Stratigraphy, Sedimentology and Provenance of the Ca. 3.26 Ga Mapepe Formation in the Manzimnyama Syncline, Barberton Greenstone Belt, South Africa
STRATIGRAPHY, SEDIMENTOLOGY AND PROVENANCE OF THE CA. 3.26 GA MAPEPE FORMATION IN THE MANZIMNYAMA SYNCLINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA
A THESIS SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
Danielle Bridgette Zentner June 2014
© Copyright by Danielle Zentner 2014 All Rights Reserved
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ABSTRACT
The Barberton greenstone belt in South Africa contains some of the oldest, well- preserved sedimentary rocks known from mid-Archean time. These rocks represent an important record of surface conditions and sedimentary processes on early Earth. A recent campaign by the International Continental Scientific Drilling Program acquired five cores in the belt, one of which, the BARB4 core, targeted a deep-water sedimentary sequence of banded iron formation (BIF), banded ferruginous chert (BFC) and siliciclastic rocks in the Manzimnyama Syncline. The Manzimnyama Syncline is located in a little studied area in the southeastern extent of the belt near the Swaziland border. Extensive commercial forest cover in the area has obscured the surface stratigraphy, but the acquisition of the ~540 m long BARB4 core permits detailed analysis of a continuous, relatively unweathered succession for the first time. This thesis combines, 1) sedimentological analysis from a high-resolution core description of the lithic sandstone section and along-strike outcrops at the surface with, 2) geochemical analysis of shale sampled along the core length. Sedimentological analysis reveals a coarse-grained, sand-rich (N/G=0.96) deep- water system dominated by sedimentation from high-density turbidity flows. The beds, which average 16 cm in thickness, have predominately massive, poorly sorted bases with thin, flat-laminated and rarely cross-laminated tops. These beds, which often contain mud rip-up clasts and chert plates, were emplaced primarily via rapid suspension settling of sediment from collapsing, high-density flows. The tabularity of the beds and the lack of large-scale scour suggests the flows probably deposited in an unconfined setting, most likely a frontal lobe location at the terminus of the feeder system. The paucity of mud suggests that the feeder system was likely not a levee-confined conduit, but was instead a canyon or similar incisional feature. Transport distance through the conduit was probably short, based on lack of grain size fractionation expected from flow filtering during a long run-out distance. The siliciclastic sediments had a coeval lateral facies relationship with the orthochemical deposits. This is attested to by, 1) gradational contacts and inter-bedded nature of the sequence, including siliciclastic turbidite beds within the BIF section, siliciclastic material in chert-plate breccia beds in the BFC section, and centimeter-scale chert horizons in the finest-grained section of the siliciclastic section, and, 2) the deformed chert plates in the turbidite beds that were clearly still soft at the time of deposition and likely represent the Archean equivalent of mud rip-up clasts. Petrographic examination of the sandstone indicates that, while the grains are pervasively seritized and silicified, grain textures and relict crystal shapes are still largely preserved. Quantitative point counting results show relative modal abundances of iv polycrystalline quartz (chert) fragments (51.8%) and volcanic lithic fragments (41.0%), with monocrystalline quartz (5.7%) and feldspar (1.5%) as minor components. Geochemical analysis of shale indicates pervasive metasomatism, which remobilized all labile species, depleting the rocks of Na2O, CaO and Sr while enriching the shale with K, Ba and SiO2. Metasomatic overprinting and Al-poor source rocks, such as komatiite and chert, complicate assessment of weathering conditions in the source area, but the preservation of volcanic grains suggests that the weathering environment was not extreme. Major oxide, trace and rare earth element (REE) abundances in shale indicate a mixed felsic and mafic source, with high Ni and Cr values suggesting some contribution of sediment from ultramafic rocks. Mafic and ultramafic material was likely sourced from proximal uplifts of Onverwacht Group strata. In contrast, the felsic signature likely originates from the tuffaceous products of explosive dacitic volcanism, as the feldspar- and quartz-poor sandstone composition indicates that the plutonic roots of the belt had not been accessed by erosion. REE analyses reveal lower ∑REE, increasing values of Eu/Eu*, and decreasing LaN/YbN ratios with stratigraphic position, pointing to an evolution from a felsic source to a more mafic source over time, which is supported by similar trends in major oxide and trace element ratios with depth. This trend may be the result of either, 1) deeper-level incision into mafic rocks in the source area with time, 2) decrease in explosive dacitic volcanism with time, or 3) a combination of both volcanism and evolution of the source area. The petrographic and geochemical results support the findings of previous studies that indicate the southern facies Fig Tree Group rocks are distinct from those north of the Inyoka fault. The sandstone composition and shale geochemistry of the Manzimnyama Syncline strata, however, compare favorably with the Mapepe Formation rocks in the Barite Syncline and the Granville Grove fault area of the Central Domain. This suggests, that despite the apparent absence in the study area of impact-related spherule beds characteristic of the basal Fig Tree Group contact, these strata are genetically related to the Mapepe Formation north of the Heights Syncline and south of the Inyoka fault.
v ACKnowledgements
I would like to thank my advisor, Donald Lowe, for introducing me to the Barberton Mountain Land and unfamiliar world of the Archean Earth. Steve Graham was a first-rate reviewer, with a great eye for writing style and provided many excellent suggestions for improving the manuscript. I had many conversations about my rocks with Tim McHargue, whose input helped enormously. Rónadh Cox was, as always, a fountain of good advice from afar. An especially big thanks goes to Christoph Heubeck, for both insightful conversations about field matters and for brightening up the field seasons with his masterfully told stories. Gary Byerly shared critical age dates from the field area. Ian Hagmann and Gail Mahood provided key input on petrography, and Sam Johnstone assisted with the FFT analysis. My “accountability buddy”, Lizzy Trower Stefurak, in particular, was a great source of support and a sounding board in the final months. Barry Shaulis provided editorial help as did my boyfriend, who is possibly the most patient man I have ever met, and was unfailingly supportive and helpful. Thank you to Glenn and Joni Sharman for watching my dog when I was away from campus. The office staff, including Stephanie, Yvonne, Alyssa, Daisy, and Lauren, all deserve thanks for their quick and professional assistance with my questions. Many good times were had in the field with the Barberton Greenstone Academy. Martin Homann, Sami Drabhan, Henry Nordhauß, Paul Fugmann, Nadja Drabon, Lizzy Trower Stefurak, Nicholas Decker, and Kimberly McManus were all excellent field assistants and are good friends. Some of my fondest memories of my time at Stanford come from our antics at the Old Coach Road Guesthouse after long field days. Thank you to Lili and Adriaan for being gracious hosts. I want to thank Sappi Forest Products for granting access permission to the field area. Much of my funding came from the Stanford Project on Deep-water Depositional Systems (SPODDS) affiliates and the Stanford School of Earth Sciences, without which I would not have had the opportunity to go to South Africa and to complete this work. Finally and above all, I would like to thank my family and especially my indomitable mother. I consider myself a very lucky person to have you all.
vi TABLE OF CONTENTS
CHAPTER 1: SEDIMENTOLOGY AND STRATIGRAPHY OF AN ARCHEAN DEEP-WATER SEQUENCE FROM THE CA. 3.26 GA MAPEPE FORMATION, MANZIMNYAMA SYNCLINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA
ABSTRACT 1
INTRODUCTION 2
GEOLOGIC SETTING 3 Study Area 6 Structure 6 Stratigraphy 7 Core 8 METHODS 9 Core statistics 10
RESULTS 10 Lithofacies La: Amalgamated sandstone 10 Interpretation 11 Lithofacies Lsm: Inter-bedded sandstone and mudstone 12 Interpretation 12 Lithofacies Lm: Inter-bedded mudstone and sandstone 13 Interpretation 14 Synthesis 15 Core to outcrop correlation 16
DISCUSSION 16
CONCLUSIONS 21
REFERENCES CITED 23
vii
CHAPTER 2: SHALE GEOCHEMISTRY AND SANDSTONE PETROGRAPHY OF THE CA. 3.26 GA MAPEPE FORMATION, SOUTHEASTERN BARBERTON GREENSTONE BELT, SOUTH AFRICA
ABSTRACT 61
INTRODUCTION 61
GEOLOGIC SETTING 62 Study Area 65
METHODS 66 Sandstone petrography 66 Shale geochemistry 67
RESULTS 67 Petrography 67 Geochemistry 70 Major elements 70 Rare earth elements 70 Trace elements 71
DISCUSSION 72 Alteration 72 Weathering 72 Provenance 73 Comparison to Fig Tree Group in other areas of the BGB 77
CONCLUSIONS 78
REFERENCES CITED 80
SUMMARY 113
APPENDIX A 115
APPENDIX B 141
viii
LIST OF TABLES
Table 1. Point counting scheme for framework grain modal abundances of 15 sandstone samples from the BARB4 core. 108
Table 2. Major, trace and REE abundances from 15 shale samples extracted from the BARB4 core. 110
ix
LIST OF FIGURES
CHAPTER 1
Figure 1. Location maps and stratigraphic columns for the Barberton greenstone belt (BGB) and in the study area. 29 Figure 2. Generalized stratigraphic columns for formation of the Fig Tree Group with age and stratigraphic relationships from Lowe and Byerly (1999). 31 Figure 3. High-resolution scans of banded iron formation (BIF) and banded ferruginous chert (BFC) from the BARB4 core. 33 Figure 4. Correlation of BARB4 core to field measured sections. 35 Figure 5. Summary figure of Lithofacies La. 37 Figure 6. Outcrop photos of lithofacies from field measured sections. 39 Figure 7. High-resolution core scans showing examples of grading profiles observed in the BARB4 core. 41 Figure 8. High-resolution scans of the BARB4 core showing chert-plates and chert layers. 43 Figure 9. Summary figure of Lithofacies Lsm. 45 Figure 10. High-resolution scans of the BARB4 core showing sedimentary structures. 47 Figure 11. Summary figure of Lithofacies Lm. 49 Figure 12. Plots showing whole core analysis of bed statistics. 51 Figure 13. Plots showing large-scale trends in maximum bed grain size along BARB4 core length. 53 Figure 14. Plots showing large-scale in bed thickness trends along the BARB4 core length. 55 Figure 15. Composite plot showing graphic core log, lithofacies and average bed thickness trends along the length of the BARB4 core. 57 Figure 16. Power vs frequency graph of bed thickness averaged over 1 m depth intervals showing no peaks above noise for any wavelength. 59
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CHAPTER 2
Figure 1. Simplified geologic map of the Barberton greenstone belt and generalized stratigraphic column modified from Lowe and Byerly (1999). 84 Figure 2. Generalized stratigraphic columns for formations of the Fig Tree Group with age and stratigraphic relationships from Lowe and Byerly (1999) and stratigraphy of the lower limb of the Manzimnyama Syncline. 86 Figure 3. Stratigraphic section of the BARB4 core described from high-resolution core scans. 88 Figure 4. QFL and QmFLt plots after Dickinson and Suczek (1979) and Dickinson et al. (1983) showing inferred provenance type from detrital modes of sandstone. 90 Figure 5. Photomicrographs of lithic grains from the BARB4 core. 92 Figure 6. Photomicrographs of tuffaceous lithic fragments from the BARB4 core. 94
Figure 7. Plots of major oxide proportions in wt% of BARB4 shale vs SiO2 content, referenced to depth. 96 Figure 8. A selected suite of elements normalized to PAAS (Taylor and McLennan, 1985). 98 Figure 9. Plots of shale sample REE abundances normalized to Boynton (1984) chondrite values. 100 Figure 10. Plots showing likelihood of felsic or mafic provenance for Mapepe Formation shale. 102 Figure 11. CN-A-K and CNK-A-FM plots after Nesbitt and Young (1982). 104 Figure 12. Major oxide and trace element plots useful for discriminating source rock compositions. 106
xi
CHAPTER 1: sedimentology and stratigraphy of an Archean deep-water sequence from the ca. 3.26 Ga Mapepe Formation, Manzimnyama Syncline, Barberton greenstone belt, South Africa
Abstract
The ca. 3.26 billion year old sedimentary rocks in the Barberton greenstone belt, South Africa, include some of the world’s oldest known deep-water deposits. The Mapepe Formation in the southeastern extent of the belt includes a deep-water sequence containing a suite of orthochemical rocks and interbedded coarse-grained turbiditic sandstone. This study combines road-cut exposures and a recently cut core to interpret the ~350 m lithic sandstone sequence. Three lithofacies were identified in the clastic turbidite section: 1) thick- bedded, very coarse-grained, amalgamated and predominantly massive beds containing abundant mud clasts and chert-plates that deposited as high-density turbidity flows, 2) medium- to thick-bedded, coarse-grained, amalgamated, massive beds capped with flat lamination and rare cross-lamination originating from high-density turbidity flows with low density flow tops, and 3) thin- to medium-bedded, fine- to medium-grained sandstone beds inter-bedded with mud and centimeter-scale chert horizons deposited by both high and low density flows and pelagic sedimentation. Overall, the sequence is sand-rich (N/ G= 0.96). The notable paucity of mud in even the lowest-energy sections may stem from processes such as flow bypass, flow stripping and flow ponding, or may alternatively reflect source rock lithology, high erosion rates or mild weathering conditions in the hinterland. This mud deficit likely precluded the building of constructional levees, suggesting that a canyon or a similar incisional feature confined the density flows during transport. Transport distance from the staging area to the site of deposition was probably short, given the angularity of the material and the poorly developed grain size fractionation. The density flows likely deposited in a frontal lobe location, based on the consistent tabularity of the beds, lack of large-scale scour and the predominance of massive, poorly sorted bases, indicating rapid fallout of sediment from suspension in an unconfined setting. A complex interplay of allogenic and self-organizational factors likely contributed to the observed overall upward- fining and- thinning stacking pattern. Compensational lobe stacking processes probably influenced stacking patterns at the tens-of-meters scale. External controls on larger scale stacking patterns—including climate, eustasy, and tectonics—remain poorly constrained for this ancient sequence. In particular, recent work indicates that “modern-style” subduction plate tectonics was not yet in operation during the time of Mapepe Formation deposition,
1 suggesting that the previously proposed foreland basin setting for these rocks may warrant reevaluation.
Introduction
Deep-water depositional systems of Archean age are unique in the rock record because of the widespread association of sedimentary rocks of precipitative orthochemical origins—such as banded iron formation—with siliciclastic material. Numerous published examples suggest this was a common deep-water lithofacies association in the past. These include such localities as the Rio das Velhas greenstone belt in Brazil (Baltazar and Zucchetti, 2007), the Beardmore-Geraldton and Abitibi greenstone belts in Canada (Barrett and Fralick, 1985; 1989; Mueller and Donaldson, 1992), the Hamersley Province and Pilbara block in Australia (Krapez et al., 2003; Eriksson, 1983) and the Barberton greenstone belt (BGB), South Africa (Heinrichs, 1980; Eriksson, 1983; Lowe and Byerly, 1999). Furthermore, the chemical and siliciclastic association spans nearly a billion years of time, strongly suggesting this was also a long-standing facies relationship in Archean deep- water settings. Despite this, and with the exception of a handful of studies (e.g., Eriksson, 1983; Simonson, 1985; Pickard et al., 2004; Bontognali et al., 2013), researchers have mainly focused on the chemical rocks, largely overlooking the associated clastic turbidite deposits. Just as in Phanerozoic deposits, these clastic turbidite deposits have the potential to provide a wealth of depositional and larger scale contextual information that is invaluable for understanding the entire deep-water succession. Fundamental questions, however, remain to be answered when it comes to clastic turbidite deposits of this age. Phanerozoic turbidite systems continue to be studied in great detail, from grain-scale sediment transport mechanics to large-scale external controls on channel and lobe behavior. As a result, a clearer picture is emerging with time of the controls, morphology and behavior of deep-water systems. In contrast, for Archean rocks of this age ambiguity exists on all levels, from the nature of fluid dynamics of Archean seawater and its effect on density flows, to the existence and style of plate tectonics (Benn et al., 2006; Condie and Pease, 2008; Fralick and Carter, 2011). Given these complexities, it becomes important to discover if Archean clastic turbidite successions display similar flow properties, architecture and stacking cyclicity as found in their Phanerozoic counterparts. If these most ancient clastic turbidite sections resemble younger turbidite successions and can be analyzed using standard techniques, their interpretation could provide insight into surface conditions, sedimentary processes and deep-water environments during Archean time. Ultimately, by providing additional contextual information, the clastic turbidites could bridge the interpretative gap for studying 2 the interbedded orthochemical facies, which largely lack modern equivalents. Here, we present the most detailed study to date of a clastic turbidite succession in one of the world’s oldest preserved deep-water depositional systems. Our study examines the ca. 3.26 Ga Mapepe Formation rocks exposed in the NE- SW-striking Manzimnyama Syncline, located in the Eastern Domain (ED) of the Barberton greenstone belt (BGB), South Africa (Fig. 1). Although these ancient sediments have undergone structural deformation and extensive metasomatism, the rocks are remarkably well preserved, to the point where sedimentary structures, textures and stratigraphic successions are still recorded. This study utilizes both recently drilled core and the associated surface exposures. With this robust data set, we apply standard analytical techniques developed from studying Phanerozoic deep-water systems to interpret the Archean clastic turbidite succession.
Geologic setting
The BGB is a structurally complex and altered volcano-sedimentary sequence located in South Africa and Swaziland. Its NE-SW-striking structural belts are separated into five distinct tectonostratigraphic structural domains comprised of approximately 12- 15 km of volcanogenic and sedimentary rocks ranging from >3.547 to <3.225 Ga in age (Armstrong et al., 1990; Kröner et al., 1991; Byerly et al., 1993, 1996; Lowe and Byerly, 1999). This stratigraphic succession is sub-divided into three major units—the Onverwacht, Fig Tree and Moodies Groups—which, taken together, make up the Swaziland Supergroup (Viljoen and Viljoen, 1969a, 1969b; Lowe and Byerly, 1999) (Fig. 1B). The oldest (>3.547-3.240 Ga) and thickest (8-12 km) unit is the Onverwacht Group, which consists primarily of komatiitic and basaltic volcanic rocks and subordinate felsic volcanic units and thin silicified sedimentary units (Lowe, 1999; Lowe and Byerly, 1999). The Mendon Formation (3.335-3.260 Ga), which caps the Onverwacht Group, is between 300 and 1000 m thick and contains thick sections of komatiitic volcanic rock intercalated with silicified sedimentary layers including black, banded black-and-white and banded ferruginous chert compositions (Kröner et al., 1991; Byerly et al., 1993, 1996; Lowe and Byerly, 1999). The Mendon Formation of the Onverwacht Group is conformably overlain by the chiefly sedimentary rocks of the Fig Tree Group (3.260-3.226 Ga). The up to ~1800 m thick Fig Tree Group is lithologically diverse and includes dacitic volcanic and volcaniclastic rocks, siliciclastic sandstone, chert clast conglomerate, shale, banded ferruginous chert, chert-plate breccia, and banded iron formation (Condie et al., 1970; Eriksson, 1980b; Heinrichs, 1980; Lowe and Byerly, 1999). Age dating and petrologic differences between 3 the Fig Tree Group rocks north and south of the Inyoka Fault suggest they represent distinct tectonostratigraphic units (Lowe and Byerly, 1999). As a result, Lowe and Byerly (1999) sub-divided the Fig Tree Group into northern and southern facies separated by the Inyoka Fault. The northern facies consists from top to bottom of the Schoongezicht, Bien Venue, Belvue Road, Sheba and Ulundi formations (Condie et al., 1970; Reimer, 1983; Lowe and Byerly, 1999; Kohler and Anhaeusser, 2002), whereas the southern facies includes the Aubier Villiers Formation and the Mapepe Formation, the latter of which is the focus of this study (Heinrichs, 1980; Lowe and Byerly, 1999) (Fig. 2). The northern facies Fig Tree formations vary enormously in stratigraphy and lithology (Fig. 2), making correlations among structural blocks problematic (Lowe and Byerly, 1999). The Ulundi, Sheba, and Belvue Road formations comprise, in some localities, a continuous succession (Condie et al., 1970; Lowe and Byerly, 2007). In ascending order, the Ulundi Formation (1-50 m thick) includes carbonaceous shale, ferruginous chert and other iron-rich strata (Reimer, 1983; Lowe and Byerly, 1999). The Sheba Formation (500- 2000 m thick) contains coarse immature sandstone with minor shale (Condie et al., 1970; Lowe and Byerly, 1999). The Belvue Road Formation is comprised largely of tuffaceous and carbonaceous shale with fine-grained sandstone and siltstone and chert (Condie et al., 1970; Lowe and Byerly, 1999). The Bien Venue Formation consists largely of quartz-muscovite schists derived from silicic volcanic and volcaniclastic rocks that conformably overly the Belvue Road Formation (Kohler and Anhauesser, 2002). Finally, the Schoongezicht Formation caps the northern facies Fig Tree Group, and is comprised of felsic volcaniclastic sandstone and conglomerate, chert and dacitic tuff (Condie et al., 1970; Lowe and Byerly, 1999). The lower contact of the Schoongezicht Formation appears to be a regional thrust fault throughout the northern part of the BGB, and the Schoongezicht Formation may actually be age-equivalent to the upper parts of the Belvue Road Formation and the Auber Villiers and Mapepe formations in the southern facies Fig Tree Group. All of the northern facies formations appear to have deposited in a deep-water environment dominated by hemipelagic sedimentation and turbidity flow deposition (Condie et al., 1970, Eriksson, 1980b). The southern facies Fig Tree sediments were deposited in a variety of depositional environments, including alluvial, fan-delta and shallow-subaqueous settings, and deep- water settings (Eriksson et al., 1980b, Heinrichs, 1980; Nocita and Lowe, 1990; Lowe and Nocita, 1999). Very limited paleocurrent data indicate that sediment was sourced from uplifts to the south and southeast towards Swaziland (Jackson et al., 1987; Lowe and Nocita, 1999). The Mapepe Formation (at least 1,300 m thick), which is the only formation of the Fig Tree Group present in the study area, includes dacitic tuff, banded iron formation, banded
4 ferruginous chert, volcaniclastic and chert grain-rich sandstone and conglomerate, minor shale and local barite deposits (Heinrichs, 1980; Nocita and Lowe, 1990; Lowe and Nocita, 1989; Lowe and Byerly, 1999). The upper contact of the Mapepe Formation is not exposed anywhere in the BGB (Lowe and Byerly, 1999). The Auber Villiers Formation (1000-1,300 m thick) overlies the Mapepe Formation at a fault contact and includes mostly volcaniclastic material, comprised of thick sections of dacitic tuff, as well as minor units of shale, black chert and conglomerate (Lowe and Byerly, 1999) (Fig. 2). Structural and stratigraphic complexity combined with sparse age-dating complicates correlation of the various formations of the northern facies Fig Tree Group (Lowe and Byerly, 1999). The limited age-dating, however, has provided a loose framework for correlation (Fig. 2). In contrast to the northern facies, single-zircon age-dating from dacitic tuff has reasonably well constrained the age of the Mapepe Formation (Lowe and Byerly, 1999). Various published reports indicate that the Mapepe Formation ranges from ca. 3.252 Ga to younger than 3.225 Ga in age (Kröner et al., 1991; Byerly et al., 1996; Lowe and Byerly, 1999). These dates, however, span the formation’s extent across structurally and stratigraphically complex domains, and do not include data from the study area. Three zircons extracted from a banded iron formation section in the Manzimnyama Syncline of the study area, however, indicate an age of 3.260 Ga (+/- 5 Ma) (personal communication, Byerly), which is among the oldest of any Mapepe Formation strata measured. Finally, the Fig Tree Group is unconformably overlain by the Moodies Group, which represents the youngest unit of the Swaziland Supergroup. The 3.223-3.210 Ga Moodies Group is 500-3000 m thick and incorporates siliciclastic rocks of lithic, feldspathic and quartzose compositions, with minor components of shale, banded iron formation and basalt (Eriksson, 1978; Heubeck, 1993; Heubeck and Lowe, 1994, 1999; Lowe and Byerly, 1999; Bontagnali et al., 2012; Heubeck et al., 2013). The Moodies Group was deposited primarily in alluvial, deltaic, tidal and other shallow-marine to terrestrial environments (Heubeck et al., 2013). Coeval, deep-level tonalite-trondhjemite-granodiorite (TTG) suites periodically intruded the Swaziland Supergroup succession from ~3.509-3.225 Ga (Lowe, 1999). These events were accompanied by co-magmatic dacitic to rhyolitic volcanism (Lowe, 1999). A granite-monzonite-syenite (GMS) suite intruded the sequence later (~3.2 to 3.1 Ga) after deposition and deformation of the BGB had ceased (Lowe and Byerly, 2007). Multiple deformation events, subsequent to the initial deposition of the Fig Tree Group, complexly faulted and folded the BGB (Lowe, 1999). The most important of these shortening events is termed D2 and occurred between ~3.240 and ~3.230 Ga, overlapping with Fig Tree sedimentation (de Ronde and de Wit, 1994; Lowe, 1999b). Compressional
5 deformation from D2 and subsequent events resulted in the NE-SW-striking tightly-folded synclines, short-wavelength anticlines and thrust faults that dominate the structural grain of the BGB (de Wit, 1982; de Ronde and de Wit, 1994; Lowe et al., 1999). In the process, most bedding and fault planes were rotated to vertical or subvertical dips (Lowe and Byerly, 2007). Notably, the strain in nearly all areas was taken up in more ductile units, allowing preservation of original rock textures in sandstone (Lowe and Byerly, 2007). These deformational events were accompanied by widespread regional metamorphism and metasomatism (maximum ~300-400 °C) (Xie et al., 1997; Tice et al., 2004). As a result, most of the original sediments have been silicified or carbonitized, or were recrystallized to carbonate, sericite, chlorite and other alteration products (Lowe and Byerly, 2007). Despite this alteration, the sediments in the BGB remain remarkably well preserved, to the point where sedimentary structures are readily recognized and original petrographic compositions and textures can be assessed (Lowe and Byerly, 2007). Furthermore, it is often possible to overcome the structural complexity with careful mapping and section measuring in order to identify coherent, genetically related sedimentary sections.
Study Area
The Mapepe Formation south of the Heights Syncline crops out in four structural belts (Fig. 1). This study focuses on the northernmost belt called the Manzimnyama Syncline. The area was originally mapped at a coarse scale by Viljoen and Viljoen (1969a), studied in the greatest detail later by Heinrichs (1980), followed by more mapping by Heubeck (1993) and Lowe et al. (2012). Broadly, the Mapepe Formation at this locality is composed of banded ferruginous chert (BFC) with horizons of chert plate breccias, banded iron formation (BIF) interbedded with lithic sandstone beds, and a clastic turbidite-dominated section ranging from mud to pebble conglomerate in grain size.
Structure
The Manzimnyama Syncline is an overturned fold striking NE-SW. This study focuses on the clastic turbidite succession of the upright lower limb, where beds strike on average 36° E of N and dip varies from 24° to 63°, averaging 43° to the SE. Numerous reverse faults dissect the area, and slip along bedding planes is presumed from the common occurrence of slickenfibers on bed surfaces. Fault-damaged zones are generally less than 20 cm thick, with the exception of a major one-meter-thick fault zone at 103 m depth in the core, and they occur along bedding planes and show little offset, indicating that the beds above and below the zone are likely genetically related. Faults that cross-cut bedding
6 are comparatively less common. Two potential parasitic folds, both bounded on one side by a fault, were identified in the road section, causing several meters of inverted section. Fracturing occurs in distinct zones and is most evident in the core. These fractures are sometimes filled with vein quartz and original orientation is difficult to ascertain from the core. Most of the shearing was accommodated in the finer-grained sections of the clastic turbidite succession and in the chemical facies located above and below the siliciclastics. The banded ferruginous chert is particularly deformed, presenting with tight chevron folds and exhibiting a highly variable lateral thickness. Ductile deformation is also evident in the banded iron formation, particularly at depth within the core (Fig. 3). The finer-grained intervals of the clastic turbidite succession also show deformation through folding and faulting of the rare, thinly laminated muddy intervals. With finer-grained and chemical facies taking up the brunt of the shearing, the clastic turbidite beds appear to form a coherent stratigraphic package. As a result of the structural complexity, however, it is not possible to correlate the road section to the core on an individual bed scale. Instead, stacking patterns and lithofacies comparisons aided in the correlation.
Stratigraphy
To date this area has been studied in the greatest detail by Heinrichs (1980), building on prior work by Anhaeusser (1976) and Viljoen and Viljoen (1969). The basal black cherts in the area were first identified by Heinrichs as representing the lowermost Fig Tree rocks, but were later assigned to the underlying Mendon Formation by Lowe and Byerly (1999). Overlying the Mendon Formation of the Onverwacht Group is siliceous black shale containing fine-grained sandstone that Heinrichs termed the Loenen Micaceous Graywacke Member. This member, however, is not present on the lower limb of the Manzimnyama syncline, as it is not penetrated by the core and appears to be sheared out of the surface section. The Loenen Micaceous Graywacke Member transitions into a zone of banded ferruginous chert interrupted periodically by meter-scale horizons of chert-plate breccias. The bands in the chert consist of alternating layers of clay, silt, white and iron-rich layers of chert that are typically less than 2 cm thick. The interbedded chert-plate breccias have a variable matrix, ranging from siderite-dominated matrix compositions to siliciclastic sand. At its upper contact, the banded ferruginous chert unit rapidly transitions into banded iron formation, which Heinrichs (1980) named the Manzimnyama Jaspillite Member. The Manzimnyama Jaspillite Member falls under the classification of oxide-facies iron formation (Beukes, 1973). The banding in the BIF is made up in some zones by 7 alternating layers of siderite, specular hematite, jasper and rarely white chert, whereas in other zones the banding is dominated by iron-rich mud, pink-to-red chert (jasper) and white chert (Fig. 3A,B). In addition, the BIF contains occasional coarse siliciclastic beds ranging from a few centimeters to one meter in thickness, which also include abundant rip- up clasts of jasper and white chert-plates (Fig. 3C). The top of the banded iron formation transitions into fine-grained ashy and silty material interbedded with chert layers less than 3 cm thick and fine grained sandstone beds up to 25 cm thick (Appendix A, Section 1). This upper contact is pervasively sheared and the fine-grained transitional zone to the amalgamated clastic turbidite section is not preserved throughout, with high lateral variability observed at the surface. This zone in the core, for example, is extensively deformed and shortened, whereas along strike in the Manzimnyama Syncline, it is entirely missing and the BIF transitions rapidly, within a few meters, to a predominately clastic turbidite section. This overlying clastic turbidite section is the focus of this study. First termed the Gelagela Grit Member by Heinrichs (1980), it is approximately 350 m thick on the lower limb of the syncline and consists of a wide range in grain sizes, from mud to pebble conglomerate, and bed thicknesses from 1 cm to 1.58 m. Grain compositions include rounded chert clasts, chert-plates, minor beta quartz, and volcaniclastic lithic grains. Chemical sedimentation is restricted to centimeter-scale chert layers that occur in the finest-grained sections. The upper contact of the clastic turbidite section is transitional, with increasing frequency of chert layers devolving into a banded ferruginous chert section, capped by a BIF section in the axis of the syncline.
Core
Core retrieval in the study area was performed in 2012 in association with the International Continental Scientific Drilling Program (ICDP) Expedition 5047. The 538.55 m long core—BARB4—was drilled into the lower limb of the Manzimnyama Syncline with a borehole trajectory dipping 55° towards the NNW at location 25°54’24.09’’ S, 31°05’48.15” E (Fig. 2). The drill site was situated within the clastic turbidite section and coring penetrated most of the turbidite sequence, all of the underlying banded iron formation and banded ferruginous chert, reaching total depth in the serpentinized ultramafic rocks of the Mendon Formation of the Onverwacht Group (Fig. 2). The surface section measurements capture the section stratigraphically above the core, which includes at least another 200 m of clastic turbidite section, banded ferruginous chert and banded iron formation (Fig. 2).
8 Methods
A total of ten sections were measured in detail along road outcrops on the Barberton- Havelock (R40) Road (Fig. 4, Appendix A). Except for section eight, these outcrops are along strike equivalents of the rocks penetrated by the BARB4 core. Painstaking searches for paleocurrent indicators only yielded five sets of ripples on bedding surfaces. Unfortunately, the small number and the position of these ripples on beds deformed in a parasitic fold, combined with the unknown plunge of the Manzimnyama Syncline, made restoration of original paleocurrent direction not feasible. The core was described and drafted from high-resolution core scans and photographs of the core boxes at 1:20 scale (Appendix B). Due to frequent errors in core depth notation made during drilling and initial core archiving, the annotated depth does not match the measured thickness. In order to preserve context to the original core, the original annotated depths that are physically marked on the core were recorded as smaller, italicized numbers on the drafted version. This study refers to annotated depth (AD) as the depth physically marked on the core piece, whereas corrected depth (CD) refers to measured depths from this study. The grain size changes in larger beds were measured in multiple places along the thickness of the bed in order to accurately reflect the grading profile. This was done in order to differentiate between different flow types and flow behavior in the section. Given the very poor sorting of the majority of the lower parts of the beds, grain size determination is partially dependent on the operator. For consistency, if the largest population of grains exceeded 10% of the total population, that grain size was recorded. Isolated outsized clasts, such as a floating pebble in coarse sand, were disregarded. This method was used in order to gain the best estimate of maximum energy the flow experienced. Grain size determination of material below fine lower sand was not possible from images alone. For this reason, grain sizes of silt and mud are grouped together on the grain size axis. Mud clasts were drawn in the correct depth location within a bed, as were chert- plates and other outsized grains. Flat laminations were also drawn as observed, with potential cross laminations and wavy bedding also recorded. Since true cross-lamination is often difficult to determine in the constricted window provided by the core, notes were made in the side margin where true ripples were thought to occur. Other examples of notations in the side margin include faults, information concerning objects drawn within beds such as chert-plates and mud clasts, unusual grading behavior, and depth notation errors made by core curators. Fault zones were marked in where the offset and shearing affected large parts of the core, making bed thicknesses and grading profiles difficult to extract. In order to reflect 9 the grain size of the material caught up in the fault zones, the right boundary of the fault zone object was placed at the maximum grain size present within the fault zone. This was done in order to help discriminate fault zones that juxtaposed genetically unrelated sections versus those that likely did not cause a great deal of offset.
Core statistics
Each bed above three centimers in thickness from the core graphic log was measured for eight attributes. These included bed thickness, maximum grain size, chert plate and mud clast occurrence, amalgamation (if the mud cap was missing), slurry and debris flows, disturbed bedding and whether the bed was bounded by faults, missing section or both. In addition, the depth position of the bed was assessed in one-meter increments. The assignment of a bed to a certain depth interval was determined by the position of the bed’s basal surface. In total 1,020 beds were measured starting from the top of the clastic turbidite section section at 38 m CD to the base of the section at 218 m CD. The statistical analysis was carried out in programs JMP® and Matlab®.
Results
The core and outcrop sediments were assigned to three lithofacies, which include amalgamated sandstone (La), inter-bedded sandstone and mud (Lsm) and inter-bedded mud and sandstone (Lm).
Lithofacies La: Amalgamated sandstone
Lithofacies La consists largely of medium- to thick-bedded beds averaging 26 cm in thickness with a standard deviation of 24 cm (Fig. 5). Around 21% of beds are in excess of 41 cm thick. Grain size ranges from mud to conglomerate, with over 50% of beds exhibiting a maximum grain size of very coarse-sized sand and coarser. Amalgamation of beds is common, as more than 64% of beds lack fine-grained tops. In outcrop, pervasive centimeter-scale erosion and scour occurs at the base of the beds, although no flute casts were observed (Fig. 6). Cross-lamination and flat-lamination is rare, as the majority of the beds are massive and exceptionally poorly sorted. Many of the beds have an extended delayed normal grading profile followed by rapid fining in the upper few centimeters of the bed (Fig. 7A). Other grading trends include a basal coarse lag followed by a rapid fining to lower grain sizes (Fig. 7B). Soft-sediment deformation and mixing is common at the tops of the beds (Fig. 7C). Around 18% of the beds contain mud clasts and chert-plates. Chert-plates are
10 clasts of chert with a tabular, platy shape ranging from a few mm to 2 cm thick (Fig. 8A,B). Total dimensions of chert-plates are best determined in surface outcrops, with the largest observed clast in the clastic turbidite section exceeding 14 cm along the a-axis. The plates range from straight, unbent morphologies to curved or folded-over, or otherwise deformed forms (Fig. 8A). The chert-plates have a variety of compositions and appearances, including banded black-and-white, pink, grey, black, and white. The plates sometimes have internal flat laminations that run parallel to the axis (Fig. 8A). In the core and outcrop, the plates do not exhibit a demonstrable preferred location within the bed, with resting places ranging from positions in the coarse base to clusters of plates at the bed tops. In general, however, the chert-plates show a preference for an a-axis orientation at or near horizontal. Mud clasts, like the chert-plates, are common in this lithofacies. Beds with mud clasts often also have chert-plates in them, although beds with mud clasts are nearly twice as prevalent in the clastic turbidite section as those with chert plates. The mud clasts typically occur in the middle to upper parts of the beds, although differentiation of mud clasts from muddy undulating tops can be difficult in core. Overall, however, there is a noticeable absence of mud clast conglomerate.
Interpretation
Lithofacies La exhibits many features of high-energy, high-density turbidity currents, as described by Lowe (1982). Scour, high degrees of amalgamation, the scarcity of cross-lamination and flat-lamination, the abundance of mud and chert-plate clasts, and the preponderance towards large grain sizes all point to turbulent, highly energetic flows. In addition, the extremely poor sorting and pronounced delayed grading profile both suggest very rapid deposition of the flow material through high sediment fallout rates. These massive bases would be considered thick Ta divisions of the Bouma sequence (Bouma, 1962) or S3 divisions under the Lowe (1982) classification scheme for high-density turbidity currents. High overall sedimentation rates are inferred by the well-developed mixing zones at the tops of beds, which imply that the sediment of the underlying bed was still soft upon the arrival of the subsequent flow. In form, habit and occurrence, the chert-plates resemble mud rip-up clasts. The similar positions and frequent co-occurrence in beds suggest that chert-plates also resemble mud clasts in their hydrodynamic properties. In addition, many of the chert-plates exhibit morphologies that indicate that they were still soft at the time of deposition (Fig. 8A). This elasticity and cohesion may explain the survival of the thin platy clasts in a turbulent flow full of coarse material, conditions under which a hardened, more brittle chert-plate might not have survived. Given the clear association and similarities between chert-plates and 11 mud clasts, it is likely that chert-plates were an Archean version of intrabasinal rip-up clasts.
Lithofacies Lsm: Inter-bedded sandstone and mudstone
Lithofacies Lsm is the most diverse and most common of the three lithofacies (Figs. 6B and 9). Bed thickness averages 16 cm with a standard deviation of 16 cm, with nearly half of the beds measuring between 3 and 10 cm. Maximum bed grain size ranges from mud to conglomerate, with average maximum grain size of beds classified as medium to coarse sand. Amalgamation is common and present in 54% of the beds. Grading profiles are varied and include normal grading, normal grading that is delayed with varying severity, and beds with thin very coarse bases grading rapidly to a delayed normal grading profile (Fig. 7A-C). Many of the beds show flat lamination above the massive, poorly sorted bases, with a much smaller number also exhibiting cross-lamination (Fig. 10). Mud clasts and chert-plates are found in approximately 8% of the beds. This lithofacies also contains the lone debris flow deposit of the entire section. None were identified in outcrop probably due its poor resistance to weathering. This 1.2 m-thick bed has sandy, sheared intraclasts supported by a muddy matrix and can be found at 160 m CD.
Interpretation
The grain size, bed thicknesses, development of cross- and flat-laminated tops in these flows, and the preservation of muddy caps in nearly half of the beds all indicate a moderate level of energy intermediate to that of lithofacies La and Lm. Since most of the beds are poorly sorted at their base and still express various forms of delayed normal grading, it is likely that many of these flows were in fact high-density flows (Lowe, 1982). In most of the high-density deposited beds, the units have thick S3 divisions at the base, which are capped by a low-density flow deposit representing Bouma divisions Tb and Td-e, and a rare few examples showing the full Tb-e subdivisions (Bouma, 1962). Even though cross-lamination occurs more often in this lithofacies, it is still relatively rare throughout the entire clastic turbidite succession. The muddy caps of the turbidites often lie upon flat laminated sections, or even directly overlie the massive base. This scarcity of cross-lamination may be the result of flow stripping, flow ponding or even relate to fundamental differences in flow variables of Archean seawater. Flow stripping occurs when a turbidity current loses its finest suspended fraction over the side of a flow container, such as a levee or similar embankment (Piper and Normark, 1983). In the process, the flow loses the fine sand and silt that is most conducive to ripple formation.
12 Conversely, flow ponding happens when the turbidity current is prevented from attaining its full run-out distance through topographic containment of some form (Sinclair and Tomasso, 2002). When the density current cannot surmount its barriers, the forward motion of the flow stalls and results in rapid fallout of sediment from suspension. Under these conditions, a high-density current would not be able to later evolve into a lower- energy, low-density flow capable of forming ripples. Instead, the massive, poorly sorted S3 unit would be capped with the mud, as observed in the clastic turbidite section (Fig. 7C). Finally, Fralick and Carter (2011) noted that Archean turbidites as a whole lack cross- lamination and ripples compared to their Phanerozoic counterparts. They attributed this to lower seawater viscosity caused by hotter ocean temperatures relative to the Phanerozoic. This lower viscosity would thin the laminar sub-layer, thereby narrowing the range of grain sizes and flow velocities conducive to ripple formation.
Lithofacies Lm: Inter-bedded mudstone and sandstone
Lithofacies Lm consists mostly of relatively thin to medium-bedded sandstone with a minor component of inter-bedded muddy sediment (Figs. 6 and 11). Bed thickness averages 9 cm with 7 cm standard deviation, although over 71% of the beds are below 10 cm in thickness. Grain size ranges from mud to conglomerate, but most of the beds have a maximum grain size of medium-sized sand or smaller. ‘Net to gross’ is still high, just above 80%, and 40% of beds are amalgamated and lack muddy tops. Mud clasts and chert-plates are absent from the beds with the exception of two examples. Faulting and deformation is present, with chevron folds and other deformation features most prevalent in the ‘muddiest’ horizons, but also expressed as fracturing and centimeter-scale offset in the sandstone beds. With few exceptions, the sandy beds are predominately normally graded, and delayed normal grading is more common in beds thicker than 10 cm. Many of the very coarsest beds of granule or pebble conglomerate are thin (<5 cm) and exhibit anomalous inverse grading or other unusual features. Disturbed bedding is not uncommon, with wavy muddy laminations and mixing occurring in the tops of the thicker units. Chert horizons are present in this lithofacies, consisting of layers 1 mm to 6 cm thick and expressing a wide compositional range from white to black, grey, pink and red jasper (Fig. 8C,D). These chert horizons are distinct from vein quartz growing in fractures or along faults. Vein quartz is typically brilliant white in color, and in unpolished core has a ragged surface and shows a highly variable thickness within one layer. Unlike the vein quartz, the chert layers with red or pink staining appear to contain sand-sized silica granules, with other layers showing internal flat laminations. The horizons of chert are generally bounded by shale units, and are sometimes associated with what appear to be 13 pod-shaped chert nodules above or below the continuous horizon (Fig. 8C). Additionally, in the surface exposures of this lithofacies the chert layers show good lateral continuity at the scale of the outcrop (Fig. 6C).
Interpretation
The thinner-bedded nature, the comparatively finer average grain size and the near- absence of mud clasts and chert plates compared to the other lithofacies suggest that these beds were deposited by lowest-energy flows in the sequence. Beds with normal grading that are thinner than 5 cm were likely deposited by low-energy density flows (Bouma, 1962), whereas the thicker beds with delayed normal grading and very poorly sorted bases deposited from high-density turbidity flows (Lowe, 1982). The relative lack of mud tops appears to suggest an erosive nature, or may reflect the scarcity of mud in the density flow. Where mud is common, however, it is typically found in the disturbed bedding at the tops of beds, which could, in some cases, represent true slurries. More likely, however, they represent a borderline hybrid transitional flow rather than a fully developed slurry flow like those described by Lowe and Guy (2000). For the lowest-energy facies of the clastic turbidite section sequence, this lithofacies is still relatively starved of mud, as the net-to-gross exceeds 80%. In Phanerozoic systems this could indicate flow stripping or flow bypass processes discussed previously in the Lithofacies Lsm subsection. Alternatively, the mud/silt deficit could be related to the source rocks, mild weathering climate or high hill-slope denudation rate. Around 20% of the source rocks for these sediments include cherts that are predisposed to forming coarse “chert grits” rather than fine sand and being severely depleted in Al, would not have broken down into mud weathering products. The remaining 80% of the source rocks for these sediments are composed of komatiitic and basaltic rocks (Lowe and Nocita, 1999). The ultramafic and mafic rocks should have largely weathered to clay or dissolved solutes, rather than producing disaggregated sand-sized quartz or feldspar crystals (Lowe and Nocita, 1999). It is possible that the climate was not conducive to mud production or, alternatively, that denudation rates were perhaps high enough that the rock exposures did not have time to break down to finer grain sizes. In a mud-depleted environment, the relatively rare chert horizons likely represent the quiet interludes that occurred between periods of siliciclastic input. The timing of the chert horizon formation is unclear. Examples from chert-horizon-rich units in the underlying Mendon Formation indicate that these bands formed relatively early at the sediment water interface, either from early silicification (Lowe, 1999a) or from the deposition of primary silica grains (Stefurak et al., 2014). Silica granules are a primary mode of silica precipitation 14 from Archean seawater and could represent a form of background pelagic deposition detectable only during periods of cessation of siliciclastic input. These thin layers of granules are good candidates for early silicification, which would lead in some cases to the formation of nodules and continuous chert horizons. In a sense, these chert horizons could potentially represent the Archean equivalent of condensed sections—an interval characterized by very slow depositional rates (Vail et al., 1984)—and/or cessation of siliciclastic sedimentation.
Synthesis
Whole-core analysis combined with the surface study reveals robust large-scale trends. Overall, the maximum grain size of all the beds shows a well-developed normal distribution, with a mean maximum bed grain size of medium to coarse sand (Fig. 12), whereas the bed thickness distribution is heavily skewed towards smaller thicknesses (Fig. 12B). Maximum grain size and bed thickness show a positive correlation (p <0.0001) (Fig. 12C). Large-scale trends in maximum bed grain size and bed thickness were observed along the core length. From bottom to top, the core shows an overall thinning and fining upward trend in the bed maximum grain sizes and bed thicknesses (Figs. 13 and 14). This trend is not a linear decay, however, and can be broadly subdivided into two large-scale thinning- and fining-upward trends, if the surface data above the core top is included. At the base, the bed thicknesses thin in a series of three progressively thinning-upward, tens- of meters-scale cycles, from 217 m CD to 104.5 m CD (Fig. 15). From this relative bed thickness minimum, the beds again coarsen and thicken as they set up the second large- scale thinning and fining upward trend, but fall short of the collectively large values seen at the base of the section. The chert layers are notably absent in the lower 40 m of the section, but appear with increasing frequency in the upper 50 m of the core, in line with this overall fining trend. Observations from the surface section that outcrops above the top of the core reveal that the clastic turbidite succession continues to fine—interrupted by occasional coarser cycles—to a banded ferruginous chert section that is ultimately capped by banded iron formation in the axis of the syncline. Cyclicity at scales smaller than these large-scale trends was explored. Transformation of the data into the frequency domain, however, did not reveal peaks in spectral power at any wavelength for grain size or bed thickness (Fig. 16). In addition, low pass filters of 2, 5, and 10 m did not recover a single spatial periodicity at those wavelengths. A band-pass filter centered at 20 m, however, did show a possible cyclicity, but this may have been a smoothing effect as it did not manifest as a peak in the frequency domain. Given the complexities of extracting and proving cyclicity from similar data sets (Chester, 1994; Sylvester, 2001), 15 these findings do not by any means confirm that cyclicity is absent, but further statistical investigations are beyond the scope of this study.
Core to outcrop correlation
Correlation of the core with the along-strike surface outcrops was complicated by structural complexity and poor outcrop quality. Comparison between the core description and the measured sections from outcrop reveal large thickness differences, particularly in the orthochemical sections and the lower contact of the siliciclastic section. For example, the banded iron formation section is approximately 70 m thick at the surface but is 125 m thick in the core. In addition, the upper contact of the banded iron formation section with the clastic turbidite succession shows high variability due to faulting. Correlation higher up in the siliciclastic section was made possible by careful attention to similarities in stacking patterns and sedimentology of the turbidite deposits in addition to identification of fault zones and overturned sections. Without distinctive marker beds, correlations remain broad. Generally speaking, however, the overall stacking pattern of banded ferruginous chert to banded iron formation to siliciclastics section is identical in core and outcrop. Relative thicknesses, however, are clearly compromised due to structural deformation and likely do not primarily reflect along-strike changes due to facies changes in the deep-water system.
Discussion
The Archean deep-water succession of this study is a record—albeit imperfect—of surface conditions and sedimentary processes during one of the most enigmatic times in Earth’s history. Although interpretation of these ancient rocks presents many challenges, nuanced application of techniques developed from studying Phanerozoic turbidite systems can yield important contextual information about local conditions and early Earth in general. Like many Archean sedimentary sequences, structural complexity compromises lateral continuity and limits stratigraphic architectural analysis. Penetration of un-weathered rock by the ICDP drilling project, however, has made high-resolution analysis of a continuous vertical succession possible for the first time. Vertical stacking patterns in deep-water systems are influenced by both small- and large-scale processes. These include allogenic controls—such as eustasy, tectonic setting and climate change—and self-organizational factors like channel migration, lobe switching and depositional topography (Kneller et al., 2009; Prélat et al., 2009). While interpretation of these rocks is constrained by the one-dimensional nature of the study, aspects of Archean
16 deep-water deposition and the feeder system associated with this sequence can be surmised. Manzimnyama sequence sediments deposited in a deep-water environment. The dominance of density flow deposition and the lack of evidence for post-depositional current reworking—such as hummocky cross-stratification—suggest that the sediment came to rest in deep, quiet water conditions below storm wave-base. These density current-deposited siliciclastic sediments had a coeval, lateral facies relationship with the orthochemical deposits of banded iron formation and banded ferruginous chert. This is supported by the presence of siliciclastic turbidites within the orthochemical successions, the co-occurrence of chert plates and mud rip-up clasts—both still soft at the time of entrainment in the flow— within turbidite beds, and the chert horizons within the lowest energy lithofacies Lm in the lithic sandstone section. Proximity of the deep-water system to the uplifted source area is inferred from the immaturity of the siliciclastic material—attested to by the presence of labile lithic grains and the angularity of the grain shapes—indicating short-distance transport from the source area to the coast. Longer transport or residence time in a fluvial system would have rounded the grains and increased the proportion of resistant chert grains over volcanic lithic grains. The scarcity of mud and very fine sand in this system, even in the thin- to medium- bedded Lm lithofacies, could have greatly affected the architecture and morphology of the deep-water depositional system. With little mud available to build constructive levees to confine flow, some other conduit—such as a canyon—is necessary to make transport of the coarse material away from the staging area possible (Lowe, 1982). Beyond the limits of that conduit, and lacking a levee system, these coarse-grained high-density flows would have collapsed rapidly onto the unconfined, flat basin floor (Lowe, 1982). With these constraints in mind, these observations suggest that the turbidity flows likely traveled down a relatively short conduit and deposited proximal to the conduit’s mouth. Other supporting evidence for a short conduit lies in the broad normal distribution of maximum grain sizes, demonstrating a lack of grain size fractionation that would be expected from flow filtering over a long run-out distance (Lowe, 1982; Sylvester, 2001). When the Manzinmyama turbidity currents exited the conduit and encountered the flat basin floor they likely formed lobe deposits. Lobes—deep-water deposits with a lobate shape—form in areas that favor deceleration of density flows and their subsequent collapse and deposition (Satur et al., 2000; Normark et al., 2003). In such a zone, depositional processes dominate, and as a result large-scale scour is uncommon and beds are tabular and laterally extensive (Deptuck et al., 2008; Prélat et al., 2009). Although along-strike bed continuity in the study area is challenging to assess given the structural complexity and the extent of commercial forest cover, no evidence for channelization was observed in
17 the clastic turbidite succession. The beds are overwhelmingly tabular with no more than a few centimeters of incision at their bases. Given the absence of major bounding surfaces or other evidence for internal organization that characterizes channelized deposits (Clark and Pickering, 1996; McHargue et al., 2011; Sylvester et al., 2011), the turbidite flows likely deposited in a lobe setting. Lobes can occur in many positions along deep-water systems, including frontal, crevasse, overbank and intra-channel locations (Posamentier and Kolla, 2003; De Ruig and Hubbard, 2006; Bernhardt et al., 2011). The thickness, amalgamation, and coarseness of the succession argues for a frontal lobe location—at the terminus of the feeder system—that received sediment for a sustained amount of time. These observations, however, cannot exclude the possibility that the turbidite sequence sits within a very large canyon or channel container as an intra-channel lobe, since the scale of this deep-water system is unknown. The thickness of the clastic turbidite succession, in excess of 350 m, implies that this is not simply one large lobe but instead comprises a series of stacked lobes. Although the thickness of individual lobes depends a great deal on the system (Mutti and Normark, 1987, 1991; Normark et al, 1993), Prélat et al. (2009) proposed lobes scale around 4-10 m in thickness, whereas Crevello et al. (2007) and Saller et al. (2008) described lobes around tens-of-meters thick. Therefore, as the siliciclastic succession is in excess of 350 m thick, it is likely that the sequence contains numerous lobes. Unfortunately, partition of the sequence into the individual-lobe and interlobe hierarchy of Prélat et al. (2009) is limited by the poor lateral continuity resulting from structural complexity and forest cover. While interpretation of individual lobes is not possible in this study, self- organizational lobe processes were undoubtedly as important in this Archean system as in Phanerozic deep-water settings. Prélat et al. (2009) demonstrated that the density flows were sensitive to subtle depositional gradients (<0.02°) and would deflect from relatively high areas to deposit in topographic depressions. Subsequent density flows would continue to fill this depression until the build-up of depositional relief was enough to cause another deflection and initiate deposition in a new location. This process operates at the meter scale and is the primary controlling factor of small-scale stacking patterns (Prélat et al., 2009). A similar process occurs at the tens-of-meters scale and is commonly referred to as compensational lobe stacking (Mutti and Sonnino, 1981). The centroid of a lobe—the volumetric center—creates topographic relief, and subsequent density flows will tend to deflect to areas of lower topography (Prélat et al., 2009). Over time, this results in different parts of lobes overlapping vertically, forming thinning-upward or fining-upward stacking patterns that can reflect purely self-organizational processes rather than external controls (Prélat et al., 2009). Many of the fining-upward stacking patterns at the tens-of-meters
18 scale in the BARB4 core may originate from this process, possibly reflecting the stacking of thin-bedded finer-grained distal parts of lobes over thick-bedded, sandy central parts of younger lobes. Attributing large-scale trends (tens-to-hundreds-of-meters scale) of stratal stacking patterns within the BARB4 core to external controls—such as climate, tectonics and sea level—is a highly speculative exercise. Even in modern and Pleistocene systems where the up-dip geomorphic elements and feeder systems are preserved, disentangling these interrelated external controls on deep-water deposits is rarely possible (Kneller et al., 2009). For this Archean deep-water system, perhaps the most critical uncertainty concerns the tectonic setting of the basin. Jackson et al., (1987), Nocita and Lowe (1990) and Lowe and Nocita (1999) proposed that the Mapepe Formation strata deposited in a foreland basin setting associated with the BGB’s first orogenic event. Under their framework, the orthochemical deposits were interpreted to represent distal foredeep sedimentation. This quiet pelagic sedimentation- dominated period was succeeded by an abrupt influx of terriginous material (e.g., the lithic sandstone section) sourced from an advancing fold and thrust belt. Taking into account the abundance of felsic volcaniclastic material, Lowe and Nocita (1999) suggested that the Mapepe Formation deposited in retroarc or backarc foreland basin settings adjacent to this backarc fold-and-thrust belt. This specific classification of the “Mapepe basin” as a backarc or retroarc foreland basin inherently assumes that the Archean plate tectonic regime did not fundamentally differ from “modern-style” plate tectonics. When plate tectonics began and at what point “modern-style” steep subduction initiated, however, remains highly controversial. Some workers advocate that modern- style plate tectonics was already in motion during the Archean (De Wit et al., 1992; Lowe, 1999b; de Ronde and Kamo, 2000; Moyen et al., 2006; Turner et al., 2014). Many invoke the combination of arc terrane accretion and regional compressional deformation as evidence for orogenic events driven by subduction (Dimroth et al., 1982; de Wit, 1998; Lowe, 1999b). In the BGB, for example, these processes are called upon to explain assembly of structural blocks, syn-orogenic deposition of the sedimentary sequences and pervasive faulting and folding of BGB rocks into tight recumbent folds after 3.230 Ga (de Ronde and Kamo, 2000; Lowe, 1999b; Lowe and Byerly, 2007). The viability of “modern-style” plate tectonics in the Archean, however, has been challenged. Recent reviews integrating geodynamic modeling results and the geochemical studies of igneous rocks suggest that the subduction that defines modern-style tectonics did not initiate until after 3.2 Ga and was not fully functional (i.e. continuous instead of sporadic) until 2.7 Ga (Condie and Pease, 2008; Condie and Kröner, 2008; Shirey and
19 Richardson, 2011; Dhuime et al, 2012; Bédard, et al., 2013). Instead, various alternative mechanisms for crustal recycling beyond subduction are proposed. One of these alternatives includes crustal underplating or “subcretion”, whereby lateral plate movement is achieved by mantle traction forces, crustal recycling occurs by lithospheric delamination and dripping off processes, and tectonic uplift from accretion episodes generates syn-compressional turbidite basins (Bédard, et al., 2013; Johnson et al., 2013; Gerya, 2014). At the other extreme, another study proposes that lateral plate movement and subsequent accretion did not occur in the Archean. Using the BGB as a case study, Van Kranendonk (2011) argues that the compressional regimes found in greenstone belts were attained in-place through partial convective overturn between the thick, dense, cool greenstone cover succession and the underlying hot granitic middle crust (“greenstone drip model”). If one of these alternative plate tectonic regimes had been in force, this would have had large implications for the type of basin in which the Mapepe Formation deposited. Instead of a classical foreland basin, which is the product of subsidence caused by orogenic loading and drag forces from the down-going plate (Allen and Allen, 2005), a subcretion marginal tectonic setting would have generated small deep basins fed by erosion of tectonic highs (Bédard et al., 2013; Harris and Bédard, 2014). Alternatively, the greenstone drip model for the BGB, proposed by Van Kranendonk (2011), suggests that the Fig Tree Group sediments deposited in a progressively deepening syncline formed by the sinking Onverwacht greenstone succession, which was accompanied by the concomitant rise of the underlying TTG plutons. These rising TTG plutons would have uplifted the Onverwacht Group overburden, erosion of which generated the Fig Tree Group sediments. By the time the Moodies Group deposited, the Onverwacht sequence was largely dissected and the TTG roots were providing most of the sediment (Van Kranendonk, 2011). The lack of consensus regarding the mode of tectonics during the time of Mapepe Formation deposition poses problems for attributing large-scale sediment stacking patterns to tectonic setting. For example, the hundreds-of-meters-scale progressively-thinning and fining-upward packages within the BARB4 are broadly compatible with the foreland basin setting proposed by Jackson et al. (1987), Nocita and Lowe (1990) and Lowe and Nocita (1999). Generation of turbidity flows in small foreland basins has been attributed to episodic oversteepening of the basin margin due to tectonic activity in the associated fold and thrust belt, which initiates slope failures that evolve down-slope into high-volume turbidity currents (Pickering, 1979; Mutti, 1992). As the basin margin slope re-equilibrates, the flows become progressively less voluminous, leading to a fining- and thinning-upward sequence characteristic of small foreland basins like those in the Apennines (Mutti, 1992). This stacking pattern could alternatively be attributed, however, to subsidence
20 behavior related to other plate regimes proposed for the Archean. For example, the thinning- and fining-upward packages could reflect episodes of enhanced subsidence related to the sinking of the underlying greenstone succession and the rise of the TTG plutons from the Van Kranendonk (2011) “greenstone drip model”. Furthermore, the other characteristics of the Mapepe Formation that resemble foreland basin deposits, including coarseness and immaturity of the sediment and the provenance from underlying rocks (e.g., unroofing sequence), primarily reflect proximal uplifts rather than confirming the presence of a fully developed fold and thrust belt. Moreover, Phanerozoic-style fold and thrust belt formation is contraindicated for the Archean, based on the supposed weakness and non-rigid nature of the hot Archean crust, conditions which do not favor the development of a décollement surface below the overthrust supracrustal stack (Bédard et al., 2013). In light of these issues surrounding the plausibility of modern-style plate tectonics during the time of Mapepe Formation deposition, the foreland basin setting proposed for these rocks might merit reevaluation in the future as the understanding of Archean plate tectonics matures.
Conclusions
The ca. 3.26 Ga sedimentary sequence in the Manzimnyama Syncline is one of the oldest, best preserved examples of a deep-water depositional system. This deep-water assemblage contains a thick siliciclastic section interbedded with orthochemical rocks of banded ferruginous chert and banded iron formation compositions. The siliciclastic section consists largely of massive, medium to thick-bedded, coarse-grained, and immature volcaniclastic and chert grit-rich material interbedded with zones containing thinner beds with fine-medium sand, some mud and chert layers. The sedimentology of the beds suggests that depositional processes were dominated by high-density turbidity flows. Mud is relatively scarce in this system, despite the presence of lower-energy states attested to by the occurrence of chert layers, which likely represent periods of quiescence. Overall the succession thins and fines upward, and although periodic pulses of relatively coarser, thicker beds do occur, none match the very high energy, chert-plate and mud-clast-rich deposits at the base of the section. Assessment of stratigraphic architecture is limited by structural complexity and poor lateral extents of outcrops. The lack of large-scale scour, however, throughout the entire >350 m section, along with the consistent tabularity of the beds, argues for a unconfined depositional setting as a lobe deposit. Given the thickness of the section, it is most likely a frontal lobe as opposed to other more short-lived, poorly developed lobe settings. In addition, the broad normal distribution of maximum bed grain sizes argues for short transport distance from the staging area to the depositional site. This short sediment 21 transport conduit must have been a canyon or similar incisional feature as the scarcity of mud precludes levee construction to confine the high-density flows. Ultimately, both self-organizational and external controls likely contributed to the fining- and thinning-upward stacking patterns found in the Manzimnyama deep-water sequence. Concerning self-organizational processes, compensational lobe stacking— although unproven in this sequence—likely operated in the Archean similarly to Phanerozoic systems and influenced stacking patterns at the tens-of-meters scale. External controls on deep-water systems—such as climate, sea level, volcanism and tectonics—that act on the tens-to-hundreds-of-meters scale are poorly constrained due to the age of the rocks. This lack of constraint is underscored by the rising consensus that modern-style plate tectonics was not yet operative at the time of Mapepe Formation deposition. Given these uncertainties, a uniformitarian approach to interpreting the large-scale controls on stacking patterns in this deep-water system is not yet possible. Although many aspects of Phanerozoic systems are recognizable in this succession— including density flow sedimentology, architecture and stacking pattern scales—some are uniquely Archean. These include the chert layers in the lowest-energy facies, the paucity of cross-lamination and ripples, and the lateral facies association with banded iron formation and banded ferruginous chert. As a whole, however, this study demonstrates that the principles of interpretation developed from studying Phanerozoic rocks can be applied to a reasonable degree to Archean turbidite sequences at the bed scale. In the process, the turbidite sequence has yielded some important contextual information concerning aspects of the Archean deep-water environment in the study area. Similar application of modern approaches to studying Archean siliciclastic deep-water deposits may one day provide a breakthrough in understanding the enigmatic origins of the inter-bedded orthochemical strata.
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27 Tice, M. M., Bostick, B. C. and Lowe, D. R., 2004, Thermal history of the 3.5−3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa: Geology, v. 32, p. 37−40.
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28 Figure 1. Location maps and stratigraphic columns for the Barberton greenstone belt (BGB) and in the study area. A. Simplified geologic map of the BGB (modified from Lowe and Byerly, 1999). The Fig Tree Group rocks are shown in dark grey, whereas the Onverwacht and Moodies Groups are light grey. The heavy black lines mark the Inyoka Fault. The red box outlines the southern-eastern area shown in greater detail in C., and the star marks the study area. B. Generalized stratigraphic column for the Swaziland Supergroup. C. Detailed geologic map (modified from Lowe et al., 2012) of the Mapepe Formation near the study area, which outcrops in four structural belts within tightly folded overturned synclines. This study focuses on the BARB4 core and the along strike surface outcrops within the red box on the northern limb of the Manzimnyama Syncline. D. Stratigraphic column for the lower limb of the Manzimnyama Syncline showing coverage for the BARB4 cored interval, the detailed core description and the field measured sections from this study.
29 A. B. Moodies sandstone Gp BGB (m) angular unconformity 1000 conglomerate sandstone Swaziland shale 500 BIF Fig Tree Gp Tree Fig Mapepe Fm sandstone SOUTH AFRICA BIF & BFC 0 chert (km)
Fm komatiite Barberton 10 Mendon basalt
mafic volcaniclastic
chert mafic volcaniclastic felsic rocks
chert Inyoka fault 5 Gp Onverwacht basalt
chert
South AfricaSwaziland
komatiite
0 10 20 km N 0 C. D. Fig Tree Group, Mapepe Formation (m) Field area BIF Sandstone & conglomerate and BARB4 BFC Banded iron formation drillsite Fine-grained ferruginous Moodies Group Sandstone and Onverwacht Group 500 conglomerate e in cl yn S ts h ig e H e e in n cli ncl y Syn
S s Field measured section ma Paulu ya n m zi n 250 Detailed core description a M
BIF BARB4 cored interval
BFC
1 km N Mapepe Fm 0 Mendon Fm 30 Figure 2. Generalized stratigraphic columns for formation of the Fig Tree Group with age and stratigraphic relationships from Lowe and Byerly (1999). A. Possible relationships based on the assumption that the succession of Fig Tree for- mations is not structurally repeated in the northern area of the BGB. B. Likely age rela- tionships based on the assumption that the Schoongezicht Formation in the BGB’s north- ernmost extent includes age equivalents of the upper sections of the Belvue Road, Auber Villiers and Mapepe formations.
31 A. 3225 3225? Fm Schoong. Belvue (Ma) Road Fm (Ma) Sheba Fm Mapepe Formation Auber Villiers Formation Auber Villiers Ulundi Fm 3243
3260
B. 3225 Fm Schoong. Belvue (Ma) Road Fm Sheba Fm Mapepe Formation Schoongezicht Formation Auber Villiers Formation Auber Villiers Ulundi Fm 3243
3260
Volcaniclastic and Lithic, chert-grit pyroclastic rocks sandstone
Chert clast Shale, mudstone, and conglomerate other argillaceous rocks, some tuffaceous
32 Figure 3. High-resolution scans of banded iron formation (BIF) and banded ferruginous chert (BFC) from the BARB4 core. A. BIF with alternating layers of siderite, specular hematite and jasper (AD 353.43 m). B. BIF with iron-rich mud, pink to red chert (jasper) and white chert layers (AD 277.81 m). C. Siliciclastic turbidite 16 cm thick interbedded within the BIF section showing chert-plate rip-up clast (AD 230.92m). D. BIF with ductile deformation (AD 310.92 m). E. BFC with alternating layers of both oxidized and not oxidized sideritic sediment with bands of white to translucent chert (AD 426.52 m).
33 1 cm E. 1 cm D. 1 cm C. 1 cm B. 1 cm A.
34 Figure 4. Correlation of BARB4 core to field measured sections. A. Correlation diagram of the described section of the BARB4 core and the measured sections from the surface. The red vertical lines denote overturned strata and the blue vertical lines show coverage of individual detailed measured sections from the field. B. Simplified geologic map (after Lowe et al., 2012) showing core drill site and projected orientation of the borehole along with the locations of measured sections from roadcut outcrops along the Barberton-Havelock Road (R40).
35 A. Field sections B.
BARB4 Core
36 Figure 5. Summary figure of Lithofacies La. A. Maximum bed grain size pie chart. B. Binned bed thickness pie chart. C. Example section showing predominantly massive beds with delayed grading, coarse lag to delayed grading, mud clast and chert-plates. D. Selected descriptive statistics from the beds within the La lithofacies.
37 Lithofacies: La A. Maximum bed grain size C. Example section Grain size 2.6% 3.5% 4.3% (m) m/s f m c vc g cg 15.7% 13.0%
3
33.0% 27.8%
Grain size Mud/silt Coarse Conglomerate Fine Very coarse Chert Medium Granular
2 B. Bed thickness
26.1% 30.4%
1
20.9% 22.8%
Bed thickness bins 3-10 cm 21-40 cm 0 11-20 cm 41-158 cm
D. Statistics
Attribute Mean Std. dev. N % of beds
Bed thickness 26 cm 24 cm 115
Grain size coarse 115
Amalgamated 74 64.3%
Mud clasts & chert plates 21 18.3% 38 Figure 6. Outcrop photos of lithofacies from field measured sections. A. Lithofacies La from the upper meters of Section 5. B. Lithofacies Lms from the upper meters of Section 3. Lithofacies Lm from Section 11.
39 A.
B.
C.
40 Figure 7. High-resolution core scans showing examples of grading profiles observed in the BARB4 core. A. Delayed normal grading rapidly fining to a flat-laminated top (AD 96.49 m, CD 95.30 m). B. Coarse lag at the base fining sharply within a few centimeters (AD 151.60 m, CD 150.60 m). C. Delayed grading with abrupt muddy top containing wavy laminations and distorted bedding (AD 184.00 m, CD 183.50 m).
41 A. B. C.
1 cm 1 cm 1 cm
42 Figure 8. High-resolution scans of the BARB4 core showing chert-plates and chert layers. A. Banded black-and-white chert-plate clast with soft, diffused edges (AD 213.27 m, CD 208.06 m). B. Chert-plates located at the base of a bed (AD 88.58 m, CD 87.45 m). C. Chert horizon with granular texture bounded by mud/silt showing chert nodule below continuous horizon (AD 108.79, CD 107.65). D. Chert horizon bounded by muddy layers showing post depositional faulting (AD 107.51, CD 106.35).
43 A. B.
1 cm 1 cm
C. D.
1 cm 1 cm
44 Figure 9. Summary figure of Lithofacies Lsm. A. Maximum bed grain size pie chart. B. Binned bed thickness pie chart. C. Example section showing predominantly massive beds with delayed grading, flat laminated to wavy laminated tops, isolated mud clast and isolated chert-plates. D. Selected descriptive statistics from the beds within the Lsm lithofacies.
45 Lithofacies: Lsm A. Maximum bed grain size C. Example section Grain size 17.8% (m) m/s f m c vc g cg 21.0%
11.4%
1.9% 2.5% 4.8% 3
27.5% 16.3% Grain size Mud/silt Coarse Conglomerate Fine Very coarse Chert Medium Granular
2 B. Bed thickness
17.4% 27.2%
7.8% 1
47.6% Bed thickness bins 3-10 cm 21-40 cm 0 11-20 cm 41-158 cm
D. Statistics
Attribute Mean Std. dev. N % of beds
Bed thickness 16 cm 16 cm 619
Grain size medium-coarse 619
Amalgamated 334 54.0%
Mud clasts & chert plates 49 7.9% 46 Figure 10. High-resolution scans of the BARB4 core showing sedimentary structures. A. Relatively rare example of cross-lamination at the top of a massive, delayed graded bed (AD 148.20 m, CD 147.25 m). B. Flat-laminated bed (AD 116.72 m, CD 115.75 m). C. Outcrop photo of pervasively rippled top of a bed within measured section 3, lithofacies Lsm.
47 A. B.
1 cm 1 cm
C.
48 Figure 11. Summary figure of Lithofacies Lm. A. Maximum bed grain size pie chart. B. Binned bed thickness pie chart. C. Example section showing beds with variable grading profiles, with beds sometimes capped with flat lamination and/or wavy lamination with occasional chert layers. D. Selected descriptive statistics from the beds within the Lm lithofacies.
49 Lithofacies: Lm A. Maximum bed grain size C. Example section Grain size (m) m/s f m c vc g cg 30.1%
20.6%
4.9% 1.4% 2.1% 3 18.5% 7.7%
14.7% Grain size Mud/silt Coarse Conglomerate Fine Very coarse Chert Medium Granular
2 B. Bed thickness
71.6%
1 0.4% 7.6%
20.5% Bed thickness bins 3-10 cm 21-40 cm 0 11-20 cm 41-158 cm
D. Statistics
Attribute Mean Std. dev. N % of beds
Bed thickness 8.9 cm 7.3 cm 286
Grain size fine 286
Amalgamated 114 39.9%
Mud clasts & chert plates 2 0.7% 50 Figure 12. Plots showing whole core analysis of bed statistics. A. Histogram of maximum bed grain size, showing a well-developed distribution with a mean in the medium-coarse range. B. Histogram of bed thickness showing skew towards the smallest bed thicknesses, with a mean of 16 cm and standard deviation of 16 cm. C. Bed thickness vs maximum bed grain size plot showing strong positive correlation (p<.0001).
51 Maximum grain size M/S F M C vC G Cg N= 992 8 7 6 5 4 3 2
80 70 60 50 40 30 20 10 200
100 Bed thickness (cm) thickness Bed C. 160 140 N= 992 Mean= medium- coarse N= 993 Mean= 16 cm Std Dev= 16 cm 120 100 80 60 Bed thickness (cm) Maximum grain size 40 20 M/S F M C VC G CG 0
50
250 200 150 100 500 400 300 200 100 Count Count A. B.
52 Figure 13. Plots showing large-scale trends in maximum bed grain size along BARB4 core length. A. Depth vs maximum bed grain size plot. B. Proportion plot showing the relative proportions of grain sizes within 1 meter depth intervals normalized to 100%. The plot shows the highest proportions of the coarsest grain size of granules and conglomerate at the base, which progressively fine upwards. A second pulse of coarse grain sizes occurs around CD 70 m, but this is largely driven by increases in the proportions of coarse to very coarse grain sizes rather than in the granular and conglomerate size range. Mud is most prevalent in the intervening zone between CD 160 and 120 m.
53 1.00 Chert Conglomerate 0.75 0.50 Coarse Very coarse Granular 0.25 Fine Medium Mud/silt Grain size proportions with depth Grain Sizee 0.00
40 60 80
100 120 140 160 180 200 220 Depth (m) Depth B. Grain Size Grain size with depth M/S F M C vC G Cg Ch
40 60 80
100 120 140 160 180 200 220 Depth (m) Depth A.
54 Figure 14. Plots showing large-scale in bed thickness trends along the BARB4 core length. A. Depth vs maximum bed grain size plot. B. Proportion plot showing the relative proportions of bed thicknesses within 1 m depth intervals normalized to 100%. The plot shows the highest proportions of the thickest beds occur at the base of the section, which progressively fine upwards to approximately CD 120 m. A second pulse of thicker beds occurs around CD 80 m, but this is largely made up of increases in the proportions of the intermediate thickness beds (11-40 cm) rather than the thickest bed range (41-158 m) seen at the base. The thinnest beds (3-10 cm) are present everywhere but make up the largest proportions between CD 90 and 130 m and then again at approximately CD 50 m.
55 41-158 cm 21-40 cm 11-20 cm Bed thickness bins 3-10 cm Bed thickness proportions with depth 0.00 0.25 0.50 0.75 1.0
40 60 80
100 120 140 160 180 200 220 Depth (m) Depth B. Bed thickness bins Bed thickness with depth 3-10 cm cm 11-20 21-40 cm 41-158 cm
40 60 80 100 120 140 160 180 200 220 Depth (m) Depth A.
56 Figure 15. Composite plot showing graphic core log, lithofacies and average bed thickness trends along the length of the BARB4 core. The average bed thickness graph was constructed using a 20 m moving window over the average bed thickness of each 1 m depth interval. This effectively smoothes the plot and can reveal thinning or thickening trends in bed thickness, although this analytic technique can make trends seem more gradual than they are in reality and causes edge effects at the very top and bottom of the core. The bed thickness values show a series of progressively thinning-upward cycles from the base of the section to approximately CD 104 m. Above CD 104 m, the bed thickness trends in general appear to be below the resolution of the 20 m smoothing window and are better represented in the lithofacies stacking log. For example, the beds appear to thicken gradually from CD 100 m to approximately CD 90 m, but inspection of the graphic log shows thinning upward cycles in lithofacies Lsm, followed by a sudden appearance of amalgamated thick beds of lithofacies La at CD 84 m. Trends continue to be poorly developed above CD 65 m on the bed thickness graph, whereas the graphic log shows fining-upward cycles ranging from 5 to 10 m in scale, corroborating with an overall fining trend within the lithofacies stacking log to approximately CD 50 m. Another pulse of thick beds occurs at the top of the core, and at the surface periodically coarse sections outcrop in the lithic sandstone section until it gradually grades into banded ferruginious chert and banded iron formation at the top of the section.
57 p
Core Lithofacies Average bed thickness (cm) 10 15 20 25 30
Lithofacies La Lsm 50 Lm
100 Depth (m)
150 >
200
58 Figure 16. Power vs frequency graph of bed thickness averaged over 1 m depth intervals showing no peaks above noise for any wavelength.
59 0.5 0.4 0.3 0.2 0.1 0 Frequency (1/m) -0.1 -0.2 -0.3 -0.4 0
-0.5
40 30 20 10 -10 -20 Power - Bed Thickness Bed - Power
60 Chapter 2: Shale geochemistry and sandstone petrography of the ca. 3.26 Ga Mapepe Formation, southeastern Barberton greenstone belt, South Africa
Abstract
The rcheanA Fig Tree Group in the Barberton greenstone belt, South Africa contains some of the oldest known well-preserved sedimentary sequences. An International Continental Scientific Drilling Program project in the belt recently acquired a core that penetrated a deep-water succession of shale, orthochemical rocks and lithic sandstone from the ca. 3.26 Ga Mapepe Formation. Major, trace and rare earth element geochemical analysis of shale and quantitative petrography of sandstone sampled from throughout the core were used to assess weathering, metasomatism and changes in provenance over time. The lithic sandstone consists largely of polycrystalline quartz (chert) fragments (51.8%) and volcanic lithic fragments (41.0%). Monocrystalline quartz (5.7%) and feldspars (1.5%) are minor components. Metasomatism remobilized all labile species, depleting the rocks of Na2O,
CaO and Sr while enriching the shale with K, Ba and SiO2. Metasomatic overprinting and Al-poor source rocks renders assessment of weathering regime in the hinterland uncertain, but the preservation of labile lithic grains suggests that the weathering environment was not extreme. Major oxide proportions indicate a mixed felsic and mafic source, with high Ni and Cr abundances suggesting ultramafic sources were an important contributor of
sediment. With the exclusion of highly siliceous (>83 wt% SiO2) shale within the banded ferruginous chert section, REE analyses show lower ∑REE, increasing values of Eu/Eu*, and decreasing LaN/YbN ratios with stratigraphic position. These observations point to a change in provenance from felsic sources to more mafic source rocks over time. This trend may be the result of either 1) erosion into increasingly mafic rocks with time, 2) decrease in explosive felsic volcanism with time, or 3) a combination of both volcanism and evolution of the source area. Petrographic and geochemical results strongly resemble analyses of the Mapepe Formation rocks located in the nearby Barite Syncline and in the Central Domain, providing additional evidence for a correlative relationship with the study area that was previously suspected but unconfirmed.
Introduction
Geochemical analysis of shale and quantitative sandstone petrography are powerful tools for understanding sediment provenance, weathering and alteration (Dickinson and
61 Suczek, 1979; Taylor and McLennan, 1985; Nesbitt and Young, 1992). Careful application of these techniques can yield information from even the oldest sedimentary rocks preserved on earth. This study applies these tools to examine sediments of the Archean Fig Tree Group in the Barberton greenstone belt (BGB), South Africa. Although several geochemical and petrographic studies have examined the Fig Tree strata in other parts of the belt, the last detailed study in this locality was the doctoral work of Heinrichs (1980). This particular area of the BGB, however, has recently attracted renewed scientific interest due to an International Continental Scientific Drilling Program (ICDP) Expedition 5047 in 2012 that resulted in the acquisition of five cores to benefit Archean research. One of these cores, the BARB4 core, targeted the sedimentary rocks exposed in the Manzimynama Syncline of the study area (Fig. 1). This study documents the petrographic and elemental characteristics of the Mapepe Formation in the Manzimnyama Syncline.
Geologic setting
The BGB of South Africa and Swaziland is a structurally complex supracrustal sequence comprised of altered volcanic and sedimentary rocks intruded by coeval tonalite- trondhjemite-granodiorite (TTG) plutons and younger granite-monzonite-syenite (GMS) suites (Viljoen and Viljoen, 1969; Anhaeusser, 1973; Lowe and Byerly, 2007) (Fig. 1). It contains five distinct tectonostratigraphic structural domains that include NE-SW striking belts of volcanogenic and sedimentary rocks ranging from ~3.2 and 3.5 Ga in age (Lowe and Byerly, 1999). This ~15 km thick stratigraphic succession is sub-divided into three major units—the Onverwacht, Fig Tree and Moodies Groups—which, taken together, make up the Swaziland Supergroup (Viljoen and Viljoen, 1969)(Fig. 1). The ~3.547-3.240 Ga Onverwacht Group is the oldest and the thickest (8-10 km) sequence of the Swaziland Supergroup (Lowe and Byerly, 1999; Lowe, 1999). It contains a diverse assemblage of predominantly volcanic rocks that span komatiitic, basaltic and rhyolitic compositions (Lowe and Byerly, 1999). Silicified sedimentary units, now represented by a variety of cherts of carbonaceous, ferruginous and aluminous compositions, are up to hundreds of meters thick. The 300-1000 m thick Mendon Formation (3.335-3.260 Ga), which caps the Onverwacht Group, is comprised of ultramafic komatiite flows and ashes intercalated with black chert, banded black-and-white chert and banded ferruginous chert (Lowe and Byerly, 1999). The Mendon Formation has a conformable upper contact with the chiefly sedimentary rocks of the Fig Tree Group (3.260-3.226 Ga)(Lowe and Byerly, 1999). The up to ~1800 m thick Fig Tree Group is lithologically diverse and includes mudstone, sandstone, conglomerate, dacitic volcanic and volcaniclastic rocks, banded ferruginous 62 chert (BFC), chert-plate breccia, and banded iron formation (BIF)(Condie et al., 1970; Heinrichs, 1980; Eriksson, 1980b; Lowe and Byerly, 1999). This group is sub-divided into two major facies, northern and southern, separated by the Inyoka fault (Fig. 1). Age dating and stratigraphic comparisons suggest that the northern and southern facies represent distinct tectonostratigraphic units (Kröner et al., 1991; Byerly et al., 1996; Lowe and Byerly, 1999). The northern facies consists, from bottom to top, of the Ulundi, Sheba, Belvue Road, Bien Venue and the Schoongezicht formations, whereas the southern facies includes the Aubier Villiers Formation and the Mapepe Formation, the latter of which is the focus of this study (Fig. 2)(Lowe and Byerly, 1999, Kohler and Anhauesser, 2002). The Ulundi Formation is the basal unit of the northern-facies Fig Tree Group and consists of black carbonaceous shale, banded cherty units, jasper and other ferruginous sediments (Reimer, 1983; Lowe and Byerly, 1999). The overlying Sheba formation is a thick sequence of turbiditic sandstone with minor shale and iron-formation (Reimer, 1967; Condie et al., 1970). The Sheba Formation is succeeded by the Belvue Road Formation, which includes interbedded fine-grained turbiditic sandtone with abundant tuffaceous and carbonaceous shale, banded ferruginous chert, iron-formation and local felsic pyroclastic rocks (Eriksson, 1980b; Lowe and Byerly, 1999). Conformably overlying the Belvue Road Formation is the relatively recently identified Bien Venue Formation, which consists largely of quartz-muscovite schists derived from silicic volcanic and volcaniclastic rocks (Kohler and Anhauesser, 2002). Finally, the Schoongezicht Formation comprises pyroclastics, volcanics, conglomerate and lithic sandstone (Eriksson, 1980b). The basal contact of the Schoongezicht Formation is interpreted to be a regional thrust fault, and these rocks may be the age equivalents of the upper section of the Belvue Road Formation and/or the southern facies Fig Tree rocks (Fig. 2)(Lowe and Byerly, 1999). The southern facies Fig Tree Group consists only of two formations—the Auber Villiers Formation and the Mapepe Formation (Lowe and Byerly, 1999). The Auber Villiers Formation contains a great deal of volcaniclastic material, including thick sections of dacitic tuffs, shale, conglomerates and volcaniclastic sandstone. The Mapepe Formation, which is the only formation of the Fig Tree Group present in the study area, includes dacitic tuffs, banded iron formation, banded ferruginous chert, and siliciclastic strata ranging in grain size from mudstone to pebble conglomerate (Heinrichs, 1980; Lowe and Byerly, 1999). Geochemical studies of shale in the northern facies Fig Tree Group include McLennan and Taylor (1983), Condie et al. (1970), Wildeman and Condie (1973), Visser (1956), Danchin (1967). Quantitative analysis of sandstone was undertaken by Condie et al. (1970), Reimer (1975) and Herget (1966). For the southern facies Fig Tree Group, geochemical analyses of shale were performed by Heinrichs (1980) and Hofmann (2005),
63 with petrographic assessment of modal abundances reported by Nocita (1989) and Lowe and Nocita (1999). Various published findings indicate that the Mapepe Formation ranges from ~3.260 Ga to somewhat younger than 3.225 Ga in age (Kröner et al., 1991; Byerly et al., 1996; Lowe and Byerly, 1999) and seem to suggest that the base of the Mapepe Formation is diachronous, becoming progressively younger to the north (Byerly et al., 1996). Unpublished dates, obtained from three zircons extracted from a BIF section in the Manzimnyama Syncline of the study area, indicates an age of ~3.260 Ga (personal communication, Byerly), which is among the oldest of any Mapepe Formation strata measured. Finally, the Fig Tree Group is unconformably overlain by the youngest unit of the Swaziland Supergroup—the Moodies Group. This ~3.223-3.210 Ga sedimentary sequence is 0.5-3 km thick and incorporates sandstone, conglomerate and siltstone of lithic, feldspathic and quartzose compositions, with minor components of shale, banded iron formation and basalt (Eriksson, 1978; Heubeck, 1993; Heubeck and Lowe, 1994, 1999; Lowe and Byerly, 1999; Bontagnali et al., 2013; Heubeck et al., 2013). Intrusion of deep-level tonalite-trondhjemite-granodiorite (TTG) suites into Swaziland Supergroup succession occurred periodically from ~3.509-3.225 Ga (Lowe, 1999a). These events were accompanied by co-magmatic dacitic to rhyolitic volcanism (Lowe, 1999a). A later granite-monzonite-syenite (GMS) suite intruded the sequence after deposition and deformation of the BGB had ceased (~3.2 to 3.1 Ga) (Lowe and Byerly, 2007). To the southeast of the study area, the deep-level Ancient Gneiss Complex of Swaziland, composed of a variety of TTG gneisses ranging in age from ~3.640-3.200 Ga, is separated from the BGB by one of these younger intrusive units (Hunter, 1978; Kröner, 2007). The pronounced change from ~300 million years of effusive volcanism represented by the Onverwacht group to the chiefly sedimentary sequences of the Fig Tree and Moodies group is thought reflect the onset of the BGB’s first orogenic event (Anhauesser, 1973; Jackson et al., 1987; Lowe and Nocita, 1999). While the tectonic mechanisms behind compressional regime remain unclear, sediment transport directions and depositional environments indicate the zone of uplift lay to the southeast of the study area (Lowe and Nocita, 1999). This uplift of the Onverwacht Group and concomitant explosive volcanism generated the sediment that deposited as the Mapepe Formation (Jackson et al., 1987; Lowe and Nocita, 1999). Regional low-grade metamorphism, resulting in alteration of the rocks to lower greenschist facies (maximum ~300-400 °C) (Xie et al., 1997; Tice et al., 2004), occurred after the deformation of belt (e.g., post 3.2 Ga)(Tice et al., 2004). The timing of metasomatism, however, is as not clear. Lowe (1999b) reports silicification occurring very early on the
64 seafloor, as a result of low-temperature sediment/sea-water interaction. Hofmann (2005) also calls on seafloor alteration but attributes the origin of the fluids to low-temperature hydrothermal activity. In addition to this early alteration, metasomatism likely continued with burial and later heating of the rocks subsequent to the formation of the BGB (Lowe, 1999). As a result of the effects of both metasomatism and metamorphism, most of the original minerals and grains have been silicified or carbonitized, or were recrystallized to carbonate, sericite, chlorite and other alteration products (Lowe and Byerly, 2007). Despite this alteration, the rocks lack shear fabrics and evidence for penetrative deformation (Tice et al., 2004). Consequently, sedimentary structures are readily recognized and original petrographic compositions and textures can be assessed (Lowe and Byerly, 2007).
Study Area
This study focuses on the Mapepe Formation exposed in the NW-SE striking Manzimnyama Syncline (Fig. 1). The area has been mapped by several workers (Viljoen and Viljoen, 1969; Heubeck, 1993; Lowe et al., 2012), but its stratigraphy and sedimentology were only studied in detail by Heinrichs (1980). Heinrichs identified several members of the Fig Tree Group in this area, which were later included in the Mapepe Formation by Lowe and Byerly (1999). The Onverwacht/Fig Tree contact in the study area is represented by komatiite flows and black cherts grading into banded ferruginous chert (BFC). In other areas, siliceous black shale containing fine-grained sandstone, termed the Loenen Micaceous Graywacke Member by Heinrichs (1980), is normally found between the Mendon Formation and the BFC section. This member is either not present in the lower limb of the Manzimnyama syncline or is very thin and lacks sandy intervals, as it is not penetrated by the core and is appears to be sheared out of the surface section. Above this unit is banded ferruginous chert, which consists of alternating layers of clay, silt, white and iron-rich chert that are typically less than 2 cm thick. This section also contains interbedded meter-scale horizons of chert-plate breccias with matrices compositions ranging from siderite to siliciclastic sand. At its upper contact, the BFC unit gradually grades into a BIF section, which Heinrichs (1980) named the Manzimnyama Jaspillite Member. The Manzimnyama Jaspillite Member falls under the classification of oxide-facies iron formation (Beukes, 1973). The banding in the BIF is made up in some zones by alternating layers of siderite, specular hematite, jasper and rarely white chert, whereas in other zones the banding is dominated by thick layers of iron-rich mud, pink to red chert (jasper). In addition, the BIF also has a few normally graded siliciclastic beds ranging from a few cm to 1 m in thickness, which are full of chert-plate clasts. 65 The BIF has a pervasively sheared upper contact with a thick section of lithic sandstone. This zone in the core is extensively deformed and damaged, but examination along strike in the Manzimnyama Syncline suggests this contact is conformable. First termed the Gelagela Grit Member by Heinrichs (1980), the siliciclastic section is approximately 350 meters thick and spans a wide range in grain sizes, from mud to pebble conglomerate, and bed thicknesses from 1 cm to 1.58 m. Chemical facies are restricted to centimeter-scale chert layers that occur in the finest grained sections. The upper contact of the siliciclastic section is gradational, with increasing frequency of chert layers devolving into another BFC section, capped by a BIF section in the axis of the syncline. The cored section does not include the entire sequence. The BARB4 core was obtained in 2012 through an ICDP drilling campaign (Expedition 5047). The drill site is situated within the siliciclastic section at 25°54’24.09’’ S, 31°05’48.15” E, and a borehole 538.55 m deep with a trajectory of 55° dip to the NNW was drilled into the lower limb of the Manzimnyama Syncline. Coring penetrated most of lithic turbidite sequence, all of the underlying BFC and BFC, reaching total depth in the serpentized ultramafic rocks of the Onverwacht Group, Mendon Formation (Fig. 3). At least another 200 m of siliciclastics, BFC and BIF lie stratigraphically above the cored interval.
Methods
Sandstone petrography
Samples of sandstone from the BARB4 core were examined using standard petrographic techniques. Modal compositions were obtained through point counting of 15 thin sections using the Gazzi-Dickenson method outlined in Ingersoll et al. (1984). Three hundred point counts per slide were performed using a computerized point counting stage and software from PETROG™. These thin sections were selected to maximize coverage through the core length. The point counting scheme is shown in Table 1. Modal analyses from Condie et al. (1970), Reimer (1975), Herget (1966), Eriksson (1980a) and Nocita (1989) were included in order to compare northern and southern facies Fig Tree Group rocks. Alteration and grain size played a strong role in the modal analysis results. Petrographic examination reveals that the matrix is largely comminuted framework grains and fine-grained tuffaceous material. At grain sizes smaller than coarse sand, discrimination between psuedomatrix and matrix becomes increasingly difficult. This is
66 further complicated by the pervasive chloritization and sericitization of the matrix and framework grains. For these reasons, thin sections of the coarser samples (coarse to very coarse sand) were prioritized over those containing finer grain sizes. Even at coarser grain sizes, however, alteration complicated point counting. For example, true cherts containing fine-grained carbonaceous or mafic oxide material are challenging to distinguish from devitrified fine-grained tuff (Condie et al., 1970). Furthermore, sand-sized grains of chlorite likely represent altered mafic lithic grains, but where no diagnostic original texture was preserved, these were counted as “unknown”. In addition, silicification and seritization has strongly affected the preservation of the feldspar grains. Feldspar crystals show degrees of alteration from sericite and silica replacement in their cores to either complete degradation to sericite or complete silicification, leaving only crystal “ghosts” in some cases. With all of these factors considered, modal counts of feldspar and volcanic lithic grains likely represent minimum abundances, whereas polycrystalline quartz grains (chert) are over represented.
Shale geochemistry
A total of 15 sub-surface shale samples were analyzed for major oxide, trace and rare earth element (REE) abundances. These samples were selected the BARB4 core, and include shale near the Mapepe Formation/Mendon Formation contact (n=2), within the banded ferruginous chert (n=5), banded iron formation (n=6) and siliciclastic (n=2) sections of the Mapepe Formation present in the lower limb of the Manzimnyama Syncline (Fig. 3). Geochemical analyses, including XRF and ICP-MS measurements, were performed at Washington GeoAnalytical Laboratory. In addition, major oxide geochemistry of shale from both the northern and southern facies Fig Tree Group was assembled from existing literature (Visser, 1956; Condie et al., 1970; Heinrichs, 1980; Hofmann, 2005).
Results
Petrography
The siliclastic section of the Manzimynama sequence is comprised of lithic arenite and plots within the “recycled orogen” field of the Dickinson and Suczek (1979) QFL diagram and the “recycled lithics” field of the Dickinson et al. (1983) QmFLt diagram (Fig. 4). The grains are overwhelmingly angular and poorly sorted, and the dominant framework types include polycrystalline quartz (chert) fragments (51.8%) and volcanic lithic fragments (41.0%) of various compositions. Monocrystalline quartz and feldspar
67 are minor components, with quartz abundances averaging 5.7% and feldspars 1.5%. The matrix appears to be made up of crushed framework lithic grains, fine-grained tuffaceous material, mafic oxides and phyllosilicates. The lithic fragments consist largely of either polycrystalline quartz grains or silicified volcaniclastic material ranging from mafic to intermediate compositions (Figs. 5,6). The polycrystalline chert fragments range from relatively pure silica compositions to those that contain fine laminations, fine-grained carbonaceous or mafic oxide matter which often clumps together, small crystals of sericite (<10% of the grain), and are sometimes cross- cut cut by quartz veins. The volcanic lithic clasts show a wide range in compositions and textures, of which tuffaceous grains predominate. Despite the state of low-grade alteration in these rocks, pseudomorphs of volcanic glass shards are still preserved, in high enough abundances in some grains to be classified as vitric tuff fragments (Fig. 6). Quartz crystals and feldspar laths are present in some of the tuff fragments and represent a more intermediate composition. The majority of tuffaceous grains, however, lack sand-sized crystals, and consist instead of a diffuse, silicified groundmass of mafic oxides in varying concentrations. This texture is often felted in appearance, which results from the devitrification of the original glassy material. Differentiation of these more mafic tuffs from detrital grains of true carbonaceous or mafic oxide-rich chert can be challenging. In general, however, the true chert grains appear have larger polycrystalline chert domains and either well-defined clumps of fine grained material floating in a “pure” chert matrix or thin laminae. Furthermore, many of these chert grains have later generations of veins cross-cutting the original fabric. In contrast, the candidates for tuffaceous origin are predominantly microcrystalline and “dirty”, with pervasive presence of fine-grained material dusting the entire grain (Fig. 6). Ultimately, however, discriminating between these grain types is exceedingly difficult, especially at smaller grain sizes. Relative abundances of lithic and chert grains therefore are only approximate. Some of the grains contain sand-sized euhedral crystals of feldspar (Fig. 5). These fragments likely eroded from an intermediate composition rock such as a dacite. Silicified mineral “ghosts “of these feldspars are not uncommon, and it is likely that many of the polycrystalline chert grains were originally volcanic lithic fragments. Given the level of preservation, however, it is difficult to confidently assess whether these dacitic fragments are explosive or intrusive in origin. Less common are mafic lithic grains with acicular texture or small plagioclase laths in a fine-grained groundmass of chlorite, sericite and opaques (Fig. 5). Especially large sand-size grains of chlorite that lack this texture, and which likely replaced mafic grains, are
68 more common in the upper siliciclastics section. Although the majority of the mafic grains have been extensively chloritized, occasional preservation of original volcanic textures is attested to the presence of rare amygdaloidal basalt fragments, still containing plagioclase laths and preserving the vesicular texture, now replaced by microcrystalline quartz (Fig. 5). A small amount of monocrystalline quartz is present in both strained and unstrained varieties and is likely volcanic in origin based on the prevalence of embayments and sharp crystal faces. Both strained and unstrained quartz grains occur within the same thin sections, consistent with deformation of the strained examples occurring prior to deposition. The low abundances of quartz grains in general, however, make it difficult to be confident that this is a statically valid distinction. Feldspar largely occurs within lithic grains (Fig. 5) and is only rarely found as a detrital grain. This nearly complete absence of feldspar as individual detrital grains outside of lithic clasts attests to their tendency towards poor preservation. Feldspar grains show a range of alteration textures ranging from the centers of the feldspars being filled with sericite or microcrystalline quartz to wholesale replacement of the entire grain (Fig. 4). Original compositions of the feldspars is difficult to assess due to the pervasive alteration, but they occur either as small laths or as tabular euhedral crystals and many show Carlsbad twinning. A very minor fraction of the total lithic grains is composed of opaque rich, fine- grained clasts with horizontal laminated fabric. Some of these grains have internal quartz veins that cross-cut this fabric. These could be phyllites, welded tuffs or pumice fragments, but this is difficult to assess due to the degree of alteration and the scarcity of such grain types. The matrix is made up of crushed framework lithic grains, fine-grained tuffaceous material, mafic oxide, phyllosilicate and iron-rich cement. Pyrite, hematite and dolomite rhombs are not uncommon and are likely the products post-depositional diagenesis based on cross-cutting relationships, perfect crystal faces and size. Dolomite is common in a few samples, in which dolomite is a large component of the matrix and sometime penetrates grains, often as well-developed rhombs. The pyrite crystals are large and outsized with respect to the detrital grains, and are generally perfectly cubic in shape. Hematite also has a well-formed crystal habit and is a cement in samples near the base of the siliciclastics section.
69 Geochemistry
Major elements
Abundances of mobile oxides are low, particularly for CaO (0.02-2.38 wt%) and
Na2O (0.00-0.09 wt%), although K2O is relatively abundant (up to 7.65 wt%) in shale within BIF and near the basal Mapepe Formation/Mendon Formation contact (Table 2). In
contrast, SiO2, FeO* (total Fe), TiO2, and Al2O3 abundances are highly variable. SiO2 ranges from 95.83 wt% in samples from the banded ferruginuous chert (BFC) section to values as low as 48.67 wt% in the shale found in the siliciclastic succession. Except for the shale
near the basal contact, FeO* values (2.38-47.45 wt%) are higher than PAAS values and TiO2
abundances (0.02-1.45 wt%) are significantly lower than PAAS. Finally, Al2O3 abundances range from 0.15% in the BFC section to 24.19 wt% in the underlying shale near the Fig Tree/ Onverwacht contact. In shale associated with siliceous orthochemical rocks, such as BFC and BIF, it is important to explore whether quartz dilution effects impact elemental abundances in order to ensure that the observed trends with depth or rock type reflect true differences in
geochemistry among samples rather than relative SiO2 content. Systematic quartz dilution effects from silification do not appear to affect major, trace and REE element abundances in
shale within the siliciclastic, BIF and near the basal Fig Tree contact (SiO2 ranging 56.3-66.6 wt%)(Figs. 7-9). This suggests that any observed geochemical trends of the shale within these sections can be attributed to provenance, weathering, and alteration effects.
The samples within the banded ferruginous chert, however, have very high SiO2 contents (81.37-95.83 wt%). The overall low abundances of oxides other than FeO* and
SiO2 in the shale from the BFC section, in addition to the very low ∑REE and trace element abundances, point to significant quartz dilution effects in these samples (Figs. 7-9). Given the high proportion of chert in these samples, their geochemistry likely largely reflects the composition of the fluid that deposited the silica rather than primary signatures from source rocks or weathering. For this reason, these samples are excluded from assessment of geochemical trends with stratigraphic position.
Rare earth elements
Overall, the REE patterns of the Mapepe Formation shale samples show highly variable Eu anomalies, moderate LREE fractionation and little fractionation in HREEs, similar to other Archean shale (Condie et al., 1970; Taylor and McLennan, 1985; Hofmann, 2005). Except for the shale at the base of the section, total abundances for all REEs (∑REE) 70 are below PAAS values. Three distinct REE patterns are apparent from the shale within different parts of the sequence (Fig. 9). The oldest shale near the basal Fig Tree contact shows fractionation in the LREE, a pronounced negative Eu anomaly, a gently sloping smooth HREE profile, and ∑REE comparable or higher than PAAS. Geochemically, shale from the Loenen Micaceous Graywacke Member from Hofmann (2005) in the Central Domain matches the basal shale samples of this study, suggesting that the basal shale may be in place rather than tectonically emplaced. In contrast to the basal shale, the directly overlying shale in the BFC section has
a positive Eu anomaly, extremely variable LREE/HREE fractionation (LaN/YbN ranges from 1.7 to 8.6) and relatively erratic abundances for HREE and much a lower ∑REE than PAAS (Fig. 9). Given the low abundances and the high degree of variability in these very siliceous BFC samples, the REE patterns likely partially reflect the composition of the fluid that deposited the chert (e.g., Eu enriched seawater) rather than original source rock geochemistry. Notably, the shale in the BIF that overlies the BFC section, with the exception of one sample (SAF649-50), shows a return to more standard REE patterns, with
lower LREE/HREE fractionation than the basal shale (average LaN/YbN= 4.6), a subtler negative Eu anomaly, gently sloping HREE profile and a lower ∑REE than the PAAS and the basal shale (Figs. 9,10). Finally, the shale within the upper section interbedded with lithic sandstone show lower LREE/HREE ratios (LaN/YbN= 4.5) and ∑REE, and a flat to slightly positive Eu anomaly (Figs. 9,10). In the Mapepe Formation of the study area, with the exclusion of the highly siliceous
(>83% SiO2) shale of the BFC, there is a clear trend towards leveling Eu anomalies, lower ∑REE, and a decrease in LREE/HREE fractionation with stratigraphic height (Fig. 9).
Comparison of Eu/Eu* shows increasing values up-section. In addition, comparison of LaN/
YbN, a measure of LREE/HREE fractionation, shows decreasing values with stratigraphic
position. These trends are independent of SiO2 enrichment.
Trace elements
Nickel and chromium have very high abundances (Ni 72-544 ppm; Cr 86-5253 ppm) in all of the shale with the exception of the highly silicified BFC samples (Table 2). The samples from the BFC section are depleted in all trace elements relative to PAAS, which likely reflects quartz dilution and/or depleted values from the precipitative fluid. All other samples are significantly enriched in Ba and Sc relative to PAAS, whereas Sr is strongly depleted (Fig. 7). Relative to the shale near the basal Fig Tree Group contact and the siliciclastic section, shale from the BIF section are especially enriched in Rb Ba, and Cr. Only the shale near the basal Fig Tree contact has elevated Y and Zr abundances relative to 71 PAAS (Fig. 7).
Discussion
Alteration
Alteration from metasomatism and low-grade metamorphism is evident from the sandstone petrography and shale geochemistry of the Mapepe Formation sediments in the study area. Petrographic analysis shows widespread replacement of primary minerals with sericite, chlorite and silica and growth of pyrite and dolomite in the matrix. In addition,
geochemical results show widespread remobilization of labile components. Na2O, CaO and
Sr were removed, whereas high Ba, Sc and K2O abundances likely reflect post-depositional enrichment. This enrichment/depletion pattern is consistent with compositional changes in Onverwacht Group volcanic rocks (Duchac and Hanor, 1987) and in Mapepe Formation rocks in the Central Domain (Hofmann, 2005). In general, the Mapepe Formation rocks, with the exception of the type section in Mapepe Valley (Heinrichs, 1980), show a higher degree of major oxide redistribution compared to the northern facies sediments (Condie et al., 1970; Heinrichs, 1980; Hofmann, 2005). In contrast, the REE abundances of both facies appear to have not been affected by the metasomatism and low-grade metamorphism (McLennan and Taylor 1983; Hofmann, 2005).
Weathering
Chemical weathering in the source area is often assessed by examining the proportions of major oxides in shale (Nesbitt and Young, 1982). Progressive weathering of source rocks results in the removal of mobile cations (e.g., Ca2+, Na+ and K+) compared to relatively immobile components (Al3+ and Ti4+)(Nesbitt and Young, 1982). Nesbitt and Young (1982) developed a ternary diagram that plots molar fractions of key major oxides important for determining weathering trends from different source rocks (e.g., the A-CN-K plot)(Fig. 11). Along the vertical axis, the chemical index of alteration (CIA) captures the resultant degree of weathering (Eq. 1).
Eq. 1. CIA = Al2O3/[Al2O3 + K2O + Na2O + CaO*] *100
With the exception of the cherty samples from the BFC, the shale samples from this study plot closely along the A-K axis, and have CIA levels (60-75) near PAAS.
72 Source rock composition and metasomatic effects, however, can complicate
assessment of weathering intensity. Komatiites and komatiitic ash are relatively poor in Al2O3 (<9 wt% for flows and <5 wt% for ash; Condie, 1993 and Lowe, 1999b), and even extreme chemical weathering of these rocks would not have generated as many alumina-rich clays compared to TTGs (15.1 wt% Al; Condie, 1993). Similarly, weathering of polycrystalline
quartz fragments from chert horizons, also depleted in Al2O3 (<1 wt%; Lowe, 1999b), would have contributed very little Al2O3 to the sediment. Both chert and komatiite make up a large proportion of the upper part of Onverwacht Group (Lowe and Byerly, 1999). Therefore
a key measure of weathering—the Al2O3 proportions—may not be a reliable indicator of weathering intensity in the source area for the Mapepe Formation rocks.
In addition, post-depositional metasomatism has stripped mobile components Na2O and CaO to trace levels, which acts to artificially raise weathering indices. Furthermore, potassium metasomatism is evident from the A-CN-K diagram, as the majority of the shale samples plot below possible weathering trends for even granitic source rocks (Fig. 11)(Fedo
et al., 1995). Metasomatic enrichment of K2O depresses calculated CIA values. Normally this effect can be corrected for if the general composition of the source rock is known (Fedo et al., 1995). Given the large degree of uncertainty, however, surrounding the possible Mapepe Formation provenance rocks—ranging from komatiite to dacitic sources—it is not possible to apply the correction. Although the major oxide geochemistry may not be useful for determining weathering in the source area, petrographic analysis of the lithic sandstone suggests conditions were not extreme. This is attested to by the abundance of volcanic lithic fragments, which would not have survived an aggressive weathering regime. Rapid erosion/remobilization rates and short transport distances, however, cannot be excluded as reasons for their preservation, especially given the angularity of the clasts and the high proportion of tuffaceous fragments. These complications, in addition to the metasomatic overprinting and alumina-poor potential source rocks, make it difficult to definitively interpret weathering conditions in the hinterland for the Mapepe Formation in our study area.
Provenance
The trace, REE and major oxide geochemistry of shale can aid in identifying source rock compositions, particularly when paired with a petrographic study of associated sandstone. For the major oxides, the A-CNK-FM diagram can be useful for determining provenance, particularly from mafic source rocks (Fig. 11)(Nesbitt and Young, 1992). The shale samples from this study do not plot along a weathering trend for any particular source rock, but instead show a range from TTG to komatiitic sources. A mixed provenance is 73 also supported by a plot of TiO2 vs Al2O3, as the majority of samples fall between fields of more mafic and more felsic end-member compositions (Fig. 12). Furthermore, the trends on the A-CNK-FM diagram indicate that, 1) the source rock compositions changed over time rather than reflecting increased weathering of the one source rock type and, 2) the decreasing proportions of alumina and increasing amounts of iron and magnesium oxides with stratigraphic position suggests evolution towards a progressively more mafic provenance. This felsic to mafic trend with stratigraphic position is also reflected in the REE and trace element behavior. REE abundances are sensitive to magmatic processes involved in source rock generation, but are relatively immobile during weathering and metasomatism (Taylor and McLennan, 1985). For this reason, REE analyses of shale can be useful for identifying source rock compositions. In particular, fractionation of LREEs vs HREEs, the development of negative Eu anomalies, and ∑REE values are directly related to igneous processes and melt behavior. LREEs are slightly more incompatible in common silicate minerals because of their slightly larger atomic sizes (Taylor and McLennan, 1985). As a result, during partial melting LREEs become concentrated in residual melt, ultimately leading to the crystallization of LREE-enriched felsic rocks that have a higher ∑REE. An exception to this partition behavior is europium (Eu2+), which readily replaces Ca+2 in plagioclase sites. As a result, the residual melt is depleted in Eu with respect to neighboring REEs, which gives the resultant felsic rocks a negative Eu anomaly. This relative depletion is captured in the Eu/Eu* ratio (Eq. 2), which is a measurement of the depletion or enrichment of europium relative to samarium and gadolinum using chondrite-normalized values (Taylor and McLennan, 1985).
1/2 Eq. 2. Eu/Eu*= EuN/[SmN)/GdN)]
Values of Eu/Eu* less than 0.95 indicate depletion, whereas at values greater than 1.05, europium is enriched relative to the neighbouring REE. Higher Eu/Eu* values therefore indicate more mafic sources, whereas values below less than 1.05, like the PAAS (Eu/Eu*= 0.65) suggest felsic sources. In comparison to felsic rocks, therefore, less differentiated and more mafic rocks like komatiites and basalts do not show as highly fractionated LREE/HREE
values (e.g., have lower LaN/YbN ratios) or develop pronounced negative Eu anomalies. The shale samples show systematic changes in REE abundances that are consistent with a mixed source that became more mafic over time. Excluding the samples from the BFC, which likely largely reflect seawater geochemistry rather than provenance rock,
74 increasing stratigraphic position is associated with decreasing ∑REE, lower LaN/YbN ratios, increased Eu/Eu* values independent of SiO2 enrichment (Figs. 9,10). This REE behavior is consistent with secular changes from a more differentiated, more felsic source to a less differentiated, more mafic provenance. Trace elemental abundances can also be useful for identifying provenance (Taylor and McLennan, 1985; Hessler and Lowe, 2006). High abundances of Ni and Cr, for example, are strongly associated with mafic to ultramafic source rocks, and the extremely high concentrations in the Mapepe Formation shale (Ni>300 ppm, Cr>600 ppm) suggest that these rocks were an important contributor of material (Taylor and McLennan, 1985; Condie and Wronkiewicz, 1990; Young and Nesbitt, 1999). Mixing calculations using a Ni- and Cr- rich komatiite end-member performed by other workers on Archean sediments (McLennan and Taylor, 1983; Hessler and Lowe, 2006), however, suggest that post-depositional enrichment must have also played a role in achieving the observed high abundances. For Cr, this authigenic enrichment process is relatively straightforward, as Cr3+ readily adsorbs onto ferric hydroxide at circumneutral pHs and higher (Konhauser et al., 2011). In the iron- rich seawater of the Archean, ferric hydroxides were a common precipitate and were readily available, particularly during time of iron formation deposition. This adsorption process is supported by the higher average Cr concentrations for the shale samples within the BIF section. With that caveat for the BIF in mind, a plot of Ni/Cr concentrations does appear to indicate a mixed source of felsic and mafic rocks, with tendency towards more mafic end- members with stratigraphic position (Fig. 12). Thorium and scandium are also useful for discriminating among potential source rocks. Thorium is highly incompatible, and as a result reaches higher concentrations in more felsic rocks such as granite. In contrast, Sc is highly compatible and achieves highest abundances in more mafic rocks like basalt and komatiite (Taylor and McLennan, 1985). A plot of Th vs Sc shows a mixed provenance of felsic and mafic rocks with mafic end- members a more important contributor of sediment over time (Fig. 12). Taking the major, trace and REE geochemical trends into consideration, a consistent felsic to mafic trend with stratigraphic position emerges, suggesting that this is a robust result despite the relatively small sample size. The increasing mafic source signatures with stratigraphic position may be the result of 1) downward erosion accessing more mafic rocks lower in the sequence, or 2) progressive decrease in explosive dacitic volcanism over time. Petrographic and geochemical results strongly indicate that the Mapepe Formation was sourced by sediment derived from the uplifted, underlying Onverwacht Group and contemporaneous dacitic volcanism (Lowe and Nocita, 1999). In terms of source material, the Onverwacht contains a diverse assemblage of rock types, including ultramafic komatiites,
75 basalts, dacitic and mafic volcaniclastics and thick sequences of intercalated cherts (Lowe and Byerly, 1999). Other deeper sources of material might include the TTG plutons in the roots of the belt and the greenstones and granites of the Ancient Gneiss Complex in Swaziland (Hunter et al., 1978). The absence of large amounts of quartz and feldspar in addition to the lack of metamorphic fragments, however, indicates that erosion levels had not yet breached the Onverwacht sequence to access the plutons. These observations also preclude the Ancient Gneiss Complex in Swaziland as a source, which leaves the volcanic rocks and cherts of the Onverwacht Group as potential source rocks. The Mendon Formation of the Onverwacht Group, which directly underlies the Mapepe Formation, was likely the first stratigraphic unit to generate sediment upon uplift of the source area. As mentioned earlier, the Mendon Formation formed through alternating episodes of komatiitic volcanism and deposition of thick chert sections of various compositions. A more mafic trend over time could therefore reflect progressive incision through chert layers into komatiic flows and ashes. This simplistic scenario, however, cannot account for the highly fractionated, PAAS-like REE abundances or the negative europium anomalies of the basal Fig Tree shale, which all point to a significant felsic source rather than an alumina-poor, REE-depleted chert source rock. In addition, the occurrence of dacite and basalt fragments suggest that the Mendon Formation did not exclusively contribute sediment, as their presence indicates incision had accessed Onverwacht rocks below the Mendon Formation. Perhaps most importantly, petrographic examination of the sandstone shows that tuffaceous products of explosive dacitic volcanism contributed a large proportion of the framework grains. Voluminous dacitic eruptions could serve to dilute an otherwise mafic signature in the sediments. Felsic volcanic rocks, including flows and ash flow tuffs, do show LREE fractionation and relatively flat HREE patterns, and a modest negative Eu anomaly, sufficient to have generated shale with that same profile at the base of the sequence (Condie, 1993). This implies that decreasing explosive volcanism over time could lead to an increasingly mafic geochemical trend even if the source rock provenance did not change. In other areas exposing the Mapepe Formation, the lower Fig Tree is characterized by up to 200 meter thick sections of tuffs (Lowe and Byerly, 1999). These purely tuffaceous layers become less common up-section (Lowe and Byerly, 1999; Nocita and Lowe, 1999), making decreasing dacitic volcanism over time a plausible scenario to explain the felsic-mafic trend. Ultimately, however, it is likely that a combination of changes in source area provenance and explosive volcanism played a role in determining the shale geochemistry.
76 Comparison to Fig Tree Group in other areas of the BGB
The Mapepe Formation of the study area compares very favorably with petrographic and geochemical studies on southern facies Fig Tree Group rocks elsewhere in the BGB. These include the Barite Syncline area studied by Nocita (1989) and Lowe and Nocita (1999), and the strata in the Central Domain near the Granville Grove Fault explored by Hofmann (2005)(Fig. 1). Although these field localities are <10 km northwest of our study area, structural complexity, differences in stratigraphy and lack of spherule beds has cast doubt on the genetic relationship of the Mapepe Formation in the study area to that north of the Heights Syncline. Framework modal abundances of sandstone in the study area, however, are nearly identical to those in the Barite Syncline reported by Nocita (1989)(Fig. 4). He also described quartz- and feldspar- poor sediments (<10% quartz and <5% feldspar) dominated by volcaniclastic lithic grains. In addition, REE abundances and patterns in the basal shale in our section and the shale of the Loenen Micaceous Graywacke Member described by Hofmann (2005) compare very favorably. This suggests that although the basal shale in our study is stratigraphically in the upper Mendon Formation, it shows a geochemical affinity for shale in the lower Fig Tree Group. Additionally, although this zone in the core is pervasively sheared, the strong match with shale studied by Hofmann (2005) implies that this interval is in the correct stratigraphic position and does not represent a shale that originated higher in the sequence. Finally, geochemical analysis by Hofmann (2005) also reported a felsic to mafic provenance trend with stratigraphic position in the Mapepe Formation of the Central Domain. These observations favor a correlative relationship between the two areas that is worth studying further in the future. Northern facies Fig Tree strata are distinct from the southern facies both in modal composition and in geochemistry. The northern facies contains significantly more feldspar and monocrystalline quartz, and on QFL diagrams plot in a field distinct from the southern facies sandstone (Fig. 4)(Herget, 1966; Condie et al., 1970; Reimer, 1975). Relatively abundant granitic and metamorphic fragments are also found in northern facies, but are largely absent from the Mapepe Formation in the Barite Syncline and in our study area (Condie et al., 1970; Reimer, 1972; Nocita, 1989; Lowe and Nocita, 1999). Furthermore, the Sheba and Belvue Road formations contain an up-section increase in granitic rock fragments and decrease in volcanic fragments (Eriksson, 1980b; Condie et al., 1970). The difference between the northern and southern facies Fig Tree Group is also apparent in the geochemistry of shale. It is apparent from the CN-A-K diagram that the northern facies Fig Tree did not experience K-metasomatism (Fig. 11). Furthermore, major oxide geochemistry points to a predominance of a felsic source, although mafic and 77 ultramafic components certainly contributed material (Condie et al., 1970; Visser, 1956). Finally, unlike the southern facies, REE analysis performed by Condie et al. (1970) showed a decrease in Ni and an increase in LREE and LAN/YbN, with stratigraphic position, which they attributed to increasing importance of granitic source rocks (Condie et al., 1970). These differences across the Inyoka fault may stem from either different source areas or reflect age differences. Age dating indicates that the southern facies is likely older than the sediments north of the Inyoka fault, with the oldest age being reported from our study area (Kröner et al., 1991; Byerly et al., 1996; Lowe and Byerly, 1999; Byerly, unpublished data). This implies that by the time the northern facies deposited, incision of the uplifted Onverwacht rocks had reached the plutonic roots of the belt (Condie et al., 1970), thereby contributing significantly more quartz and feldspar and driving the increasingly felsic geochemical signature in the shale. Furthermore, a more mature and extensive drainage system could have delivered material from the Ancient Gneiss Complex to these areas. In contrast, the study area likely represents the first sediments eroded off local uplifts of the Onverwacht sequence, and reflects a mixed source of mafic to ultramafic source rocks and contemporaneous felsic volcaniclastic debris. Preliminary field work, however, in the younger Mapepe Formation rocks of our sequence to the south indicates increasing quartz content up-section and shoaling depositional environments. Therefore, this 300 m section merely characterizes the very base of the Mapepe Formation in the southeastern extent of the BGB, and younger parts of this sequence may show a greater affinity with time to the more felsic sediments of the northern facies Fig Tree Group.
Conclusions
The ristinep subsurface samples from the BARB4 core provide an excellent basis to increase understanding of the Fig Tree Group, Mapepe Formation in this little-studied area of the Barberton greenstone belt. Petrographic analysis of framework modes shows predominance of polycrystalline quartz (chert) grains (51.8%) and volcanic lithic grains (41.0%), a large proportion of which are devitrified tuffaceous fragments. In contrast, monocrystalline quartz (5.7%) and feldspar (1.5%) are minor constituents. Metasomatic overprinting and Al-poor potential source rocks complicate assessment of weathering conditions in the hinterland, but overall indicators point to a moderate weathering environment. Major, trace and REE geochemical analysis suggests a mixed felsic and mafic provenance for shale, demonstrating a robust trend towards an increasingly mafic source signature with stratigraphic position. This may stem from either progressive excavation into more mafic source rocks, a decrease in explosive felsic volcanism, or a combination of both of these scenarios. The geochemical and petrographic profiles compare favorably with 78 Mapepe Formation strata in the Barite Valley of the Eastern Domain and in the Central Domain. This suggests that although the sediments here are older and lack spherule impact layers, these deposits are genetically related to the Mapepe Formation in other areas south of the Inyoka fault. Future studies on the BARB4 core are therefore broadly applicable and results can be compared with existing studies throughout the BGB.
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83 Figure 1. Simplified geologic map of the Barberton greenstone belt and generalized stratigraphic column modified from Lowe and Byerly (1999). A. Generalized geologic map showing Fig Tree Group rocks in dark gray and the Onverwacht and Moodies groups in light gray. The heavy black line denotes the Inyoka fault, which divides the northern and southern facies of the Fig Tree Group. The study area location, which is the drill site for the BARB4 core, is shown as a star. Other localities where the Mapepe Formation has been studied are shown with a square (Nocita, 1989; Nocita and Lowe, 1990; Lowe and Nocita, 1999) and a triangle (Hofmann, 2005). Encircled abbreviations denote plutons intruding into the belt and include the Kaap Valley Pluton (KV), the Nelshoogte Pluton (N), the Stolzburg Gneiss (S), the Theespruit Gneiss (T) and the Dalmein Pluton (D).
84
angular unconformity sandstone conglomerate sandstone shale BIF sandstone BIF & BFC chert komatiite basalt mafic volcaniclastic chert mafic volcaniclastic felsic rocks chert basalt chert komatiite
Fm
Mapepe Fm Mapepe
Mendon
Onverwacht Gp Onverwacht Gp Gp Tree Fig Moodies 0 5 0 10 (m) (km) 500 1000 B.
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SOUTH AFRICA SOUTH
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ONVERWACHT ANTICLINE ONVERWACHT L U
A T
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T S Swaziland 10 N SOUTH AFRICA km FIG TREE GROUP AND MOODIES GROUPS ONVERWACHT (BARB4 SITE), EASTERN DOMAIN AREA STUDY BARITE SYNCLINE, EASTERN DOMAIN DOMAIN AREA, CENTRAL GRANVILLE GROVE 0 A. 85 Figure 2. Generalized stratigraphic columns for formations of the Fig Tree Group with age and stratigraphic relationships from Lowe and Byerly (1999) and stratigraphy of the lower limb of the Manzimnyama Syncline. A. Possible relationships based on the assumption that the succession of Fig Tree formations is not structurally repeated in the northern area of the BGB. B. Likely age relationships based on the assumption that the Schoongezicht Formation in the BGB’s northernmost extent includes age equivalents of the upper sections of the Belvue Road, Auber Villiers and Mapepe formations. C. Stratigraphic column showing the stratigraphic interval penetrated by the BARB4 core. Note the occurrence of siliciclastic beds within the orthochemical sections.
86 BARB 4 cored interval cored 4 BARB BIF BFC Sandstone and conglomerate BIF BFC Mapepe Fm Mendon Fm 0 (m) 500 250
C.
Schoongezicht Formation Schoongezicht Auber Villiers Formation Villiers Auber 3260 (Ma) 3225?
Lithic, chert-grit sandstone Shale, mudstone, and other argillaceous rocks, some tuffaceous
Auber Villiers Formation Villiers Auber
Fm
Fm
Road Fm Road Road Fm Road
Sheba Fm Sheba Sheba Fm Sheba
Ulundi Fm Ulundi Ulundi Fm Ulundi
Schoong. Schoong. Belvue Belvue Belvue Belvue
pyroclastic rocks
Volcaniclastic and Volcaniclastic Chert clast conglomerate
Mapepe Formation Mapepe Mapepe Formation Mapepe (Ma) 3225 3243 (Ma) 3225 3243 3260 A. B.
87 Figure 3. Stratigraphic section of the BARB4 core described from high-resolution core scans. Stratigraphic positions of shale samples used in the geochemical analyses are indicated.
88 Depth (m) Depth (m) 225 0
~150 m more lithic sandstone 250 25 overlain by BFC and BIF
275 50 SAF649-21
300 75
325 100
SAF649-16
SAF649-51&15 SAF649-46 350 125 SAF649-50 SAF649-40
375 150
400 175
SAF649-7&8
SAF649-5&6 425 Basal Fig Tree Group 200 SAF649-4 (Mapepe Fm) SAF649-3 Onverwacht Group SAF649-1&2 (Mendon Fm) 450 225
Black chert Shale Banded iron formation Damaged zone Chert plate
Komatiite Sericite Banded ferruginous chert Sandstone and Shale conglomerate sample 89 Figure 4. QFL and QmFLt plots after Dickinson and Suczek (1979) and Dickinson et al. (1983) showing inferred provenance type from detrital modes of sandstone. Data includes sandstone from the BARB4 core and averages values from published examples of the northern and southern facies Fig Tree Group. Northern facies data points come from the Sheba and Belvue Road formations from the Ulundi Syncline (Condie et al.,1970), the Belvue Road and Sheba formations in the Stolzburg Syncline (Reimer, 1975), and the Sheba Formation at the Montrose Mine (Herget,1966). The southern facies examples come from the Mapepe Formation in the Barite Syncline area (Nocita, 1989) and undisclosed sample localities for the Mapepe Formation from Eriksson (1980a). A. All of the BARB4 samples and the Mapepe Formation values from Nocita (1989) plot in the recycled orogenic field of the QFL plot. The northern facies rocks of the Sheba Formation falls within the dissected arc field whereas the Belvue Road Formation plots within the recycled orogenic field. B. The Mapepe Formation from the study area and Barite Syncline (Nocita, 1989) plot very close to the Lt corner of the plot within the lithic recycled field. In contrast, the northern facies rocks of the Sheba Formation plots within the transitional recycled field and the Belvue Road Formation straddles the dissected and transitional arc fields.
90 A. Q t
Fig Tree Group craton Northern facies Belvue Road Fm Sheba Fm recycled orogenic Southern facies
transitional continental Mapepe Fm Mapepe Fm, this study Mapepe Fm, this study, average composition dissected arc
transitional arc undissected basement uplift arc F L
B. Qm Fig Tree Group craton interior Northern facies Belvue Road Fm quartzose transitional recycled Sheba Fm continental Southern facies Mapepe Fm transitional Mapepe Fm, this study mixed recycled Mapepe Fm, this study, average composition
dissected arc lithic recycled basement uplift transitional arc
F Lt undissected arc
91 Figure 5. Photomicrographs of lithic grains from the BARB4 core. A. Carbonaceous chert clast cross-cut by quartz veins, plane polarized light. B. Polycrystalline chert clast, cross nichols. C. Dacitic fragment with sand-sized euhedral feldspar grains that show Carlsbad twinning, cross nichols. D. Highly silicified volcaniclastic fragment with pseudomorphs or “ghosts” of feldspars, plane polarized light. E. Amygdaloidal basalt fragment with filled vesicles and likely laths of plagioclase feldspar, cross nichols. F. Mafic lithic grain with acicular texture of possibly small plagioclase laths in a silicified groundmass of opaques and fine-grained material, cross nichols.
92 A. B.
500 μm 100 μm
C. D.
500 μm 200 μm
E. F.
1 mm 500 μm
93 Figure 6. Photomicrographs of tuffaceous lithic fragments from the BARB4 core. A. Felsic tuff with well-preserved psuedomorphs of volcanic glass shards. B. Tuff fragmentswith feldspar and quartz crystals within a silicified matrix rich with mafic oxides. The felted texture suggests the matrix was originally volcanic glass that has devitrified. C. A grain that could be either a mafic tuff fragment or a ferruginous chert grain. The high degree of disseminated fine-grained material within the grain, however, tentatively suggests a tuffaceous origin.
94 A.
500 μm 500 μm
B.
1 mm
1 mm
C.
500 μm
95 Figure 7. Plots of major oxide proportions in wt% of BARB4 shale vs SiO2 content, referenced to depth.
96 0.08 0.07 0.06
O (wt%) 0.05 2 0.04
Na 0.03 0.02 0.01 1.5
1.0
0.5 CaO (wt%)
0.0 35 30 25 20 15 10 FeO* (wt%) 5 0 20
15 (wt%) 3 O
2 10 Al 5 1.4 1.2 1.0
(wt%) 0.8 2 0.6
TiO 0.4 0.2 50 55 60 65 70
SiO2 (wt%) Key Shale samples from: Depth (m) Lithic sandstone section 100 150 Banded iron formation 200 Basal Fig Tree Gp contact 250 300 Average composition 350 400 450
97 Figure 8. A selected suite of elements normalized to PAAS (Taylor and McLennan, 1985).
Sr is strongly depleted in all samples, as is CaO and Na2O. Rb, Ba Sc, Cr and Ni are enriched above PAAS values. The BIF, BFC and lithic sandstone section shale samples show enrichment in FeO*. Overall the BFC samples are depleted in all elements except
SiO2 and FeO*.
98 (wt %) 2 66.6% 60.6% 56.3% 83.0% SiO Y Zr Lithic sandstone section Banded iron formation Banded ferruginous chert Gp. contact Tree Basal Fig Average values for shale from: Average 2 2 FeO* MgO CaO Na O K O Sc Cr Ni Rb Sr Ba 322 TiO 2 SiO Al O
1.0 0.1 0.0
10.0 Normalized to PAAS to Normalized
99 Figure 9. Plots of shale sample REE abundances normalized to Boynton (1984) chondrite values. Plot A shows chondrite normalized REE abundances grouped into lithofacies association and colored by depth, whereas plot B shows average values for shale within these categories. The basal shale at the Mendon/Mapepe Formation contact shows REE patterns most similar to PAAS, with LREE fractionation and a negative Eu anomaly. In contrast, the shale within the banded iron formation have a smaller Eu anomaly and have slightly less LREE fractionation. The shale samples from the overlying lithic sandstone section continue this trend, with a slightly positive to no Eu anomaly and lower ∑REE. Shale within the banded ferruginous chert show the greatest variation in REE abundances with most samples exhibiting a positive Eu anomaly. The anomalous REE patterns and high variation suggest these highly silicified samples reflect the composition of the precipitative fluid rather than signatures of source rocks.
100 1000 Shale samples from: Depth (m) A. Lithic sandstone section 100 150 Banded iron formation 200 Banded ferruginous chert 250 Basal Fig Tree Gp. contact 300 350 PAAS 400 450
100
SAF649-1 SAF649-3 SAF649-2 SAF649-15 SAF649-51 SAF649-21 10 SAF649-16 SAF649-46 Chondrite normalized (ppm/ppm) SAF649-4
SAF649-40 SAF649-6 SAF649-50 SAF649-7
SAF649-5 SAF649-8
1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1000 Average values for shale samples from: B. Lithic sandstone section Banded iron formation Banded ferruginous chert Basal Fig Tree Gp. contact PAAS
100
SiO2(wt%) 66.6% 62.8% 60.6% 10
Chondrite normalized (ppm/ppm) 56.3%
83.0%
1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 101 Figure 10. Plots showing likelihood of felsic or mafic provenance forM apepe Formation shale. A. Plot of Eu/Eu* with depth. Eu/Eu* measures the relative depletion or enrichment of europium relative to the neighbouring REE, samarium and gadolinum (McLennan and Taylor, 1985), with lower values representing felsic provenance and higher values representing provenance of a more mafic composition. The shale in this study show a range of Eu/Eu* values that span depleted and enriched values, but none are as depleted Post- Archean average upper-crust (Eu/Eu* =0.65). The shale within the banded iron formation spans both the depleted and enriched fields, with most points falling within depleted field. The shale samples from the lithic sandstone section fall within the range of error zone and do not show depletion or enrichment. Notably, the shales within the banded ferruginous chert show strong enrichment in Eu/Eu*. The shale from near the basal Fig Tree Group contact has two values in the depleted zone and one within the range of error zone. Excluding the banded ferruginous chert samples, there is an increase in Eu/Eu* ratio upsection, indicated by the black arrow, suggesting increasing contributions from more mafic sources over
time. B. LaN/YbN plots are informative for tracking changes in granitic provenance with stratigraphic position. La is an highly incompatible LREE and Yb is a HREE, and partial melts that generate granitic source rocks are typically enriched in LREE with respect to
HREE. Higher LaN/YbN therefore indicate a granitic source. Only one value, a shale sample from the base of section, exceeds the PAAS value of 9.3. There is a crudely developed trend, when the anomalous banded ferruginous chert samples are excluded, towards lower LaN/
YbN values up-section.
102 A. 100 Shale samples from: Lithic sandstone section 150 Banded iron formation Banded ferruginous chert 200 Basal Fig Tree Gp. contact
250 depleted enriched
300 Depth (m)
350
400
450 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 Eu/Eu*
B. 100
150
200
250
300 PAAS Depth (m)
350
400
450 1 2 3 4 5 6 7 8 9 10
La /YbNN
103 Figure 11. CN-A-K and CNK-A-FM plots afterN esbitt and Young (1982). A. CN-A-K plot of molar fractions of key oxides in silicate minerals after Nesbitt and Young (1982). The majority of the samples from this study plot along the A-K axis and have CIA values ranging from 60-75. Weathering trends from potential source rocks are shown with black arrows. End-member compositions are from Condie (1993). Since the majority of the shale samples plot below the weathering trajectory of even a granitic source rock, it is likely that these samples were enriched with potassium through K- metasomatism (Fedo et al., 1995). In contrast, the northern facies Fig Tree shales plot along the weathering trend predicted for a rock of felsic composition, suggesting these samples did not experience post-depositional K-enrichment. B. CNK-A-FM diagram colored by sample depth, useful for identifying provenance with stratigraphic height, after Nesbitt and Young (1982). The samples do not plot along any one weathering trend for a potential source rock, suggesting the composition of the source rocks were changing over time. The increase in proportions of FeO* (FeO total) and MgO with stratigraphic position suggests the source evolved from an originally felsic composition to an increasingly mafic and ultramafic provenance.
104 A. A Southern facies 100 Shale from (this study): Lithic sandstone section Banded iron formation Banded ferruginous chert Basal Fig Tree Gp. contact Average composition 75 Heinrichs (1980) Hofmann (2005) Northern facies Visser (1956) Condie et al. (1970) Potential source rock 50 Felsic Granite CIA TTG PAAS
Basalt Komatiite
25
0 CN K
B. A Shale samples from: Depth (m) Lithic sandstone section 100 150 Banded iron formation 200 Basal Fig Tree Gp. contact 250 300 Average composition 350 PAAS 400 450 Potential source rock
mafic trend
Granite TTG Felsic
Basalt
Komatiite
CNK FM
105 Figure 12. Major oxide and trace element plots useful for discriminating source rock compositions. End member compositions are from Hessler and Lowe (2006). Color is referenced to depth/stratigraphic position. A. Thorium vs scandium plot indicating a mixed felsic and mafic provenance with a trend from felsic end-members to more mafic end members with stratigraphic position. B. Nickel vs chromium plot source rock compositions for most samples falling between the basalt to granite and komatiite fields. The banded iron
formation shale samples are particularly enriched in Cr. C. TiO2 vs Al2O3 plot showing that most samples fall among the end-member fields, suggesting a mixed felsic and mafic provenance.
106 25 20 100 150 200 250 300 350 400 450 Depth (m)
15 tonalite (wt%) 3 O
2 basalt volcanic felsic Al 10 5
Lithic sandstone section Banded iron formation Gp. contact Tree Basal Fig composition Average komatiite Shale samples from: 0 Key
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
2 2 (wt%) TiO C. 40 10000 komatiite
30 komatiite 1000 basalt 20 Scandium (ppm) Chromium (ppm) 10 100 felsic volcanic tonalite granite
0 basalt to granite to basalt 10 1 10 10
100 100
1000
Nickel (ppm) Nickel Thorium (ppm) Thorium 10000 A. B.
107 Table 1. Point counting scheme for framework grain modal abundances of 15 sandstone samples from the BARB4 core.
108 (generic, cannot distinguish) cannot (generic, Sub-type Monocrystalline quartzose grains quartzosePolycrystalline lithic fragments Feldspar Plagioclase K-Feldspar metavolcanic and Volcanic Sedimentary metasedimentary and Volcanic quartz Volcanic p s m v F P K L U Unknown/miscellaneous L Q Q Type Q F L U cL cL fU cU cU cU mL vcL vcL GS mU vcU vcU vcU vcU cU-Cg 346.46 322.41 214.77 211.67 202.69 197.64 193.38 156.51 155.69 145.67 133.61 129.19 124.45 119.17 115.52 Depth (m) Depth SAF649-15 SAF649-18 SAF649-27 SAF649-28 SAF649-30 SAF649-31 SAF649-32 SAF649-33 SAF649-34 SAF649-37 SAF649-41 SAF649-43 SAF649-45 SAF649-48 SAF649-49 in sections
109 Table 2. Major, trace and REE abundances from 15 shale samples extracted from the BARB4 core.
110 SAF649-1 SAF649-2 SAF649-3 SAF649-4 SAF649-5 SAF649-6 SAF649-7 SAF649-8
SiO2 65.79 65.97 68.08 63.18 95.83 91.99 82.39 81.37 TiO2 0.85 1.45 0.73 0.28 0.04 0.15 0.02 0.00 Al2O3 20.43 20.75 16.78 24.19 1.08 3.55 0.29 0.15 FeO* 2.94 2.50 6.31 2.38 2.51 2.78 13.16 14.26 MnO 0.01 0.01 0.14 0.01 0.02 0.05 0.51 0.59 MgO 1.60 1.56 1.69 1.23 0.18 0.29 3.37 3.35 CaO 0.49 0.02 0.19 0.03 0.02 0.06 0.15 0.18
Na2O 0.06 0.06 0.09 0.06 0.00 0.04 0.00 0.00
K2O 7.45 7.65 5.86 8.61 0.29 1.03 0.08 0.05 P2O5 0.39 0.03 0.13 0.03 0.02 0.05 0.03 0.05
Rb 153 157 137 185 8 31 3 2 Sr 96 7 12 8 4 4 2 2 Ba 1480 1485 761 823 28 90 13 11 Pb 7 7 5 4 5 4 1 1 23 13 10 28 1 3 0 0 U 5.2 4.5 2.4 8.2 0.1 0.5 0.0 0.0 Zr 345 250 142 149 10 38 5 1 Hf 8.2 7.6 3.8 5.1 0.3 1.1 0.1 0.1 Nb 19.6 17.6 11.3 15.4 0.9 2.8 0.3 0.1 Ta 1.5 1.4 1.1 1.1 0.1 0.2 0.0 0.0 Y 52.7 32.5 29.8 13.2 3.9 7.9 6.7 4.5 La 43.3 47.1 29.4 19.0 3.1 8.4 1.7 0.8 Ce 83.5 89.6 57.7 35.1 5.9 16.9 3.3 1.6 Pr 9.7 10.0 6.7 3.8 0.7 2.0 0.4 0.2 Nd 36.7 36.2 25.1 13.0 2.5 7.6 1.6 1.0 Sm 8.19 7.29 5.47 2.51 0.57 1.50 0.43 0.29 Eu 2.04 1.63 1.85 0.79 0.25 0.53 0.24 0.17 Gd 8.89 6.22 5.30 2.17 0.60 1.41 0.63 0.51 Tb 1.53 1.01 0.92 0.36 0.11 0.23 0.14 0.09 Dy 9.71 6.06 5.79 2.21 0.69 1.42 0.98 0.62 Ho 2.00 1.30 1.21 0.47 0.14 0.29 0.21 0.14 Er 5.42 3.55 3.41 1.35 0.41 0.83 0.65 0.36 Tm 0.80 0.51 0.51 0.22 0.06 0.13 0.10 0.05 Yb 4.79 3.27 3.38 1.49 0.39 0.83 0.59 0.31 Lu 0.73 0.49 0.52 0.25 0.06 0.13 0.10 0.05 Sc 11.6 15.7 17.7 5.5 2.3 4.3 1.0 1.7 V 59 134 148 29 10 29 5 6 Cr 86 119 943 53 38 132 20 13 Ni 76 72 233 82 54 55 26 42
Eu/Eu* 0.73 0.74 1.05 1.04 1.27 1.12 1.40 1.38
LaN/YbN 6.1 9.7 5.9 8.6 5.3 6.8 1.9 1.7
Major elements as weight percent oxides and trace elements in ppm; FeO*= total Fe, 1/2 Eu/Eu*=EuN/[(SmN)xGdN)]
111 SAF649-50 SAF649-51 SAF649-15 SAF649-16 SAF649-21 SAF649-40 SAF649-46
SiO2 65.24 52.41 63.49 62.45 59.45 48.67 64.00 TiO2 0.39 0.50 0.84 0.73 0.66 0.31 0.71 Al2O3 18.56 15.19 16.32 13.75 12.65 7.03 14.56 FeO* 5.44 21.73 9.13 11.68 19.03 34.72 12.21 MnO 0.06 0.21 0.21 0.21 0.24 0.63 0.13 MgO 1.91 2.90 2.09 3.29 3.58 6.76 3.99 CaO 1.60 0.63 0.24 0.89 0.32 0.39 0.12
Na2O 0.04 0.07 0.02 0.03 0.06 0.02 0.05
K2O 6.69 6.06 7.53 6.84 3.88 1.41 4.15 P2O5 0.08 0.31 0.13 0.12 0.15 0.06 0.08
Rb 312 242 241 198 132 45 137 Sr 18 16 13 16 6 6 18 Ba 3078 1678 2099 1845 615 721 1969 Pb 2 3 5 2 2 2 7 2 14 7 5 5 2 4 U 0.0 3.6 1.8 2.3 0.8 1.4 2.1 Zr 30 181 137 102.1 94.1 31.0 84.5 Hf 0.9 4.8 3.6 2.8 2.5 0.8 2.3 Nb 1.9 12.9 8.5 7.2 6.3 1.7 5.9 Ta 0.1 1.6 0.8 0.6 0.6 0.1 0.5 Y 6.1 30.2 29.2 20.8 23.7 11.0 20.1 La 2.8 17.0 23.7 16.9 16.1 6.2 14.4 Ce 5.8 35.5 46.4 33.5 31.4 13.3 28.8 Pr 0.7 4.4 5.4 4.0 3.7 1.7 3.5 Nd 3.0 17.4 20.3 15.2 14.2 6.8 13.7 Sm 0.79 4.22 4.51 3.49 3.20 1.68 3.09 Eu 0.32 1.08 1.33 0.99 1.08 0.59 1.04 Gd 0.97 4.47 4.71 3.58 3.41 1.80 3.45 Tb 0.16 0.76 0.80 0.62 0.61 0.31 0.60 Dy 1.07 4.81 5.24 3.85 4.01 2.03 3.70 Ho 0.23 1.01 1.12 0.81 0.87 0.41 0.78 Er 0.65 2.83 3.18 2.23 2.47 1.15 2.14 Tm 0.10 0.40 0.48 0.32 0.36 0.17 0.31 Yb 0.67 2.46 3.09 2.04 2.35 1.02 1.97 Lu 0.11 0.38 0.51 0.33 0.37 0.16 0.31 Sc 34.5 9.0 25.8 21.6 15.4 11.7 27.7 V 219 60 184 156 119 102 20 Cr 5253 280 931 833 752 439 934 Ni 150 112 118 311 194 177 544
Eu/Eu* 1.10 0.76 0.88 0.86 1.00 1.03 0.98
LaN/YbN 2.8 4.7 5.2 5.6 4.6 4.1 4.9
Major elements as weight percent oxides and trace elements in ppm; FeO*= total Fe, 1/2 Eu/Eu*=EuN/[(SmN)xGdN)]
112 Summary
The sedimentology and the geochemistry of the rocks in the Manzimnyama Syncline reveal interesting aspects of not only the Archean deep-water depositional system, but its feeder system and the source area. Strong evidence for a lateral, coeval facies relationship between the orthochemical rocks and the siliciclastic components lies in their gradational contacts, their inter-bedded character and the presence of chert-plates that likely represent an Archean version of mud-rip up clasts. It is probable, therefore, that the orthochemical deposits are the equivalent of distal fine-grained facies in Phanerozoic and modern systems. In contrast, the lithic sandstone sequence was primarily deposited by high-energy, high-density turbidity currents, although periods of lower energy sedimentation and even cessation of siliciclastic input—attested to by the chert horizons in lithofacies Lm—did occur. The limited along-strike exposures show that the beds are overwhelmingly tabular and lack large-scale scour and other indicators of channelization. Given these observations, it is probable that the density flows came to rest in an unconfined setting and formed a lobe deposit. The thickness of the section (~350 m) suggests a frontal lobe location, at the terminus of the feeder conduit, as opposed to other more short-lived lobe settings. While the core only penetrated the frontal lobe portion of this depositional system, aspects of the sub-aqueous feeder system can be surmised. The scarcity of mud almost certainly precluded the construction of levees up-dip, meaning the high-density flows must have been confined within incisional features—such as a canyon—during transport. The distance between the staging area and the site of deposition was likely short, as the broad normal distribution of grain sizes suggests the run-out distance was not long enough to effectively size fractionate the sediment within the flow. Some insight can also be gained concerning the terrestrial part of the sedimentary system. The angularity of the material, in addition to the abundance of labile volcanic grains, suggests both a proximal source and short residence time in the fluvial system that delivered the sediment to the coastal staging area. In addition, the great quantity of Onverwacht- sourced detrital material speaks to uplift in the source area. The uplifted Onverwacht sequence was not breached, as the low abundances of quartz and feldspar grains indicate the deep-seated TTG plutons were not yet accessed. In addition, the high proportion of tuffaceous grains suggests voluminous explosive dacitic volcanism was ongoing during deposition of this sequence. The major, trace and rare earth element geochemistry of shale points to a mixed felsic and mafic source that became more mafic over time. This could reflect excavation into deeper-seated mafic rocks or a decrease in explosive felsic volcanism over time. Confident
113 assessment of weathering conditions in the source area is compromised by metasomatic overprinting and alumina-poor source rocks, but the abundance of labile grains suggests the climate was not extreme. Finally, the geochemical and petrographic affinity of the rocks of this study with Mapepe Formation rocks in the Barite Valley and in the Central Domain suggests a correlative, genetic relationship among these areas south of the Inyoka fault.
114 APPENDIX A
Measured field sections in the lower limb of the Manzimnyama Syncline.
115 KEY TO BARB4 CORE LOG AND MEASURED SECTIONS
Mudstone/siltstone Faulted zone
Chert layers Missing, broken or otherwise damaged core
Sandstone and conglomerate 47 m Annotated depth
Sandstone with 214 m Measured depth mud clasts F Fault Sandstone with chert-plate clasts ? F Suspected fault
Sandstone with Overturned section outsized, oating grains
Sandstone with muddy wavy laminations
Sandstone with disturbed bedding
Sandstone with at laminations
Sandstone with fractures
116 Section 1 Grain size m/s f m c vc g cg 4 m
chert plates up to 12 cm long
3 m erosive base
erosive base white chert layer
laminated silty material
2 m white chert layer
laminated silty material grey chert layer white chert layer
laminated silty 1 m material
white chert layer
white chert layer
banded iron formation
0 m 1:20 vertical scale 117 Section 1 (cont.) Grain size erosive base
8 m m/s f m c vc g cg
laminated silty/ashy material
black chert plates
7 m 1:20 vertical scale black chert plates, 2 cm thick erosive base
laminated silty/ 6 m ashy material
laminated silty/ ashy material 5 m
4 m 1:20 vertical scale 118 chert plates up to 12 cm long SECTION 1 Section 1 (cont.) Grain size m/s f m c vc g cg
F faulted zone
10 m
erosive base
laminated silty/ashy material
9 m erosive base
erosive base
8 m
laminated silty/ashy material
black chert plates
7 m 1:20 vertical scale black chert plates, 2 cm thick erosive base 1:20 vertical scale 119 Section 2 cross lamination Grain size m/s f m c vc g cg 4 m abundant mud clasts
cross lamination
3 m
massive
2 m
chert
probably cross lamination 1 m glassy chert nodules
ne ferruginous sediment, probably very ne tu or tu aceous mudstone, 0 m or mudstone 1:20 vertical scale 120 undulating laminations, may be current structures, but unclear Section 2 (cont.) Grain size
8 m m/s f m c vc g cg
7 m ne ashy material
muddy undulating 6 m laminations, probably cross lamination
ashy, hard and probably not mud 5 m
cross lamination basal erosion chert clasts
cross lamination
4 m abundant1:20 vertical mud scale clasts 121 SECTION 2 Section 2 (cont.) Grain size m/s f m c vc g cg
10 m
F ill-de ned boundary
9 m
undulating laminations, may be current structures, but unclear
8 m
7 m ne ashy material
1:20 vertical scale 122 Section 3 Grain size m/s f m c vc g cg 4 m
possible cross- lamination
3 m
amalgamated with abundant basal erosion
2 m
1 m possible slurry? or rippled chert clasts
0 m 1:20 vertical scale 123 Section 3 (cont.) Grain size
8 m m/s f m c vc g cg ripples
7 m
chert clast
tops have well 6 m developed undulating mud and ne sand lenses, could be slurries or mud-draped ripples
5 m many beds have erosive bases
4 m 1:20 vertical scale
124 SECTION 3 Section 3 (cont.) Grain size m/s f m c vc g cg
10 m
F undulating muddy surfaces, may be 9 m slurry ripples
8 m ripples
7 m
chert clast
1:20 vertical scale 125 chert plate,
>12 cmSection long, 4 Grain size 5 cm thick m/s f m c vc g cg 4 m
abundant basal erosion/incision
3 m
deep basal incision
2 m
slurry?
mud clasts
1 m mud draped ripples slurry?
cross-lamination
0 m 1:20 vertical scale
126 Section 4 (cont.) Grain size rippled m/s f m c vc g cg 8 m
ripples
ripples 7 m
cross-lamination 6 m
mud draped ripples
5 m cross-lamination
chert plate, >12 cm long, 5 cm thick
4 m 1:20 vertical scale
127 Section 4 (cont.) Grain size rippled 12 m m/s f m c vc g cg
cross-lamination
11 m
cross-lamination
10 m
cross-lamination
9 m cross-lamination
cross-lamination
rippled 8 m 1:20 vertical scale
128 SECTION 4 Section 4 (cont.) Grain size m/s f m c vc g cg
13 m
cross-lamination
rippled 12 m
cross-lamination
11 m
cross-lamination
10 m
cross-lamination1:20 vertical scale
129 chert plates Section 5 Grain size chert plates
4 m m/s f m c vc g cg
possible cross- lamination
3 m
chert plates
2 m
1 m
0 m 1:20 vertical scale
130 Section 5 (cont.) Grain size 8 m m/s f m c vc g cg distinctive “tiger stripe” weathering
7 m chert plate
6 m
chert layers
1:20 vertical scale
5 m erosive base chert plate
erosive base chert plates
chert plates
4 m 1:20 vertical scale
131 Section 5 (cont.) Grain size
12 m m/s f m c vc g cg
all massive beds
11 m
10 m
9 m possible cross- lamination
8 m 1:20distinctive vertical scale “tiger stripe” 132 weathering SECTION 5 Section 5 (cont.) Grain size m/s f m c vc g cg
F
14 m
3-6 cm chert layer
13 m
12 m
all massive beds
11 m
1:20 vertical scale
133 Section 6 Grain size cross lamination at lamination 4 m m/s f m c vc g cg
cross lamination in thin beds F major cross-cutting 3 m fault ripples
2 m slicken lines on bedding surfce
cross lamination 1 m
cross lamination
0 m 1:20 vertical scale
134 SECTION 7 Section 6 (cont.) Grain size m/s f m c vc g cg
cross lamination 7 m
cross lamination
6 m
possible mud clast
rippled 5 m abundant cross and at lamination
cross lamination at lamination 4 m
cross1:20 lamination vertical scale in thin beds 135 max 1 cm (disk shaped), 5 mm rounded, loaded base F
granule conglomerate
6 m 12 m max 1.2 cm disk, 5-6 mm Section 8 lower to very ne sand, max mud ne to very ne sand ne sand and silt ne grained and mud
conglomerate clasts, max 3 mm ne to very ne sand 5 m 11 m ne sand mud to coarse sand ne sand
very coarse sand with ne sand grains up to 8-9 mm mud to ne sand
coarse sand 4 m 10 m ne to medium sand all sandstone with possibly some coarse to lower minor conglomerate very coarse, max 2-3 mm possible at lamination top to undulating ne sand coarse
3 m 9 m medium sand
max grain size = coarse to very coarse sand ne sand max = 4 cm
granular to small medium to coarse sand pebble congomerate rounded clasts 2 m granular conglomerate 8 m max 8-9 mm coarse to very coarse sand, max 2-3 mm max 4-5 mm loaded base medium sand medium to coarse sand granular conglomerate coarse grained sand looks muddy, max 4-5 cm
bedded, 1 cm ne 1 m grained sandstone 7 m ne to medium sand medium sand max 1 cm (disk shaped), 5 mm rounded, With hand lens, looks well F loaded base sorted, no oating grains. Probably little mud. granule conglomerate granular conglomerate 0 m 6 m max 1.2 cm disk, 5-6 mm 136 ne to very ne sand ne grained
conglomerate clasts = max 3 mm very ne to ne sandstone
cross-laminations ne grained
very ne, max coarse grained 18 m 24 m Section 8 (cont.) very ne sand
laminated silt and mud thinly laminated ne sand cross-laminated? at laminated, ne sand
coarse to very coarse sand 17 m 23 m F medium sand very coarse to granular conglomerate, max 5 mm very coarse sand very coarse to granular conglomerate, max 5-6 mm thinly-bedded laminated mud to silt, maybe some 16 m 22 m F ne sandstone poorly exposed but mostly ssile ne grained mudstone or siltstone white chert layers laminated mud and silt very nely laminated 15 m to some cross-laminations 21 m massive appearing medium grained sandstone, probably some sedimentary units bit still sandstone banded ferruginous chert thin chert bands massive muddy separated by sandstone mudstone layers, about 50:50 chevron folded 14 m mud 20 m
coarse to very coarse sand conglomerate to very coarse sand, max 4-5 mm chert coarse to very mud to siltstone coarse sand very ne to ne sand 13 m ne sand 19 m up to lower coarse sand ne grained siltstone and mudstone very ne to ne sandstone ne sand cross-laminations ne grained
very ne, max coarse sand 12 m 18 m 137 very ne sand
ne sand and silt laminated silt and mud and mud thinly laminated ne sand cross-laminated? at laminated, ne sand m/s f m c vc g cg Section 9 Grain size
4 m m/s f m c vc g cg F
bases of beds are well cemented
thinly laminated 3 m
crumbly red clay 2 m with chert clasts chert plates, ranging 2 mm to 3 cm in thickness mud clasts chert plates, 2 mm thick
1 m
basal incision
0 m 1:20 vertical scale
138 SECTION 10 Section 10 Grain size m/s f m c vc g cg
2 m
cross lamination
rippled
1 m zone of outsized, oating vcU grains
undulating chert plate, 5mm thick
possible mud clasts
0 m
4 m
1:20 vertical scale
139 Grain size m/s f m c vc g cg Section 11 Grain size m/s f m c vc g cg ?F 4 m deformed chert bands
black and white, chert layers 4 mm- 2 cm thick, for readability 3 m reasons all drawn as 2 cm thick and white in colour
2 m
thin layers of sand interbedded with mud
1 m
1:20 vertical scale 0 m 140 A ppendix B
BARB4 core description of the lithic sandstone section from high-resolution scans, 1:20 scale.
141 KEY TO BARB4 CORE LOG AND MEASURED SECTIONS
Mudstone/siltstone Faulted zone
Chert layers Missing, broken or otherwise damaged core
Sandstone and conglomerate 47 m Annotated depth
Sandstone with 214 m Measured depth mud clasts F Fault Sandstone with chert-plate clasts ? F Suspected fault
Sandstone with Overturned section outsized, oating grains
Sandstone with muddy wavy laminations
Sandstone with disturbed bedding
Sandstone with at laminations
Sandstone with fractures
142 Grain size 216 m m/s f m c vc g cg 216 m
erosion and mud rip ups
fractures
217 m mud clasts densely fractured
faulted and sheared, mud to mU sand and chert
218 m
damaged core, appears faulted and deformed 219 m
220 m 1:20 vertical scale
143 abrupt dip changes, possible fault breccia, fault zone? 212 m Grain size m/s f m c vc g cg 212 m
mud clast ?F vein quartz, faults? 213 m ?F lens of vcL sand 213 m banded chert clast
214 m depth notation error 214 m (11 cm)
F minor fault zone
distorted bedding
215 m 215 m
F fault zone
216 m 216 m 1:20 vertical scale
144 bent and deformed 208 m Grain size chert clast m/s f m c vc g cg 208 m mud and banded chert clasts
mud clast
209 m abrupt dip change, F 209 m vein quartz, likely is a fault zone mud rich
slurry top?
densely fractured 210 m uncut core, 210 m di cult to assess grain size and bed thickness
severely disrupted bedding, slurry?
211 m disrupted bedding, 211 m slurry? densely fractured
abrupt dip changes, F possible fault breccia, fault zone? 212 m
212 m 1:20 vertical scale
145 204 m Grain size m/s f m c vc g cg fractured 204 m
?F vein quartz, fault?
205 m estimated from 205 m broken and uncut core, slurry?
mud clasts and chert plate
206 m
206 m
pieces out of order and upside down in hi-res scan white chert plate
207 m mud clast 207 m
mud clast
grey chert clast bent and deformed 208 m chert clast
208 m mud1:20 and vertical scale banded chert 146 clasts vein quartz, 200 m Grain size fault? ?F 200 m m/s f m c vc g cg
overturned
F fault zone
distorted bedding
201 m basal erosion 201 m
core missing
202 m
202 m amalgamated
203 m mud clast
203 m
204 m fractured 204 m 1:20 vertical scale
147 F Grain size faulted zone
196 m m/s f m c vc g cg distorted bedding basal erosion
197 m wispy carbonaceous intraclasts, slurry? 197 m
erosion? disturbed bedding
198 m
198 m
faulted zone
199 m disturbed bedding 199 m
overturned F
vein quartz, 200 m fault? ?F 200 m 1:20 vertical scale
148 Grain size ne sand intraclast
192 m m/s f m c vc g cg
possibly two events
193 m high degree of basal erosion, individual bed determinations 193 m challenging
unusual grading 194 m
vein quartz
194 m
slurry top? vein quartz 195 m fractures
distorted bedding 195 m vein quartz deformed bedding
196 m mud clast
F faulted zone
196 m 1:20 vertical scale slurry? 149 Grain size missing core? 188 m m/s f m c vc g cg possible cross-lamination unusual grading 189 m
cross-lamination isolated pebble? 189 m overturned
distored bedding? ?F or faulted? 190 m
vein quartz 190 m
191 m
191 m distorted bedding
partially estimated from whole core
192 m
ne sand intraclast
192 m 1:20 vertical scale
150 Grain size
184 m m/s f m c vc g cg
core measured incorrectly (7 cm) 185 m by original workers
185 m
186 m
?F 186 m
deformed bedding
187 m
187 m
188 m
missing core? 188 m possible1:20 vertical scale cross-lamination unusual grading 189 m 151 Grain size
180 m m/s f m c vc g cg
181 m
F disturbed bedding 181 m grain size unknown
182 m
pink chert clast
either core is missing or large error in 182 m depth notation
183 m
broken core overturned? F 183 m faulted
disturbed bedding
184 m
F minor fault
184 m 1:20 vertical scale core measured incorrectly (7 cm) 152 by original workers cross-lamination Grain size
176 m m/s f m c vc g cg
177 m possible cross- lamination
sandy streak
177 m
178 m
carbonaceous mud clast
178 m convoluted bedding
179 m
179 m
180 m
muddy layers
180 m 1:20 vertical scale
153 sheared mud clasts, grading is approximation due to poor image quality Grain size
172 m m/s f m c vc g cg core misnotated
173 m poorly sorted with mud streaks, sandy intraclast? disrupted bedding
compressed and sheared mud clasts 173 m thin sand lenses 174 m and possible vein quartz
pinch and swell muddy layers
174 m
175 m
175 m
176 m
cross-lamination
176 m 1:20 vertical scale
154 Grain size
168 m m/s f m c vc g cg
minor faults F 169 m with 2 cm o set
169 m
170 m
vein quartz, ?F possible fault? 170 m lled fracture
171 m
horizon of soft sediment deformation
171 m
172 m chert plate
sheared mud clasts, grading is approximation due to poor image quality 172 m core misnotated 1:20 vertical scale 173 m poorly sorted with mud streaks, 155 sandy intraclast? F
Grain size
164 m m/s f m c vc g cg 165 m likely sheared in
?F disturbed bedding, or fault?
F abundant ?F oxidation 165 m 166 m likely sheared F
faulted F
possibly white chert, 166 m but more likely fault 167 m crystallization F faulted zone
167 m
168 m change in composition?
168 m 1:20 vertical scale minor faults F 169 m with 2 cm o set 156 Grain size m/s f m c vc g cg 160 m m/s f m c vc g cg 161 m
F minor faults
161 m depth misnotation 162 m (9 cm)
soft sediment deformation
162 m chert plate 163 m major crosscutting F fault
minor faults F
163 m 164 m
F
164 m 165 m likely sheared in 1:20 vertical scale
?F 157 disturbed bedding, or fault? Grain size m/s f m c vc g cg 156 m 157 m
cross-lamination
estimate from uncut core 157 m disrupted bedding 158 m
estimate from uncut core 158 m 159 m broken core
core pieces in hi-res scans are out of order
159 m 160 m
160 m 1:20 vertical scale
158 Grain size jasper and white 152 m m/s f m c vc g cg chert cobbles 153 m chert plate
estimated from uncut core
weakly strati ed top 153 m 154 m
ne sand intraclasts?
154 m 155 m
155 m 156 m
156 m 157 m 1:20 vertical scale
159 cross-lamination Grain size m/s f m c vc g cg 148 m 149 m
149 m 150 m
estimated from uncut core, likely not one event
150 m 151 m estimated from uncut core
F? white chert layer 151 m or vein quartz 152 m lenses of ne sand
estimated from uncut core
jasper and white 152 m chert cobbles 153 m chert1:20 plate vertical scale
160 estimated from uncut core Grain size
144 m m/s f m c vc g cg 145 m
depth misnotated
ignore 145.65m marked piece, out of place 145 m 146 m
sets of identical ?F core scans show stacking discrepancy
?F 146 m white chert or 147 m vein quartz?
depth misnotation (10 cm)
147 m 148 m rippled
148 m 149 m 1:20 vertical scale 161 end of broken Grain size section 140 m m/s f m c vc g cg 141 m estimated from broken core, looks faulted
F?
vein quartz 141 m 142 m
broken core
142 m ?F 143 m
F? ?F
?F ?F lled fractures
143 m broken core 144 m distorted bedding
144 m 145 m 1:20 vertical scale
162 green colouring Grain size
136 m m/s f m c vc g cg 137 m
estimated from uncut core 137 m 138 m
estimated from uncut core 138 m 139 m
possible 30 cm depth misnotation 139 m 140 m
white chert layer or vein quartz? end of broken section 140 m 141 m estimated1:20 vertical from scale broken core, looks faulted
163 Grain size estimated 132 m m/s f m c vc g cg from uncut core 133 m
133 m 134 m possible shear zone or slurry bed ? F
core misnotation
unusual grading behaviour
134 m 135 m
slurry bed?
core uncut, 135 m grain size unclear 136 m
F fault zone
green colouring
136 m 137 m 1:20 vertical scale
164 Grain size core piece in hi -res scan 128 m m/s f m c vc g cg upside down 129 m
lenses of medium upper size sand, loading present
F minor fault
129 m disrupted bedding 130 m
chert plate clast
130 m 131 m described from whole box core photo (14), scans not available
?F appears sheared
131 m 132 m subtle cross- lamination
estimated 132 m from uncut core 133 m 1:20 vertical scale
165 complex unit, faulted, sandy streaks, wispy laminated top Grain size
124 m m/s f m c vc g cg 125 m exceptionally poor sorting, with sand and mud lenses, erosive base subtle cross- lamination
125 m 126 m debris ow, matrix is very poorly sorted, ranging from clays to coarse lower sized sand
estimated from broken core
126 m 127 m either a debris ow ?F or fault breccia densely fractured, chert plate present
broken/missing core fractured estimated from whole, broken core 127 m 128 m
dissolved linear features, clasts or fractures? mud clast
core piece in hi -res scan 128 m upside down 129 m 1:20 vertical scale
166 soft-sediment deformation? apparent thickness, Grain size dip now 40 degrees 120 m m/s f m c vc g cg 121 m could be debris ow, F but probably faulted zone, see rapidly changing dip sheared, fU to mU F sand w/ carbonaceous layers (<1 cm thick)
121 m faulted, 1 cm o set 122 m
mud clast?
122 m uncut core, 123 m grain size unclear erosive base
F? uncut core, grain size unclear
123 m 124 m piece upside down in hi-res scan
complex unit, faulted, sandy streaks, wispy laminated top
124 m 125 m exceptionally1:20 vertical scale poor sorting, with 167 sand and mud lenses, erosive base Grain size
116 m m/s f m c vc g cg fractures 117 m cross-lamination? basal loading
erosive base
erosive base cU-sized sand lenses 117 m 118 m
estimated from uncut core
light red chert (jasper?) clast 118 m fractures 119 m
pink chert clast 119 m 120 m pink chert plate vein quartz? looks F faulted in uncut core soft-sediment deformation? apparent thickness, dip now 40 degrees 120 m 121 m could1:20 be vertical debris scale ow, but probably faulted zone, see 168 rapidly changing dip Grain size
112 m m/s f m c vc g cg 113 m minor fault F
soft-sediment deformation
113 m core mismeasured 114 m cross-lamination
fractures
114 m fractures 115 m
likely one ow 115 m 116 m estimated from uncut core sandy horizons soft-sediment deformation
116 m fractures 117 m cross-lamination?1:20 vertical scale basal loading
169 F jasper (?) chert layer chert nodules 109 m Grain size overturned?
108 m m/s f m c vc g cg white chert laminae? F
110 m
109 m
faulted and deformed F
111 m 110 m overturned?
chert laminae
white chert layer fracture
112 m 111 m depth misnotated (>20 cm)
112 m minor1:20 fault vertical scale
soft-sediment 170 deformation 105 m Grain size m/s f m c vc g cg 104 m
erosive base?
106 m white chert plate?
105 m distorted bedding distorted bedding
syn-sedimentary faulting, restored here distorted bedding
107 m sediments silici ed 106 m in place? ? F faulted, or debris ow 1 cm thick jasper layer distorted bedding grain size from fU to cU fractures and linear dissolution feature 108 m small scale faulting 107 m distorted bedding
F jasper (?) chert layer chert nodules 109 m overturned?
108 m white1:20 chert vertical laminae? scale F
171 Grain size m/s f m c vc g cg 100 m m/s f m c vc g cg
cross lamination
mud clast
102 m minor fault
101 m erosive base
piece upside down in hi-res scan
103 m ?F 102 m faulted zone F grain size up to vCu in poorly sorted beds
104 m fault zone with extensive folding 103 m and deformation
grains vary from mud to granular-sized material F
105 m
104 m 1:20 vertical scale
172 subtle cross- lamination?
97 m Grain size chert plate clast?
96 m m/s f m c vc g cg some outsized granules core pieces misnumbered in hi-res scan
98 m partially estimated from boxed core 97 m
possibly one event: 99.37-97.85 m
black chert plates or lled fractures?
99 m black chert plates 98 m and clasts
disturbed bedding
likely one event
100 m 99 m slurry bed?
*core notation 100-102 m incorrect distorted bedding
101 m depth notated 100 m incorrectly on core 1:20 vertical scale
173 sandy streaks 93 m Grain size
92 m m/s f m c vc g cg white chert lamina
disturbed bedding ne sand intraclasts
94 m
93 m
1 cm layer of vcU-sized sand
chert plate 95 m lenses of mL-mU- 94 m sized sand
basal erosion chert nodule uncut core, muddy disturbed bedding F crystallization 96 m along fault 95 m
mud rip-up clast
subtle cross- lamination?
97 m chert plate clast?
96 m some1:20 outsized vertical scale granules core pieces misnumbered 174 in hi-res scan 89 m Grain size
88 m m/s f m c vc g cg
missing and broken core
90 m
89 m broken core
chert plate?
core is either missing 91 m or mismeasured
90 m sandy streak
sheared, grain size F from mud to mU -sized sand cross-lamination F 92 m fractures 91 m rst appearance of surface oxidation at 92.47 m estimated from broken whole core basal erosion sandy streaks 93 m
92 m white1:20 chert vertical lamina scale
175 disturbed bedding ne sand intraclasts F
Grain size
84 m m/s f m c vc g cg core misnotated
86 m
compressed mud chips
faulted chert layers 85 mF iron-staining obscures grain size
F mud rip-up clast? core mismeasured by 9 cm
87 m
86 m
white chert plate
mud clast
88 m erosive base 87 m chert plate
89 m
88 m 1:20 vertical scale
176 iron staining obscures grain size changes 81 m
Grain size erosive base 80 m m/s f m c vc g cg
82 m
alteration zone at base
estimated from 81 m whole core, possibly two events
likely faulted 83 m F? estimated from broken core, cU to vcL sand F fault with chert 82 m
84 m missing and broken core
overturned 83 m
F 85 m
84 m core misnotated 1:20 vertical scale
177 77 m
broken core, Grain size mL to mU sand
76 m m/s f m c vc g cg iron-staining obscures grain size compressed mud chips? 78 m
77 m
white chert nodule 79 m or plate
78 m
80 m iron-staining obscures grain size
79 m
chert plate or nodule iron staining obscures grain size changes 81 m
erosive base 80 m 1:20 vertical scale
178 white and grey chert plates (<1.5 cm thick), with cL sand matrix mud rip up 73 m pink chert layer Grain size
72 m m/s f m c vc g cg white chert layer
layered pink chert
74 m outsized chert pebbles 73 m amalgamation
75 m lenses of cL sand chert plates fractured black chert, 74 m likely silici ed mud severely altered
erosive base? 76 m chert layer vertical lled fracture
75 m vertical lled fracture up to 2 cm wide
core depth ?F misnotated 77 m
broken core, mL to mU sand
76 m iron-staining1:20 vertical scale obscures grain size 179 compressed mud chips? broken core
68 m distorted bedding, rapidly changing dips, Grain size slurry bed? 68 m m/s f m c vc g cg mud clast
1 mm white chert laminae 69 m
69 m
core misnotated, 70 m marked as missing from 71.40 to 70.36 m, but assessed to be 70 m continuous
fault w/ minor o set F distorted bedding 72 m
71 m erosive base
white and grey chert plates (<1.5 cm thick), with cL sand matrix mud rip up 73 m pink chert layer
72 m white1:20 chert vertical layer scale
180 layered pink chert missing core
Grain size
64 m 64 m m/s f m c vc g cg estimated from scanned and broken core
likely one event
broken core
65 m 65 m
66 m 66 m
distorted bedding
67 m
67 m estimated from broken core
broken core
68 m distorted bedding, rapidly changing dips
68 m mud1:20 clast vertical scale 181 mud clast 60 m Grain size 60 m m/s f m c vc g cg
vertical, bed limited dissolution features rippled top?
61 m 61 m
according to core notation, core missing from 61.03-61.88 m
62 m 62 m
silici ed top?
pinch and swell structures missing core
63 m 63 m *discrepancy between depth notation on boxed core and hi-res scans
missing core
64 m 64 m estimated1:20 vertical from scale scanned and broken 182 core 56 m Grain size distorted bedding 56 m m/s f m c vc g cg chert layers iron staining obscures grain size
57 m
57 m
subtle at laminations
58 m
58 m
?F chert or fault? attened mud chips? 59 m
59 m ?F chert layer?
mud clast rich top
mud clast 60 m 60 m 1:20 vertical scale 183 chert layer (0.7 cm)
52 m Grain size chert layer (1 cm) 52 m m/s f m c vc g cg chert layer (0.5 cm) iron-staining obscures grain size
distorted bedding, 53 m with lenses of vcU sand 53 m chert layer distorted bedding
compressed mud chips mud clasts 54 m grey chert plate
F? 54 m distorted bedding, compressed mud chips
mud clast 55 m
uncut core 55 m fracture estimated from uncut core
56 m distorted bedding 56 m chert layers 1:20 vertical scale iron staining obscures grain size 184 48 m Grain size
48 m m/s f m c vc g cg fU sandy streak mud chip?
49 m disturbed bedding 49 m
50 m 50 m
F fault zone
iron-staining obscures grain size
51 m
51 m fractured
F minor fault
chert layer (0.7 cm)
52 m chert layer (1 cm) 52 m chert layer (0.5 cm) iron-staining1:20 vertical scale obscures grain size 185 Grain size
44 m m/s f m c vc g cg 44 m
chert clast
45 m 45 m
iron-staining obscures grain size F chert
iron-staining 46 m obscures grain size 46 m
distorted bedding, slurry bed?
47 m
47 m distorted bedding
?F abrupt 15 degree steepening of dip, fault?
48 m
48 m fU sandy1:20 vertical streak scale mud chip? 186 Grain size
40 m m/s f m c vc g cg uncut core, iron staining obscures grain size
distorted bedding
41 m distorted bedding 41 m
basal loading or erosion?
42 m
42 m
estimated 43 m from broken core 43 m
44 m 44 m 1:20 vertical scale
187 Grain size m/s f m c vc g cg
end of cut core and high-res scans 38 m
cross-lamination
39 m piece upside down 39 m on hi-res scan
? F white chert, probably fault
exceptionally poor sorting
40 m uncut core, iron staining obscures grain size
distorted bedding
41 m slurry bed? 41 m
basal loading or erosion?1:20 vertical scale
188