<<

Primary silica granules—A new mode of sedimentation

Elizabeth J.T. Stefurak1*, Donald R. Lowe1, Danielle Zentner1, and Woodward W. Fischer2 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91126, USA

ABSTRACT SAMPLES AND METHODS In the modern silica cycle, dissolved silica is removed from sea- Outcrops, polished hand samples, and polished petrographic thin water by the synthesis and sedimentation of silica biominerals, with sections were used to examine silica granules. Some samples are from the additional sinks as authigenic phyllosilicates and silica cements. BARB4 core from the 2011 International Continental Scientifi c Drilling Fundamental questions remain, however, about the nature of the Program drilling project in the Barberton Greenstone Belt (South Africa). ancient silica cycle prior to the appearance of biologically mediated Elements of interest (Ca, Mg, Fe, Al, and P or Ti) were mapped in carbon- silica removal in time. The abundance of siliceous coated (~14 nm thick) polished thin sections using a JEOL JXA-8200 sedimentary rocks in sequences, mainly in the form of chert, advanced electron probe microanalyzer at the Division of Geological strongly indicates that abiotic silica precipitation played a signifi - and Planetary Sciences Analytical Facility at the California Institute of cant role during Archean time. It was previously hypothesized that Technology (Pasadena, California, USA) and using the JEOL JXA-8230 these cherts formed as primary marine precipitates, but substantive SuperProbe electron probe microanalyzer at the School of Sciences evidence supporting a specifi c mode of sedimentation was not pro- Mineral Analysis Facility at Stanford University (Stanford, California). vided. We present sedimentologic, petrographic, and geochemical Qualitative intensity maps without background corrections were col- evidence that some and perhaps many Archean cherts were deposited lected, operating the electron probe in wavelength dispersive X-ray spec- predominately as primary silica grains, here termed silica granules, trometer mode at 15 kV accelerating voltage, 100 nA beam current, and that precipitated within marine waters. This mode of silica deposi- 100 ms dwell time. tion appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean OBSERVATIONS oceans. Understanding this mechanism promises new insights into the Silica granules are round, internally unstructured, sand-sized silica Archean silica cycle, including chert petrogenesis, microfossil preser- particles (Fig. 1). The granules are composed of essentially pure micro- vation potential, and Archean alkalinity budgets and silicate weather- crystalline quartz, although minor Fe-bearing impurities, especially sid- ing feedback processes. erite and hematite, occur locally. The occurrence of these silica grains is not limited to white chert bands. In many cases, cherty layers as much as INTRODUCTION 50 cm thick are composed largely of silica-rich grains, and many detrital It has been suggested that primary chemical precipitation of amor- sedimentary deposits include virtually pure silica grains mixed with a vari- phous silica played a major role as a silica sink during time ety of carbonaceous, volcaniclastic, and other sediment and particle types. (Lowe, 1999a; Maliva et al., 2005; Posth et al., 2008; Siever, 1992), al- Most granules display evidence of compaction (Fig. 2); granule cross sec- though unambiguous examples of primary silica phases were elusive. tions are elliptical in planes perpendicular to the bedding plane, with an Pre–3.0 Ga Archean sedimentary units include abundant chert litholo- average grain shape of an oblate spheroid. Unlike boudins, barrel-shaped gies formed through silica replacement and/or cementation of volcanic deformation structures formed during extension, compacted granule cross ash, detrital sediments, and a variety of other primary sediment types. sections are similar along any plane perpendicular to the bedding plane. One common element of these cherty sequences is the occurrence of lay- This compaction and the current microcrystalline nature of the granules ers or bands of white- to light gray–weathering chert, often translucent, exclude an origin as monocrystalline quartz sand, suggesting instead an generally <10 cm thick, and composed of nearly pure SiO2 (>99 wt%) initial composition as amorphous silica. (Lowe, 1999a). These layers are widely interbedded with carbonaceous Granules can be easily distinguished in hand specimen and thin sec- layers containing trace organic matter (Lowe, 1999a; Walsh and Lowe, tion when not compacted, but are more diffi cult to recognize when se- 1999), ferruginous bands, or sideritic layers of comparable thickness to verely fl attened (Fig. 2) or when the surrounding cement is also composed form black and white banded chert, banded iron formation, and banded of nearly pure and largely homogeneous microquartz (Figs. 1A and 2A). ferruginous chert, respectively. Some larger silica grains appear to be aggregates of individual granules Banded black and white cherts have been considered likely candi- (Fig. 1C), but the sand-sized granules are unstructured in thin section and dates for primary silica precipitates; early deformation features (Lowe, distinct from other types of subspherical grains within the same Archean 1999a) and oxygen isotopic data (Hren et al., 2009; Knauth and Lowe, sequences, such as accretionary lapilli (Lowe, 1999b) or impact spherules 1978, 2003) are consistent with primary or earliest diagenetic band for- (Lowe and Byerly, 1986). They lack relict textures that would indicate mation. Two hypothetical band formation mechanisms have been pro- diagenetic transformation or replacement of primary carbonate (Maliva et posed: (1) primary precipitation of silica on the seafl oor (Lowe, 1999a; al., 2005) or volcanic (DiMarco and Lowe, 1989) grains. Electron probe van den Boorn et al., 2007), and (2) earliest diagenetic segregation of maps of Al, Fe, Ca, Mg, P, and Ti do not reveal internal structuring (Fig. 3) adsorbed silica, originally deposited homogeneously with carbonaceous and support petrographic observations that granules are generally distin- matter and/or iron oxides, into distinct layers (Lowe, 1999a). While guishable from the surrounding microquartz matrix because they contain many white chert layers are massive, a surprising number display pre- few trace impurities. served internal granular textures characterized by sand-sized grains of Granules are common in sedimentary units representing a variety of nearly pure silica. This observation suggests that many, if not all, white depositional environments, from intertidal to deep-water basinal settings chert bands originated via a third novel mechanism, i.e., deposition of in the Barberton Greenstone Belt in South Africa and the Pilbara Craton primary silica grains. These pure to nearly pure silica particles are here in Western . In the following discussion we describe the proper- termed silica granules. ties and distribution of granules in four specifi c occurrences that provide examples of the morphology and environmental diversity of granules in *E-mail: [email protected]. the 3.2–3.5 Ga Barberton and Pilbara sequences.

GEOLOGY, April 2014; v. 42; no. 4; p. 1–4; Data Repository item 2014106 | doi:10.1130/G35187.1 | Published online XX Month 2013 GEOLOGY© 2014 Geological | April Society 2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 AB A

C D

B

E S F S S Q Q

C

G

Figure 2. Hypothetical sequence of progressive com- paction of silica granules in white chert band illustrates how matrix material can take on appearance of anas- tomosing laminations with increasing compaction. Figure 1. Silica granules. Granules are outlined (dotted lines) for clar- A: Minimally compacted. B: Moderately compacted. ity in right panels of A–C. Scale bars are 500 µm. See Table DR2 (see C: Severely compacted. Scale bars are 500 µm. See Ta- footnote 1) for more detailed stratigraphic information for samples ble DR2 (see footnote 1) for more detailed stratigraphic shown. A: Pure silica granules from within white chert band, barely and location information for samples shown. distinguishable due to presence of trace carbonate inclusions in sur- rounding material. Upper Mendon Formation, , South Africa. B: Granules (possibly including aggregates of granules, based on irregular shapes) with carbonaceous grains and matrix. Lower Mapepe Formation, Fig Tree Group, South Africa. C: Large in- traclast (dashed outline) composed of round, uncompacted granules. Upper Mendon Formation. D: Granules rimmed with diagenetic hema- A B tite (upper left) or completely fi lled with diagenetic siderite and hema- tite (lower right). Antarctic Creek Member at base of Mount Ada Basalt, , . E, F: Pure silica granules in plane-polarized light (E) and cross-polarized light (F), some rimmed with diagenetic siderite (S) or replaced with coarse quartz cement (Q). 1 cm 1 mm Antarctic Creek Member at base of Mount Ada Basalt. G: Lens of silica granules within ferruginous shale, including micron-scale hematite grains within matrix. Lower Mapepe Formation. C 50

40

Figure 3. Layer of silica granules within banded iron formation in Mapepe Formation, South Africa. A: Hand-sample scale. Granule 30 layer indicated by dashed outline. B: Magnifi ed view showing grains visible on sample surface. C: Representative electron microprobe relative intensity map of Fe (WDS—wavelength dispersive X-ray 20 spectrometer). Note that granules are distinguishable by their rela- tive lack of trace minerals. Main Fe-bearing phases in this example are siderite (larger, euhedral grains) and hematite (smaller, micron- 10 scale grains). See Table DR2 (see footnote 1) for more detailed strati- 500500 μμmm graphic information for sample shown; see Figure DR1 for additional WDS intensity - Fe relative electron microprobe maps from this sample. 0

2 www.gsapubs.org | April 2014 | GEOLOGY Within the largely basaltic Warrawoona Group (Pilbara Craton), the PROCESSES OF GRANULE FORMATION AND DEPOSITION Antarctic Creek Member is a 4–14-m-thick unit that includes silicifi ed All granules examined to date are sand-sized subspherical grains that felsic volcaniclastic sediments and a spherule bed formed by meteorite show evidence of compaction and lack internal structure. These character- impacts correlated with a similar 3472 Ma unit in the Barberton belt (Glik- istics suggest a common origin and provide several clues to the processes son et al., 2004; Hickman and Van Kranendonk, 2008; Lowe and Byerly, of granule formation and deposition. 1986). The Antarctic Creek Member includes many beds composed large- Silica granules are subrounded to rounded and subspherical in shape; ly of current-worked, subspherical, sand-sized silica granules. The gran- this could be a result of primary precipitation mechanism, abrasion of ini- ules occur as the main components of lenticular, 10–30-cm-thick, grain- tially more angular and irregular grains during transport, or some com- supported layers that locally display dune cross-bedding. Some granules bination of both. If the granules had originated as abraded rip-up clasts are composed purely of microquartz, while others contain trace amounts from a bed of amorphous silica precipitated on the seafl oor, analogous to of fi nely disseminated iron oxides and carbonates (Figs. 1D–1F). Most muddy intraclasts, we might expect at least rare occurrences of granules granules are oblong in cross section, with aspect ratios from 1:1 to 4:1. with irregular shapes or internal layers or lamination. However, the gran- The granules are cemented by a combination of microquartz and coarse ules are consistently subspherical, have rounded grain shapes, and lack in- quartz, with admixed hematite or goethite in the more reddish layers. We ternal structures. These characteristics are most consistent with formation interpret these cross-stratifi ed granule layers as current-deposited bars or in suspension, where precipitation could occur radially at subequal rates. shoals in a shallow subtidal to intertidal paleoenvironment; they demon- Granules display cross-sectional aspect ratios from 1:1 to >10:1, strate that silica granules were deposited in energetic shallow-water set- representing a range of compaction and cementation timing. The least tings, could be transported as sand-sized debris, and were suffi ciently re- compacted granules must have been cemented rapidly during earliest sistant that they could be swept into dunes and other bedforms by currents. diagenesis, implying that, under some conditions, seawater or pore fl uids Examples from the Barberton Greenstone Belt suggest that granules could promote rapid precipitation of silica. Minimally compacted gran- are not limited to shallow-water settings. We consider granules in three ules could therefore prove useful indicators for silicifi cation conditions stratigraphic units: the ca. 3400 Ma Buck Reef Chert, the ca. 3300 Ma with relatively high microfossil preservation potential. Mendon Formation, and the ca. 3260 Ma Mapepe Formation. The Buck There is no apparent relationship between granule size and deposi- Reef Chert is a 200–400-m-thick unit of banded black and white and tional water depth (Table DR1 in the GSA Data Repository1). This implies banded ferruginous cherts at the base of the Kromberg Formation (Lowe that precipitation was limited to a specifi c depth zone within the water and Byerly, 1999), and represents depositional environments ranging from column, because granule size might otherwise be predicted to increase a shallow, restricted evaporitic setting at the base (Lowe and Fisher Wor- with depth as longer settling or transport time allowed more silica to rell, 1999), to a relatively open shelf toward the middle, to a deeper, qui- precipitate. Archean seawater was likely near saturation with respect to eter setting marked by fi nely laminated, banded ferruginous cherts toward amorphous silica (Siever, 1992) and silica solubility would have decreased the top (Lowe and Byerly, 1999). The uppermost Mendon Formation is an with water depth; silica is signifi cantly less soluble at lower temperatures ~50-m-thick unit of black chert, banded black and white chert, and banded (Siever, 1962) and is not particularly sensitive to pressure (Willey, 1974). ferruginous chert deposited in relatively quiet water below storm wave Theoretical rates of aggregation or coagulation of silica via polymeriza- base (Lowe and Byerly, 1999). The basal Mapepe Formation, which rep- tion would be highest at low temperatures (<60 °C) and either (1) slightly resents a quiet and probably deep-water setting (Lowe and Nocita, 1999), acidic pH (pH range of 4–7), or (2) at higher pH (pH range of 7–10) in includes thick units of fi ne ferruginous shale, banded ferruginous chert, the presence of electrolytes (Iler, 1979; Williams and Crerar, 1985). Either and, in a few areas, hematitic banded iron formation defi ned on a lamina- of these conditions might be met in shallow-water environments, which tion scale by alternations of sideritic, hematitic, and cherty layers. could have had slightly acidic pH due to interaction with a CO2-bearing There are three types of occurrences of silica granules within black atmosphere (Hessler et al., 2004; Kasting, 1987) or high salinity (Knauth, cherts of the Buck Reef Chert and the upper Mendon Formation: (1) ma- 2005) due to evaporative concentration. The observation of silica granules trix-supported granules within a microquartz groundmass that contains in a variety of depositional water depths, including intertidal settings, also trace inclusions of carbonaceous matter, phyllosilicates, and/or carbonate supports granule formation in relatively shallow water. phases (dolomite and ankerite) (Figs. 1A and 2A–2C); (2) grain-support- ed, current-deposited granules mixed with carbonaceous and/or volcanic CONCLUSIONS grains (Fig. 1B); and (3) larger, irregularly shaped grains and intraclasts, The widespread occurrence and abundance of silica granules in these many of which appear to have formed as aggregates of sand-sized gran- pre–3.2 Ga sequences indicate that they represented a signifi cant silica ules, mostly occurring in comparatively high energy event beds mixed depositional mode during Paleoarchean time. It is also very likely that with other intraclasts in addition to carbonaceous and volcanic grains many massive, structureless silica layers and bands in these sequences (Fig. 1C). Granules are often strongly compacted, with aspect ratios in represent accumulations of granules lacking impurities that would allow excess of 10:1 (e.g., Fig. 2C). petrographic or geochemical discrimination of granules and the surround- There are two types of occurrences of silica granules within the ferru- ing matrix and/or cements. Although the silica granules described here ginous lithologies of the Mapepe Formation: (1) as centimeter-scale, len- bear some resemblance to granular iron formation (Simonson, 2003), as ticular layers interbedded with ferruginous shale (Fig. 1G), and (2) as sub- chemical grains particular to Precambrian time, their unique mineralogy centimeter-scale lenticular layers within banded iron formations (Fig. 3). and occurrence in nonferruginous settings suggest that silica granules The granule layers in the banded iron formations are less abundant than formed by mechanisms and/or conditions different from those of granular other layer types and stand out by their lenticularity, suggesting that gran- iron formation. Granule formation and deposition appear to have repre- ule deposition was locally superimposed on slow background accumula- sented a substantial sink for dissolved silica in Paleoarchean oceans, with tion of material settling out of suspension (Fischer and Knoll, 2009). Some additional sinks of banded iron formations, authigenic phyllosilicates, granules are composed purely of microquartz, while others contain trace inclusions of hematite that may represent oxidized siderite. Granules in this setting display a range of compaction effects, with maximum aspect ratios 1GSA Data Repository item 2014106, Figure DR1 (additional electron probe maps of area shown in Fig. 3C), Table DR1 (granule size data), and Table DR2 of ~5:1 (Fig. 3C). Grain-supported layers are typically silica cemented, (stratigraphic information for samples shown in fi gures), is available online at whereas matrix-supported layers contain micron-scale hematite and sider- www.geosociety.org/pubs/ft2014.htm, or on request from editing@geosociety ite grains (in addition to microquartz) in the matrix. .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY | April 2014 | www.gsapubs.org 3 and silica cements. Because the formation of clays via reverse weathering Lowe, D.R., 1999b, Shallow-water sedimentation of accretionary lapilli-bearing consumes alkalinity and short-circuits the silicate weathering feedback strata of the Msauli Chert: Evidence of explosive hydromagmatic komatiitic in (Michalopoulos and Aller, 1995), understanding the contributions of each volcanism, Lowe, D.R., and Byerly, G.R., eds., Geologic evolution of the Barberton Greenstone Belt, South Africa: Geological Society of America silica sink has important implications for climate, seawater chemistry, and Special Paper 329, p. 213–232, doi:10.1130/0-8137-2329-9.213. the carbon cycle during Archean time. Silica granules are notably absent Lowe, D.R., and Byerly, G.R., 1986, Early Archean silicate spherules of probable from younger rocks, indicating that the dominant mode of silica deposi- impact origin, South Africa and Western Australia: Geology, v. 14, p. 83– tion has evolved over geologic time, possibly driven by secular changes 86, doi:10.1130/0091-7613(1986)14<83:EASSOP>2.0.CO;2. Lowe, D.R., and Byerly, G.R., 1999, Stratigraphy of the west-central part of the in continental growth, weathering, ocean composition, and the biological Barberton Greenstone Belt, South Africa, in Lowe, D.R., and Byerly, G.R., silica cycle. eds., Geologic evolution of the Barberton Greenstone Belt, South Africa: Geological Society of America Special Paper 329, p. 1–36, doi:10.1130 ACKNOWLEDGMENTS /0-8137-2329-9.1. We thank C. Ma (electron probe, Caltech) and R. Jones (electron probe, Stan- Lowe, D.R., and Fisher Worrell, G., 1999, Sedimentology, mineralogy, and im- ford) for their assistance. Stefurak was supported by a National Science Founda- plications of silicifi ed evaporites in the Kromberg Formation, Barberton tion graduate fellowship. Fischer was supported by the National Aeronautics and Greenstone Belt, South Africa, in Lowe, D.R., and Byerly, G.R., eds., Geologic Space Administration Exobiology program (grant NNX09AM91G) and the David evolution of the Barberton Greenstone Belt, South Africa: Geological Society and Lucile Packard Foundation. The School of Earth Sciences, Stanford University, of America Special Paper 329, p. 167–188, doi:10.1130/0-8137-2329-9.167. provided funds to Lowe. We are grateful to Sappi Forest Products, the Mpumalanga Lowe, D.R., and Nocita, B.W., 1999, Foreland basin sedimentation in the Mapepe Parks Board (J. Eksteen and L. Loocks), and Taurus Estates (C. Wille) for access Formation, southern-facies Fig Tree Group, in Lowe, D.R., and Byerly, to private properties. G.R., eds., Geologic evolution of the Barberton Greenstone Belt, South Africa: Geological Society of America Special Paper 329, p. 233–258, REFERENCES CITED doi:10.1130/0–8137–2329–9.233. DiMarco, M.J., and Lowe, D.R., 1989, Petrography and provenance of silicifi ed early Maliva, R.G., Knoll, A.H., and Simonson, B.M., 2005, Secular change in the Archaean volcaniclastic sandstones, eastern Pilbara Block, Western Australia: Precambrian silica cycle: Insights from chert petrology: Geological Society Sedimentology, v. 36, p. 821–836, doi:10.1111/j.1365-3091.1989.tb01748.x. of America Bulletin, v. 117, p. 835–845, doi:10.1130/B25555.1. Fischer, W.W., and Knoll, A.H., 2009, An iron shuttle for deepwater silica in Late Michalopoulos, P., and Aller, R.C., 1995, Rapid clay mineral formation in Amazon Archean and early iron formation: Geological Society of Delta sediments: Reverse weathering and oceanic elemental cycles: Science, America Bulletin, v. 121, p. 222–235, doi:10.1130/B26328.1. v. 270, p. 614–617, doi:10.1126/science.270.5236.614. Glikson, A.Y., Allen, C., and Vickers, J., 2004, Multiple 3.47-Ga-old asteroid im- Posth, N.R., Hegler, F., Konhauser, K.O., and Kappler, A., 2008, Alternating pact fallout units, Pilbara Craton, Western Australia: Earth and Planetary Si and Fe deposition caused by temperature fl uctuations in Precambrian Science Letters, v. 7040, p. 1–14, doi:10.1016/S0012-821X(04)00104-9. oceans: Nature Geoscience, v. 1, p. 703–708, doi:10.1038/ngeo306. Hessler, A.M., Lowe, D.R., Jones, R.L., and Bird, D.K., 2004, A lower limit for Siever, R., 1962, Silica solubility, 0°–200° C, and the diagenesis of siliceous sedi- atmospheric carbon dioxide levels 3.2 billion ago: Nature, v. 428, ments: Journal of Geology, v. 70, p. 127–150, doi:10.1086/626804. p. 736–738, doi:10.1038/nature02471. Siever, R., 1992, The silica cycle in the Precambrian: Geochimica et Cosmochimica Hickman, A., and Van Kranendonk, M.J., 2008, Marble Bar geologic map: Acta, v. 56, p. 3265–3272, doi:10.1016/0016-7037(92)90303-Z. Geological Survey of Western Australia Sheet SF 50–8, scale 1:250,000. Simonson, B.M., 2003, Origin and evolution of large Precambrian iron forma- Hren, M., Tice, M.M., and Chamberlain, C.P., 2009, Oxygen and hydrogen iso- tions, in Chan, M.A., and Archer, A.W., eds., Extreme depositional environ- tope evidence for a temperate climate 3.42 billion years ago: Nature, v. 462, ments: Mega end members in geologic time: Geological Society of America p. 205–208, doi:10.1038/nature08518. Special Paper 370, p. 231–244, doi:10.1130/0-8137-2370-1.231. Iler, R.K., 1979, The chemistry of silica: New York, John Wiley & Sons, 866 p. van den Boorn, S.H.J.M., van Bergen, M.J., Nijman, W., and Vroon, P.Z., 2007, Kasting, J.F., 1987, Theoretical constraints on oxygen and carbon dioxide con- Dual role of seawater and hydrothermal fl uids in Early Archean chert forma- centration in the Precambrian atmosphere: Precambrian Research, v. 34, tion: Evidence from silicon isotopes: Geology, v. 35, p. 939–942, doi:10.1130 p. 205–229, doi:10.1016/0301-9268(87)90001-5. /G24096A.1. Knauth, L.P., 2005, Temperature and salinity history of the Precambrian ocean: Walsh, M.M., and Lowe, D.R., 1999, Modes of accumulation of carbonaceous Implications for the course of microbial evolution: Palaeogeography, matter in the early Archean: A petrographic and geochemical study of the Palaeoclimatology, Palaeoecology, v. 219, p. 53–69, doi:10.1016/j.palaeo carbonaceous cherts of the Swaziland Supergroup, in Lowe, D.R., and .2004.10.014. Byerly, G.R., eds., Geologic evolution of the Barberton Greenstone Belt, Knauth, L.P., and Lowe, D.R., 1978, Oxygen isotope geochemistry of cherts from South Africa: Geological Society of America Special Paper 329, p. 115– the Onverwacht Group (3.4 billion years), Transvaal, South Africa, with 132, doi:10.1130/0-8137-2329-9.115. implications for secular variations in the isotopic composition of cherts: Willey, J.D., 1974, The effect of pressure on the solubility of amorphous silica Earth and Planetary Science Letters, v. 41, p. 209–222, doi:10.1016/0012 in seawater at 0°C: Marine Chemistry, v. 2, p. 239–250, doi:10.1016/0304 -821X(78)90011-0. -4203(74)90018-8. Knauth, L.P., and Lowe, D.R., 2003, High Archean climatic temperature in- Williams, L.A., and Crerar, D.A., 1985, Silica diagenesis, II. General mechanisms: ferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Journal of Sedimentary Petrology, v. 55, p. 312–321, doi:10.1306/212F86B1 Supergroup, South Africa: Geological Society of America Bulletin, v. 115, -2B24-11D7-8648000102C1865D. p. 566–580, doi:10.1130/0016-7606(2003)115<0566:HACTIF>2.0.CO;2. Lowe, D.R., 1999a, Petrology and sedimentology of cherts and related silici- Manuscript received 9 October 2013 fi ed sedimentary rocks in the Swaziland Supergroup, in Lowe, D.R., and Revised manuscript received 18 December 2013 Byerly, G.R., eds., Geologic evolution of the Barberton Greenstone Belt, Manuscript accepted 20 December 2013 South Africa: Geological Society of America Special Paper 329, p. 83–114, doi:10.1130/0-8137-2329-9.83. Printed in USA

4 www.gsapubs.org | April 2014 | GEOLOGY