Biostratigraphy and Chronostratigraphy of the Cambrian–Ordovician Great American Carbonate Bank
Published:January 01, 2012
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John F. Taylor, John E. Repetski, James D. Loch, Stephen A. Leslie, 2012. "Biostratigraphy and Chronostratigraphy of the Cambrian–Ordovician Great American Carbonate Bank", Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia, James Derby, Richard Fritz, Susan Longacre, William Morgan, Charles Sternbach
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The carbonate strata of the great American carbonate bank (GACB) have been subdivided and correlated with ever-increasing precision and accuracy during the past half century through use of the dominant organisms that evolved on the Laurentian platform through the Cambrian and the Ordovician. Trilobites and conodonts remain the primary groups used for this purpose, although brachiopods, both calcareous and phosphatic, and graptolites are very important in certain facies and intervals. A series of charts show the chronostratigraphic units (series and stages) currently in use for deposits of the GACB and the biostratigraphic units (zones, subzones, and biomeres) whose boundaries delineate them. Older and, in some cases obsolete, stages and faunal units are included in the figures to allow users to relate information from previous publications and/or industry databases to modern units. This chapter also provides a brief discussion on the use of biostratigraphy in the recognition and interregional correlation of supersequence boundaries within the Sauk and Tippecanoe megasequences, and the varied perspectives on the nature of biostratigraphic units and their defining taxa during the past half century. Also included are a concise update on the biomere concept, and an explanation of the biostratigraphic consequences of a profound change in the dynamics of extinction and replacement that occurred on the GACB in the Early Ordovician when the factors responsible for platformwide biomere-type extinctions faded and ultimately disappeared. A final section addresses recent and pending refinements in the genus and species taxonomy of biostrat-igraphically significant fossil groups, the potential they hold for greatly improved correlation, and the obstacles to be overcome for that potential to be realized.
A solid foundation for biostratigraphic subdivision and correlation of lower Paleozoic carbonate strata of the Laurentian platform in North America was provided more than half a century ago in now-classic articles by Ross (1951), Hintze (1953), Palmer (1955), and— particularly relevant in this volume dedicated to James Lee Wilson—Lochman-Balk and Wilson (1958). The biozones and derivative Laurentian series and stages developed in those studies were based primarily on trilobites and brachiopods. Shortly thereafter, the extraordinary potential of conodonts for zonation and correlation was demonstrated with the description of stratigraphically restricted Upper Cambrian (Miller, 1969) and Ordovician (Bergström and Sweet, 1966; Ethington and Clark, 1971) faunas from the great American carbonate bank (GACB) strata in the United States. During the past four decades, numerous taxonomic and bio-stratigraphic studies, more tightly focused on specific fossil groups and thinner stratigraphic intervals than was possible in the aforementioned seminal studies, yielded extensive biostratigraphic data that allowed for considerable refinement of that early biostratigraphic framework. This new information also led to significant revisionsin the series and stage nomenclature used for Cambrian and Ordovician strata of the GACB.
The purpose of this chapter is to serve as a guide to these revisions and provide, through a series of up-to-date diagrams, a translational device for geologists to relate the biostratigraphic and chronostratigraphic units currently in use to those that have been superseded. A discussion on the rationale for the revisions is also included to provide some sense of how the philosophy and practice of biostratigraphy in the lower Paleozoic has evolved to the present day. Although space constraints preclude presentation of all thin biozones and biofacies recognized in recent biostratigraphic studies, access to many highly refined but perhaps only locally applicable zonations is provided through an extensive set of cited references.
Figures 1 and 2 show the chronostratigraphic units currently recognized within the Cambrian and the Ordovician of Laurentian North America, which constitute the Sauk and lower Tippecanoe sequences of Sloss (1963, 1996). Figure 1 illustrates the series and stages within the Cambrian and the Lower Ordovician, whereas Figure 2 provides the units for the Middle and the Upper Ordovician. Both depict series and stages developed within the shallow-marine successions of Laurentian North America. Figure 1 identifies the specific trilobite-based biozone and subbiozone boundaries that are used to define and correlate Laurentian stage boundaries. The related biomeres (see later) are shown in the fifth column. Figure 2 identifies the North American mid-continent conodont zones that form the basis for nearly all Laurentian stage boundaries in the Middle and Upper Ordovician. However, in one case (the base of the Chatfieldian Stage), a stage boundary has been placed at an interregionally extensive K-bentonite bed (Leslie and Bergström, 1995). For brevity, both figures show only the faunas used to define the series and stage boundaries. Numerous other intrastage macrofossil zones and subzones, along with a rich and highly refined conodont biostratigraphy for the uppermost Cambrian and Ordovician (see Miller et al., 2003) are provided in Figures 3–5.
The Laurentian series and stages are provincial in nature, and very few of their boundaries coincide precisely with the correlation events that have been selected to define the boundaries of the global series and stages currently recognized within the Cambrian and the Ordovician Systems (Webby et al., 2004; Babcock et al., 2005). These global units are shown on the right in Figures 1 and 2, with the correlation event that defines each boundary abbreviated in a rectangle at the right margin. For example, the base of the global Trem-adocian Stage (correlation event: first appearance datum [FAD] of the conodont Iapetognathus fluctivagus) has served since 2001 as the internationally recognized base of the Ordovician System (Cooper et al., 2001). Thus, the base of the Ordovician is shown some distance up into the Skullrockian Stage, well above the base of the Laurentian Ibexian Series.
The series designations (Lower, Middle, and Upper Cambrian) shownin the leftmost column in Figure 1 are convenient, widely used subdivisions of the Cambrian System in Laurentia. They are formal units in that bases of the Middle and Upper Cambrian coincide precisely with the bases of the Lincolnian and Millardan Series (respectively) of Palmer (1998). The base of the Upper Cambrian, positioned at the base of the Aphelaspis tri-lobite Zone, also coincides with the base of the global Furongian Series (Peng et al., 2004). However, the base of the Laurentian Middle Cambrian, aligned with the base of the Delamaran Stage in Figure 1, does not coincide with any global series or stage boundary. The practical and traditional rationale for this placement is discussed in greater detail in the Biostratigraphy section. In contrast, the Lower, Middle, and Upper Ordovician Series shown in the leftmost column in Figure 2 are global chronostratigraphic units whose boundaries have been formally established through the selection of a correlation event and the ratification of the Global Standard Stratotype Section and Point (GSSP) for each boundary (Webby et al., 2004; Wang et al., 2009). All seven global stages in the Ordovician have been named, and a GSSP has been ratified for the base of each. Although correlation events have been proposed for the bases of all four global series and for ten global stages within the Cambrian System (Babcock et al., 2005), names and GSSPs have been selected for only two of the series and four of the stages. Deliberations continue regarding the remaining series and stage boundaries, and some of the correlation events shown in Figure 1 might be replaced as new information becomes available. For example, considerable and substantial criticism recently has been offered regarding the use of Oryctocephalus indicus and Lotagnostusamericanus for establishment of GSSPs for the bases of stages 5 and 10, respectively.
In their synthesis of Cambrian biostratigraphy in North America, Lochman-Balk and Wilson (1958) discussed in detail the nature of the faunal units that they used to subdivide and correlate strata that formed in shallow-marine environments of the continent (their cratonic and intermediate biofacies realms) in the early Paleozoic. They made clear the practical and conceptual differences between a “teilzone” through which a species or genus ranges locally and a “biozone,” an extensive time-rock unit that approximates the total spanoftime throughwhichaparticular genus survived. The greater geographic extent of the genus-based bio-zone rendered it more useful for interregional correlation. By expanding the concept to define a time-rock unit representing the total stratigraphic range of any of several characteristic genera or species in a stratigraph-ically significant assemblage, which they termed the “faunizone,” they had some success in correlating into coeval deep-water successions (their extracratonic eux-inic realm). A similar assemblage-zone concept was used in the definition and correlation of the original alphanumeric zones established on the stratigraphic ranges of broadly defined genera and species of trilobites (Ross, 1951; Hintze, 1953) and conodonts (Ethington and Clark, 1971) in the Lower Ordovician.
Numerous studies through the latter half of the 20th century yielded considerable refinement of the early coarse biostratigraphic framework, producing many species-based subzones within the genus-based assemblage zones. Although space constraints preclude incorporation of all of these subzones in the figures of this chapter, numbered rectangles are provided in Figures 3 and 4 to direct the reader to the references that contain detailed information on the zones and sub-zones recognized in particular areas of North America. These typically include detailed range charts that show the stratigraphic distribution of species recovered from measured sections and from drill core. They also generally provide a complete listing of the species that occur within each zone or subzone and identify the species whose lowest occurrence(s) define(s) the base of the unit. It became fairly standard practice to leave the zones topless by simply defining the top of each unit as the base of the overlying unit. By this convention, barren intervals are assigned to the top of the subjacent zone or subzone. This practice has the disadvantage in studies of subsurface material recovered from well cuttings of emphasizing lowest occurrences (FADs) though highest documented occurrences (last appearance datums) are less prone to distortion by downhole transport of material by caving or fluid circulation. This distortion is less of a problem in data collected from core material, as is typically the case for macrofossils.
In concept, the boundaries of biozones and sub-biozones are independent of time (i.e., either isochronous or diachronous), a stipulation formalized in the first American Code of Stratigraphic Nomenclature (American Commissionon Stratigraphic Nomenclature, (1961) and the International Stratigraphic Guide (Hedberg, 1976). This position was reinforced by placement of bio-stratigraphic units in material categories of the 1983 and 2005 North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 1983, 2005). In practice, however, the horizons selected for use as zonal or subzonal boundaries are typically based on the FADs of widely distributed and environmentally tolerant taxa so that the boundaries are not strongly diachronous. It is, after all, commonly time control that is sought in constructing a biostratigraphic framework. Thus, the more closely a zonal boundary approximatesanisochron, the betterit serves the intended purpose. Consequently, the Cambrian–Ordovician zones and subzones shown in the figures herein are actually biochronozones as defined in the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005). This is not to say that we support the opinion expressed by some (e.g., Ludvigsen et al., 1986; Zalasiewicz et al., 2004) that chronostratigraphic units should be abandoned. Like most stratigraphers (e.g., Ferrusquia-Villafranca et al., 2009), we see the value in the conceptual separation of biostratigraphic, chronostratigraphic, and geochronol-ogic units. The undifferentiated treatment of biozones and biochronozones in the text and figures in this chapter is merely for concision.
Cambrian and Lower Ordovician Biochronozones and Biomeres
Lochman-Balk and Wilson (1958) recognized as their oldest biostratigraphic package a Lower Cambrian Olenellid fauna, which they divided into lower and upper subdivisions based on the appearance of a variety of trilobite genera other than Olenellus. Palmer and Repina (1993) delineated three intervals based on the estimated stratigraphic ranges of the Early Cambrian trilobite genera: the “Fallotaspis,” “Nevadella,” and Ole-nellus Zones. (We follow the convention used by Palmer  of using quotation marks to denote older, commonly broadly defined, taxa that are undergoing tax-onomic revision.) This tripartite division formed the basis of the new stages proposed by Palmer (1998) for the upper trilobite-bearing part of the Lower Cambrian, which he designated the Waucoban Series (Figure 1). The “Fallotaspis” Zone and overlying “Nevadella” Zone together constitute the Montezuman Stage, whereas the upper part comprises the loosely defined Olenellus Zone. Although no well-established fine-scale biozo-nation is available for the Begadean Series or lower Montezuman Stage (“Fallotaspis” Zone), recent work by Fritz (1972, 1993) and Hollingsworth (2005, 2006, 2007) on fallotaspidoid trilobites from western North America has expanded significantly what is known of the oldest trilobites in the Cambrian of Laurentia. The oldest trilobite fauna yet described is that of the Fritz-aspis Zone, which was assigned to the uppermost part of the Begadean Series (Hollingsworth, 2007). With few exceptions, the Begadean and lower Montezuman faunas have been recovered from inner-shelf clastic fa-cies. Hence, the biostratigraphic value of the fallotaspi-doids for subdivision of carbonate bank deposits is limited at best. In contrast, carbonate deposition spread across wider areas of the Laurentian shelf during the time represented by the overlying “Nevadella” and Olenellus Zones.
Among the many things revealed by higher resolution biostratigraphic studies through the 1960s and 1970s was that severe extinction events punctuated the evolutionary history of the shallow-marine fauna through the Cambrian. Each event devastated the faunas that inhabited the GACB and the nearshore clastic-dominated environments of the adjacent inner detrital belt. In the aftermath of each extinction, these areas were repopulated by a distinctive group of olenid or olenid-like trilobites (olenimorphs) that migrated inward from deep off-platform environments. This mono-generic replacement fauna invaded all shallow-marine environments across the shelf, reducing the taxonom-ic and morphologic diversitytoaminimum from which it would rise through endemic speciation, augmented by immigration of surviving taxa from distal sites, for the next few million years until the next extinction. The package of strata between the horizons that record the appearances of two successive olenimorph-dominated replacement faunasisabiomere (Palmer, 1965a, b; 1984; Stitt, 1971a, 1975, 1977; Taylor, 2006). Five of these stage-level biostratigraphic units have been recognized in the Cambrian System (Figure 1). Each biomereisnamed for a trilobite family that, although not restricted to that stratigraphic interval, diversified significantly to become a conspicuous component of the faunas that characterize its constituent biozones.
Each biomere-boundary crisis homogenized the platform biota, eliminating the pronounced differences in the taxonomic composition of faunas (biofacies) that simultaneously inhabited different environments before the extinction (Ludvigsen and Westrop, 1983; Pratt, 1992; Taylor et al., 1999; Westrop and Cuggy, 1999). The virtual absence of clearly differentiated biofacies in faunas near the biomere boundaries suits them well for use in dividing the Cambrian strata into stages whose boundaries are traceable with exceptional precision across the continent (Ludvigsen and Westrop, 1985; Pratt, 1992; Palmer, 1998). Conversely, strong biofacies differentiation that developed as faunas recovered from the biomere boundary crises continues to pose a challenge to those attempting to correlate with precision within the body of each biomere. For excellent examples, see the correlation charts provided by Westrop (1986) and Hughes and Hesselbo (1997) for the upper half of the Sunwaptan Stage (upper part of the Ptychaspid biomere). The two oldest biomere boundaries, at the base and in the middle of the Middle Cambrian, coincide precisely with the bases of the Delamaran and Marjuman Stages (Figure 1). The Middle Cambrian biomere and stage boundaries coincide because the replacement of the diverse preextinction fauna by the minimum diversity olenimorph-dominated fauna ap-pearstohave been geologically instantaneous at each of these turnovers. At each of the biomere boundaries in the Upper Cambrian, a thin “critical interval” (Taylor, 2006) with a transitional fauna separates the preex-tinction fauna from the olenimorph-dominated fauna. Thus, the stage boundaries and biomere boundaries are slightly offset. This contrast in the style of replacement at the Middle versus the Upper Cambrian biomere boundaries and the nature of the transitional fauna in the critical interval are explained more fully below.
The oldest well-documented biomere-like faunal turnover, marked by the extinction of the last olenellid trilobites in the Laurentia at the top of the Olenellus Zone, has been used to define the boundary between the Lower and the Middle Cambrian Series in North America for more than half a century (Lochman-Balk and Wilson, 1958; Palmer, 1998). Unfortunately, this series boundary is unconformable in most areas be-cause of a second-order regression knownasthe Hawke Bay event (Palmer and James, 1979; Palmer, 1981). Nonetheless, precise biostratigraphic data from intensive sampling across this boundary in the most complete sections discovered so far confirm that the faunal turnover involved the replacement of a diverse preex-tinction fauna by one of minimal morphologic and tax-onomic diversity. The replacement faunaisoverwhelm-ingly dominated by the generalized ptychopariid trilobite Eokochaspis (Sundberg and McCollum, 2000, 2003b). The appearance of this minimum-diversity replacement fauna defines both the Olenellid-Corynexochid biomere boundary and the base of the Delamaran Stage and Lincolnian Series (Palmer, 1998). A similar pattern at the base of the overlying Marjuman Stage, where the fauna of the Glossopleura Zone is replaced by a fauna of very low diversity dominated by the generalized ptychpariid Proehmaniella at the base of the Ehmaniella Zone (Sundberg, 1994), serves to define the base of the Marjumiid biomere.
As previously noted, a more complex and protracted pattern of faunal turnover has been documented by high-resolution sampling across the three bio-mere boundaries within the Upper Cambrian: the tops of Marjumiid, Pterocephaliid, and Ptychaspid bio-meres (Figure 1). For each of these crises, the diverse preextinction fauna was not replaced immediately by the minimum-diversity replacement fauna. The two are separated stratigraphically by a thin interval with a transitional fauna that is dominated by a surviving opportunistic genus or species from the preextinction fauna, but also includes a few taxa that migrated in from deeper and cooler environments. The base of this critical interval (Taylor, 2006), which Stitt (1971a, 1975) referred to as stage 4 in describing a repeating evolutionary pattern in the Upper Cambrian biomeres, records the extermination of most of the platform trilobites. It can also be recognized by its impact on other faunal groups (e.g., brachiopods and conodonts) and in some cases even in non-Laurentian successions. The breadth of its taxonomic and paleogeographic scope and apparent synchronicity across the entire shelf suit this horizon well for use as a stage boundary within the chronostratigraphic framework. For example, see Miller et al. (2006) for a summary of the attributes of the base of the critical interval of the Pty-chaspid biomere. Consequently, the bases of the critical intervals that form the uppermost parts of the Marjumiid, Pterocephaliid, and Ptychaspid biomeres are used to define the bases of the Steptoean, Sunwap-tan, and Skullrockian Stages, respectively (Ludvigsen and Westrop, 1985; Palmer, 1998). However, it is the top (not the base) of the critical interval that records the return to minimum diversity with wholesale collapse of platform biofacies and domination of the platform fauna by olenimorphs. For this reason, the top of the critical interval defines the biomere boundary; hence, the Upper Cambrian stage and biomere boundaries are offset stratigraphically from one another by the thickness of the critical interval (Figure 1).
As first noted by Stitt (1983) in discussing the “Symphysurinid” biomere, a similar crisis occurred in the platform trilobite fauna during deposition of the Skullrockian-Stairsian Stage boundary interval in the earliest Ordovician. The pattern of faunal change resembles that documented at the Upper Cambrian biomere or stage boundaries in several respects. There is a thin critical interval (the Paraplethopeltis Zone) dominated by a survivor of the stage-boundary extinction that decimated the diverse fauna of the underlying Bellefontia trilobite Zone (Figures 1, 5). In addition, a cosmopolitan open-ocean trilobite (Kainella) migrated onto the platform to join the survivors within the critical interval. However, the pattern at the top of the crisis interval (base of the Leiostegium trilobite Zone) differs from the Cambrian biomere boundaries in two critical respects (Taylor et al., 2009a; J. D. Loch and J. F. Taylor, unpublished data): (1) the trilobite genera that dominate the Paraplethopeltis Zone do not disappear but range upward into the Leiostegium Zone, where they are joined by the species used to define the base of that zone, and (2) consequently, a minimum-diversity olenimorph-dominated replacement fauna comparable to those that typify the Cambrian biomeres never developed. Unlike the Cambrian biomere boundaries, the base of the Leiostegium Zone does not mark the final stage in the extinction process, but records the beginning of the biotic recovery. Because of less severe environmental stress and/or critical zone taxa with higher tolerances, the effect was muted and the virtual depopulation of the platform that occurred during the Cambrian biomere extinction episodes was not repeated.
Regardless of the cause, the change in the dynamics of the extinction-recovery process, from the continent-wide nearly total turnover typical of the Cambrian biomeres to a less complete and more regionalized phenomenon, significantly affected the use of the benthic macrofauna for interregional correlation. Consequently, separate stage nomenclatures for the Lower Ordovician were developed for either side of the Transcontinental Arch (Flower, 1970; Ross, 1976; Ethington et al., 1987). The effects are less pronounced for the lower Ibexian, so the Gasconadian and Demingian Stages of Flower (1964, 1970) have been abandoned in favor of the Skullrockian and Stairsian Stages of Ross et al. (1997), which now can be traced from the standard Ibexian Series in Utah into the eastern successions, despite a strong contrast in lithofacies and bio-facies between the two regions. However, we retain (Figures 1, 4) a dual stage terminology for the upper Ibexian, assigning the uppermost Ibexian strata in eastern North America to the Jeffersonian and Cas-sinian Stages, and restricting the Tulean and Black-hillsian Stages to western North America. However, the bases of the Jeffersonian and Cassinian Stages (rightmost column in Figure 4) differ significantly from those proposed by Flower (1970). The sloping lines used to mark the bases of Flower’s (1964, 1970) Jef-fersonian and Cassinian Stages (fourth column from left in Figure 4)are positionedto show the approximate range of diachroneity of these boundaries that resulted from inaccurate correlation between the midcontinent and northern Appalachians.
The conodont zonation for the Upper Cambrian and the Lower Ordovician used herein was developed mostly from extensive work in the eastern Great Basin (Ethington and Clark, 1971, 1982; Miller, 1980, 1988; Ross et al., 1997), but it has been augmented by many studies elsewhere in Laurentian successions (see, for example, Repetski, 1977, 1985; Ethington and Repetski, 1984; Nowlan, 1985; Sweet and Bergström, 1986; Derby et al., 1991; Smith, 1991; Ji and Barnes, 1994; Harris et al., 1995; Repetski et al., 1995; Landing et al., 2003). The Sauk conodont zonation (Figures 2, 5) essentially follows that in Ross et al. (1997). Two boundaries within this zonation represent profound crises in the conodont faunas of the GACB, both closely associated with major turnovers in the macrofauna: one in the latest Cambrian and the other in the earliest Ordovician. The base of the Cordylodus proavus Zone coincides precisely with the base of the Eurekia apopsis trilobite Zone, which is the base of the critical interval of the Ptychaspid biomere. The extinction of numerous species of trilobites, bra-chiopods, and conodonts at this horizon, along with the concurrent appearance of many new species of all three groups, makes it one of the most recognizable and traceable boundaries in the lower Paleozoic (Miller et al., 2006). Because of its use for correlation throughout Lau-rentian North America and for recognition of coeval strata deposited on other paleocontinents, Ross et al. (1997) selected it to define the base of the Skullrockian Stage and the Ibexian Series.
The other major crisis recorded in the succession of conodont zones within the Sauk megasequence of the Laurentian platform is at the base of the low-diversity interval, just above the base of the Stairsian Stage (Figures 2, 5). The extinction of many long-ranging conodont species of the underlying Rossodus mani-touensis Zone makes this a very significant event in the early evolution of conodonts (Ethington et al., 1987; Ji and Barnes, 1994). Unlike the latest Cambrian crisis at the base of the Skullrockian Stage, in which the cono-dont and trilobite extinctions apparently were simultaneous, the conodont extinctions at the base of the low-diversity interval postdated the disappearance of the Bellefontia Zone trilobite taxa at the base of the Stairsian Stage (Ethington et al., 1987; Taylor et al., 2009a). Some conodont species of the R. manitouensis Zone range upward a short distance into the Leioste-gium trilobite Zone, producing a very thin overlapping range zone (Figure 5). The presence or absence of these lowermost strata of the Leiostegium Zone that yield conodonts of the R. manitouensis Zone has been used in some studies (e.g., Myrow et al., 2003) to assess the magnitude of the stratigraphic gap in sections where the boundary between the Skullrockian and Stairsian Stages is unconformable.
Middle and Upper Ordovician Faunas
Conodonts have become the dominant tool in zo-nation and correlation of the Middle and the Upper Ordovician carbonates because of pronounced biofacies-to province-level differentiation that developed in the shelly macrofauna of the GACB during the Middle and the Late Ordovician. The Middle and the Upper Ordovician conodont zones used herein follow those of Webby et al. (2004). These zones have evolved from the numbered faunas of Sweet et al. (1971) into the succession of named conodont-based chronozones recognized within the composite standard created by Sweet (1984, 1995) through graphic correlation using species range data from more than 100 Laurentian measured sections. Other important references for conodont faunas of this age include Bergström (1971) and Harris et al. (1979).
Although homotaxial successions of trilobite species useful for local intrabasinal correlation have been documented in some studies (e.g., Fisher, 1977; Ludvigsen, 1977, 1979; Shaw, 1991), no interregional zonation has been developed for these time intervals. Instead, the most recent work on the Middle and the Upper Or-dovician shelly macrofaunas has focused mostly on reconstruction of environmental gradients preserved in the lateral arrangement of biofacies or faunas within conodont- or graptolite-based chronozones or between broadly distributed K-bentonite beds or sequence boundaries (e.g., Ludvigsen, 1978; Patzkowsky and Holland, 1996, 1999; Amati and Westrop, 2006; Holland and Patzkowsky, 2007). The influence of environmental contrasts across the broad carbonate platform created by the Middle and Late Ordovician sea level highstands was compounded further by the development and migration of the Taconic foreland basin in the Appalachians and by associated environmental stresses imposed by episodic tectonics in that region. However, the westward spread of organic-rich deep-water facies did expand the distribution of environments favorable for the preservation of graptolites on the outer margins of the GACB. In the successions that accumulated in such environments, detailed sampling and the integration of biostratigraphic (conodont and graptolite) and geochemical (carbon isotopic and K-bentonite chemistry and age dating) data have produced some of the most finely calibrated chronostratigraphic frameworks and detailed paleoceanographic and tectonostratigraphic models ever constructed for lower Paleozoic strata (e.g., Finney and Bergström, 1986; Mitchell et al., 1994, 2004; Ganis and Wise, 2008). Such facies are also of particular interest to the petroleum geologist for their importance as source beds.
Cambrian Series and Stages
The information on the global series and stages and defining species for Figures 1 and 2 was extracted primarily from Shergold and Geyer (2003), Peng et al. (2004), Webby et al. (2004), and Babcock et al. (2005, 2007). The Laurentian Cambrian series and stages shown in Figure 1 are primarily those proposed by Palmer (1998), with revisions in the biostratigraphic designations at some boundaries. These include:
Replacement of the Elvinia Zone by the Cliffia lataegenae Subzone (of the Elvinia Zone [Taylor et al., 1999]) beneath the Irvingella major Subzone at the base of the Sunwaptan Stage. This reflects our preference that the very thin (always <2 m [6.6 ft]) interval characterized by the I. major fauna be included in the Elvinia Zone as its uppermost subzone, instead of being set apart as a separate superja-cent zone. Similarly, we prefer that the very thin critical interval to which the opportunistic species Coosella perplexa is restricted at the top of the Marjumiid biomere remains a subzone instead of being elevated to full zonal status as was done by Palmer (1998). Accordingly, it is shown in Figure 4 as a basal subzone of the Aphelaspis Zone, as originally presented by Palmer (1979). Given the dominance by species of genera that range upward from the Crepicephalus Zone (Blountia, Coosella, and Glaphyraspis) and extreme scarcity of Aphelaspis in that critical interval in many sections, one can argue that the Coosella perplexa Subzone should instead be assigned to the top of that subjacent zone. However, the appearance and abundance (locally) of both Aphelaspis and Cheilocephalus in the interval in many sections support its assignment to the Aphelaspis Zone. Moreover, the top of the subzone is difficult to establish in many sections where Coosella is not present, and both Glaphyraspis and Cheilocephalus range well upward into the higher parts of the Aphelaspis Zone (Palmer, 1955; Rasetti, 1965). For a thorough discussion on these issues and an exhaustive compilation of detailed trilobite range data through the Marjumiid-Pterocephaliid biomere boundary interval in the western United States, see Thomas (1993).
The Saukiella serotina Subzone has been renamed the Prosaukia serotina Subzone to accommodate the provisional reassignment of the eponymous species to a different saukiid genus by Adrain and Westrop (2004). More significantly, we divide the slightly younger interval previously known as the Missisquoia Zone into two separate zones: a lower Tanghanaspis depressa Zone and an upper Apopla-nias Zone. This is done to eliminate confusion created by the abandonment of the name Missisquoia as a junior synonym of the Asian genus Parakoldi-nioidia (Fortey, 1983) and reassignment of its species to several different missisquoid genera (Lee et al., 2008). It also emphasizes the significance of the olenid genus Apoplanias, whose appearance in monogeneric concentrations across all Laurentian facies belts marks the Ptychaspid-“Symphysur-inid” biomere boundary (Taylor, 2006). Although they bear different names, the two new zones correspond precisely to the two subzones of the former Missisquoia Zone. The T. depressa Zone is exactly the interval that previously was called the Missisquoia depressa Subzone, and the Apoplanias Zone is what formerly was referred to as the Missisquoia typicalis (Parakoldinioidia stitti) Subzone.
The species whose FAD defines the base of the Delamaran Stage is changed from Eoptychparia piochensis to Eokochaspis nodosa because of taxo-nomic revision of the ptychopariid trilobites that appear directly above the highest occurrence of olenelloid trilobites (Sundberg, 2005). We have opted not to recognize the Topazan Stage, proposed by Sundberg (2005) for the interval between the FAD of Eokochaspis nodosa and the FAD of Ptychagnostus atavus and still assign that interval to the base of the Marjuman Stage. Although it is unquestionably a very useful horizon for intercontinental correlation, serving as the event used to correlate the base of the global Drumian Stage (Babcock et al., 2007), the FAD of Ptychagnostus atavus has not yet been established with precision in most areas of Laurentian North America.
Lower Ordovician Series and Stages
The Lower Ordovician series and stage names for Lau-rentia are those proposed by Ross et al. (1997), with the following modifications:
The base of the Stairsian Stage is repositioned from the base of the Leiostegium Zone (top of the Paraplethopeltis Zone) downward to the base of the Paraplethopeltis Zone (top of the Bellefontia Zone) (Figure 4). This is done to be consistent with the approach taken at all Upper Cambrian stage boundaries, wherein the extinction horizon at the base of the thin critical interval at the top of each biomere serves as the stage boundary (Taylor, 2006). This revision lowers the Skullrockian-Stairsian boundary only a few meters because the Paraplethopeltis Zone, like the Cambrian critical intervals (e.g., the Coosella perplexa and Irvingella major Subzones), is quite thin.
A dual stage nomenclature isretained for the upper half of the Ibexian Series to acknowledge the difficulty (or impossibility) of tracing the faunal changes that define the bases of the Tulean and Blackhillsian Stages in western North America eastward into coeval strata in the mid-continent and Appalachians. A very strong lithofacies and related biofacies contrast between the eastern and western areas late in the Early Ordovician warrants a conservative approach to the problem and retention, at least temporarily, of the redefined Jeffersonian and Cassinian Stages of Loch (1995, 2007) for use in the eastern sponge-microbial reef facies. Recent redefinition of the base of the Tulean Stage (Adrain et al., 2009) to coincide with a biomere-like mass extinction horizon in western North America does offer promise for continent-wide recognition of that stage boundary. However, no conclusive evidence has yet been found that the same event can be recognized in eastern North America. The pronounced faunal change in the northern Appalachians that Adrain et al. (2009) identified as perhaps recording the same faunal turnover as the base of the Tulean in Idaho and Utah occurs across an unconformity in the Boat Harbour Formation in Newfoundland (Boyce, 1989) that omits four trilobite zones (including the entire Jeffersonian Stage) described by Loch (2007) in southern Oklahoma.
Middle and Upper Ordovician Series and Stages
Figure 2 is patterned after part of figure 2.2 in Webby et al. (2004) that covers the series, stages, and North American midcontinent province conodont zones of the uppermost Lower and the Middle and the Upper Ordovician. A column is added to show how the zones currently in use relate to the original alphanumeric faunal units introduced by Ethington and Clark (1971) and Sweet et al. (1971). It also shows the positions and defining biozonal boundaries of the global stages recently established for the Middle and the Upper Ordovician Series (Bergström et al., 2006, 2008; Wang et al, 2009). Other modifications include the addition of the Chazyan Stage and specification of the Millbrig K-bentonite Bed as the base of the Chatfieldian Stage (Leslie and Bergström, 1995) because of their regional use in correlation.
Megasequences and Supersequences
The sequence-stratigraphic revolution of the last few decades, with the innovative adaptation of the methodology for analysis of mostly carbonate successions (Loucks and Sarg, 1993; Harris et al., 1999), has provided modern carbonate sedimentologists and strati-graphers with improved insight to the response of carbonate systems to eustasy and tectonics. One derivative of these new methods is an enhanced ability to identify horizons or thin intervals that represent particularly significant paleoceanographic events that affected the style and extent of carbonate deposition on the GACB. In many cases, such events also severely impacted the marine biota, providing a unique faunal signature that expedites recognition and correlation of these intervals on a continental or greater scale. Some also were accompanied by changes in ocean chemistry that left their mark on the carbonates produced, providing a chemical fingerprint in the form of carbon isotopic excursionsortrends. Consequently, manysuch horizons coincide with or lie close to the series and stage boundaries in recently developed geologic time scales (Ross et al., 1997; Webby et al., 2004).
Although a powerful tool, sequence stratigraphy benefits greatly from supplemental time control for reliable correlation between basins. This is particularly true when correlations are extended into areas where the rocks are poorly exposed and supportive bio Stratigraphic and chemostratigraphic control is limited. For example, Goldhammer et al. (1993) equated the base of the Signal Mountain Limestone in Oklahoma and the base of the Stonehenge Limestone in the Appalachians with the base of their supersequence SC-1 (their 505 Ma boundary). Although they were correct in identifying the base of the Stonehenge as a major (second-order) sea level event, a comparison of biostratigraphic data (trilobites and conodonts) from several studies (Stitt, 1983; Taylor et al., 1992; Brezinski et al., 1999; Taylor et al., 2004; Pojeta et al., 2005) has revealed that the age-equivalent horizon in Oklahoma is the base of the McKenzie Hill Formation (the top of the Signal Mountain, not the base). Accurate and precise interbasinal correlation of sequence boundaries is best accomplished through an integrated approach wherein fine-scale sequence-stratigraphic, biostratigraphic, and che-mostratigraphic data are collected concurrently. In this way, the supplemental time control provided by the faunas and isotopes ensures that the same sequence boundary is identified in separate sedimentary basins.
However, even the most accurate and refined correlations still leave room for highly varied opinions on precise placement of important boundaries. A very large gap (hiatus) is shown between the Sauk and Tip-pecanoe megasequences in Figure 2 to emphasize the magnitude of the event that created the unconformity between them and also to acknowledge that perfectly valid rationales exist for placing this megasequence boundary at several different levels within the uppermost Lower Ordovician and basal Middle Ordovician. Some practitioners (e.g., Finney et al., 2007) placed the boundary just below the base of the Middle Ordovi-cian, where the earliest evidence of the drawdown is found. Others (e.g., Derby et al., 1991) specified a higher level in the uppermost part of the Histiodella holodentata conodont Zone, interpreting that horizon as representing the peak of the regression. Still others (e.g., Landing and Westrop, 2006) have extended the Sauk megase-quence upward to the base of transgressive facies at or near the base of the Chazyan Stage. The purpose of this chapter and its figures is to provide the temporal framework in which those various options can be debated, not to stipulate the option that we prefer. Accordingly, nothing younger than the uppermost Ibexian strata of the middle to upper Reutterodontus and inus conodont Zone has been assigned to the Sauk megasequence, and nothing older than basal Chazyan (Cahabagnathus friendsvil-lensis conodont Zone) to the Tippecanoe in Figure 2.
We have taken a similarly conservative approach with respect to subdivision of the Sauk megasequence. The only supersequences shown in Figure 1 are those differentiated by Palmer (1981) as Sauk I, II, and III on the basis of major unconformities that occur at or near the Lower–Middle Cambrian and the Middle–Upper Cambrian boundaries in most areas of North America. This is not meant to imply that these three packages cannot be further subdivided. Indeed, what appear to be legitimate second-order cycle boundaries have been used to divide one or more of these packages into separate supersequences, in particular, sedimentary basins (e.g., Read, 1989; Goldhammer et al., 1993; Golonka and Kiessling, 2002; Miller et al., 2003; Dewing and Nowlan, 2012). However, the biostratigraphic data available to expedite interbasinal correlation of these boundaries are sparse at best. Consequently, their lateral persistence and geographic range of use have not yet been rigorously tested.
Progress and Problems in Refinement of Species and Biozones
At various levels within the Cambrian and the Ordovician, recent systematic and biostratigraphic research has resurrected a problem confronted by biostratigra-phersin the 1940s and 1950s. Lochman-Balk and Wilson (1958), and those who followed their lead in the next few decades, accomplished continent-wide correlation of many Cambrian and Ordovician zones by applying relatively broad species concepts that attributed appreciable morphologic variation to within-species geographic variation (Rasetti, 1961; Robison, 1964, 1976; Grant, 1965; Palmer, 1965a; Winston and Nicholls, 1967; Longacre, 1970; Stitt, 1971b, 1977, 1983; Ludvigsen, 1982; Ludvigsen and Westrop, 1983; Westrop, 1986, 1995; Boyce, 1989; Palmer and Repina, 1993; Taylor et al., 1999; among others). In the last decade, more rigorous mor-phometric analysis of large collections from tightly constrained collection horizons has greatly narrowed the accepted range of morphologic variation for many Cambrian and Ordovician species. Although this has had the beneficial effect of diminishing the stratigraphic range of many taxa, thereby producing a more refined zonation, it also has reduced the geographic range of most of the revised genera and species. Consequently, the refined zonalsuccessionsso farhave demonstrated to be of only local or regional use.
In the Lower Cambrian, for example, exhaustive quantitative characterization of olenelloid trilobites from the western United States by Webster (2003, 2005, 2007) has refined the low-level taxonomy of the group and provided for finer scale subdivision and correlation of strata within the Dyeran Stage. In the Middle Cambrian, precise sampling and systematic reevaluation of lower to middle Middle Cambrian tri-lobite faunas through the Delamaran and basal Marju-man Stages has produced a highly refined species-based biostratigraphy for that interval in the southern Great Basin (Sundberg and McCollum, 2000, 2002, 2003a, b; Sundberg, 2005; McCollum and Sundberg, 2007). Similarly narrowed species and genus concepts have emerged from studies of silicified Upper Cambrian (Adrain and Westrop, 2004; Westrop and Adrain, 2007) and Lower Ordovician (Adrain, 2005; Adrain and Westrop, 2006) trilobite faunas. Recent work on Lower Ordovician co-nodonts (Nicoll et al., 1999; Miller et al., 2003, 2006; Pyle and Barnes, 2003; Lehnert et al., 2005) also holds promise for subdivision of some of the thicker conodont zones in that series. However, in most instances, the more restrictive definitions of the genera and species under study havecalled into question the interregional correlations made in previous work, instead of reinforcing or improving on them.
The challenge for the next few decades will be to connect these refined local or regional zonations. This will require integration of biostratigraphy with non-paleontological correlation methods, which have been steadily improving in accuracy and precision. It will also require thorough documentation of inadequately described faunas in areas that lie between the relatively few and widely separated locations (e.g., the Great Basin in Utah and Nevada and the southern Oklahoma aulacogen) where work has been focused historically because of thick, well-exposed, and highly fossilifer-ous successions well suited for recovery of precise biostratigraphic data. For examples of substantially improved correlations and more accurate paleogeo-graphic reconstructions produced by integration of precise biostratigraphic data (for multiple faunal groups) and geochemical data from areas where the units are poorly exposed or only sparsely fossiliferous, see Taylor et al. (1996, 2009b); Myrow et al. (2003), Landing and Westrop (2006), and Runkel et al. (2007).
In some instances, fossil groups other than trilobites, conodonts, and graptolites might prove instrumental in bridging the gaps between regional zonations. Although the time and space limitations in preparing this chapter allowed only a scant reference to other groups, recent studies have shown that much untapped bio-stratigraphic potential still exists in such groups as brachiopods (Freeman and Stitt, 1996; Robson et al., 2003; Holmer et al., 2005), cephalopods (Flower, 1970, 1976, 1988; Frey, 1995; Frey and Norford, 1995), echi-noderms (Sprinkle and Guensburg, 1997), radiolar-ians (Mun-zu and Iams, 2002), and even microbialites (Shapiro and Awramik, 2006) in the lower Paleozoic carbonates of Laurentian North America. Organic-rich subtidal facies, which formed in deeper areas of the GACB where the influx of fine siliciclastics resulted in deposition of dark shale interbedded with the carbonate lithologies, are noteworthy not only for the potential recovery of graptolites, but also for the possibility of extraction of other organic-walled fossils such as ac-ritarchs (Wood and Stephenson, 1989; Strother, 2008; Wicander and Playford, 2008) and chitinozoans (Achab, 1989; Paris et al., 2004). Although such facies typify the Middle and the Upper Ordovician of the Appalachian foreland basin, they are not restricted to that region or time interval. Similar depositional conditions also prevailed throughout much of the Cambrian and earliest Ordovician at the seaward margins of the GACB and at the inner edge of the carbonate belt, where proximal carbonate deposits interfinger with shale that accumulated in the deepest offshore parts of the adjacent inner detrital belt (Aitken, 1966, 1978; Palmer, 1971; Lohmann, 1976).
To illustrate the fine biostratigraphic precision already attainable through use of just the boundaries of the species-based zones and subzones established on trilobites and conodonts through the Cambrian-Ordovician boundary interval, Figure 5 was designed to include all of the biostratigraphic units recognized interregionally in the upper half of the Sunwaptan Stage, the entire Skullrockian Stage, and the base of the Stairsian Stage. The columns on the right and left show how some of the trilobite faunas and zonal or subzonal boundaries have been traced even into coeval deep-water successions deposited on the southern (Iapetus) and northern (Panthalassa) margins of the Laurentian continent. As shown with gray and white rectangles in the column entitled BDI (for bio Stratigraphically discriminated intervals, which are for the most part overlapping range zones), the platform tri-lobite and conodont zonations, used in conjunction, allow discrimination of seven different intervals in the upper Sunwaptan and 18 separate packages within the Skullrockian and basal Stairsian intervals covered in the figure. Note that the number of BDIs within the Skullrockian Stage will increase as the boundaries of the subzones recognized within the Rossodus manitouensis conodont Zone by Lehnert et al. (2005) are recognized in more areas. Similarly, recent work on Skullrockian trilobite genera such as Clelandia (Taylor, 1999), Symphy-surina, and numerous new genera being extracted from the previously undersplit genus Hystricurus (Adrain et al., 2003; Adrain and Westrop, 2006) should yield finer subdivision of the relatively thick Symphysurina and Bellefontia trilobite Zones. Moreover, there is no reason tobelieve that the Cambrian-Ordovician boundary interval is unique in this regard. Similar efforts toward refinement of the low-level taxonomy of faunas in other intervals of the Cambrian and the Ordovician almost certainly will produce comparable improvements in biostratigraphic precision. Although much has been accomplished in building on the foundation presented by Lochman-Balk and Wilson (1958) more than half a century ago, much more work remains to be done by future generations of stratigraphers to take full advantage of the information encoded in the remains of the Cambrian and the Ordovician inhabitants of the GACB.
We thank L. E. Edwards and R. C. Orndorff for thorough and helpful reviews of the manuscript. Additional constructive criticism was provided by J. R. Derby, R. L. Ethington, S. C. Finney, E. Landing, J. F. Miller, A. R. Palmer, and B. R. Pratt. However, the authors assume full responsibility for the content and design of the final text and figures. B. Pedder assisted with review of the literature on acritarchs and other palynomorphs.
Figures & Tables
Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia
The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.