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Salterella in the Argentine Precordillera: an Early Cambrian palaeobiogeographic indicator of Laurentian affinity
Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 125 – 132 www.elsevier.com/locate/palaeo
Salterella in the Argentine Precordillera: an Early Cambrian palaeobiogeographic indicator of Laurentian affinity Ricardo A. Astinia,*, William A. Thomasb, Ellis L. Yochelsonc a
CONICET, Ca´tedra de Estratigrafı´a y Geologı´a Histo´rica, Universidad Nacional de Co´rdoba, Av. Velez Sarsfield 1611, Co´rdoba X5016GCA, Argentina b Department of Gelogical Sciences, University of Kentucky, Lexington, KY 40506-0053, United States c Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0121, United States Received 31 October 2003; received in revised form 3 June 2004; accepted 6 July 2004
Abstract This paper documents Salterella maccullochi (Murchison) in the Precordillera of western Argentina in the present-day southern hemisphere. The record of these Early Cambrian small conical-shaped fossils, assigned to the extinct phylum Agmata, provides further palaeobiogeographic evidence to consider the Argentine Precordillera an exotic terrane to Gondwana that originally rifted from Laurentia. For palaeogeographic reconstructions, Salterella can be considered as an organism equally important and biogeographically as meaningful as the olenellid trilobites due to their limited dispersal and demonstrated link to Laurentian shallow-water inner-shelf facies. It has not been found anywhere in Gondwana. Salterella existed for only a very short interval of time, but spread almost instantaneously around Laurentia. On this basis, the Early Cambrian of the Precordillera terrane remains the more conclusive palaeobiogeographical link to Laurentia. Salterella provides another line of evidence supporting palaeontological, sedimentologic, stratigraphic, palaeomagnetic, and isotopic evidence for a Laurentia origin of the Precordillera terrane. D 2004 Elsevier B.V. All rights reserved. Keywords: Salterella; Early Cambrian; Paleobiogeography; Argentine Precordillera; Laurentia
1. Introduction The Lower Cambrian beds of the Argentine Precordillera (Fig. 1A) contain Salterella, a small, * Corresponding author. E-mail addresses: [email protected] (R.A. Astini)8 [email protected] (W.A. Thomas)8 [email protected] (E.L. Yochelson). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.07.008
cone-shaped fossil of the extinct phylum Agmata. During the past decade, a wide range of geological studies (e.g., Astini et al., 1995; Kay et al., 1996; Thomas and Astini, 1996; Astini, 1998; Rapalini and Astini, 1998; Benedetto et al., 1999; Keller, 1999; Thomas et al., 2001) has produced increasingly strong evidence that the Argentine Precordillera was part of the Laurentian continent into Early Cambrian time. However, a contradictory view based on preliminary
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Fig. 1. (A) Location map of the Argentine Precordillera in the present-day Andean region (abbreviations: PR, Precordillera; F, Famatina; WP, Western Pampean Ranges; FC, Frontal Cordillera; PC, Principal Cordillera. Locations: 1, Ancaucha block; 2, Cerro Totora area; 3, Villicum Range in the eastern Precordillera). (B) Time-stratigraphic chart for Cambrian and Ordovician across the Argentine Precordillera (abbreviations: CTF, Cerro Totora Formation; LLF, La Laja Formation; ZF, Zonda Formation; LFF, La Flecha Formation; LSiF, La Silla Formation; SJF, San Juan Formation; LSF, Los Sombreros Formation; modified from Astini, 1998). Ancaucha block (1) with Salterella shown in the stratigraphy of western Precordillera.
analyses of detrital zircons (Finney et al., 2003) has recently questioned the Laurentian origin of the Argentine Precordillera (for a thorough discussion, see Acen˜olaza et al., 2002; Astini and Rapalini, 2003). Cambrian-age platform carbonates overlying Grenville-age basement rocks in the Precordillera first invited direct comparisons to the basement and cover of Laurentia. Salterella is locally abundant in Lower Cambrian beds of North America and other places considered original parts of Laurentia. Except for the new collections from the Precordillera, Salterella is unknown in the present southern hemisphere and has not been recorded in any Gondwanan location. The purpose of this paper is to highlight the importance of Salterella as a palaeobiogeographic indicator strongly linking the Argentine Precordillera to Laurentia.
2. Location of Salterella collections The specimens described and illustrated herein are from the western Precordillera, where an Ordovician slope-facies mudstone (Los Sombreros Formation) contains large olistoliths of the early Paleozoic platform-carbonate strata, as well as older synrift strata and
basement rocks (Fig. 1B). Salterella is identified in limestone beds within the Ancaucha olistolith (Fig. 2) of Lower Cambrian synrift strata (Astini and Thomas, 1999; Astini et al., 2000). The Ancaucha block (olistolith) is more than 500 m in length and includes a stratigraphic succession nearly 80-m thick, similar to that exposed at the base of the autochthonous succession (Cerro Totora Formation) to the east (Fig. 1B). The lower part of the block contains a clastic succession of maroon and red coarse sandstones and siltstones with subaerial features (e.g., wrinkle marks, mud cracks, flat-top wave ripples). The succession grades upward into a mixed carbonate-clastic rhythmic interval, where Salterella and fragmented trilobites are common constituents of graded and massive thin-bedded packstones interbedded with muddy background sediments (Fig. 2). Bed geometries are both tabular and lenticular. One previous short report (Bordonaro and Martos, 1985) identified Salterella from the lower member of the La Laja Formation in the eastern Precordillera (Fig. 1B). The lower member of the La Laja Formation includes shaly beds that grade up to a thick carbonate sequence, indicating evolution of a passive margin across the Precordillera terrane (Astini et al., 1995; Keller, 1999). The Salterella
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further, except to note that small hyoliths may be confused with Salterella, especially if the specimens have been sorted and transported.
3. Description of fossils Salterella maccullochi (Murchison) from the Precordillera comprises a relatively thick calcium carbonate integument with an exterior conical shape, having an angle of expansion near 108, so that shape resembles a pencil point (Fig. 3). Under low magnification, the outer shell wall shows two prominent divisions (sometimes better seen in cross section, e.g., upper center in Fig. 3). The outer layer has several more compact laminations, and from a point about two thirds of the distance from apex to aperture, the laminated section thins toward the rim at the apertural opening (see longitudinal section left center in Fig. 3). The thicker inner division of the cone wall appears more amorphous. From a point about two fifths of the distance between apex and aperture, the inner division thins toward the aperature, and it does not extend beyond the point where the Fig. 2. Stratigraphic columnar section of the Salterella-yielding Early Cambrian olistolith within the Los Sombreros Formation in the western Precordillera (see location in Fig. 1). Note the abundance of shallow and very shallow water features and the mixed nature of sedimentation. Salterella is usually a more abundant component in the thin carbonate layers (modified from Astini et al., 2000).
specimens are associated with olenellid trilobites, indicating an Early Cambrian age (Bonnia–Olenellus Zone). Bordonaro and Martos (1985) described noncurved, sharp-edged, small, cone-like fossils, which they identified as Salterella sensu lato. The specimens range from 8 to 12 mm length and from 2 to 4 mm diameter, and have apical angles between 148 and 248. Bordonaro and Martos (1985) compared the smaller specimens with Salterella mexicana Lochman (1952) and the larger ones with Salterella pulchella Billings (1861); both are now considered subjective synonyms of Salterella maccullochi (Murchison, 1859). Hyolithids from the same outcrops (Bordonaro and Martos, 1985) have maximum dimensions of 24-mm length and 7-mm diameter. The only illustration provided is hand-drawn. We have not restudied their material and cannot comment
Fig. 3. Thin section from the Salterella packstones in the Argentine Precordillera showing Salterella maccullochi (Murchison) in various orientations, including oblique and transverse cross sections. The central tube in longitudinal V-shaped sections and in circular cross section may be seen. To the left, a longitudinal section shows the thinness of the outer layer at the apertural margin and the gradual thinning of the inner deposits toward the apertural cavity. Near the bottom left corner, a cross section of two imbricated (telescoped) specimens is evident. Curved trilobite fragments are also present (CEGH-UNC 20220).
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outer layer begins to thin. Secondary deposits within the cone are seen as concentric circles in cross section and as a contrasting pattern within the lower part of the longitudinal section. Laminae of secondary deposits are at an angle to the outer cone wall. Secondary deposits vary in length along the cone and appear to increase in thickness slightly faster than the cone grows. A central tube is present, which in a perfectly oriented longitudinal section bisects the secondary deposits (Fig. 3). All specimens recovered from the western Precordillera belong to the species Salterella maccullochi (Murchison) (Fig. 3). The outer wall slightly exceeds 10 mm in length and 2 mm in thickness. Growth lines on the exterior surface are represented by transverse rings preserved only in isolated external molds on shale. Laminae in the inner more compact cone are seen, in thin section, to curve concave upward converging toward the outer wall. Neomorphism obscures details among successive layers in some specimens, but two size components seem to alternate. A central tube filled with coarser calcite crystals, extending down from the apertural cavity and apparently not reaching the apex, is readily observed in cross and longitudinal sections. The apertural rim is sharp and well preserved, indicating little postmortem reworking. No apertural cover can be observed. The apertural cavity is small and occupies one third to one fourth of the length of the shell. Secondary dolomite is nearly absent in small specimens and increases with increasing size, as has been noted in specimens from elsewhere (e.g., Griffin and Yochelson, 1975; Osborne and Yochelson, 1991; Yochelson and Kisselev, 2003).
All apparent variations seen in thin section in angle of expansion, geometry of cross section, presence or absence of a central tube, and width (Fig. 3) result from random orientation of specimens relative to the plane of the thin section. The configuration of the conch and the secondary deposits, as well as other morphological features, have been compared to topotype specimens of S. maccullochi from northern Scotland to confirm assignment of the Argentine specimens to this species. Although individuals are abundant, no distinct orientation is evident. Relative abundance has no evident relation to preservation and fabric. Although the specimens are wellpreserved, some are affected by abrasion (Fig. 3), suggesting a shallow-water habitat. Rare telescoping results in a cone-in-cone arrangement. Fragments of trilobites, mostly oriented parallel to bedding, are common but are a minor constituent of the rock. Fig. 3 shows longitudinal, oblique, and transverse cross sections. The range of diameters in cross sections is (the largest about 7 mm in length) mostly related to the position at which the specimen is intercepted in sectioning. Nevertheless, some cross sections show the inner deposits in a bbull’s-eyeQ pattern, and others of the same size do not, demonstrating that larger and smaller specimens are intermixed. Random arrangement of specimens within the limestone, lack of size sorting, and intervals of solution and nondeposition imply relatively gentle water movement, suggesting that the fossils were not transported a long distance.
5. Stratigraphic significance 4. Discussion Although a number of specific names have appeared in the literature, the genus Salterella Billings (1861) (Phylum Agmata Yochelson, 1977; Family Salterellidae Walcott, 1886) seems to contain only two species (Yochelson and Kisselev, 2003). Salterella conulata Clark (1925) (redescribed by Yochelson, 1970) is known from only a few localities. Where S. conulata is found in place, it is only a few meters below Salterella maccullochi. This latter species is exceedingly widespread and abundant. Only S. maccullochi is represented in the specimens from the Precordillera.
In a review of Salterella as an Early Cambrian index fossil (Yochelson, 1977), Fritz and Yochelson (1988) considered geographic distribution and biostratigraphic range within the Early Cambrian, concluding that the genus might be restricted to the medial part of the Bonnia–Olenellus Zone (525 Ma). The association with olenellid trilobites in Argentina (Vaccari, 1994) confirms the age and provides a more restricted time line for correlation. Because Salterella can easily be identified in the field with a hand lens, it has the characteristics of an ideal bguide fossilQ. In the northern Precordillera at Cerro Totora (Fig. 1A), the uppermost part of the Early Cambrian Cerro
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Totora Formation has a lithological association similar to the Salterella-bearing succession in the Ancaucha block. The lower Cerro Totora Formation contains abundant evaporites, interlayered with red clastic rocks and overlain by Olenellus-bearing green shales (Vaccari, 1990; Astini and Vaccari, 1996). The physical stratigraphy of both the Ancaucha and Cerro Totora successions is comparable to that of the Early Cambrian Rome Formation in outcrop and subsurface in the southern Appalachians. Thomas and Astini (1999) have considered these two areas as counterparts from the rifted margin of Laurentia. Strontium isotopic ratios of the evaporites in the Cerro Totora and Rome Formations are indistinguishable and are consistent with Early Cambrian sea water, further confirming the Early Cambrian age of these successions (Thomas et al., 2001). Salterella in the Rome Formation in Alabama (Osborne and Yochelson, 1991) is restricted to several thin beds within a range of not more than 4 m. Although the stratigraphic position closely matches that in the Ancaucha block in the Argentine Precordillera, and the host sedimentary facies are nearly identical, some cautions need to be raised in order not to suggest strict facies specificity or taphonomic artifacts.
high concentration of individuals leads to the speculation that Salterella was gregarious. In North America, Salterella has been found in limestones, sandy limestones, and sandstones, collectively suggesting a shallow-water habitat (Fritz and Yochelson, 1988). The shallow-water setting limits to a small range the possibilities for the habitat of Salterella during life. Inner-detrital shelf belts, carbonate shoals, and lagoonal environments are so far the most likely paleoenvironmental settings where these organisms lived. Common transported and aligned individuals, few of which are imbricated, support the interpretation of a near-shore environment. No evidence supports an infaunal or semi-infaunal habitat, and the organism may have used tentacles to collect grains on which bacteria and algae lived and placed them within the cone to strip off the nutrients (Yochelson, 1977). There is not necessarily any relationship between a high degree of specialization and a short stratigraphic range, but for Salterella, the two points may be linked (Yochelson and Kisselev, 2003). In the Early Cambrian, apart from trilobites, there were few shelled organisms in the near-shore environment and presumably no competition for food or space with which this animal had to contend.
6. Environmental setting and life habitat
7. Palaeogeography
In the Argentine Precordillera, Salterella is mostly concentrated in thin-bedded coarse skeletal packstones with no obvious fabric and minor detrital content. Salterella forms 50–60% of the total volume of some beds. Conchs are generally not imbricated, and they vary from random arrangement to poorly oriented along the long axis. The limestone contains trilobite spines, along with rare glauconite and phosphate grains. The Salterella-rich limestones are found within a silty bioturbated succession containing a few quartzrich sandstone beds with rippled tops. Symmetrical inplane geometry suggests oscillatory water flow. Interference patterns on bed tops and scattered salt casts indicate very shallow water. In addition to the fossil shells in limestone, some of the silty–sandy beds contain a few external molds of Salterella. The packstones are interpreted as hydrodynamic concentrations. However, in the absence of conclusive indications of strong currents or vigorous storms, the
In North America, Salterella maccullochi ranges from Caborca, Mexico, and California, to the Northwest Territories of Canada, from Alabama to Newfoundland, and beyond North America through Greenland and Scotland to Svalbard. Although pelagic organisms tend to be widespread, benthic forms, particularly those inhabiting inner-shelf areas, less commonly have such a wide distribution, except in times when supercontinents with long coastlines existed. To the best of our knowledge, Salterella maccullochi has not been found anywhere in Gondwana (Australia, New Zealand, India, China, Africa, or South America apart from the Argentine Precordillera) or in Siberia. Reports of Salterella from Australia were rejected as spurious (Clark, 1925; Spath, 1936; Kobayashi, 1937). Yochelson (1983) placed Volborthella Schmidt (1888) in synonymy with Salterella. New data (Yochelson and Kisselev,
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2003) now suggest that Volborthella is indeed both slightly morphologically different and stratigraphically slightly older. The distributions of Salterella and Volborthella apparently are mutually exclusive, in that Salterella extends through North America, Greenland, Scotland, and Svalbard, whereas Volborthella extends across Europe from Norway through Estonia to Poland (Yochelson, 1981). Like the restricted Olenellus trilobite fauna (McKerrow et al., 1992), Salterella evidently belongs to the Laurentian faunal realm. Salterella existed for only a very short interval of time, but spread almost instantaneously around Laurentia. This genus may have originated along present-day western Laurentia, which was open to the sea before the opening of Iapetus. Progressively, Salterella invaded the eastern Laurentian margin after the Iapetus Ocean (Fig. 4) had opened enough to represent a barrier to inner-shelf forms. Evolution of the eastern Laurentia–Iapetus passive margin began in the earliest Cambrian (~543 Ma) along the Blue Ridge rift (Thomas, 1991). A widespread passive margin around Laurentia was initiated at approximately 525 Ma (Thomas and
Astini, 1999; Cawood et al., 2001), but before that, Salterella had expanded along the shallow Laurentian shelf during late-stage rifting (Rome facies and analogues). Salterella in Greenland, Svalbard, Scotland, the central and southern Appalachians, and the Precordillera are all in similar shallow-water mixed facies deposited before widespread passive-margin carbonates spread along eastern Laurentia. Both movement of Laurentia into low latitudes soon after break-up (Torsvik et al., 1996; Cocks and Torsvik, 2002) and global eustatic rise led to a climax in carbonate sedimentation, smoothing differences along the Laurentian passive margin and helping to expand the Iapetus shelf biotas (Erwin et al., 1997), possibly leading to a faunal turnover and consequent demise of Salterella. For palaeogeographic reconstructions, Salterella can be considered as an organism equally important and biogeographically as meaningful as the olenellid trilobites due to their limited dispersal and demonstrated link to Laurentian shallow-water inner-shelf facies. The olenellid trilobites of Laurentia show minimal links with the Redlichiid trilobite Realm of
Fig. 4. Schematic paleogeographic reconstruction of Iapetus and bordering continents (modified from Cawood et al., 2001), showing the Precordillera rifted block as suggested by Astini et al. (1995) and Thomas and Astini (1996). Paleoequator and paleopole according to Torsvik and Rehnstrfm (2001) and Torsvik et al. (1996). Abbreviations: SP, South Pole; AM, Amazonia; ANT, Antartica; AUS, Australia, AV, Avalon; BA, Baltica; C-SF, Congo-Sao Francisco; IND, India; K, Kalahari; LAUR, Laurentia; RP, Rı´o de La Plata; SIB, Siberia; WA, West Africa; Pc, Precordillera; Ox, Oaxaca; Ar, Arequipa.
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Australia–Antarctica, both strongly marked for the Early Cambrian (McKerrow et al., 1992). This means that by then, separation of Laurentia and Gondwana had to be sufficiently great for the Laurentian ollenelids to develop in isolation (Dalziel, 1997). From the paleontological viewpoint, the restricted Olenellus trilobite fauna in Precordillera, as pointed by Vaccari (1994) and Astini et al. (1995), is still an irrefutable argument to hold that this region had a connection with Laurentia during the Early Cambrian (Borrello, 1971; McKerrow et al., 1992; Palmer et al., 1996; Astini and Rapalini, 2003). As stated by Benedetto (2003), the inner-shelf Olenellid trilobite Realm in the Argentine Precordillera cannot be explained by patterns of oceanic circulation, due to the fact that dispersion of benthic organisms across oceanic basins can only be achieved, at most, by a few eurytopic genera but never by the fauna as a whole. Independent studies on Mid and Late Cambrian lingulate brachiopods (Holmer et al., 1999) and trilobites (Vaccari, 1994; Bordonaro and Banchig, 1995) recovered from the Precordillera also suggested strong affinity to those found in North America and Greenland, reinforcing either the geographic continuity or the close proximity of the Argentine Precordillera and Laurentia during the rest of the Cambrian. However, the Early Cambrian remains the more conclusive on a palaeobiogeographical basis.
8. Conclusion The distribution of Salterella further strengthens the interpretation of a Laurentian provenance for the Argentine Precordillera terrane. Salterella provides another line of evidence supporting palaeontological, sedimentologic, stratigraphic, palaeomagnetic, and isotopic evidence for rifting of the Precordillera from Laurentia during the Early Cambrian.
Acknowledgements We are grateful to the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (PIP-2000-02971 CONICET), the Agencia Nacional de Promocio´n de Ciencia y Tecnologı´a (PICT-2002/07-11741 FONCYT) and the Secretarı´a de Ciencia y Tecnologı´a of
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the Universidad Nacional de Co´rdoba (62/03 SECyTUNC) that fund our Research Projects in west Argentina. Part of the research in Argentina was supported by a grant from the National Science Foundation (EAR-0229522). We thank Duncan McIlroy and Gregory Edgecombe for their helpful reviews. Peter Cawood and Juan Luis Benedetto provided encouragement after reading an early version of the manuscript. References Acen˜olaza, F.G., Miller, H., Toselli, A.J., 2002. Proterozoic–Early Paleozoic evolution in western South America—a discussion. Tectonophysics 354, 121 – 137. Astini, R.A., 1998. Stratigraphical evidence supporting the rifting, drifting and collision of the Laurentian Precordillera terrane of western Argentina. In: Pankhurst, R.J., Rapela, C.W. (Eds.), The Proto-Andean Margin of Gondwana. Spec. Publ.-Geol. Soc. Lond., vol. 142, pp. 11 – 33. Astini, R.A., Rapalini, A.E., 2003. Proterozoic–Early Paleozoic evolution in western South America—a discussion. Tectonophysics 366, 143 – 148. Astini, R.A., Thomas, W.A., 1999. Un bloque Ca´mbrico Inferior en la Precordillera occidental de San Juan con afinidad apalachiana: Nueva evidencia de la vinculacio´n PrecordilleraLaurentia. Proc. 14th Congreso Geolœgico Argentino, Artes Gra´ficas, Salta, Argentina, pp. 56. Astini, R.A., Vaccari, N.E., 1996. Sucesio´ n evaporı´tica del Ca´mbrico Inferior de la Precordillera: significado geolo´gico. Rev. Asoc. Geol. Argent. (Argentina) 51, 97 – 106. Astini, R.A., Benedetto, J.L., Vaccari, N.E., 1995. The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: a geodynamic model. Geol. Soc. Amer. Bull. 107, 253 – 273. Astini, R.A., Ma´ngano, M.G., Thomas, W.A., 2000. El icnoge´nero Cruziana en el Ca´ mbrico Temprano de la Precordillera Argentina: el registro ma´s antiguo de Sudame´rica. Rev. Asoc. Geol. Argent. (Argentina) 55, 111 – 120. Benedetto, J.L., 2003. Paleobiogeography. In: Benedetto, J.L. (Ed.), Ordovician Fossils of Argentina, Secretarı´a de Ciencias y Tecnologı´a, Universidad Nacional de Co´rdoba, Argentina, pp. 75 – 90. Benedetto, J.L., Sa´nchez, T.M., Carrera, M.G., Brussa, E.D., Salas, M.J., 1999. Paleontological constraints on successive paleogeographic positions of Precordillera terrane during the early Paleozoic. In: Ramos, V.A., Keppie, J.D. (Eds.), Laurentia– Gondwana Conections Before Pangea. Special Paper, Published by Geological Society of America Inc., Boulder, CO, vol. 336. pp. 21 – 42. Billings, E., 1861. On some new or little known species of Silurian fossils from the Potsdam group (Primordial zone). In: Hitchcock, C., et al. (Eds.), Report on the Geology of Vermont, Descriptive, Theoretical, Economical, and Scenographical, vol. 2. Claremont Manufacturing Company, Claremont, New Hampshire, pp. 942 – 955. Published under the authority of legislature.
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chian Fold–Thrust belt, Central Alabama. Southeast. Geol. 32, 107 – 114. Palmer, A.R., Vaccari, N.E., Astini, R.A., Can˜as, F.L., Benedetto, J.L., 1996. Cambrian history of the Argentine Precordillera: Adios Laurentia! Geol. Soc. Amer. 28, 57 (Abst. with Prog). Rapalini, A.E., Astini, R.A., 1998. Paleomagnetic confirmation of the allochthony of the Argentine Precordillera. EPSL 155, 1 – 14. ¨ ber eine neuentadeckte untercambrische Fauna Schmidt, F., 1888. U in Estland. Acade´mie Impe´riale des Sci. de St. Pete´rsbourg Me´moires, 7, 36 (2), 1–27. Spath, L.F., 1936. So-called Salterella from the Cambrian of Australia. Geol. Mag. 73, 433 – 440. Thomas, W.A., 1991. The Appalachian–Ouachita rifted margin of southeastern North America. Geol. Soc. Amer. Bull. 103, 415 – 431. Thomas, W.A., Astini, R.A., 1996. The Argentine Precordillera: a traveler from the Ouachita embayment of North American Laurentia. Science 273, 752 – 757. Thomas, W.A., Astini, R.A., 1999. Simple-shear conjugate rift margins of the Argentine Precordillera and the Ouachita embayment of Laurentia. Geol. Soc. Amer. Bull. 111, 1069 – 1079. Thomas, W.A., Astini, R.A., Denison, R.E., 2001. Strontium isotopes, age, and tectonic setting of Cambrian salinas along the rift and transform margins of the Argentine Precordillera and southern Laurentia. J. Geol. 109, 231 – 246. Torsvik, T.H., Rehnstrfm, E.F., 2001. Cambrian paleomagnetic data from Baltica: implications for true polar wander and Cambrian paleogeography. J. Geol. Soc. (Lond.) 158, 321 – 329. Torsvik, T.H., Smethurst, M.A., Meert, J.G., Vand der Voo, R., McKerrow, W.S., Brasier, M.D., Sturt, B.A., Walderhaug, H.J., 1996. Continental break-up and collision in the Neproterozoic and Paleozoic—a tale of Baltica and Laurentia. Earth-Sci. Rev. 40, 229 – 258. Vaccari, N.E., 1990. Primer hallazgo de trilobites del Ca´mbrico Inferior en la Provincia de La Rioja (Precordillera Septentrional). Rev. Asoc. Geol. Argent. (Argentina) 43, 558 – 561. Vaccari, N.E., 1994. Las faunas de trilobites de lasa sucesiones carbona´ticas del Ca´mbrico y Ordovı´cico Temprano de la precordillera septentrional, Repu´blica Argentina. PhD thesis, Universidad Nacional de Co´rdoba, 271 p. Walcott, C.D., 1886. Second contribution to the studies on the Cambrian faunas of North America, United States. Geol. Surv. Bull. 30, 1 – 369. Yochelson, E.L., 1970. The Early Cambrian fossil Salterella conulata Clark in eastern North America. U. S. Geol. Surv. Prof. Pap. 638-B B1–B10. Yochelson, E.L., 1977. Agmata, a proposed extinct phylum of Early Cambrian age. J. Paleontol. 51, 437 – 454. Yochelson, E.L., 1981. A survey of Salterella (Phylum Agmata): short papers for the Second International Symposium on the Cambrian System. U. S. Geol. Surv. Open-file Report 81-743, 244 – 248. Yochelson, L.E., 1983. Salterella (Early Cambrian: Agmata) from the Scottish Highlands. Palaeontology 26, 253 – 260. Yochelson, L.E., Kisselev, G.N., 2003. Early Cambrian Salterella and Volborthella (Phylum Agmata) re-evaluated. Lethaia 36, 8 – 20.
Salterella in the Argentine Precordillera: an Early Cambrian palaeobiogeographic indicator of Laurentian affinity Ricardo A. Astinia,*, William A. Thomasb, Ellis L. Yochelsonc a
CONICET, Ca´tedra de Estratigrafı´a y Geologı´a Histo´rica, Universidad Nacional de Co´rdoba, Av. Velez Sarsfield 1611, Co´rdoba X5016GCA, Argentina b Department of Gelogical Sciences, University of Kentucky, Lexington, KY 40506-0053, United States c Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0121, United States Received 31 October 2003; received in revised form 3 June 2004; accepted 6 July 2004
Abstract This paper documents Salterella maccullochi (Murchison) in the Precordillera of western Argentina in the present-day southern hemisphere. The record of these Early Cambrian small conical-shaped fossils, assigned to the extinct phylum Agmata, provides further palaeobiogeographic evidence to consider the Argentine Precordillera an exotic terrane to Gondwana that originally rifted from Laurentia. For palaeogeographic reconstructions, Salterella can be considered as an organism equally important and biogeographically as meaningful as the olenellid trilobites due to their limited dispersal and demonstrated link to Laurentian shallow-water inner-shelf facies. It has not been found anywhere in Gondwana. Salterella existed for only a very short interval of time, but spread almost instantaneously around Laurentia. On this basis, the Early Cambrian of the Precordillera terrane remains the more conclusive palaeobiogeographical link to Laurentia. Salterella provides another line of evidence supporting palaeontological, sedimentologic, stratigraphic, palaeomagnetic, and isotopic evidence for a Laurentia origin of the Precordillera terrane. D 2004 Elsevier B.V. All rights reserved. Keywords: Salterella; Early Cambrian; Paleobiogeography; Argentine Precordillera; Laurentia
1. Introduction The Lower Cambrian beds of the Argentine Precordillera (Fig. 1A) contain Salterella, a small, * Corresponding author. E-mail addresses: [email protected] (R.A. Astini)8 [email protected] (W.A. Thomas)8 [email protected] (E.L. Yochelson). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.07.008
cone-shaped fossil of the extinct phylum Agmata. During the past decade, a wide range of geological studies (e.g., Astini et al., 1995; Kay et al., 1996; Thomas and Astini, 1996; Astini, 1998; Rapalini and Astini, 1998; Benedetto et al., 1999; Keller, 1999; Thomas et al., 2001) has produced increasingly strong evidence that the Argentine Precordillera was part of the Laurentian continent into Early Cambrian time. However, a contradictory view based on preliminary
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Fig. 1. (A) Location map of the Argentine Precordillera in the present-day Andean region (abbreviations: PR, Precordillera; F, Famatina; WP, Western Pampean Ranges; FC, Frontal Cordillera; PC, Principal Cordillera. Locations: 1, Ancaucha block; 2, Cerro Totora area; 3, Villicum Range in the eastern Precordillera). (B) Time-stratigraphic chart for Cambrian and Ordovician across the Argentine Precordillera (abbreviations: CTF, Cerro Totora Formation; LLF, La Laja Formation; ZF, Zonda Formation; LFF, La Flecha Formation; LSiF, La Silla Formation; SJF, San Juan Formation; LSF, Los Sombreros Formation; modified from Astini, 1998). Ancaucha block (1) with Salterella shown in the stratigraphy of western Precordillera.
analyses of detrital zircons (Finney et al., 2003) has recently questioned the Laurentian origin of the Argentine Precordillera (for a thorough discussion, see Acen˜olaza et al., 2002; Astini and Rapalini, 2003). Cambrian-age platform carbonates overlying Grenville-age basement rocks in the Precordillera first invited direct comparisons to the basement and cover of Laurentia. Salterella is locally abundant in Lower Cambrian beds of North America and other places considered original parts of Laurentia. Except for the new collections from the Precordillera, Salterella is unknown in the present southern hemisphere and has not been recorded in any Gondwanan location. The purpose of this paper is to highlight the importance of Salterella as a palaeobiogeographic indicator strongly linking the Argentine Precordillera to Laurentia.
2. Location of Salterella collections The specimens described and illustrated herein are from the western Precordillera, where an Ordovician slope-facies mudstone (Los Sombreros Formation) contains large olistoliths of the early Paleozoic platform-carbonate strata, as well as older synrift strata and
basement rocks (Fig. 1B). Salterella is identified in limestone beds within the Ancaucha olistolith (Fig. 2) of Lower Cambrian synrift strata (Astini and Thomas, 1999; Astini et al., 2000). The Ancaucha block (olistolith) is more than 500 m in length and includes a stratigraphic succession nearly 80-m thick, similar to that exposed at the base of the autochthonous succession (Cerro Totora Formation) to the east (Fig. 1B). The lower part of the block contains a clastic succession of maroon and red coarse sandstones and siltstones with subaerial features (e.g., wrinkle marks, mud cracks, flat-top wave ripples). The succession grades upward into a mixed carbonate-clastic rhythmic interval, where Salterella and fragmented trilobites are common constituents of graded and massive thin-bedded packstones interbedded with muddy background sediments (Fig. 2). Bed geometries are both tabular and lenticular. One previous short report (Bordonaro and Martos, 1985) identified Salterella from the lower member of the La Laja Formation in the eastern Precordillera (Fig. 1B). The lower member of the La Laja Formation includes shaly beds that grade up to a thick carbonate sequence, indicating evolution of a passive margin across the Precordillera terrane (Astini et al., 1995; Keller, 1999). The Salterella
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further, except to note that small hyoliths may be confused with Salterella, especially if the specimens have been sorted and transported.
3. Description of fossils Salterella maccullochi (Murchison) from the Precordillera comprises a relatively thick calcium carbonate integument with an exterior conical shape, having an angle of expansion near 108, so that shape resembles a pencil point (Fig. 3). Under low magnification, the outer shell wall shows two prominent divisions (sometimes better seen in cross section, e.g., upper center in Fig. 3). The outer layer has several more compact laminations, and from a point about two thirds of the distance from apex to aperture, the laminated section thins toward the rim at the apertural opening (see longitudinal section left center in Fig. 3). The thicker inner division of the cone wall appears more amorphous. From a point about two fifths of the distance between apex and aperture, the inner division thins toward the aperature, and it does not extend beyond the point where the Fig. 2. Stratigraphic columnar section of the Salterella-yielding Early Cambrian olistolith within the Los Sombreros Formation in the western Precordillera (see location in Fig. 1). Note the abundance of shallow and very shallow water features and the mixed nature of sedimentation. Salterella is usually a more abundant component in the thin carbonate layers (modified from Astini et al., 2000).
specimens are associated with olenellid trilobites, indicating an Early Cambrian age (Bonnia–Olenellus Zone). Bordonaro and Martos (1985) described noncurved, sharp-edged, small, cone-like fossils, which they identified as Salterella sensu lato. The specimens range from 8 to 12 mm length and from 2 to 4 mm diameter, and have apical angles between 148 and 248. Bordonaro and Martos (1985) compared the smaller specimens with Salterella mexicana Lochman (1952) and the larger ones with Salterella pulchella Billings (1861); both are now considered subjective synonyms of Salterella maccullochi (Murchison, 1859). Hyolithids from the same outcrops (Bordonaro and Martos, 1985) have maximum dimensions of 24-mm length and 7-mm diameter. The only illustration provided is hand-drawn. We have not restudied their material and cannot comment
Fig. 3. Thin section from the Salterella packstones in the Argentine Precordillera showing Salterella maccullochi (Murchison) in various orientations, including oblique and transverse cross sections. The central tube in longitudinal V-shaped sections and in circular cross section may be seen. To the left, a longitudinal section shows the thinness of the outer layer at the apertural margin and the gradual thinning of the inner deposits toward the apertural cavity. Near the bottom left corner, a cross section of two imbricated (telescoped) specimens is evident. Curved trilobite fragments are also present (CEGH-UNC 20220).
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outer layer begins to thin. Secondary deposits within the cone are seen as concentric circles in cross section and as a contrasting pattern within the lower part of the longitudinal section. Laminae of secondary deposits are at an angle to the outer cone wall. Secondary deposits vary in length along the cone and appear to increase in thickness slightly faster than the cone grows. A central tube is present, which in a perfectly oriented longitudinal section bisects the secondary deposits (Fig. 3). All specimens recovered from the western Precordillera belong to the species Salterella maccullochi (Murchison) (Fig. 3). The outer wall slightly exceeds 10 mm in length and 2 mm in thickness. Growth lines on the exterior surface are represented by transverse rings preserved only in isolated external molds on shale. Laminae in the inner more compact cone are seen, in thin section, to curve concave upward converging toward the outer wall. Neomorphism obscures details among successive layers in some specimens, but two size components seem to alternate. A central tube filled with coarser calcite crystals, extending down from the apertural cavity and apparently not reaching the apex, is readily observed in cross and longitudinal sections. The apertural rim is sharp and well preserved, indicating little postmortem reworking. No apertural cover can be observed. The apertural cavity is small and occupies one third to one fourth of the length of the shell. Secondary dolomite is nearly absent in small specimens and increases with increasing size, as has been noted in specimens from elsewhere (e.g., Griffin and Yochelson, 1975; Osborne and Yochelson, 1991; Yochelson and Kisselev, 2003).
All apparent variations seen in thin section in angle of expansion, geometry of cross section, presence or absence of a central tube, and width (Fig. 3) result from random orientation of specimens relative to the plane of the thin section. The configuration of the conch and the secondary deposits, as well as other morphological features, have been compared to topotype specimens of S. maccullochi from northern Scotland to confirm assignment of the Argentine specimens to this species. Although individuals are abundant, no distinct orientation is evident. Relative abundance has no evident relation to preservation and fabric. Although the specimens are wellpreserved, some are affected by abrasion (Fig. 3), suggesting a shallow-water habitat. Rare telescoping results in a cone-in-cone arrangement. Fragments of trilobites, mostly oriented parallel to bedding, are common but are a minor constituent of the rock. Fig. 3 shows longitudinal, oblique, and transverse cross sections. The range of diameters in cross sections is (the largest about 7 mm in length) mostly related to the position at which the specimen is intercepted in sectioning. Nevertheless, some cross sections show the inner deposits in a bbull’s-eyeQ pattern, and others of the same size do not, demonstrating that larger and smaller specimens are intermixed. Random arrangement of specimens within the limestone, lack of size sorting, and intervals of solution and nondeposition imply relatively gentle water movement, suggesting that the fossils were not transported a long distance.
5. Stratigraphic significance 4. Discussion Although a number of specific names have appeared in the literature, the genus Salterella Billings (1861) (Phylum Agmata Yochelson, 1977; Family Salterellidae Walcott, 1886) seems to contain only two species (Yochelson and Kisselev, 2003). Salterella conulata Clark (1925) (redescribed by Yochelson, 1970) is known from only a few localities. Where S. conulata is found in place, it is only a few meters below Salterella maccullochi. This latter species is exceedingly widespread and abundant. Only S. maccullochi is represented in the specimens from the Precordillera.
In a review of Salterella as an Early Cambrian index fossil (Yochelson, 1977), Fritz and Yochelson (1988) considered geographic distribution and biostratigraphic range within the Early Cambrian, concluding that the genus might be restricted to the medial part of the Bonnia–Olenellus Zone (525 Ma). The association with olenellid trilobites in Argentina (Vaccari, 1994) confirms the age and provides a more restricted time line for correlation. Because Salterella can easily be identified in the field with a hand lens, it has the characteristics of an ideal bguide fossilQ. In the northern Precordillera at Cerro Totora (Fig. 1A), the uppermost part of the Early Cambrian Cerro
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Totora Formation has a lithological association similar to the Salterella-bearing succession in the Ancaucha block. The lower Cerro Totora Formation contains abundant evaporites, interlayered with red clastic rocks and overlain by Olenellus-bearing green shales (Vaccari, 1990; Astini and Vaccari, 1996). The physical stratigraphy of both the Ancaucha and Cerro Totora successions is comparable to that of the Early Cambrian Rome Formation in outcrop and subsurface in the southern Appalachians. Thomas and Astini (1999) have considered these two areas as counterparts from the rifted margin of Laurentia. Strontium isotopic ratios of the evaporites in the Cerro Totora and Rome Formations are indistinguishable and are consistent with Early Cambrian sea water, further confirming the Early Cambrian age of these successions (Thomas et al., 2001). Salterella in the Rome Formation in Alabama (Osborne and Yochelson, 1991) is restricted to several thin beds within a range of not more than 4 m. Although the stratigraphic position closely matches that in the Ancaucha block in the Argentine Precordillera, and the host sedimentary facies are nearly identical, some cautions need to be raised in order not to suggest strict facies specificity or taphonomic artifacts.
high concentration of individuals leads to the speculation that Salterella was gregarious. In North America, Salterella has been found in limestones, sandy limestones, and sandstones, collectively suggesting a shallow-water habitat (Fritz and Yochelson, 1988). The shallow-water setting limits to a small range the possibilities for the habitat of Salterella during life. Inner-detrital shelf belts, carbonate shoals, and lagoonal environments are so far the most likely paleoenvironmental settings where these organisms lived. Common transported and aligned individuals, few of which are imbricated, support the interpretation of a near-shore environment. No evidence supports an infaunal or semi-infaunal habitat, and the organism may have used tentacles to collect grains on which bacteria and algae lived and placed them within the cone to strip off the nutrients (Yochelson, 1977). There is not necessarily any relationship between a high degree of specialization and a short stratigraphic range, but for Salterella, the two points may be linked (Yochelson and Kisselev, 2003). In the Early Cambrian, apart from trilobites, there were few shelled organisms in the near-shore environment and presumably no competition for food or space with which this animal had to contend.
6. Environmental setting and life habitat
7. Palaeogeography
In the Argentine Precordillera, Salterella is mostly concentrated in thin-bedded coarse skeletal packstones with no obvious fabric and minor detrital content. Salterella forms 50–60% of the total volume of some beds. Conchs are generally not imbricated, and they vary from random arrangement to poorly oriented along the long axis. The limestone contains trilobite spines, along with rare glauconite and phosphate grains. The Salterella-rich limestones are found within a silty bioturbated succession containing a few quartzrich sandstone beds with rippled tops. Symmetrical inplane geometry suggests oscillatory water flow. Interference patterns on bed tops and scattered salt casts indicate very shallow water. In addition to the fossil shells in limestone, some of the silty–sandy beds contain a few external molds of Salterella. The packstones are interpreted as hydrodynamic concentrations. However, in the absence of conclusive indications of strong currents or vigorous storms, the
In North America, Salterella maccullochi ranges from Caborca, Mexico, and California, to the Northwest Territories of Canada, from Alabama to Newfoundland, and beyond North America through Greenland and Scotland to Svalbard. Although pelagic organisms tend to be widespread, benthic forms, particularly those inhabiting inner-shelf areas, less commonly have such a wide distribution, except in times when supercontinents with long coastlines existed. To the best of our knowledge, Salterella maccullochi has not been found anywhere in Gondwana (Australia, New Zealand, India, China, Africa, or South America apart from the Argentine Precordillera) or in Siberia. Reports of Salterella from Australia were rejected as spurious (Clark, 1925; Spath, 1936; Kobayashi, 1937). Yochelson (1983) placed Volborthella Schmidt (1888) in synonymy with Salterella. New data (Yochelson and Kisselev,
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2003) now suggest that Volborthella is indeed both slightly morphologically different and stratigraphically slightly older. The distributions of Salterella and Volborthella apparently are mutually exclusive, in that Salterella extends through North America, Greenland, Scotland, and Svalbard, whereas Volborthella extends across Europe from Norway through Estonia to Poland (Yochelson, 1981). Like the restricted Olenellus trilobite fauna (McKerrow et al., 1992), Salterella evidently belongs to the Laurentian faunal realm. Salterella existed for only a very short interval of time, but spread almost instantaneously around Laurentia. This genus may have originated along present-day western Laurentia, which was open to the sea before the opening of Iapetus. Progressively, Salterella invaded the eastern Laurentian margin after the Iapetus Ocean (Fig. 4) had opened enough to represent a barrier to inner-shelf forms. Evolution of the eastern Laurentia–Iapetus passive margin began in the earliest Cambrian (~543 Ma) along the Blue Ridge rift (Thomas, 1991). A widespread passive margin around Laurentia was initiated at approximately 525 Ma (Thomas and
Astini, 1999; Cawood et al., 2001), but before that, Salterella had expanded along the shallow Laurentian shelf during late-stage rifting (Rome facies and analogues). Salterella in Greenland, Svalbard, Scotland, the central and southern Appalachians, and the Precordillera are all in similar shallow-water mixed facies deposited before widespread passive-margin carbonates spread along eastern Laurentia. Both movement of Laurentia into low latitudes soon after break-up (Torsvik et al., 1996; Cocks and Torsvik, 2002) and global eustatic rise led to a climax in carbonate sedimentation, smoothing differences along the Laurentian passive margin and helping to expand the Iapetus shelf biotas (Erwin et al., 1997), possibly leading to a faunal turnover and consequent demise of Salterella. For palaeogeographic reconstructions, Salterella can be considered as an organism equally important and biogeographically as meaningful as the olenellid trilobites due to their limited dispersal and demonstrated link to Laurentian shallow-water inner-shelf facies. The olenellid trilobites of Laurentia show minimal links with the Redlichiid trilobite Realm of
Fig. 4. Schematic paleogeographic reconstruction of Iapetus and bordering continents (modified from Cawood et al., 2001), showing the Precordillera rifted block as suggested by Astini et al. (1995) and Thomas and Astini (1996). Paleoequator and paleopole according to Torsvik and Rehnstrfm (2001) and Torsvik et al. (1996). Abbreviations: SP, South Pole; AM, Amazonia; ANT, Antartica; AUS, Australia, AV, Avalon; BA, Baltica; C-SF, Congo-Sao Francisco; IND, India; K, Kalahari; LAUR, Laurentia; RP, Rı´o de La Plata; SIB, Siberia; WA, West Africa; Pc, Precordillera; Ox, Oaxaca; Ar, Arequipa.
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Australia–Antarctica, both strongly marked for the Early Cambrian (McKerrow et al., 1992). This means that by then, separation of Laurentia and Gondwana had to be sufficiently great for the Laurentian ollenelids to develop in isolation (Dalziel, 1997). From the paleontological viewpoint, the restricted Olenellus trilobite fauna in Precordillera, as pointed by Vaccari (1994) and Astini et al. (1995), is still an irrefutable argument to hold that this region had a connection with Laurentia during the Early Cambrian (Borrello, 1971; McKerrow et al., 1992; Palmer et al., 1996; Astini and Rapalini, 2003). As stated by Benedetto (2003), the inner-shelf Olenellid trilobite Realm in the Argentine Precordillera cannot be explained by patterns of oceanic circulation, due to the fact that dispersion of benthic organisms across oceanic basins can only be achieved, at most, by a few eurytopic genera but never by the fauna as a whole. Independent studies on Mid and Late Cambrian lingulate brachiopods (Holmer et al., 1999) and trilobites (Vaccari, 1994; Bordonaro and Banchig, 1995) recovered from the Precordillera also suggested strong affinity to those found in North America and Greenland, reinforcing either the geographic continuity or the close proximity of the Argentine Precordillera and Laurentia during the rest of the Cambrian. However, the Early Cambrian remains the more conclusive on a palaeobiogeographical basis.
8. Conclusion The distribution of Salterella further strengthens the interpretation of a Laurentian provenance for the Argentine Precordillera terrane. Salterella provides another line of evidence supporting palaeontological, sedimentologic, stratigraphic, palaeomagnetic, and isotopic evidence for rifting of the Precordillera from Laurentia during the Early Cambrian.
Acknowledgements We are grateful to the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (PIP-2000-02971 CONICET), the Agencia Nacional de Promocio´n de Ciencia y Tecnologı´a (PICT-2002/07-11741 FONCYT) and the Secretarı´a de Ciencia y Tecnologı´a of
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the Universidad Nacional de Co´rdoba (62/03 SECyTUNC) that fund our Research Projects in west Argentina. Part of the research in Argentina was supported by a grant from the National Science Foundation (EAR-0229522). We thank Duncan McIlroy and Gregory Edgecombe for their helpful reviews. Peter Cawood and Juan Luis Benedetto provided encouragement after reading an early version of the manuscript. References Acen˜olaza, F.G., Miller, H., Toselli, A.J., 2002. Proterozoic–Early Paleozoic evolution in western South America—a discussion. Tectonophysics 354, 121 – 137. Astini, R.A., 1998. Stratigraphical evidence supporting the rifting, drifting and collision of the Laurentian Precordillera terrane of western Argentina. In: Pankhurst, R.J., Rapela, C.W. (Eds.), The Proto-Andean Margin of Gondwana. Spec. Publ.-Geol. Soc. Lond., vol. 142, pp. 11 – 33. Astini, R.A., Rapalini, A.E., 2003. Proterozoic–Early Paleozoic evolution in western South America—a discussion. Tectonophysics 366, 143 – 148. Astini, R.A., Thomas, W.A., 1999. Un bloque Ca´mbrico Inferior en la Precordillera occidental de San Juan con afinidad apalachiana: Nueva evidencia de la vinculacio´n PrecordilleraLaurentia. Proc. 14th Congreso Geolœgico Argentino, Artes Gra´ficas, Salta, Argentina, pp. 56. Astini, R.A., Vaccari, N.E., 1996. Sucesio´ n evaporı´tica del Ca´mbrico Inferior de la Precordillera: significado geolo´gico. Rev. Asoc. Geol. Argent. (Argentina) 51, 97 – 106. Astini, R.A., Benedetto, J.L., Vaccari, N.E., 1995. The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: a geodynamic model. Geol. Soc. Amer. Bull. 107, 253 – 273. Astini, R.A., Ma´ngano, M.G., Thomas, W.A., 2000. El icnoge´nero Cruziana en el Ca´ mbrico Temprano de la Precordillera Argentina: el registro ma´s antiguo de Sudame´rica. Rev. Asoc. Geol. Argent. (Argentina) 55, 111 – 120. Benedetto, J.L., 2003. Paleobiogeography. In: Benedetto, J.L. (Ed.), Ordovician Fossils of Argentina, Secretarı´a de Ciencias y Tecnologı´a, Universidad Nacional de Co´rdoba, Argentina, pp. 75 – 90. Benedetto, J.L., Sa´nchez, T.M., Carrera, M.G., Brussa, E.D., Salas, M.J., 1999. Paleontological constraints on successive paleogeographic positions of Precordillera terrane during the early Paleozoic. In: Ramos, V.A., Keppie, J.D. (Eds.), Laurentia– Gondwana Conections Before Pangea. Special Paper, Published by Geological Society of America Inc., Boulder, CO, vol. 336. pp. 21 – 42. Billings, E., 1861. On some new or little known species of Silurian fossils from the Potsdam group (Primordial zone). In: Hitchcock, C., et al. (Eds.), Report on the Geology of Vermont, Descriptive, Theoretical, Economical, and Scenographical, vol. 2. Claremont Manufacturing Company, Claremont, New Hampshire, pp. 942 – 955. Published under the authority of legislature.
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