EAGE
Basin Research (2014) 1–18, doi: 10.1111/bre.12095
Multicyclic sediment transfer along and across convergent plate boundaries (Barbados, Lesser Antilles) ,* Maria Boni† and Mara Limonta,* Eduardo Garzanti,* Alberto Resentini,* Sergio And o € dt‡,§ Thilo Bechst a *Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Universita di MilanoBicocca, Milano, Italy †Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Universita Federico II di Napoli, Napoli, Italy ‡GeoResources STC, Heidelberg, Germany §Institute of Geological Sciences, Jagiellonian University, Krakow, Poland
ABSTRACT The main source of siliciclastic sediment in the Barbados accretionary prism is off-scraped quartzose to feldspatho-litho-quartzose metasedimentaclastic turbidites, ultimately supplied from South America chiefly via the Orinoco fluvio-deltaic system. Modern sand on Barbados island is either quartzose with depleted heavy-mineral suites recycled from Cenozoic turbidites and including epidote, zircon, tourmaline, andalusite, garnet, staurolite and chloritoid, or calcareous and derived from Pleistocene coral reefs. The ubiquitous occurrence of clinopyroxene and hypersthene, associated with green-brown kaersutitic hornblende in the north or olivine in the south, points to reworking of ash-fall tephra erupted from andesitic (St Lucia) and basaltic (St Vincent) volcanic centres in the Lesser Antilles arc. Modern sediments on Barbados island and those shed by larger accretionary prisms such as the Indo-Burman Ranges and Andaman-Nicobar Ridge define the distinctive mineralogical signature of Subduction Complex Provenance, which is invariably composite. Detritus recycled from accreted turbidites and oceanic mudrocks is mixed in various proportions with detritus from the adjacent volcanic arc or carbonate reefs widely developed at tropical latitudes. Ophiolitic detritus, locally prominent on the Andaman Islands, is absent on Barbados, where the prism formed above a westward subduction zone with a shallow decollement plane. The four-dimensional complexities inherent with multicyclic sediment dispersal along and across convergent plate boundaries require quantitative provenance analysis as a basic tool in paleogeographic reconstructions. Such analysis provides the link between faraway factories of detritus and depositional sinks, as well as clues on subduction geometry and the nature of associated growing orogenic belts, and even information on climate, atmospheric circulation and weathering intensity in source regions.
The beasts that talk, The streams that stand, The stones that walk, The singing sand. . . Josephine Tey, Singing sands
INTRODUCTION Sediments sourced in large orogenic belts generated by oceanic or continental subduction are conveyed longdistance by major river systems across foreland basins, and eventually supplied to continental margins at specific deltaic or estuarine entry points (Potter, 1978; Correspondence: Eduardo Garzanti, Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Universita di Milano-Bicocca, 20126 Milano, Italy. E-mail:
[email protected]
Dickinson, 1988; Hinderer, 2012). Sediment dispersal continues via turbidity currents for hundreds to thousands of kilometers beyond the river mouth, and huge masses of sediment are thus transferred from the continent to the deep ocean (Ingersoll et al., 2003). This typically occurs along the trend of major Himalayan-type continent–continent collision zones, where huge turbiditic successions accumulate on remnant-ocean floors destined to be subsequently subducted, while the clastic cover is detached and progressively accreted at the front of a growing fold-thrust belt (Fig. 1a; Morley et al., 2011). Geologically and geometrically distinct is the case of the Caribbean accretionary prism (Fig. 1b). Here, detritus generated in the Andean Cordillera and carried along the retro-belt basin by the Orinoco River finally reaches the Atlantic Ocean, and is deposited by turbidity
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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M. Limonta et al. (b)
(a) Fig. 1. The two conceptual diagrams illustrate opposite cases of long-distance multistep sediment transport and final incorporation of orogenic turbidites at convergent plate margins. (a) Himalayan case: detritus is transferred from the Gangetic pro-belt basin to the Bengal remnant-ocean basin, both lying on the northeastward-subducting Indian lower plate (Ingersoll et al., 2003). (b) Andean case: Orinoco sediments carried along the retro-belt basin resting on the upper plate feed ocean floors on the westward-subducting Atlantic Plate (Velbel, 1985).
currents on sea-floors actively subducting westwards beneath the Caribbean plate (Velbel, 1985). Diverse multistep trajectories in space and time are thus followed during sediment transport along and across convergent plate boundaries (Zuffa, 1987; Zuffa et al., 2000). Only by thoroughly studying modern environments, where such source-to-sink complexities can be physically traced and understood, can we acquire the necessary experience and sharpen our conceptual tools to solve the provenance conundrums posed by ancient clastic successions, where the original geometry of converging plates has been obscured by the subsequent geological evolution. In this article, we present a regional provenance study of the compositional variability and long-distance multicyclic transport of terrigenous sediments along the convergent and transform plate boundaries of Central America, from the northern termination of the Andes to the Lesser Antilles arc-trench system (Fig. 2). We specifically focus on the petrographic composition and heavymineral assemblages characterizing modern beach and fluvial sediments as well as Cenozoic sandstones of Barbados island, one of the places in the world where an active accretionary prism is subaerially exposed (Speed, 1994; Speed et al., 2012). This study extends the regional database on the petrology of modern sands across the South American and Caribbean regions (e.g. Johnsson et al., 1991; Morton & Johnsson, 1993; Potter, 1994), and represents a complement of the thorough investigation carried out with the same methods, rationale and goals on sediments produced and recycled along the Himalayan collision system, from the Bengal Basin to the Indo-Burman Ranges and the Andaman and Nicobar ridge (Garzanti et al., 2013a).
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BARBADOS ISLAND AND ADJACENT SEDIMENT SOURCES Barbados lies 150 km east of the active magmatic arc of the Lesser Antilles, at a latitude of ca. 13° N in the western Atlantic Ocean outside of the principal hurricane strike zone. Climate is warm tropical of the ‘trade-wind littoral’ type, with mean annual temperatures of 24–28°C relatively constant throughout the year. Annual precipitation, increasing with elevation, ranges from 1.1 m at sea level to 2.1 m in the Scotland District (Fig. 3), and is mostly concentrated in the wet season between June and December. The dry season lasts from January to May. The trade winds blow from the northeast at altitudes between 1.5 and 4 km, and are responsible for long-range transport of Saharan dust across the Atlantic (particles < 20 lm in diameter, 30–50% clay; Muhs et al., 2007). The year-round dominance of northeast trade winds in this region prevents ash transport from the Lesser Antilles volcanoes except during breaks in the flow, and the dominant role in eastward ash dispersal is played by westerly anti-trade winds in the upper troposphere (6– 18 km; Carey & Sigurdsson, 1978). Soil composition on Barbados testifies to the importance of trade winds in the dispersal of Saharan dust out of northern Africa or loess from the Mississippi Valley of North America, and of anti-trade winds in ash-fall transport across the Caribbean (Muhs, 2001).
Geology of Barbados Accretionary prisms display a great range in width, depending on their stage of development and sedimentary influx, which may be derived from fluvio-deltaic
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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Fig. 2. Tectonic framework of the Caribbean plate and location of Barbados island (DEM modified from www.geomapapp.org). Sampling sites for South American rivers are indicated.
sediments delivered directly to the upper plate or from deep-water sediments scraped off the lower plate and accreted to the upper plate. The northern Barbados Ridge has an east–west cross-sectional area of only 100 km2, whereas the frontal 50 km of the Italian Apennines display an average cross-sectional area of 500 km2. Such a large variation in volume and elevation is the result of a shallower detachment plane beneath Barbados than beneath the Apennines (Bigi et al., 2003). Emergent subduction complexes formed on the trend of major plateconvergence zones and fed by large volumes of orogenic sand, such as the offshore island of Barbados and the Myanmar-Andaman-Nicobar ridge, display signs of hydrocarbon generation with oil and gas seeps from deepwater sediments, often from mud volcanoes (Hill & Schenk, 2005). Since 1896, Barbados has had a small hydrocarbon production, much less important than in the transpressional setting of Trinidad to the south (Speed et al., 1991; Boettcher et al., 2003; Vincent et al., 2014). Barbados island is the emerged part of a large subduction complex, narrowing from 250 km in the south to 100 km
in the north (Fig. 2). The marked change in thickness of Cenozoic sediments, decreasing northwards from >6 km on the Venezuelan passive margin to 0.5 km on the Tiburon Ridge that constitutes a topographic barrier for turbidites, reflects dominant supply from the Orinoco Delta (Faugeres et al., 1993). Turbiditic sandstone and mudrock, more abundant during the middle Eocene to late Oligocene but continuing throughout the Neogene, are being deposited on the floor of piggyback basins bounded by anticlinal ridges, and are fed by erosional canyons cutting across the tectonic structures (Beck et al., 1990; Mascle et al., 1990; Faugeres et al., 1991). Sediment entering the subduction zone includes calcareous mud overlying pillow basalt, Eocene-Oligocene quartz-turbidite and interbedded mud, Lower Miocene radiolarianbearing ooze, and Middle Miocene to Pleistocene calcareous mud and volcanic ash (Underwood, 2007). The Barbados accretionary prism, passing laterally to the transpressional setting of Trinidad in the south and to the northern Caribbean strike-slip margin in the north (Fig. 2; Pindell & Kennan, 2007; Vincent et al., 2014),
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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M. Limonta et al.
(b)
(c)
(a)
Fig. 3. Geological sketch map (a), topography (b) and stratigraphy (c) of Barbados island (after Pudsey & Reading, 1982; Donovan, 2005). Sampling sites are indicated.
has been developing in the frontal part of the Lesser Antilles arc-trench system since the middle Eocene, during westward subduction of the Atlantic plate beneath the Caribbean plate (Weber et al., 2001). With the exception of minor ash-fall tephra, the rock succession in Barbados is entirely sedimentary, demonstrating its geological independence from the volcanic arc of the Lesser Antilles. Exposed in an erosional window of ca. 40 km2 in the Scotland District of north-eastern Barbados are deformed deep-sea turbidite, radiolarite and hemipelagite of Cenozoic age, representing ca. 15% of total outcrops (Fig. 3a; Donovan, 2005). Two tectono-stratigraphic levels can be identified: (1) the Basal Complex and its cover strata; and (2) the overlying allochthonous oceanic units (Speed & Larue, 1982). The unconformably overlying Pleistocene and Holocene limestone cap represents the remaining 85%. The Basal Complex, a stack of fault-bounded packets ≥4.5-km thick, includes hemipelagic radiolarian-rich clay, quartzose turbiditic sandstone and mudrock of the Scotland Formation, and the Joes River Melange (Senn, 1940). These rocks, originally deposited in deep-sea-fan and trench settings and subsequently off-scraped to form the accretionary prism around the end of the Eocene, have
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undergone modest tectonic deformation since then (Speed et al., 1991). The ca. 1700 m-thick Scotland Formation was deposited during the Paleocene to early?-middle Eocene (Jones, 2009). The Lower Scotland Formation includes: (1) sandstone and mudrock of the Walkers Member; (2) mudrock of the Morgan Lewis Member, containing intercalated sandstone. The Upper Scotland Formation includes: (3) Murphys Member, an upward thickening and coarsening sequence including slumped beds; (4) pebbly sandstone of the Chalky Mount Member, containing large slide blocks and minor interbedded mud; (5) sandstone and mudrock of the Mount All Member (Fig. 3c; Senn, 1940; Pudsey & Reading, 1982). Petrographic studies have long documented the quartzose to litho-quartzose composition of Scotland Formation turbidites (Velbel, 1980). This signature, anomalous for an arc-trench system, is similar to that of Orinoco sand (Johnsson et al., 1988, 1991), suggesting provenance and long-distance turbiditic transport from northern South America (Velbel, 1985). Heavy-mineral assemblages in these rocks include zircon, tourmaline, rutile, epidote, clinozoisite, apatite, garnet, chloritoid, kyanite, staurolite, andalusite, sillimanite and titanite (Senn, 1940; Pindell et al., 2009; Vincent et al., 2014). The Joes River
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
Sediment transfer across active margins Melange contains blocks dominantly from the Scotland Formation, embedded in a foliated organic-rich sandstone/mudstone matrix with fossils of Paleocene to middle Eocene age. The melange cuts bedding and is interpreted as formed by diapiric processes (Poole & Barker, 1983; Torrini et al., 1985; Babaie et al., 1992). After accretion, the Basal Complex was unconformably overlain by a trench-slope apron of Oligocene to Neogene hemipelagic mudrock interbedded with turbiditic sandstone and debris flow (Speed et al., 1991). These deposits are structurally intercalated with the Basal Complex to a depth of 3 km (Torrini & Speed, 1989). The Basal Complex and prism cover are structurally overlain by the Oceanic Allochthon, a ≤1.5-km thick sequence of thrust sheets including deep-water clay, nannofossil and radiolarian ooze, volcanogenic turbidite and tuffaceous beds of late middle Eocene to early Miocene age (Saunders et al., 1984; Jones, 2009). These rocks, partly coeval with the Basal Complex, are interpreted to have been deposited at bathyal to abyssal depths along the outer margin of the forearc basin, and thrusted onto the accretionary prism for ≥20 km on a shallow west-dipping fault beginning at 16 Ma (Torrini & Speed, 1989). The Barbados accretionary prism is unconformably capped by a southwestward-dipping arch of Pleistocene and Holocene back-reef, reef, and fore-reef sediments (Mesolella et al., 1970; Schellmann & Radtke, 2004). The upraised coral reefs, completely eroded in the Scotland District, include three main terraces formed during the long-term uplift of the Barbados Ridge and documenting middle and late Pleistocene glacio-eustatic fluctuations. The older coral reefs are locally folded and reach their highest elevation in the central-northern part of the island (Taylor & Mann, 1991). The two main reef steps are known as the First High Cliff, located close to the coastline at 20–60 m a.s.l., and the Second High Cliff at 130– 200 m a.s.l. (Fig. 3a; Donovan, 2005).
The Antilles arc The Lesser Antilles volcanic arc extends for ca. 800 km (Fig. 2). Three groups of islands can be distinguished petrologically. The northern group (Saba to Montserrat) is dominated by andesite, with minor dacite and rare rhyolite; small volumes of basalt (≤5 km3) occur locally as blocks in pyroclastic deposits (Rea & Baker, 1980). The central group includes the largest islands (Guadeloupe, Dominica, Martinique, St Lucia) and is also characterized by predominant andesite, with some basalt, dacite and rare rhyolite. The southern group includes St Vincent, dominated by basalt and basaltic andesite, and the Grenadine islands, where rock types range from picritic or ankaramitic basalts to andesite and rare dacite (Rea, 1982; Macdonald et al., 2000). Explosive eruptions of Lesser Antilles volcanoes (e.g. Mount Pelee on Martinique in 1902; Lacroix, 1904; La Soufriere on St Vincent in 1902, 1979 and 1995; Brazier et al., 1982) have long been documented to be the source
of ash-fall deposits on Barbados. Air-fall tephra, which are produced by different mechanisms (Vulcanian activity, dome collapse, ash-venting, phreatic explosion; Bonadonna et al., 2002), represented most ejecta during the La Soufriere eruption in 1902, when ≥28% of the air-fall tephra were carried eastward towards the Atlantic Ocean by anti-trade winds, at altitudes between 7 and 16 km (Carey & Sigurdsson, 1978).
The Orinoco sedimentary system The Orinoco River catchment occupies ca. 1.1 106 km2 in northern South America (Fig. 2). The Andes and Caribbean Mountains in the west and north (ca. 15% mountainous part of the basin) have strong relief with steep slopes and sharp peaks with active alpine glaciers. They are made of terrigenous and carbonate sedimentary and metasedimentary rocks, along with felsic to mafic plutonic rocks. Orogeny began in the late Oligocene and peaked in the Pliocene, exerting a major influence in the development of the Amazon and Orinoco River systems (Hoorn, 1993; Hoorn et al., 1995). The Guyana Shield (ca. 35% of the basin) is largely of low relief with dense vegetation and deeply weathered soils, but locally approaches 3000 m a.s.l. It consists of felsic to intermediate granitoids and gneisses overlain by quartzites (Gibbs & Barron, 1983). The Llanos region (50% of the basin) is a retro-belt foreland basin filled with sediments derived mostly from the rising Andean orogen in the west, and reworked widely by fluvial processes during the flood season and by eolian processes during the dry winter season (Johnsson et al., 1991). The Orinoco is the third largest river in the world in terms of annual water discharge (36 000 m3 s 1); the estimated annual sediment flux (150 50 106 t/a, 50% of which deposited in the delta) consists of 80% mud and 20% sand (Meade, 1994; Warne et al., 2002). The delta has a strongly seasonal tropical climate, with uniform temperatures throughout the year (25–28°C) and annual rainfall ranging from 1.2 to 3.6 m. The low- to moderateenergy broad shelf is affected by a combination of easterly trade winds, tides (1.4–1.9 m) and littoral currents, with rare major storms and hurricanes (Aslan et al., 2003). The northwest-directed Guyana Current, responsible for the presence of mud banks along much of the northeast coast of South America (Allison & Lee, 2004), transports between 100 and 200 106 t/a of suspended sediment from the Amazon delta to the Orinoco region, an amount comparable to the total sediment discharge of the Orinoco (Van Andel, 1967; Eisma & Van Der Marel, 1971; Eisma et al., 1978; Kuehl et al., 1986; Warne et al., 2002). The Orinoco deep-sea fan is characterized by a welldeveloped braided pattern (Belderson et al., 1984). This relatively sand-rich turbidite system is controlled by the compressional structures of the Barbados prism, and consequently does not display the classic fan geometry (Callec et al., 2010). A channel-levee complex characterizes the upper part close to the Trinidad-Venezuelan
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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M. Limonta et al. continental slope. Fold-thrust deformation, mud diapirism and mud volcanism induce frequent and massive gravity deposits (Deville et al., 2006). Orinoco sands are quartzose, with a few feldspars (Kfeldspar ≫ plagioclase) and metasedimentary to minor plutonic and sedimentary rock fragments (Potter, 1978; Johnsson et al., 1988, 1991). Sands of the Apure River tributary are quartzo-lithic in the Andean upper reaches, with epidote, amphibole, zircon and garnet as main heavy minerals. Due to reworking of weathered foreland-basin sediments, unstable components are progressively diluted with increasing distance from the mountain front, and Apure sand becomes eventually almost as quartzose as that of the Orinoco in the distal course (Johnsson et al., 1988). Epidote and amphibole do not show downstream changes, whereas zircon tends to be enriched and apatite and garnet selectively depleted (Morton & Johnsson, 1993). Heavy-mineral analyses of modern sediments on the Guyana Shelf allowed recognition of different provinces (Nota, 1958; Imbrie & Van Andel, 1964): (1) the area offshore of the Orinoco Delta, characterized by an epidoteamphibole suite; (2) the adjacent shelf in the west offshore Trinidad, characterized by a zircon-epidote suite; (3) the area offshore the Guyana Shield in the east, characterized by a staurolite-rich suite. Other heavy minerals, found also farther east offshore the Essequibo and Demerara deltas, include sillimanite, tourmaline, rutile and andalusite. Garnet and kyanite are minor; hypersthene occurs locally.
SAMPLING AND ANALYTICAL METHODS To investigate the compositional variability in modern sediments derived from the Barbados accretionary prism and to compare the composition of daughter sands and parent sandstones, in July 2011 to March 2013 we collected 22 medium to coarse-grained sand samples on beaches and river beds (mostly gully to creek) from coastal Barbados, five silt samples of river mud, and 13 bedrock samples from the Basal Complex, ranging from coarse siltstone to medium-grained sandstone. Eight sand samples from main rivers of northern South America (Orinoco, Maroni, Amazon) and beaches from the Lesser Antilles (Dominica, Martinique) were also analyzed for comparison. Petrographic analysis of modern sands and Cenozoic sandstones was carried out by counting 400 points in each thin section by the Gazzi–Dickinson method (Ingersoll et al., 1984). Also, we counted ca. 100 granules on the >2 mm class of two coarse-sand samples, and ca. 100 points within each of three representative sandstone granules. Heavy-mineral analyses were carried out on 22 bulk-sand samples, on a quartered aliquot of the 32–355 or 32–500 lm class obtained by dry-sieving for another eight sand samples and by wet sieving after crushing or gentle disaggregation for the 13 sandstone samples, and on a quartered aliquot of the 15–63 lm class obtained by wet-sieving for the five mud
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samples. Heavy minerals were separated by centrifuging in Na polytungstate (density ca. 2.90 g cm 3), and recovered by partial freezing with liquid nitrogen. On grain mounts, 200–250 transparent heavy-mineral grains were either point-counted at suitable regular spacing to obtain real volume percentages (samples rich in transparent heavy minerals) or counted by the area method (all sandstones and modern sediments rich in Fe-carbonate or altered grains; Mange & Maurer, 1992). Dubious grains were checked with Raman spectroscopy (Ando & Garzanti, 2014). The ZTR index (sum of zircon, tourmaline and rutile over total transparent heavy minerals; Hubert, 1962) defines the ‘mineralogical stability’ of the assemblage. Heavy-mineral concentration was calculated as the volume percentage of total (HMC) and transparent (tHMC) heavy minerals (Garzanti & Ando, 2007a). In Barbados sands and sandstones, the tHMC index does not exceed 1. Transparentheavy-mineral concentration thus ranges from extremely poor (tHMC < 0.1) to very poor (0.1 ≤ tHMC < 0.5) and poor (0.5 ≤ tHMC < 1). Heavy-mineral assemblages are somewhat richer in river sands from northern South America, and extremely rich (20 ≤ tHMC < 50) in volcaniclastic beach sands of the Lesser Antilles. Specific attention was dedicated to the surface features of detrital minerals; for each species, the percentage of corroded, etched, deeply etched and skeletal grains was quantitatively recorded following the operational classification of Ando et al. (2012). Size and elongation of detrital grains were measured by image analysis of digital photographs using ImageJ software (http://rsbweb.nih.gov/ij). Grain roundness was assessed by visual comparison with classical charts (Krumbein, 1941; Powers, 1953). Detrital components are listed in order of abundance throughout the text. Chemical analyses of two volcaniclastic beach sands of the Lesser Antilles were carried out at ACME Laboratories (Vancouver) on a quartered aliquot of the 63– 2000 lm class separated by wet sieving. Following a lithium metaborate/tetraborate fusion and nitric acid digestion, major oxides and several minor elements were determined by ICP-ES and trace elements by ICP-MS. A separate split was digested in aqua regia and analyzed for Mo, Ni, Cu, Ag, Au, Zn, Cd, Hg, Tl, Pb, As, Sb, Bi, Se. For detailed information on adopted procedures, standards used and precision for various elements of group 4A–4B see http://acmelab.com. The Chemical Index of Alteration was calculated following Nesbitt & Young (1982).
PETROGRAPHY AND HEAVY MINERALS We will illustrate here the petrographic and mineralogical features of Cenozoic sandstones of the Basal Complex and of the modern sands derived from them on Barbados island. Next, we will describe the composition of carbonaticlastic beach sands in the rest of Barbados, of volcaniclastic beach sands in Martinique and Dominica islands, and of river sands from northern South America (Fig. 4, Table 1).
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
Sediment transfer across active margins
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4. Petrography of Orinoco and Barbados sands (all photos with crossed polars; blue bar = 250 lm). Grain size control on Orinoco bedload sand: (a) slate (Lms) and phyllite rock fragments (white arrows), as well as epidote (black arrow), are concentrated in the fine sand fraction; (b) quartz (Q) with minor feldspars (P = plagioclase) make up the medium sand fraction. (c) Recycled quartz associated with siltstone (Ls) and radiolarite (ch) rock fragments derived from turbidites and hemipelagic oozes of the Scotland District. (d) Siltstone rock fragments and quartz grains – displaying etch pits and lobate outlines indicative of weathering in subequatorial soils – document recycling of turbidites fed long distance from South America. (e) Radiolarian-rich clasts. (f) Calcareous grains eroded from Pleistocene carbonates (Lc), mixed with allochems (a = coralline alga; b = bioclast; i = intraclast) and euhedral plagioclase and pyroxene (p) derived from ash layers in beach sand of western Barbados.
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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0 0 0 0 1 1
5 1 5
3 2 3 2
3 3 4 0 88 6
88 83 70
72 17 79 14
KF
6 4 3 3
1 0 6
36 6 0 0 1 1
P
5 3 3 3
0 0 9
7 4 0 0 0 1
Lv
0 0 0 0
0 0 0
0 0 90 7 7 7
Lc
2 2 0 0
1 0 2
0 0 0 0 1 2
Ls
10 7 8 6
3 0 7
0 0 0 0 0 0
Lm
0 0 0 0
0 0 0
0 0 0 0 0 0
Lu
1 2 2 4
0 0 1
0 0 0 0 0 0
Mi
1 1 2 2
2 16 1
54 10 5 7 1 1
HM
100.0
100.0
100.0 100.0 100.0
100.0
100.0
100.0
210 6 184 17
222
208
181 22
MI*
0.6 0.8
44 6 0.4 0.5 0.2 0.2 1.4 1.4 0.5 7.9 0.7
tHMC
Zircon 44 29
0 0 0 0 15 10 5 5 46 1 1
Tourmaline 12 10
0 0 0 0 7 4 4 2 2 7 2
Ti oxides 6 7
0 0 0 0 1 1 5 4 5 0 0
Titanite 0 0
0 0 0 0 1 1 3 2 0 0 0
Epidote 27 32
0 0 1 1 33 22 74 15 24 0 24
Garnet 3 2
0 0 0 0 4 2 1 1 2 35 4
Chloritoid 3 3
0 0 0 0 3 4 3 3 0 0 0
Staurolite 1 1
0 0 0 0 1 1 0 0 1 55 0
Al2SiO5 0 0
0 0 1 4 5 7 0 0 0 2 7
Amphiboles 1 4
22 19 3 4 1 2 1 1 18 0 30
Clinopyroxene 0 0
7 3 42 14 12 8 2 2 1 0 15
0 0
71 16 44 10 14 9 1 2 0 0 18
Hypersthene
0 0
0 0 8 5 1 1 0 0 0 0 0
1 1
0 0 0 0 1 1 1 1 0 0 0
100.0
100.0 100.0 100.0
100.0
100.0
100.0
100.0
Q, quartz; KF, K-feldspar; P, plagioclase; L, aphanitic lithic grains (Lv, volcanic; Lc, carbonate; Ls, terrigenous and chert; Lm, metamorphic; Lu, ultramafic); MI, mica; HM, heavy minerals; tHMC, transparent heavy-mineral concentration (Garzanti & Ando, 2007a). The Metamorphic Index MI* expresses the average rank of metamorphic rock fragments in each sample, and varies from 100 (very low-grade metamorphic source rocks) to 500 (high-grade metamorphic source rocks; Garzanti & Vezzoli, 2003). The Al2SiO5 mineral group includes andalusite, kyanite and sillimanite; other heavy minerals are mostly apatite.
Sandstones
Modern sediments Lesser Antilles (Martinique, Dominica) Barbados sands (Quaternary reefs) Barbados sands (Scotland Distrinct) Barbados muds (Scotland Distrinct) Orinoco River Maroni River Amazon River Basal complex Granules
Q
Olivine
8 Others
Table 1. Petrography and heavy minerals in Cenozoic sandstones of the Basal Complex, and in modern sediments of Barbados island, Lesser Antilles arc, and rivers of northern South America (mean values in bold; standard deviation in italics)
M. Limonta et al.
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
Sediment transfer across active margins
Parent sandstones and sandstone clasts Very-fine-grained Cenozoic sandstones of the Basal Complex are quartzose to feldspatho-litho-quartzose metasedimentaclastic and mica-rich, whereas fine to mediumgrained sandstones are quartzose and may contain variable amounts of intrabasinal grains, including bioclasts (nummulitids, echinoids, bryozoans, algae). Glaucony is locally common and phosphates may occur, as well as calcite cement and replacements. Heavy-mineral assemblages range from poor and epidote-dominated in finer-grained feldspatho-litho-quartzose sandstones (tHMC up to 1.2, ZTR as low as 7) to extremely poor and zircon-dominated in coarser quartzarenites (tHMC as low as 0.1, ZTR up to 94). Associated minerals are tourmaline, rutile, garnet, chloritoid, and minor staurolite, anatase, brookite, apatite, titanite, lawsonite (Fig. 5a), andalusite, sillimanite, and kyanite. Beta-fergusonite (Y) – a rare mineral found in pegmatites and granites (Fig. 5b) – and xenotime were detected with Raman spectroscopy and with a pocket spectrometer in one sample each. Hornblende or clinopyroxene occur rarely, suggesting chemical weathering at subequatorial latitudes and extensive post-depositional dissolution. Textural evidence indicates that epidote is detrital and not grown during burial diagenesis. Sandstone granules – representing the coarsest tail of modern fluvial sands – contain monocrystalline and polycrystalline quartz (Q 74 16, Qp/Q 14 3), significant plagioclase and K-feldspar (F 9 6, P/F 60 25), shale/slate to phyllite/schist/metapelite lithics, and felsitic volcanic rock fragments (Lsm 12 9, Lv 5 3).
Composition is more variable than quartzose Orinoco sand and ranges from quartzose to litho-quartzose and feldspatho-litho-quartzose (Velbel, 1980; Johnsson et al., 1988, 1991).
Daughter sands and muds The medium- to coarse-grained river and beach sands of the Scotland District, principally derived from recycling of Cenozoic sandstones of the Basal Complex, are mainly quartzose (Fig. 4c). Predominant monocrystalline quartz (Q ≤ 95, Qp/Q ≤ 10) commonly shows rounded outlines and deep etching features (Fig. 4d), as a consequence of both recycling and chemical weathering in the mature soils developed in subequatorial climate. A few plagioclase and K-feldspar grains are invariably present (F 2 1, P/ F 50 20), and low-rank metasedimentary (slate, phyllite, metasiltstone), mafic to felsic volcanic, and radiolarian-chert grains occur (Lm ≤ 4; Lv ≤ 3; Lch ≤ 2). Calcareous grains derived from the Pleistocene reefal cap may be quite abundant (Lc ≤ 55), as well as shale/siltstone grains recycled from turbiditic mudrocks (Lp ≤ 15). Granules in coarse fluvial sands include monocrystalline and subordinately polycrystalline quartz, sandstone, siltstone and shale clasts derived from Cenozoic turbidites, calcareous grains eroded from the Pleistocene reefal cap, and locally abundant clasts rich in radiolaria (Fig. 4e). Soil clasts are also common. Heavy-mineral assemblages range from epidote-dominated to zircon-rich with metasedimentary minerals recycled from accreted turbidites (tourmaline, andalusite, garnet, staurolite,
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(b)
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Fig. 5. Raman spectra of particular heavy minerals in Cenozoic Basal Complex turbidites (a, b) and modern Martinique beach sand (c, d). (a) Detrital lawsonite, indicating ultimate provenance from blueschist-facies rocks of the Caribbean Mountains (Morton & Johnsson, 1993; Pindell et al., 2009; Vincent et al., 2014); (b) Detrital mineral determined as beta-fergusonite (Y) by comparison with Raman spectrum RRUFF R070600 (shown in orange for comparison; www.rruff.info); (c) green brown kaersutitic hornblende; (d) reddish-brown oxy-hornblende. Microphotographs show lawsonite and beta-fergusonite (Y) grains at parallel and crossed polars (a, b) and pleochroism of amphibole grains (c, d). © 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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M. Limonta et al. chloritoid, rare sillimanite; tHMC 0.2 0.2). Fluvial muds are epidote-dominated (tHMC 1.4 1.4). Rare amphiboles, together with very low heavy-mineral concentrations, reflect intense diagenetic dissolution in parent sandstones. Epidote grains are invariably corroded in both modern sediments (96 3%) and bedrock sandstones (93 10%). In beach sand of the Scotland District, garnet is mainly of upper-fine sand size, zircon, tourmaline and epidote mainly of lower-fine sand size, and rutile and apatite of upper very-fine sand size. Garnet, tourmaline and rutile are mainly angular, epidote mainly subangular, and zircon and apatite mainly subrounded.
Carbonaticlastic beaches Beach sands along the north-eastern, south-eastern, southern, and western coast of Barbados consist monotonously of reworked bioclasts and other allochemical detritus, reflecting erosion of up-raised Pleistocene reef terraces and recent reefs (Fig. 4f). These calcareous sands include very poor to extremely poor heavy-mineral assemblages, dominated by augitic clinopyroxene and
hypersthene in subequal amounts (tHMC 0.1 0.1). Variable amounts of the same pyroxene types occur also in sands of the Scotland District, revealing reworking of widespread ash-fall tephra derived from Lesser Antilles volcanoes. Minor green-brown kaersutitic hornblende occurs in the northeast, whereas olivine is ubiquitous but most abundant in beach sands to the west and south (Fig. 6). This reflects the trend displayed along the Lesser Antilles arc, from dominant andesites and subordinate mafic and felsic products in the central group of islands including St Lucia, to increasing abundance of basalts in the southern group of islands including St Vincent. The few zircon grains in calcareous beach sands are of upper-fine sand size and subrounded to rounded. On both sides of Barbados, pyroxene and olivine grains are commonly euhedral, range from very angular to subrounded, and are coarser-grained and less corroded than other detrital minerals, suggesting a distinct, neovolcanic origin (Fig. 7e). They are mostly of lower-medium sand size, and range from upper-fine sand to lower-coarse sand. Pyroxenes are mostly corroded (81 13%), subordinately unweathered (15 13%), and rarely etched (4 6%).
Fig. 6. Petrography and heavy minerals in modern sediments and Cenozoic sandstones from Barbados, the Lesser Antilles, and rivers of northern South America. Q = quartz; F = feldspar; L = lithic fragments (Lv = volcanic; Lc = carbonate; Lm = metamorphic). ZTR = zircon + tourmaline + rutile; Grt = garnet; MM = chloritoid + staurolite + andalusite + kyanite + sillimanite; Ep = epidotegroup minerals; Amp = amphibole; Cpx = clinopyroxene; Hy = hypersthene; Ol = olivine; & = other transparent heavy minerals, including titanite, apatite and lawsonite.
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© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
Sediment transfer across active margins
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Fig. 7. Bulk petrography (a, b) and heavy minerals (c, d, e, f) reveal multiple sources for modern Barbados sediments and superimposed chemical effects during weathering and diagenesis. (a) Barbados sands, sharply distinct from volcaniclastic sands generated in the Lesser Antilles arc, are chiefly recycled from Cenozoic turbidites or eroded from Quaternary reefs. (b) Relative to parent sandstones, Barbados sands are depleted in metamorphic lithics and enriched in sedimentary lithics eroded from Cenozoic turbiditic mudrocks or Quaternary calcarenites. (c) Heavy-mineral suites of Barbados sediments: muds are markedly enriched in epidote, whereas medium and coarse-grained sands are heavy-mineral-poor and relatively enriched in more durable zircon, tourmaline, Ti oxides (ZTR), garnet and metasedimentary minerals (MM). (d) Neovolcanic heavy-mineral suites vary from hypersthene-dominated and amphibole-bearing in Dominica and Martinique to richer in augite and olivine-bearing in southern Barbados, reflecting the southward trend from andesitic to more mafic activity along the Lesser Antilles arc. (e) In the Windy Hill river-mouth sand (Scotland District), euhedral shape reveals the neovolcanic origin of pyroxene grains (Aug = augite; Hy = hypersthene); other heavy minerals are subangular to subrounded and recycled from Cenozoic turbidites. (f) As in equatorial Africa and contrasting with cold mountain regions (Alps, Himalaya) or arid deserts (Namibia), sands of northern South America and sands and sandstones of Barbados (sandstone data after Vincent et al., 2014) have low garnet/MM ratio and high andalusite/MM ratio, pointing to selective dissolution of garnet and relative enrichment in andalusite in hot-humid subequatorial climates (Garzanti et al., 2013b). Ls = sedimentary lithics; other parameters as in Fig. 6.
Volcaniclastic beaches The studied beach sands from Martinique island are litho-feldspathic volcaniclastic. They consist largely of single minerals (plagioclase, pyroxene, amphibole), suggesting selective destruction of labile pyroclastic rock fragments by wave action in the coastal environment. Volcanic lithics are mainly microlitic, but mafic types also occur, reflecting andesitic to basaltic volcanic activity. The studied beach sand from Dominica island is quartzolitho-feldspathic volcaniclastic. Relatively high quartz content reflects occurrence of andesite-dacite lava domes and pyroclastic flows. Heavy-mineral assemblages are invariably extremely rich (tHMC 37 1) and pyroxenedominated (Fig. 6). In the St Pierre beach, hypersthene is dominant, augitic clinopyroxene subordinate, and kaersutitic hornblende minor. In southern Martinique and Dominica, hypersthene still dominates over augite, but green-brown kaersutitic hornblende is much more common (Fig. 5c), and reddish-brown oxy-hornblende may occur (Fig. 5d). Pyroxenes and amphibole grains are
mostly unweathered (85 10%), subordinately corroded (14 9%) and rarely etched (1 1%). Volcaniclastic beach sands have 52.4 0.3 SiO2. Most alkaline and alkaline-earth elements (Na, K, Rb, Sr, Ba), as well as REE, Th, U, Zr, Hf, Nb, Ta, Cr and Ni, are low relative to sands produced by erosion of andesitic volcanoes in Mediterranean settings (own unpublished data). The Eu anomaly is mildly negative (Eu/Eu* 0.88). Fe and Mn are relatively high, and negative loss on ignition in St Pierre beach indicates occurrence of magnetite, oxidized during heating in the laboratory. CIA values of 46 indicate negligible weathering.
Rivers of northern South America The studied bedload sand of the Orinoco River is quartzose. The fine tail of the sand distribution contains less feldspar and a few more low-rank metasedimentary rock fragments (Fig. 4a), whereas the medium to coarse sand fraction chiefly consists of quartz with K-feldspar, plagioclase and few sandstone/metasandstone rock fragments
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
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M. Limonta et al. (Fig. 4b). The poor heavy-mineral population includes zircon associated with epidote, amphibole and minor rutile, tourmaline, garnet, staurolite and clinopyroxene. The medium sand fraction (>250 lm) includes mainly amphiboles, tourmaline, fibrolitic sillimanite, zircon, andalusite, augite, garnet and staurolite, and does not contain epidote. Concentration of less dense heavy minerals (2.9–3.2 g cm 3; fibrolitic sillimanite, tourmaline, andalusite, amphibole) in the coarse tail of the size distribution is explained by the settling-equivalence principle (Rubey, 1933; Garzanti et al., 2008). Epidote, however, is unexpectedly finer-grained than much denser minerals such as garnet and zircon, and its anomalous concentration in the fine tail of the size distribution indicates provenance from a distinct orogenic source (i.e. the low-grade rocks of the Caribbean Mountains). The studied bedload sand of the Maroni River, which drains the Guyana Shield, is quartzose with a rich heavymineral assemblage dominated by garnet and staurolite (Fig. 6). The sampled bedload sands of the Amazon River, including its Solim~oes and Madeira tributaries, are feldspatho-litho-quartzose with common felsitic and microlitic volcanic lithics and sedimentary/metasedimentary (shale/slate, siltstone/metasiltstone, phyllite/schist) rock fragments. The poor heavy-mineral assemblages include amphiboles, epidote, hypersthene, augitic clinopyroxene, andalusite, garnet, tourmaline and zircon.
SUBDUCTION COMPLEX PROVENANCE Modern sands on Barbados island reflect mixing in various proportions of detritus eroded from different siliciclastic, volcaniclastic and carbonate sources (Fig. 7). These include Cenozoic turbidites ultimately derived long-distance from the northernmost segment of the Andean orogen, the Caribbean mountains and the Guyana Shield, ash-fall tephra ejected from the Antilles island arc, and the reefal cap, reflecting local biogenic production during the Pleistocene and Holocene. Such a composite signature – with a dominance of detritus recycled from Cenozoic turbidites ultimately derived from a large orogenic belt mixed in variable amounts with detritus reworked from melange, oceanic sediments and younger volcaniclastic and reefal units – represent the typical mark of sediments derived from subduction complexes formed by tectonic accretion above trenches choked with thick sections of remnant-ocean turbidites, and thus large enough to be exposed subaerially (Subduction Complex Provenance of Garzanti et al., 2007, 2013a). Although geological processes and geometries of convergence are varied, and every subduction system with its associated wedge of accreted oceanic rocks displays peculiar features, the comparison of modern sands on Barbados with those shed by larger accretionary wedges such as the Indo-Burman Ranges and Andaman-Nicobar Ridge can illuminate the distinctive signatures of Subduction Complex Provenance. This may help us to infer the
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occurrence and reconstruct the polarity of such paleotectonic settings from provenance analysis of ancient stratigraphic successions. Mixing of detritus largely recycled from offscraped turbidites (Recycled Clastic Provenance) with volcanic and ophiolitic detritus (Volcanic Arc and Ophiolite Provenance) is observed commonly, but mixing proportions and end-member compositions may vary considerably. On Barbados as on the Andaman-Nicobar Ridge, chert grains do occur but only locally (Fig. 4), and are never dominant as predicted in the original definition of Subduction Complex Provenance (Dickinson & Suczek, 1979, p. 2176).
The recycled-clastic component Subduction complexes large enough to reach above sealevel – and thus become a source of sediments – must be tectonically fed by huge volumes of turbidites progressively off-scraped at the trench. In turn, such a large amount of detritus implies that the ultimate source should be a large and actively uplifted and eroded mountain belt such as an Himalayan-type orogen or an Andean-type cordillera (Garzanti et al., 2007). Sediment is transferred from the eroding orogen via an integrated fluvio-deltaicturbiditic conveyor belt to another subduction zone with either the same eastward/northeastward polarity (e.g. Himalayan detritus supplied to the Makran and BurmaSunda trenches; Fig. 1a) or opposite westward polarity (e.g. Alpine detritus supplied to the Apenninic foredeep, or Andean detritus supplied to the Barbados trench via the Orinoco River and Fan; Fig. 1b). Turbiditic rocks representing the bulk of material accreted in subduction complexes range from mud-dominated distal-fan units to generally younger sand-dominated proximal-fan units (Garzanti et al., 2013a); also calcareous turbidites may be prominent, as for the Helminthoid Flysch of the northern Apennines (Zuffa, 1987; Garzanti et al., 1998, 2002a; Di Giulio et al., 2003). The mineralogy of such parent siliciclastic rocks is controlled by their source-rock lithology and geodynamic setting, as well as by the intensity of chemical processes during the sedimentary cycle, including both pre-depositional weathering and diagenesis (Johnsson, 1993; Morton & Hallsworth, 2007). Daughter sands on Barbados are quartzose, and contain mostly monocrystalline quartz with rounded outline or deep etch pits (Fig. 4d) and depleted heavy-mineral assemblages with mainly stable and semi-stable minerals including andalusite, one of the most stable minerals in equatorial climates (Garzanti et al., 2013b). This reflects both subequatorial weathering in the paleo-Orinoco catchment and diagenetic dissolution in Cenozoic sandstones, but potentially also weathering in the modern humid tropical climate of the Caribbeans (Fig. 7f; Johnsson & Stallard, 1989).
The volcanic-arc component Neovolcanic detritus derived from active volcanoes and transported across the forearc region, as well as
© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists
Sediment transfer across active margins paleovolcanic detritus recycled from older volcaniclastic units, occur commonly in accretionary prisms (Garzanti et al., 2013a). Mineralogical signatures characterized by augitic clinopyroxene associated with frequently abundant or even dominant hypersthene reflect the character of subduction-related magmas (Garzanti & Ando, 2007b). Basaltic source rocks may be documented by the abundance or even local predominance of olivine over pyroxene and of durable lathwork lithics over plagioclase, reflecting scarcity of plagioclase phenocrysts in mafic parent lavas. Provenance from andesitic and more felsic products is instead revealed by abundant single minerals – also because of extensive occurrence of friable pumicerich pyroclastic deposits – and by presence of reddishbrown oxy-hornblende, green-brown kaersutitic hornblende and biotite.
The ophiolitic component The presence or absence of ophiolitic detritus in sediments derived from subduction complexes is strictly related to the polarity of the subduction zone. In convergent settings associated with eastward/north-eastward subduction (Doglioni et al., 2007), complete sections of forearc lithosphere may be obducted during early collision stages, as for peri-Arabic and peri-Indian ophiolites exposed along the Alpine-Himalayan orogenic system. Oceanic crustal rocks are predominant in ophiolitecapped accretionary wedges from Cyprus to Makran and the Andaman Islands (Ricou, 1972; Moores et al., 1984; Pedersen et al., 2010). Detritus consequently includes abundant mafic volcanic/metavolcanic to igneous/metaigneous rock fragments (lathwork to boninite, diabase/ metadiabase, epidosite, plagiogranite, gabbro, mafic/ ultramafic cumulate) and heavy-mineral suites dominated by clinopyroxene, epidote and actinolite (Garzanti et al., 2000). Mantle peridotites are instead widely unroofed in the northern Oman mountains, which represent a fully developed obduction orogen formed during north-eastward continental subduction of the Arabian continental margin. Sands shed by the Sama’il Ophiolite are thus characterized by widespread ultramafic rock fragments, whereas enstatite associated with subordinate olivine and spinel dominates the heavy-mineral assemblages (Garzanti et al., 2002b). Instead, ophiolitic units do not occur on Barbados island, which is part of an arc-trench system associated with westward subduction, slab roll-back and back-arc extension (Bigi et al., 2003). The oceanic plate is subducted steeply, and the decollement plane is much too shallow (
