42. A SUMMARY OF THE SEDIMENTOLOGY OF DEEP SEA DRILLING PROJECT LEG 67 SITES:THE MIDDLE AMERICA TRENCH AND SLOPE OFF GUATEMALA—

AN ACTIVE MARGIN TRANSECT1

William T. Coulbourn, Deep Sea Drilling Project, Scripps Institute of Oceanography,University of California, San Diego, La Jolla, California

Reinhard Hesse, Department of Geological Sciences, McGill University, 3450 University St., Montreal,PQ Canada H3A 2A7, and Technische Universitàt, Lichtenbergstrasse 4, 8046 München, West GermanyJacques Azema, Université Pierre et Marie Curie, Laboratoire de Géologie Structurale, 4 Place Jussieu,

Paris 75230, Franceand

Tsunemasa Shiki, Department of Geology and Mineralogy, Kyoto University, Kyoto, Japan

ABSTRACT

The transect of drill sites across the Middle America Trench offshore Guatemala contains portions of the sedi-mentary record from the Early Cretaceous to the Recent. Hemipelagic muds cover both slopes of the Trench; the thickaccumulation on the Cocos Plate indicates transport of these sediments hundreds of kilometers seaward of their land-ward-slope source area. Beneath those muds, the sedimentary section of the Cocos Plate is a classic, basalt-chalk-abys-sal clay sequence that records the passage of Site 495 from the East Pacific Rise to depths greater than the calcite com-pensation depth.

The eight holes drilled at Sites 499 and 500 contain portions of the sequence cored at Site 495, only the abyssal redbrown clays of Site 495 are completely missing and a deposit of turbidites caps the hemipelagic muds originally depos-ited seaward of the Trench. A horst is buried beneath the turbidites ponded in the Trench axis. Facies changes are ab-sent within the turbidites, and biogenic components indicate transport from upslope.

The sediments, sedimentary features, and microfossils cored from the landward slope generally indicate depositionbeneath nutrient-rich equatorial waters and subsequent downslope displacement. Tranquil accumulation was inter-rupted by deposition of pebbly conglomerates at Sites 496 and 497. Sigmoidal veinlets are common and probablyformed as the slope sediments dewatered. Numerous gas voids and greatly expanded core sections characterize thesesamples. Gas hydrates occurred in the deeper sandy layers at both sites. Beneath the blanket of hemipelagic mud at Site494, deposits older than the Miocene have a varied lithology and an age sequence reminiscent of deposits outcroppingon the Nicoya Peninsula of Costa Rica. The differences between the lithologic section at Site 498 (drilled just to the eastof Site 494) and the section at Site 494 indicate that the structure of the lower slope is more complicated than suggestedby interpretive models derived from geophysical data. The lithology at both Sites 494 and 498 puts severe restrictions onthe volume that an accretionary prism can occupy offshore Guatemala, the type locality for the Trench-Slope Model.

INTRODUCTION

Modern active continental margins are among thegeologically least known areas on the globe. Unlike pas-sive continental margins they have not been a prime tar-get for hydrocarbon exploration, because the structuralcomplexities of these zones have evaded successful reso-lution by seismic reflection profiling and because of lowhydrocarbon potential (perhaps a result of depressedgeothermal gradients). Great water depth and irregularbathymetry complicate conventional sampling methods.Most theories regarding tectonic processes at convergentmargins, therefore, are speculative and are based ongeophysical surveys and widely scattered core samples.

One popular hypothesis is that of Seely, Vail, andWalton (1974), the "Trench-Slope Model," which de-picts convergent margins as the product of progressiveoffscraping of sediment and rock from the surface ofthe subducting oceanic lithospheric plate (Fig. 1). The

Aubouin, J., von Huene, R., et al., Init. Repts. DSDP, 67: Washington (U.S. Govt.Printing Office).

concept is important to our story because it is basedprimarily on Exxon seismic reflection data and drillingresults from a well on the Guatemalan margin. Accord-ing to this interpretation, the Guatemalan margin re-sembles most other convergent margins in that it is an"accretionary prism" (Karig and Sharman, 1975). Theconcept explains the sequence of landward-dipping re-flectors observed on multichannel profiles (Ladd et al.,this volume) and the geometry of fore-arc basins com-mon to convergent margin settings (Seely, 1979).

Geological and geophysical results, however, do notlend universal support to this concept. For example,computations of sediment budget and convergence rates,and seismic refraction surveys of sections of the Peru-Chile subduction zone fail to support the Trench-SlopeModel (Katz, 1971; Scholl et al., 1977; Hussong et al.,1976; Shepherd, 1979). The first results from IPODconvergent margin drilling off Japan and the Marianashave further complicated the picture. The results fromLegs 56 and 60 were surprising in that off scraped sedi-ment and rock were not found in the fore-arc setting,greatly reducing the volume that a hypothetical accre-tionary prism might occupy (von Huene et al., 1980;

759

W. T. COULBOURN, R. HESSE, J. AZEMA, T. SHIKI

Seaward slopeof the Trench

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Figure 1. Trench-Slope Model. (The drawing and terminology are adapted from Seely et al. [1974] and Dickinson and Seely [1979].)

Hussong et al., 1982). A summary of lithologic sequenc-es illustrates some of the drilling results from similartectonic settings (Coulbourn, Introduction, this volume).Where pelagic sediment has been sampled, at Sites 127(in the Hellenic Trench) and 460 (on the landward slopeof the Mariana Trench), its provenance is unclear. Like-wise, sands drilled from the landward slope of the Nan-kai Trough at Site 298 and from the landward slope ofthe Middle America Trench at Leg 66 sites were perhapsraised from the Trench axis, as Karig et al. (1975) and asMoore et al. (1979) propose, but perhaps were simplyponded in catchments on the slope and never reachedthe Trench axis. Drilling has not yet achieved the goal ofpenetration of a complete stratigraphic section throughto the subducting oceanic lithospheric plate.

Earthquake foci define the Benioff zone and the ac-tive continental margin of Central America (Minsterand Jordan, 1978; Carr et al., 1974) where the descend-ing Cocos Lithospheric Plate forms the Middle AmericaTrench and is associated with active volcanism (Fig. 2).How do sediment and rock caught between the volcanicarc and the subducting plate respond to plate conver-gence? Their distribution within the continental marginmay restrict some of the hypotheses describing con-vergent margin tectonics. The landward dipping reflec-tors seen on multichannel seismic profiles are not inthemselves proof of accretion (Fig. 3). Thick sections ofdeformed hemipelagic sediment, the absence of pelagicsediment, and the presence of microfauna reworkedfrom upslope would suggest that a variety of "tectonicerosion," perhaps as advocated by Hussong et al. (1976)

and Scholl et al. (1977), could be applied to the Guate-malan margin. Repeated lithologic and age sequencescould document either imbricate thrusting or slumping.Recovery of pelagic sediment in slope sediments, how-ever, would imply the transfer and incorporation ofthose sequences into a deforming, accreting prism.

The Guatemalan continental margin is a suitable placeto pursue these questions. Precruise site surveys locateda pronounced sequence of landward-dipping seismic re-flectors thought to represent a stack of imbricate rockand sediment (Ladd et al., 1978; Ibrahim et al., 1979;and Fig. 3). We expected the incoming foraminifer-nan-nofossil chalk riding on the Cocos Plate to contrast withthe diatomaceous hemipelagic muds deposited beneaththe zone of coastal up welling. We anticipated that thediversity of tropical microfossil assemblages should af-ford easy resolution of stratigraphic reversals. And theopportunity to drill offered us once again the chance ofpenetrating to the subducting oceanic slab.

During Leg 67, 15 holes were drilled at 7 sites locatedin three tectonic environments: Holes 496, 497, 498,498A, 494, and 494A (in order of increasing water depth)on the continental slope, Holes 499, 499A, 499B, 499C,499D, 500, 500A, and 500B on the floor of the MiddleAmerica Trench, and Hole 495 located on the CocosPlate (see Fig. 4).

GEOLOGIC SETTING

The Middle America Trench is a major feature of theeastern Pacific ocean basin. Having a length in excess of2500 km, a width of about 5 to 15 km, and in places

760

SUMMARY OF SEDIMENTOLOGY

25°N

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Figure 2. Major tectonic features of Central America and the Middle America subduction zone.

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Figure 3. Summary of seismic-reflection and -refraction data off Guatemala, and location of Leg 67 drill sites(after Ladd et al., this volume). (The profile shown is a depth section from multichannel seismic lineGUA-13. Numbers refer to refraction velocities, and shaded patterns represent the possible location ofoceanic crust. Vertical exaggeration (VE) is ×2.5.)

761

W. T. COULBOURN, R. HESSE, J. AZEMA, T. SHIKI

91°00'14° 00' 90° 00'

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Middle AmericaTrench Axis

Figure 4. Location of Leg 67 drill sites with respect to the bathymetry of the Guatemalan continentalmargin.

reaching water depths as great as 6000 meters, it is acomposite of two geologic provinces (Aubouin, Azéma,et al., this volume). Along its northern segment theTrench borders southern Mexico, the southernmost partof the North American continent, whereas along itssouthern part it borders the Central America isthmus, avolcanic chain. As Coats (1962) illustrated in his generaldiagram relating volcanism and subduction, the vol-canism in Central America is probably linked to the sub-duction of the Cocos Plate.

The Middle America Trench is bordered to the northby the Tamayo Transform Fault and to the south by theaseismic Cocos Ridge (Fig. 2). The Tehuantepec Ridgebisects the Trench, and although it is a major feature ofthe Cocos Plate, it has no obvious expression in thestructural trends of southern Mexico. The North Amer-ican continent probably extends as far south as thePolochic and Montagua fault zones in Guatemala andperhaps even as far as the Honduran platform. The por-

tion of the Trench to the north of the TehuantepecRidge cuts the structural trends of Mexico obliquely andborders a very narrow continental shelf. In contrast, thesouthern portion of the Trench parallels the Mesozoicstructural trends of Central America and borders a verywide continental shelf, perhaps formed by the prolonga-tion towards the northwest of formations like those ofthe Nicoya Complex, exposed on the Santa Elena Penin-sula of Costa Rica. Like the Tehuantepec Ridge, thePolochic-Montagua Fault Zone of Guatemala is trun-cated by the Middle America Trench and has no obviousextension onto the adjacent lithospheric plate.

The contrast between the northern and southern seg-ments could result from the westward movement ofboth the North and South American lithospheric platesrelative to the Caribbean Plate. The extent of the west-ern edge of the Caribbean Plate is perhaps related to thiswide section of continental margin. Whatever the cause,the difference between the northern and southern parts

762

SUMMARY OF SEDIMENTOLOGY

of the Middle America Trench has been confirmed bydrilling transects of Legs 66 and 67 (Aubouin et al.,1979).

MINERALOGIC COMPOSITION ANDLITHOFACIES

During DSDP Leg 67, holes were drilled through thepelagic section of the Cocos Plate at Site 495, throughthe trench-fill sediments to underlying basalts at Sites499 and 500, into the lower Trench slope at Sites 494and 498, and into the middle slope at Sites 496 and 497.The stratigraphy of these drill sites is summarized inFigure 5.

Pelagic and Hemipelagic Sediments of the CocosPlate (Site 495)

Site 495 is located on a horst 22 km seaward of theMiddle America Trench axis at 4140 meters water depthand 1915 meters above the Trench floor (Fig. 6). Thehorst trends 130°N to 140°E and intersects the strike ofthe Trench axis at an angle of 15° to 25°. Similarly,mapping of fault trends for portions of the Nazca Platerevealed oblique intersection of those trends with seg-ments of the Peru-Chile Trench axis (Prince et al., 1974;Coulbourn and Moberly, 1977; Coulbourn, 1981). TheLeg 56-57 and the Leg 67 study areas are yet other ex-amples of this geometry (Honza, 1980; Aubouin, Ste-phan, et al., this volume; Coulbourn, Stratigraphy andStructure of the Middle America Trench, this volume).

Site 495 provides a complete record of pelagic faciessuccessions expected in the eastern equatorial PacificOcean. Lithology at this site is based on sharp lithologicchanges from basalt, to carbonates, to red brown clays,and finally to olive green hemipelagic muds.

Basalts occupy the sub-bottom interval from 428 to447 meters (Cores 495-46 to -49) and were formed dur-ing a time span of 7 to 8 m.y. at a location near the pa-leomagnetic equator (Gose, this volume).

A basal sedimentary sequence of 228 meters of lowerMiocene pelagic carbonates was rapidly deposited (50m/m.y.), indicating an origin beneath the equatorialzone of high productivity. The first core of basalt con-tained two pieces of very firm, lithified, manganese-stained limestone that included fragments of volcanicglass. The basal 22.5 meters of this sequence owe theirbrown and purplish stain to manganese and iron oxideemanations from ridge-crest hydrothermal activity. Theferromanganese in these chalks often forms dendrites.Lithification and cementation decreases upward; thenumber of chalk layers decreases and the number of for-aminiferal and nannofossil ooze layers per core in-creases. Siliceous microfossils, radiolarians, diatoms,and sponge spicules are absent in the lower part butcommon above Core 495-37. This contrast coincides withthe first appearance of nodules and thin (less than 5 cmthick) stringers of chert or porcellanite at the top ofCore 38 (352 m sub-bottom). At this level a markeddownward decrease occurs in the dissolved silica in thepore fluids, corresponding to its reprecipitation as por-cellanite. These calcareous oozes and chalks are biotur-

bated except at certain levels in Cores 495-34, -36, and-38, where distinct primary laminations are preserved.Those laminations attest to the temporary absence of abottom-dwelling infauna. Accompanying a decrease insedimentation rates to 16 m/m.y. near the top of thecarbonate unit, colors change from pale green, greenishgray, or very pale bluish green to very pale brown, pink-ish white, or light brownish gray in the upper part of thelower Miocene and in the middle Miocene, signalling afurther decrease in sedimentation rate as the minimumof 9 m/m.y. for the deposition of brown clay is ap-proached. An upward increase in radiolarian contentand concomitant loss of foraminiferal biostratigraphicresolution in the carbonate section is due to increaseddissolution and corresponds to the subsidence predictedfor a lithospheric plate as it moves away from a spread-ing center (Sclater et al., 1971).

Other components of the clay-size fraction of the pe-lagic carbonate invariably include mixed-layer smec-tite—illite and traces of illite (Heinemann and Fücht-bauer, this volume; Latouche and Maillet, this volume).Zeolites, feldspar, quartz, and volcanic glass occur inthe silt-size fraction. The number of volcanic glass lay-ers reaches one of three maxima near the boundary be-tween the lower and middle Miocene (Cadet et al., thisvolume). The boundary between the chalk and overlyingclay units occurs in Core 495-19 and is marked by colorand compositional changes (Fig. 7), a drop in densityand shear strength, an increase in water content, and achange from over- to undercompacted sediment (Faas,Gravitational Compaction Patterns chapter, this vol-ume). A small reverse fault, of less than 1 cm displace-ment, occurs in Section 495-30-1.

Seven meters of brown abyssal clay cap the pelagiccarbonates. According to radiolarian-based age deter-minations, the ridge flank subsided beneath the calcitecompensation depth (CCD) in the middle Miocene. Agedata from poorly preserved calcareous nannofossils andforaminifers also date Core 19 as middle Miocene. Min-eralogically, sediments of this core contain the sameclays as the carbonate unit, but with the addition ofkaolinite, abundant volcanic glass, only traces of feld-spar and quartz, and complete absence of zeolites (Heine-mann and Füchtbauer, this volume). The equivalent pres-ent-day area of brown abyssal clay sedimentation is northof the tropical carbonate belt at about 10°N to 15°Nlatitude.

A cover of 171 meters of diatomaceous, green andolive gray, hemipelagic mud completes the section. Theappearance of these kaolinite, illite, and mixed-layerclay-bearing muds announces the arrival of density flows,suspension clouds, or nepheloid layers at Site 495. Thesesediments arrived at Site 495 in the middle Miocene andpredate an increased abundance of volcanic glass in thelate Miocene (Cadet et al., this volume; Heinnemannand Füchtbauer, this volume). That volcanic glass pre-sumably marks the onset of volcanism in Guatemalaand Nicaragua. By the early Pliocene, coarse silt-sizegrains were accumulating at Site 495 (Shiki, et al., thisvolume). In some cores, for example, Core 495-4, theterrigenous sediment fraction, composed of feldspars,

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SUMMARY OF SEDIMENTOLOGY

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Figure 7. Lithology of Core 495-19. (Hemipelagic clays overlie abyssal brown clays, which in turn overlie pelagic carbonates.)

765

W. T. COULBOURN, R. HESSE, J. AZEMA, T. SHIKI

volcanic glass, detrital clay minerals, and quartz, pre-dominates. The frequency of occurrence of ash layersreaches a maximum of 15 m/m.y. in the Pleistocene sed-iments (Cadet et al., this volume). These new compo-nents boost the sedimentation rate from 9 m/m.y. inCore 19 to about 41 m/m.y. in the Pliocene and Quater-nary cores from Site 495. Another small reverse fault,like its counterpart in Section 495-30-1, occurs in Sec-tion 495-17-6, and its origin is also attributed to soft-sediment deformation.

The biogenic fraction of the uppermost lithologicunit contains abundant diatoms, radiolarians, spongespicules, and lesser amounts of calcareous nannoplank-ton and foraminifers. The upper Miocene to Pliocenesediments in Cores 14 to 18 also contain cold water dia-toms diagnostic of neritic upwelling zones (Jousé et al.,this volume). The appearance of lower bathyal, calcare-ous benthic foraminifers suggests a slight shallowing ofthe sea floor as the site approached the outer swell of theTrench. Coincident with that relatively exposed posi-tion, radiolarian stratigraphy indicates that the upper-most two radiolarian zones are missing, probably be-cause of erosion (Westberg and Riedel, this volume).

Turbidites, Pelagic and Hemipelagic Sediments of theMiddle America Trench Axis (Sites 499 and 500)

The network of eight closely spaced holes drilled atSites 499 and 500 provides a unique data base for thesedimentary sequence in the Trench. We are able tostate that the surface of the Cocos Plate is offset by nor-mal faults as it enters the Trench and that the sedi-mentary section it carries into the subduction zone ispassively buried beneath a wedge of axial turbidites(Aubouin, Stephan, et al. this volume; Coulbourn, Stra-tigraphy and Structure, this volume). Compressive struc-tural features are absent, even at the foot of the conti-nental margin. Also absent are veinlets, scaly fractures,and other fabric that might account for significant com-pression registered on a small scale.

The lithologies at Site 499 match those at Site 495,our reference site for pelagic sedimentation, except forthe addition of a cover of trench-fill deposits, specifical-ly, interbedded turbidites and hemipelagic muds. As atSite 495, lithology at Sites 499 and 500 shows abruptchanges. The succession of pelagic sediment facies over-lying rubbly basalt includes calcareous chalk and oozeoverlain by burrow-mottled hemipelagic muds. The cal-careous section at Site 499 is only one-third the thick-ness of that at Site 495, and solution of foraminifers ismore pronounced than at Site 495, indicating depositionnearer the CCD. Pyrite concentrations and the odor ofH2S, even in the chalky sections, indicates that reducingconditions prevail. The contact between nannofossilooze and hemipelagic mud occurs in Core 499-23 (Fig.8). No red brown clays comparable to those of Core495-19 were recovered.

The turbidites are undercompacted (Faas, Gravita-tional Compaction Patterns, this volume) and includenannofossil muds, laminated sandy muds, muddy sands,and medium to coarse sands. Though drilling distur-

499 499A

50

100

150

200

250

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Figure 8. Lithology of Core 499-23. (Sharp contact between hemipe-lagic clays and pelagic carbonates. The red brown clays of Core495-19 are missing in the Trench axis lithologic sections. Refer toFig. 7 for an explanation of symbols.)

bance often obscured sedimentary structures within spe-cific turbidite beds, the systematic change of grain-sizedistributions in each bed of the trench-fill deposits iscomparable to that of well-preserved turbidites piston-cored from the Okinawa Trough (Shiki, et al., this vol-ume) and from the Japan Trench (Hesse, 1977). Biogen-ic components, including wood fragments, diatoms, andbenthic foraminifers, indicate rapid transport from up-slope. Diatom assemblages include species typical ofbrackish-water habitats (Jousé, et al., this volume). Ben-thic foraminifers indicate progressively shallower sourc-es ranging from lower bathyal in cores deeper than Core499A-12, to upper bathyal in Cores 499-1 to -9, to uppershelf fauna in Cores 499-1 to -7 and Cores 499A-1 to -5(Thompson, this volume). Rapid accumulation (300 m/m.y.), preserving a high concentration of organic mat-ter, is probably responsible for the H2S odor, and thatgas content in turn could generate the undercompactionregistered in the physical properties analyses (Faas, Grav-itational Compaction Patterns, this volume).

The heavy mineral composition of the sand fractioncontains hypersthene, augite, green brown hornblende,and lesser amounts of epidote and olivine. This mixturestrongly resembles that deposited in the Quaternary andPliocene samples from Site 494, located on the lowerslope. The pyroxenes and amphiboles, which together

766

SUMMARY OF SEDIMENTOLOGY

constitute more than 90% of the heavy mineral concen-trates, are present in an average 3:1 ratio (Prasad andHesse, this volume).

Volcanic glass fragments, labradoritic plagioclases,and montmorillonite are the dominant components ofthe silt and clay fractions of Site 499 sediments (Heine-mann and Füchtbauer, this volume). Other constituentsinclude zeolites and quartz. The number of ash layersper million years reaches a maximum in the Pleistocenesediments of Site 499 (Cadet et al., this volume), how-ever, all may not be accounted for, as Heinemann andFüchtbauer (this volume) attribute the abundance ofzeolites to the alteration of volcanic glass depositedwithin the Trench axis turbidites. Kaolinite and illite donot appear before the late Pliocene, whereas at Site 495on the Cocos Plate they occur as early as the middleMiocene. This discrepancy may be a sampling artifact,because no middle Miocene, late Miocene, or early Plio-cene samples were analyzed for the Trench sites.

The lithologies at Site 500 are basically the same as atSite 499, however, basalt cobbles were found at relative-ly high levels within the hemipelagic mud at Holes 500Aand 500B. A contact between Quaternary turbidites fill-ing the Trench axis and underlying middle Miocene for-aminifer-nannofossil ooze in Section 500-10-2 is prob-ably the result of normal faulting; in this process thehemipelagic sediments have been cut out of the strati-graphic sequence (Fig. 9). Cobbles of micritic limestonewere also recovered at high levels at Hole 500A (105 msub-bottom), and benthic foraminifers indicate trans-port of sediment to the Trench axis from the entire depthrange of the continental margin (Thompson, this vol-ume). Two small reverse faults, of less than 0.5-cm dis-placement, occur in Section 500-13-2, and one normalfault offsets the foraminifer-nannofossil chalk of Sec-tion 500-14-1 (Dengo, this volume).

The geometry of the lithologies encountered in oureight Trench axis holes coupled with the results of aSeabeam survey across the Trench axis demonstrate thatthe Cocos Plate is broken by a network of normal faults(Aubouin, Stephan, et al., this volume; Coulbourn,Stratigraphy and Structure, this volume). These trendsare probably a composite of "tectonic fabric" gener-ated at mid-ocean ridge crests (Rona et al., 1976), reacti-vation of those faults in mid-Plate settings (Luyendyk,1970), and, finally, generation of new faults and re-opening of old scarps as the Plate bends downward intothe subduction zone (Coulbourn, Stratigraphy and Struc-ture, this volume). The time range of possible tectonicmovements allows ample chance to disturb an otherwisenormal pelagic sedimentary sequence.

Sediments of the Lowermost Continental Slope(Sites 494 and 498)

Sites 494 and 498 are located on a bench or terrace onthe landward side of the Middle America Trench, about580 meters above the surface of the turbidites ponded inthe Trench axis (Fig. 6). Bathymetry suggests that theterrace may be formed as either a slump block, as ablock fault, or as a result of a large landward-dipping

Section 500-10-2

50

ε IOOo

150

200

Figure 9. Lithology of Core 500-10. (Contact between Quaternaryturbidites filling the Trench axis and underlying foraminifer-nan-nofossil ooze. See Fig. 7 for an explanation of symbols.)

thrust (Aubouin, Stephan, et al., this volume). The se-quence of lithologies recovered does not preclude any ofthese interpretations.

Drilling at Hole 494A ended with long drilling timesand recovery rates of about 3%. Only 1 meter of rockand sediment was recovered in the sub-bottom intervalfrom 321.5 to 366.5 meters (Cores 31-35). The alteredmafic and intermediate composition of rocks from thesecores, however, raises some important questions abouttheir origin. Their feldspar composition indicates an"active margin basalt parentage," but their depletion inthe Rare Earths is typical for oceanic tholeiites; theirorigin therefore remains problematic (see Maury et al.,this volume).

Late Cretaceous medium gray mudstone composesmost of the drill breccia in Core 30 (312.5-321.5 m sub-bottom). Some of these pebbles contain relatively di-verse foraminiferal assemblages, including Globotrun-cana. Benthic foraminifers are sparse, and small plank-tonic taxa are abundant, indicating deposition underopen-marine conditions at depths well above the CCD(Thompson, this volume). Paleomagnetic latitudes aver-

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age 12° for the Cretaceous samples and suggest that thesediment was deposited near its present-day latitude of12.7°N (Gose, this volume).

A mixture of middle Eocene dark gray mudstone andLate Cretaceous blue gray micritic limestone composethe 3 meters of sediment recovered in Cores 28 and 29(294-312.5 m sub-bottom). Radiolarians first appear inCore 494A-29 and indicate that Eocene drill chips are aminor component of this mixture (Westberg and Riedel,this volume). The pattern of mixing of Cretaceous andEocene microfauna could belie a stratigraphic reversal;however, it probably arises from drilling contamination(see Site 494 report core photographs, this volume).

Dark gray middle Eocene mudstone, the likely sourceof the chips in Cores 494A-28 and -29, were recovered inCores 22 to 27 drilled from 240 to 294 meters below theseafloor. Recrystallization and poor preservation of for-aminifers and relatively abundant calcareous nannofos-sils suggest that deposition of this unit occurred abovethe nannofossil lysocline, but below the foraminiferallysocline (Thompson, this volume). Incipient cleavagein Core 494A-25 and healed fractures in Core 494A-27provided us some indication of features lost in the manycores of pure drill breccia (Fig. 10).

An unconformity at a sub-bottom depth of 240.5 me-ters marks the onset of accumulation of a blue gray clayand claystone containing lower Miocene to upper Oli-gocene nannofossils attributed to open-ocean environ-ments rather than to regions of coastal upwelling (Cores20-22, 223-240 m sub-bottom). Dengo (this volume)and Faas (Gravitational Compaction Patterns, this vol-ume) discuss the plasticity and remarkable deforma-tional features of these clays.

An unconformity at 223 meters sub-bottom is thebase of a blanket of dark gray diatomaceous mud (Cores494A-1 to -20). Only sediment of the uppermost 50 me-ters is normally compacted; the interval from 50 to 233

Figure 10. Structure of Section 494A-27-1. (Closely spaced faults off-set light gray sandy mudstone [stippled].)

meters sub-bottom is overcompacted, and is marked byabrupt increases in density and shear strength and a de-crease in water content. Overcompacted sediment atshallow depth can occur by removal of sediment over-burden, dessication, or cementation and recrystalliza-tion (Faas, Gravitational Compaction Pattern, this vol-ume). The number of ash layers per million years reach-es a maximum near the Pliocene/Pleistocene boundaryat Cores 494A-16 to -17 (Cadet et al., this volume). Dur-ing the Pliocene, diatoms representative of tropical as-semblages accumulated at Site 494, while cold waterforms, typical of coastal upwelling regions, were accu-mulating seaward at Site 495. Benthic foraminifers inthis uppermost unit are typical of assemblages normallycored from a water depth of 1000 meters.

Site 498 was drilled in the hope of improving the per-centage recovery achieved at Site 494 and in the hope ofextending that section to deeper levels. Neither of theseaspirations was realized. The uppermost units of olivegray, diatomaceous mud represent a drape of hemipe-lagic sediment, 213 meters thick, reworked from up-slope. As in the uppermost unit at Site 494, there is noevidence that these are turbidites.

Beneath that depth, a heterogeneous sequence of fa-des documents the complexity of the landward slope ofthe Middle America Trench. A comparison of the sam-ples recovered at Sites 494 and 498, moreover, docu-ments a lateral variability of structure. The blue graymudstone found at 213 to 321 meters sub-bottom inHole 498A is the equivalent of that occurring at 213 mto 241 meters sub-bottom in Hole 494A. The remainingfour lithologic units found at Hole 494A are not repre-sented at Hole 498A (494A, 249-358 m sub-bottom).Gas hydrates were encountered in volcanic ash and sandat 310 meters sub-bottom, associations that occurred atSite 496 and again at Site 497. Figure 11 illustrates thevigorous effervescing of methane-rich gas that promp-ted us to abandon the hole. Curiously, gas hydrateswere not encountered at Site 494.

Hemipelagic Sediments of Mid-SlopeSites 496 and 497

Sites 496 and 497 are located at water depths of 2062meters and 2358 meters, respectively, seaward of a fore-arc basin in the vicinity of the San José Canyon. Thesediments at the two sites are similar, except that theQuaternary deposits are thicker and the Pliocene sedi-ments thinner at Site 496 than at Site 497. Also, drillingat Site 496 reached lower Miocene sediments, whereas atSite 497 drilling reached only lower Pliocene sedimentsat similar sub-bottom depths. The sediments are olivegray to dark gray, hemipelagic, diatomaceous muds withinterbedded volcanic ash that usually occurs in patchesand stringers deformed by drilling. Our lithologic sub-divisions are based primarily on grain-size changes rath-er than contrasting sediment composition. At both sitesthe lower lithologic unit is differentiated from the upperon the basis of downward increasing sand and pebblecontent.

The lower unit is overcompacted at Site 496 and nor-mally compacted at Site 497 (Faas, Gravitational Com-

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figure 11. Gas hydrates effervescing from Section 498-15-2

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W. T. COULBOURN, R. HESSE, J. AZEMA, T. SHIKI

paction Patterns, this volume). The pebbles occur in in-distinctly graded, intraformational pebbly mud and mud-stone layers at about 280 meters sub-bottom in the mid-dle Miocene sediments of Site 496 and at about 200 me-ters sub-bottom in the upper Pliocene sediments of Site497, and are made up of mud and mudstone intraclasts.The sand grains in the sandy muds include volcanicglass, glauconite, pyrite, numerous fossil remains, feld-spar, and rarely quartz. Sands are perhaps underrep-resented in our cores by preferential loss during drilling.For example, the recovery of the intraformational peb-bly conglomerate in Cores 497-23 and -24 was preceededby 19 meters penetration with no recovery. Gas hydrateswere particularly noticeable in the coarse layers at thebase of both sites but probably occur throughout the en-tire section at both Sites 496 and 497 (Harrison et al.,this volume).

The silts at Site 496 contain generally more than 50%volcanic glass, the remainder consisting of feldspar andsmall amounts of quartz, opaline silica, and zeolites(Heinemann and Füchtbauer, this volume). Comparableresults are not available for Site 497. The number of ashlayers at both Sites 496 and 497 increased sharply fromearly Pliocene to Pleistocene (Cadet et al., this volume).

At both Sites 496 and 497, primary sedimentary struc-tures indicative of depositional mechanisms are lacking.The clay fraction consists predominantly of montmoril-lonite, with some illite and kaolinite occurring in aboutthe same amounts. Chlorite was identified in Core 496-3. In addition, the fine-grained sediments include twothin layers of micrite in Section 497-26-3 and Section497-36-1. Specimens examined in thin section lackedmicrofossils and microfossil traces, and resemble thelight colored, homogeneous, barren dolomicrites dredgedfrom the Peruvian continental margin (Kulm et al.,1981). Microcrystalline dolomitic nodules were alsocored at Sites 434 and 435 on the Japan margin (Okada,1980) and at Sites 490 and 493 off southern Mexico(Wada et al., 1982). Throughout most of the sections atboth sites the populations of benthic foraminifers re-semble assemblages accumulating at comparable depthstoday. Below 308 meters sub-bottom at Site 496, how-ever, the assemblages indicate paleodepths somewhatshallower than their present 2400 meters (Thompson,this volume).

Possible causes for the absence or paucity of sedi-mentary structures are: (1) bioturbation (rarely identi-fied at Sites 496 and 497, in contrast to Site 495), (2)drilling disturbance (most common), (3) slumping (littleevidence at Sites 496 and 497, in contrast to Site 494),(4) occurrence of gas hydrates and associated sedimentdegassing, and (5) tectonic deformation evidenced bythe development of sigmoidal veins (Fig. 12), scaly clays,and cleavage. Sigmoidal veins that are filled with darkclay appear at about 150 meters sub-bottom and repre-sent a semipenetrative deformational structure causedby or, more likely, causing and enhancing sediment de-watering (Cowan, this volume; Dengo, this volume).These deformation features are not unique to the Guate-malan margin; their similarity to those described fromcomparable sub-bottom levels at Japan Trench DSDPsites is striking (Arthur et al., 1980).

Figure 12. Sketches of veinlet patterns in Site 497 samples. A. Section497-17-3. B. Section 497-34-2. C. Section 497-36-1. D. Section497-36-4.

PROVENANCE AND TRANSPORTATION

The stratigraphy of Leg 67 drill sites documents thereworking of sediment from shallow to deeper slope en-vironments, the redeposition of slope sediments over thepelagic sediments of the Cocos Plate, and the conspicu-ous absence of graded beds and current bed forms inmost of these displaced sediments.

Site 495Site 495 is not only 22 km from the Trench but is also

2000 meters above the Trench floor. At the onset ofhemipelagic sedimentation the paleolocation of Site 495was even more distant from the continental margin. As-suming a spreading rate of 9 cm/yr. for the last 10 m.y.,hemipelagic sedimentation began when Site 495 was 900km seaward of its present position. In terms of present-day sedimentation patterns this distance seems exces-sive, if the source of sediment is attributed to coastal up-welling. It does not seem excessive, however, if it isattributed to suspended or diluted density currents cas-cading over the slope, currents that could spread outlike a cloud at shallow or intermediate water depths(e.g., Drake and Gorsline, 1973) and extend far enoughseaward to deposit this blanket of mud (Shiki et al., thisvolume; Heinemann and Füchtbauer, this volume). Asimilar deposit exists east of Japan where a wedge ofhemipelagic and volcanogenic silty sediment extends

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seaward at least 800 km into the West Pacific (Hesse etal., 1974). Whatever the cause may be, the result has thepotential of complicating our interpretation of the sedi-mentary section at other Leg 67 sites, because the hemi-pelagic section at Site 495 would be virtually indistin-guishable from the slope sediments were it mixed tec-tonically with the hemipelagic drape covering the con-tinental margin.

Sites 499 and 500

The sharp contact between nannofossil-foraminiferooze and hemipelagic mud in Core 499-23 contrasts withthe sequence shown in Core 495-19 (compare Figs. 7 and8). The absence of red brown clays in the Trench axisholes suggests that nepheloid-layer transport affectedthe paleolocation of Site 499 at about the same time thesite subsided beneath the CCD or that bottom currentshad sufficient velocity to erode any red brown clay thathad been deposited.

The trench-fill seaward of San José, Guatemala, in-cludes turbidites, hemipelagic sediments, and eupelagicdeposits. Except for the pebbly mudstones at Sites 496and 497, these are the only drill sites on the Guatemalantransect that have provided clear evidence for the occur-rence of sedimentary mass flow deposits such as turbid-ites. These turbidites contain

1) Components derived from the Neogene volcanicprovince of Guatemala.

2) Diatoms and benthic foraminifers reworked fromnearshore environments.

3) Plant fragments.These constituents indicate supply from land and possi-ble temporary redeposition on shallower parts of themargin. The unique occurrence of some of these compo-nents in turbidites of the Trench floor and the apparentabsence of turbidites on the slope, moreover, suggesttransportation by "bypassing" through canyons. SanJosé Canyon, situated close to Sites 499 and 500, is anappropriate conduit for "bypassing." The pebbly mud-stone layers encountered at Sites 496 and 497, on theother hand, may indicate spillover of particularly volu-minous and vigorous massflows from the nearby can-yon to the open slope.

Exotic pebbles were recovered at Sites 499, 500, and494. The lithologic assemblage includes: peridotites, al-tered basalts, micritic calcareous rocks, and a quartzdiorite pebble. With the exception of the quartz dioritepebble, the assemblage resembles rocks recovered dur-ing other oceanographic cruises to the region, suggest-ing a source in outcrops that may have been exposedonce before, as the Nicoya and Santa Elena peninsulasto the south are exposed today.

The quartz diorite pebble was found in Pleistocenesediment (Section 494-16-1), and its occurrence mayhave a different significance. This rock is poor in potashfeldspar, and iron is disseminated throughout. It is, infact, richer in iron than the rock name suggests. Frac-tures are well-developed, and the quartz and feldspargrains have wavy or mosaic extinction. Some of thedark minerals, mainly hornblende, also have mosaic ex-tinction characteristic of protoclastic texture. Similar

acidic rocks have not yet been observed in the NicoyaPeninsula, however, our sample indicates that at somepast time these rocks outcropped along the Guatemalanmargin.

Sites 494 and 498

The mode of emplacement of the rock and sedimentat Site 494 is a matter of conjecture. Poor recovery andlong sections of drill breccia preclude definite conclu-sions. The differences in lithologies between Sites 494and 498 are suggestive of either postdepositional slump-ing, local tectonic displacement by transverse faulting,variably oriented planes along which imbricate thrustinghas occurred, or the chaotic distribution of rocks thatmight be encountered in a melange of large proportions.

Neither repeated lithologic and age sequences, pe-lagic brown clay, nor chalk were encountered at Site494. The pelagic sediments recovered contain species ofnearshore microflora and terrigenous components thatindicate a slope position of the entire sequence sincethe Cretaceous (Muzylöv, this volume; Heinemann andFüchtbauer, this volume). Paleomagnetic results, more-over, indicate that Late Cretaceous rocks cored at Site494 have not travelled long distances to their present lo-cation (Gose, this volume). If accretion and imbricationare occurring at this lowermost part of the slope offGuatemala, the process is limited to the space betweenthe base of Hole 494A and the top of the seismic reflec-tor thought to represent Cocos Plate basalt (Ladd et al.,this volume). Based on the interpretation of these multi-channel records, that space is 500 to 800 meters thick.Chalks deposited in the early Miocene are now in theTrench axis. Assuming that sediment supply is constantand that subduction has occurred since the time of de-position of the oldest recovered slope sediments (i.e.,the Late Cretaceous to early Eocene), some 40 m.y. oftime must be contained in the undrilled interval. A sub-stantial amount of pelagic sediment must have been sub-ducted.

Terrigenous sands are absent at Site 494 below 18.5meters sub-bottom; therefore we lack the evidence forthe uplift and incorporation of trench-fill turbidites intothe trench slope so strongly advocated by the Leg 66 sci-entific party (Moore et al., 1979). The lithologic sectionsof Sites 494 and 498 do not support the "Trench-SlopeModel" for convergent margin tectonics as proposed bySeely et al. (1974).

Sites 496 and 497

The terrigenous, volcanogenic, and pelagic biogeniccomponents of the sediments at Sites 496 and 497 sug-gest a trench-slope environment having two dominantsediment sources since the early Miocene: the nearbyNeogene volcanic province of Guatemala and the bio-genic production in surface water layers strongly in-fluenced by upwelling. Upwelling throughout that inter-val is indicated by the abundance and composition ofthe diatom and nannofossil assemblages. Jousé et al.(this volume), however, report abundant tropical dia-toms in Cores 496-1 and -3, suggesting a variable paleo-environment. High abundance of organic carbon, as

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much as 5% but on the average 2%, is largely due toland-derived organic matter (Curiale and Harrison, thisvolume).

As within the uppermost lithologic unit at Sites 494and 498 (0-213 m sub-bottom), there is no evidence ofgraded beds at Sites 496 and 497, except in the pebblymudstones. Microfaunal and microfloral populationsindicate varying degrees of downslope reworking of thissediment drape (Jousé et al., this volume; Muzylöv, thisvolume; Thompson, this volume) Similarly, Bein andFütterer (1977) found evidence for downslope rework-ing coupled with absence of bed forms in sediments ofthe passive continental margin of northwestern Africa.They invoked McCave's (1972) suggestion of reworkingby currents and cascades of lutite flows and SeibokTs(1976) combination of resuspension through bioturba-tion and downslope gravity transport as possible mecha-nisms. Our results suggest that mechanisms such asthese may also exert the most dominant influence on thesedimentary drape of the active continental margin ofGuatemala.

DIAGENESISDetailed mineralogic study of the clay- and silt-sized

fractions (Heinemann and Füchtbauer, this volume) hasrevealed few mineralogical changes attributable to dia-genetic reactions. Optical investigation of glass shardsdid not show signs of smectitization. On the other hand,the mixed-layer smectite-illite minerals present in thelower to middle Miocene pelagic carbonates of Site 495must have formed by the alteration of volcanic glass. Itis possible, therefore, that small glass shards, few mi-crons in diameter, were the main source of the swellingclay minerals in the Miocene and also in the youngerhemipelagic sediments. Also, the zeolites found in mostsamples of the silt fraction are likely a diagenetic prod-uct (Heinemann and Füchtbauer, this volume). Disap-pearance of olivine in the heavy mineral concentrates ofthe sand fraction in Miocene sediments of Hole 498Asuggests intrastratal dissolution of mafic minerals (Pra-sad and Hesse, this volume). The only other mineralogiceffect of early diagenetic processes is the formation oflutitic limestones as a by-product of organic matter de-cay and pyrite formation.

Whereas mineral reactions during early diagenesis aresluggish, particularly in a relatively cold geothermal en-vironment, the organic-matter-rich hemipelagic sedi-ments encountered on the trench slope at Sites 496 and497 undergo pronounced alterations. Those changes re-sult from the oxidation of organic matter, a processmost rapid in the upper 50 to 100 meters of the sedimentcolumn. Oxidation is reflected by strong gradients in thepore-water compositions of the rapidly deposited sedi-ments. The dramatic rise in alkalinity observed in theupper 25 to 50 meters of Holes 496 and 497, reaching amaximum of 120 meq/1 at 22 meters sub-bottom depthin Hole 496, is the immediate consequence of this oxida-tion process (Harrison et al., this volume). This alkalin-ity increase occurs after sulfate depletion; it is thereforeassociated with deeper zones of organic matter oxida-tion, in which methane is produced.

The concentration of magnesium in the interstitialwaters of Holes 496 and 497, after a rapid initial in-crease due to release from swelling clays by cation-ex-change for NH4, decreases downward in the holes, prob-ably reversing the clay-exchange reaction. Dolomite for-mation would also serve as a sink for Mg + 2, but nodolomite was identified in the sediment samples. Theconcentrations of Cl~, Na + , and, to a lesser extent, K +decrease downhole. Calcium, however, does not showthe expected downward increase. These variations addup to an overall lowering of salinity with depth. Thissalinity decrease, especially the chlorinity decrease, can-not be explained by conventional mechanisms such asthe influx of and mixing of fresh water from the nearbycontinent, because of the observed increase of δ 1 8 θ withdepth, which reaches values of +2.5‰ (SMOW). Simi-lar chemical trends of pore-water composition occur onother continental margins and have been interpreted asresulting from the decomposition of gas hydrates (Hesseand Harrison, 1981).

Gas hydrates, like ice, exclude salt ions from theircrystal structure. They also fractionate isotopes by pref-erentially withdrawing heavy isotopes of H, C, and Ofrom the pore fluids. During burial and compaction, thesolids, including the hydrate crystals, sink relative to thepore fluids. The hydrates carry isotopically heavy andfresh water to levels of decreased porosity at greatersub-bottom depths, leaving lighter isotopes, salts, andremaining pore fluids behind. Thawing of the hydratesat the base of the hydrate zone or during core recovery,and remixing with the smaller volumes of pore waterthat remain in the deeper subsurface levels, will decreasechlorinity and increase positive isotope values. The oc-currence of gas hydrates may control the early diagenet-ic evolution of pore fluids. By causing downward fresh-ening of the pore waters, a diagenetic regime may be es-tablished that is significantly different from that of hy-drate free sediments. In the future this contrast in pore-water composition could serve as a prospecting tool forhydrates.

None of the changes described for Sites 496 and 497are observed in the pelagic and hemipelagic sediments ofSite 495, because their initial organic matter content of1% to 2% is about half that of the mid-slope sites, andtheir sedimentation rates are less than the pore-fluid dif-fusion rate. Pore-water chemistry of this hole conse-quently does not show any appreciable variations withdepth except for Sr2+ and dissolved SiO2. The latter oc-cur in the pelagic chalk section and indicate carbonaterecrystallization and porcellanite precipitation.

Chemical trends of pore waters from Trench Sites499 and 500 show strong fluctuations between the sandyportions of turbidites and the intercalated silts and clays.Without the sand layers, however, vertical trends in pore-fluid composition would be similar to those observed inhydrate-free lower-slope Site 494 (Harrison et al., thisvolume).

In summary, the chemistry of the pore fluids of theseyoung, Quaternary to Miocene sediments from the ac-tive margin off Guatemala provides some importantclues as to the early diagenetic processes that are pres-ently in progress.

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PALEOGEOGRAPHY

One of the most important targets of Leg 67 was thesearch for sediments comparable to those known onland in the Nicoya Complex of Costa Rica, whose north-westward prolongation to Guatemala is suggested bygeophysical data (Ladd et al., 1978; Ibrahim et al.,1979). Unfortunately, the occurrence of high gas con-centrations on the Trench slope prevented drilling deepenough sections to achieve this goal. The oldest forma-tions recognized in the different sites of the slope wereUpper Cretaceous (494), lower Miocene (496), and lowerPliocene (497). The most complete sequence from thiscontinental margin was identified at Site 494, from bot-tom to top: Upper Cretaceous, middle Eocene, upperOligocene-lower Miocene, Pliocene, and Pleistocene.This sequence, except for minor differences in thick-ness, is similar to the Esso Petroleum Well on the con-tinental platform off Guatemala (Seely, 1979) and canalso be compared, by ages more than by facies, with thesequence known on land along the Pacific coast of Nica-ragua and Costa Rica.

These two sequences, at sea and on land, have onecharacteristic in common: both show an Upper Creta-ceous unconformity at the base of the thick Tertiary de-posits devoid of coarse volcanogenic detritus. The un-conformity at Site 494, compared with those known onland, can be explained by the distance from the volcanicarc of south Central America. Thus the Tertiary se-quence of Site 494 shows more distal, more abyssal fa-cies than those of the Tertiary sequences in Nicaraguaand Costa Rica, which are close to the source area forthe volcanogenic detritus and may correspond to sedi-mentation in a fore-arc basin.

CONCLUSIONSShipboard visual and microscopic observations, and

subsequent shore-based studies of Leg 67 samples, high-light the unexpected thickness and prevalence of thedrape of hemipelagic sediment blanketing both the land-ward and the seaward slopes of the Middle AmericaTrench at the Guatemala transect. An increase in grainsize of terrigenous minerals from the early Miocene toRecent at the oceanic reference site suggests sedimen-tation by low-density suspension currents or nepheloidlayers reaching hundreds of kilometers seaward of theTrench axis. A few missing radiolarian zones indicateerosion of surface sediment as Site 495 passed over theouter swell seaward of the Trench. Small reverse faultsat Site 495 and 500 are attributed to soft-sediment de-formation. In contrast, the stratigraphy of the eight holesdrilled in the Trench axis documents normal faulting ofthe subducting plate and of its overburden of pelagicsediments and turbidites. Lithologic and age sequencesare neither repeated nor reversed at any of our sites, andpure pelagic sediments are absent in all sites landward ofthe Trench axis. Displaced benthic foraminifers suggestdownslope transport of the slope sediments by variousmechanisms. Leg 67 drill sites did not reach an "accre-tionary prism" because either none exists off Guate-mala or we failed to penetrate deep enough. If accretion

of slivers of oceanic crust with pelagic sediments is tak-ing place at the base of the slope, it is occurring in the500 to 800 meters of rock not drilled between the masterthrust separating the upper and lower plates and thebottom of Site 494 at about 400 meters sub-bottomdepth.

Our results are suprising in that the drape of hemipe-lagic sediments and turbidites off Guatemala is thicker,gas hydrates more prevalent, and the "accretionaryprism" more elusive than we anticipated.

ACKNOWLEDGMENTS

We thank Juliana Fenner, Glenn Shepherd, Greg Moore, and Jo-hanna Resig for their readings of various versions of the manuscript.

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