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Microstructural Geochronology: Planetary Records Down to Atom Scale, Geophysical Monograph 232, First Edition. Edited by Desmond E. Moser, Fernando Corfu, James R. Darling, Steven M. Reddy, and Kimberly Tait. © 2018 American Geophysical Union. Published 2018 by John Wiley & Sons, Inc.

9.1. INTRODUCTION TO DETRITAL SHOCKED MINERALS

Meteorite impacts produce irreversible microstructural deformation in various minerals at high pressure. Such minerals are subsequently referred to as shocked minerals,

and they have provided key diagnostic evidence to confirm hypervelocity processes at many terrestrial impact structures [French, 1998; French and Koeberl, 2010; Ferrière and Osinski, 2013]. The recognition that shock microstructures in certain minerals can survive post‐impact thermal conditions, uplift, erosion, fluvial transport, and can be preserved in sediments and sedimentary rocks has provided new avenues for documenting evidence of both recent and ancient impact processes in the sedimentary record.

Quartz is likely the most widely studied shocked mineral, and has been extensively reviewed. Shock deformation microstructures in quartz include planar microstructures (PMs), twinning, high‐pressure phase transformations, and melting [e.g., Stöffler and Langenhorst, 1994; Grieve et al., 1996; French, 1998]. The most commonly cited evidence of shock deformation in quartz are planar deformation

The Rietputs Formation in South Africa: A Pleistocene Fluvial Archive of Meteorite Impact Unique to the Kaapvaal Craton

Aaron J. Cavosie1,2,3, Timmons M. Erickson1, Pedro E. Montalvo3, Diana C. Prado4, Nadja O. Cintron3, and Ryan J. Gibbon5

9

1 TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Western Australia, Australia

2 NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin‐Madison, Madison, Wisconsin, USA

3 Department of Geology, University of Puerto Rico, Mayagüez, Puerto Rico, USA

4 Department of Civil & Environmental Engineering, Polytechnic University of Puerto Rico, San Juan, Puerto Rico, USA

5 Department of Geological Sciences, University of Cape Town, Cape Town, South Africa

ABSTRACT

Reconstructing the terrestrial impact cratering record is a fundamental goal of planetary science. However, erosion, burial, and deformation can obscure or destroy impact records. A sedimentary record of impact is provided by detrital shocked minerals, which have been shown to survive erosion and transport in modern alluvium and Holocene glacial deposits. Here we describe detrital shocked minerals from a known impact structure in sediments that were transported to distal locations and buried. The Rietputs Formation is a Pleistocene fluvial terrace of the Vaal River in South Africa, and shocked minerals were found in the terrace at locations up to 750 km downstream of the 2020 Ma Vredefort impact structure. Optical and electron microscopy, and U‐Pb geochronology, were used to establish microstructural and isotopic provenance indicators that demonstrate the detrital shocked minerals originated from the Vredefort impact structure. The Rietputs Formation contains fluvial diamonds and Acheulean (ca. 1.7–1.3 Ma) artifacts at sites such as Canteen Kopje, a South African National Monument. The assemblage of detritus in Rietputs Formation gravels, including shocked minerals from Earth’s largest impact structure, diamonds from Cretaceous kimberlites, and Stone Age artifacts, comprises a unique sedimentary archive of the Kaapvaal craton, and may have geoheritage significance.

204 MICROSTRUCTURAL GEOCHRONOLOGY

features (PDFs) and planar fractures (PFs); they differ principally in that PDFs originally form as thin, <1 µm, amorphous lamellae, whereas PFs are open fractures. The amorphous component of PDFs can anneal over time, leaving tell‐tale planar trails of fluid inclusions that “decorate” the PDFs and provide optically visible evidence for the former presence of PDFs in shocked quartz from ancient impact sites [Leroux et al., 1994; Hamers and Drury, 2011]. Formation conditions of PMs in quartz have been experimentally calibrated; PDFs record impact pressures ranging from 5 to 35 GPa, while PFs form in the range from 5 to 10 GPa [Stöffler and Langenhorst, 1994].

Detrital shocked quartz grains with PDFs have been reported in Miocene fluvial sediments near the Ries crater (Germany) [Buchner and Schmieder, 2009], in modern alluvium eroded from the Vredefort Dome in South Africa [Cavosie et al., 2010; Erickson et al., 2013a], and in modern alluvium and Holocene glacial deposits at the Sudbury impact basin in Canada [Thomson et al., 2014]. Detrital shocked quartz grains with PFs have thus far only been reported from the Rock Elm crater (USA), where they occur in modern alluvium downstream from the impact structure [Roig et al., 2013].

Accessory minerals used as U‐Pb geochronometers have also been reported as detrital shocked grains. Zircon is the most widely studied accessory mineral that records shock deformation. Shock features in zircon include crystal‐plastic deformation, planar deformation bands (PDBs), PMs, twinning, granular texture, and high‐pressure phase transformations, and it ultimately dissociates into constituent oxides [Wittmann et al., 2006; Timms et al., 2012, 2017]. Of all the above features, only the presence of {112} twinning and the high‐pressure polymorph reidite provide diagnostic evidence of shock deformation. Shock twins and PMs have been experimentally shown to form in zircon at 20 GPa [Leroux et al., 1999; Morozova, 2015]. Shock‐twinned zircon grains with {112} lamellae have been reported in bedrock from the central uplift of the Vredefort Dome [Moser et al., 2011; Erickson et al., 2017b], and other localities, including the Rock Elm impact structure [USA, Cavosie et al., 2015a], the Ries impact structure [Germany, Erickson et al., 2017a], and in lunar impact breccia [Timms et al., 2012].

Detrital shock‐twinned zircon has been found in modern alluvium of the Vaal River within the Vredefort Dome [Cavosie et al., 2010, 2015a], up to 750 km further downstream in the Vaal River [Erickson et al., 2013a, 2013b], as inclusions in detrital shocked monazite from the Vaal River [Erickson et al., 2016], and in beach sand on the Atlantic coast of South Africa, ~1940 km downriver from the Vredefort Dome [Montalvo et al., 2017]. Detrital shock‐twinned zircon was also documented in modern alluvium and Holocene glacial deposits within the Sudbury impact basin [Thomson et al., 2014].

The high‐pressure ZrSiO4 polymorph reidite has been experimentally shown to form at ~30 GPa [Leroux et al., 1999; Morozova, 2015], and has been reported in impact ejecta [Glass and Liu, 2001] and also bedrock from several impact structures, including Rock Elm [Cavosie et al., 2015a], Xiyuan [China, Chen et al., 2013], and Ries [Wittmann et al., 2006; Erickson et al., 2017a]. The only report of reidite‐bearing detrital shocked zircon grains is from the ca. 1200 Ma Stac Fada impactite [Scotland, Reddy et al., 2015]. The extent of sedimentary transport experienced by the Stac Fada grains is not known given that a source crater has not been identified.

Monazite is a rare earth phosphate accessory mineral that is widely used for U‐Pb geochronology, and, like zircon, is ubiquitous as detrital grains in siliciclastic sediments. Shock deformation of monazite has long been recognized [Schärer and Deutsch, 1990], but has only recently been systematically characterized [Erickson et al., 2016, 2017b]. Shock features in monazite include crystal‐plastic deformation, PMs, twinning, and formation of granular texture. Shocked monazite grains with PMs and granular texture have been reported in bedrock at Vredefort [Moser, 1997; Flowers et al., 2003; Erickson et al., 2017b], and also at the Araguainha impact structure in Brazil [Tohver et al., 2012; Silva et al., 2016; Erickson et al., 2017b]. Detrital shocked monazite has been reported in modern alluvium in the Vaal River [Cavosie et al., 2010; Erickson et al., 2013a], including an occurrence of shock‐twinned monazite grains that contain shock‐twinned zircon inclusions [Erickson et al., 2016].

With the exception of the examples at Ries and Stac Fada described above, most detrital shocked mineral studies have focused on modern alluvium. Here we describe the microstructure and U‐Pb ages for a suite of distally transported detrital shocked minerals in Pleistocene fluvial deposits of the Vaal River in South Africa, and discuss their significance as a fluvial archive of the unique geologic history of the Kaapvaal craton.

9.2. GEOLOGICAL BACKGROUND

9.2.1. Kaapvaal Craton and the Vredefort Dome Impact Structure

The Kaapvaal craton is a large Archean block located predominantly in South Africa, and contains the Vredefort Dome World Heritage Site (WHS) (Fig. 9.1a). The Vredefort Dome WHS, located south of Johannesburg, encompasses part of the central uplift of a giant 2020 Ma meteorite impact crater that is currently the oldest and largest (250–300 km) confirmed terrestrial impact structure [Gibson and Reimold, 2008]. The impact structure is deeply eroded; thermobarometry studies on exposed

THE RIETPUTS FORMATION IN SOUTH AFRICA 205

rocks estimate that the upper 8–11 km of the structure have been removed by erosion [Gibson et al., 1998]. Shocked minerals are ubiquitous in Vredefort bedrock, and include quartz [e.g., Grieve et al., 1990; Leroux et al., 1994], zircon [e.g., Kamo et al., 1996; Moser et al., 2011], monazite [Moser, 1997; Flowers et al., 2003], and others [Gibson and Reimold, 2005].

Further west on the Kaapvaal craton along the Vaal River near Barkly West is Canteen Kopje, a South African National Monument and Provincial Heritage site [Oberholster, 1972] (Fig. 9.1b). Canteen Kopje exposes a Pleistocene fluvial terrace of the Vaal River, the Rietputs Formation, and is the site of the first diamond mine in South Africa [de Wit, 2008]. In addition, Canteen Kopje hosts one of the richest deposits of Earlier Stone Age tools (Acheulean) in southern Africa [Helgren, 1979]. In this study, we use microstructural analysis and U‐Pb geochronology to document the preservation of far‐traveled

detrital shocked minerals from the Vredefort Dome WHS in the same Pleistocene terrace throughout the lower Vaal River basin to highlight the terrace as a fluvial archive that contains a sedimentary assemblage not likely to occur elsewhere in the geological record.

9.2.2. Vaal River and the Rietputs Formation

The Vaal River is a right‐bank tributary to the Orange River, the largest fluvial system in southern Africa [Bremner et al., 1990]. The Vaal River originates east of Johannesburg on the South African Highveld, flows west across the Kaapvaal craton (Fig. 9.1a and b), and is incised in Archean bedrock at the Vredefort Dome WHS [Gibson and Reimold, 2008; Moser et al., 2011]. As described above, the Vaal River is the first modern fluvial system where detrital shocked minerals were reported. Detrital shocked grains of quartz, monazite, and zircon are present in Vaal River alluvium within the Vredefort Dome [Cavosie et al., 2010], further downriver to the confluence with the Orange River [Erickson et al., 2013a], and also within the lower Orange River and on the Atlantic coast near the river mouth, ~1940 km from the Vredefort Dome [Montalvo et al., 2017] (Fig. 9.1). The Vredefort Dome is thus a “point source” for detrital shocked minerals in the greater Orange River basin.

The Rietputs Formation is a ~7‐ to 16 m‐thick fluvial terrace of the Vaal River comprising principally coarse gravel and sand [Helgren, 1979]. The dominant sedimentary components in the Rietputs Formation include cobble‐ to pebble‐sized fragments of various locally derived Archean bedrock and sand‐sized grains of quartz. The coarse gravels of the Rietputs Formation have been interpreted to record a time of higher energy flow in the Pleistocene as compared to the modern river [Butzer et al., 1973; Helgren, 1979]. Rietputs Formation gravels have been extensively exposed by mining along the Vaal River below Bloemhof (Fig. 9.1b) [Helgren, 1979]. Deposition ages of the Rietputs Formation near Wind sorton ranging from ca 1.8 to 1.0 Ma were reported based on cosmogenic nuclide burial dating using 26Al and 10Be in detrital quartz [Gibbon et al., 2009]. The cosmogenic nuclide data from Gibbon et al. [2009] were subsequently reprocessed to account for advances in data reduction methodology; the revised deposition age for Rietputs Formation coarse alluvium (gravels) at Windsorton now ranges from 1.73 ± 0.16 to 1.26 ± 0.21 Ma [Leader et al., in press]. The Rietputs Formation contains important evolutionary artifacts, including hand axes, cleavers, picks, and other Earlier Stone Age (Acheulean) tools [Gibbon et al., 2009; Leader et al., in press] (Fig. 9.2a). In addition, Rietputs Formation gravels have long been recognized as containing economic concentrations of fluvial diamonds sourced from Cretaceous kimberlites within the Kaapvaal craton

Vaal River

100 km

Johannesburg

N

Shocked mineral sites:

Vaal River

Orange River

25°E 30°E25°S

30°S

Canteen Kopje

Vredefort Dome

Rietputs terrace

modern alluviumRietputs Formation

Bloemhof

N

Douglas

Windsorton

500 km

Botswana

NamibiaMozam-

bique

Vaal River

Orange

South Africa

S

L

Kaapvaal craton

20°E 30°E

25°S

30°S

Vredefort Dome

River basins

Orange

Vaal

CanteenKopje

(a)

(b)

b

Z

River

Figure 9.1 Regional geological setting. (a) Map showing features of the Kaapvaal craton in southern Africa. (b) Vaal River basin showing location of Rietputs Formation and samples analyzed in this study. Alluvium sites from Cavosie et al. [2010] and Erickson et al. [2013a]. L, Lesotho; S, Swaziland; and Z, Zimbabwe. (See electronic version for color representation.)


(Fig. 9.2b). The Rietputs Formation at Canteen Kopje is estimated to have yielded 10,000–15,000 carats of fluvial diamonds [de Wit, 2008].

9.3. SAMPLES AND METHODS

9.3.1. Sample Collection

Eight 1–2 kg sand samples were collected in situ from Rietputs Formation gravels exposed on excavated walls in fluvial diamond mines along the Vaal River to survey for the presence of detrital shocked minerals (Fig. 9.3). The sites range from 506 to 759 km downriver of the Vredefort Dome WHS, and are located near the towns of Windsorton (Fig. 9.3b and c), Barkly West (Canteen Kopje) (Fig. 9.3a), and Douglas (Fig. 9.3d). Four samples were collected during field work in 2009, and the other four were unprocessed material remaining from

the study of Gibbon et al. [2009] (Table 9.1). In general, the sampling strategy focused on collecting sand‐size matrix material located between large cobbles (Fig. 9.3). Detrital grains of quartz were prepared as thin sections of loose grains mounted in epoxy. Zircon and monazite were separated using conventional techniques, including heavy liquids and a Frantz magnetic separator. Shocked minerals were searched for using optical microscopy for quartz, and scanning electron microscopy (SEM) for monazite and zircon. Selected shocked monazite and zircon grains were further analyzed by electron backscatter diffraction (EBSD) and sensitive high‐resolution ion microprobe (SHRIMP) for U‐Pb age, as described below.

9.3.2. Scanning Electron Microscopy

Backscattered electron (BSE) imaging of the exterior surfaces of grains was done using SEM with a Hitachi

(a)

(b)

3 cm

2 mm 2 mm

Figure 9.2 Objects of archeological and economic interest from the Rietputs Formation. (a) A Stone Age Acheulean cleaver, shown from three perspectives (photos courtesy of Kathleen Kuman). (b) Rounded (left) and euhedral (right) fluvial diamonds from the Rietputs Formation (photos courtesy of Tania Marshall). (See electronic version for color representation.)


S3400 W‐filament SEM at the University of Wisconsin‐Madison. Further SEM analysis was conducted using a Tescan MIRA3 field emission gun (FEG) SEM at the Microscopy and Microanalysis Facility at Curtin University. The FEG‐SEM was used for BSE and panchromatic cathodoluminescence (CL) imaging, and EBSD. Automated EBSD maps of regions of interest for three zircon grains and one monazite grain were generated by indexing EBSD patterns on user‐defined

grids. Whole‐grain maps and smaller regions of interest were collected using step sizes ranging from 350 to 750 nm. EBSD analyses were collected with a 20 kV accelerating voltage, 70° sample tilt, 20.5 mm working distance, and 18 nA beam intensity. Electron backscatter patterns were collected with a Nordlys Nano high‐resolution detector and Oxford Instruments Aztec system using routine data acquisition and noise reduction settings [Reddy et al., 2007]. EBSD maps and pole

(a) (b)

(d)(c)

Figure 9.3 Examples of fluvial diamond mines where the suite of Rietputs Formation sediment samples were col-lected. (a) Canteen Kopje, near Barkly West (site of sample 09VD25). (b) Private mine on west side of Vaal River near Windsorton (site of sample 09VD29). (c) Private mine on east side of Vaal River, near Windsorton (site of samples Sec‐02F and Sec‐02H). (d) Private mine on south side of Vaal River near Douglas (site of sample 09VD40). Increments on left side of scale in (a) and (b) are 1 cm. Stars in (c) and (d) indicate sample location. (See electronic version for color representation.)


figures were processed using the Tango and Mambo modules in the Oxford Instruments/HKL Channel 5 software package. Full EBSD analysis conditions are shown in Table 9.2.

9.3.3. SHRIMP‐RG Ion Microprobe

Zircon and monazite grains were analyzed in Au‐coated epoxy mounts in October 2010 using the SHRIMP‐RG (reverse geometry) at the Stanford‐USGS Micro Analysis

Table 9.1 Locations of Rietputs Formation Samples Analyzed in this Study

Sample Nearest TownDistance from Parys Bridge (km)a Sample Locationb Commentc

09VD31 Windsorton 506 S28°19.825′ Mine wallE24°42.930′

Sec 01C Windsorton 506 Gibbon et al. [2009] Mine wall, pit 1, 7 mSec 02F Windsorton 506 Gibbon et al. [2009] Mine wall, pit 2, 13.5 m (Fig. 9.3c)Sec 02H Windsorton 506 Gibbon et al. [2009] Mine wall, pit 2, 16 m (Fig. 9.3c)Sec 03D Windsorton 506 Gibbon et al. [2009] Mine wall, pit 3, 14 m09VD29 Windsorton 509 S28°21.114′ Mine wall (Fig. 9.3b)

E24°43.714′09VD25 Barkly West 579 S28°32.508′ Canteen Kopje, pit 6, 5.3 m (Fig. 9.3a)

E24°31.846′09VD40 Douglas 759 S29°03.503′ Mine wall (Fig. 9.3d)

E23°42.242′

a The bridge over the Vaal River at Parys approximates the closest point of the Vaal River to the center of the Vredefort Dome impact structure.b Map datum: WGS84.c Meter values in comment column indicate depth below surface, where recorded.

Table 9.2 EBSD Analysis Conditions

Grain Mnz9 Zrn 44 Zrn 130 Zrn 182Shown in figure(s) 9.10d and e 9.6d and e 9.7b 9.7a

Epoxy mount SA5 SA5 SU2 SU2

Acquisition speed (Hz) 40 40 40 40Background (frames) 64 64 64 64Binning 4 × 4 4 × 4 4 × 4 4 × 4Gain High High High HighHough resolution 60 60 60 60Band detection min/max 6/8 6/8 6/8 6/8Mean angular deviation (phase) 0.39 0.24 0.35 0.40X steps 972 405 332 331Y steps 570 372 420 423Step distance (nm) 350 750 700 700

Noise reduction methodsWildspike Yes Yes Yes Yesn neighbor zero solution extrapolation 0 0 0 0Kuwahara filter No No No No

SEM model: Tescan Mira3 FEG‐SEMEBSD system: Nordlys Detector, Aztec

Match units:Zircon: Zircon 5260 [Reddy et al., 2007]Monazite: Monazite [Erickson et al., 2015]Analyzed grains were in epoxy, mounted flat, and grounded with Cu tapeSamples were rotated to a 70° tilt by moving the stageThe samples were coated with a thin (<5 nm) carbon coatThe accelerating voltage was 20 kV for all samplesThe working distance was 20.5 mm for all samples

Note: Mnz, monazite and Zrn, zircon.


Center. Grains were cast in epoxy, ground, and polished to a submicron finish. Secondary ions were generated from the target spot with an O2

– primary ion beam varying from 4 to 6 nA. The primary ion beam produces a spot with diameter of 20 µm and a depth of 1–2 µm for an analysis time of 9–12 minutes. The full set of analyzed peaks begins with a high mass normalizing species (90Zr2

16O+), followed by 204Pb+, a background measured at 0.050 mass units above 204Pb+, 206Pb+, 207Pb+, 208Pb+, 238U+, 232Th16O+ and 238U16O+, and 232Th+, and 270UO2. A complete analysis consisted of five cycles of measurements at each mass. Measurements are made at mass resolutions of M/ΔM = 7500–8500 (10% peak height), which eliminates all interfering molecular species. Concentration data for U and Th were standardized against zircon standard MAD‐green (4196 ppm U) [Mazdab and Wooden, 2006]. Zircon age data were standardized against zircon R33, whose U‐Pb age is 419 Ma [Black et al., 2004], which was analyzed repeatedly throughout the analytical session. Monazite age data were standardized against monazite 44069 [2500 ppm U and 425 Ma, Aleinikoff et al., 2006]. Zircon grains were analyzed over the course of three sessions on different days, whereas monazite grains were analyzed in two sessions. Standard results (R33) for the three analytical runs were (Pb/U calibration error in 2σ): 0.81% (Oct. 10), 0.82% (Oct. 14), and 0.78% (Oct. 15). Standard results (44069) for the two analytical runs were (Pb/U calibration error in 2σ): 0.89% (Oct. 13) and 0.44% (Oct. 14). The analytical procedures used are similar to those described by Kennedy and De Laeter [1994], Nelson [1997], De Laeter and Kennedy [1998], and Williams [1998]. Data reduction for geochronology followed methods described by Williams [1998] and Ireland and Williams [2003], and used the MS Excel add‐in Squid 1, Squid 2, and Isoplot programs of Ludwig [2001a, 2001b, 2003, 2009].

9.4. RESULTS

Detrital shocked minerals were identified in each of the Rietputs Formation samples investigated, and are summarized in Table 9.3. Shocked quartz and zircon were found in all eight samples, and shocked monazite grains were identified in the four samples that yielded monazite. Geochronology results for detrital shocked zircon and monazite are presented in Tables 9.4 and 9.5.

9.4.1. Detrital Shocked Quartz

Two thin sections of detrital quartz grains were analyzed per sample. Each section was estimated to contain ~2500 grains, which results in ~5000 grains surveyed per sample. The yield of shocked quartz grains ranged from one to nine grains per sample, which equates to abundances of 0.02 to 0.18% (Table 9.3). Similar values (one to three grains per two thin sections) were found in modern Vaal River alluvium at comparable distances (469–759 km) from the Vredefort Dome [Erickson et al., 2013a]. Detrital shocked quartz grains in the Rietputs Formation all contain one set of decorated PDFs (Fig. 9.4). Shocked quartz grains with similar microstructures have been described previously from Vredefort bedrock [Grieve et al., 1990] and in detrital suites from the modern Vaal River [Cavosie et al., 2010; Erickson et al., 2013a].

9.4.2. Detrital Shocked Zircon

A total of 640 zircon grains were surveyed from the eight Rietputs Formation samples (31–160 grains per sample). Based on the presence of PMs on exterior surfaces (Fig. 9.5), 137 grains (21%) were identified as shocked, with abundances ranging from 3 to 46% per

Table 9.3 Summary of Shocked Minerals Identified in Rietputs Formation Samples

SampleFrom Parys (km)

Detrital Quartz

Detrital Zircon Detrital Monazite(Two Thin Sections)

Rietputs Fm.Shocked Grains Inv.

Shocked Grains Inv. %

Shocked Grains Inv. %

09VD31 506 7 5k 10 45 22 3 3 100Sec 01C 506 5 5k 27 111 24 22 27 81Sec 02F 506 8 5k 14 31 45 — —Sec 02H 506 2 5k 28 160 18 — —Sec 03D 506 9 5k 41 89 46 33 33 10009VD29 509 4 5k 4 36 11 9 10 9009VD25 579 1 5k 3 91 3.3 — —09VD40 759 2 5k 10 77 13 — —Total 38 ~40k 137 640 21 67 73 92

Notes: Inv., investigated.Each thin section is estimated to contain ~2500 grains.

Table 9.4 SHRIMP‐RG U‐Th‐Pb Isotopic Data for Rietputs Formation Detrital Shocked Zircon Grains

U Th% comma 204Pb/ 206pb*/238U 207Pb*/206Pb* %

DiscaGrain.spot (ppm) (ppm) Th/U 206Pb 206Pb 208Pb*/232Th 207Pb*/235U 206pb*/238U 207Pb*/206Pb* Age (Ma)b Age (Ma)b

Sec‐01C‐ml‐101 39 27 0.72 0.43 2.4E−4 0.1510 ± 0.0074 18.00 ± 0.56 0.5744 ± 0.0141 0.22730 ± 0.00432 2926 ± 58 3033 ± 30 4Sec‐01C‐m2‐63.1 74 63 0.88 0.10 5.5E−5 0.1577 ± 0.0042 18.32 ± 0.38 0.5795 ± 0.0100 0.22923 ± 0.00273 2947 ± 40 3047 ± 20 3Sec‐02F‐ml‐22.1 38 30 0.82 0.10 5.4E−5 0.1442 ± 0.0052 12.65 ± 0.42 0.4964 ± 0.0127 0.18478 ± 0.00380 2598 ± 54 2696 ± 34 4Sec‐02H‐ml‐10.1 119 119 1.03 0.05 2.9E−5 0.1587 ± 0.0062 19.76 ± 0.36 0.6048 ± 0.0092 0.23689 ± 0.00232 3049 ± 36 3099 ± 16 2Sec‐02F‐ml‐44.1 122 98 0.83 0.01 7.9E−6 0.1567 ± 0.0030 18.50 ± 0.32 0.5873 ± 0.0084 0.22844 ± 0.00215 2979 ± 34 3041 ± 16 2Sec‐02H‐ml‐106.1 68 31 0.47 2.58 1.4E−3 0.1103 ± 0.0090 9.14 ± 0.28 0.3341 ± 0.0061 0.19843 ± 0.00495 1858 ± 30 2813 ± 40 51Sec‐02H‐m2‐148.1 49 23 0.48 0.41 2.3E−4 0.1437 ± 0.0084 16.84 ± 0.48 0.5488 ± 0.0123 0.22252 ± 0.00376 2820 ± 52 2999 ± 28 6Sec‐02F‐ml‐109.1 175 51 0.30 0.25 1.4E−4 0.1527 ± 0.0058 17.85 ± 0.24 0.5679 ± 0.0064 0.22796 ± 0.00181 2899 ± 26 3038 ± 12 5Sec‐03D‐ml‐71.1 367 263 0.74 5.60 3.1E−3 0.0751 ± 0.0048 8.87 ± 0.18 0.2860 ± 0.0030 0.22491 ± 0.00372 1621 ± 16 3016 ± 26 8609VD29‐ml‐l.l 71 81 1.18 0.05 2.6E−5 0.1567 ± 0.0038 18.32 ± 0.50 0.5788 ± 0.0104 0.22953 ± 0.00466 2944 ± 42 3049 ± 32 409VD29‐ml‐31.1 103 33 0.34 0.42 2.3E−4 0.1175 ± 0.0064 12.38 ± 0.26 0.4890 ± 0.0075 0.18360 ± 0.00251 2566 ± 32 2686 ± 22 5Sec‐03D‐ml‐24.1 26 27 1.06 1.14 6.3E−4 0.1408 ± 0.0090 12.11 ± 0.56 0.4828 ± 0.0152 0.18190 ± 0.00612 2540 ± 66 2670 ± 56 5Sec‐03D‐ml‐17.1 137 61 0.46 0.03 1.6E−5 0.1534 ± 0.0032 17.90 ± 0.28 0.5693 ± 0.0073 0.22799 ± 0.00198 2905 ± 30 3038 ± 14 5Sec‐02F‐ml‐120 68 68 1.03 0.08 4.2E−5 0.1598 ± 0.0040 19.50 ± 0.44 0.6038 ± 0.0116 0.23417 ± 0.00294 3045 ± 46 3081 ± 20 109VD31‐ml‐9.1 48 42 0.91 0.80 4.5E−4 0.1414 ± 0.0080 18.28 ± 0.58 0.5837 ± 0.0143 0.22709 ± 0.00443 2964 ± 58 3032 ± 32 209VD31‐ml‐9.2 54 47 0.90 0.90 5.0E−4 0.1501 ± 0.0076 19.39 ± 0.70 0.6068 ± 0.0136 0.23176 ± 0.00665 3057 ± 54 3064 ± 46 009VD31‐ml‐7.1 357 291 0.84 0.10 5.5E−5 0.1627 ± 0.0026 19.84 ± 0.24 0.6141 ± 0.0064 0.23430 ± 0.00144 3087 ± 26 3082 ± 10 009VD31‐ml‐7.2 324 233 0.74 0.14 7.6E−5 0.1616 ± 0.0026 19.55 ± 0.26 0.6099 ± 0.0068 0.23248 ± 0.00151 3069 ± 28 3069 ± 10 009VD31‐ml‐7.3 146 65 0.46 0.44 2.4E−4 0.1406 ± 0.0058 18.29 ± 0.34 0.5738 ± 0.0090 0.23123 ± 0.00238 2923 ± 36 3061 ± 16 5Sec‐03D‐ml‐55.1 277 64 0.24 0.11 5.9E−5 0.1542 ± 0.0044 19.79 ± 0.28 0.6017 ± 0.0067 0.23851 ± 0.00205 3037 ± 26 3110 ± 14 2Sec‐03D‐ml‐61.1 107 60 0.58 0.17 9.4E−5 0.1513 ± 0.0048 18.72 ± 0.42 0.5851 ± 0.0111 0.23206 ± 0.00264 2969 ± 46 3066 ± 18 3Sec‐03D‐ml‐61.2 92 71 0.79 0.12 6.6E−5 0.1615 ± 0.0056 19.24 ± 0.58 0.6021 ± 0.0127 0.23182 ± 0.00507 3038 ± 50 3065 ± 34 1Sec‐03D‐ml‐61.3 90 46 0.53 0.03 1.7E−5 0.1590 ± 0.0048 19.35 ± 0.46 0.6032 ± 0.0123 0.23270 ± 0.00265 3043 ± 50 3071 ± 18 1

a comm = common; disc = discordance.b Age in Ma. Uncertainty in ages and isotope ratios is listed at 2σ.* Indicates a 204Pb corrected Pb value.

Table 9.5 SHRIMP‐RG U‐Th‐Pb Isotopic Data for Rietputs Formation Detrital Shocked Monazite Grains

Grain.spotU (ppm)

Th (ppm) Th/U

% comma

204Pb/206Pb 208Pb*/232Th 207Pb*/235U 206pb*/238U 207Pb*/206Pb*

206pb*/238U 207Pb*/206Pb*

% Disca206Pb Age (Ma)b Age (Ma)b

09VD29‐30.1 117 22509 198.7 0.34 1.9E−4 0.1385 ± 0.0046 11.72 ± 0.42 0.4743 ± 0.0158 0.17929 ± 0.00239 2502 ± 68 2646 ± 22 609VD29‐33.1 48 17198 369.5 3.12 1.7E−3 0.1264 ± 0.0066 9.85 ± 0.61 0.4377 ± 0.0227 0.16326 ± 0.00552 2340 ± 102 2490 ± 56 609VD29‐33.2 45 11950 276.4 1.18 6.6E−4 0.1226 ± 0.0065 8.45 ± 0.52 0.4146 ± 0.0218 0.14779 ± 0.00461 2236 ± 100 2320 ± 54 409VD31‐47.1 44 25022 588.3 5.56 3.1E−3 0.1326 ± 0.0071 9.16 ± 1.95 0.4498 ± 0.0296 0.14772 ± 0.02992 2394 ± 132 2320 ± 347 −309VD31‐47.2 102 33885 341.7 4.72 2.6E−3 0.1451 ± 0.0105 11.09 ± 1.41 0.5153 ± 0.0387 0.15601 ± 0.01607 2679 ± 164 2413 ± 175 −1009VD31‐47.3 78 24031 318.8 12.50 6.9E−3 0.1323 ± 0.0073 4.95 ± 4.10 0.4239 ± 0.0419 0.08462 ± 0.06967 2278 ± 190 1307 ± 1599 −43Sec‐01C‐112.1 84 16828 207.4 0.30 1.7E−4 0.1553 ± 0.0062 15.29 ± 0.67 0.5411 ± 0.0214 0.20498 ± 0.00385 2788 ± 90 2866 ± 31 3Sec‐03D‐07.1 47 8414 184.1 0.71 4.0E−4 0.1698 ± 0.0092 18.63 ± 1.01 0.6132 ± 0.0318 0.22039 ± 0.00330 3083 ± 128 2984 ± 24 −3Sec‐03D‐09.1 36 9898 287.0 74.29 4.1E−2 0.0792 ± 0.0058 4.22 ± 3.23 0.2731 ± 0.0345 0.11200 ± 0.08457 1557 ± 174 1832 ± 1368 18Sec‐03D‐09.2 37 8902 251.8 45.24 2.5E−2 0.0979 ± 0.0062 6.83 ± 1.86 0.3512 ± 0.0269 0.14095 ± 0.03677 1940 ± 128 2239 ± 451 15Sec‐03D‐33.1 57 13356 240.0 0.89 5.0E−4 0.1575 ± 0.0077 16.40 ± 0.83 0.5339 ± 0.0259 0.22283 ± 0.00345 2758 ± 108 3001 ± 25 9Sec‐03D‐33.2 71 14693 214.9 5.11 2.8E−3 0.1398 ± 0.0070 13.41 ± l.ll 0.4947 ± 0.0252 0.19667 ± 0.01274 2591 ± 108 2799 ± 106 8Sec‐03D‐33.3 67 14397 222.1 4.87 2.7E−3 0.1515 ± 0.0080 14.72 ± 0.94 0.5275 ± 0.0278 0.20232 ± 0.00731 2731 ± 118 2845 ± 59 4Sec‐03D‐33.4 79 15520 202.0 0.76 4.2E−4 0.1568 ± 0.0065 16.02 ± 0.67 0.5389 ± 0.0214 0.21564 ± 0.00266 2779 ± 90 2948 ± 20 6Sec‐03D‐35.1 48 49031 1056.0 4.60 2.6E−3 0.1322 ± 0.0086 8.59 ± 0.98 0.4344 ± 0.0290 0.14336 ± 0.01327 2325 ± 130 2268 ± 160 −2Sec‐03D‐35.2 57 48619 886.1 10.89 6.0E−3 0.1276 ± 0.0076 4.74 ± 2.26 0.3997 ± 0.0314 0.08609 ± 0.04040 2167 ± 144 1340 ± 907 −38Sec‐03D‐35.3 63 73252 1196.1 5.71 3.2E−3 0.1327 ± 0.0078 8.41 ± 1.76 0.4321 ± 0.0301 0.14109 ± 0.02779 2315 ± 136 2241 ± 341 −3Sec‐03D‐35.4 101 82657 842.8 9.34 5.2E−3 0.1573 ± 0.0073 4.82 ± 1.41 0.3640 ± 0.0209 0.09601 ± 0.02763 2001 ± 98 1548 ± 541 −23Sec‐03D‐71.1 322 27531 88.2 9.45 5.2E−3 0.1539 ± 0.0035 16.55 ± 0.44 0.5329 ± 0.0117 0.22519 ± 0.00344 2754 ± 50 3018 ± 25 10

a comm = common; disc = discordance.b Age in Ma. Uncertainty in ages and isotope ratios is listed at 2σ.* Indicates a 204Pb corrected Pb value.


100 µm 100 µm

100 µm100 µm

100 µm

(c)

(d)

100 µm

(a) (b)

Figure 9.4 Transmitted light images of detrital shocked quartz grains from the Rietputs Formation. All grains contain one orientation of decorated PDFs, indicated by arrows. Grains in (a) (Sec‐02F), (b) (09VD31), and (c) (Sec‐03D) are from Windsorton; grains in (d) (09VD40) are from Douglas. (See electronic version for color representation.)


sample (Table 9.3). The average shocked zircon abundance of 21% in the Rietputs Formation is significantly higher than the average abundance of ~5% reported in modern alluvium in the Vaal River at comparable

distances (469–759 km) downstream from the Vredefort Dome [Erickson et al., 2013a]. Three zircon grains analyzed by EBSD contain various impact‐related microstructures, including deformation twins and low angle

(a) (b)

(c) (d)

(e) (f)

(b)

100 µm

(d)

(f)

100 µm

100 µm

90 BSECOMP

10 µm

25 µm

10 µm

Figure 9.5 Exterior BSE images of detrital shocked zircon grains from the Rietputs Formation at Windsorton. (a and b) Sample 09VD29 grain 31, showing three orientations of PMs. (c and d) Sample Sec‐01C grain 63, showing one orientation of PM. (e and f) Sample Sec‐02H grain 106, showing three orientations of PM. Arrows indicate orientations of planar features. (See electronic version for color representation.)


(<10°) boundaries accommodated by PDBs. Each of the three analyzed grains contain {112} twins in 65°<110> orientation relative to the host grain. The twins are typically <1 µm in width and tens of microns in length, and occur as sets of lamellae in multiple orientations. A Rietputs Formation zircon from Windsorton with an anomalously high density of PMs on the exterior surface

(Fig. 9.6a and b) and igneous zoning (Fig. 9.6c) contained four orientations of {112} twins (Fig. 9.6d–f), which has only been described in two other shocked zircon grains, both detrital [Cavosie et al., 2015a; Timms et al., chapter 8, this volume]. Two other Rietputs Formation zircon grains from Barkly West and Douglas each contain two orientations of {112} twins (Fig. 9.7). PDBs occur in

10 µm50 µm 50 µm

(a) (c)

(d)

0° 8°

(b)

3041 ±16 Ma

(001) {110} {100} {112}

(e)

t1

t2

t3

t4

65°/<110>(twin <110> labeled)

Four twin orientations

{100}(not aligned)

Twin/host aligned {112} (twin <112> labeled)

t3

t2t4

t1

t2,3

t1,4

t2

t1

t4

t3

n= 85,772

(001)

(110)

(010)

host

(b)

TC IPF

(f)

BSE CL

reference =Twin boundary

PDB

c-ax

is

Figure 9.6 Detrital shock‐twinned zircon from the Rietputs Formation (sample Sec‐02F grain 44). (a and b) Exterior surface BSE images. (c) Interior CL image, showing location of SHRIMP spot and 207Pb/206Pb age. (d) Orientation map showing crystallographic misorientation, relative to a reference (red cross). Red lamellae are twins, indicated by arrows. (e) Orientation map with an inverse pole figure color scheme. (f) Pole figures showing crystallographic relations between the host grain and different twin (t) orientations. (See electronic version for color representation.)

{110} {112}

t1

t2

t2

t1

(001) (010)

(110)

IPF

t1

t2

CL TC IPF

Pole figures(001)

Pole figures(001)

{110}

(a) Sample 09VD25, grain182 (Barkly West, Canteen Kopje) (t = twin).

0° 15°

t1

t2

t1

t2

Aligned {112}

50 µm

(b) Sample 09VD40, grain130 (Douglas)

0° 15°(001) (010)

(110)

IPF

t1

t2

CL TC IPF

50 µm

Misorientation

{112}

Host

65°<110>

Misorientation

65°<110> Aligned {112}

Host

n= 65,861 pts

n= 72,218 pts

Twin

PDB

c-axis

Twin

c-axis

= reference

= reference

Figure 9.7 Shock‐twinned detrital zircon grains from Rietputs Formation samples. (a) CL images, EBSD orientation maps, and pole figures for grain 182 from Canteen Kopje at Barkly West (sample 09VD25). Two orientations of twins (t) are indicated by arrows. (b) CL images, EBSD orientation maps, and pole figures for grain 130 from Douglas (sample 09VD40). Two orientations of twins (t) are indicated by arrows. TC = texture component; IPF = inverse pole figure. Arrows in {110} pole figures show apparent rotation of twin about the host. Arrows in {112} pole figures indicate shared axes between the twin and host. (See insert for color representation of the figure.)


two of the three grains and are parallel to the c axis (Figs. 9.6d and 9.7a), as has been observed previously in zircon [e.g., Erickson et al., 2013b] and other shocked accessory minerals with tetragonal symmetry [Cavosie et al., 2016a]. Single‐spot analyses for U‐Pb age made on 18 detrital shocked zircon grains range from concordant to highly discordant, and reveal age groups at 2755 and ca. 3090 Ma (Fig. 9.8). Three grains with multiple analyses yield ages of 3068 ± 80, 3071 ± 24, and 3076 ± 8 Ma, indicating the ca. 3090 Ma group is sourced from rocks of different but broadly similar age. Lower Concordia intercept ages are variable, but are all significantly younger than 2020 Ma, and thus do not record evidence of impact age resetting (Fig. 9.8).

9.4.3. Detrital Shocked Monazite

Four samples, all from Windsorton, yielded a total of 73 detrital monazite grains (3–33 grains per sample). Of the 73 grains, 67 (92%) preserve PMs on exterior surfaces and were identified as shocked (Fig. 9.9). Shocked monazite abundances are high, ranging from 81 to 100% per sample. The high abundances are similar to values of 89% (n = 89/100) reported from a Vaal River tributary within the Vredefort Dome [Cavosie et al., 2010], and 82% (n = 42/51) in the modern Vaal River near Windsorton, 469 km downstream [Erickson et al., 2013a]. One monazite analyzed by EBSD, grain 9, contained an anomalously dense network of intersecting PMs on both exterior and polished surfaces (Fig. 9.10a–c). Orientation mapping revealed the PMs to be deformation twins in seven unique orientations, including 180°<100>, 180°<001>, 95°<201>, 180°<101>, 147°<101>, 180°<201>, and 107°<411> (Fig. 9.10d and e), corresponding to twin planes in (001), (100), {122}, (101), {212}, and (102) orientations [Erickson et al., 2016]. Twins in (001), (100), and {122} have been reported in experimentally indented [Hay and Marshall, 2003] and tectonically deformed monazite [Erickson et al., 2015]. However, twins in (101), {212}, and (102) have only been reported in shock‐deformed grains [Erickson et al., 2016; Erickson et al., 2017b]. The U‐Pb data for nine shocked monazite grains are complex and variably discordant. Five grains yield Pb‐Pb spot ages from 2650 to 3018 Ma; the other four grains contain spots whose maximum ages range from 2239 to 2490 Ma. The data show that the population mostly comprises Archean grains that show variable Pb‐loss (Fig. 9.11); no indication of impact age resetting was detected.

9.5. DISCUSSION

9.5.1. Provenance of Detrital Shocked Minerals

The identification of shocked quartz, combined with EBSD and in situ SHRIMP U‐Pb analysis of zircon and monazite reported here, allows for the determination of the unique microstructural and isotopic provenance of detrital shocked minerals in Rietputs Formation gravels. The shocked minerals described here were transported from ~500 to ~750 km from their source at the Vredefort impact structure and are the first detrital shocked minerals described in Pleistocene sedimentary deposits in southern Africa. Overall, higher abundances of detrital shocked minerals were found in the Rietputs Formation as compared to modern alluvium of the Vaal River at comparable distances downstream from the Vredefort Dome. While abundances of detrital shocked quartz (1–3 grains per two thin sections) and detrital shocked monazite (92 vs. 65%) were similar between this study and Erickson et al. [2013a], the most notable difference is the higher abundance of detrital shocked zircon in the Rietputs Formation. The study by Erickson et al. [2013a] focused primarily on sediment collected from overbank deposits and sandbars, which are conspicuously finer grained when compared to the coarse cobble‐bearing gravels sampled in this study (Fig. 9.3). However, the modern alluvium samples yielded abundant zircon grains; an average of 372 grains per sample were analyzed by Erickson et al. [2013a] as compared to an average of 80 grains per sample here (Table 9.3). Thus, differences in heavy mineral abundance do not appear to explain observed differences in shocked zircon abundance. One possible explanation for the apparent higher abundance of shocked zircon in Rietputs Formation gravels is if there was increased erosion of Vredefort bedrock and subsequent transport of shocked minerals in the Vaal River during times of higher energy flow in the Pleistocene that were required to transport the coarse gravels sampled for this study [Helgren, 1979]. The anomalously high abundances of shocked monazite in detrital populations in the Vaal River [Cavosie et al., 2010; Erickson et al., 2013a, this study] indicate that exposed bedrock at the Vredefort Dome is likely the main source of detrital monazite in the Vaal River basin.

If the Vredefort Dome is indeed the dominant source of monazite in the Vaal River, the abundance of detrital shocked monazite will remain relatively constant (and high) regardless of flow regime. This is in contrast with detrital shocked zircon abundance, which is likely to be high during periods of enhanced weathering of Vredefort bedrock, and potentially low during quiescent periods of


2880

2920

2960

3000

3040

3080

3120

2550

2570

2590

2610

2630

2650

2670

2690

2100

2300

2500

2700

2900

3100

Rietputs Formation18 zircons, 23 analyses

Intercepts (unanchored):1487 ± 360 & 3091 ± 19 Ma (2SD)

(MSWD = 2.7, n= 18) data for 13 grains

Group 1: ca. 3090 Ma

Group 2: ca. 2755 Ma(single-spot analyses)


(MSWD = 0.02, n= 3 grains)

b

c

3000

3020

3040

3080

3100

2980

3020

3060

3100

2960

3000

3040

3080

3120

Sec-03D, grain 61Windsorton


MSWD = 0.09 (n= 3)

09VD31, grain 7Windsorton


MSWD = 2.1 (n= 3)

09VD31, grain 9Windsorton


(n= 2)

Error ellipses are 2σ

0.63

0.55

0.57

0.59

0.61

17.4 17.8 18.2 18.6 19.0 19.4 19.8 20.2 20.6

0.59

0.61

0.63

0.57

0.51

0.53

0.55

15.5 16.5 17.5 18.5 19.5 20.5

18.0 18.4 18.8 19.2 19.6 20.06 10 14 18 22

0.62

0.57

0.58

0.59

0.60

0.61

0.5

0.6

0.7

0.2

0.3

0.4

11.2 11.6 12.0 12.4 12.8 13.2 17 18 19 20 210.55

0.57

0.59

0.61

0.63

0.46

0.47

0.49

0.50

0.51

0.48

207Pb/235U207Pb/235U

206 P

b/23

8 U(a)

(b)

(c)

(d)

(e)

(f)

3060

Figure 9.8 U‐Pb concordia diagrams for detrital shocked zircon grains from the Rietputs Formation. (a) Complete data set. (b) Group 1, ca. 3090 Ma grains. (c) Group 2, ca. 2755 Ma grains. (d–f) Three detrital shocked zircon grains from Windsorton samples with multiple spot analyses. (See electronic version for color representation.)


weathering, when input of detrital zircon grains from other (non‐Vredefort) source rocks in the Vaal River basin may result in lower abundances of detrital shocked grains in the Vaal River.

Ages of shocked zircon grains from the Archean core of the Vredefort Dome have been described in several bedrock studies, and mostly range from ca. 3150 to 2950 Ma [Kamo et al., 1996; Armstrong et al.,

2006; Moser et al., 2011; Wielicki et al., 2012; Wielicki and Harrison, 2015]. Minor populations of grains outside the above range are also present, including younger grains with ages from ca. 2600 to 2850 Ma [Kamo et al., 1996; Armstrong et al., 2006; Wielicki et al., 2012; Wielicki and Harrison, 2015], and grains older than ca. 3400 Ma [e.g., Armstrong et al., 2006; Moser et al., 2011].

(a) (b)

(b)

(c) (d)

(d)

(e) (f)

(f)

Figure 9.9 Exterior BSE images of detrital shocked monazite grains in the Rietputs Formation at Windsorton. (a and b) Sample Sec‐03D, grain 35, showing two orientations of PMs. (c and d) Sample Sec‐03D, grain 41, show-ing two orientations of PM. (e and f) Sample 09VD29, grain 16, showing four orientations of PM. Arrows indicate orientations of PMs. (See electronic version for color representation.)


The presence of {112} deformation twins and the U‐Pb age range (ca. 2755–3091 Ma) of the Rietputs Formation shocked zircon grains are indistinguishable from ages reported in Vredefort bedrock, and similar to those reported in detrital shocked zircon populations from modern Vaal River alluvium [ca. 2650–3420 Ma, Erickson et al., 2013a], and from Atlantic coast beach sand [ca. 3040–3092 Ma, Montalvo et al., 2017].

Relatively few geochronology studies have been conducted on shocked monazite from Archean bedrock at the Vredefort Dome. Monazite grains yielding Archean ages from 3180 to 2569 Ma were reported by Hart et al. [1999] and Flowers et al. [2003], with conspicuous evidence of discordance. Age data for the detrital shocked monazite grains reported here are also complex, and do not define unique populations (Fig. 9.11). However, five of nine grains analyzed yield Archean ages from 2650 to

3018 Ma; the other four, all discordant, yield ages from 2239 to 2490 Ma, indicating they are likely Archean grains that experienced Pb‐loss. The ages reported here are broadly similar to ages reported for shocked monazite grains from Vaal River alluvium by Erickson et al. [2013a], which ranged from 2454 to 3044 Ma. In summary, the overall similarity in microstructure and Archean U‐Pb ages solidifies the provenance of the Rietputs Formation shocked minerals grains as originating from the Vredefort Dome impact structure.

In addition to preserving lasting evidence of shock deformation in the sedimentary record, the Rietputs Formation detrital shocked minerals provide tantalizing new insights into impact conditions of exposed rocks at the Vredefort Dome. The {112} deformation twins in zircon are recognized as a diagnostic shock microstructure, and have only been reported from the Vredefort Dome

0° 100°Reference =

Misorientation

(a)

(b)

(b) (c)

(d) (e)

(d) (e)

Euler orientations:

Twin boundaries:

95° <201>

180° <100>

<411>107°

180° <001> 180° <101>360°0°

E3E2180°0°180°0°

180° <201>147°

E1

<101>

Figure 9.10 Detrital shock‐twinned monazite from Rietputs Formation (sample Sec‐03D, grain 9). (a and b) Exterior surface BSE images. (c) BSE image of the polished interior. (d) misorientation map (from inset in (c)) rela-tive to a reference point (red cross, upper left). TC, texture component. (e) Euler orientations (from inset in (c)) showing seven orientations of twin boundaries. (See electronic version for color representation.)


[Moser et al., 2011; Erickson et al., 2013a, 2013b, 2016, 2017b; Cavosie et al., 2015b; Montalvo et al., 2017; Timms et al., chapter 8, this volume] and other impact environments [Timms et al., 2012; Thomson et al., 2014; Cavosie et al., 2015a, 2016b; Erickson et al., 2017a]. Erickson et al. [2013a] demonstrated that formation of up to four unique orientations of {112} twins is possible. However, individual grains with four {112} twin orientations have thus far not been reported in Vredefort bedrock [c.f., Moser et al., 2011; Erickson et al., 2017b]. The Rietputs Formation zircon with four twin orientations (Fig. 9.6) represents a new occurrence of a four‐twin grain [Cavosie et al., 2015a; Timms et al., chapter 8, this volume], and it is currently not known what impact environment such grains form in, as all grains described thus far with four twin orientations are detrital. The four‐twin microstructure in zircon may record a higher degree of shock intensity as compared to grains with fewer twin orientations. A higher density of PDFs in shocked quartz is known to scale with increasing shock pressure [Stöffler and Langenhorst, 1994], and this relationship was recently proposed for shock twins in monazite [Erickson et al., 2016]. Thus, the four‐twin detrital shocked zircon grain (Fig. 9.6)

highlights the potential for new shocked bedrock discoveries. Similarly, shock‐twinned monazite grains (Fig. 9.10) are known from modern alluvium in the Vaal River [Erickson et al., 2016], but similar shock‐twinned grains have yet to be described in Vredefort bedrock [Flowers et al., 2003; Erickson et al., 2017b]. The Rietputs Formation shocked minerals thus provide hitherto undescribed records of impact deformation, with information that contributes to ongoing studies of the Vredefort Dome WHS and the overall geologic history of the Kaapvaal craton. More broadly, occurrences of detrital shocked minerals, such as described here, may have the ability to preserve evidence of crater environments that have eroded and are no longer preserved at the source impact structure.

9.5.2. The Rietputs Formation as a Unique Archive of the Kaapvaal Craton

The occurrence of shocked minerals, Stone Age tools, and fluvial diamonds together in Rietputs Formation gravels may be unique in the sedimentary record. Fluvial diamonds are sourced from diamondiferous rocks, and are thus generally restricted in occurrence to Archean cratons and their surroundings. They have been reported from Brazil, Canada, Russia, Australia, and different regions of Africa [e.g., Corbett and Burrell, 2001; Wendland et al., 2012], as well as other sites where their origin is less certain [e.g., Smith et al., 2009]. Unlike diamonds, occurrences of Earlier Stone Age tool deposits are not limited to Archean cratons, but instead are found in areas occupied by humans’ earliest ancestors; these include Pleistocene deposits in Europe, Africa, the Middle East, and southeast Asia [Mishra et al., 2007]. In contrast, detrital shocked minerals are not restricted in time, space, or by age of source rock, and can occur wherever impact structures form and are subject to erosion; in this regard, detrital shocked minerals are likely to occur throughout the sedimentary record [e.g., Cavosie et al., 2010; Thomson et al., 2014]. The sedimentary assemblage of fluvial diamonds, Stone Age tools, and detrital shocked minerals together in Rietputs Formation gravels is thus an unusual convergence of detritus representative of non‐uniformitarian processes, and is unlikely to occur anywhere outside the Kaapvaal craton based on the geological considerations discussed above. The Rietputs Formation gravels further record a temporally unique period of deposition within the Vaal River basin during the Pleistocene [Butzer et al., 1973; Helgren, 1979], as older Cenozoic terraces along the Vaal River do not contain Stone Age tools, and abundances of detrital shocked zircon grains in the modern Vaal River are lower [Erickson et al., 2013a].

1200

1600

2000

2400

2800

3200

Single spots (n= 4)

09VD29-33 (n= 2)

Sec-03D-33 (n= 4)

Sec-03D-9 (n= 2)

09VD31-47 (n= 3)

Sec-03D-35 (n= 4)

Rietputs Formation9 monazite grains; 19 analyses

206 P

b/23

8 U

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0 4 8 12 16 20 24207Pb/235U

Error ellipses are 2σ

Figure 9.11 Composite concordia diagram showing U‐Pb data for nine detrital shocked monazite grains from the Rietputs Formation. The data for all nine grains do not define a coherent age group, and instead spread in a poorly defined array along the concordia curve, yielding 207Pb/206Pb ages from 3018 to 1307 Ma. The population of monazite grains comprises mostly low [U] grains (36–322 ppm, avg. = 79 ppm), which yields large uncertainties in age calculations and complex age systematics, including evidence of reverse discordance. The age distribution is similar to detrital shocked monazite grains from the Vaal River [Erickson et al., 2013a], with the majority of the grains yielding Archean ages, consistent with derivation from the Vredefort Dome. (See electronic version for color representation.)


9.5.3. Geoheritage Significance of Detrital Shocked Minerals

The Rietputs Formation gravels represent a unique fluvial archive in the geological record and thus may have geoheritage significance. Geoheritage refers to geological features at a range of scales that represent “…intrinsically or culturally important sites for understanding the evolution of the Earth” [Brocx and Semeniuk, 2007]. Microscopic geoheritage further focuses on features of interest that are generally only visible through instrumental magnification or analysis [Brocx and Semeniuk, 2010]. Detrital zircon populations have previously been proposed to constitute microscopic geoheritage. The Jack Hills metasedimentary rocks in Western Australia were deposited at ca. ~3000 Ma, but contain Hadean detrital zircon grains as old as ca. ~4400 Ma [Cavosie et al., 2005, 2007; Harrison, 2009; Valley et al., 2014]. Jack Hills zircon grains have thus been proposed as a globally significant geoheritage record, as they are the oldest intact geological objects from Earth and contain unique information on the early operation and evolution of the planet [Brocx and Semeniuk, 2010].

Detrital shocked minerals in the Rietputs Formation are here proposed as a globally significant microscopic geoheritage record, as the microstructural and geochronological information they contain provides a record of, as well as new information about, the Vredefort Dome, a WHS of international geological interest and one crucial for understanding the impact history of Earth. At the Vredefort Dome, shocked minerals within bedrock are an integral part of the heritage story, as they provide the crucial diagnostic evidence for confirmation of hypervelocity impact [Gibson and Reimold, 2008]. After erosion and transport, detrital shocked minerals retain key information about the Vredefort impact, including a record of high‐pressure shock deformation and the age of shocked bedrock, and are here considered to constitute a secondary geoheritage deposit in the Rietputs Formation. Canteen Kopje, a National Monument and Provincial Heritage Site in South Africa, is thus unique as an established heritage site, as its primary heritage status is based on the occurrence of fluvial diamonds and Stone Age tools. Also as shown here, it also contains secondary geoheritage deposits in the form of shocked detritus from a WHS located ~580 km upstream (Fig. 9.1b).

ACKNOWLEDGMENTS

B. Hess cast and polished the epoxy mounts containing zircon and monazite. C. Johnson, J. Valley, and J. Wooden provided assistance and access to facilities. Tania Marshall and Kathleen Kuman are thanked for providing photos featured in Figure 9.2. We thank Associate Editor

James Darling, Matthew Wielicki, and an anonymous reviewer for helpful comments that improved the manuscript. Support was provided by the National Science Foundation (EAR‐0838300, EAR‐1145118), the NASA Astrobiology program, a Curtin Research Fellowship, and the Microscopy and Microanalysis Facility at Curtin University.

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