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Organic fades in Recent sediments of carbonate platforms: Southwestern Puerto Rico and Northern Belize
Advances in Organic Geochemistry
1985
Vol. IO. Pp. 717-724, 1986 Printed in Great Britain. All rights reserved Org. Geochem.
0146-6380/86
Copyright
f3.00
+O.OO
0 1986 Pergamon Journals Ltd
Organic facies in Recent sediments of carbonate platforms: Southwestern Puerto Rico and Northern Belize JANINA RAFALSKA-BLOCH’* and ROBERT CUNNINGHAM JR* ‘School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A. 2Exxon Production Company, P.O. Box 2189, Houston, TX 77001, U.S.A. (Received
16 September
1985; accepted
6 March
1986)
Abstract-Our organic facies model of a platform environment is based on the analysis of (1) total organic carbon, (2) visual protokerogen types and hydrogen indices of the sediments, (3) sediment mineralogy and grain size, and (4) reef patterns, water circulation, and relief of the adjacent land. TOC values are lowest on reefs and outer platforms (0.2%); intermediate in sediments fringing carbonate islands (0.52.0%), on sea-grass meadows (0.5-0.7%), and in forereef lagoons (0.1-0.7%); highest in inner platform lagoons (0.5-5.0X) and mangrove swamps (2.7-3.5%). Particulate organic matter comprises structured terrestrial, marine, and amorphous types. On average, terrestrial material accounts for 28% of OM in Puerto Rico and 12% in Belize. The mean hydrogen index of the sediment is 355 (mg hydrocarbons/g TOC) in Puerto Rico and 719 in Belize. This study indicates that TOC values are mainly related to platform subenvironment, while OM types are controlled by distance from the shoreline. Organic facies dominated by structural terrestrial OM occur closer to the shore, whereas toward the shelf margin marine and amorphous OM contents increase. Key words:
organic facies, carbon, protokerogen, Rock-Eval, reefs, carbonate sediments, Puerto Rico,
INTRODUCTION
algal mat collectedin LagartosLagoon nearEl Cuyo (Yucatan Peninsula,Mexico). This study showsthat, asin the caseof carbonate inorganic facies distributions, organic facies are a function of terrigenous influx and intrabasinal productivity. Depositional input is modified by the chemical and, mechanical breakdown of organic particles and their subsequentredistribution on carbonateplatforms.
The development of methods for predicting the quality and quantity of organic matter in the potential source rocks requires a knowledge of organic facies distribution in various source rock-forming environments. Organic facies may be characterized as mappable units of similar organic richness and organic matter type (Rogers, 1980). Studies of the organic facies properties of ancient and recent carbonate sediments have received little attention in the literature with notable exceptions of contributions by McKirdy et al. (1984) and Jones (1984). This is in sharp contrast to the economic potential of these rocks as hydrocarbon sources. Here the distribution and control of organic facies on the Puerto Rican (Fig. 1) and Belizian (Fig. 2) carbonate platforms is addressed. Both are excellent settings for determining what kind of organic facies patterns, if any, exist because they provide a variety of depositional subenvironments. Differences between the two settings are terrigenous influx, which is more substantial in Puerto Rico than in Northern Belize and the energy of the depositional environments. Circulation is less vigorous along the Belizian shelf due to the presence of a barrier reef in contrast to Puerto Rico where patchy reef development allows
METHODS
more vigorous water circulation. In addition, for comparison,a sedimentsamplewasanalyzedfrom an
The subaqueous sediment samples were collected using either SCUBA equipment or a Peterson grab sampler. In some casesa piston corer was used. The samples were stored in glass jars, treated with a bactericide (sodium azide) and frozen within 6 hr of collecting. Only the samples representing the top IOcm of the sediment are discussed here. The samples were subsequently analyzed for grain-size distribution and petrographic composition using standard sedimentological techniques. The TOC (total organic carbon) values were measured with a LECO analyzer. Organic matter types were determined using microscopic observations of isolated organic particles in transmitted light. Isolation of organic matter for optical studies was accomplished using the chemical method of the destruction of the mineral phase with HCl and HF; the liberated organic fraction was then concentrated by centrifuging in ZnBr,. Hydrogen and oxygen indices were determined using the Rock-Eva1 pyrolysis technique on whole sediment samples (Espitalie et al., 1977).
Total
*Author
organic
RESULTS
AND
carbon
(TOC)
DISCUSSION
The organic carbon content of carbonate sedi-
to whom correspondence should be addressed. 717
JANINA RAFALSKA-BLOCH
718
0
1
0
0.5
INNER
1
and ROBERT CUNNINGHAM
2, KILOMETERS <
PUERTO
RICO
ISMILES
SHE:;
_
.;, ENRIQUE
D
LAUREL MEDIA
LUNA l 1.s
TURRUMOTE
0
OUTER
SHELF
%i
/ l P REEF SUBMERGED +
SAMPLE
REEF LOCATION
Fig. 1. Studyareawith locationof sample sitesin southwestern PuertoRico(afterMorelocket al., 1977). ments on Puerto Rican and Belizian platforms is related to depositional environment, content of inorganic insoluble residuum of the sediment,and grain size (Table 1). The insular shelf off La Parguera, southwestof Puerto Rico, is 8-10 km wide. Reefson the shelfare alignedapproximatelyeast-westand are separatedby backreef and inter-reef lagoons (Morelock et al., 1977).The coast is dominated by mangrove forests (Fig. 1). The TOC values average3.5% for the sediments of mangrove-linedcoastal channelsand 2.79% for Phosphorescent Bay, a mangrove-borderedbay. Sedimentsassociatedwith Halimeda-Thalassia meadows contain between0.5 and 0.7% of TOC. Backreef and inter-reef lagoons sedimentsrange from 0.13 to 0.68% TOC. The TOC abundancesin inter-reef troughs, 0.08-0.24%, are characteristicallylow. This may be attributed to the patchy distribution of reefs
which allows relatively free water circulation. Low abundancesof TOC are also typical of reef crests, flats, and forereef areas.This is in spite of the fact that reefsare one of the most metabolicallyefficient of productive environments,and that their metabolismrate exceedsthat of grassmeadows(Odum et al., 1959). The secondlowest range of organic carbon values, from 0.1 to 0.3%, occurs in the outer-shelf province and shelf-edgereef. The scarcity of organic material is a reflection of low productivity and high depositional energy, factors which are detrimental both to accumulation and preservation of organic material. In summary, total organic carbon values for samplesfrom the insular shelf of southwestPuerto Rico are controlled by biological productivity and water energy which affects the rate of mechanical breakdown,chemicaldecomposition,and winnowing of organic detrius.
Organic facies in Recent carbonates
OCKY
719
POINT
NC “LLILL SHELF
LEGEND 17%’
---AXIS
OF LIMESTOI(E RIOOE
-C*RsOHATEl
SHOAL SAMPLlNG
SCALE NAUTICAL
The Northern Belizeshelf consistsof severalzones paralleling the coast (Fig. 2). The narrow body of water (l-2 miles)betweenthe barrier reefand the line of mangrove-sandcays is called the outer (reef) lagoon and the westernpart of the shelfis called the inner (shelf) lagoon (its averagewidth is 10 miles). AmbergrisCay, a carbonateisland,separatesthe two lagoons. The barrier reef-produced sediments(reef facies of Pusey, 1975)are depletedin organiccarbon (0.1l0.17%). Betweenthe wave-agitatedreef and the very low energycurrentsof the inner lagoon,intermediate energy conditions coexist with Halimeda facies. Presenceof water currents coupled with intense burrowing result in very low abundancesof TOC (0.14%). Ambergris Cay consists of several subenvironments. Only two of them, the supratidal flats and intra-island lagoons, were sampled.Lamination in supratidal sedimentsattests to an absenceof burrowing organisms.Periodical subaerialexposureof thesesediments,however, may have oxidized some of the organic matter (mainly algal mat). The TOC valuesrange from 0.5 to 0.8%. Intra-island lagoons, which have limited exchangewith the outer or inner
SITES
15 MILES
lagoons,accumulategray or black muddy sediments. Therefore, their TOC content, at 2.06%, is relatively high. Inner lagoon TOC values vary accordingto subenvironment.Valueswererecordedas follows: 0.4% closestto the mainland,0.8-5.03% in the vicinity of mangrove cays, 1.73-2.13% on mud shoals, and O&l-0.79% in other areasof the lagoon.The highest quantities of organic carbon occur in miliolid-mud facies.Much of the organic material, as previously observedby Pusey(1975),is in the form of mucus-like slime. Thus, aswasthe casein Puerto Rico, environments characterizedby high physicalenergy,i.e. barrier reef and outer lagoon, have low abundancesof organic carbon. In contrast, quiet and restrictedlagoonsare richer in organic carbon. TOC us sediment textures and composition
The energy level of the environment dictatesnot only the distribution of organic matter but also the distribution of inorganic particles. The relationship betweenorganicand inorganicaspectsof sedimentis expressedby plotting sedimentparametersagainst total organiccarbon and calculatingPearson’scorre-
720
JANINA RAFALSKA-BLOCH and ROBERTCUNNINGHAM Table
I. Total organic carbon contenl and general sedimentological
Sample Number Southwestern A 8 C 0 E F G H I J K
2
1 Puerto
of sediments data
3
4
Rico
S :
3.51 0.70 0.54 0.61 0.51 0.58 0.67 0.45 0.22 0.21 0.13 0.89 0.19 0.12 0.12 0.17 0.27 0.41 0.18 2.79 0.15
54.99 15.35 10.87 15.63 16.32 15.82 17.30 11.98 8.27 9.02 7.20 14.28 8.06 6.72 6.05 3.33 10.29 13.37 6.73 14.44 6.94
I-A !-B 1-c 1-o 1-E 1-F 1-L 1-M
0.08 0.10 0.21 0.24 o.i4 0.68 0.17 0.12
5.19 5.75 5.27 11.74 5.20 16.12 3.74 2.55
Northern
Belize
A 8
2.84 0.80
23.57 12.39
oc F
0.40 5.03 1.73 0.44
31.64 33.71 15.25 16.59
G H I J
2.13 0.79 0.80 0.50
12.77 15.08 14.89 20.67
: M N
2.06 0.14 0.17 0.11
18.36 9.03 12.74 7.90
2.64
24.23
.
s-
I1.00 3.50 3.12 5.05 5.62 5.38 5.96 3.51 2.43 2.73 4.00 7.29 1.60 2.22 0.76 0.21 1.80 4.12 -0.35 0.24 8.22 -0.07 1.01 -1.05 l:o 3.0
45.00 26.78 29.51 65.52 82.88 69.31 83.76 46.17 10.93 12.99 10.00 8.20 2.90 0.38 0.13 0.00 0.30 59.41 0.57 0.21 99.09 0.00 0.03 0.24 1.00 0.20 20.00
3-
.
.
. .Y”CATAN
2-
.
l-
.5-
.PUErlTO n 0ELIZf
. .
. d,, :I:. 10
. 20
30
% Insoluble 3.34 1.82 3.60 0.38 0.99 2.64 2.12 1.42 1.73 1.96 3.07 1.10 0.82 -0.20
18.77 9.00 63.65 0.27 0.53 21.39 33.26 1.97 8.27 3.64 37.73 0.29 9.51 0.07
3.10
1.73
lnco
I
I,,
40
50
, 60
70
10
Residuum
Fig. 3. Relationship betweenthe weightpercentof organic carbonand the content of inorganic insoluble residuum. organic matter doesnot show a direct influenceof biomass,a phenomenonthat has beenreported by others, for examplein the study examining seasonal variations in organic matter concentrations on an intertidal flat in Maine (Mayer et al., 1985). 5-
Yucatan A 1 2 3 4 -
total organic carbon mineral, HCl-insoluble mean grain size (phi grains smaller than not determined
4-
(X) residuum (%) units) l/16 mm (weight X)
AYUCAYAN *PUERTO . BELIZE
3-
RICO
. .
lation coefficients (r). The amount of organic carbon in the Puerto Rican samples increases both with the content of mineral insoluble residuum (r = 0.94; Fig. 3) n . 2and with decreasinggrain size of sediment(r = 0.55; Fig. 4). However, correlation betweenTOC and the . content of silt and clay fractions (r = 0.18; Fig. 5) is poorer. Adsorption of organic matter onto clay minerals, therefore, cannot explain an association lbetween TOC and insoluble residuum. Quartz in Puerto Rican sedimentsclustersaround the 3.54.04 .s grain size range (Breyer and Ehlmann, 1981).Interestingly, high TOC contentsappearto be associated with the samegrain sizerange(Fig. 4). It seems likely that most organic carbon is presentin organic particles that are capable of surviving transport from Fig. 4. Relationship between the grain size of the sediments land with quartz. In this respect,the abundanceof the and their organic carbon content.
Organic facies in Recent carbonates
from land plants, this material will be darker yellow to amber color than if derived from marine algae. Under reducingconditions,however, the product of biodegradationis colorlessor gray, whetherderived from marine or non-marine environments.Finally, “structured aqueousmatter” includesthe unaltered remainsof marineorganismssuchasalgae,liningsof foraminifera, and marine grasses. Estimatesof the different organic matter types weremadeon sampleswith a fair amount of organic matter. The organic matter contained on the slides from the Belize sampleswas too sparseto enable percentageestimatesto be made (Table 2). Particulate organic matter in Puerto Rican sediments consistsof structured terrestrial material in the form of woody tissues,epidermal tissueswith well preservedstomata,and shorelinegrassremains.
8,
. .
. BELIZE . PUERTO RICO .YUCATAN .
.
.
, :.
.
.
l
:= t,
l
.
:
.
.
Table
2. Tvoes
of
Darticulate organic
matter
.,
IO
,
20
% Fines
30
40
50
I
60
10
80
(<‘/SW+)
Fig. 5. Relationship betweenthe weightpercentof fines (grainsize< l/16 mm)and organiccarboncontentof the sediments. In contrast, the percent TOC in Belizian samples correlateswith the percent of fines(r = 0.93; Fig. 5), meangrain size (r = 0.85; Fig. 4), but not with the percent of insoluble mineral residuum (r = 0.58; Fig. 3). Thus the bulk of organic matter is associated with fine-grained carbonates,produced in situ. As previously discussed,the miliolid mud shoal, which supportsrich, highly productive communities,is the main source of organic carbon. The cohesiveness of sedimentsand lack of bioturbation in this subenvironment enhance the preservation of organic matter. Organic
721
matter types
Masran and Pocock (1981) have designeda simple classification of plant-derived particulate organic matter. It is comprised of several general classes delineated on the basis of origin and degree of alteration of organic particles. For example, the “structured-terrestrial” OMT includesplant remains (mostly wood fragments)that, althoughmechanically cornminuted still retain their cellular structure and are not significantlyaltered by biodegradation.“Biodegraded-terrestrial material” designates organic matter that has undergoneintense biodegradation but is still discernible as land-derived in origin. “Charcoal” is the product of the oxidation of structuredmaterials,mainly of stemand root tissues. Some workers, e.g. Cope (1981), maintain that charcoal is the product of forest fires. “Amorphous material” is completely structureless.When derived
Sample Nunber
ParticFlate ST
Southwestern A E 0 E i H I J K
10.0 i:: 5.0
:-05 1o:o -
2+5 1:5 -
so
l
k
:
-
;
-
_
10.0 -
-
SM
20.0 19.0 28.5 35.0 25.0 38.0 32.0 + + +
20.0 24.0 21.0 10.0 15.0 12.0 19.0
22.0 15.0 5.0 38.0 26.5 21.0
+
+
21.0 +
79fo
32fO
+
+
Northern
Belize
A
1.0 5.0 -
20.0 30.0 24.5 30.0 18.5 14.0 17.0 + + + 2OfO + + +
50fo
10.0
17.0 35.0
30.0 24.0 10.0
: ,“A 1-F 1-L
(X)
Rico
;:“5 15.0
13fo 10.0 -
Ma;t$r
PS Puerto
30.0
OrEnit
3oro 50.0 25.0
20fo +
1oro
30.0 +
+ +
: H I J K
+ _ + -
_ + -
:
:
-
+ +
-
-
++
: E
+ +
+ +
+ + +
+ +
Yucatan A ST PS C hII GA SM +
structured terrestrial oollen and soores kharcoal andrsemi-charcoal amorphous gray amorphous structured marine not determined determined. but too sparse
to estimate
the
percent
122
JANINA
RAFALSKA-BLOCH
and
Although pollen and sporeswere found in most samples,charcoal and semi-charcoaldominate the land-derivedparticles.The well-preservedstomataor pits observedin charcoal are likely to have resulted from fire. Theseforms of organic particles account for 15% of the total organic matter and 53% of the terrigenousinput. Terrestrial plant remainsaccount for 28% of the organic matter. Structured marine material and marine-derived amorphous matter occur in all samples. The structured marine material consistsof cysts, algal spores,calcareousalgal remains,seaweedtissuesand foraminifera linings. The amorphous matter is a mixture of non-descript yellow and grey material. The terrigenousimprint on OMT in Belizeis minor and restrictedto somewoody tissues(found on small mangrovecays), shorelinegrassremains(on supratidal flats), a few pollen and spores(on carbonate shoals),and somecharcoal or semi-charcoal. Structured marine material is present in most samples.It includesthe remainsof calcareousalgae, seaweedtissues, cysts, and foraminifera linings. Finely disseminatedamorphousmaterial is ubiquitous, most being a light grey-yellow color. Samples from mud shoalsappear to contain fecal pellets. Pyrolysis characterization
The Rock-Eva1 pyrolysis is consideredinadequate for oxygen-rich organic matter, especiallyin carbonate sediments.Indeed, the values of oxygen indices of organic matter from Puerto Rican and Belizian samples are very high and dispersed, reflecting perhapsthe matrix effect (Table 3). Therefore, since T,,,, is influencednot only by maturation (negligible
Table
3. Results
Sample NUllbPl-
of Rock-Ed HI
Southwestern
analvsis
01
Puerto
ToFAX Rico
A
428
829
417
CB 0 F
1685 1535 1804 1634
u 1-o
414 375 324 346 324 300 328 250 270 508
1-F
391
413 414 416 421 423 409 418 414 422 405 435
H I
J L
Northern A C 0 F G
I J K L
1515 2477 2004
1329 960 2237 1457
Eel ize 878 780 172 874 984 750 572
714
918
1137
550
3307
433 430 430 431 430 429 426 430 411
990
1269
434
917 2052 1143
1357 1465 2014
Yucatan A
ROBERT
CUNNINGHAM
Fig. 6. Organicmatter typesdeterminedfrom hydrogen indicesandmaximumtemperature of Rock-Eva1 pyrolysis.
for recent sediments)but also by the type of organic matter (Tissotand Welte, 1984),the hydrogen indices (HI) were plotted againstthe T,,,,, valuesrather than the oxygen indices(Fig. 6). Organic matter from Belize sedimentsis much more hydrogen-rich [HI ranges from 172 to 878 (mg hydrocarbons/gTOC), with the meanHI = 7191 than organic matter from Puerto Rico sediments (HI rangesfrom 250 to 508 with the meanHI = 355; Fig. 6). A plot of HI vs T,, (Fig. 6) indicatesthat BelizesamplesclusterbetweenType I and Type II of sedimentaryorganic matter (is characteristicof algal and/or microbial origin). The Puerto Rico samples plot betweenTypesII and III implying mixed marineterrestrial provenance. For comparison, hydrogen index of algal mat from Yucatan is 990, which is suggestiveof Type I protokerogen. Organic facies and dktance from shoreline
It is evident that there are two major types of organic faciesin thesecarbonateenvironments.One is dominatedby the mixture of structured terrestrial plant remains,pollen and spores,and charcoal. Another has a more amorphousor marine structured character. Both types of organic faciesare found in the sedimentsfrom Belize and Puerto Rico. It is usuallyassumedthat most of the terrigenousmaterial is trapped in the coastal zone. However, a plot of marine organic matter vs distance from shoreline revealsthat substantialamountsof the land-derived organic pdrticlespersistfrom one-third to one-half of the extent of the Puerto Ricancarbonateshelf,i.e. for about 5 km (Fig. 7). The percentageof marine organic matter is derived from optical microscopic studies. The persistenceof terrestrial organic materialsin reef environmentsoff the shore of southwestPuerto Rico may be related to one or more factors. A large influx of resistantorganic matter suchascharcoalor semi-charcoalis supplied from the adjacent, highrelief mainland. Alternatively, the lack of a strong
Organic facies in Recent carbonates
723
l
0 SHORELINE
.25
.5
PUERTO RICO BELIZE YUCATAN
35
1 PLATFORM MARGIN
RELATIVE
DISTANCE
ACROSS
PLATFORM
Fig. 7. The transitionbetweenterrestrialandmarineorganicfacieson carbonateplatforms. hydrographic barrier resultsin high-energytransport and deposition conditions which cause the mechanical and chemical degradation of labile organic materials.The sameconditionscontribute to reworking and spreadingof the resistantorganic particles over the shelf. The transition from terrestrial or terrestrial-mixed organic faciesto marine organic faciesoccursover a much shorter distanceon the Belize carbonateplatform than on the Puerto Rico carbonate platform. The rapid disappearanceof terrestrial organicmatter in Belizian sediments(at about 1/4th of the platform distance)(Fig. 7) is causedby a sparsesupply of terrestrial material. The adjacent land is of low relief and has very low surfacerunoff. The barrier reef restricts water agitation in the lagoons very effectively, thus limiting the extent of chemicalor mechanicalbreakdown and the possibtlity of transporting any land plant fragmentsvery far from the shore. SUMMARY
The abundancesof organicmatter in sedimentsof carbonateplatforms arestrongly relatedto individual subenvironments.Higher valuesof TOC are characteristic of sedimentscollected from mangrove-lined coasts, algal-grassmeadows,mud shoals, lagoons and tidal flats. Whereastheserelationshipsare true both for Belize and Puerto Rico, Puerto Rican sedimentsin generalshowa depletion in organiccarbon for any given subenvironment,relative to Belize. Thesetwo areasalso differ in the relative proportions of organic matter types present.Land-derived (particularly charcoal) organic particles are prominent in Puerto Rican reef settings,whereasin Belize organic matter is dominated by the marine and amorphousorganic types. Sedimentaryorganic par-
ticlesfrom Belizeare more hydrogen-richthan those from Puerto Rico. In both Belizeand Puerto Rico the influx of terrigenousorganic matter types diminishes with distance from land, however, this transition takes placemuch more rapidly in Belize. The differing distribution of organic faciesin the carbonateplatform environmentsof Puerto Rico and Belizeis a function of the complex interplay between source terrain, mode of transport, energy level, depositionalconditions,and preservationpotential of any given area. The substantialrelief of the Puerto Rican hinterland results in an increasedinput of land-derivedplant material. In addition, the patchy distribution of reefs off Puerto Rico permits free circulation of water. Organic particles are thus continually subjectedto processes of transport, oxidation, or mechanicalbreakdown.The low relief of the Belizemainlanddoesnot provide a significantsource of terrestrial organic particles. Additionally, the presenceof a barrier reef and largecarbonateislands createa quiet, restrictedenvironment, with excellent preservationpotential for organic matter. Acknowledgements-This researchwascarried out with support from the American Chemical Society, Exxon Research and Production Company, ARC0 Research Foundation, Mobil Foundation, Olympics Exploration, NSF Grant EAR-8352055 (Dr M. H. Engel), the Energy Resource Center, and the School of Geology and Geophysics of the University of Oklahoma. The Department of Marine Sciences at the University of Puerto Rico, and the United States Geological Survey at San Juan, Puerto Rico, kindly provided one of us (JRB) with accessto their research vesselsand labs. Several individuals extended their help. The prime acknowledgement goes to Th. C. Masran, Dr B. Silver, Dr J. Morelock, and K. R. Hahn. I am also very grateful to Dr L. Ciereszko, S. Imbus, L. Hyman, G. Stewart, M. Stearns, D. Gilbert and Dr C. P. Summerhayes. Many thanks to M. Starr for typing the manuscript and J. Pianalto for drafting.
JANINA
124
RAFALSKA-BLOCH and ROBERT CUNNINGHAM
REFERENCES
an intertidal mudflat. Estuarine,
Coastal
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Sri.
20,
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Breyer J. A. and Ehlmann A. J. (1981) Mineralogy of arc-derived sediment: siliciclastic sediment on the insular shelf of Puerto Rico. Sedimentology 28, 61-74. Cope M. J. (1981) Products of material burning as a component of the dispersed organic matter of sedimentary rocks. In Organic Maturation Studies and Fossil Fuel Exploration (Edited by Brooks J.), pp. 89-109. Academic Press, New York. Espitalit J., Laporte J. L., Madec M., Marquis F., LePlat P., Paulet J. and Boutefeu A. (1977) Mtthode rapide de caracterisation des roches-meres, de leur potentiel p&rolier et de leur degre d’evolution. Rev. Inst. Fr. P&. 32, 23-42. Jones R. W. (1984) Comparison of carbonate and shale source rocks. In Studies in Geology (Edited by Palacas J. G.), No. 18, pp. 163-180. Am. Assoc. Pet. Geol., Tulsa. Masran T. C. and Pocock S. J. A. (1981) The classification of plant-derived particulate organic matter in sedimentary rocks. In Organic Maruration Studies and Fossil Fuel Exploration (Edited by Brooks J.), pp. 145-159. Academic Press, New York: Maver L. M.. Rahaim P. T.. Guerin W.. Macko S. A.. Wading L. ‘and Anderson F. E. (1985) Biological and granulometric controls on sedimentary organic matter of
McKirdy D. M., Kantsler A. J., Emmett J. K. and Aldridge A. K. (1984) Hydrocarbon genesis and organic facies in Cambrian carbonates of the Eastern Officer Basin, South Australia. In Studies in Geology (Edited by Palacas J. G.), No. 18, pp. 13-3 I. Am. Assoc. Pet. Geol., Tulsa. Morelock J., Schneidermann N. and Bryant W. R. (1977) Shelf reefs, southwestern Puerto Rico. In Studies in Geoloav (Edited bv Frost S. H.. Weiss M. P. and Saunders J. B.):‘No. 4, pp.- 17-55. Am.’ Assoc. Pet. Geol., Tulsa. Odum H. T., Burkholder P. R. and River0 J. (1959) Measurements of productivity of turtle grass flats, reefs, and the Bahia Fosforescente of southern Puerto Rico. Pub/.
Inst.
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Sci.,
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6,
105-170. Pusey W. C. III (1975) Holocene carbonate sedimentation on Northern Belize shelf. In Studies in Geology (Edited by Wantland K. F. and Pusey W. C. III), No. 2, pp. 134-226. Am. Assoc. Pet. Geol., Tulsa. Rogers M. A. (1980) Application of organic facies concepts to hydrocarbon source rock evaluation. Proc. f&h World Pefroleum Congr., Bucharest, 1979, pp. 23-30. Heyden, London. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, 699 pp. Springer, Heidelberg.
1985
Vol. IO. Pp. 717-724, 1986 Printed in Great Britain. All rights reserved Org. Geochem.
0146-6380/86
Copyright
f3.00
+O.OO
0 1986 Pergamon Journals Ltd
Organic facies in Recent sediments of carbonate platforms: Southwestern Puerto Rico and Northern Belize JANINA RAFALSKA-BLOCH’* and ROBERT CUNNINGHAM JR* ‘School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A. 2Exxon Production Company, P.O. Box 2189, Houston, TX 77001, U.S.A. (Received
16 September
1985; accepted
6 March
1986)
Abstract-Our organic facies model of a platform environment is based on the analysis of (1) total organic carbon, (2) visual protokerogen types and hydrogen indices of the sediments, (3) sediment mineralogy and grain size, and (4) reef patterns, water circulation, and relief of the adjacent land. TOC values are lowest on reefs and outer platforms (0.2%); intermediate in sediments fringing carbonate islands (0.52.0%), on sea-grass meadows (0.5-0.7%), and in forereef lagoons (0.1-0.7%); highest in inner platform lagoons (0.5-5.0X) and mangrove swamps (2.7-3.5%). Particulate organic matter comprises structured terrestrial, marine, and amorphous types. On average, terrestrial material accounts for 28% of OM in Puerto Rico and 12% in Belize. The mean hydrogen index of the sediment is 355 (mg hydrocarbons/g TOC) in Puerto Rico and 719 in Belize. This study indicates that TOC values are mainly related to platform subenvironment, while OM types are controlled by distance from the shoreline. Organic facies dominated by structural terrestrial OM occur closer to the shore, whereas toward the shelf margin marine and amorphous OM contents increase. Key words:
organic facies, carbon, protokerogen, Rock-Eval, reefs, carbonate sediments, Puerto Rico,
INTRODUCTION
algal mat collectedin LagartosLagoon nearEl Cuyo (Yucatan Peninsula,Mexico). This study showsthat, asin the caseof carbonate inorganic facies distributions, organic facies are a function of terrigenous influx and intrabasinal productivity. Depositional input is modified by the chemical and, mechanical breakdown of organic particles and their subsequentredistribution on carbonateplatforms.
The development of methods for predicting the quality and quantity of organic matter in the potential source rocks requires a knowledge of organic facies distribution in various source rock-forming environments. Organic facies may be characterized as mappable units of similar organic richness and organic matter type (Rogers, 1980). Studies of the organic facies properties of ancient and recent carbonate sediments have received little attention in the literature with notable exceptions of contributions by McKirdy et al. (1984) and Jones (1984). This is in sharp contrast to the economic potential of these rocks as hydrocarbon sources. Here the distribution and control of organic facies on the Puerto Rican (Fig. 1) and Belizian (Fig. 2) carbonate platforms is addressed. Both are excellent settings for determining what kind of organic facies patterns, if any, exist because they provide a variety of depositional subenvironments. Differences between the two settings are terrigenous influx, which is more substantial in Puerto Rico than in Northern Belize and the energy of the depositional environments. Circulation is less vigorous along the Belizian shelf due to the presence of a barrier reef in contrast to Puerto Rico where patchy reef development allows
METHODS
more vigorous water circulation. In addition, for comparison,a sedimentsamplewasanalyzedfrom an
The subaqueous sediment samples were collected using either SCUBA equipment or a Peterson grab sampler. In some casesa piston corer was used. The samples were stored in glass jars, treated with a bactericide (sodium azide) and frozen within 6 hr of collecting. Only the samples representing the top IOcm of the sediment are discussed here. The samples were subsequently analyzed for grain-size distribution and petrographic composition using standard sedimentological techniques. The TOC (total organic carbon) values were measured with a LECO analyzer. Organic matter types were determined using microscopic observations of isolated organic particles in transmitted light. Isolation of organic matter for optical studies was accomplished using the chemical method of the destruction of the mineral phase with HCl and HF; the liberated organic fraction was then concentrated by centrifuging in ZnBr,. Hydrogen and oxygen indices were determined using the Rock-Eva1 pyrolysis technique on whole sediment samples (Espitalie et al., 1977).
Total
*Author
organic
RESULTS
AND
carbon
(TOC)
DISCUSSION
The organic carbon content of carbonate sedi-
to whom correspondence should be addressed. 717
JANINA RAFALSKA-BLOCH
718
0
1
0
0.5
INNER
1
and ROBERT CUNNINGHAM
2, KILOMETERS <
PUERTO
RICO
ISMILES
SHE:;
_
.;, ENRIQUE
D
LAUREL MEDIA
LUNA l 1.s
TURRUMOTE
0
OUTER
SHELF
%i
/ l P REEF SUBMERGED +
SAMPLE
REEF LOCATION
Fig. 1. Studyareawith locationof sample sitesin southwestern PuertoRico(afterMorelocket al., 1977). ments on Puerto Rican and Belizian platforms is related to depositional environment, content of inorganic insoluble residuum of the sediment,and grain size (Table 1). The insular shelf off La Parguera, southwestof Puerto Rico, is 8-10 km wide. Reefson the shelfare alignedapproximatelyeast-westand are separatedby backreef and inter-reef lagoons (Morelock et al., 1977).The coast is dominated by mangrove forests (Fig. 1). The TOC values average3.5% for the sediments of mangrove-linedcoastal channelsand 2.79% for Phosphorescent Bay, a mangrove-borderedbay. Sedimentsassociatedwith Halimeda-Thalassia meadows contain between0.5 and 0.7% of TOC. Backreef and inter-reef lagoons sedimentsrange from 0.13 to 0.68% TOC. The TOC abundancesin inter-reef troughs, 0.08-0.24%, are characteristicallylow. This may be attributed to the patchy distribution of reefs
which allows relatively free water circulation. Low abundancesof TOC are also typical of reef crests, flats, and forereef areas.This is in spite of the fact that reefsare one of the most metabolicallyefficient of productive environments,and that their metabolismrate exceedsthat of grassmeadows(Odum et al., 1959). The secondlowest range of organic carbon values, from 0.1 to 0.3%, occurs in the outer-shelf province and shelf-edgereef. The scarcity of organic material is a reflection of low productivity and high depositional energy, factors which are detrimental both to accumulation and preservation of organic material. In summary, total organic carbon values for samplesfrom the insular shelf of southwestPuerto Rico are controlled by biological productivity and water energy which affects the rate of mechanical breakdown,chemicaldecomposition,and winnowing of organic detrius.
Organic facies in Recent carbonates
OCKY
719
POINT
NC “LLILL SHELF
LEGEND 17%’
---AXIS
OF LIMESTOI(E RIOOE
-C*RsOHATEl
SHOAL SAMPLlNG
SCALE NAUTICAL
The Northern Belizeshelf consistsof severalzones paralleling the coast (Fig. 2). The narrow body of water (l-2 miles)betweenthe barrier reefand the line of mangrove-sandcays is called the outer (reef) lagoon and the westernpart of the shelfis called the inner (shelf) lagoon (its averagewidth is 10 miles). AmbergrisCay, a carbonateisland,separatesthe two lagoons. The barrier reef-produced sediments(reef facies of Pusey, 1975)are depletedin organiccarbon (0.1l0.17%). Betweenthe wave-agitatedreef and the very low energycurrentsof the inner lagoon,intermediate energy conditions coexist with Halimeda facies. Presenceof water currents coupled with intense burrowing result in very low abundancesof TOC (0.14%). Ambergris Cay consists of several subenvironments. Only two of them, the supratidal flats and intra-island lagoons, were sampled.Lamination in supratidal sedimentsattests to an absenceof burrowing organisms.Periodical subaerialexposureof thesesediments,however, may have oxidized some of the organic matter (mainly algal mat). The TOC valuesrange from 0.5 to 0.8%. Intra-island lagoons, which have limited exchangewith the outer or inner
SITES
15 MILES
lagoons,accumulategray or black muddy sediments. Therefore, their TOC content, at 2.06%, is relatively high. Inner lagoon TOC values vary accordingto subenvironment.Valueswererecordedas follows: 0.4% closestto the mainland,0.8-5.03% in the vicinity of mangrove cays, 1.73-2.13% on mud shoals, and O&l-0.79% in other areasof the lagoon.The highest quantities of organic carbon occur in miliolid-mud facies.Much of the organic material, as previously observedby Pusey(1975),is in the form of mucus-like slime. Thus, aswasthe casein Puerto Rico, environments characterizedby high physicalenergy,i.e. barrier reef and outer lagoon, have low abundancesof organic carbon. In contrast, quiet and restrictedlagoonsare richer in organic carbon. TOC us sediment textures and composition
The energy level of the environment dictatesnot only the distribution of organic matter but also the distribution of inorganic particles. The relationship betweenorganicand inorganicaspectsof sedimentis expressedby plotting sedimentparametersagainst total organiccarbon and calculatingPearson’scorre-
720
JANINA RAFALSKA-BLOCH and ROBERTCUNNINGHAM Table
I. Total organic carbon contenl and general sedimentological
Sample Number Southwestern A 8 C 0 E F G H I J K
2
1 Puerto
of sediments data
3
4
Rico
S :
3.51 0.70 0.54 0.61 0.51 0.58 0.67 0.45 0.22 0.21 0.13 0.89 0.19 0.12 0.12 0.17 0.27 0.41 0.18 2.79 0.15
54.99 15.35 10.87 15.63 16.32 15.82 17.30 11.98 8.27 9.02 7.20 14.28 8.06 6.72 6.05 3.33 10.29 13.37 6.73 14.44 6.94
I-A !-B 1-c 1-o 1-E 1-F 1-L 1-M
0.08 0.10 0.21 0.24 o.i4 0.68 0.17 0.12
5.19 5.75 5.27 11.74 5.20 16.12 3.74 2.55
Northern
Belize
A 8
2.84 0.80
23.57 12.39
oc F
0.40 5.03 1.73 0.44
31.64 33.71 15.25 16.59
G H I J
2.13 0.79 0.80 0.50
12.77 15.08 14.89 20.67
: M N
2.06 0.14 0.17 0.11
18.36 9.03 12.74 7.90
2.64
24.23
.
s-
I1.00 3.50 3.12 5.05 5.62 5.38 5.96 3.51 2.43 2.73 4.00 7.29 1.60 2.22 0.76 0.21 1.80 4.12 -0.35 0.24 8.22 -0.07 1.01 -1.05 l:o 3.0
45.00 26.78 29.51 65.52 82.88 69.31 83.76 46.17 10.93 12.99 10.00 8.20 2.90 0.38 0.13 0.00 0.30 59.41 0.57 0.21 99.09 0.00 0.03 0.24 1.00 0.20 20.00
3-
.
.
. .Y”CATAN
2-
.
l-
.5-
.PUErlTO n 0ELIZf
. .
. d,, :I:. 10
. 20
30
% Insoluble 3.34 1.82 3.60 0.38 0.99 2.64 2.12 1.42 1.73 1.96 3.07 1.10 0.82 -0.20
18.77 9.00 63.65 0.27 0.53 21.39 33.26 1.97 8.27 3.64 37.73 0.29 9.51 0.07
3.10
1.73
lnco
I
I,,
40
50
, 60
70
10
Residuum
Fig. 3. Relationship betweenthe weightpercentof organic carbonand the content of inorganic insoluble residuum. organic matter doesnot show a direct influenceof biomass,a phenomenonthat has beenreported by others, for examplein the study examining seasonal variations in organic matter concentrations on an intertidal flat in Maine (Mayer et al., 1985). 5-
Yucatan A 1 2 3 4 -
total organic carbon mineral, HCl-insoluble mean grain size (phi grains smaller than not determined
4-
(X) residuum (%) units) l/16 mm (weight X)
AYUCAYAN *PUERTO . BELIZE
3-
RICO
. .
lation coefficients (r). The amount of organic carbon in the Puerto Rican samples increases both with the content of mineral insoluble residuum (r = 0.94; Fig. 3) n . 2and with decreasinggrain size of sediment(r = 0.55; Fig. 4). However, correlation betweenTOC and the . content of silt and clay fractions (r = 0.18; Fig. 5) is poorer. Adsorption of organic matter onto clay minerals, therefore, cannot explain an association lbetween TOC and insoluble residuum. Quartz in Puerto Rican sedimentsclustersaround the 3.54.04 .s grain size range (Breyer and Ehlmann, 1981).Interestingly, high TOC contentsappearto be associated with the samegrain sizerange(Fig. 4). It seems likely that most organic carbon is presentin organic particles that are capable of surviving transport from Fig. 4. Relationship between the grain size of the sediments land with quartz. In this respect,the abundanceof the and their organic carbon content.
Organic facies in Recent carbonates
from land plants, this material will be darker yellow to amber color than if derived from marine algae. Under reducingconditions,however, the product of biodegradationis colorlessor gray, whetherderived from marine or non-marine environments.Finally, “structured aqueousmatter” includesthe unaltered remainsof marineorganismssuchasalgae,liningsof foraminifera, and marine grasses. Estimatesof the different organic matter types weremadeon sampleswith a fair amount of organic matter. The organic matter contained on the slides from the Belize sampleswas too sparseto enable percentageestimatesto be made (Table 2). Particulate organic matter in Puerto Rican sediments consistsof structured terrestrial material in the form of woody tissues,epidermal tissueswith well preservedstomata,and shorelinegrassremains.
8,
. .
. BELIZE . PUERTO RICO .YUCATAN .
.
.
, :.
.
.
l
:= t,
l
.
:
.
.
Table
2. Tvoes
of
Darticulate organic
matter
.,
IO
,
20
% Fines
30
40
50
I
60
10
80
(<‘/SW+)
Fig. 5. Relationship betweenthe weightpercentof fines (grainsize< l/16 mm)and organiccarboncontentof the sediments. In contrast, the percent TOC in Belizian samples correlateswith the percent of fines(r = 0.93; Fig. 5), meangrain size (r = 0.85; Fig. 4), but not with the percent of insoluble mineral residuum (r = 0.58; Fig. 3). Thus the bulk of organic matter is associated with fine-grained carbonates,produced in situ. As previously discussed,the miliolid mud shoal, which supportsrich, highly productive communities,is the main source of organic carbon. The cohesiveness of sedimentsand lack of bioturbation in this subenvironment enhance the preservation of organic matter. Organic
721
matter types
Masran and Pocock (1981) have designeda simple classification of plant-derived particulate organic matter. It is comprised of several general classes delineated on the basis of origin and degree of alteration of organic particles. For example, the “structured-terrestrial” OMT includesplant remains (mostly wood fragments)that, althoughmechanically cornminuted still retain their cellular structure and are not significantlyaltered by biodegradation.“Biodegraded-terrestrial material” designates organic matter that has undergoneintense biodegradation but is still discernible as land-derived in origin. “Charcoal” is the product of the oxidation of structuredmaterials,mainly of stemand root tissues. Some workers, e.g. Cope (1981), maintain that charcoal is the product of forest fires. “Amorphous material” is completely structureless.When derived
Sample Nunber
ParticFlate ST
Southwestern A E 0 E i H I J K
10.0 i:: 5.0
:-05 1o:o -
2+5 1:5 -
so
l
k
:
-
;
-
_
10.0 -
-
SM
20.0 19.0 28.5 35.0 25.0 38.0 32.0 + + +
20.0 24.0 21.0 10.0 15.0 12.0 19.0
22.0 15.0 5.0 38.0 26.5 21.0
+
+
21.0 +
79fo
32fO
+
+
Northern
Belize
A
1.0 5.0 -
20.0 30.0 24.5 30.0 18.5 14.0 17.0 + + + 2OfO + + +
50fo
10.0
17.0 35.0
30.0 24.0 10.0
: ,“A 1-F 1-L
(X)
Rico
;:“5 15.0
13fo 10.0 -
Ma;t$r
PS Puerto
30.0
OrEnit
3oro 50.0 25.0
20fo +
1oro
30.0 +
+ +
: H I J K
+ _ + -
_ + -
:
:
-
+ +
-
-
++
: E
+ +
+ +
+ + +
+ +
Yucatan A ST PS C hII GA SM +
structured terrestrial oollen and soores kharcoal andrsemi-charcoal amorphous gray amorphous structured marine not determined determined. but too sparse
to estimate
the
percent
122
JANINA
RAFALSKA-BLOCH
and
Although pollen and sporeswere found in most samples,charcoal and semi-charcoaldominate the land-derivedparticles.The well-preservedstomataor pits observedin charcoal are likely to have resulted from fire. Theseforms of organic particles account for 15% of the total organic matter and 53% of the terrigenousinput. Terrestrial plant remainsaccount for 28% of the organic matter. Structured marine material and marine-derived amorphous matter occur in all samples. The structured marine material consistsof cysts, algal spores,calcareousalgal remains,seaweedtissuesand foraminifera linings. The amorphous matter is a mixture of non-descript yellow and grey material. The terrigenousimprint on OMT in Belizeis minor and restrictedto somewoody tissues(found on small mangrovecays), shorelinegrassremains(on supratidal flats), a few pollen and spores(on carbonate shoals),and somecharcoal or semi-charcoal. Structured marine material is present in most samples.It includesthe remainsof calcareousalgae, seaweedtissues, cysts, and foraminifera linings. Finely disseminatedamorphousmaterial is ubiquitous, most being a light grey-yellow color. Samples from mud shoalsappear to contain fecal pellets. Pyrolysis characterization
The Rock-Eva1 pyrolysis is consideredinadequate for oxygen-rich organic matter, especiallyin carbonate sediments.Indeed, the values of oxygen indices of organic matter from Puerto Rican and Belizian samples are very high and dispersed, reflecting perhapsthe matrix effect (Table 3). Therefore, since T,,,, is influencednot only by maturation (negligible
Table
3. Results
Sample NUllbPl-
of Rock-Ed HI
Southwestern
analvsis
01
Puerto
ToFAX Rico
A
428
829
417
CB 0 F
1685 1535 1804 1634
u 1-o
414 375 324 346 324 300 328 250 270 508
1-F
391
413 414 416 421 423 409 418 414 422 405 435
H I
J L
Northern A C 0 F G
I J K L
1515 2477 2004
1329 960 2237 1457
Eel ize 878 780 172 874 984 750 572
714
918
1137
550
3307
433 430 430 431 430 429 426 430 411
990
1269
434
917 2052 1143
1357 1465 2014
Yucatan A
ROBERT
CUNNINGHAM
Fig. 6. Organicmatter typesdeterminedfrom hydrogen indicesandmaximumtemperature of Rock-Eva1 pyrolysis.
for recent sediments)but also by the type of organic matter (Tissotand Welte, 1984),the hydrogen indices (HI) were plotted againstthe T,,,,, valuesrather than the oxygen indices(Fig. 6). Organic matter from Belize sedimentsis much more hydrogen-rich [HI ranges from 172 to 878 (mg hydrocarbons/gTOC), with the meanHI = 7191 than organic matter from Puerto Rico sediments (HI rangesfrom 250 to 508 with the meanHI = 355; Fig. 6). A plot of HI vs T,, (Fig. 6) indicatesthat BelizesamplesclusterbetweenType I and Type II of sedimentaryorganic matter (is characteristicof algal and/or microbial origin). The Puerto Rico samples plot betweenTypesII and III implying mixed marineterrestrial provenance. For comparison, hydrogen index of algal mat from Yucatan is 990, which is suggestiveof Type I protokerogen. Organic facies and dktance from shoreline
It is evident that there are two major types of organic faciesin thesecarbonateenvironments.One is dominatedby the mixture of structured terrestrial plant remains,pollen and spores,and charcoal. Another has a more amorphousor marine structured character. Both types of organic faciesare found in the sedimentsfrom Belize and Puerto Rico. It is usuallyassumedthat most of the terrigenousmaterial is trapped in the coastal zone. However, a plot of marine organic matter vs distance from shoreline revealsthat substantialamountsof the land-derived organic pdrticlespersistfrom one-third to one-half of the extent of the Puerto Ricancarbonateshelf,i.e. for about 5 km (Fig. 7). The percentageof marine organic matter is derived from optical microscopic studies. The persistenceof terrestrial organic materialsin reef environmentsoff the shore of southwestPuerto Rico may be related to one or more factors. A large influx of resistantorganic matter suchascharcoalor semi-charcoalis supplied from the adjacent, highrelief mainland. Alternatively, the lack of a strong
Organic facies in Recent carbonates
723
l
0 SHORELINE
.25
.5
PUERTO RICO BELIZE YUCATAN
35
1 PLATFORM MARGIN
RELATIVE
DISTANCE
ACROSS
PLATFORM
Fig. 7. The transitionbetweenterrestrialandmarineorganicfacieson carbonateplatforms. hydrographic barrier resultsin high-energytransport and deposition conditions which cause the mechanical and chemical degradation of labile organic materials.The sameconditionscontribute to reworking and spreadingof the resistantorganic particles over the shelf. The transition from terrestrial or terrestrial-mixed organic faciesto marine organic faciesoccursover a much shorter distanceon the Belize carbonateplatform than on the Puerto Rico carbonate platform. The rapid disappearanceof terrestrial organicmatter in Belizian sediments(at about 1/4th of the platform distance)(Fig. 7) is causedby a sparsesupply of terrestrial material. The adjacent land is of low relief and has very low surfacerunoff. The barrier reef restricts water agitation in the lagoons very effectively, thus limiting the extent of chemicalor mechanicalbreakdown and the possibtlity of transporting any land plant fragmentsvery far from the shore. SUMMARY
The abundancesof organicmatter in sedimentsof carbonateplatforms arestrongly relatedto individual subenvironments.Higher valuesof TOC are characteristic of sedimentscollected from mangrove-lined coasts, algal-grassmeadows,mud shoals, lagoons and tidal flats. Whereastheserelationshipsare true both for Belize and Puerto Rico, Puerto Rican sedimentsin generalshowa depletion in organiccarbon for any given subenvironment,relative to Belize. Thesetwo areasalso differ in the relative proportions of organic matter types present.Land-derived (particularly charcoal) organic particles are prominent in Puerto Rican reef settings,whereasin Belize organic matter is dominated by the marine and amorphousorganic types. Sedimentaryorganic par-
ticlesfrom Belizeare more hydrogen-richthan those from Puerto Rico. In both Belizeand Puerto Rico the influx of terrigenousorganic matter types diminishes with distance from land, however, this transition takes placemuch more rapidly in Belize. The differing distribution of organic faciesin the carbonateplatform environmentsof Puerto Rico and Belizeis a function of the complex interplay between source terrain, mode of transport, energy level, depositionalconditions,and preservationpotential of any given area. The substantialrelief of the Puerto Rican hinterland results in an increasedinput of land-derivedplant material. In addition, the patchy distribution of reefs off Puerto Rico permits free circulation of water. Organic particles are thus continually subjectedto processes of transport, oxidation, or mechanicalbreakdown.The low relief of the Belizemainlanddoesnot provide a significantsource of terrestrial organic particles. Additionally, the presenceof a barrier reef and largecarbonateislands createa quiet, restrictedenvironment, with excellent preservationpotential for organic matter. Acknowledgements-This researchwascarried out with support from the American Chemical Society, Exxon Research and Production Company, ARC0 Research Foundation, Mobil Foundation, Olympics Exploration, NSF Grant EAR-8352055 (Dr M. H. Engel), the Energy Resource Center, and the School of Geology and Geophysics of the University of Oklahoma. The Department of Marine Sciences at the University of Puerto Rico, and the United States Geological Survey at San Juan, Puerto Rico, kindly provided one of us (JRB) with accessto their research vesselsand labs. Several individuals extended their help. The prime acknowledgement goes to Th. C. Masran, Dr B. Silver, Dr J. Morelock, and K. R. Hahn. I am also very grateful to Dr L. Ciereszko, S. Imbus, L. Hyman, G. Stewart, M. Stearns, D. Gilbert and Dr C. P. Summerhayes. Many thanks to M. Starr for typing the manuscript and J. Pianalto for drafting.
JANINA
124
RAFALSKA-BLOCH and ROBERT CUNNINGHAM
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