Age of the Precambrian Roraima Formation in Northeastern South America: Evidence from Isotopic Dating of Roraima Pyroclastic Volcanic Rocks in Suriname
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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 Age of the Precambrian Roraima Formation in Northeastern South America: Evidence from Isotopic Dating of Roraima Pyroclastic Volcanic Rocks in Suriname 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/5/1677/3443505/i0016-7606-84-5-1677.pdf by guest on 28 September 2021 1678 PRIZM AND OTHERS VENEZUELA GEORGETOWN ATLANTIC OCEAN GUYANA PARAMARIBO SURINAME ^ - a® ^ _ aa <S W BOA VISTA t BRASIL RORAIMA FM. 200 km 1 Mt. RORAIMA 2 AUYAN TEPUI 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/5/1677/3443505/i0016-7606-84-5-1677.pdf by guest on 28 September 2021 RORAIMA FORMATION IN NORTHEASTERN SOUTH AMERICA 1679 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.