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6-1984

Barbados Ridge; Inner Trench Slope Sedimentation

William J. Cleary University of North Carolina at Wilmington

H. Allen Curran Smith College, [email protected]

Paul A. Thayer University of North Carolina at Wilmington

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Recommended Citation Cleary, William J.; Curran, H. Allen; and Thayer, Paul A., " Ridge; Inner Trench Slope Sedimentation" (1984). Geosciences: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/geo_facpubs/57

This Article has been accepted for inclusion in Geosciences: Faculty Publications by an authorized administrator of Smith ScholarWorks. For more information, please contact [email protected] BARBADOS RIDGE: INNER TRENCH SLOPE SEDIMENTATION'

WILLIAM J. CLEARY Departmentof Earth Sciences University of North Carolina at Wilmington Wilmington, North Carolina 28403 H. ALLEN CURRAN Department of Geology Smith College Northampton, Massachusetts 01063 AND PAULA. THAYER' Department of Earth Sciences University ofNorth Carolina at Wilmington Wilmington, North Carolina 28403

ABSTRACT: A study of 27 piston cores from the Barbados Ridge shows that Quaternary sediments of this inner trench slope are chiefly foraminiferal-pteropod-nanno oozes that have been reworked and/or emplaced by gravity currents. These oozes are mixed with volcanogenic sediments and minor amounts of material derived from the insular shelf. Irregular topography greatly influences the type and dispersal of bottom sediments, particularly on the east side of the ridge. Reworking occurs on topographic highs where fines are winnowed out and mass wasting and gravity currents are initiated; these processes ultimately result in the leveling of irregular floors of deeper, intervening slope basins. Petrographic analyses of graded intervals from the basin floors and slopes reveal the sands to be mixtures ofplanktic foraminifers (35%), pteropods (10%), and fresh, angular volcanic detritus (10- 90%). Planktic foraminifers are commonly form-segregated on the basis of test shape in the graded intervals, On the east flank of the Barbados Ridge, graded sands of insular-shelf origin are locally important; these sands consist of molluscan, algal, and barnacle fragments (0-25%), quartz (0-30%), and minor metaquartz, K-feldspar, and gneissic fragments. The presence of these terrigenous con­ stituents suggests reworking of sediment from Tertiary land or subsea outcrops; additional evidence in favor of reworking is the occurrence of recrystallized radiolarians (Oceanic Group) and biomicrite gravels. Volcanogenic sediments are areally widespread and dispersed throughout the upper 6 m of the cores and consist of fresh calcic plagioclase (5-40%), together with tuffaceous and pumaceous fragments. We believe these sediments have experienced reworking and resedimentation by bottom currents, with the initial deposition related to volcanic activity on nearby St. Vincent in the .

INTRODUCTION deposited. Sediment and acoustical studies in lower-trench slope settings (Moore and The identification of different styles of Karig 1976; Moore 1979), such as those of sedimentation in complex deep-sea settings the Sunda Arc System, have shown the ex­ is vital to the recognition of ancient analogs istence of sediment ponds in this type of de­ in the rock record. The Barbados Ridge rep­ positional environment. Sediment in these resents an inner trench slope (as defined by ponds consists of redeposited hemipelagic Karig 1974), and it offers a unique oppor­ clays, pelagic oozes, and shallow-water ma­ tunity to study a tectonically active conti­ terial emplaced by slumping and turbidity nental margin area where a wide variety of current activity. In some cases, lithified de­ biogenic and terrigenous sediments are being posits similar to the above occur on conti­ nents (Underwood and Bachman 1982). Me­ 1 Manuscript received 31 December 1982; revised 8 lange material and the lithified counterparts September 1983. of trench slope ponded sediments have been 2 Present address: Amoco Production Company (USA), mapped in an uplifted slope basin on the is­ P.O. Box 50879, New Orleans, Louisiana 70150. land of Nias near Sumatra (Moore 1979), as

JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 54, No. 2, JUNE, 1984, P. 0527-0540 Copyright © 1984, The Society of Economic Paleontologists and Mineralogists 0022-4472/84/0054-0527/$03.00 528 WILLIAM J. CLEARY. H. ALLEN CURRAN. AND PAUL A. THAYER

every 10 cm over the entire length of each � Puerto Rico core studied. Sand, silt, and clay percentages were de­ Caribbean termined using a combination of data ob­ Sea tained by standard sieving techniques and pipette analysis. The petrographic nature of the sand fractionwas ascertained from anal­ yses of 150 samples. Samples were washed, and the sand-sized fraction(2.0 to 0.125 mm) ° 13 15' was embedded in a polyester resin and thin­ sectioned. Point counts were made on 200 grains in each thin section. The reliability of feldspar identification was checked using standard staining techniques (Hayes and 13°!0' Klugman 1959; Bailey and Stevens 1960). Five representative cores were processed BARBADOS for microfossils in order to establish a bio­ stratigraphic framework for the cores. Sub­

° samples of 4 grams taken at 10 or 20 cm 13 05' intervals throughout the cores were washed over sieves to retain microfossils. The bio­ 0 I 2 3 4 stratigraphic subdivision of the cores was l,,,J I I I KM based on planktic foraminifers, following the �----�----�----�13° 00' method of Emiliani (I 969). Presence or ab­ ° ° ° ° 59 40' 59 35' 59 30' 59 25' sence of benthic foraminifers, pteropods, os­ tracodes, shell and coral fragments, and echi­ F10. 1.-lndex map for Barbados. Inset map shows location of Barbados in the easternpart of the Caribbean noid spines also was noted for each sample. Sea. Internal structures were studied by means of X-radiography. Cores were slabbed into sections, usually 20 to 25 cm in length and well as numerous other orogenic belts (Un­ 1.5 cm thick, and placed on an 8 x 10 Kodak derwood and Bachman 1982). "M"-type film-pack for exposure. The ex­ This reconnaissance study is concerned posure was varied according to composition with the diverse nature of sediments along of the sediment. one of the salient tectonic features of the Ca­ ribbean Sea, the Barbados Ridge. The paper REGIONAL GEOLOGY describes the mineralogy, petrology, and mi­ crofaunalcharacteristics of the sediments and Barbados is a coral-capped, nonvolcanic interprets the dominant sedimentary pro­ situated at the lead edge of the Carib­ cesses active on the Barbados Ridge (Fig. 1). bean plate, immediately east of a group of There is a complex interaction between tec­ volcanic , the Lesser Antilles. Barba­ tonic processes, volcanism, and multiple sed­ dos represents one of the few places in the iment sources in this area; and knowledge of world where the trench-slope break emerges the resulting patternsof sedimentation should as an island (Karig and Sharman 1975; Dick­ be of use in attempts to interpret similar an­ inson and Seely 1979). The island is approx­ cient deposits. imately 37 km long and 23 km wide, with a maximum elevation of 340 m. It is located METHODS some 340 km northeast of Trinidad and 190 km east of the Lesser Antilles at approxi­ An assessment of the diversity of sediment mately 13°1 0'N latitude and 59'30'W longi­ types and microfaunalconstituents was made tude (Fig. 1). Eighty-five percent of the is­ by sampling a suite of 27 piston cores that land's surface is covered by a veneer of were raised aboard the R/V Eastward during uplifted Pleistocene coral rock that reaches 1969-70 (Fig. 2). Subsamples were taken 90 m in thickness (Senn 1940; Baadsgaard BARBADOSRIDGE SEDIMENTATION 529

13° 30'

13° 00'

0 10 Nautical Miles L______c 1. . = 1oo _F ---1._uc._=z::_"""=�..L-'--L.::;====s.__1_--- ______J 12,30, ° 1 ° 1 ° 1 1 60 30 60 00 59 00 58° 30 Fm. 2.-Bathymetric chart of the Barbados Ridge showing locations of cores used in this study. Cores identified with an asterisk and number are portrayed in Figure 5. (Modified after unpublished data fromJ. G. Newton, 1972.)

1960;Macintyre 1967;andJames 1972). The terbedded with clay and conglomeritic san1s. remaining area, the Scotland District, is an The Joes River Formation is a chaotic de­ erosional window into underlying folded and posit of huge, exotic blocks of Paleocene faulted strata of Tertiary age. limestone along with smaller clasts and sand­ Deep test wells confirm the presence of a sized grains set in a dark gray silt matrix. thick, structurally complex group of Tertiary These rocks have been interpreted by Speed strata beneath the entirety of Barbados (1981) to be slumped, lower slope deposits. (Baadsgaard 1960). Although 4,583 m ofTer­ The Oceanic Group consists of an interbed­ tiary sediments were encountered during ded pelagic suite of Early Eocene to Late Oli­ drilling, the true thickness is unknown since gocene age (Speed 1981; Speed and Larue the section is often repeated by isoclinal fold­ 1982). This deep-water sequence is mainly ing, faulting,and bedding plane shearing (Senn intercalated foraminiferal-nannoplankton­ 1940, 1947; Baadsgaard 1960; and Daviess radiolarian marls and volcanogenic turbi­ I 97 l). dites (Pudsey and Reading 1982; Speed l 983). The Tertiary strata have been grouped into The island of Barbados is an emergent por­ three major lithostratigraphic units (Fig. 3): tion of the broad, north-south trending Bar­ Scotland Group, Joes River Formation, and bados Ridge which parallels the Antillean Arc Oceanic Group (Senn 1940, 1947; Baads­ and runs fromthe Venezuelian-Trinidad shelf gaard 1960; Kugler 1961; Saunders and Cod­ northeast beyond the Island of . ry I 96 5). The basal complex, the Scotland The Barbados Ridge is the only example of Group, of Paleocene to Late Eocene age, is a compressional continental margin pro­ dominantly a elastic turbidite sequence in- duced by plate collision in the , 530 WILLIAM J. CLEARY, H. ALLEN CURRAN, AND PAULA. THAYER

LJ REEFALCARBONATES (PLEISTOCENE) A -- MIXED CARBONATES (PLIO-PLEISTOCENE)

.,,.,,.. ffiOCEANIC FM, (LATE EOCENE-MIOCENE) H,}}j JOES RIVER FM, /SCOTLAND GROUP ( PALEOCENE-LATE EOCENE) ,,,,------AXIS OF STRUCTURAL HIGHS ··· POSITION OF RAISED, WAVE-CUT TERRACES

FIG. 3.-Generalized geologic map and cross section of Barbados (modified from James 1972). and the island has been considered by some of Hess (1938). Some investigators (Chase investigators (Moore et al. 1980; Westbrook and Bunce 1969; Schneider I 969; and Dav­ 1982) to be geologically similar to the island iess 1971) have viewed Barbados and the of Nias, which rests atop a non volcanic arc Barbados Ridge as having originated by un­ in the Indonesian Archipelago in association derthrusting of the westward-drifting Atlan­ with the Sunda Trench. Both islands and as­ tic Ocean floor beneath the eastern margin of sociated ridges have similar features, includ­ the Caribbean plate. It is thought that this ing uplifted Pleistocene reefs. Seismic studies process produced the large negative gravity of the Barbados Ridge (Ewing et al. 1957; anomaly and provided the means for crum­ Officer et al. 1959; Hurley 1966; Chase and pling the thick accumulation of Tertiary sed­ Bunce 1969; Eaton and Driver 1969; Moore iments that originally filled the southern por­ et al. 1982; and Westbrook 1982) have re­ tion of the Puerto Rican Trench. Speed (1981) vealed strongly deformedstrata similar to the suggested that the ridge as a whole originated Tertiary sequence found on the island. In as part of an accretionary prism. some areas the folded and faulted strata are Chase and Bunce (I 969), Schneider (1969), overlain by relatively undisturbed, bedded and Fox (personal communication, 1973) sediments. Core and dredge data of Hurley proposed that the thick sedimentary fill in (1966), Ramsay (1968), and Moore et al. the original trough is autochthonous and was (1982) along the ridge north and south of derived from three sources: 1) continental Barbados support the contention that the ridge sediments from the South American Conti­ is composed of deformed Tertiary strata. nental Platform carried into the trough by Gravity data (Hess 1938; Daviess 1971) sug­ axial turbidity currents; 2) pelagic sedimen­ gest that the Barbados Ridge represents a tation; and 3) volcanic sediments from the southern extension of the Puerto Rico Trench Lesser Antilles Island Arc. Daviess (1971), along the approximate axis of a major neg­ on the other hand, held that the original fill ative gravity anomaly. is chiefly of allochthonous origin. He pro­ The origin of Barbados Ridge has been the posed that sediments present along the east­ subject of much debate since the early work ern border of the Caribbean block (Scotland BARBADOS RIDGE SEDIMENTATION 531

Group) slid downslope (olistostrome) in Ear­ (Fig. 5). Several processes are active along ly Tertiary time into the Puerto Rico Trench. the ridge and have resulted in the deposition Results from the Deep Sea Drilling Project of unique units with a complicated sedimen­ (DSDP) Leg 78A, on a track northeast of tary history. The sediments within these units Barbados, have indicated that pelagic and hemi­ can be grouped into three categories and in­ pelagic sediments are being both accreted clude: 1) pelagic; 2) hemipelagic (volcanic); and subducted along the inner trench wall. and 3) recycled sediments (Fig. 6). The stratigraphic column of Site 541 (ap­ The pelagic sediments, which are domi­ proximately 260 km northeast of Barbados) nant, are normally light brown to tan, the shows a stratigraphic inversion of Miocene lighter color generally reflecting the high car­ and younger sediments in a folded, faulted, bonate content. Sand- and silt-sized grains and thickened unit (Moore et al. 1982). which are included as pelagic constituents are Over the last ten years, a large quantity of volumetrically less important and consist geophysical data has been obtained from primarily of pteropods and foraminifers. the Barbados Ridge (Westbrook 1975, 1982; Nannoplankton are important clay- and silt­ Peter and Westbrook 1976; Biju-Duval et al. sized constituents of the pelagic oozes, but 1978, 1982; Moore et al. 1982). These data their presence was only noted during cursory and DSDP core data have been used to sup­ examination and not studied. port the previously mentioned idea that the Hemipelagic sediment consists primarily ridge represents part of a wide accretionary of volcanogenic sediments that have been sediment pile that developed in conjunction spread across the Barbados Ridge as a result with the evolution of the Lesser Antilles Arc of eruptions in the nearby Antillean Arc to (Westbrook 1975, 1982; Speed 1981). Seis­ the west. Terrigenous clays from the mic studies suggest that the Tobago Trough island of Barbados may be delivered to the and the Barbados Ridge together comprise deep-sea and may be locally important, par­ an exceptionally broad forearc region (West­ ticularly on the east side of the island, but brook 1975, 1982; Speed I 981). The island volumetrically, the terrigenous clays are of Barbados represents an exposure of this probably not significant. wide, mature forearccomplex near its thick­ Recycled sediments include the redeposi­ est point (Speed 1981). tion by gravity currents of the above pelagic Although much work has been done on and hemipelagic sediments that accumulate Barbados subaerial geology, fewstudies have on topographic highs, plus terrigenous ma­ dealt with the offshore sediments, submarine terials that are derived fromthe island itself rock outcrops, or subsea depositional envi­ (Fig. 6). These sediments are generally darker ronments. Macintyre (1967, 1970) studied in color (gray to green) and include clay- to reefs and shelf sediments on the west side of gravel-sized material that typically is graded the island, and Hurley (1966) and Ramsay (Fig. 5). While recycled sediments, particu­ (1968) did limited dredging and coring on the larly biogenic oozes, are distributed over the Barbados Ridge north and south of the is­ ridge, those with a detrital-dominant assem­ land. To date, there have been no published blage are most abundant on the northeast studies concerned with the eastern shelf and flank of Barbados where maximum local re­ slope of Barbados, or with river and beach lief occurs (Fig. 4). Sandy cores recovered sediments of the island. from bathymetric depressions on the north­ east flank consist largely of recycled sediment SEDIMENTS derived from the island or from the many subsea outcroppings of Tertiary rocks (Fig. The general surface of the Barbados Ridge 5). (Figs. 2, 4) is blanketed by a light to dark Reworking and winnowing of fine sedi­ brown, thin (10-100 cm) calcilutite layer. ment from topographic highs by bottom cur­ These sediments are generally fine-grained rents may be an important process that re­ and of Holocene age. A variety of interbed­ sults in the concentration of coarse ded, gray to brown, fine- and coarse-grained microfossils(primarily planktic foraminifers) gravity current emplaced sequences (10-280 and the redistribution of calcareous fines to cm thick) occur below this upper mud cover intervening closed depressions. Here is a sit- 532 WILLIAM J. CLEARY, H. ALLEN CURRAN, AND PAUL A. THAYER

F1G. 4.-Block diagram illustrating subsea topography of the northeast flank of Barbados. Core l I 276 (see Fig. 5) was taken from floorof the depression area. (Modified after unpublished data from J. G. Newton 1972.) uation where topography not only influences amination of fullymature tests permits more the types and patterns of sedimentation, but consistent recognition of species and intra­ also plays a feedback role resulting in the specificvariation (Emiliani 1969). Use of the elevation of intervening basin floors. larger test fraction tends to eliminate species with generally smaller size, such as Globig­ CORE BIOSTRATIGRAPHY erinoides ruber, a major constituent of the total foraminiferalpopulation, but the species Assemblages ofplanktic foraminiferafrom most important forbiostratigraphic zonation the five cores shown in Figure 5 were ana­ are retained. More information concerning lyzed to establish a working biostratigraphic the total foraminiferal populations of these frameworkfor this study. Samples were taken core samples can be obtained from Mitchell at approximately 20-cm intervals fromthese (I 974). cores and wet-sieved to isolate the bioclastic Initial analysis of the foraminifersrevealed components. For the biostratigraphic analy­ that none of the core samples were older than sis, only foraminiferal tests larger than 0.5 Pleistocene in age, although some of the sam­ mm in diameter were studied because ex- ples rarely contained older planktic foramin- BARBADOS RIDGE SEDIMENTATION 533

WEST f--FLANK"" 1----- EAST FLAN K------1

SW SE EAST NE EAST HIGH HIGH HIGH DEPRESSION 11267 13761 I D:: 1410 m

11257 I

11262 I

D::2200 m

.. ., .. ,;.tJ• • .....� . :.4: .. :.: .. :._li-,£t.5.tif ·'."',·.� · - ,-.--�- 7:..!.-::::::- -11- .. - - -',.._-jj 11276 -�- ,..- - .. -,1- .. _,--jj---,..- j -..E_ il -11 D=2450 m jj- �- 7-t:JC -Jl-.!...Jf -w\1-.. -�--�... -�\j F1G. 5.-Bedding and lithologic characteristics of cores from the Barbados Ridge. The five cores illustrated are representative of the suite raised from the ridge. Those shown represent the three basic topographic divisions of the Barbados area: West Flank, East Flank Plateau, and Northeast Flank Depression.

ifers reworked from Tertiary outcrops on or sence of Globigerinoides fistulosus, and the around Barbados. The Pleistocene-Holocene absence of discoasters. age is based on the presence of G/oborota/ia Following the criteria ofEricson et al. (l 963, truncatulinoides throughout the cores. This l 964), the Holocene-Pleistocene boundary species is cited by Hays and Berggren (l971) was identified in 4 of the 5 cores analyzed as the most reliable criterion for recognition (see Fig. 5) on the basis of the occurrence of of deep-sea Pleistocene-Holocene sediments, abundant specimens of the G/oborotalia and it is used as the Pleistocene zone fora­ menardii-tumida species complex (Holocene miniferby Blow (Zone N. 22, l 969) and Bolli beds); these occur overlying sediments in and Silva (l 973). Other criteria first used by which G. menardii-tumida specimens are to­ Ericson et al. (l 963, 1964) for recognition of tally absent. This interval has a variable Pleistocene deep-sea sediments also apply to thickness of 90 cm ( core 137 6 l) to 600 cm these core samples, namely the presence of (core 11262). According to Ericson et al. the Globorotalia menardii-tumida species (l 963, l 964), the absence ofthe G. menardii­ complex with 95% sinistral coiling, the ab- tumida species complex represents a time of 534 WILLIAM J. CLEARY, H. ALLEN CURRAN, AND PAUL A. THAYER

VOLCANIC SAND PETROLOGY 100% o Topographic High The sands of the Barbados Ridge belong • West Flank to a diverse compositional suite of sediments • Northeast Flank (Depression) that has been derived froma variety of source 6'. Core No, 11262 areas within the Antillean Arc system (Fig. 6). Generally sediments are fine-grained, al­ though a few thin ( < IO cm) gravel beds do occur, particularly on the northeast flank of • • • • the ridge (e.g., 11267, 90-130 cm, Fig. 5). Planktic tests, primarily foraminifers (5- • .,, • 25%) and pteropods (0-50%) characteristic • • ,,. •• of open marine sedimentation generally IOO%L------��-.c,100% dominate the sand-sized sediments (Fig. 7A). BIOGENIC (PELAGIC) REWORKED SHALLOW SHELF Foraminiferal tests within the same sand sample show a wide variety of diagenetic FJG. 6.-Triangular plot for composition of the sands. stages, which suggests some mixing of spec­ Reworked shallow shelf grains include dctrital quartz (all types), potassic feldspar, rock fragments, and shallow­ imens of differentages. Moreover, some tests water carbonate constituents. Volcanic grains include show complete recrystallization with calcite plagioclase (angular, fresh), pumaceous, and lithic frag­ or dolomite rhombs infilling chamber areas; ments. Biogenic grains are principally planktic foramin­ others are infilled with a ferruginous clay. ifers and pteropods. Some biogenic material may be re­ worked, but all is Pleistocene or younger in age, This The second major sand assemblage frac­ plot does not include heavy minerals. tion has a first-cycle volcanic or pyroclastic origin. Heavy mineral analyses of fine sands show that most belong to a single composi­ tional suite which includes magnetite, hy­ glacial conditions (in this case, Wisconsinan persthene, augite, hornblende, and olivine. time). Sediments below this interval are char­ These minerals suggest a close affinity to the acterized by the abundant occurrence of spec­ nearby Antillean Arc system 180 km to the imens of the extinct foraminiferal subspecies west. Other volcaniclastic sand constituents Globorotalia tumida j/exuosa, which is include pumaceous and tuffaceouslithic frag­ thought to mark warm interglacial conditions ments (1-75%, Fig. 7B, C), basalt fragments in the Atlantic and Caribbean (Ericson et al. (0-50%), and angular, fresh calcic plagio­ 1963, 1964). clase (l-40%), some zoned. In some cases, Material of Pleistocene age occurs at the brown, translucent glass shards coat euhedral top of core 11267 ( Globorotalia tumidaflex­ crystals of augite or plagioclase. These frag­ uosa present in the uppermost sediments) im­ ments are commonly fresh and angular, sug­ plying the presence of a scoured surface near gesting that little reworking has occurred (Fig. the base of the northeast flankdepression area. 7B). Volcanic components rarely occur as Graded beds probably deposited by turbidity discrete laminae but rather are dispersed currents are common in these cores (partic­ throughout the cores; the volcanics comprise ularly cores I 1262, 11267, and l 1276). The between 6 to 98% of some layers and are coarse material at the bases of graded se­ ubiquitous on both ridge flanks. This indi­ quences consists primarily of large forami­ cates that burrowing by bottom organisms niferal tests with rounded morphologic form, (see Fig. 8) has been extensive and/or re­ particularly Orbulina universa. G/obigeri­ working and redistribution by bottom and noides conglobatus, and Sphaeroidinella de­ gravity currents has occurred subsequent to hiscens (minor). Beds with these rounded the initial deposition of the sediments. Both forms are almost always immediately over­ processes probably were operative in con­ lain by beds characterized by abundant spec­ structing the observed distribution patterns. imens of Globorotalia menardii-tumida. This Zeolites (clinoptilite) are also present in most phenomenon probably can be attributed to a of the fine-grained, clay-rich sediments and more rapid settling velocity for the forami­ most likely represent volcanic glass that was niferal tests with rounded form than for the devitrified on the seafloor. flatter tests of G. menardii-tumida. That part of the sand fraction with conti- BARBADOSRIDGE SEDIMENTATION 535

A 0,5mm B

C D

Fm. 7 .-Thin-section photomicrographs of sands showing: A) Shallow-water carbonate grains. Chambers of the foraminiferAmphistegina sp . are partially infilled with micrite. Opaque masses are micritized algal fragments.Plane polarized light, Core 11264-240 cm. B) Assemblage of pumaceous fragments and augite or piagioclase. Plane polarized light, Core 13753-190 cm. C) Mixture ofvolcanogenic sands and planktic foraminifers. Plane polarized light, Core 11267-50 cm. D) Sand assemblage with continental affinities. Typical suite consists of granite gneiss Ii tho fragments, rounded and weathered orthoclase, and metaquartz fragments. Crossed nicols, Core 11273-220 cm. 536 WILLIAM J. CLEARY, H. ALLEN CURRAN, AND PAULA. THAYER

Minor sand constituents include recrystal­ lized radiolarians (0-8%) and small amounts of brown-to-red, opaque masses. The former are solely restricted to the northeast-flank depressions, while the latter appear to be more widespread. Presumably, these opaque masses are manganiferous-rich pebbles or represent highly altered pyroclastic detritus.

PROVENANCE In general, the provenance determination for deep-sea sands is hampered when one F1G. 8.-Bottom photograph from the Barbados Ridge, showing a muddy bottom with numerous mounds, tracks, must rely on sparse mineralogical data. trails, and irregular markings. These features indicate Provenance considerations also are difficult that the upper surface is well bioturbated. This photo, when dealing with sand assemblages whose taken in one of the northeast flank depressions, shows constituent fractions are derived from mul­ no evidence of current activity. tiple sources often widely spaced but with similar lithologic characteristics. Of impor­ tance to this study are data from the few iso­ nental affinities(Fig. 7D) constitutes the third lated dredge samples on the ridge (Hurley major assemblage; this component most 1966) and data from an insular shelf study probably was derived from the island itself (Macintyre 1970) that provide some infor­ or from northeast-flank subsea outcrops. mation on the nature of shallow-water sands Monocrystalline quartz (0-20%), which is that are potential contributors to the sedi­ typically highly strained, is forthe most part ment of downslope environments. restricted to sand layers on the northeast flank. Macintyre (1970) recognized three distinct Polycrystalline quartz subtypes (0-22%; sedimentary facies bordering the west coast stretched, recrystallized vein) are foundonly of Barbados, consisting of I) an insular shelf in the coarsest sand layers. Gneissic frag­ facies (0-70 ft); 2) an upper slope facies(70- ments (0-17%) also can be common constit­ 400 ft); and 3) a slope facies (400-1,300 ft). uents in these same sand layers (Fig. 7D). The sands are typically fine- to medium­ Feldspars, other than fresh, angular plagio­ grained and are dominated by carbonate clase varieties, are generally subrounded to components, consisting of corals, algae, ben­ subangular, slightly altered orthoclase and thic foraminifers,and mollusks, the last being microcline (0-12%). The close association in most prevalent in the slope region. The pro­ terms of frequency of occurrence and abun­ portion of relict (fossil) and residual sands dance of these constituents on the northeast (lithic fragments) increases seaward, and in flank suggests a common upslope source. the slope facies,fossil material comprises be­ Carbonate particles indicative of an insu­ tween 73 to 95% of the sand fraction. Non­ lar-shelf origin include thick-walled benthic carbonate material is usually 10% or more foraminers (0-5%, Fig. 7A), calcareous algae and is composed of plagioclase, glass, shards, (0-25%) commonly highly micritized, and and minor quartz (Macintyre 1970). bored molluscan debris (0-5%), also diage­ The petrographic nature of sand assem­ netically altered. Other skeletal allochems blages on the Barbados Ridge is typical of consist of barnacles, corals, and bryozoan what one might expect in open ocean sub­ fragments which usually occur in minor duction zones (Stewart 1978; Valloni and amounts (less than 2%). Maynard 1981; Maynard et al. 1982). The Nonskeletal allochems are principally car­ mineralogically immature nature of the vol­ bonate lithoclasts (0-20%) and unidentifiable canogenic suite of sands has attached its own masses of micrite and neomprphic spar (0- provenance signature. The abundance of an­ 8%). Lithic fragments are principally fora­ gular, fresh, zoned plagioclase, unaltered glass miniferal biomicrite, coral-algal biomicrites, shards, and both pumaceous and tuffaceous and arenaceous molluscan biomicrites. fragments suggests a close affinityto the vol- BARBADOS RIDGE SEDIMENTATION 537 canic islands to the west. Preliminary heavy identified by the continental suite of detrital mineral analyses support this contention. Re­ grains. The second source is the volcanic An­ cent ash falls (1812 and 1902) related to vol­ tillean Arc to the west, which contributes the canic activity on St. Vincent account for the immature, volcanogenic, plagioclase-en­ presence of pyroclastic debris found in shelf riched sands. and slope sands. Samples from both the ash Ash fallssimilar to those described by Teal fall and shelf pyroclastics have similar char­ (1902), Diller and Steiger (1902), and Sig­ acteristics (Macintyre 1970). Although the urdsson et al. (I 980) were responsible forthe exact source of the volcanogenic sands found widespread distribution and deposition of the in the cores cannot be determined, it seems pyroclastic sand assemblages. Their initial most likely that Mt. Soufriere on the island deposition resulted in the draping of a series of St. Vincent or another volcanic center in of thin-sediment layers over the existing to­ the Lesser Antilles chain has been the source. pography identical to normal pelagic settling. The presence of mixtures ofrelatively fresh, Subsequent winnowing by bottom currents shallow-water carbonate constituents, worn transported much of the fine material, re­ carbonate shells, and shallow-water lithic sulting in redeposition in other areas. In ad­ fragments (coral-algal biomicrites) indicates dition, mass wasting along topographic highs the immediate source area for these sands may rework some of these discrete layers and is the insular shelfand slope surroundingBar­ transport the pelagic grains (foraminifersand bados. This is to be expected, considering the pteropods) together with the volcanogenic close proximity of the island. sediment into the bathymetric depressions Sand grains with continental affinities, such that occur along the ridge. A similar sequence as granite-gneiss, orthoclase, metaquartz, and of events has been suggested as the cause for strained monocrystalline quartz, suggest a the emplacement of graded, foraminifer-rich derivation from the Tertiary rocks that com­ calcilutite turbidites in lower slope basins now prise the core ofBarbados. The Early Tertiary exposed on Nias Island, Indonesia (Moore et Scotland Group, in particular, is comprised al. I 980). of folded, interbedded turbidite sequences. Mass wasting on Barbados Ridge is prob­ Seismic profiles across the Barbados Ridge ably similar to that which occurs subaerially north of Barbados reveal a thick sequence of on Barbados within the Scotland District (Fig. highly contorted rocks (Westbrook 1982). 1 ). Here mass wasting is an important process Limited dredging in this area recovered in modification of the landscape. The Scot­ flyschlike rocks similar to those that com­ land District, more completely described ear­ prise the St. Andrews Formation on Barba­ lier, is partly composed of folded, interbed­ dos (Hurley 1966). Submarine erosion or mass ded, poorly consolidated silts and clays that wasting of subsea outcrops within canyons are highly plastic. By their very nature these and slope gullies may account for the pres­ rocks are prone to slope failure, which leads ence of these "continental" sands. The pres­ to the formation of amphitheater-shaped ence of recrystallized radiolarian frustules scarps and hummocky topography where (I 0%) suggests the Tertiary outcrops of the flowscome to rest. One such flow, known as Scotland Group are the immediate source theBascobel slip, was induced by heavy rains rocks. in 190 I and removed one square mile of ma­ terial, ultimately leading to massive soil de­ SAND DISPERSAL AND SEDIMENTARY PROCESSES struction (Price 1962). Events such as these may have been quite common along the ridge The recognition of source areas for sands during the Pleistocene sea-level oscillations. found on the Barbados Ridge provides a It seems plausible then to assume that mass good basis for interpreting both the dispersal wasting such as this has occurred on the upper history of these sediments and the deposi­ slope offBarbados and in deeper-water en­ tional processes. We can recognize two source vironments where subsea outcrops of Ter­ areas based on detrital sand-grain composi­ tiary rocks are reported to exist. At depths of tion within this open-ocean environment 2,000 m on the insular slope offthe northeast where pelagic contributions are common­ coast ofBarbados, a number of steep escarp­ place. The first of these is theBarbados Ridge ments occur (J. Newton, pers. comm.; and 538 WILLIAM J. CLEARY, H. ALLEN CURRAN, AND PAUL A. THAYER

RIV Eastward cruises, 1969-70). These have topographic highs where fines are winnowed been interpreted to be slump features with and gravity sedimentation is initiated. De­ arcuate forms, and all have precipitous slopes, position resulting from these downslope relief up to 130 m, and talus piles at their movements ultimately produces the flatfloors bases. These formsare similar to document­ of the small sediment ponds. ed features related to slumping in a variety 5) The ubiquitous occurrence of fresh vol­ of other submarine tectonic settings (Moore canogenic sediments throughout the cores 1977; Dingle 1977; Embley and Jacobi 1977; suggests that (a) bottom currents have been and Embley 1982). an important process in redistributing these Inspection by the authors of echograms sediments since their initial deposition, and (RIV Eastward cruises (1969-70) shows that (b) burrowing by bottom organisms is im­ similar features abound along the northeast portant in mixing volcanogenic sediments flank. Coring at the base of the steep slopes throughout the upper sediment column. (Fig. 5) or in intervening depressions that sep­ arate the numerous topographic highs has re­ ACKNOWLEDGMENTS vealed sections of graded bioclastic and de­ trital turbidites (Fig. 5; e.g., 11267, 90-130 We wish to express our appreciation to the cm). The emplacement of these beds results Duke University Oceanography Program for in the infillingof the depressions and leveling use of the R/V Eastward. The project was of the floors of those basins closest to the originally initiated by 0. H. Pilkey Jr., Duke island. University, from whom the cores were ob­ tained. CONCLUSIONS Cooperation and suggestions during var­ ious phases of the study from J. G. Newton, This study, while descriptive in nature, I. G. Macintyre, and 0. H. Pilkey Jr. are shows that sediments of the Barbados Ridge gratefullyacknowledged. Numerous students are diverse in terms of provenance, dispersal, at the University of North Carolina-Wil­ and mode of emplacement. The nature of mington and Smith College assisted during depositional processes active withinthis geo­ various phases of the study, including D. A. logic setting can be deduced from sediment Mitchell (Smith), H. Whitlley, J. Hostettler, texture, compositional analyses of sand con­ J. Burnette, G. Pyles, and S. Smith (all stituents, and the nature of the microfaunal UNC-W). We thank M. E. Field, U.S. Geo­ assemblages. logical Survey, G. F. Moore, Cities Service The followingconclusions are drawn from Company, and M. B. Underwood, University analyses of the observed sedimentary pat­ of Missouri, forreviewing the manuscript and terns: providing helpful criticism. I) The distribution and abundance of lu­ tites and biogenic oozes in the upper IO m of the sediment column indicate that pelagic REFERENCES settling has been the dominant sedimentation BAADSGAARD, P.H., 1960, Barbados, W. I.: Exploration process operativeon the Barbados Ridge from results 1950-1958: Copenhagen, 21st Internat. Geol. Cong. Rept., pt. 18, p. 21-27. Late Pleistocene through Holocene time. BAILEY, E. H., AND STEVENS, R. F., 1960, Selective stain­ 2) Carbonate sands of insular shelf origin ing of K-feldspar and plagioclase on rock slabs and and rework,d terrigenous material found in thin sections: Am. 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