SUPPLEMENTARY MATERIALS

Transcription

1 GSA DATA REPOSITORY Rubidge et al. SUPPLEMENTARY MATERIALS Lithostratigraphic and biostratigraphic information on volcanic ash beds Samples were collected from key volcanic ash layers ranging from 4mm to 400mm in thickness, interstratified with vertebrate fossil bearing strata of the lower Beaufort Group (Fig, DR1). The dated samples vary from fine- to coarse-grained, vitric and crystal-vitric tuffs that have undergone post-emplacement alteration and devitrification. In thin section, shard-like quartz fragments are present in various grain sizes and proportion (Fig. DR2); biotite, feldspar and volcanic lithic fragments are observed in the coarser tuffs. A microlitic fabric of sericite- and carbonate- altered feldspar (and possibly glass shards) is preserved in the matrix of some samples. Bruce-1 This is stratigraphically the lowermost ash analyzed from the Beaufort Group. Bruce-1 is a green-grey siliceous chert found on the farm Uintjies Vlakte 118, Jansenville, Eastern Cape Province (S /E ). This layer occurs 2010m stratigraphically above the contact between the Ecca and Beaufort Groups. It is mapped as upper Koonap Formation and is 340m below the base of the overlying Middleton Formation. While the area is heavily vegetated, the relatively high topographic relief has revealed extensive outcrops of sandstones and mudrocks of the lower Beaufort, which facilitated the collection of fossils. In the immediate vicinity several dinocephalian fossils were recovered up to 1230m above the Ecca-Beaufort contact. Stratigraphically above the dinocephalians, but still below the ash, a number of small dicynodont skulls were recovered, as well as two scylacosaurid therocephalians. Only fragmentary dicynodont remains were found in the immediate vicinity of Bruce-1, but extensive fossil collecting at the same stratigraphic level on the neighbouring Farm Hilary resulted in the recovery of the dicynodont Diictodon and several scylacosaurid therocephalians. The absence of dinocephalians despite extensive searching at this stratigraphic level, and the relative abundance of the dicynodont Diictodon and scylacosaurid therocephalians, has allowed us to positively correlate this ash with the Pristerognathus Assemblage Zone (Keyser and Smith, 1979; Smith and Keyser, 1995b). K This is a green-grey siliceous chert on an isoalted conical hill on the Farm Hilary 53, Jansenville, Eastern Cape Province (S /E ). This chert occurs 30m stratigraphically below a prominent sandstone which caps the hill. Although the entire hill is mapped as Koonap Formation, our estimation is that the prominent sandstone could be the lowermost sandstone of the overlying Middleton Formation as this is mapped from

2 outcrops 1 km farther on the other side of the valley. Fossil skulls of Diictodon are abundant immediately below the chert. The absence of dinocephalians, as well as the relative abundance of scylacosaurid therocephalians and the dicynodont Diictodon permit us to positively correlate this ash with the Pristerognathus Assemblage Zone (Keyser and Smith, 1979; Smith and Keyser, 1995b). Tortoise ash This ash was recovered from the lower Middleton Formation (375m from the base of the Formation) at the northernmost side of the Farm Hilary (S /E ). The ash is mm thick and is capped by a prominent mm thick siliceous siltstone that may be a reworked volcaniclastic rock. Three separate samples were collected on two visits (Tortoise, K B and K T). Diictodon was found above and below this ash horizon, and a skull of the dicynodont Endothiodon from 10m below the ash establishes this to be from either the Pristerognathus, Tropidostoma or the Cistecephalus Assemblage Zone. Scylacosaurid therocephalians are present in relative abundance up to 375m below this horizon. Apart from Endothiodon and Diictodon, no additional diagnostic fossils of either the Cistecephalus or the Tropidostoma Assemblage Zones have been found. The relatively short stratigraphic distance of the ash above the last occurrence of scylacosaurid therocephalians (375m) indicates that this ash is in the Tropidostoma Assemblage Zone rather than the Cistecephalus Assemblage Zone (Smith and Keyser, 1995c). Bruintjieshoogte An ash bed was recovered from a roadside cutting on the Bruintjieshoogte Pass on the farm Rietfontein 102 (S /E ) in the district of Somerset East. The locality is mapped as Middleton Formation and is 220m below the Oudeberg Member of the Balfour Formation. A snout of Aulacephalodan was recovered 90 m below the ash, a single specimen of Oudendon came from the same level as the ash, and a skull of Pristerodon was collected 80m above the ash. The co-occurrence of Aulacephalodon and Oudenodon below the Oudeberg Member places the locality of this ash in the lower Cistecephalus Assemblage Zone (Smith and Keyser, 1995a). At this stage, because the Bruintjieshoogte locality is situated more than 200km to the east of the Hilary locality, it is not possible to accurately determine the statigraphic distance between the localities of the Tortoise and Bruintjieshoogte ashes. Doornplaats In 1996 Ken MacLeod, now of the University of Missouri, recovered a 40mm thick, very friable ash at Farm Doornplaats, Graaff-Reinet (S /E ), labeled as Da-1. We recollected the same ash, labeled as Doornplaats. The ash is laterally restricted in outcrop and is found above the Oudeberg Sandstone Member of the Balfour Formation

3 in the Cistecephalus Assemblage Zone (Smith and Keyser, 1995a). The abundance of Cistecephalus fossils from this locality suggests that this corresponds to Kitching s Cistecephalus Acme Zone at the top of the Cistecephalus Assemblage Zone (Kitching, 1977). U-Pb analytical procedures, data reduction and age calculation methods Zircon was separated from the volcanic ash samples by standard methods of crushing, pulverizing and heavy mineral separation, and hand-selected for analysis using a binocular microscope. The U-Pb analytical techniques are similar to those described in detail in Ramezani et al. (2011). Accordingly, preference was given in grain selection to distinct populations of prismatic zircon containing elongate glass (melt) inclusions parallel to their crystallographic c axis. This has proven effective in minimizing selection of older xenocrystic and/or detrital zircons present in tuffs with mixed grain populations. All analyzed single zircon grains were pre-treated by the thermal annealing and acid-leaching (CA-TIMS) technique of Mattinson (2005) in order to overcome the effects of radiation-induced Pb loss which typically results in anomalously young measured dates. We report 206 Pb/ 238 U, 207 Pb/ 235 U and 207 Pb/ 206 Pb dates for each zircon analysis (Table DR1) and calculate weighted mean 206 Pb/ 238 U dates (Table 1) based on coherent clusters of data for which scatter does not exceed what can be explained by analytical uncertainty alone. The U-Pb age calculation methodology has been laid out in detail in Bowring et al. (2006). Uranium and lead isotopic data reduction, error propagation and age calculation follows the algorithm of McLean et al. (2011) and its associated software applications (Bowring et al., 2011). Results are plotted with 2σ uncertainty on standard concordia diagrams (Fig. DR3-A to F). Most high-precision zircon data exhibit a slight but systematic age discordance (with 206 Pb/ 238 U < 207 Pb/ 235 U < 207 Pb/ 206 Pb) likely related to inaccuracy in one or both of the U decay constants (Mattinson, 2010; Schoene et al., 2006). We consider the weighted mean 206 Pb/ 238 U date to be generally the most precise and accurate representation of the zircon crystallization age, which we herein interpret as the eruption age of the ash bed. All calculated weighted mean dates and associated uncertainties are reported as X/Y/Z in Table 1, where X is the internal (analytical) uncertainty in the absence of all external errors, Y incorporates the U-Pb tracer calibration error and Z includes the latter as well as the decay constant errors of Jaffey et al. (1971). The external uncertainties should be taken into account if the results are to be compared with dates obtained in other laboratories (Y) or dates derived from other isotopic systems such as 40 Ar/ 39 Ar (Z). For the purpose of calculating the duration of fossil zones in the Beaufort based on U-Pb results of this study alone, all external sources of error can be ignored (Fig. 2). However, for comparing them to dates from global marine sections (data from multiple U-Pb labs) and to SHRIMP ion-probe U-Pb dates

4 from the Emeishan (e.g., He et al., 2007), only the decay constant uncertainties can be ignored. The latter must be incorporated if one compares U-Pb and 40 Ar/ 39 Ar dates. Our reported ages are based on the weighted mean 206 Pb/ 238 U date of the youngest coherent cluster of data consisting of three or more zircon analyses from each sample, which we interpret as the age of eruption and hence a maximum limit for the time deposition. Demonstrably older analyses representing pre-existing grains incorporated during eruption and/or deposition were excluded from weighted mean calculations. Although there is strong petrologic evidence for the pyroclastic origin of the dated tuff samples (see above), the presence of reworked volcaniclastic material including zircon in the sampled beds cannot be unequivocally ruled out. Therefore, it is conceivable that a bias exist between the maximum depositional age, as determined by U-Pb geochronology, and the true depositional age of the bed. Our strategy for assessing any age bias consists of a) dating a suite of closely spaced tuff samples with well-constrained stratigraphic positions and, b) testing the consistency of the measured ages relative to their stratigraphy (Ramezani et al., 2011). When all samples produce mutually resolvable U-Pb dates (within 2σ analytical uncertainties) that are consistent with stratigraphic superposition, it can be concluded that any possible bias between the maximum and true depositional ages would have to be smaller than the age difference between the adjacent dated beds. In this case, any bias would be less than 700 k.y. based on the average age difference between our adjacent dated samples. References Cited Bowring, J.F., McLean, N.M., and Bowring, S.A., 2011, Engineering cyber infrastructure for U-Pb geochronology: Tripoli and U-Pb_Redux: Geochem. Geophys. Geosyst., v. 12, p. Q0AA19. Bowring, S.A., Schoene, B., Crowley, J.L., Ramezani, J., and Condon, D.J., 2006, Highprecision U-Pb zircon geochronology and the stratigraphic record: progress and promise, in Olszewski, T.D., ed., Geochronology: Emerging Opportunities, The Paleontological Society Papers Volume 12, p He, B., Xu, Y.-G., Huang, X.-L., Luo, Z.-Y., Shi, Y.-R., Yang, Q.-J., and Yu, S.-Y., 2007, Age and duration of the Emeishan flood volcanism, SW China; geochemistry and SHRIMP zircon U-Pb dating of silicic ignimbrites, postvolcanic Xuanwei Formation and clay tuff at the Chaotian section: Earth and Planetary Science Letters, v. 255, p Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., and Essling, A.M., 1971, Precision Measurement of Half-Lives and Specific Activities of 235 U and 238 U: Physical Review C, v. 4, p

5 Keyser, A.W., and Smith, R.M.H., 1979, Vertebrate biozonation of the Beaufort Group with special reference to the western Karoo Basin: Annals of the Geological Survey, Pretoria, v. 12, , p Kitching, J.W., 1977, The distribution of the Karroo vertebrate fauna; with special reference to certain genera and the bearing of distribution on the zoning of the Beaufort Beds: Memoir Bernard Price Institute Palaeontological Research, University Witswatersrand, v. 1, p Mattinson, J.M., 2005, Zircon U/Pb chemical abrasion (CA-TIMS) method; combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages: Chemical Geology, v. 220, p Mattinson, J.M., 2010, Analysis of the relative decay constants of 235 U and 238 U by multistep CA-TIMS measurements of closed-system natural zircon samples: Chemical Geology, v. 275, p McLean, N.M., Bowring, J.F., and Bowring, S.A., 2011, An algorithm for U-Pb isotope dilution data reduction and uncertainty propagation: Geochem. Geophys. Geosyst., v. 12, p. Q0AA18. Ramezani, J., Hoke, G.D., Fastovsky, D.E., Bowring, S.A., Therrien, F., Dworkin, S.I., Atchley, S.C., and Nordt, L.C., 2011, High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs: Geological Society of America Bulletin, v. 123, p Schoene, B., Crowley, J.L., Condon, D.J., Schmitz, M.D., and Bowring, S.A., 2006, Reassessing the uranium decay constants for geochronology using ID-TIMS U/Pb data: Geochimica et Cosmochimica Acta, v. 70, p Smith, R.M.H., and Keyser, A.W., 1995a, Biostratigraphy of the Cistecephalus assemblage zone, in Rubidge, B.S., ed., Biostratigraphic Series - South African Committee for Stratigraphy, Report: 1: Pretoria, Council for Geoscience, p Smith, R.M.H., and Keyser, A.W., 1995b, Biostratigraphy of the Pristerognathus assemblage zone: Biostratigraphic Series - South African Committee for Stratigraphy, Report: 1, p Smith, R.M.H., and Keyser, A.W., 1995c, Biostratigraphy of the Tropidostoma assemblage zone, in Rubidge, B.S., ed., Biostratigraphic Series - South African Committee for Stratigraphy, Report: 1: Pretoria, Council for Geoscience, p

6 Figure Captions Figure DR1. Outcrop view of the Tortoise ash bed from the lower Middleton Formation at Farm Hilary, Jansenville, South Africa. Figure DR2. Photomicrographs of the Bruintjieshoogte crystal-lithic ash tuff showing pyrogenic quartz fragments (qtz) and biotite (biot) in an altered quartzofeldspathic (and vitric?) matrix. Altered volcanic lithic fragments (lith) of variable sizes and shapes are present in the rock. Figure DR3. U-Pb concordia plots for dated volcanic ash beds from the Karoo Supergroup, South Africa. Error ellipses represent 2σ internal uncertainties; shaded ellipses mark analyses used for age calculation. Dashed lines parallel to solid black concordia represent 95% confidence envelope associated with uncertainties in the uranium decay constants (Jaffey et al., 1971). See Table 1 for details of error reporting. MSWD mean square of weighted deviates.

7 Table DR1. U-Pb data for analyzed zircon from the ash beds of the Karoo Supergroup, South Africa Composition Ratios Age (Ma) Sample Pb c b Fractions a (pg) Pb c U Bruce-1 Pb* b Th 206 Pb c 208 Pb d 206 Pb e err 207 Pb e err 207 Pb e err 206 Pb err 204 Pb 206 Pb 238 U (2 %) 235 U (2 %) 206 Pb (2 %) 238 U (2 ) 207 Pb 235 U 207 Pb corr. 206 Pb coef. z (.06) (.30) (.27) z (.08) (.65) (.60) z (.07) (.47) (.43) z (.09) (.69) (.63) z (.11) (1.23) (1.17) K z (.08) (.41) (.38) z (.08) (.36) (.32) z (.08) (.52) (.48) z (.06) (.29) (.26) z (.12) (1.18) (1.10) z (.07) (.46) (.44) z (.05) (.35) (.33) Tortoise z (.06) (.28) (.26) z (.09) (.70) (.66) z (.06) (.19) (.16) z (.06) (.23) (.20) z (.07) (.41) (.38) z (.13) (1.00) (.95)

8 z (.07) (.45) (.42) z (.07) (.31) (.27) z (.06) (.29) (.26) K B z (.06) (.26) (.22) z (.06) (.25) (.22) z (.06) (.39) (.36) z (.09) (.63) (.58) z (.07) (.35) (.31) z (.07) (.37) (.33) z (.09) (.74) (.69) z (.12) (.36) (.32) K T z (.06) (.42) (.38) z (.19) (1.54) (1.45) z (.14) (1.41) (1.32) z (.15) (1.78) (1.67) z (.11) (1.10) (1.03) z (.19) (2.16) (2.04) z (.14) (1.31) (1.22) Bruintjieshoogte z (.06) (.26) (.24) z (.07) (.55) (.51) z (.09) (.72) (.67)

9 z (.07) (.46) (.44) z (.14) (1.65) (1.56) z (.10) (1.09) (1.03) z (.09) (.93) (.87) z (.08) (.50) (.46) z (.11) (.92) (.88) Doornplaats z (.09) (.73) (.69) z (.14) (1.44) (1.34) z (.06) (.47) (.44) z (.13) (.97) (.91) z (.06) (.31) (.29) z (.06) (.11) (.08) Da-1 z (.17) (1.96) (1.84) z (.07) (.54) (.51) Notes: Corr. coef. = correlation coefficient. Age calculations are based on the decay constants of Jaffey et al. (1971). a All analyses are single zircon grains and pre-treated by the thermal annealing and acid leaching (CA-TIMS) technique. Data used in age calculations are in bold. b Pb c is total common Pb in analysis. Pb* is radiogenic Pb concentration. c Measured ratio corrected for spike and fractionation only. d Radiogenic Pb ratio. e Corrected for fractionation, spike, blank, and initial Th/U disequilibrium in magma. Mass fractionation correction of 0.25%/amu ± 0.04%/amu (atomic mass unit) was applied to single-collector Daly analyses. All common Pb is assumed to be laboratory blank. Total procedural blank less than 0.1 pg for U. Blank isotopic composition: 206 Pb/ 204 Pb = ± 0.21, 207 Pb/ 204 Pb =15.34 ± 0.15, 208 Pb/ 204 Pb = ± 0.50.

10 Rubidge et al. Karoo Geochronology Figure DR1 05/14/2012

11 qtz biot lith biot 200μ Rubidge et al. Karoo Geochronology Figure DR2 05/14/2012

12 Pb/ 238 U Bruce-1 A Pb/ 238 U K B Weighted Mean 206Pb/ 238U age: ± Ma MSWD = 0.56, n = 5 207Pb/ 235U Weighted Mean 206Pb/ 238U age: ± Ma MSWD = 1.3, n = 6 207Pb/ 235U Pb/ 238 U Tortoise C Pb/ 238 U K B D Weighted Mean 206Pb/ 238U age: ± Ma MSWD = 1.5, n = Weighted Mean 206Pb/ 238U age: ± Ma MSWD = 1.5, n = Pb/ 235U Pb/ 235U Rubidge et al. Karoo Geochronology Figure DR3 1 of 2 05/14/2012

13 Pb/ 238 U K T E Pb/ 238 U Bruintjiershoogte F Weighted Mean 206Pb/ 238U age: ± 0.23 Ma MSWD = 1.5, n = Weighted Mean 206Pb/ 238U age: ± Ma MSWD = 2.1, n = Pb/ 235U Pb/ 235U Rubidge et al. Karoo Geochronology Figure DR3 2 of 2 05/14/2012