Paleogeographic and tectonic implications of the first paleomagnetic results from the Middle–Late Cambrian Mesón Group: NW Argentina

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Paleogeographic and tectonic implications of the first paleomagnetic results from the Middle–Late Cambrian Mesón Group: NW Argentina

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  TECTONICS, VOL. 5, NO. 6, PAGES 837-854, OCTOBER 986 PALEOGEOGRAPI•C AND TECTONIC IMPLICATIONS OF QUARTZOSE SANDSTONES OF BARBADOS D.C. Kasper and D. K. Larue Department of Geology, Stanford University, California Abstract. The provenance of Paleogene sandstones n the island of Barbados is relevant to various models of the tectonic evolution of the southern Caribbean area. Modal point counts of 26 sandstones rom Barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin. Abundant detrital polycrystalline quartz grains have affinities to a spectrum of low- to high-rank metamorphic/plutonic crystalline source rocks. Microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances. The complex heavy-mineral suite of the terrigenous sandstones, ncluding glaucophane, chloritoid, titanire, zircon, and aluminosilicates, supports derivation from orogenic and cratonal terranes. Due to the complex history f ihe South American-Caribbean late boundary and poor age constraints on timing of deposition, widely contrasting models have been proposed concerning he provenance of the sandstones. Convergence between the Aruba-Orchila arc and continental South America during the Paleogene resulted in the formation of an E-W trending foreland fold-thrust belt and foredeep. Dominant sediment dispersal was to the east-northeast along tectonic strike. The present study shows that the terrigenous sandstones on Barbados probably were deposited n deep-sea ans to the north of present-day Araya Peninsula during and shortly after the late middle Eocene. Probable source terranes include the Guayana 1Now t Exxon USA, Houston, exas. Copyright 1986 by the American Geophysical Union. Paper number 6T0310. 0278-7407 86/006 T-0310510.00 Shield, he Caribbean Mountain System/interior oreland fold-thrust belt, and possibly to a lesser extent) the Lesser Antilles magmatic arc or predecessor rc. INTRODUCTION Poorly dated Paleogene quartzose turbidires on Barbados are assumed to have a cratonal South American provenance Senn, 1940; Chase and Bunce, 1969; Speed, 1979; Velbel, 1980; Speed and Larue, 1982; Pudsey and Reading, 1982; Westbrook, 1982; Baldwin, 1984; Baldwin et al., 1986]. However, the complex history of the Caribbean-South American plate boundary poses problems for correlation of Paleogene sandstones on Barbados to specific source areas on the mainland. Two models have been published •vhich attempt to tie the Paleogene turbidires of Barbados to cratonal South Aanerica. n model I (Figure la), proposed by Dickey [1980] and Burke et al. [1984], a Paleocene o middle Eocene proto-Orinoco River system flowed to the north and northwest from the Guayana Shield to a delta-fed deep-sea an near presesnt-day Lake Maracaibo in western Venezuela. Portions of the delta and deep-sea fan complex of the proto-Orinoco River system were incorporated nto the subduction complex of the Lesser Antilles magmatic arc and translated 1500 km eastward to its present geographic position. This model requires large post-Eocene dextral strike-slip offsets which are seen neither onshore nor offshore but which may be masked by the South Caribbean deformed belt lying offshore of northern Venezuela [Dewey and Pindell, 1985]. In model 2 (Figure lb), advocated by Velbel [1980], Pudsey and Reading [1982], and Westbrook [1982], he mouth of the proto-Orinoco iver system was located north of its present position on the northeastern passive margin of the continent during the Paleocene to  838 Kasper and Larue: Provenance f Barbados Sandstones A MODEL I (Barbados) e •'--• m• (Colombia) aribb ste mE ' '• (Trinidad) (Lak Maracaibo) (V 0 ø o Guayana Shield B MODEL •. (Barbados) (Colombia) , (Lake Maracaibo) (Venezuela) o 0 o • Guayana hield cz 0 l•'ig. 1. (a) Generalized model of Dickey [1980] suggesting arge-scale extral strike-slip ffset of Paleogene terrigenous sandstones on Barbados relative to South America on the Oca-San Sebastian-E1 Pilaf fault system. Deep-sea fan complex fed by large deltaic complex near present-day Lake Maracaibo at approximately 0 Ma. Inferred source e,','anes: ndean Central Cordillera and Guayana Shield. (b) Generalized odel of Velbel [1980], Pudsey nd Reading 1982], and Westbrook 1982] advocating delta-fed deep-sea an system generated off northeast, rn South America at approximately 50 Ma. Inferred source erranes: Caribbean Mountain System, Guayana Shield, and (possibly) sland arc. Present-day South American coastline and Barbados drawn in for reference.  Kasper and Larue: Provenance of Barbados Sandstones 839 8O 7O 6O I I i I 2O 10- _Jamaica • '•::.Hispaniola •"'•.. Puerto ico '•' CARIBBEAN EFORMCU • Gulf of ß •;•'• •:;' • ,• 2 .;•.• '/ • Venezuela • ::. • • e o ..:. • • • • ß •'•• • . • • • • SOUTH M R CAN A E .... m l. "' • ' •"/• •L'• ¾. u , • u .{ , •,,o ere .- 2O 10 80 70 60 Fig. 2. Regional setting of Barbados and possible present-day ectonic configurations of Oaribbean, South American, nd North American plates after Jordan 1975], (ellogg and Bonini 1982], Case et al. [1984], and Dewey and Pindell [1985]. Allochthonous aribbean Mountain System llustrated n diagonal ars. middle Eocene. Detritus from the low-lying Guayana Shield was shed to the north through a delta-fed deep- sea fan complex. Subsequently, the deep-sea fan was accreted into the subduction copmplex of the Lesser Antilles magmatic arc during the late Eocene. This model suffers from a lack of any exposed stratigraphic evidence, suggesting the presence of a large deltaic system in northeastern South America during the early Paleogene. It also fails to explain the petrography of Barbados sandstones, s discussed n this paper. Based on detrital zircon ages, Baldwin 1984] nd Baldwin et al. [1984] suggest a paleogeographic cenario which resembles model 2 but assumes younger, Oligocene age of deposition of Barbados sandstones. Although petrographically sound, this model has significant problems. Dominant sedimentary dispersal systems n the Oligocene probably developed to the E-NE along tectonic strike (H. Healberg, personal communication, 1985), not to the north as illustrated by Baldwin et al. [19861 Based on the provenance signature of Paleogene sandstones of Barbados and northern South America and ancillary paleogeographic information concerning northern South America, we propose a third tectonosedimentary model linking Barbados to specific source provinces on the mainland. GEOLOGIC SETTING The island of Barbados is the only subaerial exposure of the massive arbados idge complex see Figure 2), a prominent 20-km thick N-S trending physiographic feature, thought by many authors to be the accretionary complex f the Lesser ntilles magmatic rc [Speed nd Larue, 1982; Westbrook, 1982; I•{oore and Biju-Duval, 1982]. The subduction omplex, omposed f deformed low-velocity material Chase nd Bunce, 1969], generally thickens to the south toward the continental shelves of Trinidad and northeastern Venezuela. In outcrop and in the subsurface, Barbados displays a three-tiered stratigraphic architecture defined by Speed and Larue [1982]). A thin, weakly deformed Pleistocene coral cap blankets an intermediate zone of deformed nappes consisting predominantly of Upper Eocene to Miocene deep-marine pelagites and locally intercalated volcanigenic turbidires. The basal complex consists mostly of highly deformed Paleocene o Oligocene ?) quartzose turbidites and crops out in a 50-km erosional window in the Scotland district of northeastern Barbados. These terrigenous beds are thought to represent rench and/or base-of-slope urbidires rom one or more deep-sea fans emanating from northern South America [Speed, 1979; Westbrook, 1982; Pudsey and  8/40 Kasper and Larue: Provenance of Barbados Sandstones Sample Location Map i 13 20 - 7 0 • . 9 40 59 25' Fig. 3. Samples ocation map. Each number refers to a specific location and appears as the flint numeral of individual sample numbers see Table 1): (1) Bawden's, (2) Chalky Mount, a) St. Simons, 4) Mount All, (5) Spa House, 6) Walker's Savannah, 7) Cattle Wash, and 8) Ragged Point. Terrigenous andstone basal complex), stippled; Oceanic Nappe, horizontal dashes; Bissex Hill Nappe, vertical bars. Reading, 1982]. Recent fission rack •vork on detrital zircons Bald•vin, 984] suggested hat some of the basal complex may be as young as Oligocene --• 30 Ma) in age. Baldwin [1984] ascribed he Eocene ossil ages o re•vorking. At this time it is probably safest to merely assume hat the sandstones re of Paleogene age. Although tated othersvise Senn, 940; Pudsey nd Reading, 1982], turbiditic successions annot be correlated across faults on Barbados and therefore do not merit formal stratigraphic nomenclature Larue and Speed, 1983]. We •vill refer to these strata, formerly classified s the Scotland ormatiron Schomburgk, 847], as the terrigenous sandstones of Barbados. SAMPLE COLLECTION AND PREPARATION Numer6us geologists rom North•vestern University and Stanford University collected t•venty-six Paleogene terrigenous sandstone samples along the east coast of Barbados during the 1977-1984 field seasons. Massive to plane-laminated ouma Ta and Tb beds vere selected for study from various deep-sea an associations xposed in outcrop. These associations vere defined by Larue and Speed [1983] and Larue [1985] after sedimentological conventions f Mutti and Ricci-Lucchi 1972] nd Walker [1978]. Care •vas taken to choose clean, unaltered sandstones from the middle of bedding units unaffected by scour. Nine calcite-cemented samples and 17 uncemented and unindurated sandstones •vere chosen at random ntervals or thin seciton tudy (see Figure 3 for samples ocations). In the laboratory, unconsolidated ands mpregnated with epoxy and calcite-cemented samples •vere cut into standard thin sections. Six polished sections •vere prepared for electron microprobe analysis of detrital feldspars. For comparative analysis, •ve studied twelve Lo•ver to Middle Eocene Misoa Formation sandstones and three Lower to Upper Eocene Mirador Formation sandstones from the Maracaibo basin of north•vestern Venezuela. These samples •vere collected srcinally by van Andel [1958] or a petrographic tudy of Cretaceous o Upper Eocene sandstones f •vestern Venezuela. See Figure 4 for generalized results of van Andel's study. PETROGRAPHIC METHODS Employing the Gazzi-Dickinson point counting method [Gazzi, 1966; Dickinson, 1970; Ingersoll et al., 1984], •ve analyzed the terrigenous sandstones f Barbados and the broadly coeval sandstones of the Lake Maracaibo region for the purpose of determining provenance.. Because light frame•vork grain size rarely exceeded coarse sand (mean -- medium sand), 400 points vere counted for each thin section at I mm spacing on a standard point counting stage. T•vo hundred points •vere counted by each author. When expressed as a standard Quartz >80% f ata ts. :•' •• o, . . % 50 50 Feldspar 5'0 RFs & Chert of Cretaceous through Eocene terrigenou• sandstones of western ¾enezue]a not including rkosic Cretaceous io •e•o • Co]o• •o•m•io• s•nd•o•e•). Da• s•]'on•]y su•e• m•xed c•o•M • •ecyc]ed oro•cn pro•enm•ce •or mo• •]eo•e•e s•mp]es •cr D•c]d•son m•a Suc•ek, (•b]e Z) co•rm Qm• compos•ion•] po]e• •ppro•im•e  Kasper and Larue: Provenance of Barbados Sandstones 841 Fig. 5. Photomicrograph of Paleogene tcrrigenous sandstone on Barbados. Note common quartz, metasedimentary-sedimentary lithic fragments, minor m crocline, and chert. Sample is uncemented. Scale: I cm --0.5 mm. deviation, the counting error encountered for each sample was approximately 5% or less or each parameter [Van de Plas and Tobi, 1965]. Reproducibility and operator error were less than 5%. Total framework counts were incorporated into Q : F : L triangle diagrams, reflecting compositional maturity, and Qm: F: Lt triangle diagrams, eflecting provenance Graham et al., 1976; Dickinson nd Suczek, 1979]. Additional point counts of 25 medium-grained polycrystalline quartz grains per terrigenous sandstone slide were carried out as an aid to interpreting crystalline source ock provenance after method of Young [1976]). Due to extensive quartz cementation, this procedure was not carried out on Mirador and Misoa Formation sandstones. Light-framework rains >0.03 mm) counted nclude monocrystalline uartz (Qm); aphanitic polycrystalline quartz without mica, including chert (Qp); feldspar, both plagioclase nd alkali feldspar F -- P + K); and total unstable aphanitic lithic fragments including metamorphic, edimentary, and volcanic L -- Lm + Ls + Lv). Matrix and cement percentages or each thin section were calculated independently of the aforementioned point counts. Volumetrically insignificant carbonate fragments, glauconite, organic detritus, and heavy minerals were noted but disregarded in the framework counts. Recent studies have shown the advantages of using the electron microprobe for chemical analysis of feldspar composition n sandstone rovenance tudies Trevena and Nash, 1981; Maynard, 1984]. A modicum f feldspar in the study samples enabled us to point count polished
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