Foraminiferal Sand Research Articles

Evidence for topography- and current-controlled deposition on the Reykianes Ridge between 59°N and 60°N

Sediment patterns derived from sediment sampling and acoustic subbottom profiling were mapped on the Reykjanes Ridge (North Atlantic) between 59°N and 60°N. Five discrete sediment echo patterns were distinguished and mapped on a regional scale. The prolonged and layered echo facies, which mainly reflect sediment filled basins on the ridge flanks, indicate deposition of predominantly fine-grained sediments deposited by the Iceland-Scotland Overflow Water. A combination of westward flowing currents spilling over the ridge crest due to the Coriolis force together with the existing morphology probably caused the N-S trending facies distribution pattern on the northwest flank. Furthermore, the modern surface sediment distribution is controlled by biological productivity, which is closely related to the mixing zone of cold subpolar surface water masses and the warm North Atlantic Current, and bottom water transport processes. The effect of bottom current transport is reflected in the pattern of settling velocity and sediment grain size. The clay mineral composition indicates that most of the fine-grained material is supplied predominantly from the Icelandic province by the Iceland-Scotland Overflow Water. Erosional processes are concentrated on narrow zones on top of the axial ridges and on the steep flanks of the Catalonia Seamount. Well-sorted foraminiferal sands on these exposed regions are assumed to represent residual sediments.

Composition and provenance of turbidite sands: Late quaternary, Madeira Abyssal Plain

Late Quaternary sediments from the Madeira Abyssal Plain (MAP) consist of metre-thick fine-grained distal turbidites, separated by centimetre-thick layers of pelagic clay, marl and/or ooze. Three major compositional groups of turbidites have been defined by previous work: volcanic, organic-rich and calcareous. The coarse to medium-grained sand fraction from the basal parts of the volcanic turbidites is predominantly composed of volcanic glass, pumice and foraminifera, whereas heavy mineral and plagioclase feldspar grains, plus basalt and trachyte clasts, become more abundant in the overlying fine-grained sands and silts. The bases of organic-rich turbidites consist of fine-grained sands and silts which are rich in foraminifera, quartz and alkali feldspar grains and altered lithic clasts. Calcareous turbidites have bases composed of medium to fine-grained foraminiferal sand devoid of terrigenous coarse-grained material. New mineralogical data are used to define grain assemblages in the sand/silt fraction from these turbidite groups. These assemblages are comparable to mineral assemblages from alkali olivine basalts, trachybasalts to trachyandesites and trachytes/phonolites on the Azores, Canary Islands and Madeira, and with sediments from the continental margin of northwestern Africa. Volcanic glass chemistry, however, distinguishes the Canary Islands as the principal island provenance of the volcaniclastic sands. Volcaniclastic material probably originated either from reactivation of basic volcanism on Tenerife and Gran Canaria in the Quaternary, or during the development of the western islands of La Palma and Hierro (5 Ma-Recent). Trachytic and phonolitic material was derived from the more fractionated volcanic complexes on the central and eastern islands of the Canary Islands (Tenerife, Gran Canaria, Fuerteventura and Lanzarote). Organic-rich turbidites consist of two mineralogical subgroups, which originated from two distinct source areas along the continental margin of northwestern Africa: (1) north of the Canaries off Morocco; (2) south of the Canaries off Senegal. Differences in the mineralogy between source areas reflect geographical variations in the weathering of continental granitic rocks which were subsequently transported off-shore mainly by aeolian processes. Calcareous turbidites were derived from the Great Meteor-Cruiser Seamount Chain, west of the MAP.

Distribution of epibenthic megafauna and lebensspuren on two central North Pacific seamounts

The abundance, composition and spatial distribution of megafaunal communities and lebensspuren assemblages at three sites on two deep seamounts in the central North Pacific were surveyed photographically using still cameras mounted on the research submersible Alvin. Photographic transects were made on the summit cap (∼1500 m depth) and summit perimeter (∼ 1800 m depth) of Horizon Guyot and on the summit cap (∼3100 m depth) of Magellan Rise. The summit caps of both seamounts were covered with foraminiferal sand, while the summit perimeter of Horizon Guyot was characterized by numerous rock outcroppings (basalt and chert encrusted with ferromanganese oxides) on which was situated a speciose assemblage of suspension-feeding organisms. The most abundant megafauna at all three sites were large, sediment-agglutinating protists belonging to the class Xenophyophorea. Among the three sites, the Horizon Guyot summit cap supported the highest densities of fishes and lebensspuren and the fewest echinoderms, while the Magellan Rise summit cap was populated by a diverse community of deposit-feeding echinoderms. Megafaunal abundances on Horizon Guyot were lower than those at equivalent depths on the western North Atlantic continental slope, while those on Magellan Rise were higher. The faunal differences observed between the two seamounts were attributed primarily to differences in hydrodynamic conditions, substrate availability and nutrient availability. Most of the lebensspuren on these seamounts appeared to be patchily distributed on spatial scales of 10–1000 m, while xenophyophore distributions were predominantly random on the same spatial scales. Biogeographically the species identified exhibited predominantly widespread to cosmopolitan distributions with Indo-West Pacific faunal affinities, typical of other seamounts in the same depth range and biogeographic province.

Internal tides and sediment movement on Horizon Guyot, Mid-Pacific Mountains

Internal tidal currents are the likely cause of erosional features such as current ripples, sand waves, and truncated bedding horizons on the sediment cap of Horizon Guyot. Current meter data obtained over a 9 month period in 1983–1984 at about 213 m above the guyot show that the tidal currents are anomalously strong for mid-oceanic depths, probably the result of topographically induced generation of internal tidal waves. An analysis of the initiation of motion of the foraminiferal sand by the internal tidal currents indicates that these currents, particularly during the months of March–May, are likely to transport the surficial sediment and generate the observed bedforms.

Carbonate Facies and Stratigraphic Framework of Middle Magdalena (Middle Pennsylvanian), Hueco Mountains, West Texas: ABSTRACT

The middle Magdalena of the Hueco Mountains, west Texas, is best exposed in the vicinity of Pow Wow Canyon, particularly along the western scarp of the range. It can be divided into two major depositional sequences, the lowermost of which consists predominantly of alternating bank, interbank, and shoal deposits of Atokan age. These banks are low-relief accumulations of Donezella, with coeval slackwater skeletal wackestones and interbedded deposits of foraminiferal sands. A prominent zone of intertonguing Chaetetes biostromes punctuates the middle part of this sequence. The Desmoinesian sequence begins as a series of rubbly limestones in association with abundant silicified plant remains and is interpreted as a set of paleosols. These are overlain by argillaceous, low-diversity wackestones and packstones of lagoonal origin, followed by carbonates of more open-marine circulation with abundant corals and other stenotypic fossils. In turn, these deposits are succeeded by a sequence of phylloid algal banks that increase in resistance upward to where they represent strata of the upper Magdalena. At this locality, however, the upper Magdalena is very thin because it is truncated by the pre-Hueco unconformity which, so prominent at the head of the canyon, can now be traced to the western scarp. These deposits aremore » directly analogous to subsurface reservoir facies of the same age on the opposite side of the Diablo uplift in the Permian basin and thus provide an opportunity to generate reservoir models based on extensive outcrop exposure.« less

Sedimentary environments of the Central Region of the Great Barrier Reef of Australia

The sediments and calcareous organisms on the outer reefal shelf of the Central Region of the Great Barrier Reef were collected and observed by SCUBA diving and research vessel techniques (including underwater television) to understand the production and processes of deposition of the sediment. The carbonate grains are mainly sand and gravel size and solely of skeletal origin. Over the whole area the major CaCO3 producers, in order of decreasing importance are: benthic foraminiferans (chiefly Operculina, Amphistegina, Marginopora, Alveolinella and Cycloclypeus), the calcareous green alga Halimeda, molluscs and corals. Coral abundance is high only close to reefs and submerged rocky substrates. Benthic foraminiferal sands dominate the inter-reef areas i.e. the bulk of the shelf, and Halimeda gravels form an outer shelf band between 60 and 100 m depths. Seven distinct facies are recognised after quantitative analyses of the sediments. These are: A. Shelf edge slope (>120 m depth); B. Shelf edge (with rocky outcrops); C. Outer shelf with high Halimeda (>40%); D. Inter-reef I; E. Inter-reef II ( 100 m depth but >2% pelagics); F. Lee-ward reef talus wedge (<2 km from sea level reefs); G. Lagoonal.

Facies and sediment dynamics in Charlie-Gibbs fracture zone during the Late Quaternary

Analysis of five cores from the Charlie-Gibbs fracture zone enables us to define the sedimentary characteristics in four different types of environments: a sill disrupting the southern valley, open channels in the northern and southern valleys, a flanking plateau of the northern side wall in the northern valley, and an enclosed depression in the northern valley. Five lithofacies have been recognized: clayey-silty mud (40%), clayey-silty calcareous mud (25%), calcareous sands (20%), sedimentary breccia (10%) and silty-sandy calcareous mud (5%). The predominance of terrigenous clayey material is evidence for a mainly continental origin of the fracture-zone sediments. Yet another striking feature is the abundance of foraminiferal sands rich in pelagic calcareous and siliceous components associated with coarser ice-rafted elements. Despite the pelagic origin of components, a uniform draping of sediments — which kind of sedimentation we call “ubiquiste” (Faugères et al., 1979) — has not been noted in the survey area, due to the reworking processes of already deposited materials by bottom geostrophic currents or gravity flows. The influence of the bottom-current processes on sedimentation is confirmed by the following characteristics: (1) the presence of sharp or erosional contacts between two facies; (2) low clay fraction in certain levels, caused by winnowing of sedimentary materials; (3) high clay fraction in other levels, considered to be the result of dilution of pelagic material by clay particles transported in the nepheloid layer and trapped in a depression downstream of a sill; and (4) Icelandic smectites transported by Iceland—Scotland Overflow Water (I.S.O.W.). The influence of gravity flow dynamic processes is indicated by sandy or silty-sandy centimetric turbidites, very thick mega-sequences (8 m) transported by a single mass flow, and important sedimentary gaps, whence gravity flows originated.

Vicksburg Group Carbonates--A Look at Gulf Coast Paleogene Carbonate Banks: ABSTRACT

Throughout the Paleogene of the northern Gulf coastal plain large deltaic systems dumped thick sequences of lithoclastic sediments onto the shelf of the Gulf of Mexico. In between major deltaic pulses, thick carbonate banks or reefs developed in the vicinity of the broad Wiggins uplift of southern Mississippi and Alabama. Because the present-day outcrop of the Oligocene Vicksburg Group in Mississippi and Alabama cuts across sedimentary strike, all of the component lithofacies of a typical Paleogene carbonate bank complex of the northern Gulf coastal plain are exposed. By relating outcrop lithology to electric log character, the various lithofacies of the Vicksburg Group can be mapped throughout its subsurface extent. Similar inferences and maps can be made for the other P leogene carbonate complexes. Outcrop sediments of the Vicksburg Group comprise five lithofacies, all of which show variations: (1) molluscan, glauconitic, foraminiferal, quartz silty sand/wackestone; (2) foraminiferal, algal mudstone; (3) (quartzose) bryozoan, foraminiferal silty sand; (4) glauconitic, Lepidocyclina, algal(?), silty sand; (5) skeletal grainstone/coarse sand. These lithofacies suggest a set of depositional environments (corresponding lithofacies number in parentheses): (A) destructional delta (1); (B) algal muddy shelf bottom (2); (C) regressive carbonate shelf (1); (D) carbonate bank (back-bank primarily) (3,4); (E) regressive carbonate shoal/shoreline (5). All of these environments, with their slightly different faunal constituents, can be found in the other carbonate units of the Gulf Coast Paleogene. The Tatum and the Salt Mountain Limestones have coral-algal community shelf margins, whereas the Cook Mountain, Ocala, and Vicksburg have a slightly deeper water foraminiferal-algal community at the margin. Time-equivalent lithoclastic deposition to the west prevented westward expansion much beyond central Louisiana. During the Miocene, bank complex communities were displaced farther to the east by continued Rocky Mountain-derived sediments coupled with a major epeirogenic uplift of the southern Appalachians. End_of_Article - Last_Page 1463------------

Extrapolation of physical properties of sediments from a localized area in the Gulf of Mexico, based on a conceptual geological model

Conceptual models of depositional systems may permit extensive extrapolation of physical properties for a relatively small amount of detailed correlation among physical, sedimentological and acoustic properties. For example, in a 9-m midslope core (IG19-39, lat. 28°51.2′N, long. 87°14.0′W, depth 1006 m) in the northeastern Gulf of Mexico there are two approximately 60-cm zones (at the core top and from 750 to 810 cm) which have distinctly higher foraminiferal sand and clay contents than the rest of the core. In the foram-rich zones, the relative increase in both clay- and sand-sized particles leads to poorer sorting, lower silt/clay ratios, more negative skewness, less platykurtosis, and higher fine-clay/total-clay ratios than in the silty clay which comprises the rest of the core. Within the silty clay, bulk densities and in situ acoustic velocities increase with depth from 100 to 425 cm, remain at or near peak values to 700 cm, and then drop sharply into the lower foram-rich zone. Although high impedence contrasts within this sequence do not correlate well with the 3.5-kHz reflection pattern, the pattern is distinctive, and its extent suggests that the core's sedimentary pattern extends over about 4000 km 2 on the northeastern continental slope of the Gulf of Mexico. The foram-rich zones have a predominance of warm-water species and correlate closely with higher carbonate content (in the mud as well as in the sand fraction) and more positive δ 13C values (for organic carbon), which are typical of marine-derived organic matter. On the other hand, the silty clay has few if any warm water indicating forams and much less carbonate. It has organic carbon with more negative δ 13C values, indicative of a terrestrial origin. Several investigators have concluded that, in the Gulf of Mexico, the foram-rich zones represent warm, interglacial or interstadial periods dominated by pelagic sedimentation in the deep-sea, whereas the siltier zones represent glacial periods of low sea-level stands during which more terrigenous sediment was delivered to the continental margins and transported to adjacent ocean basins by various gravity flow mechanisms. Since the causes of the changes in depositional processes are glacio-eustatic, the change in sediment types should be widespread with good lithostratigraphic correlations. Published data exist for over 100 cores from the deep Gulf of Mexico wherein only one or a few sediment properties are reported. In a large number of these cores the reported property correlates well with the pattern in the core studied and is explained by the glacio-eustatic model. This suggests that the relationships between physical, chemical, textural, and acoustic properties in core IG19-39 may be used to predict similar relationships over a widespread area. The model seems to apply to the abyssal plains of the Gulf of Mexico, the margins of the Mississippi Fan, and various parts of the northern continental slope.

A modern oceanic hardground on the Carnegie Ridge in the eastern Equatorial Pacific

ABSTRACTFossil surfaces of erosion and non‐deposition are common in limestone sequences from the Mesozoic in western Europe and in the Tethys and have been described under the name ‘hardgrounds’. They are of shallow water as well as of oceanic origin. A modern example in the Pacific is described in this paper. The Carnegie Ridge, an east‐west trending shallow ridge between South America and the Galapagos Islands, has a central, deeper saddle where erosion has removed most of the sediment cover down to a hard chalk and chert bed (acoustic basement), and has cut intricate channel patterns on the south flank and two deep canyons on the north. The erosion has produced a karst‐like relief of steep‐walled channels, cliffs, and corroded chalk remnants. The floors of the channels are covered with ferromanganese oxide crusts or crust fragments over which loose sediment is being transported. In the two canyons on the north flank, this sediment consists of foraminiferal sand travelling downslope in the form of barchan dunes. All sediment down to acoustic basement has been stripped from the Carnegie Ridge crest except where it is protected behind basement ridges and pinnacles. Surface features of the eroded chalk are strongly reminiscent of features observed in Mesozoic hardgrounds. Current measurements over several days indicate a net northward movement, slow but possibly adequate to keep the sea floor free of fresh deposits. The rates, however, seem inadequate to explain the formation of the deep channels, and there is no evidence for the southward flow which is implied by the southern channel system. A process of combined carbonate dissolution and removal by the current of fresh sediment and dissolution residues can account for the required erosion in about two million years. Regional unconformities identifiable in seismic reflection profiles and dated in cores are of middle to late Pliocene age, suggesting that the formation of the erosion surface began 2‐3 million years ago. Buried Miocene unconformities of local extent show that the present erosion period had minor precursors possibly related to short‐lived increases in current action and carbonate dissolution.

Sediment Transport Over the Hatton and Gardar Contourite Drifts

ABSTRACT The contrasted modes of transport and deposition of sandy and muddy contourites were examined by hydrographic, photographic, lithologic, and seismic data from Hatton and Gardar sediment drifts in the Iceland Basin, northeastern Atlantic Ocean. Gardar Drift is made of muddy contourites deposited from Iceland-Scotland Overflow Water with a thick and relatively intense nepheloid layer whose particles originate from Iceland. Hatton Drift is also mainly mud, possibly from Rockall Bank, but at its foot a strong northeast-flowing boundary current moves winnowed foraminiferal sand as ripples and low sand waves. The nepheloid layer on Hatton Drift is locally generated and is mainly in a stratum of North Atlantic Deep Water that is contaminated with Antarctic Bottom Water. This stratum, with it entrained particles, spreads across the basin as far as the crest of Gardar Drift. The bottom mixed layer over Gardar Drift is mainly thin (30-40 m) and indicative of depositional shear-stresses (u* 0.85 cm/sec over sands at the foot of Hatton Drift, the mixed layer is 90 m thick, but this thins to less than 40 m higher up on the drift as the environment changes from winnowing towards deposition.

Manganese-Nodule Bedforms and Thermohaline Density Flows in a Deep-Sea Valley on Carnegie Ridge, Panama Basin

ABSTRACT Arrangements of manganese nodules in rows and cellular networks decorate Miocene chalk surfaces stripped by erosion at depths of 2700-3000 m on the north flank of Carnegie Ridge, eastern equatorial Pacific. The patterns are interpreted as bedforms, similar to the transverse ribs and stone cells of fast, shallow rivers in appearance, but probably not in origin. The powerful currents responsible for shaping these deep-sea bedforms and adjacent fields of foraminiferal sand waves are believed to be density flows of Peru Basin water that episodically spills into the warmer Panama Basin. These thermohaline density currents cascade down their erosional valley with many of the characteristics and some of the geologic competence of channelled turbidity currents.

The development of sediment threshold curves for unusual environments (Mars) and for inadequately studied materials (foram sands)

ABSTRACTGeneralized curves for grain threshold under liquid flow and under gas flow (air), based on laboratory data summarized in a previous paper, are utilized to generate a series of threshold curves for unusual sedimentary environments and little‐studied sedimentary materials of interest to geologists. These are aeolian and water flow threshold on the planet Mars, and the threshold of foraminiferal sands in the deep‐sea. Such extrapolations from the generalized curves are necessary because of the difficulty or impossibility of obtaining satisfactory data for these sediments or environments, and the procedures demonstrate the importance of the general threshold curves.

Abyssal Dunes of Foraminiferal Sand on the Carnegie Ridge

Research Article| November 01, 1974 Abyssal Dunes of Foraminiferal Sand on the Carnegie Ridge PETER LONSDALE; PETER LONSDALE 1University of California at San Diego, Marine Physical Laboratory of the Scripps Institution of Oceanography, La Jolla, California 92037 Search for other works by this author on: GSW Google Scholar BRUCE MALFAIT BRUCE MALFAIT 2Oregon State University, School of Oceanography, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1974) 85 (11): 1697–1712. https://doi.org/10.1130/0016-7606(1974)85<1697:ADOFSO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation PETER LONSDALE, BRUCE MALFAIT; Abyssal Dunes of Foraminiferal Sand on the Carnegie Ridge. GSA Bulletin 1974;; 85 (11): 1697–1712. doi: https://doi.org/10.1130/0016-7606(1974)85<1697:ADOFSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A field of sand dunes was discovered by the side-looking sonars of a deep-towed geophysical instrument package at a water depth of 2.65 km on the north flank of the Carnegie Ridge in the eastern equatorial Pacific. The dunes are moving down the flat floor of an erosional valley that has been excavated through calcareous ooze to a resistant, manganese-encrusted chalk stratum. Transverse dunes (mean wave length 20 m) anc barchans (length 6 to 80 m) are arranged in long, narrow belts. Stereo photographs (875 oriented pairs) of the dunes and of the current-swept rock floor show that average dune height is —0.6 m, the steep (25° to 30°), lee side is generally smooth, and the upstream slope is covered with short-crested current ripples. A core from a transverse dune recovered moderately sorted sand, composed of broken fragments (70 percent) and intact tests of Quaternary age Foraminifera. Photographs and side-looking sonar records both indicate a northwestward, down-valley sediment transport. The dunes are thought to be formed and propelled by fast (>30 cm/sec) currents of dense water, which spill episodically over the Carnegie Ridge into the Panama Basin. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sedimentation and Erosion on Horizon Guyot

var googletag = googletag || {}; googletag.cmd = googletag.cmd || []; googletag.cmd.push(function () { googletag.pubads().disableInitialLoad(); });

Sedimentation in the area of Peake and Freen Deeps (Mid-Atlantic Ridge)

Peake and Freen Deeps are two flat-bottomed depressions situated in the eastern foothills of the Mid-Atlantic Ridge near 43°N, 20°W. Seismic reflection profiling shows that they are partially filled with horizontally stratified sediments. Small-scale faulting within the sediments and tilting of the floors of flat-bottomed sediment pockets to the north and south of the deeps indicate only minor structural adjustments in the area since the onset of deposition in the two troughs. Closely-spaced core samples from the region contain two types of material. The first kind is a modified pelagic sediment in which climatic changes are recorded by variations in the foraminiferal fauna and in the amount and composition of mineral debris. The second is a coarse foraminiferal sand which occurs as thin layers in the bottom of the deeps. These sand layers are interpreted as turbidity current deposits composed of locally derived material. The histories of sedimentation in the two deeps appear to have been different, in spite of their close proximity.

A cretaceous (Turonian) core from the Naturaliste Plateau southeast indian ocean

An Upper Cretaceous (Turonian) sediment core was recovered from the northeastern s slope of the Naturaliste Plateau, eastern Indian Ocean. At the core site, coarse grained foraminiferal sand of Recent-Pleistocene age disconformably overlies a white Cretaceous chalk consisting largely of planktonic organisms. The pelagic nature of the sediment suggests that the water depth at this core site was approximately the same in both Cretaceous and Pleistocene time. Bottom photographs and seismic profile records were also taken across this feature. The seismic profile records show a rather thin sediment layer with some thicker sedimentary pockets, possibly due to faulting. The south side of the Plateau is marked by a steep scarp with a well developed sediment “ramp” at its base. This sediment “ramp” probably owes its origin to longshore currents which sweep around the southwest tip of Australia. The Naturaliste Plateau is similar to other plateaus marginal to continents in that: (a) they have more or less flat summits, (b) Pleistocene overburden is thin or lacking, and (c) steep scarps are present on at least one side.

Black and Brown Speckled Foraminiferal Sand from the Southern Part of the Great Barrier Reef

ABSTRACT Black and brown speckled foraminiferal sand occurs in the western part of the Capricorn Reef Complex and on adjacent parts of the Inner Shallow Shelf in the zone of mixing of terrigenous and carbonate sediments. The colored materials, which are found preferentially on foraminifera tests, are iron and manganese sulphides, hydroxides and/or oxides. The iron and manganese appear to be derived from associated terrigenous sediments and are being reprecipitated under reducing conditions and in the presence of sulphate-reducing bacteria around centers of organic material such as the foraminifera tests in the sediment. Removal of sediment by erosion and the activity of burrowing organisms brings reduced sediment back into the oxidizing zone where sulphides (black) are oxidized to ferric hydro ides and/or oxides (brown).

Sands of the Mid-Atlantic Ridge

Sands collected at 24 locations along the crest of the Mid-Atlantic Ridge between 57 degrees S and 38 degrees N consist predominantly of olivine, diopsidic augite, hypersthene, enstatite, amphibole, quartz, plagioclase, and volcanic glass, suggesting an olivine tholeiitic source. Eight cores contain relatively pure mineral sands; three of these cores reflect local volcanic activity. In 16 cores the manganese-coated mineral grains are mixed in a current-winnowed foraminiferal sand or ooze.

Stages of Glauconite Formation in Modern Foraminiferal Sediments

ABSTRACT Examination of 325 bottom samples taken at 68 stations off the southeast coast of the United States by the United States Fish and Wildlife Service revealed the common occurrence of glauconite in foraminiferal sands. The glauconite is variable in color and in morphology, ranging from dark greenish-black to greenish-white. The dark green glauconite generally occurs in the smooth, bulbous pellet form, whereas the lighter green glauconite commonly occurs as rough, porous pellets in the form of internal molds of microorganisms. However, in some internal mold pellets, dark green and light green glauconite occur together. Magnetic fractions were taken from all samples. The samples composed primarily of foraminiferal tests yielded a glauconite-rich sparate. In the magnetic separates occurred many intact shells of foraminifers which, upon solution in acid, generally yielded internal molds of the light-colored glauconite. Analysis by x-ray diffraction of single pellets and of composite samples of hand-picked grains revealed the glauconite to be essentially a mica-type clay varying in degree of hydration and disorder. Examination of oriented slides of the composite samples following vapor glycol treatment showed that the dark-green glauconite was unaffected, but that the pale green glauconite was expandable. Debye-Scherrer films of single pellets of the light green glauconite showed the presence of kaolinite which was not found in the darker green glauconite. A decrease in sharpness and a shift of the basal mica diffraction line to higher d values as well as a decrease in the potassium content and an increase in the water content were directly related to the color change of the glauconite from dark green to pale green. The glauconite in these foraminiferal sands is interpreted as representing various stages of glauconitization of clay material which was originally enclosed within the tests of microorganisms.