- in order to form the tight, interlocking mosaics of mi- clusively of neomorphic(?) calcite micrite with less than crite and sparry cement shown in Figures 10, 11, and 12. 5 per cent recognizable cocolith material (Figure 9). The The porosity of the micritic limestones decreases from texture is an interlocking amoeboid mosaic, generally about 12 per cent at 1450 meters to less than 5 per cent considered a final stage of micrite diagenesis (Fischer et at 1600 meters. I attribute this loss of porosity to an in- al., 1967; Bathurst, 1971, p. 502; see Figure 12). Stain- crease in pervasive pressure solution. ing reveals an iron content in the micrite similar to that of the blocky calcite cement. The marlstone, on the oth- Diagenetic Environments er hand, has a porosity of 10 to 25 per cent and most of Burial diagenesis is by far the most prominent diage- its carbonate fraction is still-recognizable Coccolith ma- netic environment recognizable in the rocks. It caused terial; only a small portion is neomorphic calcite mi- not only the compaction and pressure solution in the crite. grainstone and mudstone, but also provided over 90 per cent of the pore-fill in the form of blocky, slightly fer- Pressure Solution roan calcite cement that postdates compaction and at Three groups of pressure-solution features can be least incipient pressure solution. The similar iron con- recognized in the calciturbidites: tent in blocky calcite cement and neomorphic micrite of 1. Microstylolites, horse-tails (Roehl, 1967), and su- the fine turbidite tails suggests a burial diagenetic origin tured grain contacts occur frequently throughout the for most of this micrite too. The micrite of peloids is Hauterivian-Tithonian interval. non-ferroan and thought to be the result of shallow-wa- 2. Well-developed stylolites with amplitudes of 5 mm ter lithification. The iron content of the burial cements or more occur from Core 40 (1460 m) down into the mi- indicates reducing conditions within the turbidite layer critic limestones. They are vertical, and when taken in spite of the high oxidation state of the perennial sedi-

743 W. SCHLAGER

5 µm Figure 12. Tightly interlocking mosaic offerroan micrite from fine tail of shallow- water turbidite. Limestone is rich in strontium and magnesium and is thought to be a neomorphic alteration of aragonite and magnesian calcite. Sample 57, CC. SEM micrograph of broken surface. ment. The allochthonous material was probably buried also marks the disappearance of the micritic limestone. with abundant organic matter swept in from the shallow This correlation suggests that the micritic limestone was source area. derived from the same shallow-water source as the car- Diagenesis at the deep-sea floor was of minor impor- bonate sand, and was not, or only partly, cocolith mud tance. It caused some dissolution at the tops of the calci- picked up on the way downslope. turbidites, indicated by a drop in carbonate content. 3. The limestone is considerably less porous and bet- The thin druse of microspar that predates compaction ter lithified than pure Coccolith ooze under comparable probably also formed at least close to the sea floor. overburden and conditions. Its porosity at Shallow-water diagenesis is represented by micrite 1500 meters is between 3 and 10 per cent, while the trend rims on skeletal grains and ooids (characteristically established by Scholle (1977, p. 992) and Schlanger and lacking on deep-water aptychi and Saccocoma frag- Douglas (1974) would predict values of 15 to 25 per cent ments), by micrite cement in grapestones and peloids, (Figure 13). and by ghosts of fibrous cement in lithoclasts. With the 4. Atomic-absorption analyses of the limestone (Ta- exception of a few doubtful cases of selective leaching ble 2) show abnormally high contents of strontium. It of aragonite, no indications of fresh-water diagenesis seems impossible that this amount of strontium can be have been found. accommodated in the lattice of low-temperature calcite. Rather, it may occur as strontianite and (or) celestite ORIGIN OF THE MICRITIC LIMESTONE formed during the transformation of strontium-rich Several independent arguments suggest that the mi- aragonite to strontium-poor calcite. Neither strontium critic limestone was originally not Coccolith sediment could be identified by X-ray diffraction, how- but was composed largely of shallow-water carbonate ever; only the presence of dolomite could be verified. mud: Dolomite accounts for the high magnesium content of 1. The total content of recognizable nannoplankton the limestone and may have formed as a by-product of in the micritic limestones is less than 10 per cent, much the recrystallization of magnesian calcite to calcite. lower than in the nannofossil marlstone that forms the All these observations have led me to believe that the fine tails of the calciturbidites in Cores 36 and 37 (Fig- micrite limestone—contrary to the nannofossil marl- ure 9), notwithstanding the higher total carbonate con- stone above Core 37—consisted largely of platform- tent of the limestones. derived mud with a minor portion of calcareous nanno- 2. The micritic limestone occurs always as the fine plankton. If the recent is in any way a key to the past, tails of calciturbidites of mainly shallow-water detritus. then this mud must have consisted of aragonite and The abrupt decrease in shallow-water detritus and the some magnesian calcite. In modern carbonate plat- change to phosphorite-rich relict sands above Core 37 forms, the mud consists of aragonite and magnesian

744 MESOZOIC CALCITURBIDITES

80• Curves Circles Nannofossil ooze, chalk, limestone Nannofossil ooze (after Schlanger ar d Douglas 1974; O Nannofossil marlstone Hole416A Scholle 1977) O Micritic limestone (Data from Bovce, this volume)

60• o 8 Θ o

40 • Sco'ian s No'th

'/16•\

o° ^r—. 20 • -A. o o ^ o•»•* ' depth of bur Figure 13. Curves of porosity versus burial depth for Coccolith sediments in DSDP holes and oil wells. Coccolith sediments of Hole 416A follow this trend down to about 1350 meters. Influx of shallow-water carbonate mud below this level is thought to have caused the remarkably low porosity.

TABLE 2 susceptible to pressure solution and matured to tight Atomic Absorption Analyses of Some Limestone Samples limestone earlier than purely calcitic rocks. 2. The metastable stabilized rapidly at or Sample Insoluble (Interval in cm) Residue Mgo CaO Sr FeO MnO near the sea floor and were replaced by calcite of 4 to 5 mole per cent MgCO3, which seems to be the least- 37-1, 10 22.48 1.9 37.5 0.67 1.55 0.12 soluble form of calcite in sea water (Plummer and Mc- 46-2, 18-21 10.1 0.79 47.5 0.93 0.78 0.12 Kenzie, 1974; Füchtbauer and Hardie, 1976; Schlager 47-1, 12-15 11.1 1.05 46.9 0.91 0.80 0.065 and James, 1978). With change in pore fluids during burial diagenesis, this magnesium-bearing calcite may calcite in a ratio of 1:1 to 9:1 (Husseini and Matthews, have become less stable than pure calcite and may have 1972; Matthews, 1966; Taft and Harbaugh, 1964). been converted to pure calcite by pressure-solution re- Most, if not all, of this lime mud is of organic origin precipitation. In this way, the last pore space may have (Stockman et al., 1967; Neumann and Land, 1975). been closed. Green algae, the main mud producers in the recent, oc- DOWNHOLE VARIATION OF THE curred in abundance throughout the Mesozoic (Wray, CALCITURBIDITES 1977, p. 153). The question remains, why are the micritic limestones Change In Carbonate Input so tight and so highly altered compared to nannofossil The layers with carbonate detritus vary considerably ooze under similar overburden (Figure 13). The micritic throughout the hole. The most prominent change is up- limestone is now a tightly interlocking mosaic of calcite ward disappearance of the gray carbonate turbidites and crystals with sutured boundaries. It seems that the inevi- their replacement by the green, phosphoritic type (Fig- table merry-go-round of pressure solution and reprecip- ure 14). Aboard ship, this change was used to define the itation that controls burial diagenesis of limestone boundary between lithologic Units VI and VII (see Site (Scholle, 1977; Logan and Semeniuk, 1976) has gone to 416 Report this volume). Another, more gradual change completion earlier than in Coccolith sediments. The is the upward disappearance of the green turbidite. The original content of aragonite and magnesian calcite is topmost beds of carbonate elastics are two breccias in the most likely cause of this diagenetic "prematurity." Core 7 which differ from all other beds in their coarse The path of diagenesis to this final stage of tight micrite grains (up to 2 cm in diameter). They contain mainly li- may have followed either one of the following two alter- thoclasts of shallow-water limestone and of sucrosic do- natives: lomite, as well as finer layers of quartz sand with some 1. The mud was deposited so rapidly that the meta- carbonate grains and phosphorite. stable minerals were little affected by dissolution at the The most important changes throughout the turbidite sea floor. During burial diagenesis, the turbidite sand- sequence are summarized in Figure 14. Grain types and wiched between clay was a closed system in which some grain sizes were point counted in thin section. Mean bed aragonite may have survived until the onset of pressure thickness was measured aboard the ship. The section solution. Because of the higher solubility of aragonite has a minimum mean bed thickness at 1420 meters, over calcite, these aragonite-bearing rocks were more where the gray turbidites with their thick layers of micri-

745 W. SCHLAGER

Age Types of Mean thickness of Phosphorite Ooids with Ratio of to Carbonate turbidites turbidites in sand fraction quartz nuclei elastics in gray and source ε 2 green turbidites

1000 r

1100

1200 Hauterivian

c I

1300

Valanginian 1400

1500

Berriasian

16001- Tithonian

meters 15 cm 5 10 15% 10 % 20 0.1 1.0 10.0 100 . I • Figure 14. Growth and drowning of the carbonate platform as interpreted from the turbidite record. Period of pre- sumed platform growth (Tithonian-Valanginian) is marked by abundant micritic limestone of shallow-water (?) mud, by high carbonate-to-quartz ratio and by scarcity of phosphorite. Drowning of platform during Valanginian causes disappearance ofmicritic limestone, decrease in carbonate-to-quartz ratio, and increase of phosphorite and of number of ooids with quartz nuclei. Bed thickness of total core drops with drowning of platform because of disappearance of thick beds ofmicritic limestones. Increase in bed thickness above 1400 meters is probably caused by greater terrigenous supply. Bed thickness and thickness of micritic limestone are on basis of shipboard core description; other columns on basis of point counts in thin sections: 100 points per sample for carbonate-to-quartz ratio, 100-300 points for ooids with quartz nuclei, 2000 points per sample for phosphorite.

tic limestones disappear. (The upward increase in bed Interpretation—Drowning Of A Carbonate Platform thickness above this minimum is probably the result of an increase in terrigenous supply, rather than a change Clearly during the Valanginian the carbonate contri- in carbonate contribution.) The disappearance of the bution ("green" and "gray" turbidites) changed in micritic limestone coincides with the increase in phos- character, whereas the siliciclastic contribution ("brown phorite in the sand fraction from traces below Core 37 turbidites") varied only in distance from source. In to values around 1 per cent and higher in Cores 36 to 7. Cores 57 through 37 the high carbonate content of the At the same time, larger amounts of quartz appear in gray turbidites in combination with the abundance of the green turbidites, reducing the ratio of carbonate to green algae and shallow-water mud indicate that they siliciclastic grains. The increase in ooids with quartz nu- were derived from a carbonate source area in warm clei, and the persistent and clear separation of green and shallow water, within the euphotic zone, in which al- brown terrigenous turbidites in the cores suggest en- most no siliciclastic material was present. The green lay- croachment of elastics on the former carbonate source ers of Cores 37 through 12 are probably derived from a area, rather than increased mixing during downslope winnowed shelf below the euphotic zone, as indicated transport. None of the major lithologic changes shown by the scarcity of green algae, the lack of shallow-water in Figure 14 correlates with a biostratigraphic bound- mud, and the abundance of phosphorite—a rock that ary. It is thus unlikely that the changes in lithology are deserves special attention in this discussion. The precipi- caused by hiatuses in sedimentation. tation of calcium in the marine environment

746 MESOZOIC CALCITURBIDITES is favored by the same conditions as the formation of eastward, and the lagoonal deposits were covered by calcium carbonate, but in addition requires a high con- ammonite-bearing marl, sandstone, and detrital lime- centration of phosphate ion in sea water. Consequently, stone. Condensed faunas, hardgrounds, and hiatuses in- form in shallow of high fertility but, dicate slow and intermittent deposition (Ambroggi, unlike carbonates, in depths just below the euphotic 1963, p. 123-124; Societe Cherifienne, 1963; Wiedmann zone, where the phosphate ions are not depleted by liv- et al., 1978; Behrens et al., 1978; Price, this volume). ing organisms, and rates of carbonate deposition are The rapid change from a highly restricted to an open sufficiently low so as not to mask the relatively slow de- deeper-water marine environment in the Western Atlas position of phosphate (Gulbrandsen, 1969; Blatt et al., and the Essaouira Basin coincides with the postulated 1972, p. 551-555). drowning of the carbonate platforms on the continental This change in carbonate contribution could have slope. One is tempted to link the transgression in the on- been brought about either by a shift to a different shore basins with the drowning of the offshore plat- source area or by a change in the depositional environ- forms and assume that a rise in of a period of ment within the same source area. The abruptness of the increased was the common cause of both. change and the fact that the carbonate-rich ''green" tur- The intermittent sedimentation in the onshore sections bidites as a group remain distinctly separate from the may indicate that sea level rose in steps, possibly terrigenous "brown" turbidites argue against a shift to separated by episodic lowering. A slight drop followed another source area and point to an environmental by a rapid rise in sea level is probably the most efficient change within the carbonate source. way to drown a carbonate platform. I believe that Drowning of the shallow-water carbonate source area healthy platforms can keep up with very high rates of and its transformation into a current-swept, deeper-wa- subsidence and rises of sea level; their tolerances is ter shelf below the euphotic zone best explains the disap- reduced, however, in the critical time span when car- pearance of green algae and shallow-water-mud contri- bonate production is slowly resumed after a period of bution, as well as the appearance of phosphorite. The exposure. ooids, hardened peloids, and lithoclasts of Cores 36 On the basis of West African and global data, Todd through 12 may be relict sands that can persist in such and Mitchum (1977) propose a rapid and deep drop of an environment for considerable time (e.g., the relict sea level in the Valanginian, separating periods of rise in sands on present-day open shelves; Pilkey et al., 1966; the Portlandian-Berriasian and in the late Valanginian- Fabricius et al., 1970; Ginsburg and James, 1974, p. Hauterivian. Our data on Hole 416A are compatible 137-144). On isolated highs of the former platform, ac- with this hypothesis, inasmuch as they, too, point to an tive ooid shoals may have persisted long after the main important intra-Valanginian event. However, the conti- part of the platform had subsided below the zone of nuity of the turbidite record, as well as the onshore stra- maximum carbonate production. tigraphy by Ambroggi (1963) are not easily reconciled The drowned platform continued to shed carbonate with an extreme Valanginian low stand. With Price (this detritus, albeit at a steadily decreasing rate, until the late volume) I favor the interpretation of a Stepwise rise in Hauterivian. The uppermost beds of carbonate clastic sea level during the whole Portlandian-to-Valanginian sediments are two breccias in Core 7. Their granule-size interval, interrupted only by minor low stands. clasts of shallow-water limestone and of sucrosic dolo- mite, and the predominance of quartz in the terrigenous SITE 416—THE FIRST TITHONIAN SEQUENCE finer layers, indicate that by Hauterivian time erosion BELOW THE CCD IN THE ATLANTIC had cut into the lithified and dolomitized part of the The shale-sandstone-limestone alternation of the platform and that terrigenous sediments were encroach- Tithonian-Hauterivian interval at Site 416 was depos- ing on it. ited below calcite-compensation depth, with brown clay The picture of a carbonate fits as perennial sediment and turbidite input of varying well with the regional geology. Throughout the Jurassic provenance in episodic pulses. This statement, made the shallow seas in northwestern Africa were largely a tentatively after shipboard examination of the cores (see domain of carbonate deposition (Dresnay, 1971). Upper Site 416 Report, this volume), has been fully confirmed Jurassic shallow-water carbonates crop out on the Maz- by the present study. The carbonate occurs in graded agan escarpment, 100 km east of Site 416 (Renz et al., beds, deposited in one pulse, with all the characteristic 1975) and are widespread in the subsurface of the conti- features of turbidites. Even the fine carbonate fraction nental slope (Lehner and de Ruiter, 1977; Todd and consisted largely of shallow-water material, and the Mitchum, 1977). coarse layers contain all transitions from pelagic lime- On shore, in the Essaouira basin, the Souss trough stone to brown clay in the form of "rip-up" clasts, and the Tarfaya basin, the shoreward part of these car- picked up during transport downslope. Site 416 lay be- bonate platforms is exposed. It consists of dolomitic low the compensation depth, but the continental slope limestone with red beds (Ambroggi, 1963, p. 123-134; adjacent to it was covered with carbonate sediment and Societe Cherifienne, 1966; Wiedmann et al., 1978; Beh- crowned by carbonate platforms at the continental rens et al., 1978). Sparse biota and occasional evaporites shelf. indicate their deposition in restricted shallow-water en- Other DSDP holes that reached Tithonian strata in vironments. During late Berriasian and Valanginian the Atlantic include those drilled at Sites 4 and 5 (Ew- time, the open marine environment expanded rapidly ing, Worzel, et al., 1969); Sites 99, 100, 101, and 105

747 W. SCHLAGER

(Hollister, Ewing, et al., 1972), and Site 391 (Benson, REFERENCES Sheridan, et al., 1978) in the western Atlantic; and Site Ambroggi R., 1963. Etude géologique et Versant Meridional 367 (Lancelot, Seibold, et al., 1978) in the eastern At- du Haut-Atlas Occidental et de la Plaine du Sous, Notes et lantic. In all these holes the Tithonian sequence com- Mém. Serv. Géol. Maroc, v. 157, p. 1-321. prises pelagic carbonate sediments; turbidites with shal- Ames, L. L., 1959. The genesis of carbonate apatites, Econ. low-water debris are scarce, although extensive carbon- Geol., v. 54, p. 829-941. ate platforms existed on both sides of the young ocean. Bathurst, R. G. C, 1971. Carbonate sediments and their dia- The difference of facies between the sediments of Site genesis, developments in sedimentology, no. 12: Amster- 416 and the rest is consistent with the location of Site dam (Elsevier). 416 far within the magnetically quiet zone, on crust that Behrens, M., Krumsiek, K., Meyer, D. E., Schàfer, A., Siehl, could be as old as Early Jurassic (Pittman and Talwani, A., Stets, J., Thein, J., and Wurster, P., 1978. Sedimenta- tionsablàufe im Atlas-Golf (Kreide Küstenbecken Marok*- 1972; Lancelot, Seibold, et al., 1978). By Tithonian time ko), Geol. Rundschau, v. 67, p. 424-453. this crust must have subsided below the calcite-compen- Benson, W. E., Sheridan, R. E., et al., 1978. Initial Reports of sation depth; it lay adjacent to the continental rise and the Deep Sea Drilling Project, v. 44: Washington (U. S. was thus within reach of shallow-water debris from the Government Printing Office). . The other sites, on younger crust Blatt, H., Middleton, G., and Murray, R., 1972. Origin of (mostly Callovian-Oxfordian), still stood high on the sedimentary rocks: Englewood Clifss (Prentice Hall). flanks of the mid-ocean ridge, above the compensation Bouma, A. H., 1962. Sedimentology of some flysch deposits: depth, and beyond reach of turbidity currents sweeping Amsterdam (Elsevier). down the continental slope. This explanation holds for Dresnay, R. du, 1971. Extension et dévelopement des phéno- all sites except for Site 391 in the Blake-Bahama basin of mènes récifaux jurassiques dans le domaine atlasique maro- the western Atlantic. It is located within the magnetical- cian, particuliérement au Lias moyen, Bull Soc. Géol. ly quiet zone, at about the same distance from its France, v. 7, p. 46-56. Dunham, R. J., 1962. Classification of carbonate rocks ac- boundary as Site 416, and very close to what has been cording to depositional texture. In Ham, W. E. (Ed.), shown to be a Neocomian (and possibly older) carbon- Classification of carbonate rocks: Tulsa (Am. Assoc. Pet- ate-platform margin (Benson, Sheridan, et al., 1978). rol. Geol.), p. 108-121. The setting of Site 391 is therefore an almost exact mir- Evamy, B. D., 1963. The application of chemical staining ror image of Site 416. Yet the Tithonian-Neocomian of technique to a study of dedolomitisation, Sedimentology, Site 391 is almost purely pelagic limestone, deposited v. 2, p. 164-170. above calcite-compensation depth and without shallow- Ewing, M., Worzel, J. L., et al., 1969. Initial Reports of the water-carbonate influx. This discrepancy needs further Deep Sea Drilling Project, v. 1: Washington (U. S. Govern- investigation. ment Printing Office). Fabricius, F. H., Berdau, D., and Münnich, K. O., 1970. Early Holocene ooids in modern littoral sands reworked CONCLUSIONS from a coastal terrace, southern Tunisia, Science, v. 169, p. 757-760. Calciturbidites, although only a fraction of the terrig- Fischer, A. G., Honjo, S., and Garrison, R. E., 1967. Electron enous contribution in the Jurassic-Cretaceous sequence micrographs of limestones: Princeton (Princeton Univ. of Site 416, provide a considerable amount of informa- Press). tion about the adjacent continental shelf and slope. In Füchtbauer, H. and Hardie, L. A., 1976. Experimentally de- particular, they seem to record fairly accurately the termined homogeneous distribution coefficients for precip- growth and the drowning of a carbonate platform. itated magnesian calcites, Abstr. with Programs, Ann. Carbonate platforms are common along passive con- Meeting Geol. Soc. Am., p. 877-878. tinental margins and are probably the most sensitive in- Ginsburg, R. N. and James, N. P., 1974. Holocene carbonate dicator of sea-level changes along the seaboard of conti- sediments of continental shelves. In Burk, C. A. and Drake, C. L. (Eds.), The geology of continental margins: nents. Their initiation, the rate of their growth, and the New York (Springer-Verlag), p. 137-155. timing of their death can often provide a key to both the Gulbrandsen, R. A., 1969. Physical and chemical factors in subsidence history of passive margins and eustatic sea- the formation of marine apatite, Econ. Geol., v. 69, p. 365- level fluctuations in the . 382. Turbidite sequences not only allow us to recognize Hesse, R. 1973. Diagenesis of a seamount oolite from the these events at some distance from the platform, but al- West Pacific, Leg 20, DSDP. In Heezen, B. C, MacGreg- so to date them fairly accurately by means of the inter- or, I. D., et al., Initial Reports of the Deep Sea Drilling calated pelagic sediments. Project, v. 20: Washington (U. S. Government Printing Office), p. 363-387. Hollister, C. D., Ewing, J. I., et al., 1972. Initial Reports of ACKNOWLEDGMENTS the Deep Sea Drilling Project, v. 11: Washington (U. S. I thank my colleagues of the scientific party aboard the Government Printing Office). Glomar Challenger for stimulating discussions and exchange Husseini, S. I. and Matthews, R. K., 1972. Distribution of of ideas and observations during Leg 50. I thank O. Joensuu high-magnesium calcite in lime muds of the Great Bahama (University of Miami) for help with atomic-absorption and Bank: diagenetic implications, /. Sediment Petrol., v. 42, X-ray analyses; R. N. Ginsburg (University of Miami) and E. p. 179-182. A. Shinn (United States Geological Survey) kindly reviewed Illing, L. V., 1954. Bahaman calcareous sands, Am. Assoc. the manuscript. Petrol. Geol. Bull., v. 38, p. 1-95.

748 MESOZOIC CALCITURBIDITES

Jenkyns, H. C, 1972. Pelagic "oolites" from the Tethyan Florida and the Bahamas — Sedimenta VI: Miami (Univer- Jurassic, /. Geol., v. 80, p. 21-23. sity of Miami). Lancelot, Y., Seibold, E., et al., 1978. Initial Reports of the Schlager, W. and James, N. P., 1978. Low-magnesian cal- Deep Sea Drilling Project, v. 41: Washington (U. S. Gov- cite limestones forming at the deep-sea floor (Tongue of the ernment Printing Office). Ocean Bahamas), Sedimentology, v. 25, p. 675-702. Schlanger, S. O. and Douglas, R. G., 1974. The pelagic ooze- Lees, A. and Buller, A. T., 1972. Modern temperate-water and chalk-limestone transition and its implications for marine warm-water shelf carbonate sediments contrasted, Marine stratigraphy. In Hsü, K. J. and Jenkyns, H. C. (Eds.), Geol., v. 13, p. M67-M73. Pelagic sediments: on land and under the sea: Spec. Publ. Lehner, P. and Ruiter, P. A. C. de, 1977. Structural history of Int. Assoc. Sedimentol., v. 1, p. 117-148. Atlantic margin of Africa, Am. Assoc. Petrol. Geol. Bull., Scholle, P. A., 1977. Chalk diagenesis and its relation to pe- v. 61, p. 961-981. troleum exploration: Oil from chalks, a modern miracle? Logan, B. W. and Semeniuk, V., 1976. Dynamic metamorph- Am. Assoc. Petrol. Geol. Bull., v. 61, p. 982-1009. ism: Processes and products in carbonate rocks, Société Chérifienne des Pétroles, 1966. Le Bassin du Sud-Ouest Canning Basin, Western Australia: Sydney (Geological So- Marocain. In Reyre, D. (Ed.), Sedimentary basins of the ciety of Australia), Spec. Publ. 6. African coasts, v. 1, p. 5-12. Lohmann, K. C. and Meyers, W. M., 1977. Microdolomite Sorby, H. C, 1879. The structure and origin of limestones, inclusions in cloudy prismatic calcites: a proposed criterion Proc. Geol. Soc. London, v. 35, p. 56-95. for former high-magnesium calcites, J. Sediment Petrol., Stockman, K. W., Ginsburg, R. N., and Shinn, E. A., 1967. v. 47, p. 1078-1088. The production of lime mud by algae in south Florida, /. Loreau, J. P. and Purser, B. 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