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Calcified Metazoans in Thrombolite-Stromatolite Reefs of The Paleobiology, 26(3), 2000, pp. 334±359 Calci®ed metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia John P. Grotzinger, Wesley A. Watters, and Andrew H. Knoll Abstract.ÐReefs containing abundant calci®ed metazoans occur at several stratigraphic levels with- in carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia. The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefs are composed of thrombolites (clotted internal texture) and stromatolites (laminated internal tex- ture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occur in depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefs occur broadly across the ramp pro®le and are well developed as pinnacle reefs in downdip posi- tions. The three-dimensional morphology of reef-associated fossils was reconstructed by computer, based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens and additionally supported by observations of over 90 sets of serial sections. Most variation observed in outcrop can be accounted for by a single species of cm-scale, lightly calci®ed goblet-shaped fos- sils herein described as Namacalathus hermanastes gen. et sp. nov. These fossils are characterized by a hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular opening with a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlying biological structure. The goblets lived atop the rough topography created by ecologically complex microbial-algal carpets; they appear to have been sessile benthos attached either to the biohermal substrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phy- logenetic af®nities of Namacalathus are uncertain, although preserved morphology is consistent with a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unminerali- zed, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appear to be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans. Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming mi- croorganisms antedate the Cambrian Period. John P.Grotzinger and Wesley A. Watters. Department of Earth, Atmospheric and Planetary Sciences, Mas- sachusetts Institute of Technology, Cambridge, Massachusetts 02139. E-mail: [email protected] and E-mail: [email protected] Andrew H. Knoll. Botanical Museum, Harvard University, Cambridge, Massachusetts 02138. E-mail: [email protected] Accepted: 8 March 2000 Introduction regarded as the exception that proves the rule or as a hint that more diverse animals inhab- Siliciclastic successions of terminal Prote- ited carbonate platforms before the dawn of rozoic age contain moderately diverse, com- the Cambrian? monly problematic fossils of soft-bodied or- Here, we report evidence for a paleoecol- ganisms, collectively known as the Ediacaran ogically distinctive assemblage of calci®ed biota. Many rocks that host Ediacaran assem- metazoans in thrombolite-stromatolite reefs blages also contain a limited diversity of and associated facies of the terminal Protero- metazoan trace fossils. In contrastÐand in zoic Nama Group, Namibia, where Cloudina strong contradistinction to Cambrian and was originally discovered. Particularly abun- younger successionsÐterminal Proterozoic dant are goblet-shaped fossils described in limestones have been thought to yield only this paper as Namacalathus hermanastes gen. et limited evidence of animal life. A major ex- sp. nov., and interpreted as lightly mineral- ception to this pattern is provided by Cloudina, ized, attached benthos comparable to simple a globally distributed calci®ed fossil known cnidarians in bodyplan. almost exclusively from uppermost Protero- Archaeocyathans and corallimorph meta- zoic carbonate rocks. But should Cloudina be zoans formed persistent ecological associa- q 2000 The Paleontological Society. All rights reserved. 0094-8373/00/2603-0003/$1.00 CALCIFIED PROTEROZOIC METAZOANS 335 thickens northward toward the Damara oro- genic belt. The Osis Arch represents a site of depositional thinning of all Nama units that separates the two subbasins (Fig. 1). In general, the Nama Group consists of a number of marine-shelf siliciclastic and car- bonate sequences (Kuibis and Schwarzrand Subgroups) overlain by alluvial to shallow marine molasse (Fish River Subgroup) that documents unroo®ng of the Damara/Gariep hinterlands. Near the subbasin axes, thick- nesses are on the order of 2±3 km, thinning to less than 1 km farther onto the craton toward FIGURE 1. Geological map of southern Namibia, show- the Osis Arch (Germs 1974, 1983). Thrombol- ing the distribution of major sedimentary and tectonic elements, including the Nama Group, discussed in the ite-stromatolite reefs are well developed in the text. Kuibis Subgroup of the northern, Zaris sub- basin, and in the Huns platform of the south- ern, Witputs subbasin (Fig. 2). tions with microbial communities in Early Geochronologic constraints (Grotzinger et (but not earliest) Cambrian reefs (e.g., Riding al. 1995) are provided by U-Pb zircon ages on and Zhuravlev 1995), and stromatolite-throm- several units within the Nama Group (Fig. 2). bolite reefs have been reported from upper An ash bed within the northern Nama basin (but not uppermost) Proterozoic successions yields an age of 548.8 6 1 Ma for the middle (e.g., Turner et al. 1993, 2000). Therefore, in Kuibis Subgroup (Grotzinger et al. 1995). In addition to contributing evidence of diversity southern exposures of the Nama, the overly- among terminal Proterozoic skeletons, the ing Schwarzrand Subgroup contains ash beds fossiliferous bioherms reported here help to that yield, in ascending order, ages of 545.1 ®ll a gap in our understanding of the sedi- Ma, 543.3 Ma, and 539.4 Ma (all 61 Ma) (Grot- mentary transition from Proterozoic reefs ac- zinger et al. 1995). The Proterozoic/Cambrian creted by physical and microbial processes to boundary in Namibia is bracketed by the 543.3 Phanerozoic structures in which mineralized Ma and 539.4 Ma ages, although this also in- algae and animals play important construc- cludes a signi®cant unconformity; on a global tional roles. basis the Proterozoic/Cambrian boundary is currently regarded to be on the order of 543 Geological Setting of Nama Reefs Ma. The geology of southern Namibia, includ- ing the distribution of the Nama Group, is Zaris SubbasinÐKuibis Platform shown in Figure 1. The Nama Group has been The basal unit of the Nama Group, the Kui- interpreted as a foreland basin ®ll (Germs bis Subgroup is regionally widespread, con- 1983; Gresse and Germs 1993) related to con- sisting of a thin, basal, transgressive sand- vergence along the western and northern mar- stone that grades upward into a carbonate gins of the Kalahari craton and overthrusting platform 150±500 m thick (Figs. 2, 3A). In the in the Gariep and Damara orogens (Miller Zaris subbasin, Kuibis rocks form a well-de- 1983). The general stratigraphy of the Nama veloped carbonate ramp that thickens from Group (Fig. 2) was outlined by Martin (1965) the Osis Arch northward to the Naukluft and developed in a series of papers by Germs Mountains (Germs 1983; Saylor et al. 1998; (1972, 1974, 1983). Regional isopachs and fa- Smith 1998). This change in thickness is ac- cies distributions de®ne two subbasins; the companied by a gradation from shallow-water Witputs subbasin, located in southern Namib- facies in the south to deeper-water, basinal fa- ia, thickens toward the Gariep orogenic belt, cies near the Naukluft Mountains (Germs while the more northerly Zaris subbasin 1983). Germs (1972b, 1974, 1983) subdivided 336 JOHN P. GROTZINGER ET AL. FIGURE 2. Generalized stratigraphy of the Nama Group for the Zaris (north) and Witputs (south) subbasins, show- ing major lithostratigraphic, chemostratigraphic, biostratigraphic, and sequence stratigraphic attributes. Note the positions of thrombolite reef complexes above sequence boundaries. CALCIFIED PROTEROZOIC METAZOANS 337 the Kuibis platform into two members lief (Fig. 5A). In general, the cores of domes (Omkyk and Hoogland; Fig. 2) that corre- and columns are characterized by a throm- spond approximately to stratigraphic se- bolitic texture, whereas the margins become quences (Grotzinger et al. 1995; Saylor et al. progressively more stromatolitic in nature, in 1995, 1998; Smith 1998). The surfaces de®ning the sense that they exhibit crude lamination the member boundaries, however, represent (Fig. 5A; equivalent to the stromatolitic ¯ooding events rather than sequence bound- thrombolites of Kennard and James 1986). In aries (Smith 1998). Detailed mapping of these the Kuibis reefs, columns are consistently sequences (Smith 1998) shows that the reefs elongated with an azimuth of 2708±3108. Cal- described here, including the Driedoornvlagte ci®ed macrofossils occur within thrombolite bioherm identi®ed by Germs (1972a, 1974), all domes and columns, and the intrachannel ®ll nucleated within the Transgressive Systems to between domes and columns consists of early Highstand Systems Tract (TST to HST) trough cross-bedded, fossiliferous packstone at the base of their respective sequences (Fig. and grainstone containing
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