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Taphonomic Trends of Macrofloral Assemblages Across the Permian-Triassic Boundary, Karoo Basin, South Africa

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Taphonomic Trends of Macrofloral Assemblages Across the Permian-Triassic Boundary, Karoo Basin, South Africa RESEARCH REPORT 479 Taphonomic Trends of Macrofloral Assemblages Across the Permian-Triassic Boundary, Karoo Basin, South Africa ROBERTA. GASTALDO Department of Geology, Colby College, Waterville, ME 04901; E-mail:[email protected] ROSE ADENDORFF, MARION BAMFORD Bernard Price Institute for Palaeontology, Witwatersrand University, Johannesburg, 2050 South Africa CONRAD C. LABANDEIRA Department of Paleobiology, NMNH, Smithsonian Institution, Washington, DC 20013-7012 JOHANN NEVELING Council for Geosciences, Silverton, Pretoria, South Africa HALLIE SIMS Department of Geoscience, The University of Iowa, Iowa City, lA 52242 PALMOS, 2005, V. 20, p. 479^97 DOI 10.2110/palo.2004.P04-62 on the literature, parautochthonous assemblages reappear in the Upper Triassic Molteno Formation. Hence, the The terrestrial crisis that reportedly parallels the P ¡Tr ma- change in taphonomic regime to poorly preserved alloch- rine mass extinction is based mainly on Northern Hemi- thonous assemblages (dispersed, fragmentary adpressions) sphere microfloral assemblages and Southern Hemisphere at the critical interval on either side of the P/Tr extinction Gondwanan macrofloral collections. It is well established event, but not coincident with, requires extreme caution that taphonomic filters control the ultimate collectable fos- when interpreting global patterns from these data. Addi- sil assemblage in any depositional regime. Recognition and tionally, the presence of plant fossils in the Early Triassic comparison of isotaphonomic assemblages are critical be- provides evidence for a vegetated landscape during a time fore conclusions can be drawn about evolutionary trends when sedimentation patterns are interpreted to be the result over time. Such an approach has been taken in the investi- of a land-plant die-off gation of pre-boundary, trans-boundary, and post-bound- ary plant-fossil assemblages in the Karoo Basin, South Af- rica. INTRODUCTION Fourteen stratigraphie sections were evaluated in the Bal- Biodiversity loss as a consequence of the Mother of All four and Normandien formations (Lower Beaufort Group), Mass Extinctions (Erwin, 1993), marking the Permian- Katberg Formation, and overlying Burgersdorp Formation Triassic boundary at the end of the Paleozoic Era (251 ± (Upper Beaufort Group). These include previously pub- 0.4 Ma; Gradstein et al., 2005), is estimated to range be- lished (e.g., Bulwer, Bethulie, Carlton Heights, Wapadsberg, tween 75•90% of known organisms in the marine fossil re- Commando Drift) as well as newly discovered (e.g.. Clous- cord. Similar estimates have been made for both land ton Farm) localities, and span the Late Permian to Middle plants and vertebrates (e.g., Retallack, 1995; Visscher et Triassic. Fossiliferous intervals were characterized with re- al., 1996; Looy et al., 1999; Rubidge and Sidor, 2001; Rees spect to their sedimentology and plant taphonomy and et al., 2002), and recent proposals suggest that the deci- bulk collections were made at several stratigraphie levels for mation in the oceans was accompanied by a synchronous future evaluation offloristic and plant-insect associational collapse in the terrestrial realm (Twitchett et al., 2001). trends. There is some agreement that woody plants experienced a The depositional regimes and plant taphonomic charac- decline regionally (Retallack, 1995; Visscher et al., 1996; ter of assemblages change through time. Much of the Lower Looy et al., 1999; Kerp, 2000; Rees, 2002), which would Beaufort Group is characterized by parautochthonous as- have had direct effects on erosional rates, changes in at- semblages within oxbow-lake channel fills. Below the P/Tr mospheric CO2 levels, carbon cycling, and ecosystem boundary, these are replaced by allochthonous assemblages, structure (MacLeod et al., 2000; Ward et al., 2000). How- poorly preserved in lateral-accretion deposits and barforms ever, these S3mtheses are based on macrofloral data origi- of relatively shallow fluvial nature. Allochthonous assem- nating from localities that are isolated spatially (mainly blages within the same fluvial context continue across the South Africa and Australia), and not well constrained in boundary into the earliest Triassic (Palingkloof Member time, and that may or may not actually contain the terres- and Katberg Formation, and typify the Middle Triassic trial P/Tr boundary interval (De Kock and Kirschvink, where scour-and-flll structures preserve plant debris. Based 2004; Steiner et al., 2003; Ward et al., 2005). Therefore, it Copyright © 2005, SEPM (Society for Sedimentary Geoiogy) 0883-1351/05/0020-0479/$3.00 480 GASTALDOETAL ME S is possible that the extinction event may have varied re- BEAUFORT •^-^^^^"'"^ • * . • --.•rr-r^ Burgeisdotp, , AniSJOn ^^^ ^ ff^f^ri^^A^ " »"*""» > * « '*\KatbQi« !nduön-0!eriek[aii gionally in magnitude and timing, and the effects on land M/^E5cöJrf" %V.'?l'°.">:.'.-'."-;,j'fi¿¿¿¿\cudeberg Changhsingion- may pre-date, be coincident with, or post-date the effects •¿- -^- --^y •\-^. • •^.'^•r^'r'•f^°.'°'.''s-^ Capitanian documented in the oceans. Hence, it is essential that high- / .* - " • .H^^:^^-::^____ ICollinQhcim I i I I i II i JJ 270.6 quality, comparable data sets be acquired and evaluated [•. ;. yrvhe¡d/Xil^^^-==- / whitehq Kungurioîi to understand continental biodiversity loss at this critical pz^~ZZ^-P-^^^"^^=r"^!I^^SÊ?;îw)iTK.«n Pietei-mariizbijrg PririceAiüefl \ "îAftinskaOn 7- " i" " " 275.6 time in Earth history. DWYIÍA i* * * '^ • r 2Sa.4 The Karoo Basin of South Africa has been the focus of I ^ * * * -k \ ( I i f i f I H 1 1• intense activity because it is one of the few basins in which 900 800 700 6C0 500 400 300 200 10C 0 k;ri the terrestrial record across this critical interval is pre- served and exposed. To date, most of the attention has Hsiaciai nEsr B°-«-= raPííiK been paid to the changes in tetrapod assemblages Eïll PtaiÎ&'laois'frîne^' ^^°°'^ H ^i•«! B^ided g Diasîem/Hiatus throughout the section (e.g., Rubidge, 1995; Smith, 1995; FIGURE 1•Time-space diagram of tine Karoo Basin sliowing tine MacLeod et al., 2000; Smith and Ward, 2001; Ward et al., overall stratigraphy and generalized depositional environments into 2005), with the documented replacement of the Dicynodon the Early Triassic (Anisian), where a hiatus with overlying Late Triassic Assemblage Zone (latest Permian) with an Early Triassic rocks occurs. Diagram modified and updated from Cole (1992) and Lystrosaurus Assemblage Zone (Groenewald and Kitch- Veevers et al. (1994), and based on Catuneanu et al. (1998) and ing, 1995; Kitching, 1995a, b). Recent data presented on unpublished Council for Geoscience data. Age assignments and age/ palynomorphs (Steiner et al., 2003) and plant macrofossils stage names from Gradstein et al. (2005). and paleosols (Retallack et al., 2003) both supplement and confound the interpreted trends. It is well established Karoo Supergroup, with the most basal, the Dwyka (Behrensmeyer et al., 2000; Gastaldo, 2001) that before di- Group, consisting of Pennsylvanian and Early Permian versity measures can be compared across space and time, glacial tillites, glacial outwash, and lake deposits formed it is essential that isotaphonomic assemblages be identi- from the retreat of continental glaciers that had spread fied and assessed. This contribution addresses the plant across Gondwana (Visser, 1991; Fig. 1). This thick se- taphonomic character of assemblages in the non-marine quence is overlain by the Ecca Group, a time-transgressive Beaufort Group, which spans the Late Permian (Wujia- unit that becomes younger across the basin from south to pingian; Gradstein et al., 2005) to Middle Triassic (Ani- northeast in both its lower and upper contacts. These sian), to place the preserved megafiora into such a context. rocks accumulated in deep-water, pelagic, and reducing To date, no additional published data exist on the plant environments (Visser, 1992), including distal submarine taphonomic character of this, or any part of the Karoo, al- fans in which volcanic-ash deposits have been identified though interpretations of plant assemblages and the eco- (e.g., Johnson et al., 2001). Towards the northeast, a ter- systems they represent have been made for the Late Tri- restrial sequence is notable with the presence of coals and assic Molteno based on paleontological and sedimentolog- transitional fiuvial-deltaic environments (Johnson et al., ical evidence (Cairncross et al., 1995; Anderson et al., 1997; Cairncross, 2001; Catuneanu et al., 2002). 1998). The Ecca Group is superceded by the Permian•Triassic Beaufort Group, which prograded over the former from KAROO BASIN STRATIGRAPHY the south to north. By the latest Permian, accumulation throughout the basin took place under nonmarine condi- The Karoo Basin formed in southwestern Gondwana tions, which continued until the Early Jurassic. Several (South America, Africa, Antarctica, India, and Australia) upwards-fining sequences have been documented in the in response to collision and subduction of the paleo-Pacific uppermost formations (Balfour and Normandien forma- plate under the Gondwanan plate during the Pennsylva- tions) of the lower Beaufort Group (Visser and Dukas, nian (Veevers et al., 1994). A mountain chain, the Cape 1979; Groenewald, 1989; Catuneanu and Elango, 2001). Fold Belt (which formed part of the Pan-Gondwanan The coarse-grained portions of these cycles are interpreted Mountain Belt), consisting of folded and thrust-faulted as sandy braided or sand-bedload meandering channel rock, formed ~1500
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