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Biostratigraphy of the Vendian–Cambrian Sukharikha River section, northwestern Siberian Platform Stephen M. Rowland, Veronica A. Luchinina, Igor V. Korovnikov, Dmitri P. Sipin, Alexander I. Tarletskov, and Artem V. Fedoseev
Abstract: The Sukharikha River section contains more than 800 m of fossiliferous Vendian and Lower Cambrian carbonate rock deposited in ramp, shelf, and slope environments. A diverse fauna of small shelly fossils, calcibionts, brachiopods, trilobites, and archaeocyaths has allowed us to develop a multi-taxa biostratigraphic framework for this section. A dearth of distinctive fossils low in the Sukharikha Formation prevents us from determining the position of the Vendian–Cambrian boundary. Abundant small shelly fossils and archaeocyaths in the uppermost Sukharikha Formation and low in the Krasnoporog Formation provide ample biostratigraphic control near the base of the Tommotian Stage, but the Nemakit–Daldynian – Tommotian boundary, as defined at Ulakhan–Sulugur on the Aldan River, is temporally ambiguous. For this reason there is no precise definition of this boundary. In the Sukharikha River section we have provisionally placed the base of the Tommotian Stage at the first occurrence of Nochoroicyathus sunnaginicus Zone archaeocyaths, about 1.5 m below the top of the Sukharikha Formation. However, we suppose that this horizon actually predates the deposition of nominally basal Tommotian taxa in the Aldan region. A new global stratotype section for the Nemakit–Daldynian – Tommotian boundary should be selected, and this section may turn out to be a good candidate. The paleontological richness (especially small shelly fossils and archaeocyaths), the apparent absence of long depositional hiatuses, and the presence of well-preserved limestones suggest that the Sukharikha River section contains the combination of paleontological, sedimentological, and isotopic data to resolve some fundamental problems in Early Cambrian stratigraphy. Résumé : La coupe de la rivière Sukharikha expose plus de 800 m de roches carbonatées fossilifères du Vendien et du Cambrien inférieur, déposées sur rampe, plate-forme et talus continentaux. Une faune diversifiée de fossiles coquilliers, calci-organismes, brachiopodes, trilobites et archéocyathes nous a permis de dresser un canevas biostratigraphique des nombreux taxons rencontrés dans cette coupe. La pénurie de fossiles distinctifs au bas de la Formation de Sukharikha empêche de définir la position de la limite Vendien–Cambrien. Le foisonnement de petits fossiles coquilliers et les archéocyathes dans le portion sommitale de la Formation de Sukharikha et dans la portion inférieure de la Formation de Krasnoporog assurent un bon contrôle biostratigraphique près de la base de l’Étage tommotien, mais la limite Nemakit–Daldynien – Tommotien, telle que définie à Ulakhan–Sulugur sur la rivière Aldan, est temporellement ambiguë. Pour cette raison, il n’existe pas de définition précise de cette limite. Dans la coupe de la rivière Sukharikha, nous avons provisoirement placé la base de l’Étage tommotien au niveau de la première apparition de la Zone à Nochoroicyathus sunnaginicus des archéocyathes, à environ 1,5 m en dessous du sommet de la Formation de Sukharikha. Cependant, nous croyons que cet horizon prédate actuellement le dépôt du taxon basal assigné nominalement au Tommotien dans la région d’Aldan. Une nouvelle coupe stratotype globale pour la limite Nemakit–Daldynien – Tommotien doit être sélectionnée, et la coupe décrite ici pourrait s’avérer un excellent choix. La richesse paléontologique (spécialement les petits fossiles coquilliers et archéocyathes), l’absence apparente de lacunes de sédimentation de longue durée, et la présence de calcaires bien préservés, font que la coupe de la rivière Sukharikha réunit une combinaison de données paléontologique, sédimentologique et isotopique pouvant contribuer à solutionner certains problèmes fondamentaux de la stratigraphie du Cambrien précoce. [Traduit par la rédaction]
Received April 17, 1997. Accepted December 18, 1997. S.M. Rowland.1 Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, U.S.A. V.A. Luchinina, I.V. Korovnikov, D.P. Sipin, and A.V. Fedoseev. Unified Institute of Geology, Geophysics, and Mineralogy, Russian Academy of Science, 630090 Novosibirsk, Russia. A.I. Tarletskov. Paleontological Institute, Russian Academy of Science, 117868 Moscow, Russia. 1
Corresponding author (e-mail:
[email protected]).
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Introduction On the northwestern margin of the Siberian Platform, near the town of Igarka (Fig. 1), Vendian and Paleozoic strata are exposed in stream cuts of tributaries of the Yenisey River. The Precambrian–Cambrian boundary beds of this region are among the least studied on the Siberian Platform. The stratigraphic nomenclature and macrofossil biostratigraphy were worked out in the 1950s and 1960s (Datsenko 1963; Datsenko et al. 1968; Grigor’ev 1958; Luchinina and Meshkova 1969; Savitsky et al. 1964, 1967; Rozanov 1973), but these sections © 1998 NRC Canada
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Fig. 1. Map of the Siberian Platform, showing major modern rivers, distribution of major facies belts in the Early Cambrian, and location of the study area. Adapted from Rozanov and Zhuravlev (1992).
have largely been untouched for the past 30 years (see summaries by Astashkin et al. 1991 and Rozanov et al. 1992). Consequently, such aspects of these strata as the ranges of small shelly fossils, sedimentology, and isotope stratigraphy are poorly known or completely unstudied. The best known exposures of this region are along the Suk-
harikha River, which cuts across mostly gently folded strata of the Izluchin, Sukharikha, Krasnoporog, and Shumny formations (Fig. 2). The upper three of these units make up a Vendian through Botomian carbonate interval that represents the westernmost occurrence of platform-margin and slope facies on the Siberian Platform (Fig. 1). The Sukharikha River sec© 1998 NRC Canada
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Fig. 2. (A) Map of the Sukharikha River area showing fold axes. (B) Simplified east–west vertical structure section showing main lithologies and structures. Numerous north-striking, high-angle faults (not shown) also occur in this region.
tion is particularly important because it is the only section in the Igarka–Norilsk region known to contain archaeocyaths, thus allowing correlation across the approximately 1400 km width of the Siberian Platform with archaeocyath-rich sections in the Lena–Aldan region to the southeast (Fig. 1) (Rozanov 1984). In the 1970s, when the Precambrian–Cambrian Boundary Working Group was established, Soviet workers considered proposing the Sukharikha River section as the boundary global stratotype section. They instead proposed the Ulakhan–Sulugur section on the Aldan River (Fig. 1), in part because Soviet military restrictions at that time effectively precluded foreigners from visiting arctic Siberia. With the easing of access to this part of Russia, we have begun a reexamination of the Precambrian–Cambrian boundary sections of this region to address current questions. In this paper we summarize the principal biostratigraphic and sedimentologic features of the Sukharikha River section. Luchinina et al. (1997) provide more detailed data on calcibionts, small shelly fossils, and trace fossils. Among the coauthors of this paper, calcibionts have been studied by V.A.L., brachiopids by I.V.K., small shelly fossils by D.P.S., archaeocyaths by A.I.T., trilobites by both A.V.F. and I.V.K., and sedimentology by S.M.R.
Description of rock units The Sukharikha River section is more than 1000 m thick, consisting of an unmeasured interval of Proterozoic red argillites (Izluchin Formation) overlain by more than 800 m of Vendian and Lower Cambrian carbonates (Fig. 3). The carbonate interval is subdivided into three formations, the Sukharikha, Krasnoporog, and Shumny formations (Fig. 3), all of which are named for stratotype sections along the Sukharikha River. Fig-
ure 4 shows a tentative correlation of these units with the southeastern portion of the Siberian Platform (Lena–Aldan region) and with Avalonia. Brief descriptions and interpretations of the three carbonate units are provided below. Sukharikha Formation: deposition on a proximal carbonate ramp By our measurement, the Sukharikha Formation consists of 619 m of subtidal and peritidal limestones and dolostones (Fig. 3), compared with 570 m reported by Grigor’ev (1958). Desiccation-cracked, tidal-flat dololaminites are a recurring facies, as are domal stromatolites and flat pebble conglomerates. The uppermost 200 m of the Sukharikha Formation is characterized in Fig. 5. The facies are very similar to carbonate portions of the Vendian Turkut Formation in the Olenek Uplift (Fig. 1), as descibed by Pelechaty et al. (1996). Lithologies and sedimentary structures indicate that the Sukharikha Formation represents sedimentation on the proximal portion of a carbonate ramp, but more sections need to be examined to determine the orientation of this ramp and other sedimentological details. Luchinina et al. (1997) describe a diverse flora of calcareous algae and calcibionts from the Sukharikha Formation. The Sukharikha Formation does not contain conspicuous, karstic sequence boundaries, although its upper contact is probably a sequence boundary defined by an unconformity. We interpret sequence boundaries within this formation to be contained within intervals of supratidal facies. These are sequence boundary zones, typically several metres thick, similar to those described in the Middle Cambrian Bonanza King Formation of the Great Basin by Montañez and Osleger (1993). One probable 40 m thick, third-order sequence can be identi© 1998 NRC Canada
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Fig. 3. Generalized stratigraphic column of the Sukharikha River section. The Russian coauthors of this paper reject placement of the Nemakit–Daldynian Stage in the Cambrian System, preferring to define the base of the Cambrian System as the base of the Tommotian Stage.
fied between 520 and 560 m above the base of the formation (Fig. 5). This is a distinctive shallowing-upward interval in which the lower portion consists of metre-scale, peritidal parasequences of fissile green shale overlain by buff dolostone caps. These shale–dolostone couplets grade into decimetrethick dololaminite beds separated by thin shale partings. A cherty stromatolite bed caps the entire sequence. This is a rare instance in this section in which a single major flooding event can be recognized, followed by parasequences that culminate in intertidal or supratidal facies. The contact between the Sukharikha Formation and the overlying Krasnoporog Formation is marked by an abrupt change from light gray lime mudstone below to red, bioturbated, stylolitic lime mudstone above (Figs. 5, 6). Although this contact is not conspicuously karstic in the Sukharikha River exposures, we interpret it to be a sequence boundary
because (i) it is a very sharp contact in which proximal carbonate ramp deposits are abruptly overlain by open shelf sediments and thus records a conspicuous flooding event, and (ii) the stratigraphic position and lithologic characteristics suggest that it is equivalent to the prominent erosion surface at the Manykai–Medvezhya contact in the western Anabar region (Kaufman et al. 1996; Knoll et al. 1995b; Landing 1994). Krasnoporog Formation: reefs and bioturbated carbonate shelf deposits The Krasnoporog Formation consists of at least 148 m of bioturbated, small shelly fossil-bearing, lime mudstone (Figs. 3, 6). It is conspicuously stylolitic parallel to bedding. In Sukharikha Anticline exposures, where this formation was studied most extensively, few primary sedimentary structures are preserved, apparently due to intensive bioturbation fol© 1998 NRC Canada
Rowland et al. Fig. 4. Correlation between Siberian Platform and Avalonian sections, based on Brasier (1992), Knoll et al. (1995b), and Landing (1994). Radiometric dates from Bowring et al. (1993) and Isachsen et al. (1994). GSSP, global stratotype section and point.
343 Fig. 5. Upper 210 m of the Sukharikha Formation and basal portion of the Krasnoporog Formation.
lowed by stylolitization. Pseudobedding is produced by cracks that occur where stylolites are particularly dense, resulting in the appearance of a thick-bedded limestone. Some intensively stylolitic horizons are cherty. One interval that escaped bioturbation and stylolitization is a well-preserved, 4 m thick, boundstone-bioherm interval described below. The basal few metres of the Krasnoporog Formation consists of red to pink lime mudstone. The intensity of the color increases when the rock is wet. The remainder of the formation is banded pink and gray; each color band is typically 2–5 m thick, with gradational boundaries between colors. Figure 6 characterizes the lower 15 m of the Krasnoporog Formation. Between 10.5 and 14.5 m from the base of the formation is a 4 m thick archaeocyath-rich interval (Fig. 7). The lower 2 m of this interval is a light gray biostrome of archaeocyathan– calcibiont boundstone without discrete bioherms (unit 5 of Fig. 12 of Rozanov et al. 1969). The upper 2 m consists of two 1 m thick layers of pink lime mudstone with irrregularly shaped archaeocyath–calcibiont bioherms (unit 6 of Fig. 12 of Rozanov et al. 1969). The boundary between the gray boundstone below and the pink, bioherm-bearing unit above is very irregular, and some bioherms appear to have grown from the boundstone upward into the biostrome interval. The boundary between the two bioherm-bearing beds, in contrast, is abrupt, and could represent a depositional hiatus. © 1998 NRC Canada
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Fig. 6. Small shelly fossil ranges and biozonation in uppermost Sukharikha Formation and lower 15 m of the Krasnoporog Formation. Lithologic symbols as in Fig. 5.
The Krasnoporog Formation represents shallow shelf environments. We interpret the 2 m thick biostrome, 10.5–12.5 m from the base, to represent a laterally continuous reef complex. This interval was documented by Rozanov et al. (1969, Fig. 12) to extend for many kilometres along the Sukharikha River. The overlying bioherms were probably discrete
patch reefs in the back-reef lagoon. The bedding plane within the 2 m thick patch reef facies may represent a brief interval of subaerial exposure or a major storm event that locally disrupted sedimentation. The total thickness of the Krasnoporog Formation probably exceeds the 148 m shown in Fig. 3. Astashkin et al. (1991) © 1998 NRC Canada
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Fig. 7. Archaeocyathan ranges and biozonation in uppermost Sukharikha Formation and lower 15 m of the Krasnoporog Formation. Lithologic symbols as in Fig. 5.
report a thickness of 160–200 m, measured near Krasniy Porog (Red Rapid) (Fig. 2). Our thickness of 148 m was measured on the east limb of the Sukharikha Anticline, several kilometres to the east (Fig. 2), where the Krasnoporog–Shumny contact is not exposed and is very likely a fault. At Krasniy Porog, the lower portion of the Krasnoporog Formation is not well exposed, but the uppermost 52 m of the formation is completely exposed and consists of more variable lithologies than described above. In addition to some intervals of typical intensively burrowed, stylolitic lime mudstone, there are also beds of true, thick-bedded, ledgy, gray, lime mudstone without conspicuous burrowing textures. This uppermost 52 m lacks the
wide pink bands that characterize the Krasnoporog Formation generally, but instead contains a few, very thin, vivid red beds. These thin red beds are up to 1.0 m thick, but are commonly much thinner. This uppermost, vividly red striped, lithologically variable interval of the Krasnoporog Formation is probably faulted out of the Sukharikha Anticline exposure; it represents a transition from the intensively bioturbated shelf carbonates of the Krasnoporog Formation to the deeper-water slope deposits of the overlying Shumny Formation. Shumny Formation: carbonate slope deposits The Shumny Formation consists of a minimum of 73 m of © 1998 NRC Canada
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deeper water carbonate lithologies, including carbonate turbidites, nodular limestones, and a metre-thick bed of internally chaotic limestone conglomerate. The top of this unit is not preserved, so its complete thickness is not known. The Krasnoporog–Shumny transition is gradational, as described above; the base of the Shumny Formation is chosen as the base of the lowest dark, recessive, limestone bed, which is a carbonate turbidite. The presence of turbidites and conglomeratic slump deposits clearly record sedimentation on a slope. This interpretation is reinforced by the presence of an intraformational truncation surface in a cliff exposure on the right bank, about 500 m upstream from the mouth of the Shumny River. The Shumny Formation contains a diverse fauna of trilobites, brachiopods, archaeocyaths, ostracods, hyoliths, and problematica (Luchinina et al. 1997; Rozanov et al. 1992). The Botomian trilobite Calodiscus schucherti occurs 5 m above the base of the formation. Other trilobites that occur low in the formation include Pagetiellus anabarus, Pagetiellus lenaicus, Triangulaspis lermontovae, Hebediscus attleborensis, ?Neocobolldia dentata, Erbiella musta, and Lermontovia grandis. The Shumny Formation extends into the Toyonian Stage, as documented by the presence high in the formation of the Toyonian trilobites Lermontovia grandis, Lermontovia cf. Lermontovia clavata, Kooteniella acuta, and E. musta. In the present study a variety of brachiopods were found. Lingulella variabilis occurs throughout the formation, and Lingulella linguata and several undescribed species of Lingulella occur scattered throughout the lower and middle portions. Other brachiopods include Botsfordia caelata and an unidentified species of acrotretid, both occurring near the middle of the formation. The Shumny Formation contains a fauna of at least seven species of archaeocyaths, Stapicyathus stapiporus, Tumulifungia datzenkoi, Irinaecyathus inoratus, Gagarinicyathus ethmophylloides, Carinacyathus squamosus, Mennericyathus grigorievi, and Rozanovicythus alexi (Astashkin et al. 1991; Rozanov 1992). All of the Shumny Formation archaeocyaths observed in this study were isolated clasts, presumably transported downslope from shelf-margin settings.
Biostratigraphy The ranges and proposed zones of small shelly fossil taxa are shown in Fig. 6, and archaeocyath ranges and zones in Fig. 7. Figure 8 summarizes the biostratigraphic results of this study and compares our proposed zonation with those of previous workers. Small shelly fossil zones The Sukharikha small shelly fauna is still under study, and the results presented here are preliminary. Anabarites-like small shelly fossils occur as low as 240 m above the base of the Sukharikha Formation, which is about 380 m below the appearance of a diverse small shelly fauna (Fig. 3). At least 32 taxa occur higher in the section. All of these except one, Lapworthella bella, first appear within a 3 m interval near the top of the Sukharikha Formation and the base of the Krasnoporog Formation (Fig. 6). We propose a small shelly fossil zone, the Ladatheca annae Zone, which is correlative with the wellknown Nochoroicyathus sunnaginicus archaeocyath zone. Small shelly fossils within the Sukharikha Formation exhibit the same quality of preservation as those in the Krasnoporog Formation, and there are no glauconitic lenses or
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evidence of karst at the Sukharikha–Krasnoporog contact, as there are at Ulakhan–Sulugur on the Aldan River (Cowie 1989; Khomentovsky and Karlova 1993; Landing 1994). Comparison of the Sukharikha section small shelly fauna with those of the Aldan and Anabar regions is discussed below, concerning the placement of the Nemakit–Daldynian – Tommotian boundary. Calcibiont zones The upper 381 m of the Sukharikha Formation, excluding the uppermost 1.5–2.0 m, contains a complex of calcibionts, including Hedstroemia sp., Renalcis polymorphus, Proaulopora glabra, and Subtifloria gracila. Sometimes these taxa occur in buildups in which Hedstroemia forms the framework. Because of the ubiquity and important sedimentological role of this form in the upper Sukharikha Formation, Luchinina et al. (1997) propose a Hedstroemia calcibiont zone, which includes the upper two thirds of the Sukharikha Formation (Fig. 8). The Hedstroemia Zone extends well below the base of the N. sunnaginicus archaeocyath Zone (Fig. 8). Luchinina et al. (1997) further propose a Proaulopora Zone that overlies the Hedstroemia Zone and includes the Krasnoporog and Shumny formations (Fig. 8). Archaeocyath zones In the Sukharikha River section archaeocyaths occur in the uppermost 1.5–2.0 m of the Sukharikha Formation, the lower 20 m of the Krasnoporog Formation, and as bioclasts in the slope facies of the Shumny Formation (Fig. 4). According to Rozanov et al. (1969), the taxa in the Sukharikha Formation include Archaeolynthus polaris, Cryptaporocyathus junicanensis, Nochoroicyathus virgatus, N. sunnaginicus, Nochoroicyathus dragunovi, and Nochoroicyathus igarcaensis (Fig. 7). The first four of these species occur within the N. sunnaginicus Zone in the southeastern Siberian Platform (Rozanov et al. 1969). In the present study, the only exposures of the uppermost Sukharikha Formation that we examined in detail are those in the east limb of the Sukharikha Anticline (Fig. 2), where we did not see archaeocyaths. Thus, we are unable to corroborate the presence of these N. sunnaginicus Zone archaeocyaths below the Sukharikha–Krasnoporog contact. The archaeocyaths collected by Rozanov et al. (1969) are not reported to be abundant in the outcrop and apparently did not form reefs or bioherms in this section. However, one member of this fauna, A. polaris, is a reef-building member of the oldest known Cambrian reef, in the Pestrotsvet Formation at Ulakhan–Sulugur (Riding and Zhuravlev 1995). The presence of archaeocyaths in the Sukharikha Formation has taken on special significance because of the possible importance of archaeocyaths in a revised definition of the Nemakit– Daldynian – Tommotian boundary, and because of the nowpopular view that the “basal Tommotian” fauna in the Aldan region was deposited on a significant unconformity (see below). The presence of in situ archaeocyaths in the Sukharikha Formation should be confirmed with additional fieldwork. The Krasnoporog Formation contains a diverse fauna of at least 23 archaeocyath taxa (Fig. 7) (Astashkin et al. 1991; Rozanov et al. 1969, 1992; this study). Five of these (A. polaris, N. sunnaginicus, N. virgatus, N. dragunovi, and N. igarcaensis) also occur in the underlying Sukharikha Formation (Fig. 7). Archaeocyaths are concentrated in two © 1998 NRC Canada
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Rowland et al. Fig. 8. Summary of biostratigraphy of Sukharikha River section showing biozones proposed by previous workers and results of the present study. The Russian coauthors of this paper would prefer a definition of the Vendian–Cambrian boundary at the Nemakit–Daldynian – Tommotian boundary.
intervals, the basal metre and between 10.5 and 14.5 m above the base (Fig. 7). Above this level they occur sporadically in some exposures (Rozanov et al. 1969, Fig. 12). As discussed in detail below, the placement of the Nemakit–Daldynian – Tommotian boundary is problematical due to ambiguities in the stratotype section on the Aldan River. Here we place this boundary at the first occurrence of archaeocyaths, as reported by Rozanov et al. (1969), 1.5–2.0 m below the top of the Sukharikha Formation, although these beds probably predate the deposition of basal Tommotian fossils at Ulakhan–Sulugur. We place the Tommotian–Atdabanian boundary at 12.5 m above the base of the Krasnoporog Formation, just below the top of the gray boundstone bed (Fig. 6). Datsenko et al. (1968) and Zhuravleva and Korshunov (1976) placed this boundary at the very top of the boundstone bed, whereas Rozanov et al. (1969) placed it several metres higher. The new placement, at 12.5 m, is based on the discovery during this study of Rotundocyathus dotatus and Pliocyathus cf.
Pliocyathus platyseptatus in the upper part of this gray boundstone bed, both of which are known only from the Atdabanian.
The Vendian–Cambrian boundary problem Until 1992 the base of the Tommotian Stage was considered by most workers to define the base of the Cambrian System (Brasier et al. 1994b; Landing 1994). Although the Tommotian Stage is defined by the abrupt appearance of a large and diverse small shelly fauna, it does not include the oldest shelly fauna on the Siberian Platform. Several small shelly taxa occur lower in the section, most notably Anabarites trisulcatus and several species of the conodont-like genus Protohertzina (Brasier 1989). These pre-Tommotian small shelly fossils characterize the Nemakit–Daldynian Stage (sometimes also called the Manykaian Stage), the type section of which is in the Anabar Uplift (Cowie 1989), a few hundred kilometres to the northeast of Igarka (Fig. 1). In 1992 the International Stratigraphic Commission ratified © 1998 NRC Canada
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the Precambrian–Cambrian Boundary Working Group’s selection of a point in the Chapel Island Formation at Fortune Head in southeastern Newfoundland as the global stratotype section and point (Fig. 4). By chosing the base of the Phycodes pedum Zone to define the Precambrian–Cambrian boundary, the Working Group had effectively lowered the base of the Cambrian System and included within it rocks that most workers had previously considered to be Proterozoic in age. On the Siberian Platform, the newly defined Precambrian–Cambrian boundary has been tentatively correlated with the base of the Nemakit–Daldynian Stage (Fig. 4). The evidence for this correlation is based on the reported (but not precisely documented) occurrence of Phycodes sp. within Nemakit– Daldynian beds in the Olenek region, and also the occurrence within the P. pedum Zone in southeastern Newfoundland of the organic-walled tube Sabellidites cambriensis, which, on the Siberian Platform, is restricted to the Nemakit–Daldynian Stage (Brasier et al. 1994b). Although the base of the Cambrian Period is now formally defined to include sub-Tommotian strata, many Cambrian workers are uncomfortable with this definition; the Russian coauthors of this paper would support a redefinition of the Vendian–Cambrian boundary such that the Nemakit–Daldynian Stage would be the terminal stage of the Vendian Period. In the Anabar Uplift, the base of the Nemakit–Daldynian Stage, as originally defined by Savitsky (1975), is a conspicuous unconformity with breccia clasts and solution features at the base of the Manykai Formation (Kaufman et al. 1996; Knoll et al. 1995b). On the basis of the somewhat tenuous criteria summarized above, and in the absence of a more precise correlation tool, this sub-Manykai sequence boundary now serves as the de facto Precambrian–Cambrian boundary reference horizon on the Siberian Platform. In the Sukharikha River section the correlative Vendian–Cambrian boundary horizon presumably occurs within the Sukharikha Formation; however, there are no conspicuous karstic unconformities within this formation, and therefore no obvious surface to correlate with the base of the Manykai Formation at Anabar. Small shelly fossils that may be Anabarites occur in thin sections of samples collected about 380 m below the top of the Sukharikha Formation. Because Anabarites is not known to occur below the base of the Nemakit–Daldynian Stage on the Siberian Platform, we have very tentatively placed the Vendian–Cambrian boundary at the lowest occurrence of these Anabarites-like small shelly fossils (Fig. 3). However, in Mongolia Brasier et al. (1996) have identified Anabarites and Cambrotubulus in beds that they interpret to be sub-Nemakit–Daldynian. Also, Cambrotubulus has been reported below the base of Nemakit–Daldynian equivalent strata in the Olenek Uplift (Khomentovsky and Karlova 1993; Knoll et al. 1995a). Thus, the placement of the Vendian– Cambrian boundary in the Sukharikha section cannot be determined on the basis of current information, and may not be determinable with paleontological data alone.
The Nemakit–Daldynian – Tommotian boundary problem The interregional and intercontinental correlation of the Nemakit–Daldynian – Tommotian boundary has emerged as one of the outstanding problems in Cambrian stratigraphy.
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Even within the Siberian Platform there are conflicting patterns of fossil occurrences at the base of the Tommotian Stage. In the southeastern part of the platform (Aldan region), where the Tommotian Stage is formally defined, some 100 taxa of small shelly fossils and archaeocyaths appear very abruptly at the base of the stage (Rozanov 1984; Rozanov et al. 1969, 1992). Until recently, most Cambrian workers accepted the nearly simultaneous appearance of so many small shelly taxa and archaeocyaths as a burst of evolution signaling the ignition of the Cambrian explosion. Others, however, expressed concern that this abrupt appearance of the Tommotian fauna in the southeastern part of the Siberian Platform was due to the presence of a significant unconformity at the base of the Pestrotsvet Formation. According to this view, the basal Pestrotsvet unconformity represents a significant interval of time and resulted in the condensation of a gradual evolutionary history into one horizon (Knoll et al. 1995b, 1996; Landing 1994, 1996). In the Anabar region of the northwestern Siberian Platform the pattern of first appearances is quite different. There the Tommotian small shelly fossils enter the record more gradually; some 50 of the small shelly taxa that occur abruptly together in the Aldan region make their first appearances sequentially over more than 30 m of section (Kaufman et al. 1996; Khomentovsky and Karlova 1993; Knoll et al. 1995b; Missarzhevsky 1983). Rozanov (1992) interpreted the abrupt appearance of so many taxa in the southeast to faithfully record evolutionary history, and the gradual appearance at Anabar to reflect environmental factors that had prevented the local presence of some species. A recent study by Knoll et al. (1995b), discussed below, strongly supports the gradual model as the true reflection of evolutionary events (but see Landing 1996). Reconstructing the actual temporal and spatial distribution of organisms across the Nemakit–Daldynian – Tommotian transition on the Siberian Platform has been hampered by an unfortunate choice of global stratotype section and point (GSSP). The boundary is defined by a point at the base of bed 8, 1.3 m below the top of the Yudoma Formation in the Ulakhan–Sulugur section on the Aldan River (Fig. 1) (Rozanov et al. 1969; Rozanov 1984). Of the approximately 100 taxa that appear near the base of the Tommotian Stage in this section, about 30 small shelly fossil taxa and one species of archaeocyath occur in glauconitic lenses of bed 8, and an additional 70 species of small shelly fossils and archaeocyaths occur in the basal beds of the overlying Pestrotsvet Formation. The fossils in bed 8 of the Yudoma Formation are restricted to karst-filled glauconitic lenses (Khomentovsky and Karlova 1993), so it now appears that the fossils that define the base of the Tommotian Stage were actually deposited well after the horizon in which they are found. Thus, although 100 species appear essentially simultaneously in the rock record at the GSSP, they presumably did not all appear simultaneously on Earth; rather, they evolved during the unknown interval of time represented by the Yudoma–Pestrotsvet unconformity and were deposited together as the Pestrotsvet sea transgressed across the eroded pre-Tommotian surface. Brasier et al. (1996) have shown that in Mongolian sections the Tommotian taxa appear much less abruptly than in the southeastern region of the Siberian Platform. With a surfeit of nominally basal Tommotian taxa to chose from in the boundary stratotype section, various workers have used different fossils to correlate the Nemakit Daldynian – © 1998 NRC Canada
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Tommotian boundary from Ulakhan–Sulugur to other sections. In the Anabar region, for example, Rozanov et al. (1992) placed the base of the Tommotian at the point where the first basal Tommotian taxa appear, Khomentovsky and Karlova (1993) placed the base of the Tommotian at the point where the most basal Tommotian taxa appear, and Missarzhevsky (1989) placed the base of the Tommotian at the point where the last basal Tommotian taxa appear (Kaufman et al. 1996). The problem, of course, is that the Nemakit–Daldynian – Tommotian GSSP is temporally ambiguous. Basal Tommotian has no precise meaning and therefore cannot be meaningfully correlated. Clearly, a new Nemakit–Daldynian – Tommotian GSSP needs to be selected to overcome the fundamental problems at Ulakhan–Sulugur. We suggest that the Sukharikha River section may turn out to be a good replacement. Small shelly fossil first appearances in Igarka, Aldan, and Anabar sections In addition to the erosional ambiguities associated with the Nemakit–Daldynian – Tommotian GSSP at Ulakhan–Sulugur, the first appearance of small shelly taxa is very inconsistent across the Siberian Platform. In the Sukharikha River section we have identified 28 small shelly taxa (Fig. 6), of which 20 also occur in the Aldan sections. Five of the species that occur in both areas (Tiksitheca licis, Torellella curva, Torellella lentiformis, Hyolithellus vladimirovae, and Aldanella rozanovi) first appear in bed 8 of the Yudoma Formation at Ulakhan– Sulugur (Rozanov et al. 1992). In the Sukharikha section all five of these taxa first appear in the upper metre of the Sukharikha Formation (Fig. 6). Thirteen of the taxa that occur in both areas (Spinulitheca billingsi, L. annae, Hyolithellus tortuosus, Hyolithellus grandis, Tommotia admiranda, Tommotia kozlowskii, Archiasterella tetractina, Bemella jacutica, Chancelloria simmetrica, Conotheca mammilata, Heraultipegma sibirica, Camenella garbowskae, and Lapworthella tortuosa) appear in basal Pestrotsvet beds in the Aldan sections (Rozanov et al. 1992), and in the Sukharikha section they appear either in the upper 1 m of the Sukharikha Formation or the lowermost 2 m of the Krasnoporog Formation (Fig. 6). Finally, two of the species that occur in both areas (Halkieria sacciformis and Hyolithellus tenuis) first appear in the uppermost Sukharikha Formation in the Sukharikha River section (Fig. 6), whereas on the Aldan River they first appear several metres above the base of the Pestrotsvet Formation (Rozanov et al. 1992). Thirteen of the small shelly taxa that occur in both the Sukharikha and Aldan sections also occur in the Kotuikan River section of western Anabar (Kaufman et al. 1996; Knoll et al. 1995b). Figure 9 shows the position in the Sukharikha and Anabar sections where these 13 taxa first appear. In the Aldan sections all 13 occur either at the base of the Pestrotsvet Formation or in bed 8 of the underlying Yudoma Formation and are therefore members of the basal Tommotian fauna at the GSSP. Figure 9 shows that there is not a consistent pattern of first appearances of small shelly taxa between the Aldan, Anabar, and Igarka regions. These 13 taxa that are basal Tommotian at Ulakhan–Sulugur have first appearances spread over several metres in the Igarka region and over several tens of metres in western Anabar. Most significantly, there is no consistent sequence of first appearances between Anabar and Igarka. These
349 Fig. 9. Position of first appearance of 13 small shelly taxa that occur in the western Anabar, Sukharikha River (Igarka), and Ulakhan–Sulugur sections. In the Ulakhan–Sulugur section all 13 taxa were deposited with the basal Tommotian onlap.
results leave little room for doubt that the first appearances of at least some small shelly taxa are facies controlled. Archaeocyath first appearances in Igarka and Aldan sections In contrast to the small shelly fossils, first appearances of archaeocyaths appear to be quite consistent across the Siberian Platform. In the Sukharikha section, six species of archaeocyaths appear in the uppermost Sukharikha Formation, marking the base of the N. sunnaginicus Zone (Fig. 7). Four of these (A. polaris, C. junicanensis, N. sunnaginicus, and N. virgatus) also occur in the Aldan sections; N. virgatus first appears in the glauconitic, karst-fill lenses of bed 8 of the Yudoma Formation, whereas the other three first appear at the base of the Pestrotsvet Formation (Rozanov et al. 1992). In the Sukharikha River section four additional species (Robustocyathus robustus, Dictyocyathus translucidus, Cambrocyathellus tschuranicus, Paranacyathus tuberculatus) appear together about 1.0 m above the base of the © 1998 NRC Canada
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Krasnoporog Formation, marking the base of the Dokidocyathus regularis Zone (Fig 7). Two of these species (R. robustus and D. translucidus) also occur in the Aldan region, where they likewise appear together, along with two other archaeocyath species, about 4.5 m above the base of the Pestrotsvet Formation (Rozanov et al. 1992). Thus, the pattern of first occurrences of archaeocyaths appears more consistent between the Igarka and Aldan regions than does the patten of first appearances of small shelly taxa. Archaeocyaths are absent from the western Anabar region; they appear in northeastern Anabar only at the base of the Atdabanian (Kaufman et al. 1996; Knoll et al. 1995b). Knoll et al. (1995b) suggested that archaeocyaths may be more robust markers of the early Tommotian than are small shelly fossils, and the pattern of first occurrences in the Sukharikha River section appears to support this view. The center of origin of archaeocyaths was apparently in the Anabar– Sinyaya facies belt of the Siberian Platform (Rozanov 1992), but their lowest occurrence is unclear. Archaeocyath fragments are reported from the Olenek Uplift in the upper Kessusa Formation (Rozanov et al. 1992), which Kaufman et al. (1996) interpret to lie beneath a sub-Tommotian unconformity. The Igarka section, being the only archaeocyath-rich section in the northwestern Siberian Platform, provides an opportunity to test this correlation.
Can. J. Earth Sci. Vol. 35, 1998 Fig. 10. Hypothesized correlation of Nemakit–Daldynian – Tommotian boundary beds between the Sukharika River section (Igarka region) and the GSSP section at Ulakhan–Sulugur (Aldan region). The irregular surface at the top of the Yudoma Formation in the Aldan section represents the pre-Pestrotsvet, karstic, erosion surface, within which the fossiliferous, glauconitic lenses of bed 8 were deposited. This model implies that the first appearance of N. sunnaginicus Zone archaeocyaths and small shelly fossils in the Sukharikha River section actually predates the deposition of these same taxa at Ulakhan–Sulugur.
Hypothesized correlation between Igarka and Aldan Figure 10 shows our hypothesized correlation of Nemakit– Daldynian – Tommotian boundary beds between the Sukharikha River and Aldan River sections. The two regions are superficially very similar. In both cases red carbonates containing diverse Tommotian faunas unconformably overlie gray, sparsely fossiliferous carbonate. Many of the same taxa of archaeocyaths and small shelly fossils occur in both regions. Moreover, the earliest basal Tommotian taxa occur in both sections just below the gray–red unconformity. The key difference is that in the Aldan sections the Tommotian fauna is associated with the pre-Pestrotsvet unconformity and is therefore temporally ambiguous. In the Sukharikha section, in contrast, there is no evidence of karst; the inferred Sukharikha–Krasnoporog unconformity is a very subtle feature with no conspicuous evidence of erosion. According to this interpretation, no deposition was occurring in the Aldan region when the earliest Tommotian taxa evolved; they are preserved in situ in the uppermost Sukharikha Formation. If this interpretation is correct, then the Sukharikha–Krasnoporog unconformity represents a relatively short time interval, because very few archaeocyaths and small shelly taxa occur in the uppermost Sukharikha Formation without also occurring in the overlying Krasnoporog Formation (Fig. 6, 7), i.e., the N. sunnaginicus Zone straddles the boundary. An alternative hypothesis would be that first appearances of both archaeocyaths and small shelly faunas are temporally capricious, such that no precise correlation is possible across the Siberian Platform. The similar pattern of archaeocyath ranges in the Igarka and Aldan sections, discussed above, argue against this view. A third view might be that the Sukharikha–Krasnoporog
and Yudoma–Pestrotsvet unconformities both represent the same interval of missing section, but that archaeocyaths and early Tommotian small shelly fossils were environmentally excluded from the Aldan region while they thrived in the Igarka region. This would explain the presence of these taxa in situ in the uppermost Sukharikha Formation and their absence in the uppermost Yudoma Formation (except for the lenses of bed 8). Both of these regions lie within the Anabar– Sinyaya facies belt (Fig. 1), at least in part, and there is no a priori reason that small shelly fossils and archaeocyaths would have been completely excluded from the Aldan region if they existed elsewhere in this facies belt. All of these hypotheses can be tested. The Sukharikha River section is especially promising for resolving problems associated with the ambiguities of the Nemakit–Daldynian – Tommotian boundary. As the only archaeocyath-rich section in the northwestern Siberian Platform, it offers a rare opportunity to test conflicting hypotheses concerning the distribution of archaeocyaths and their possible value as index fossils of the basal Tommotian. To objectively test the hypothesis presented in Fig. 10, and other interpretations concerning the correlation of Vendian and Cambrian strata on the Siberian Platform and beyond, correlation tools that are independent of the fossils themselves must be used. Carbon and strontium chemostratigraphy is the most promising approach. Isotopic reference data are now available from a variety of fossiliferous sections (e.g., Brasier et al. 1994a, 1994c; Kaufman et al. 1996; Knoll et al. 1995a, 1995b), conflicting hypotheses are ready for testing (e.g., Knoll et al. 1995b, 1996; Landing 1996; © 1998 NRC Canada
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this study), and the well-preserved limestones that occur throughout most of the Sukharikha River section (Fig. 3) should contain their original isotopic signatures.
Conclusions The Sukharikha River section is a thick, carbonatedominated, fossiliferous section in a corner of the Siberian Platform which has not yet been well studied. It offers the potential of resolving some fundamental problems of correlation and the temporal and spatial distribution of animals on the Siberian Platform, and it may turn out to be a good candidate to replace the Ulakhan–Sulugur section as the global stratotype for the Nemakit–Daldynian – Tommotian boundary. Carbon and strontium isotopic data are needed to realize this potential.
Acknowledgments We thank Andy Knoll, Ed Landing, and Godfrey Nowlan for insightful reviews that greatly helped improve the clarity of the manuscript, and Marc Rowland for assistance in the field. Fieldwork was supported by a National Science Foundation EPSCoR grant to S.M.R. under Cooperative Agreement 9353227. A.I.T. gratefully acknowledges support from Russian Basic Research Foundation grant number 95-05-14519.
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