Geology

doi: 10.1130/G35886.1 published online 26 September 2014;Geology

Ian W.D. Dalziel

connection?Cambrian transgression and radiation linked to an Iapetus-Pacific oceanic

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Cambrian transgression and radiation linked to an Iapetus-Pacific oceanic connection?Ian W.D. Dalziel*Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, USA

ABSTRACTThe geologically abrupt appearance in the fossil record of almost

all animal phyla is referred to as the Cambrian radiation or explo-sion of life on Earth. Also known as Darwins dilemma, because it seemingly posed a major problem for his theory of gradual evo-lution, it coincided with the initiation of the first of the two princi-pal global marine transgressions of the Phanerozoic. Although now seen as more protracted, it is still one of the most striking and criti-cal events in the history of the biosphere. Almost all paleogeographic reconstructions for the early Cambrian feature a previously isolated Laurentia, the core of ancestral North America. Yet geological evi-dence from five continents, integrated here for the first time, indicates that the present-day southern cone of Laurentia was still attached to the newly amalgamated supercontinent of Gondwanaland into Cambrian times. Laurentia was then isolated by the development of a major deep oceanic connection between the opening Iapetus Ocean basin and the already well-developed paleo-Pacific. As the marine transgression advanced, major changes in ocean chemistry occurred, upwelling generated phosphorite deposits, and the number of fossil-ized metazoan phyla exploded with morphologic disparity between Laurentia and Gondwanaland already established. The development of this deep oceanic gateway, and of an ocean floorconsuming and arc-generating subduction zone along virtually the entire margin of Gondwanaland shortly thereafter, need to be taken into account in consideration of the global environmental and biotic changes associ-ated with the Neoproterozoic-Phanerozoic transition.

NEOPROTEROZOIC AND CAMBRIAN PALEOGEOGRAPHYThe importance of paleogeography for understanding the Cambrian

explosion has been recognized for many years, as has the probability that the initiation of the Cambrian transgression, like that of the Cretaceous, was related to the breakup of a supercontinent (Valentine and Moores, 1970; Brasier, 1992; Fortey et al., 1996). One of the most striking aspects of the Cambrian fossil record is the geographic differentiation of the first benthic trilobite faunas on Laurentia and the newly amalgamated super-continent of Gondwana, pointing to a missing ancestry. This led to the suggestion of an unknown vicariant event intervening between the devel-opment of an unfossilized ancestral trilobite clade and the higher members that are well represented by the fossil record (Fortey et al., 1996; Lieber-man, 2003; lvaro et al., 2013).

The absence of preserved pre-Jurassic ocean floor renders the recon-struction of supercontinents that amalgamated prior to the late Paleo-zoic assembly of Pangaea subject to numerous uncertainties. Nonethe-less, paleomagnetic data are widely accepted to indicate that following the Neoproterozoic breakup of the late Mesoproterozoic supercontinent Rodinia to form the paleoPacific Ocean basin, Laurentia was in a tropi-cal to equatorial location during the Cambrian, as was the Antarctic-Australian portion of the paleo-Pacific margin of the newly amalgamated Gondwanaland. Hence, almost all published reconstructions for the early Cambrian show the present southern cone of a previously isolated Lau-rentia opposed to the Antarctic margin of Gondwanaland across the Iape-tus Ocean (Meert and Lieberman, 2004; Pisarevsky et al., 2008; Fig. 1A).

However, changing only the unconstrained paleolongitude, the paleomag-netic data can also be satisfied with present-day southern Laurentia juxta-posed with Patagonia, southernmost Africa, and the Ellsworth-Whitmore Mountains crustal block of Antarctica (restored to its original position in between Africa and East Antarctica; see the GSA Data Repository1). This is the Pannotia supercontinental arrangement that followed initial breakup of the Rodinian supercontinent, opening of the paleoPacific Ocean basin, and amalgamation of Gondwanaland (Dalziel, 1997; Fig. 1B).

Strongly supporting such a juxtaposition, recent studies of late Mesoproterozoic volcanic rocks in East Antarctica that form isolated nunataks in Coats Land at the head of the Weddell Sea (Fig. 2) indicate that they are likely a former fragment of the Laurentian craton (Loewy et al., 2011). Comparison of lead isotopic data from these granophyres and rhyolites with those of contemporaneous (ca. 1112 Ma) igneous rocks of the Umkondo large igneous province of southern Africa and the Keween-awan large igneous province of the mid-continent rift system of Laurentia revealed that the Coats Land crustal block is likely to have been part of the latter craton in Mesoproterozoic times. Paleomagnetic data show that juxtaposition with the southern cone of Laurentia is possible at that time.

GROWTH OF THE IAPETUS OCEAN BASINThe existence of Pannotia, the supercontinental assembly formed

by Laurentia and the newly completed Gondwanaland, was fleeting. The Iapetus Ocean basin between proto-Appalachian Laurentia and proto-Andean South America had started to open even before Gondwanaland amalgamation was complete. Precise timing of rift-drift transition along the margins of the Iapetus Ocean is difficult to determine. However, the ages of rift-related igneous rocks are well known and indicate that rela-tive to present-day North America, rifting was initiated in the north. Igne-ous rocks related to the Central Iapetus magmatic province extend from Newfoundland to the Chesapeake Bay area and range in age from 615 to 564 Ma (Mitchell et al., 2011), an interval that includes rift-related vol-canic rocks in the Dalradian of Scotland (601 4 Ma; Dempster et al., 2002). In striking contrast (Fig. 3), equivalent rift magmatism in present-day southern Laurentia (Ouachita embayment [Hanson et al., 2013]; New Mexico [Amato and Mack, 2012]), the Sierra de la Ventana of Argentina (Rapela et al., 2003), southernmost Africa (Kisters et al., 2002), and the Ellsworth-Whitmore Mountains crustal block of West Antarctica (Rees et al., 1997; Curtis et al., 1999) did not occur until the Cambrian. This strongly suggests that the Iapetus Ocean opened from north to south rela-tive to present-day North America, and that the present southern cone of Laurentia was still attached to the newly amalgamated Gondwanaland, even as the Iapetus Ocean basin started to open further north during the Neoproterozoic (Fig. 4). An analogous situation occurred in the Mesozoic South Atlantic Ocean basin where magnetic anomalies indicate that open-ing began in the south. The initial rift-related magmatism formed the Early Cretaceous Parana-Etendeka large igneous province (ca. 132 Ma); addi-tional rifting and associated magmatism in northeastern Brazil followed during the Late Cretaceous (Fig. 4, inset).

*E-mail: ian@ig.utexas.edu.

GEOLOGY, November 2014; v. 42; no. 11; p. 14; Data Repository item 2014346 | doi:10.1130/G35886.1 | Published online XX Month 2014 2014 Geological Society of America. For permission to copy, contact editing@geosociety.org.

1GSA Data Repository item 2014346, table of rotation poles used in the reconstruction shown in Figure 4, is available online at www.geosociety.org/pubs /ft2014.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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Separation of Gondwana and Laurentia in the area of Patagonia, southernmost Africa, and the restored Ellsworth-Whitmore Mountains does not seem to have occurred until close to the time of the main Cam-brian marine transgression and the radiation of metazoan life (Erwin and Valentine, 2013). Notably, recent detrital zircon studies have demon-strated that the Sauk Sequence resulting from the transgression of cratonic Laurentia was not initiated in west Texas (USA) until the Cambrian, sig-nificantly later than along its eastern and western margins (Spencer et al., 2014). The fact that the first benthic trilobites reflect geographic differen-tiation between the two continental masses, the olenellids in Laurentia and the redlichiids in Gondwana (Brasier, 1992; Fortey et al., 1996; Lieber-man, 2003; lvaro et al., 2013), suggests significant separation prior to the start of Cambrian Stage 3 (ca. 521 Ma; Fig. 3). Phylogenetic analysis indicates that while the trilobites of northeastern Laurentia had affinities with those of Baltica, the southwestern forms were closely related to those of Gondwana (Lieberman, 2003).

This separation reflects the initiation of a major deep oceanic con-nection between the opening Iapetus Ocean basin and the previously established paleoPacific Ocean basin (Dalziel, 1997; Fig. 4). The peak of Neoproterozoic-Cambrian phosphorite production, ascribed to upwelling associated with development of a low- to mid-latitude seaway (Cook and Shergold, 1984; Brasier, 1990), and significant changes in oceanic circula-tion, chemistry, and the marine biosphere, including oxygenation, occur at the Neoproterozoic-Cambrian transition (Maloof et al., 2010; Sperling et al., 2013; Marshall, 2006). Such changes, together with faunal isolation, are well established as accompanying development of Mesozoic-Ceno-zoic oceanic gateways (Dalziel et al., 2013, and references therein).

THE CAMBRIAN TRANSGRESSION AND RADIATIONA myriad of explanations has been offered to explain the Cambrian

radiation of metazoan life (Smith and Harper, 2013). It seems likely that there was no one single cause. Nonetheless, major geographic changes have the potential to set the framework for global environmental and hence ecological changes involving sea level and oceanic circulation, as

A B

L

GR

CU

LF

NNK

E GM

C

GR

60S

30S

OESC

D

South America

Africa

Figure 2. Present-day geography showing localities mentioned in text. Areas shown in black have Grenvillian (ca. 1.0 Ga) basement. CCoats Land crustal block; CIMPCentral Iapetus magmatic province; CUCuyania terrane (greater Argentine Precordillera); DDalradian (Tayvallich) volcanics, Scotland; EEllsworth Moun-tainsHaag Nunataks portion of Ellsworth-Whitmore Mountains crustal block; GGrunehogna terrane; GRtype Grenville orogen; KKalahari craton; LLaurentian craton; LFLafonian terrane (Falkland-Malvinas Islands crustal block); MMaud orogen; NNNamaqua-Natal orogen; OEOuachita embayment; SCsouthern cone of Laurentia (west Texas, New Mexico, USA); SVSierra de la Ventana (Argentina). Red lettering indicates areas of rift-related igneous rocks identified in Figure 3.

Figure 1. Comparison of paleogeographic reconstructions for the Precambrian-Cambrian transition. Note that relative positions of Lau-rentia and Gondwana differ only in paleolongitude. A: Meert and Lieberman (2004). LauLaurentia. B: Dalziel (1997). AArequipa/Anto-falla terrane; AMAmazonian craton; CCongo craton; EEllsworth-Whitmore Mountains crustal block (restored to its position prior to Gondwana fragmentation); F/MPFalkland-Malvinas plateau; KKalahari craton; RPRio de la Plata craton; SFSo Francisco craton; TxPhypothetical Texas Plateau; WAWest African craton. Cross-hatching denotes East AfricanEast Antarctic orogen.

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well as ocean and atmospheric chemistry as noted by Meert and Lieber-man (2008).

The most striking geological feature of the Precambrian-Cambrian boundary is the surface known as the Great Unconformity that separates Cambrian strata from underlying, mainly crystalline, rock in many parts of the world. It has been suggested recently that, because of prolonged continental denudation before the deposition of the overlying Phanerozoic strata, this paleogeomorphic surface represents a unique physical environ-ment that affected seawater chemistry during the ensuing transgression and expansion of shallow marine habitats (Peters and Gaines, 2012).

Turning to the cause of the two principal marine transgressions of the Phanerozoic (Fischer, 1984), that of the Cretaceous transgression is still debated even though it can be linked to the seafloor spreading history. There are two related hypotheses: rapid production of warm, low-density oceanic lithosphere that displaced seawater at spreading ridges during the Cretaceous normal superchron, and the supercontinent breakup effect, specifically creation of mid-Atlantic and Indian Ocean ridges at the expense of subduction of older, dense lithosphere in Tethys and the Pacific realm (Conrad, 2013). Whereas seafloor spreading rates cannot be determined for Neoproterozoic to early Paleozoic Iapetus, there is a similarity between the early Cambrian paleogeography proposed here, with the development of an Iapetus-Pacific oceanic gateway (Fig. 4), and mid-Cretaceous open-ing of a connection between the northward-propagating South Atlantic Ocean basin and the previously formed Central Atlantic Ocean basin (Fig. 4, inset). Both involved considerable increase in length of young spreading ridges. Hence the same fundamental mechanism may have caused both of these great transgressions. In the case of the Cambrian event, this mecha-nism, the propagation of an Iapetus spreading ridge into the pre-existing paleoPacific Ocean basin, seems likely to have been at least partly respon-sible for the striking geographic differentiation of faunal provinces at the time when the first fossilized benthic trilobites lived at ca. 520 Ma.

The absence of ancestors to the phyla that explosively radiated in the Cambrian fossil record, Darwins dilemma, may also be related to paleogeography. The Delamerian-Ross subduction zone along the Pacific-Iapetus margin of Gondwanaland from Australia to South America was initiated in the Cambrian, soon after the connection of the two ocean basins (Cawood, 2005), destroying older oceanic lithosphere and hence contributing to transgression. Much of the sediment deposited on oceanic crust and on continental slopes and rises in the latest Precambrian to early Cambrian would have been subducted directly, or else accreted as wedges subsequently subducted, or uplifted and eroded together with any fossils or traces of ancestors of the phyla represented in the Cambrian radiation.

CONCLUSIONThe geological evidence indicates that paleogeographic changes

at the Neoproterozoic-Phanerozoic transition appear to have been much

first trilobite fossils and

Sauk transgression west Texas

Ellsworth Mts

Sierra de la Ventana

Southern Africa

Ouachita embayment

CIMP flood basalts

Dalradian volcanics X

542

530

521

515

510

499

489

peak phosphorites

Sauk transgression

E and W Laurena

New Mexico

PALEOPACIFIC OCEAN

IAPETUS OCEAN

CA

SA

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AUS

EANT

IND

AFR

SAM

LAU

GR

KNN

EC

MG

CU

530 Ma

LF

[olenellids]

[redlichiids]

Figure 3. Timing of synrift magmatism on margins of Iapetus for the latest Precambrian and Cambrian (see text). Time scale is from Er-win and Valentine (2013). CIMPCentral Iapetus magmatic province.

Figure 4. Cambrian paleogeography proposed here on geological grounds for ca. 530 Ma as an Iapetus-Pacific deep oceanic connec-tion developed (see the Data Repository [see footnote 1] for rotation parameters). The reconstruction is generally compatible with paleo-magnetic reconstructions. In detail, data are somewhat ambiguous and are shown here mainly for reference: pole for Gondwana at 530 Ma (southern African coordinates) based on a spherical spline path (Torsvik et al., 2012; gray cross); poles for Laurentia at 533 Ma (gold cross; Wichita granites from the Ouachita embayment) and Cuyania restored to the Ouachita embayment of Laurentia at 525 Ma (blue cross) (Rapalini, 2012). a95 circles of confidence are shown, and the grid is based on the Laurentian pole. The only other pole for Lauren-tia in this age range, the Mont RigaudChatham Grenville pole (532 Ma) that forms the sole basis for the Laurentian spline pole for 530 Ma (Torsvik et al., 2012), does not appear in the global stereographic projection shown here. Baltica had separated from Laurentia by ca. 550 Ma (Pisarevsky et al., 2008) and was an isolated continent outside the view shown here. Areas with Grenvillian basement (ca. 1.0 Ga) are shown in black. Legend as in Figure 2, with addition of: AFRAfrica; AUSAustralia; EANTEast Antarctica; INDIndia; LAULaurentia; SAMSouth America. Area of likely extended con-tinental crust is shown with black arrows and includes the hypo-thetical Texas Plateau that incorporates the Cuyania terrane (Dalziel, 1997). Blue cones mark extent of Cambrian Delamerian-Ross conti-nental margin arc (Cawood, 2005). Inset: Mid-Cretaceous paleoge-ography from database of PLATES project, Institute for Geophysics, The University of Texas at Austin. CACentral Atlantic Ocean; SASouth Atlantic Ocean. Red stars show location of Parana (South America)Etendeka (Africa) large igneous province.

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more profound than previously recognized. They need to be taken more fully into account in consideration of possible environmental factors influencing the Cambrian transgression and radiation of metazoan life on Earth. Beyond the marine transgression itself, they are likely to have had a major influence on oxygenation, oceanic circulation and chemistry, bioge-ography, and the preservation of fossils in the oceanic realm.

ACKNOWLEDGMENTSMy work in the Antarctic was funded by the United States Antarctic Pro-

gram of the National Science Foundation. I thank Lisa Gahagan for help with the reconstructions. This is Institute for Geophysics, The University of Texas at Austin, contribution #2721.

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Manuscript received 19 May 2014 Revised manuscript received 13 August 2014 Manuscript accepted 14 August 2014

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