Mineralogic, elemental, and stable isotopic composition of bulk carbonate sediments from ODP Leg 166 sites

An intensive mineralogic and geochemical investigation was conducted on sediments recovered during Ocean Drilling Program Leg 166 from the western Great Bahama Bank at Sites 1006, 1008, and 1009. Pleistocene through middle Miocene sediments recovered from Site 1006, the distal location on the Leg 166 transect, are a mixture of bank-derived and pelagic carbonates with lesser and varying amounts of siliciclastic clays. A thick sequence of Pleistocene periplatform carbonates was recovered near the platform edge at Sites 1008 and 1009. Detailed bulk mineralogic, elemental (Ca, Mg, Sr, and Na), and stable isotopic (d18O and d13C) analyses of sediments are presented from a total of 317 samples from all three sites.

(Table T1) Coral taxa and their ecological significance in ODP Site 194-1196 and Hole 194-1199A sediments

The Southern Marion Plateau (SMP) represents a vertical stacking of Miocene carbonate platform deposits. Two sites (1196 and 1199) were drilled on top of this plateau, penetrating a 663-m carbonate succession of bioclastic and reefal sedimentary bodies. The study focuses on the least dolomitized 410-m-thick upper part of the succession, which is middle to late Miocene in age. Sedimentological and paleontological studies were conducted at both sites in order to propose a paleoenvironmental model and its evolution through the Miocene age. Six main microfacies of possible environmental significance were defined using statistical multivariate analyses, based on the recognition and point counting of 24 biogenic components. Depositional environment reconstructions are proposed as well as the biosedimentary and the environmental evolution regarding seismic architectures, stratigraphy, biosedimentology, and microfacies analysis. The SMP platform mainly results from a vertical stacking of lens-shaped bodies in homoclinal to distally steepened ramp settings.

Chemistry of siliciclastic and volcanoclastic sediments of DSDP Site 47-397

The Lower Cretaceous and Miocene sequences of the NW African passive continental margin consist of siliciclastic, volcaniclastic and hybrid sediments. These sediments contain a variety of diagenetic carbonates associated with zeolites, smectite clays and pyrite, reflecting the detrital mineralogical composition and conditions which prevailed during opening of the North Atlantic. In the Lower Cretaceous siliciclastic sediments, siderite (-6 per mil to +0.7per mil d18O PDB, -19.6 per mil to +0.6 per mil d13C PDB) was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter. Microcrystalline dolomite layers, lenses, nodules and disseminated crystals (-3.0 per mil to +2.5 per mil d18O PDB, -7.2 per mil to +4.9 per mil d13C PDB) predominate in slump and debris-flow deposits within the Lower Miocene sequence. During the opening of the Atlantic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Calcite is the dominant diagenetic carbonate in the siliciclastic-bioclastic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite (-22 per mil to +1.6 per mil d18O PDB, -35.7 per mil to +0.8 per mil d13C PDB) was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters. Phillipsite is confined to the alteration of volcaniclastic sediments, whereas clinoptilolite is widely disseminated, occurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.

Composition of bioclastic sands, carbonates and pyroclastic rocks of the Great Meteor and Josephine Seamounts, eastern North Atlantic

1. Great Meteor Seamount (GMS) is a very large (24,000 km**3) guyot with a flat summit plateau at 330-275 m; it has a volcanic core, capped by 150-600 m of post-Middle-Miocene carbonate and pyroclastic rocks, and is covered by bioclastic sands. The much smaller Josephine Seamount (JS, summit 170- 500 m w. d.) consists mainly of basalt which is only locally covered by limestones and bioclastic sands. 2. The bioclastic sands are almost free of terrigenous components, and are well sorted, unimodal medium sands. (1) "Recent pelagic sands" are typical of water depths > 600 m (JS) or > 1000 m (GMS). (2) "Sands of mixed relict-recent origin" (10-40% relict) and (3) "relict sands" (> 40% relict) are highly reworked, coarse lag deposits from the upper flanks and summit tops in which recent constituents are mixed with Pleistocene or older relict material. 3. From the carbonate rocks of both seamounts, 12 "microfacies" (MF-)types were distinguished. The 4 major types are: (1) Bio(pel)sparites (MF 1) occur on the summit plateaus and consist of magnesian calcite cementing small pellets and either redeposited planktonic bioclasts or mixed benthonic-planktonic skeletal debris ; (2) Porous biomicrites (MF 2) are typical of the marginal parts of the summit plateaus and contain mostly planktonic foraminifera (and pteropods), sometimes with redeposited bioclasts and/or coated grains; (3) Dense, ferruginous coralline-algal biomicrudites with Amphistegina sp. (MF 3.1), or with tuffaceous components (MF 3.2); (4) Dense, pelagic foraminiferal nannomicrite (MF 4) with scattered siderite rhombs. Corresponding to the proportion and mineralogical composition of the bioclasts and of the (Mgcalcitic) peloids, micrite, and cement, magnesian calcite (13-17 mol-% MgCO3) is much more abundant than low-Mg calcite and aragonite in rock types (1) and (2). Type (3) contains an "intermediate" Mg-calcite (7-9 mol-X), possibly due to an original Mg deficiency or to partial exsolution of Mg during diagenesis. The nannomicrite (4) consists of low-Mg calcite only. 4. Three textural types of volcanic and associated gyroclastic rocks were distinguished: (1) holohyaline, rapidly chilled and granulated lava flows and tuffs (palagonite tuff breccia and hyaloclastic top breccia); (2) tachylitic basalts (less rapidly chilled; with opaque glass); and (3) "slowly" crystallized, holocrystalline alkali olivine basalts. The carbonate in most mixed pyroclastic-carbonate sediments at the basalt contact is of "post-eruptive" origin (micritic crusts etc.); "pre-eruptive" limestone is recrystallized or altered at the basalt contact. A deuteric (?hydrothermal) "mineralX", filling vesicles in basalt and cementing pyroclastic breccias is described for the first time. 5. Origin and development of GMS andJS: From its origin, some 85 m. y. ago, the volcano of GMS remained active until about 10 m. y. B. P. with an average lava discharge of 320 km**3/m. y. The volcanic origin of JS is much younger (?Middle Tertiary), but the volcanic activity ended also about 9 m. y. ago. During L a t e Miocene to Pliocene times both volcanoes were eroded (wave-rounded cobbles). The oldest pyroclastics and carbonates (MF 3.1, 3.2) were originally deposited in shallow-water (?algal reef hardground). The Plio (-Pleisto) cene foraminiferal nannomicrites (MF 4) suggest a meso- to bathypelagic environment along the flanks of GMS. During the Quaternary (?Pleistocene) bioclastic sands were deposited in water depths beyond wave base on the summit tops, repeatedly reworked, and lithified into loosely consolidated biopelsparites and biomicrites (MF 1 and 2; Fig. 15). Intermediate steps were a first intragranular filling by micrite, reworking, oncoidal coating, weak consolidation with Mg-calcite cemented "peloids" in intergranular voids and local compaction of the peloids into cryptocrystalline micrite with interlocking Mg-calcite crystals up to 4p. The submarine lithification process was frequently interrupted by long intervals of nondeposition, dissolution, boring, and later infilling. The limestones were probably never subaerially exposed. Presently, the carbonate rocks undergo biogenic incrustation and partial dissolution into bioclastic sands. The irregular distribution pattern of the sands reflects (a) the patchy distribution of living benthonic organisms, (b) the steady rain of planktonic organism onto the seamount top, (c) the composition of disintegrating subrecent limestones, and (d) the intensity of winnowing and reworking bottom current

(Table T1) Calcium carbonate mineralogy and stable isotopic composition of ODP Hole 182-1132C sediments

Petrographic observation and carbonate mineralogic and stable isotopic investigation were conducted on lower Oligocene to middle Miocene sediments recovered during Ocean Drilling Program Leg 182 from Site 1132, located at a water depth of 218.5 m immediately seaward of the shelf-slope break of the eastern Eyre Terrace in the western Great Australian Bight. The middle Miocene section consists of bioclastic packstone and grainstone with an interval of partially silicified nannofossil-foraminiferal chalk and is slightly to densely dolomitized. By contrast, the lower Oligocene to lower Miocene section is characterized by a predominance of planktonic and benthic foraminifers, high porosity, absence of chert, and weak dolomitization. The carbon and oxygen isotopic composition of calcites and dolomites between two sections, however, shows no significant difference.