Geologica Acta: an international earth science journal ISSN: 1695-6133 [email protected] Universitat de Barcelona España Finney, S. Global Series and Stages for the Ordovician System: A Progress Report Geologica Acta: an international earth science journal, vol. 3, núm. 4, december, 2005, pp. 309-316 Universitat de Barcelona Barcelona, España Available in: http://www.redalyc.org/articulo.oa?id=50530402 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geologica Acta, Vol.3, Nº4, 2005, 309-316 Available online at www.geologica-acta.com Global Series and Stages for the Ordovician System: A Progress Report S. FINNEY Department of Geological Sciences, California State University at Long Beach Long Beach, CA, USA 90840. E-mail: [email protected] ABSTRACT There is no global standard set of chronostratigraphic/geochronologic subdivisions for the Ordovician System/Period. British series/epochs are often used as de facto nomenclature on stratigraphic correlation charts and geologic time scales. However, they were not widely adopted outside of the British Isles because the high degree of biogeographic provincialism and ecologic differentiation of Ordovician faunas prevent the British series from being correlated with precision and high resolution. As a result, several independent and very differ- ent regional sets of series and constituent stages were established for the Ordovician System with each generally applicable to a different paleoplate or modern continent. Of course, this has greatly confused Ordovician chronostratigraphy/geochronology and often results in imprecise correlations of Ordovician stratigraphic suc- cessions. An example is the Argentine Precordillera where British series are traditionally used to correlate Ordovician strata that contain faunas of predominately Laurentian affinity. The Subcommission on Ordovician Stratigraphy of the International Commission on Stratigraphy is addressing this problem by developing a stan- dard set of Global Series and Stages for the Ordovician System. The Subcommission has made considerable progress, but it must complete its work expeditiously. The new global standard will facilitate reliable global cor- relation. It will provide a common language for discussing Ordovician strata, fossils, and geologic events. It will be of fundamental importance in advancing research on Ordovician rocks worldwide. KEYWORDS Ordovician. Chronostratigraphy. Precordillera. Argentina INTRODUCTION organisms included the planktic graptolites, chitinozoans, and acritarchs and the nektic conodonts, nautiloid The Ordovician System has an exceptionally diverse cephalopods, phyllocarid crustaceans, and some trilobites. and rich fossil record. Land plants and animals were vir- Given the widespread distribution of fossil-rich strata, tually non-existent during the Ordovician Period, yet a biostratigraphy should provide a reliable means for pre- great radiation occurred in the marine realm (Webby, cise, high resolution correlations between Ordovician 2004), while several continents were at low and mid pale- stratigraphic successions worldwide. It does, but the situ- olatitudes and extensively covered by warm, shallow ation is very complicated. epeiric seas (Fig. 1). Benthic faunas were dominated by trilobites, brachiopods, corals, bryozoans, sponges, Ordovician continents ranged in size from microcon- bivalves, gastropods, and ostracods. Common pelagic tinents to the super-continent of Gondwana and were dis- © UB-ICTJA 309 S. FINNEY The Ordovician system: A progress report tributed across a wide range of paleolatitude (Fig. 1). As a the same reason, there has been no global standard set of result, Ordovician faunas show a high degree of biogeo- chronostratigraphic/geochronologic units for the Ordovi- graphic provincialism in addition to ecologic differentia- cian System/Period. British series are often used, because tion. Benthic faunas display considerable endemism, and of historical priority, as de facto nomenclature on geolo- thus generally are useful only for regional correlations. gic time scale and stratigraphic correlation charts. How- Planktic graptolites are considered to be cosmopolitan ever, they were not widely adopted outside of the British and are the primary index fossil for global correlations Isles because the British series can not be correlated with (fig. 2.1 in Webby et al., 2004), yet they were differentiat- precision and high resolution. Instead, several indepen- ed into distinct Atlantic (high latitude) and Pacific (low dent and very different regional sets of series and con- latitude) paleobiogeographic provinces during the Late stituent stages were established, each generally applicable Early and Mid Ordovician (Finney and Chen, 1990). In to a different paleoplate or modern continent (Webby, addition, their biostratigraphic usefulness generally is 1998), e.g. Laurentia, Baltica, Avalonia (Britain), North restricted to strata deposited on continental margins (outer Gondwana, Australia, China (Fig. 1). Of course, this has shelf, slope, and rise) and in ocean basins (Finney and greatly complicated Ordovician chronostratigraphy/geo- Berry, 1997). Conodonts are the most useful fossils for chronology, and confused those who do not have exper- correlation of platform deposits, in particular those rich in tise with Ordovician chronostratigraphy yet wish to study endemic benthic faunas, but their potential for global cor- tectonics, paleogeography, and other geologic aspects of relation is also limited by their differentiation into pelagic Ordovician rocks. Consequently, the Subcommission on and nektobenthic habitats and deep/cold and Ordovician Stratigraphy of the International Commission shallow/warm water biofacies. As a result the succession on Stratigraphy was established with the primary goal of of Ordovician conodonts is expressed in terms of two establishing a single, standard framework of Series and very different conodont zonations – the North American Stages for use in global correlation to the highest resolu- Midcontinent Province zonation and the North Atlantic tion possible. The process began with the formal defini- Province zonation (fig. 2.2 in Webby et al., 2004). tion of lower and upper boundaries to the Ordovician Sys- tem and is being followed with determination of Series Accordingly, there is no single zonation that can be and Stages and their boundaries. This paper is a progress used for global correlation of Ordovician successions. For report on the work of the Ordovician Subcommission, and Early Late Ordovician 1 8 3 4 7 2 3 6 5 FIGURE 1 Christopher Scotese’s global paleogeographic reconstruction for Early Late Ordovician time showing the distribution of major lithospheric plates, plate boundaries, continents, oceans, shallow epeiric seas, exposed land, and mountains. Numbered arrows indicate locations of stratotype sections for 1) base of Ordovician System, Lower Ordovician Series, and Tremadoc Stage at Green Point, Newfoundland, Fig. 3A; 2) Base of Second Stage at Diabasbrottet, Sweden, Fig. 3B; 3) Possible bases of Middle Ordovician Series and Third Stage at either Huanghuachang, China or Niquivil, Argentina; 4) Base of Darrwilian Stage at Huangnitang, China, Fig. 3C; 5) Base of Upper Ordovician Series and Fifth Stage at Fågelsång, Sweden, Fig. 3D; 6) base of Sixth Stage at Black Knob Ridge, USA; 7) base of Hirnantian Stage at Wangjiawan, China, Fig. 3E; 8) base of Silurian System (top of Ordovician System) at Dob’s Linn, Scotland. See Fig. 2 for futher detail on the selected biohorizons. Geologica Acta, Vol.3, Nº4, 2005, 309-316 310 S. FINNEY The Ordovician system: A progress report it offers the Ordovician stratigraphy of the Precordillera (Fig. 2), as originally defined (Fortey et al., 1995). As a of northwestern Argentina as an example of the need for result, the biostratigraphic extent of the original series has the Subcommission to complete its work expeditiously. been substantially modified (Fig. 2). The number of series was reduced to five by combining the Llanvirn and lower Llandeilo series into a single Llanvirn Series, and the THE ORDOVICIAN SYSTEM IN BRITAIN boundaries of some series, in particular the base of the Caradoc, were significantly changed. With such substan- In its type area in Britain, the Ordovician System has tial modification, the historical precedence of the British traditionally been divided into six series : Tremadoc, series must be questioned. Furthermore, the redefinition Arenig, Llanvirn, Llandeilo, Caradoc, and Ashgill. As of the British series has been largely conceptual rather originally defined, the British series were based on local than real. Because graptolites are uncommon in type stratigraphic successions and their contained benthic fau- areas of most British series, the boundaries of these series nas, which were highly endemic, except for one series, are not defined on graptolite occurrences in stratigraphic the Llanvirn, which was based on Atlantic province grap- sections. Instead, series boundaries are correlated into tolites. The series were defined on unit stratotypes in geo- graptolite successions elsewhere in the world, which graphically separate areas and with their boundaries often is not precise, or a graptolite zonal level that corre- marked by major unconformities or stratigraphic