Age of the Precambrian Roraima Formation in Northeastern South America: Evidence from Isotopic Dating of Roraima Pyroclastic Volcanic Rocks in Suriname

Home , Roraima

H.N.A. PRIEM N.A.I.M. BOELRIJK Z.W.O. Laboratorium voor Isotopen-Geologie, De Boelelaan 1085, E. H. HEBEDA Amsterdam-11, The Netherlands E.A.Th. VERDURMEN R. H. VERSCHURE

ABSTRACT Brazil, and Guyana join. The Roraima Forma- tion stands out in the landscape as impressive, Pyroclastic acidic volcanic rocks are inter- rugged, and forest-covered plateaus bounded calated between sedimentary rocks of the by spectacular cliffs. Present-day outliers of the Roraima Formation at the Tafelberg in formation are scattered over a total area of Suriname. From these rocks, a 14-point whole- some 1,200,000 sq km in Venezuela, Guyana, rock Rb-Sr isochron of 1,599 + 18 m.y. was Brazil, and Suriname (Fig. 1). It is impossible obtained with initial 87Sr/86Sr = 0.7075 ± to decide whether these occurrences represent 0.0032 (47.2 X 109 yrs 87Rb half-life; errors erosional remnants of a once-continuous cover with 95 percent confidence level). K-Ar or sediments deposited in a number of isolated measurements on five samples give dates rang- basins. ing from 1,573 to 1,681 m.y. Some 200 m.y. In the area of Mount Roraima, Gansser have thus elapsed between the consolidation of (1954) divided the Roraima Formation into the Guiana Shield during the ' 'Trans-Amazo- three members, that is, the Upper Member nian orogeny" 1,810 + 40 m.y. ago and the (thick-bedded sandstone horizons forming development upon it of the Roraima basin (or spectacular cliffs), the Middle Member (char- basins) of deposition. Isotopic age measure- acterized by frequent intercalations of "jasper" ments on dolerite dikes and sills cutting the beds), and the Lower Member (including the Roraima sedimentary rocks (the Roraima basal conglomerates, if present, which are Intrusive Suite) indicate that the basalt thought to be the source of the diamonds found intrusions took place within a relatively short in river deposits derived from them). A similar time interval (some 50 m.y. at most) after the division is adopted in the Pakaraima Moun- deposition of the Roraima Formation. tains in western Guyana (McConnell and Williams, 1971). The Roraima Formation, INTRODUCTION which reaches its maximum thickness of about The Precambrian Roraima Formation over- 2,400 m in the Auyan Tepui in southeastern lies some 450,000 sq km of crystalline basement Venezuela, is predominantly a monotonous in the western part of the Guiana Shield, the series of well-bedded quartz sandstones. Judging nucleus of South America north of the Amazon. from their sedimentary characteristics (cross- The formation is made of a tabular sequence of beddings, ripple marks, slumping features, and flat-lying, unmetamorphosed, mainly arena- so forth), they represent mainly stream and ceous sedimentary rocks intruded by dolerite delta deposits laid down in continental to sills and dikes. It is named after Mount epicontinental environments. Roraima, the highest elevation (2,810 m) in In Suriname, the Roraima Formation the whole Guiana Shield, where Venezuela, occupies some 280 sq km in the area of the

Geological Society of America Bulletin, v. 84, p. 1677-1684, 3 figs., May 1973 1677

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VENEZUELA GEORGETOWN ATLANTIC OCEAN

GUYANA PARAMARIBO

SURINAME

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66" 62" 58 Figure 1. Areas of the Guiana Shield covered by 2,400 m in the Auyan Tepui. In the Tafelberg area, the Roraima Formation. Named after Mount Roraima, the formation is up to 7(13 m thick. the formation reaches its maximum thickness of about Tafelberg (Dutch for "Table Mountain") in 1963; McDougall and others, 1963; McDougall, the central part of the country, while another, 1968) and in Suriname (Priem and others, very small remnant has been preserved below 1968a) indicated that their deposition took an inclined dolerite sheet in the Emmaketen place in Proterozoic time. The best approxima- (Dutch for "Emma Range") north of the tion for the age of t lis basaltic magmatism is Tafelberg (IJzerman, 1931; Bisschops, 1969; given by the Rb-Sr isochron date of 1,593 + 66 Fig. 2). These are the easternmost occurrences m.y. obtained on whole rocks and separated of the Roraima Formation (Fig. 1). A: the minerals in western Guyana (Snelling and Tafelberg (altitude 1,026 m), where the McConnell, 1969). This date also sets a sequence of Roraima sediments reaches a minimum age for the time of Roraima sedi- thickness of some 700 m and forms spectacular mentation. The maximum age is firmly escarpments, the formation is predominantly established by Rb-Sr isochron investigations a tabular succession of white- to pink-colored in Suriname on granitoid and acidic volcanic bedded sandstones. Near its base, the sand- rocks of the underlying shield, that is, 1,810 ± stones contain variable amounts of feldspar 40 m.v. (Priem and others, 1971). From these and sericite (Bisschops, 1969). Conglomerate data it is clear that the Roraima sediments were layers, some of them several tens of meters laid down between about 1,800 and 1,600 m.y. thick, are interbedded in the upper 400 m of ago. the sequence. Thin horizons containing pyro- The horizons of pyroclastic volcanic rocks at clastic volcanic rocks (ignimbrites and tuffs) the Tafelberg offer an excellent opportunity are intercalated in the lower and middle parts for a direct radiometric dating of the time of of the series. An exposure recently blasted in Roraima sedimentation. The present paper the Elfriede Falls below the Tafelberg shows discusses the results of a Rb-Sr isochron study that the base of the Roraima Formation is made on whole-rock samples from these horizons, of sandstone containing sparse granitic pebbles together with a few K-Ar measurements. Also, overlying an irregular surface of granitic- a K-Ar date is reported of one whole-rock rhyolitic agglomerate. At the plateau of the sample from the dolerite dike cutting the Tafelberg, the Roraima sediments are cut by Roraima Formation at the Tafelberg. Two age a dolerite dike. measurements have been reported previously: For many years there has been considerable K-Ar and Rb-Sr measurements on a single speculation about the age of the Rcraima loose block of ignimbritic "porphyry-tuff" sediments, but isotopic age measurements on (Priem and others, 1968b). The sampling sites intrusive dolerites in western Guyana (Snelling, of the investigated rocks are shown in Figure

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2. Two sites (243 and 244) are blasted outcrops, supposedly being of predominantly sediment- to ensure that unweathered material was col- ary origin, the latter representing altered lected. pyroclastics. Bailey (1964) described "fossil- like objects" from chert and jasper beds in RORAIMA PYROCLASTIC Guyana, which, on a closer look (Allen, 1967), VOLCANIC ROCKS AT THE turned out to be typical pyroclastic shards TAFELBERG and bubbles of vesiculated glass. Similarly, a Although loose blocks of ash-flow tuffs are pyroclastic origin may be attributed to the frequent in conglomeratic river deposits below "spongilites" reported by Barbosa and Ramos the Tafelberg escarpments (they were already (1959) from jaspers in the Roraima Formation reported in 1931 by IJzerman), in situ outcrops in Brazil. of these rocks interbedded between Roraima Pyroclastic deposits thus seem to be wide- sediments have been found for the first time spread in the jasper-bearing Middle Member during two geochronological sampling expedi- of the Roraima Formation. A genetic relation tions in 1971. The highest pyroclastic horizon between volcanism and silica sedimentation is was observed at the plateau of the Tafelberg, generally accepted (for example, Krauskopf, approximately 430 m above the base of the 1959; Strakhov, 1967), so the Roraima vol- Roraima sequence (sampling site 244 in Fig. 2; canism probably supplied the silica for the site 27 probably lies in the same horizon). Its jasper beds that characterize this part of the thickness could not be established, but is prob- sequence. The occurrence of the horizon with ably in the order of a few meters. These pyro- ash-flow tuffs and associated thin chert beds clastic rocks (as well as loose block 240, which in the lower and middle parts of the Roraima may have been derived from the same horizon) Formation at the Tafelberg could suggest that are extremely fine-grained to cryptocrystalline they may be correlated with the Middle banded rocks with grayish, white, or reddish Member of the formation in the area of Mount colors. Their chemistry corresponds to a Roraima and in western Guyana. leucorhyolitic composition (fohannsen, 1932, p. 259). A conspicuous feature is formed by DOLERITE DIKE AT THE bubbles of original vesiculated glass (infilled with granular quartz or matrix material) up to TAFELBERG several millimeters in diameter. Due to its extremely fine grain size, only quartz, sericite, The dolerite dike cutting the Roraima sedi- some feldspars, and ore grains can be recog- ments at the Tafelberg has obviously to be nized under the microscope. Some bands of correlated with the doleritic-gabbroic sills typical ignimbritic character (shards, flow and dikes of the Roraima Intrusive Suite in structures, and so forth) display a rhombohedric Guyana and Venezuela, for which a Rb-Sr columnar jointing. Thin chertlike layers are isochron age of 1,593 + 66 m.y. has been intercalated between the tuffaceous beds. reported (Snelling and McConnell, 1969). K-Ar measurements on this suite, as well as Other thin tuffaceous layers were found at those on older gabbroic-doleritic intrusions in lower levels in the eastern escarpment of the western Suriname, often display the presence Tafelberg, roughly between 100 and 200 m of varying amounts of excess radiogenic argon. above the base of the Roraima Formation (sites This phenomenon is probably due to the 236 and 238 in Fig. 2). These tuffs are fine- incorporation of radiogenic argon expelled grained rocks of rhyolitic composition, con- from the basement rocks during a tectono- taining fragments of Roraima sediments (which thermal event 1,200 ± 100 m.y. ago, the were carefully removed before crushing and Nickerie Metamorphic Episode (Hebeda and pulverizing the samples). Priem, 1970; Priem and others, 1971; Hebeda The ash-flow tuffs of the Tafelberg resemble and others, in prep.). However, the Tafelberg very much some of the so-called jasper hori- area seems to lie just east of the area affected zons reported by Gansser (1954) from the by this event (Priem and others, 1971). Middle Roraima Member in southeastern K-Ar measurements were made on a single Venezuela, western Guyana, and the adjoining sample from this dike, a rock composed of

Brazilian territory. Gansser distinguished two calcic plagioclase (An8o-6o), augite (partly main types of jasper beds, that is, the "red uralitized), and some orthopyroxene, horn- jaspers" and the "green jaspers," the former blende, iron ore, chlorite, and quartz.

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~~4°00'N 4°00'N~~

56° i'w Figure 2. Geological sketch map of the Tafelberg the locations of the samples investigated. area, central Suriname (after Bisschops, 1969), showing EXPERIMENTAL PROCEDURES AND contents and Rb/Sr ratios by x-ray fluorescence CONSTANTS USED spectrometry. Three samples were also measured by standard isotope dilution techniques, 87 84 Splits of crushed and pulverized whole-rock employing spikes enriched in Rb and Sr. samples were analyzed for their Rb and Sr A Philips PW 1220 semiautomatic x-ray

~~1.400~~

~~1.300~~

~~1.200~~

~~7 /86 Sr/ Sr~~

~~1.100~~

~~1.000~~

~~0.900~~

~~0.800~~

~~0.700~~

~~87 /86 Rb/ Sr Figure 3. Whole-rock Rb-Sr isochron plot of the tion at the Tafelberg, central Suriname. (Half-life acidic pyroc stic volcanic rocks in the Roraima Forma- 87Rb = 47.2 X 10® yr.)~~

spectrometer was used, equipped with a 2-kw whenever isotope dilution analyses were made, Mo x-ray tube and a (200) LiF analyzing this ratio was also calculated from the isotope crystal. Mass absorption coefficients for both dilution run. The Sr ratios are normalized to sample and external standard were estimated 88Sr/86Sr = 8.3751 and adjusted to 87Sr/86Sr

by measuring the intensity of the Compton = 0.7081 in the Eimer and Amend SrC03 scattering of the Mo Ka primary beam. The standard. isotope measurements were made on a 20-cm The Rb-Sr isochron and the error were 60° Nier-type mass spectrometer with digital computed by means of a least-squares regression output, utilizing thermal ionization and analysis as the best-fitted straight line through multiplier detection. 87Sr/86Sr ratios were the 87Sr/86Sr - 87Rb/86Sr data points, follow- measured directly on unspiked strontium; ing the computation method of York (1966,

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~~TABLE 1. Rb-Sr DATA RORAIMA PYROCLASTIC VOLCANIC ROCKS, TAFELIiERG~~

~~Sample Rb/ir* (ppm wt). (ppm •7Rb/"6Sr e7Sr/e8Sr No. (wt/v.t) XR* IDt «R* XR» IDT~~

67 Sur 27 0.9534; 0.9536 3.51 82.2 82.5 ,!3.4 0.9517» 3.47 83.2 ;!3.9 71 Sur 236 0.7693 0.934 218 233 0.937 217 2:M 71 Sur 238 0.7381 0.427 133 313 0.432 135 31' 71 Sur 240/1 0.9826 3.82 87.£ 23.0 3.82 87.8 2:.o 71 Sur 244/1 1.354 8.82 143 15.3 8.73 142 15.3 71 Sur 244/2 1.329 8.27 117 14.1 8.50 119 13.9 71 Sur 244/4 1.275 7.90 114 K.4 7.80 114 K.5 71 Sur 244/5 1.387 9.35 156 16.5 9.37 155 16.4 71 Sur 244/7 1.419 9.85 151 15.3 9.61 150 15.7 71 Sur 244/8 1.151 6.04 73.5 75.7 12.2 12.8 1.500; 1.148s 6.11 74.0 75.0 12,2 12.7 71 Sur 244/11 1.321 8.32 138 16 5 8.38 139 16. 6 71 Sur 244/12 1.278 7.91 110 14.0 7.91 111 14.' 71 Sur 244/13 1.372 9.22 127 13.7 9.18 127 13.7 71 Sur 244/14 1.536 11.31 221 225 19.6 20.3 1.533; 1.539s 11.43 223 225 19.1 20.3

* x-ray fluorescence spectrometry. The directly determined Rb/Sr ratios are used in the calculations contents are rounded-off figures. + Isotope dilution. § Calculated from isotope dilution runs.

1967). To each pair of coordinates the relative All Rb-Sr dates quoted in this paper are weight was assigned based upon estimated based upon the above 87Rb decay constant. maximum relative errors of 0.5 percent and 2.0 percent for the measured 87Sr/86Sr and 87Rb/ RESULTS AND DISCUSSION 86Sr ratios, respectively. The errors for the The analytical Rb-Sr and K-Ar data are isochron age and the initial 87Sr/86Sr ratio are listed in Table 1 and Table 2, respectively. A quoted with 95 percent confidence limits as diagram plotting 87Sr/86Svr versus 87Rb/86Sr is calculated from the analytical data. shown in Figure 3. The following geochrono- K-Ar measurements were made on +125 to logical results were obtained: (1) the 14 samples 250ju sieve fractions of the crushed whole-rock of Roraima pyroclastic volcanic rocks are very samples. Potassium was analyzed by flame well linearly correlated in the 87Sr/86Sr- photometry with a lithium internal standard 87Rb/86Sr diagram. They define an isochron and a CsAl buffer. Argon was extracted in a of 1,599 ± 18 m.y. within an initial 87Sr/86Sr bakeable glass vacuum apparatus and deter- ratio of 0.7075 ± 0.0032. (2) K-Ar dates meas- mined by standard isotope dilution techniques 38 ured on five of the samples show a considerable (using Ar as a tracer) in a Reynolds-type spread, ranging from 1,573 to 1,681 m.y. (3) glass mass spectrometer; the measurements The sample from the dolerite dike cutting the were made by the static method. Roraima sediments gives a K-Ar age of 1,544 For the age calculations the following con- ± 50 m.y. stants were used: From the Rb-Sr data it is evident that the 87Rb : X^ = 1.47 X lO-^/yr; Roraima pyroclastic volcanic rocks in the 40 K : Ae = 5.85 x 10-»/yr; Tafelberg area were erupted 1,599 ± 18 m.y. 87 86 X(5 = 4.72 X 10~10/yr; and ago. The initial Sr/ Sr ratio lies in the abundance 40K = 0.0118 atom percent total K. normal range of magmatic rocks. As these

TABLE 2. K-Ar DATA RORAIMA PVROCLASTIC VOLCANIC ROCKS AND DOLERITE, TAFELBERG Intrusive Suite in Guyana. This date is fairly close to the age of the Roraima Formation, so there can have been only a relatively short time interval between the deposition of the Roraima sediments and the intrusion of the basaltic magma. •r- O U T3 E» >- rO TO Regarding the geological history of the

Pyroclastic volcanic rocks Guiana Shield, it may thus be concluded that after the consolidation of the shield (during 1.78; 1.79 0.310; 0.315 8; 5 1,592 66 Sur 27 the Trans-Amazonian orogeny, 1,810 + 40 71 Sur 244/1 2.89; 2.89 0.501 3 1,573 m.y. ago) some 200 m.y. elapsed before the 71 Sur 244/2 2.37; 2.37 0.454 3 1,681 continental to epicontinental basin (or basins) 71 Sur 244/12 2.20; 2.19 0.417; 0.415 3; 1 1,669 71 Sur 244/14 4.69; 4.69 0.832 2 1,596 of deposition developed in which the sedimentary sequences of the Roraima Formation were Dolerite dike laid down (very close to 1,600 m.y. ago). This 71 Sur 243 0.850; ; 0.849 0.144; 0.143 5; 3 1,544 precludes an interpretation of the Roraima sediments as representing a stage of post- * Maximum analytical error estimated at ±3 percent. orogenic ("molasse-type") sedimentation in the Trans-Amazonian orogenic cycle. Volcanic volcanic rocks are interbedded between the eruptions of acidic magma, in part having an Roraima sedimentary rocks, it is clear that an ignimbritic character, were taking place age very close to 1,600 m.y. has to be attributed contemporaneously with the Roraima sedi- to the deposition of the Roraima Formation. mentation. Dikes and sills of basaltic magma intruded the Roraima Formation within a Three K-Ar dates of the pyroclastic vol- relatively short time interval (some 50 m.y. at canic rocks are fairly well concordant with the most) after its deposition. Rb-Sr isochron age, but two K-Ar dates are significantly higher. This phenomenon is difficult to explain. Presence of excess radiogenic argon is highly improbable in these ACKNOWLEDGMENTS -11 rocks. Use of the value 1.39 X 10 per yr for This study was made in co-operation with 87 the Rb decay constant instead of the value the Surinam Government Geological and -11 1.47 X 10 per yr applied in this paper, Mining Service (GMD). The authors are which would raise the Rb-Sr isochron age to particularly indebted to their colleagues E. H. 1,695 m.y., could give amenable results: the Dahlberg and L. Krook (both with GMD) two high K-Ar dates would then be concordant and A. C. Tobi (Utrecht University) for con- with the Rb-Sr isochron age, while the lower tributing to this investigation through their K-Ar dates could be held to indicate some argon advice and constructive discussions. Krook loss. However, there still is a strong division and Tobi also accompanied two of us (Priem 87 of opinion concerning the correct Rb half- and Verschure) in the 1971 Tafelberg expedi- life, and both decay constants are presently in tion, organized by GMD, during which most general use in geochronology. All direct of the samples used in this study were collected. physical measurements point to the higher Thanks are due to S. A. Bouterse (GMD), who decay constant, whereas evidence from geo- supervised the expedition team and took some logical sources is conflicting (see the discussion samples in the eastern escarpment of the by Lambert, 1971). For the time being, the Tafelberg during a reconnaissance expedition present authors therefore adhere to the higher earlier in 1971, and to G. J. Keijzer and 87 Rb decay constant, although it is then not J.F.H.D. van der Plas (Royal Netherlands possible to combine satisfactorily all K-Ar Army) for blasting some sampling outcrops. dates and the Rb-Sr isochron age of the The preparatory work for the Tafelberg Roraima pyroclastics. expedition by P. den Hengst (GMD) is The K-Ar age of 1,544 ± 50 m.y. measured sincerely appreciated. The computer calcula- on the dolerite dike accords with the Rb-Sr tions for the isochron analysis were made at the isochron age of 1,593 + 66 m.y. reported by Mathematical Centre, Amsterdam. This work Snelling and McConnell (1969) for the forms part of the research program of the gabbroic-doleritic sills and dikes of the Roraima "Stichting voor Isotopen-Geologisch On-

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derzoek," supported by the Netherlands McDougall, I., 1968, Isotopic dating of Roraima Organization for the Advancement of ?ure dolerites: Royal Astron. Soc. Geophys. Jour., Research (ZWO). v. 16, p. 141-145. McDougall, 1., Compston, W., and Hawkes, D. D., REFERENCES CITED 1963, Leakage of radiogenic argon and stron- Allen, J. B., 1967, Pyroclastic origin of supposed tium from minerals in Proterozoic dolerites microfossils in the Roraima Formation, from British Guk.na: Nature, v. 198, p. Guyana: Nature, v. 215, p. 1261-1262. 564-567. Bailey, P.B.H., 1964, Possible microfossils found in Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E. H., the Roraima Formation in British Guiana: and Verschure, R. H., 1968a, Isotopic age Nature, v. 202, p. 384. determinations on Surinam rocks, 3: Proter- Barbosa, O., and Ramos, J. R. de A., 1959, Ter- ozoic and Permo-Triassic basalt magmatism in ritorio do Rio Branco: [Brazil] Div. Geologia the Guiana Shield: Geologie en Mijnbouw, v. e Mineralogia Bol. 196, 46 p. 47, p. 17-20. Bisschops, J. H., 1969, The Roraima Formation in Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E. H., Surinam: Proceedings of the 7th Guiana Verschure, R. H., and Verdurmen, E.A.Th., Geological Conference (Paramaribo, 1966): 1968b, Isotopic age determinations on Surinam Kon. Ned. Geol. Mijnbouwk. Genoot., Ve:h., rocks, 4: Ages of basement rocks in north- Geol. Ser., v. 27, p. 109-118. western Surinam ard the Roraima tuff at Tafelberg: Geologie en Mijnbouw, v. 47, p. Gansser, A., 1954, The Guiana Shield (S. America), 191-196. Geological observations: Eclogae Geol. Kel- vetiae, v. 47, p. 77-112. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E. H., Hebeda, E. H., and Priem, H.N.A., 1970, Geo- Verdurmen, E.A.Th., and Verschure, R. H., chronology and K-Ar studies of the I're- 1971, Isotopic ages of the Trans-Amazonian cambrian doleritic-gabbroic intrusions in acidic magmatism and the Nickerie Meta- Suriname: Amsterdam, Ann. Prog. Rept. morphic Episode in the Precambrian basement 1970, ZWO Lab. voor Isotopen-Geologie of Suriname, South America: Geol. Soc. (Internal Rept.), p. 21-34. America Bull., v. 82, p. 1667-1680. IJzerman, R., 1931, Outline of the geology and Snelling, N. J., 1963, Age of the Roraima Forma- petrology of Surinam (Dutch Guiana) tion, 3ritish Guiana: Nature, v. 198, p. 1079. [D. Sc. thesis]: Utrecht, Utrecht Univ., 519 p. Snelling, N. J., and McConnell, R. B., 1969, The Johannsen, A., 1932, A descriptive petrography of geochronology of Guyana: Geologie en the igneous rocks, Vol. 2, The quartz-bearing Mijnbouw, v. 48, p. 201-213. rocks: Chicago, The University of Chicago Strakhov, N. M., 1967, Some aspects of the geo- Press, 428 p. chemistry of silica: Ir ternat. Geology Rev., Krauskopf, K. B., 1959, The geochemistry of silica v. 9, p. 25-30. in sedimentary environments, in Silica in York, D., 1966, Least-squares fitting of a straight sediments: Soc. Econ. Paleontologists and line: Canadian Jour. Physics, v. 44, p. 1079- Mineralogists Spec. Pub. 7, p. 4-19. 1086. Lambert, R.St.J., 1971, The half-life of "Rb, in 1967, The best isochron: Earth and Planetary The Phanerozoic time-scale, a supplement: Sci. Letters, v. 2, p. 479-482. Geol. Soc. London Quart. Jour., v. 5, p. 27-29. McConnell, R. B., and Williams, E., 1971,Distribu- tion and provisional correlation of the Pre- MANUSCRIPT RECEIVED BY THE SOCIETY JULY 25, cambrian of the Guiana Shield: Georgetown, 1972 Proceedings of the 8th Guiana Geol. Conf, REVISED MANUSCRIPT RECEIVED NOVEMBER 17, (1969). 1972

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