Interpreting conodont communities, tectonics and eustasy of the Ordovician – Early Silurian Laurentian margins
Christopher R. Barnes1, Shunxin Zhang1 and Leanne J. Pyle2
1 School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada. E–mail: email@example.com
2 Department of Geological Sciences, Queens University, Kingston, Ontario K7L 3N6.
Key words: Conodont paleoecology. Tectonics. Eustasy. Ordovician–Silurian. Laurentia.
The record of eustatic change in the stratigraphic record continues to attract attention of researchers. For the Lower Paleozoic, many papers address changes ranging from the most extreme trangressive/regressive events down to small scale oscillations reflected in metre-scale lithostratigraphic variations (e.g., Pope and Read, 1998). The causes of eustatic variations have been the subject of many interpretations, with the challenge to discriminate between major factors such as variations in sea-floor spreading, ice volume and climate change, underplating by plumes and superplumes, and regional tectonism and orogenesis (Pousart et al., 1999; Barnes, in press a, b). Lower Paleozoic sea level change is characterized by some of the largest Phanerozoic transgressions and establishment of widespread epeiric seas. Terminal Ordovician glaciation produced major eustatic changes, but its timing and duration remain in question (cf., Brenchley et al., 2003, and Pope and Steffan, 2003). The broadly peneplaned Laurentian craton was especially susceptible to flooding and the development of epeiric seas, which effectively submerged the entire craton during the Late Ordovician. The pattern of regional stratigraphic sequences, punctuated by hiatuses, has allowed general interpretations of the eustatic signal through time. However, precise biostratigraphy and an understanding of the local and regional tectonic processes are required. An additional approach is to establish the pattern of marine biotic communities along a nearshore to offshore environmental gradient and determine the changing spatial and temporal distribution of the communities that will reflect eustatic changes. The present short paper integrates and summarizes a number of such studies recently undertaken, on the ancient margins of Laurentia, now preserved in the Canadian Appalachian and Cordillera orogens, from which eustatic curves can be compared.
The Ordovician-Lower Silurian of the Canadian Appalachians is exposed best in western Newfoundland and Anticosti Island, Quebec. Over 1200 conodont samples have been amassed, yielding over 150,000 conodonts. Following taxonomic and paleoecological publications, more quantitative studies have better defined the conodont communities and interpreted the eustatic signal through long sequences. Lower and Middle Ordovician sequences in western Newfoundland represent the carbonate platform and the continental slope environments with the intervening shelfbreak facies reconstructed from a series of major breccia units. Four cluster analyses were undertaken using 18,468 identifiable conodont specimens from 230 samples from four sections covering the Cambrian-Ordovician boundary interval (Zhang and Barnes, submitted a). A further four cluster analyses involved 69,598 identifiable conodont specimens from 153 samples from four sections covering the Arenigian (Early Ordovician) interval. These eight sections represent different facies of the platform, upper proximal slope, lower proximal slope, and distal slope environments. Eleven and seventeen conodont communities are recognized in the Cambrian-Ordovician and Arenigian intervals, respectively. Their distribution shows some gradational relationships, with evidence of only minor downslope transportation. Eustatic sea level curves derived from the distribution pattern of the evolving conodont communities show that sea level changes affected both the development and replacement of the conodont communities. This study reveals 1) within the Cambrian-Ordovician interval: a) a major sea level drop in the upper C. proavus Zone and b) two major sea level rises in the lower C. lindstromi (or Iapetognathus) and lower C. angulatus zones, respectively; and 2) within the Arenigian interval: a) a gradual transgression lasted most of the T. approximatus Zone, which was followed by a brief regression; b) a transgressive-regressive cycle occurred in the T. akzharensis Zone; c) a major transgression produced a highstand during the entire T. fruticosus Zone, i.e., "O. evae transgression", followed by a major regression in the lower D. bifidus Zone; d) in the I. v. lunatus Zone a series of alternating conodont communities indicates an oscillating sea level; e) a severe regression in the lowest I. v. victoriae Zone is represented by the St. George unconformity on the shelf and by the Bed 12 megaconglomerate on the slope, when sea level gradually dropped to its lowest level during the I. v. maximus Zone. The Late Cambrian-Early Ordovician sea level curve developed by this study reflects a combination of global eustatic changes and local eustatic effects of the early phases of the Taconic Orogeny.
The Upper Ordovician-Lower Silurian interval, of predominantly subtidal carbonates, is superbly exposed on Anticosti Island. Conodont diversity was severely reduced during the terminal Ordovician mass extinction. The nature of the post-extinction recovery and diversification has been previously difficult to assess due to limited knowledge and preservation of earliest Silurian faunas. Recent documentation of early Llandovery Anticosti conodont faunas (Zhang and Barnes, 2002a), together with the data re-examined from earlier studies, has facilitated a new analysis. Five evolutionary cycles are recognized through the Llandovery, together with a distinct set of bioevents that are interpreted as immigration, emigration, origination, and extinction events (Zhang and Barnes, 2002b). These events are supported by the detailed sampling and stratigraphic range data, as well as cladistic analysis of four key genera: Oulodus, Ozarkodina, Pterospathodus, and Rexroadus (Zhang and Barnes, submitted b). Many faunal variations appear to be correlated to eustatic events as well as the changing ocean-climate state through this interval. The complete Anticosti sequence provided a database of over 77,600 conodonts from 272 samples through the 800-1100m sequence. Statistical analysis established the pattern of conodont communities and their interpreted water depths (Zhang and Barnes, 2002b), producing a detailed sea level curve. The curve reveals more sea level oscillations and different water depths for certain intervals than those established earlier. A spatial study (Zhang et al., 2002) of Gun River Formation conodont communities indicates quantitatively that Icriodella deflecta had a nearshore environmental preference, whereas Rexroadus kentuckyensis tended to reside in offshore environments.
In addition to documenting the temporal and spatial distribution of Llandovery communities linked to eustatic change (Zhang and Barnes, 2002b, in press b; Zhang et al., 2002), a set of 107 conodont samples was taken from four cliff sections in the lower and upper Becscie, lower Gun River, and upper Jupiter formations to examine the relationship of conodont elements and communities with microfacies through the Llandovery. Over 9300 conodonts elements were identified and subjected to cluster analysis (Zhang and Barnes, 2002c). The sections comprise mainly the background sedimentation of lime mudstone, deposited in 30-100m of water on a gentle ramp. These mudstones are variably affected by interbeds or channel fill deposits interpreted as tempestites. Different communities are found in the tempestite deposits that do not suggest a reworking of the mudstones, but rather the importation of deeper offshore faunas during periods of storm surge onto the ramp. The tempestites contain the highest species diversity and show no evidence of reduced diversity through post-mortem selective sorting by the storms.
The ancient Cordilleran margin rifted in the latest Neoproterozoic to early Cambrian and experienced repeated phases of extension with minor faulting, volcanism, basin foundering and platform flooding. The stratigraphic and conodont biostratigraphic information from four platform-to-basin transects across this ancient Laurentian margin have greatly advanced our knowledge of the evolution of the Cordilleran margin during the early Paleozoic. In total, 26 stratigraphic sections were studied, over 25 km of strata were measured and described (Pyle and Barnes, 2000, 2001), and over 1,200 conodont samples yielded more than 100,000 conodont elements. Key zonal species enable regional correlation of uppermost Cambrian to Middle Devonian strata along the Cordillera (Pyle and Barnes, 2002a, b, 2003, in press; Pyle et al., 2003). In northeastern British Columbia, two northern transects span the Macdonald Platform to Kechika Trough and Ospika Embayment in the northern Rocky Mountains, and a third transect spans the parautochthonous Cassiar Terrane. In the southern Canadian Rocky Mountains, new Ordovician conodont biostratigraphic data from the Bow Platform is correlated to coeval basinal facies of the White River Trough (Pyle et al., in press). The stratigraphic framework records an evolutionary history of a margin that is more complex than a simple passive margin (Pyle and Barnes, submitted). The detailed conodont biostratigraphy temporally constrains at least two periods of renewed extension along the margin, in the latest Cambrian and late Early Ordovician. A well-defined ancient shelfbreak persisted from the late Early Ordovician to Devonian, possibly due to tectonic steepening. The platform-to-basin succession contains several intervals of slope debris breccia deposits, distal turbidite flows and associated alkalic volcanics. Siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the Peace River Arch. Prominent hiatuses punctuate the succession, including unconformities of latest Ordovician-early Silurian, early-middle Silurian and sub-Devonian age. The pattern of conodont communities is deduced by cluster analysis of the abundant faunas a sea level curve established.
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Pyle, L. J and Barnes, C. R. 2001. Conodonts from the Kechika Formation and Road River Group (Lower to Upper Ordovician) of the Cassiar Terrane, northern British Columbia. Canadian Journal of Earth Sciences, 38: 1387-1401.
Pyle. L. J and Barnes, C. R. 2002. Taxonomy, evolution and biostratigraphy of conodonts from the Kechika Formation, Skoki Formation and Road River Group (Upper Cambrian to Lower Silurian), northeastern British Columbia. National Research Council of Canada Press: 224.
Pyle, L.J. and Barnes, C.R. 2003. Conodonts from a platform-to-basin transect, Lower Ordovician to Lower Silurian, Northeastern British Columbia. Journal of Paleontology, 77: 146-171.
Pyle, L.J., Orchard, M.J., Barnes, C.R. and Landry, M.L. 2003. Biostratigraphy and conodont fauna from the Lower-Middle Devonian Deserters Formation (new), Road River Group, northeastern British Columbia. Canadian Journal of Earth Sciences, 40: 99-113.
Pyle, L.J., Barnes, C.R. and Ji, Z. In press (2003). Conodont fauna of the Outram, Skoki and Owen Creek formations (Lower to Middle Ordovician), Wilcox Pass, Alberta, Canada. Journal of Paleontology, 77.
Pyle, L. J. and C. R. Barnes. Submitted. Early Paleozoic tectonic evolution of the Canadian Cordilleran margin based on regional stratigraphic and biostratigraphic correlations. Canadian Journal of Earth Sciences .
Zhang, S and Barnes, C. R. 2002a. New Llandovery conodont biozonation and pectiniform-ramiform conodonts from the Becscie, Merrimack and Gun River formations, Anticosti Island, Quebec. Journal of Paleontology v. 76, suppl. to Nº 2, Paleontological Society, Memoir 57, 46 p.
Zhang, S. and Barnes, C. R. 2002b. Eustatic sea level curve for the Ashgillian-Llandovery derived from conodont community analysis, Anticosti Island, Québec. Paleogeography, Paleoclimatology, Paleoecology, 180: 5-32.
Zhang, S., and Barnes, C. R. 2002c. Paleoecology of Llandovery conodonts, Anticosti Island, Quebec. Paleogeography, Paleoclimatology, Paleoecology, 180: 33-55.
Zhang, S., Barnes, C.R. and Pohler, S.M.L. 2002. Relationships between lithofacies belts and conodont faunas, Gun River Formation (Lower Silurian), Anticosti Island, Quebec: a statistical approach. Canadian Journal of Earth Sciences, 39: 1767-1782.
Zhang, S. and Barnes, C. R. Submitted a. Late Cambrian and Early Ordovician conodont communities from platform, shelfbreak and slope facies, western Newfoundland: a statistical approach. Special Publication, Geological Society of London.
Zhang, S. and Barnes, C. R. Submitted b. The post-mass extinction bioevents, cladistics and response to glacio-eustasy of conodonts across and after the Ordovician-Silurian boundary, Anticosti Basin, Quebec. Special Publication, Geological Society of London.
Received: February 15, 2003
Accepted: June 15, 2003