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2 Paleogeography of the Great American Carbonate Bank of Laurentia in the Earliest Ordovician (Early Tremadocian): The Stonehenge Transgression

James R. Derby1

Consultant, Leonard, Oklahoma, U.S.A.

Robert J. Raine2 and M. Paul Smith3

Lapworth Museum of Geology, University of Birmingham, Edgbaston Birmingham, United Kingdom

Anthony C. Runkel Minnesota Geological Survey, Saint Paul, Minnesota, U.S.A.

ABSTRACT

This chapter describes and presents a newly compiled map illustrating the paleogeography of Laurentia during the earliest Ordovician, a time when the great American carbonate bank was at one of its greatest extents and a period for which the most is understood. The map depicts the known or postulated extent of the inner detrital belt, the great American carbon- ate bank and the more problematic (commonly structurally relocated) outer detrital belt. The period on which the map is based and discussed in the accompanying text is based on the Early Ordovician (early Ibexian) (early Tremadocian) Stonehenge transgression.

INTRODUCTION

In 1980, while Derby and Ginsburg were preparing Ginsburg for his talk at the 1980 Society of Economic Paleontologists and Mineralogists Research Confer- ence on the Carbonate and Orthoquartzite Suite (Dott and Byers, 1981), Ginsburg asked the questions, ‘‘Just what was the extent of the great American (carbonate)

bank (GA[C]B)?’’ and ‘‘How broad was the GA(C)B?’’ At that time, no single map existed. In the process of answering those questions again in 2010, Derby real- ized that no modern platewide map of the original extent of the GACB exists, 30 yr later.

To this day, authors continue to use a series of out- dated maps, all of which were also available in 1980. Many authors continue to use Kay’s famous map (1951).

Derby, James R., Robert J. Raine, Anthony C. Runkel, and M. Paul Smith, 2012,

Paleogeography of the great American carbonate bank of Laurentia in the earliest

Ordovician (early Tremadocian): The Stonehenge transgression, in J. R. Derby,

R. D. Fritz, S. A. Longacre, W. A. Morgan, and C. A. Sternbach, eds., The great

American carbonate bank: The geology and economic resources of the Cambrian–

Ordovician Sauk megasequence of Laurentia: AAPG Memoir 98, p. 5–13.

1Present address: Department of Geosciences, University of Tulsa, Tulsa, Oklahoma, U.S.A. 2Present address: Ichron Ltd., Norwich, Cheshire, United Kingdom. 3Present address: Oxford University Museum of Natural History, Oxford, Oxfordshire, United Kingdom.

DOI:10.1306/13331487M983496

However, this map actually shows paleographic realms of the lower Tippecanoe megasequence and nothing at all of the Sauk megasequence. More accurate Sauk maps by Reuben Ross (1976) also are widely used by authors, including somein this volume. However, the Ross (1976) map portrays only the western United States and parts of Mexico.

All of North America is illustrated on the magnif- icent maps in the Stratigraphic Atlas of North and Central America (Cook and Bally, 1975). These maps do an excellent job of portraying the present extent of existing strata of the GACB, with some exceptions; they do not include areas where the GACB sediments have been removed by later erosion, nor those parts that have drifted away. However, these maps do include areas that were appended to North America much after Sauk deposition, and, somewhat misleadingly for the pur- poses of this volume, they show the facies of these appendages to Laurentia. In many respects, these maps are superior in detail to our current effort, but they also contain some serious omissions and miscorrelations, which the chapters in this volume will demonstrate. For example, the Reelfoot rift basin and its sediments were completely omitted.

CONSTRUCTION OF THE PALEOGEOGRAPHIC MAP

This map (Figure 1) was originally intended to be used in an introductory summary chapter by Derby and Ginsburg, with a review of the naming of the great American carbonate bank (GACB) and the original dilemma posed by Ginsburg in 1980 a review of what we have learned to date, and a discussion of possible depositional mechanisms for the sequences we observe. The compilation of the map soon exceeded that original intent and was judged to deserve a separate chapter.

The schematic map of Byers and Dott, presented here as Figure 2, represents a period in the latest Cam- brian and earliest Ordovician when the inner detrital belt was at its maximum that likely corresponds, at least in part, to a widely recognized lowstand—the Lange Ranch lowstand (Miller et al., 2012). This leaves the impression, correct for the period portrayed, that the GACB was a narrow belt fringing Laurentia. For a stratigraphic cross section illustrating this period, see figure 3 of Runkel et al. (2012).

The earliest Ordovician map represents, however, the probable near-maximum extent of the GACB dur- ing the Stonehenge transgression. Figure 3 of Runkel et al. (2012) also illustrates the extent of the Lower Ordovician carbonates of the Gasconade Formation (and its equivalents), which were deposited during the Stonehenge transgression.

The schematic paleographic map of Figure 1 is highly generalized, except in a few areas, where fairly precise information is available. This map attempts to portray the extent of the GACB at the time of the early Trem- adocian or Stonehenge transgression (Taylor et al., 1992). This probably does not represent the greatest maximum transgression of the Sauk megasequence, but it is one of the intervals that is most extensively preserved and widely recognized. Because later maximum transgres- sions are poorly preserved, if at all, on the cratonic interior, the transition from the carbonate bank to the inner detrital belt (IDB) is poorly known. For compar- ison of well-documented Sauk sea level curves, see the well-studied and nearly 100% exposed sections in west- ern Utah’s Ibex-House Range, sections presented in Miller et al. (2012), and the extrapolation of the Utah curves into the Missouri Ozarks sequences presented by Miller, in Palmer et al. (2012).

The generalized map of Figure 1 is not intended to be a Global Information System-registered accurate- to-the-mile summary of final results. State lines and in- ternational borders are intentionally left off to under- score an absence of precision. We have attempted to summarize the general state of our knowledge, as dem- onstrated in much greater detail in this volume.

Two possibly controversial innovations are on this map. One is the lack of defined siliciclastic source area. As Runkel points out in the discussion below, without outcrop of the paleoshoreline, we really have no way of knowing wherethe IDB ends and the source area begins, so he showed the IDB and exposed land combined into one map unit. The second issue is the removal of the Transcontinental arch (TA) from this paleogeographic map. For years, several experienced stratigraphers have questioned the validity of the TA as a control on depo- sition during the Cambrian and Ordovician. Myrow et al. (2003) finally dispelled the myth. The TA is mostly an artifact of uplift and erosion after Sauk deposition.

Following is a discussion of the degree of restoration and sources of information for the map and the three major facies belts as shown on the map (Figure 1). In this discussion, compass directions and coordinates will refer to modern maps, not to paleoreconstructions.

PALINSPASTIC AND CONTINENTAL DRIFT RESTORATION

We have taken some liberties with this map, with the result that the end product is somewhat uneven. The North Atlantic region is shown with major restoration of parts of Laurentia that have been dislocated sig- nificantly by continental drift. In contrast, the western

6 Derby et al.

Figure 1. Restored extent of the great American carbonate bank (GACB) during the earliest Ordovician (early Tremadocian) Stonehenge transgression. This transgression occurred during deposition of the Early Ordovician Symphysurina trilobite Zone or during deposition of the Cordylodus angulatus and basal Rossodous manitouensis conodont Zones. This period probably does not represent the maximum transgression during deposition of the Sauk III but is commonly preserved and recognized across Laurentia. The records of younger transgressions, which may have been more extensive, are commonly removed by erosion. Consequently, this map records the maximum documented (at this time) extent of the GACB across Laurentia.

Paleogeography of the Great American Carbonate Bank of Laurentia 7

part of North America (modern coordinates) is shown with no restorations at all because of the complexity of the area and the mixture of translocated parts of Laurentia and exotic terranes. In Alaska, some of the terranes, although accurately dated and clearly delin- eated, contain mixed faunas that cannot be ascribed to Laurentia with certainty, so these terranes are omitted from the map and indicated by question marks.

In other areas, a modest amount of palinspastic res- toration is effected as in the Appalachians and in the southern Oklahoma–Arkansas Ouachita region, where seismic data provide a fair estimate of the amount of subsequent overthrusting of the outer detrital belt (ODB) over the great American carbonate bank (GACB) car- bonate shelf.

Cuyania, the Argentine pre-Cordillera, is restored to its postulated position adjacent to the Marathon and Ouachita basins. This area clearly contains Cambrian trilobites of Laurentian affinity but differs sufficiently in lithofacies and its Ordovician fauna to suggest it is slightly detached from the contiguous GACB (Dickerson, 2012; Keller, 2012). The outer margin of the Cuyania little American carbonate bank is delineated by the Valle Fertil lineament (VFL on Figure 1), beyond which no terranes of Laurentian affinity are known.

Two parts of North America that were not part of Laurentia are eliminated from the map: the Suwannee terrane of Florida, Georgia, and Alabama and the region of southern Mexico. The Suwannee terrane lies south of the Suwannee terrane suture (STS on Figure 1). As sug- gested by Bass (1969) and Rodgers (1970) and supported by paleontologic evidence (Pojeta et al., 1976; Derby, 1982), the area is a fragment of western Africa left at-

tached to North America following the opening of the modern Atlantic Ocean.

The complex southern margin of Laurentia in Mexico is approximated by the position of the Walper mega- shear, a Mesozoic feature. The map shows that position, as drawn by Pessagno and Martin (2003).

THE INNER DETRITAL BELT AND SOURCE AREA

The inner detrital belt (IDB) (Palmer, 1960) represents a suite of typical nearshore marine siliciclastic facies that contain features reflecting the importance of both wave- and tide-generated currents in the depositional system. The facies range from relatively coarse-grained shoreline deposits to offshore deposits dominated by very fine grained sandstone, siltstone, and shale that accumulated below fair-weather wave base. The IDB is shown combined with exposed land in Figure 1 be- cause the Lower Ordovician record is inadequate to distinguish a discrete paleoshoreline, except in rela- tively rare and localized areas of the continent.

The transitional area between the IDB and the GACB throughout much of the Cambrian and earliest Or- dovician (Figure 2) was commonly characterized by relatively deep-water deposition recorded by mixed carbonate and siliciclastic facies with condensation fea- tures and other attributes indicative of suppressed car- bonate productivity and starvation of siliciclastic sand. The interfingering of both siliciclastic and carbonate strata makes it difficult to establish an objective de- finitive boundary between the IDB and GACB. The boundary shown in Figure 1 is intended to generally

Figure 2. Byers and Dott (1995) map of Laurentia during deposition of the latest Cambrian Jordan Sandstone, published with permission of SEPM. This map fairly represents, at the degree of detail intended, a period of the maximum extent of the inner detrital belt and, therefore, a mini- mum extent of the great American car- bonate bank (GACB). 1000 km (621 mi).

8 Derby et al.

correspond to the interior edge of where Lower Or- dovician strata are dominated by stacks of oolitic, ribbon-rock, and microbialite carbonate facies that typify much of the GACB.

Depiction of the outboard (paleoseaward) extent of the IDB during the Stonehenge transgression is more problematic in some respects than for previous times. The pronounced continental-scale flooding during the Stonehenge transgression led to onlap and deposition of classic GACB facies across much more of craton than during Cambrian flooding events. As a result, in many places, the boundary between the IDB and GACB dur- ing the Early Ordovician was located in relatively far interior areas that no longer have a rock record for this period. The cratonic interior extent of the GACB across much of the map shown in Figure 1 might thus best be considered a minimum landward extent.

GREAT AMERICAN CARBONATE BANK

The greatest dimensions of the GACB lie almost en- tirely in the United States and Mexico. This truly great carbonate bank has maximum dimensions of more than 3000 km (1860 mi) east to west (Nevada–Tennessee) and 1500 km (930 mi) north to south (Texas–Minnesota) (Figure 1). Thickness ranges considerably depend on location and stratigraphic units, from more than 5000 m (>17,000 ft) in western Utah to a few hundreds of meters in thinner parts. The carbonate bank apparently nar- rows considerably in the western United States and Canadian Cordillera and in the eastern United States and the Canadian northern Appalachians, to 100 to 300 km (60–185 mi). The carbonate bank is once again much broader in northern Canada and Greenland, reaching a width of about 1000 km (�620 mi).

The continuity of the GACB along the Laurentian margin from western Newfoundland, through Scot- land to Greenland, can be deduced from the similarity of the conodont fauna (Smith, 1991; Ji and Barnes, 1994; Raine, 2010) and its position before opening of the North Atlantic has been based on the reconstruc- tion by Higgins et al. (2001).

Scotland and Western Newfoundland

The eastern Laurentian (southern paleomargin) con- sists of a series of promontories and reentrants (Lavoie et al., 2003); with Scotland situated on a promontory that marks a significant inflexion in the continental margin (Thomas, 1977; Soper, 1994). These paleogeo- graphic features suggest that the original shape of the Iapetus margin reflects the interplay of rifting and oce- anic transform faults (Soper, 1994; Lavoie et al., 2003).

The Baie Verte line represents the margin of Lauren- tia in western Newfoundland and preserves ophiolite suites and volcanic and volcaniclastic rocks thrust against the GACB strata.

The Hebridean terrane rocks’ Durness Group of the GACB in northwestern Scotland can be correlated with the external Humber zone (Port au Port Group) of the northern Appalachian orogen in western Newfound- land (Williams, 1979). In northwestern Scotland, GACB facies are preserved in the Hebridean terrane, one of several terranes that lie north of the Iapetus suture. Clo- sure of the Iapetus Ocean resulted in collision of island arcs and, finally, Avalonia and Baltica with the margin of Laurentia, forming these terranes. The exposed rocks represent a succession preserved within the Caledonian foreland. The Highland Boundary Fault marks the south- ern margin of autochthonous Laurentian crust during the Early Ordovician (Armstrong and Owen, 2001) and continues into Ireland, where it is represented by the Fair Head-Clew Bay line (Harper et al., 1989; Chew, 2003). With closure of the Iapetus Ocean, the Caledonide mountain belt developed, and much of the GACB and ODB were removed by thrusting and erosion.

A Scandian thrust fault (the Moine thrust) over- rides the Ordovician GACB sediments of the Caledo- nian foreland in Scotland, and although the margin likely lays some distance to the southeast of the pre- served Durness Group, it is now overlain by Precam- brian metasediments and gneisses. The Durness Group extends approximately 180 km (112 mi) in a northeast– southwest direction and, because of the uniformity of the stratigraphy and facies, the outcrop is interpreted to be close to depositional strike.

Greenland and Svalbard

The GACB sediments of Greenland and Svalbard Ar- chipelago (Stouge et al., 2012) present a complex picture requiring considerable reconstruction.

The GACB deposits in Greenland are principally dis- tributed along the modern-day north and east coasts, although Ordovician sea level maximums did result in the deposition of an unconformity-dominated succession at Fossilik, West Greenland (Smith, 1988). On the east coast, peritidal and shallow-subtidal carbonates ex- tend in a belt for 1300 km (808 mi) from Scoreseby Sund in the south to Kronprins Christian Land in the north (Stouge et al., 2012). The succession is deformed by a single phase of deformation, the Scandian, associated with the main collision of Baltica and Laurentia (Smith and Rasmussen, 2008). This deformation disrupted the margin into a series of thrust sheets that accommodated shortening of several hundred kilometers. The GACB deposits are present in both the foreland, where they

Paleogeography of the Great American Carbonate Bank of Laurentia 9

constitute an attenuated succession a few tens of meters thick, and in the highest thrust sheet, where the subtidally dominated GACB component of the Kong Oscar Fjord Group is in excess of 4 km (13,000 ft) in thickness (Smith and Rasmussen, 2008).

In North Greenland, from Kronprins Christian Land in the east to Nares Strait in the west, GACB deposits are part of the Franklinian Basin, which extends across the strait into Arctic Canada for approximately 2000 km (�1240 mi) in total. The Franklinian Basin in North Greenland is a relatively undeformed part of the Lau- rentian margin, with an infill that was deformed by the Ellesmerian orogeny in the Devonian, but which has not been affected by major collisions or margin-parallel strike- slip tectonics (Higgins et al., 1991). The Franklinian mar- gin preserves an inboard shallow-water shelf area with GACB deposits up to 1 km (3300 ft) thick and an out- board deep-water component of the basin, which was al- ternately starved or filled by sand deposited by turbidity currents and representative of the ODB. Although now partoftheSvalbardArchipelago,theCambrian–Ordovician rocks of Bjørnøya were deposited on the northeastern extremity of this Franklinian shelf (Smith, 2000).

The Svalbard Archipelago is composed of Spitsbergen, Nordaustlandet, Bjørnøya, and several other islands and is generally considered to comprise three principal terranes: the western, central, and eastern provinces of Harland (1997). Bjørnøya has been considered to be part of the central province but, as noted above, the stra- tigraphy indicates that it was a constituent part of the Laurentian margin until it was displaced on a strike-slip fault before Atlantic opening (Smith, 2000).

Good evidence exists that the eastern province was the outboard extension of the East Greenland margin at the time of GACB deposition and was displaced to its current location by late Caledonian strike-slip tec- tonics. The succession preserves typical Cambrian– Ordovician GACB carbonates, similar to the upper allochthonous sheet in the Greenland Caledonides, that are overlain by the ODB deep-water sediments following the foundering of the shelf late in Sauk IV deposition (Fortey and Barnes, 1977; Harland, 1997; Smith and Rasmussen, 2008).

The western province of Spitsbergen has been com- pared with North Greenland, but the correlation is not secure and lacks stratigraphic detail. The most poorly reconstructed terrane is the central province, which has a thick Cambrian–Ordovician carbonate succession with Laurentian affinity (Szaniawski, 1994; Harland, 1997), capped by even more poorly known turbidites (Major and Winsnes, 1955). Its structural position sug- gests that it lays inboard of the eastern province, but it remains one of the most poorly known fragments of the GACB.

Canada and Alaska

Dewing and Nowland (2012) describe five sequences of GACB sediments in the Canadian Arctic, the fourth of which contains Stairsian evaporites. In general, a clearly defined shelf margin is preserved, with ODB sediments occupying the off-shelf area. These sequences extend from northwestern Greenland onto the Arctic Lowland of mainland Canada and into the Jones Ridge area of Alaska (Dumoulin and Harris, 2012; Pyle, 2012). Several well-defined carbonate terranes in Alaska either contain faunas of Siberian affinity or are of such uncertain affinity that they are not ascribed herein to Laurentia.

In the Cordillera of western Canada, and south into Montana and Idaho, the GACB constitutes a narrow belt, but with a well-defined ODB to its west.

OUTER DETRITAL BELT

The ODB represents deeper water slope-and-ocean- basin detrital sediments deposited outboard of the GACB. The overall record of the ODB is somewhat sketchy, as much of the ODB has been thrusted over GACB sediments and subsequently removed by ero- sion. We shall not attempt to document the ODB in detail, but mention some major features and/or areas of uncertainty.

The (modern) southern margin of the GACB is marked by the Mesozoic Walper megashear in Mex- ico and by the Marathon-Ouachita-Cuyania Basin ODB sediments in the southern United States.

In the Appalachian trend, the ODB can be approx- imated in the south by facies reconstruction (Read and Repetski, 2012) but is represented by the Conestoga, Lancaster, and Frederick Valley slope deposits in Penn- sylvania and Maryland (Brezinski et al., 2012).

From eastern Pennsylvania to western Newfound- land, the ODB is well preserved in the Taconic al- lochthon (Landing, 2012; Lavoie et al., 2012). In New- foundland, the ODB is also preserved. These sediments comprise the allochthonous Cow Head and Northern Head Groups (James and Stevens, 1986; Jameset al., 1989), which represent ramp-margin and slope deposits.

The ODB sediments of Early Ordovician age are not preserved in Scotland, but parts of the Dalradian Supergroup (including the Cambrian Leny Limestone, which comprises thinly bedded turbidite-deposited limestones) within the Highland Border complex rep- resent an older ODB of Laurentia (Fletcher and Rushton, 2008).

The transition from the GACB to the ODB is well defined—from Northwest Greenland westward and southward from the Arctic Islands of Canada through

10 Derby et al.

the Canadian Cordillera. Bush et al. (2012) describe the complex transition from GACB sediments to the poorly defined transition to the earliest Ordovician ODB in northwestern United States. Similarly, the location of the transition from the carbonates of the GACB to the ODB during the Stonehenge transgression is shown as poorly defined in most of western United States because of the complexities of the region mentioned earlier.

ACKNOWLEDGMENTS

This map was compiled by the authors with advice from Martin Keller, Pat Dickerson, Keith Dewing, George Dix, John Taylor, John Repetski, and Svend Stouge, and consultation with the many manuscripts for this volume.

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Paleogeography of the Great American Carbonate Bank of Laurentia 11

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