to Devonian graptolite distributions along the Cordilleran margin of Laurentia
C. Finney1 and William B.N. Berry2
of Geological Sciences, California State University, Long Beach, CA 90840, USA.
2 Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA.
Key words: Graptolites. Laurentia. Ordovician. Silurian. Devonian.
Graptolites have been collected for more than 150 years from Ordovician-Devonian strata that accumulated along the Cordilleran margin of Laurentia and today are exposed extensively in the mountains of western North America, that is, in the Great Basin of Nevada, Utah, eastern California, and Idaho, as well as in eastern Washington, British Columbia, and Sonora. We have been studying these graptolites since the 1960s for our own research interests and also to provide age determinations for U.S. Geological Survey and mining company geologists. We note that graptolite occurrences, as well as diversity and abundance, vary significantly through the stratigraphic succession and across depositional facies, yet these distributions are well known and predictable, being limited to specific stratigraphic intervals and depositional facies that persist great distances along depositional strike. New collections present no surprises; they merely duplicate what is already known in terms of graptolite distributions. We conclude that these occurrences reflect the original living distribution of graptolites which were controlled by environmental (oceanic) conditions that varied over time and across the continental margin and that, in turn, were affected by profound changes to the continental margin.
Figure 1 shows the number of graptolite species in each depositional setting in a transect of the continental margin and in each zone from the lowest Ordovician to the Lower Devonian. These numbers indicate relative diversity, but by no means are they to be taken as accurate measures of biodiversity to be used, for example, in calculating rates of species origination or extinction. Zonal duration, sampling intensity, and biostratigraphic ranges vary significantly among zones, localities, and species, and these factors were not taken into acocunt in deriving species numbers per zone per depositional setting. On the other hand, these numbers accurately reflect the number of species typically found within a zone within a depositional setting based on our experience. This information was taken from a great number of sources with the most important being Ross and Berry’s (1963) monographic study of all Ordovician graptolites from the Basin Ranges of the western United States in the collections of the U.S. Geological Survey and the Departments of Geology at the University of California at Los Angeles and Utah State University and made in the period 1872-1958. Other sources of information are Finney and Perry (1991), Finney et al. (1997, 1999), Kay (1962), McKee (1976), and Mitchell (1991), and unpublished collections of Finney and Berry for Ordovician graptolites of Nevada and eastern California; Berry and Murphy (1975) and Finney (unpublished data) for Silurian and Devonian graptolites of Nevada; Braithwaite (l976) for Ordovician graptolites of Utah; Bartolini et al., (1995) and Riva and Ketner (1989) for Ordovician graptolites of Sonora, Mexico; Carter and Churkin (1977) and Dover et al., (1980) for Ordovician and Silurian graptolites of Idaho; Carter (1988a, 1988b) for Ordovician and Silurian graptolites of Washington state; and Norford et al., (2002) for Ordovician graptolites of British Columbia.
Figure 1. Distribution of graptolites from Ordovician to Lower Devonian strata deposited along the Cordilleran margin of Laurentia and in a range of depositional settings across the continental margin (shown in profiles and listed in columns). Continental margin profiles are represented by sections in Great Basin for Ordovician to early Silurian time (lower profile) and, following the mid-Llandovery collapse, for upper Llandovery to lower Devonian time (upper profile); arrow with RM indicates position of Roberts Mountains in central Nevada in both profiles. Graptolite zonation for Ordovician is simplified version used by Finney and Berry in Great Basin with lower Isograptus Zone equivalent to the zones of I. v. lunatus to I. v. maximus and the upper Isograptus Zone equivalent to the Oncograptus and Cardiograptus zones. Silurian zonation used is standard reference zonation of Koren’ (1989) with the zones of Berry and Murphy (1975) for Nevada shown in parentheses. Numbers are for graptolite species found in a zone in a depositional setting. See text for discussion of data compilation. Two or three sets of numbers are shown in the Slope and Rise setting for some zones. In these cases, the first number is primarily for central Nevada, <#> is for the Glenogle Formation of British Columbia, [#] is for Carter and Churkin’s (1977) Trail Creek data.
Ordovician graptolites in the Inner Shelf setting are those found in the shallow water limestones of the Ibex area of Utah. The best collections of Ordovician graptolites of the Outer Shelf setting are from the Ninemile, Antelope Valley, and Hanson Creek formations of the Monitor Range, central Nevada. Ordovician graptolites of the Slope and Rise setting are very widespread and abundant and occur primarily in the Vinini Formation and its many correlative units (e.g., Petes Summit, Basco, Palmetto, Toquima formations) throughout central Nevada, the Phi Kappa Formation of Idaho, the Ledbetter Slate of Washington state, the Glenogle Formation of British Columbia, and the lower Guayacan Group of Sonora. Graptolites of the Valmy Formation of central Nevada and possibly the lower Guayacan Group of Sonora represent the Ocean Floor setting. Silurian and Lower Devonian graptolites of the Slope and Rise setting are known primarily from the Roberts Mountains, Windmill, and Rabbit Hill formations of central Nevada, and from the Trail Creek and Ledbetter formations of Idaho and Washington, respectively, for the lower Silurian. Silurian graptolite collections from the Elder Formation of central Nevada are assigned to the Ocean Floor setting.
The graptolite zonation used for the Ordovician is slightly modified from that of Berry (1960). Problems have been recognized with some parts of this zonation; however, it is the most applicable zonation to use. Only two stratigraphic intervals (upper part of Isograptus Zone thorugh P. tentaculatus Zone; D. ornatus Zone to N. persculptus Zone) are represented by continuous graptolite successions that allow for the evaluation of biostratigraphic distributions and the definition of precise zonal boundaries. Graptolites generally occur in very thin stratigraphic intervals separated by thick barren intervals in sections that are structurally complex. Thus, zonal assignments are made by correlation to existing zonations, and that part of Berry’s (1960) zonation in most need of revision (post-O. amplexicaulis Zone, pre-D. ornatus Zone) is represented by few collections in much of the western United States.
The number of species listed in Figure 1 for each zone in each depositional setting was determined by counting the number of different species in all collections from that same zone within a particular area (e.g., Sonora, Roberts Mountains, Washington state) and then selecting the number from the area with the greatest number of species. Central Nevada tended to have the maximum number of species except for the Slope and Rise setting where the numbers of species for each zone are also shown for the Glenogle Formation of British Columbia and the Phi Kappa and Trail Creek formations of Idaho. Such a compilation has several inherent shortcomings. The numbers are an approximation of diversity and do not represent specimen abundance. Any given number is not always the number in a single collection. The number may be that of a rare, unusually diverse collection with most collections having many fewer species. One region may have much greater diversity than other areas, e.g., the Upper Ordovician Hanson Creek Formation of the Outer Shelf setting, and the P. tentaculatus Zone of the Glenogle Formation of the Slope and Rise setting. Different graptolite zonations have been used in different areas, for example between British Columbia, central Nevada, and Utah, which limits the accuracy of correlations of collections between geographic regions and depositional settings. Assignment of some collections to depositional settings is uncertain. Nevertheless, in spite of these shortcomings, Figure 1 shows interesting overall patterns of graptolite distributions with direct implications for graptolite paleoecology.
From Figure 1, it is very apparent that the Late Ordovician extinction event (Finney et al., 1997, 1999) and the mid-Llandovery collapse of the continental margin in central Nevada (Hurst et al., 1985; Sheehan, 1989) greatly affected graptolite distributions. Diversity and facies distribution of Silurian-Lower Devonian graptolites differ substantially from those of Ordovician graptolites.
Although the continental margin faced open ocean and was the site of continuous marine deposition since well before Ordovician time, graptolites do not appear in the succession until the approximatus Zone, and diversity remains low, and fossiliferous horizons are rare until the Isograptus Zone. Diversity, abundance, and number of collections are highest in the Slope and Rise setting. Graptolites are rare in the Ocean Floor facies. Collections from the Inner Shelf succession are limited to rare thin shale partings and include few, largely endemic graptoloid species along with abundant dendroid graptolites. From the Isograptus Zone to the uppermost Ordovician, diversity and abundance are very high in the Slope and Rise setting, although occurrences within local successions are often sporadic with thick barren stratigraphic intervals between collections. In the Isograptus zones, species diversity and abundance is unusually high in occasional beds of turbiditic sandstone, suggesting that the species lived in water above the inshore part of the Slope and Rise setting with the skeletal remains transported downslope (Finney and Berry, 1997). In the Vinini and Glenogle formations of central Nevada and British Columbia, respectively, graptolites are extremely abundant continuously through thick intervals (~100 m) of organic-rich black shale in the P. tentaculatus Zone, where diversity is also very high, especially in the Glenogle Formation. Graptolite diversity is generally lower in the H. teretiusculus to O. quadrimucronatus zones, but increases substantially in the D. ornatus and P. pacificus zones before decreasing dramatically at the level of the Late Ordovician extinction. In the Ocean Floor setting, graptolites are rare and diversity is consistently low. It is surprising that the Isograptus and P. tentaculatus zones are not represented in the Ocean Floor setting, given their high diversity and abundance in the Slope and Rise setting. Also of note is that the highly diverse fauna of the P. pacificus Zone of the Slope and Rise setting is represented by only 4 species in the Ocean Floor setting. In successions of the Outer Shelf setting, graptolite collections are few and diversity is very low in the Middle Ordovician, reflecting the general scarcity of graptolites, but diversity is very high in the Upper Ordovician zones of D. ornatus and P. pacificus.
Following the Late Ordovician extinction and associated perturbations to the Earth system, graptolites were very scarce to absent from the Cordilleran margin until the convolutus Zone. However, immediately following this reappearance, the mid-Llandovery collapse of the continental margin greatly changed the distribution of graptolites. Graptolites are common only locally in upper Llandovery to Lower Devonian successions and diversity is very low. Most occurrences are in the Slope and Rise facies, and these are sporadic with graptolites absent from the upper Ludlow to upper Pridoli. In the Open Ocean facies, graptolites are found at only four levels in the Silurian, and graptolites are not known in Outer Shelf successions.
Implications for graptolite paleogeography
The Slope and Rise and Ocean Floor successions are composed of lithologies deposited in deep marine settings (shale, siltstone, turbiditic sandstone, fine-grained limestone) that at some levels and some localities contain graptolites, in some instances abundant and diverse assemblages, but lack graptolites all together at other localities and levels. From this, we conclude that the control on graptolite distributions was not taphonomic, but instead was ecologic.
In many areas of the world, graptolite successions are continuous, specimens are abundant, and diversity is high, especially for the Silurian. On the Cordilleran margin, however, Silurian times were not particularly favorable for graptolites even though they existed there. Diversity was low and graptolites were absent for substantial intervals. During the Ordovician, conditions were much more favorable especially during the periods represented by the Isograptus to P. tentaculatus zones, the C. bicornis Zone, and, interestingly enough, the D. ornatus and P. pacificus zones that directly preceded the Late Ordovician extinction.
Given the distribution pattern, the most favorable habitat for graptolites was over the continental slope and rise. As proposed by Finney and Berry (1997, 1998), building on earlier ideas of Berry et al., (1987), graptolites likely flourished in the denitrification zone at the margins of oxygen minimum zones, which develop beneath zones of upwelling along continental margins. With changes in oceanic circulation, sea level, surface winds, and continental margin topography, upwelling zones can expand and contract and even disappear and later appear. The temporal and geographic distribution patterns of graptolites along the Cordilleran margin may reflect, therefore, changes in the parameters affecting coastal upwelling. During the Ordovician, the range of graptolites also extended far offshore and invaded shallow seas on the craton. In the Silurian to early Devonian, however, the habitat range was substantially reduced, perhaps reflecting mid-Llandovery changes to the morphology of the continental margin. The collapse of the continental margin also resulted in a craton-ward shift of habitats.
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Received: February 15, 2003
Accepted: June 15, 2003