Articleshttps://doi.org/10.1038/s41561-018-0131-7

1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA. 2The Denver Museum of Nature and Science, Denver, CO, USA. 3Department of Geosciences, University of Arizona, Tucson, AZ, USA. 4Department of Geoscience, University of Calgary, Calgary, Alberta, Canada. 5Boise State University, Boise, ID, USA. 6Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia. *e-mail: kek1@unm.edu

Neoproterozoic to Cambrian rocks record dramatic changes in Earth systems such as extreme climate changes during Snowball Earth events1, oxygenation events2 and the diver-

sification of microbial life leading to animals3. Steadily improving geochronology is helping to document the interaction of protracted and abrupt events within these entwined systems4,5. Rifting of Rodinia has been implicated with these system changes6, but the timing and duration of rifting remain incompletely understood. Palaeomagnetic data suggest that western Laurentia had separated from adjacent conjugate continents by 750 million years ago (Ma)7, but geologic data suggest a much longer history of incipient rift-ing beginning at 780 Ma8, rift-related shallow seaways during 720–630 Myr glaciations9 and thermal subsidence in the Cambrian10,11.

Cambrian strata globally record the transgressive inundation of many continents by advancing oceans that deposited sheet sands, muds and carbonates12. In Laurentia, the term Sauk sequence was applied13 and was envisioned as a cumulative transgression across the basement unconformity that may have lasted from the Neoproterozoic near the rift margins to the Early Ordovician in the mid-continent. This prolonged sedimentary onlap took place in pulses, each followed by sedimentary offlap, such that the Sauk ‘megasequence’14 is bounded by and includes important internal unconformities. The timescale linking Cambrian unconformities, extinctions and global change15,16 is a globally important subject that needs refinement.

We demonstrate the effectiveness of using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of large numbers of detrital zircon grains to identify the youngest population, then extraction of these from grain mounts to produce more precise results using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-IDTIMS; see

Methods and Supplementary Table 1). These dates from strata of the Grand Canyon region in Arizona help calibrate and redefine the Sauk transgression across the classic Great Unconformity17. This region is about 200 km inboard of the rifted margin and exposes three important successions. The 780–729 Myr Chuar Group was deposited during early Rodinia rift phases18. The previously undated Sixtymile Formation was speculatively linked to a late Proterozoic ‘Grand Canyon disturbance’19 or sea-level draw-down20. The Tonto Group (Tapeats Sandstone, Bright Angel Shale and Muav Limestone) represents the classic west-to-east Sauk marine transgression21 previously thought to have spanned more than 60 Myr22.

Here we present an unexpected stratigraphic and geochronologic revision of classic Grand Canyon stratigraphy by showing that the Sixtymile Formation is Cambrian (not Neoproterozoic) and records a previously unknown episode of early Cambrian faulting and epeirogeny. U–Pb detrital zircon analysis of the overlying Tapeats Sandstone does not show easterly younging and instead 505–501 Ma dates are identical within error across Nevada, Arizona and New Mexico. Combined detrital zircon and calibrated fossil evidence bracket the duration of the dominant marine transgression onto the craton to the interval 505–500 Ma. We apply the term Sauk I23 for the Sixtymile Formation and Sauk II for the rapid late Cambrian Tonto Group continental onlap in southwestern Laurentia. Multiple mechanisms are implicated: Neoproterozoic rift basins formed dur-ing separation of conjugate continents, early Cambrian epeirogeny accompanied inboard faulting and the rapid late Cambrian (Sauk II) marine transgression is attributed to rapid continental flooding driven by abrupt post-rift thermal subsidence following final rifting of southern Rodinia, probably interacting with punctuated global eustatic events15.

Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zirconsKarl Karlstrom1*, James Hagadorn2, George Gehrels3, William Matthews4, Mark Schmitz5, Lauren Madronich4, Jacob Mulder6, Mark Pecha3, Dominique Giesler3 and Laura Crossey   1

The Sauk transgression was one of the most dramatic global marine transgressions in Earth history. It is recorded by deposi-tion of predominantly Cambrian non-marine to shallow marine sheet sandstones unconformably above basement rocks far into the interiors of many continents. Here we use dating of detrital zircons sampled from above and below the Great Unconformity in the Grand Canyon region to bracket the timing of the Sauk transgression at this classic location. We find that the Sixtymile Formation, long considered a Precambrian unit beneath the Great Unconformity, has maximum depositional ages that get younger up-section from 527 to 509 million years old. The unit contains angular unconformities and soft-sediment deformation that record a previously unknown period of intracratonic faulting and epeirogeny spanning four Cambrian stages. The overlying Tapeats Sandstone has youngest detrital zircon ages of 505 to 501 million years old. When linked to calibrated trilobite zone ages of greater than 500 million years old, these age constraints show that the marine transgression across a greater than 300-km-wide cratonic region took place during an interval 505 to 500 million years ago—more recently and more rapidly than previously thought. We redefine this onlap as the main Sauk transgression in the region. Mechanisms for this rapid flooding of the continent include thermal subsidence following the final breakup of Rodinia, combined with abrupt global eustatic changes driven by climate and/or mantle buoyancy modifications.

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Sixtymile Formation detrital zircon resultsBelow the Great Unconformity, the Sixtymile Formation20,24,25 is exposed in the eastern Grand Canyon along the Chuar syncline, which formed during west-down normal slip on the Butte fault20 (Supplementary Fig. S1). It underlies the Tapeats Sandstone with angular unconformity in places but can be locally disconformable (Supplementary Figs. S2 and S3). It includes lacustrine, shallow marine and fluvial units, with numerous landslides or subaqueous slumps in lower units suggesting that its deposition took place in a fault-controlled basin during reactivation of the Butte fault20,25. Before this study, the age of the Sixtymile Formation was brack-eted by the 729.0 ± 0.9 Ma ash of the underlying Chuar Group26 and 515–507 Ma Bolbolenellus euryparia to Glossopleura walcotti Zone trilobites of the overlying Bright Angel Shale27,28.

Five stratigraphic levels of the Sixtymile Formation were selected for U–Pb dating of detrital zircons (Samples A–E of Fig. 1; see the Supplementary Information for measured sections and outcrop photos). At the base of the Sixtymile Formation, sample A (K16-60-19; Supplementary Fig. S5A) was collected from mature quartz sandstone that fills karst cavities in a dolostone that has variably been assigned to the lower Sixtymile25 or upper Walcott20 forma-tions. The detrital zircon spectrum (Fig. 1) has a dominant mode at 1,081 Ma; a weighted average of the four youngest grains yields with an LA-ICPMS age of 536.5 ± 12.6 Ma and a youngest CA-ID-TIMS age of 527.4 ± 0.7 Ma (Supplementary Table S2). Sample B (K16-60-18; Supplementary Fig. S5B) is from an approximately 1-m-thick sandstone dike that cross-cuts the cherty siltstone interval of the lower member several metres above the base of the section. This dike is consistent with the other evidence for early soft sediment deformation. The detrital zircon spectrum has a prominent mode at 1,086 Ma; the weighted average of the five youngest grains yields an LA-ICPMS age of 535.6 ± 11.8 Ma and youngest two CA-ID-TIMS ages of 526.7 ± 0.7 Ma. Sample C (K16-60-4; Supplementary Fig. S6) is from a maroon sandstone lens ~47 m above the base of the section. Its detrital zircon spectrum contains modes at 1,700 and 1,433 and a prominent mode at 1,085 Ma; the weighted average of the youngest four grains yields an LA-ICPMS age of 525.1 ± 11.1 Ma and youngest three CA-ID-TIMS ages of 523.3 ± 0.7 Ma.

Sample D (K15-52; Supplementary Fig. S7) is from subrounded 2–4-cm-scale sandstone pebbles within the upper Sixtymile brec-cia that fills steep-walled channels on Nankoweap Butte about 56 m above the base of the section (Supplementary Fig. S12). Its detrital zircon spectrum (Fig. 1) has a few pre-1,800 Ma grains, prominent modes at 1,690 and 1,428 Ma, smaller modes at 1,210 and 1,083 Ma and a young grain population of 510.5 ± 7.7 Ma (LA-ICPMS weighted mean age of 17 grains). Five of the young grains were also dated by CA-ID-TIMS and gave concordant and equivalent isotope ratios with a weighted mean U–Pb age of 512.4 ± 0.7 Ma, which provides a reproducible conservative maximum depositional age. One grain gave a concordant date of 508.6 ± 0.8 Ma that we use as a precise maximum depositional age under the assumption that chemical abrasion was effective in removing any Pb-loss domains. Lower Th/U values for this grain compared with the 512 Ma grains also supports its unique provenance (Supplementary Table S2). Thus, CA-ID-TIMS analysis confirms the results of LA-ICPMS, but resolves two discrete ages within the Cambrian population and thus refines the stratigraphic interpretation. Sample E is a compos-ite of three samples from maroon sandstones near the top of the exposed Sixtymile Formation at Nankoweap Butte (K06-13, K10-53 and K12-100; Supplementary Fig. S8). The detrital zircon spectrum (n = 228; Fig. 1) is dominated by modes at 1,722 and 1,441 Ma, with minor modes between 1,180 and 1,030 Ma and no Cambrian grains.

The up-section decrease in age of youngest grains in the Sixtymile Formation raises the possibility that detrital zircon ages may approximate depositional age and that deposition could have spanned much of the early Cambrian, from 527 to 509 Myr

(Fig. 1). The composite spectrum of the Sixtymile Formation (Fig. 2, n = 1,774) shows prominent age probability modes: > 1,600 (31%), 1,600–1,300 Ma (19%), ~1,250 Ma (4%), 1,200–1,000 Ma (43%) and younger modes at 700 Ma and 512 Ma. The grains dated to 510 Ma are within uncertainty of and may have been sourced from the 510 ± 5 Ma Florida Mountains granite in New Mexico29. A pos-sible source for the 535–525 Myr grains in the lower Sixtymile is the 539–528 Myr magmatism in the Wichita Mountains of Oklahoma30.

tapeats Sandstone detrital zircon resultsThe Tapeats Sandstone is a semi-contiguous sand body that records marine transgression across southwestern Laurentia (Fig. 2)31. Unlike most western North American Cambrian sections that progressively thicken westwards across the Cordilleran hingeline (Fig. 2)31,32, the Tapeats Sandstone is variable in thickness owing to bevelling and infill of basement relief33. Previous detrital zircon studies found no Cambrian grains (n = 200)34.

Sample F is from a coarse sandstone 2 m above the base of the Tapeats Sandstone in Hermit Creek, eastern Grand Canyon. It has modes at 1,680 and 1,412 Ma and the youngest grains yield a weighted mean LA-ICPMS maximum depositional age of 505.4 ± 8.0 Ma (n = 12). Sample G is from the westernmost limit of Tapeats expo-sures near Las Vegas, Nevada, where we sampled a coarse-grained cross-bedded quartzite 30 m above the base of the unit35. Its detrital zircon spectrum contains prominent modes at 1,692 and 1,444 Ma, smaller modes at 1,239 and 1,078 Ma and a youngest detrital zir-con population of 504.7 ± 2.1 Ma (n = 28). Sample H is from the southeastern limit of Tapeats exposures in central Arizona where we sampled a coarse-grained, pebbly cross-bedded sandstone ~19 m above the unconformity with granitic basement35. Its detrital zircon spectrum contains modes at 1,669, 1,420 and 1,222 Ma; the young-est grains yield a weighted mean maximum depositional age of 501.4 ± 3.8 Ma (n = 19).

Sauk transgression(s)Sixtymile Formation outcrops reveal several previously unrec-ognized middle Cambrian angular unconformities that reflect faulting and epeirogenic uplift on the craton. The 1,080 Ma and 1,200 Ma detrital zircon modes in the lower Sixtymile Formation may indicate significant recycling of the Proterozoic Chuar and Unkar groups into Cambrian rocks and an increased proportion of 1,700 and 1,400 Ma basement ages (Fig. 1) up-section suggests an unroofing sequence. The 530–520 Myr middle Sixtymile Formation is similar in age to the middle/upper Wood Canyon Formation (ref. 36; their data repository) west of the continental hingeline (Fig. 2). The 510 Ma upper Sixtymile Formation is time correlative with Tapeats and Bright Angel sections of western Grand Canyon that contain ‘Olenellus Zone’ trilobites and hence are probably older than 509 Ma. Based on the unconformity relationships we see in the Sixtymile Formation, we infer it was deposited in fault-controlled 530–520 Ma rift basins that we relate to other Sauk I successions in the region14.

A rapid late Cambrian Sauk II transgression is documented by the 505–501 Ma detrital zircon maximum depositional ages for basal Tapeats Sandstone. This age is also similar to the Bliss Sandstone of New Mexico that was deposited after 504 ± 12 Ma29 (Fig. 2). These data, when combined with the oldest overlying biostratigraphically useful fossils, help constrain the onset, termination and duration of Sauk II subsidence in this region (Fig. 1). Western Grand Canyon and Lake Mead region trilobites correspond to the upper half of Stage 4 of Cambrian Series 237. Glossopleura walcotti Zone trilobites of the overlying Bright Angel Shale in eastern Grand Canyon38 cor-relate with Stage 5 of Cambrian Series 3 as do Solenpleurella tri-lobites from the uppermost Muav Limestone. Numerical ages for Cambrian Stage 4 and Stage 5 boundaries have not yet been estab-lished by the International Union of Geological Sciences, so ages

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depicted for these boundaries (Fig. 1; ref. 16) are working hypotheses. However, ages of fossils from these successions can be constrained by correlation of Laurentian trilobite zones to trilobite provinces from other continents and by integrating recalibrated ages of Stage 3–5 ashes globally39 with revised fossil zonation40 and chemostrati-graphic and magnetostratigraphic correlation41. Peachella iddingsi

to Bolbolenellus euparyia Zone trilobites from upper Tapeats expo-sures near Las Vegas are probably 508.1–503.8 Ma42,43. Viewed in light of the young detrital zircon populations from underlying strata, the Tapeats and Bright Angel formations were deposited 505–501 Myr across a width of 300 km, a much shorter duration than previously envisioned. These data challenge traditional models

501.

4 ±

3.8

504.

7 ±

5

505.

4 ±

8

Sau

k II

D1 508.6 ± 0.8

D2 512.4 ± 0.7

H

G

F

C 523.3 ± 0.7

B 526.7 ± 0.7

A 527.4 ± 0.7

BA

C

H(AZ)

F(GC)

G(NV)

D

Chu

arG

roup

Six

tym

ile F

orm

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n~6

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Che

rt

Tap

eats

San

dsto

ne~7

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Brig

ht A

ngel

Sha

le~1

50 m

Mua

v Li

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~180

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to G

roup

0

20

40

60

135

285

465

Neoproterozoic

Cam

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n

Ter

rene

uvia

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ies

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gian

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iddl

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ge 3S

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Sta

ge 5D

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79

810

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ma-

doci

anF

loria

n

Per

iod

Epo

ch

Sta

geGSSP

Numericalage

Time

Rock Height (m)

Maximumdepositional

ages(this study)

458.4 ± 0.9

467.3 ± 1.1

470.0 ± 1.4

477.7 ± 1.4

485.4 ± 1.9

541.0 ± 1.0

~529

~521

~514

~509

~504.5

~500.5

~497

~494

~489.5

460

470

480

Age

(M

a)

490

500

510

520

530

540

E

AZ502.1

–506.7

NV503.8

–508.1

IDTIMS ages: errors ~height of box

UA-ICPMS age and uncertainty

A Samples

Tapeats SandstoneSample F(Grand Canyon)<504 ± 11 Maєzr = (23/453)

502

506

504

1,406 1,691

1,441

1,722

5101,2101,083

1,4281,690

525

1,085

1,4331,700

540

1,086

536

1,081Sixtymile FmSample A<536.5 ± 9.7 Maєzr = (4/317)

Sixtymile FmSample B<540.3 ± 8.9 Maєzr = (6/309)

Sixtymile FmSample C<525.1 ± 9.0 Maєzr = (4/307)

Sixtymile FmSample D<510.5 ± 4.0 Maєzr = (18/592)

Sixtymile FmSample Eєzr = (0/228)

Tapeats SandstoneSample H(central Arizona)<501.8 ± 6.7 Maєzr = (69/726)

600400 800 1,000 1,200 1,400 1,600 1,800 2,000

Zircon age (Ma)

Tapeats SandstoneSample G(Nevada)<506.0 ± 5.2 Maєzr = (61/764)

1,661

1,419

1,200

1,670

1,440

Fig. 1 | Detrital zircon ages from the Sixtymile Formation and tapeats Sandstone. Left, ages are plotted on the Cambrian chronostratigraphic chart; GSSP, global boundary stratotype section and point16. Estimates of the ages of the trilobite zones (in blue boxes) are based on global correlations. The stratigraphic column of the Cambrian system in the Grand Canyon region is shown on the right (note the breaks in the scale); purple, Sixtymile Formation; yellow, Tapeats Sandstone; blue, Bright Angel Shale; green, Muav Limestone; errors for IDTIMS ages are black and are smaller than the red dots. Right, normalized probability density plots (y axis is relative probability) show main age peaks; maximum depositional ages were calculated using the weighted mean LA-ICPMS age of the youngest grain population (see Supplementary Table 1); єzr =  4/317 =  number of Cambrian grains/total number of grains dated. AZ, Arizona; NV, Nevada; GC, Grand Canyon; Fm, Formation.

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for time-transgressive eastward shoreline migration in a deepening sea for the Tonto Group and for a long-lived Sauk transgression21.

Driving mechanismsNew timing data allow refined discussion of the driving mecha-nisms for different stages of rifting of Laurentia from 780 Ma to the Cambrian, and for the Sauk transgressions. Palaeomagnetic stud-ies suggest early separation of western continents from Rodinia by 750 Ma7, but age, geometry and areal extent of strata west of the Cordilleran hingeline9,44 indicate continued 750–600 Myr basin deposition. The extraordinary amount of time recorded by these interpreted rift basins is challenging to explain mechanistically as drift-phase thermal subsidence does not readily explain multiple

sedimentary onlaps10,11 and drift-phase subsidence is typically observed to lag initial development of rift basins (for example, during Mesozoic rifting of eastern Pangea) by only 65–45 Myr, not hundreds of millions of years45. Models involving more than one continental separation along western Laurentia are possible, with Cambrian final separation following Neoproterozoic failed rift episodes43. But none of these models have been unequivocally sup-ported palaeomagnetically or geologically, meaning that conjugate continents to western Laurentia and rift timing remain in dispute.

Regional data from Laurentia’s rift margins suggest that drift-phase rifting and tectono-thermal subsidence were diachronous. In both northwestern and northeastern Laurentia, the onset of drift phase subsidence may have progressed north to south from

Bliss sandstone<504 ± 12 Ma

єzr = (109/924)Van Horn Fm

<521 ± 14 Maєzr = (5/130)

Sawatch Fmєzr = (0/202)

Coronadoand Bolsa

sandstones<488 ± 3 Maєzr = (2/29)

1,000 1,400 1,800600

Age (Ma)

ProveedoraQuartziteєzr = (0/160)

1,000 1,400 1,800600

Age (Ma)

ZabriskieQuartziteєzr = (0/247)Woodcanyon Fm<525 ± 9 Maєzr = (4/607)

California OffcratonHarkless Fmєzr = (0/67)Rift sequences<527 ± 13 Maєzr = (7/517)

WesternNevadaPrebble Fmєzr = (0/254)OsgoodMountains Fmєzr = (0/473)

1,000 1,400 1,800600

Age (Ma)

Grand CanyonSixtymile Fm<508.6 ± 0.5 Maєzr = (31/1774)

1,000 1,400 1,800600

Age (Ma)

Grand CanyonTapeats Sandstone<505.4 ± 8 Maєzr = (12/567)

Tapeats Sandstonecentral Arizona<501.4 ± 3.8 Maєzr = (19/556)

1,000 1,400 1,800600

Age (Ma)

1,000 1,400 1,800600

Age (Ma)

Worm Creek Fm<498.5 ± 5.0єzr = (426/630)Upper BrighamGroupєzr = (0/544)

Tapeats Sandstone (Nevada)<504.7 ± 2.1 Maєzr = (28/680)

12 1 2

78

11

10

9 6

5

4

3

OklahomaRift

MSM

FMt

Mio

geoc

line

Cra

toni

c co

ver

Transcontin

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arch?

12

1211

10

9

6

5

54 4

3

8

7

2 1

200 km 12

Hinge

line

EastAntarctica EB LT

KalahariCLB

CT

Grenville Orogen

Fig. 2 | Detrital zircon probability density plots and youngest grain ages from about 10,000 dated zircons from Cambrian successions of southwestern uSa. Data sources are summarized in the Methods section. Red spectra are from inferred Sauk I rift basins; blue spectra are from Sauk II transgressive sequences as defined here. Colours on the map: white, no preserved Cambrian strata; orange, deeper water Cambrian strata (miogeocline) deposition; pale orange, shallow water Cambrian strata (cratonic cover); MSM, Mojave-Sonora megashear. New data are indicated by stars on the maps and spectra. Palaeocurrents (arrows) and the hingeline between the cratonic and rift sections are from ref. 32, transcontinental arch and Cambrian magmatism (triangles; FMt, Franklin Mountains) are from ref. 29. Conjugate cratons at 530 Ma (shown in green) are from ref. 6: CLB, Coats Land block; EB, Ellsworth Mountains block; LT, Lafonian terrane; CT, Cuyania terrane.

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615 to 564 Ma6. Marine transgression in the Death Valley region of California could have started 560–540 Ma as recorded by the Stirling or Wood Canyon formations46. However, recalibration of western Laurentia subsidence curves11 to the updated geologic tim-escale suggests an onset of drift-phase thermotectonic subsidence ~525–515 Ma45.

Our data for rapid 505 to 501 Myr marine transgression and sedi-mentary onlap in southwestern Laurentia leads to a redefinition of the Sauk transgression at least for this region. We infer that Sauk I involved fault-bounded basins recording progressive but incipi-ent rifting, and Sauk II involved abrupt flooding of the craton that was driven, at least in large part, by thermal subsidence related to final separation of the east Gondwana blocks from southernmost Laurentia (Fig. 2). Diachronous drift-phase subsidence along differ-ent parts of the Laurentian rift margins may explain different onset times of Sauk sequence transgressions. But the short duration of the late Cambrian Sauk II transgression in southwestern Laurentia may also reflect a global eustatic event. For example, similar to the Grand Canyon region, transgression of the Potsdam Sandstone of New York was 510–505 Myr and followed earlier incipient rifting47; fine-grained clastic units have recently been identified in Jordan that suggest maximum continental flooding took place about 509–505 Myr48; and drift-phase subsidence and transgression took place in Antarctica40 and Australia49 in the late Cambrian. A globally young and synchronous Sauk II transgression, if confirmed, may imply punctuated climate forcings15 or tectonically driven global sea-level changes due to changes in mantle heat flow13, mantle dynamics50, true polar wander51 or crystallization of the inner core52. Testing of such global mechanisms for different pulses of Suak transgression will benefit from additional precise dating of global Cambrian successions.

MethodsMethods, including statements of data availability and any asso-ciated accession codes and references, are available at https://doi.org/10.1038/s41561-018-0131-7.

Received: 30 October 2017; Accepted: 16 April 2018; Published: xx xx xxxx

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acknowledgementsAnalytical support was in part from National Science Foundation (NSF) Division of Earth Sciences (EAR) grants EAR-1119629, 1145247 and 1348007 from the Tectonics Program (to K.K. and L.C.). Support for J.M. was from Australian Research Council grant FL160100168. NSF grant EAR-1338583 provided support for the Arizona LaserChron Center. Analyses conducted at the University of Calgary were obtained at the Centre for Pure and Applied Tectonics and Thermochronology, a new LA-ICP-MS laboratory funded by the Canadian Foundation for Innovation (CFI project 30696). Funding for the analytical infrastructure of the Boise State Isotope Geology Laboratory was provided by the NSF Major Research Instrumentation grants EAR-0521221 and EAR-1337887, and NSF EAR Instrumentation and Facilities Program grant EAR-0824974. We thank J. Foster, E. Rose, F. Sundberg and M. Webster for insights on fossils and facies of the Tonto Group, K. Honda for references, and patrons of the Denver Museum of Natural History for support of J.H.’s fieldwork. We thank C. Dehler for an informal review that helped improve the paper. We thank B. Guest for helping to forge the UNM-UC collaboration. Samples were collected under Research and Collecting agreements with Grand Canyon National Park.

author contributionsK.K., J.M. and L.C. synthesized the data. J.H. contributed the palaeontology. J.H., J.M. and L.C. contributed the stratigraphy and sedimentology. G.G., J. M., M.P. and D.G. conducted the ICPMS analysis of Sixtymile Formation samples A–E. M.S. conducted CA-ID-TIMS analysis of Sixtymile Formation samples A–D. W.M. conducted the ICPMS analysis of Tapeats Sandstone samples G and H. L.M. conducted the ICPMS analysis of Grand Canyon Tapeats Standstone sample F.

additional informationSupplementary information is available for this paper at https://doi.org/10.1038/s41561-018-0131-7.

Reprints and permissions information is available at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to K.K.

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MethodsUniversity of Arizona Laserchron (Sixtymile samples). Detrital zircon methods for the LA-ICPMS analyses of samples of the Sixtymile Formation (A–E) follow those summarized by an earlier study53. Detrital zircon sample descriptions, including GPS coordinates of sample localities, are presented in Supplementary Table 1. Zircon grains were separated from whole rock samples using traditional methods of jaw crushing and pulverizing, followed by density separation using a Wilfley Table and heavy liquids (methylene iodide) and magnetic separation using a Frantz LB-1 magnetic separator. A representative split of the entire zircon yield of each sample was incorporated into a 1” epoxy mount along with multiple fragments of the primary Sri Lanka (SL) zircon standard. The mounts were sanded down ~20 μ m, polished using a 9 μ m polishing pad and imaged by back-scattered electron and colour cathodoluminescence imaging using a Hitachi S-3400N scanning electron microscope. Before isotopic analysis, the mounts were cleaned in an ultrasound bath of 1% HNO3 and 1% HCl to remove any residual common Pb from the surface of the mount.

U–Pb geochronology of individual zircon crystals was conducted by laser ablation multicollector inductively coupled mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center. The isotopic analyses involved ablation of zircon using a Photon Machines Analyte G2 excimer laser coupled to a Nu Instruments high-resolution MC-ICPMS. Analyses were conducted with a 30 μ m laser spot diameter using an acquisition routine consisting of one 15 s integration on peaks with the laser off (for backgrounds), 15 1 s integrations with the laser firing and a 30 second delay to purge the previous sample. Faraday detectors with 3 × 1011 ohm resistors measure 238U, 232Th and 208–206Pb and discrete dynode ion counters measure 204Pb and 202Hg, all in static mode. Drill rate is ~1 μ m s−1, resulting in a final ablation pit depth of ~15 μ m.

The errors in determining 206Pb/238U and 206Pb/204Pb resulted in a final measurement uncertainty of ~1–2% (at the 2σ level) in the 206Pb/238U age for each individual analysis. The common Pb correction was accomplished using the Hg-corrected 204Pb. Uncertainties were applied (1.5 for 206Pb/204Pb and 0.3 for 207Pb/204Pb) to these compositional values based on the variation in Pb isotopic composition in modern crystalline and sedimentary rocks. Interference of 204Hg with 204Pb was accounted for through measurement of 202Hg during laser ablation and subtraction of 204Hg according to the natural 202Hg/204Hg of 4.35.

The primary standard consists of fragments of a single large zircon crystal (SL-2) with an ID-TIMS age of 563.5 ± 3.2 Ma (2σ ). In-run analyses of these fragments are conducted every fifth measurement and the results are used to correct for interelement fractionation of Pb/U. For these particular analyses, the uncertainty resulting from this calibration correction is between 0.8 and 1.4% (2σ ) for both 206Pb/238U and 206Pb/207Pb. Concentrations of U and Th are calibrated relative to measurements of SL-2, which contains ~518 ppm U and ~68 ppm Th.

Approximately 100 laser analyses were completed on each sample, with one U–Pb measurement per grain. Grain selection was conducted randomly, with rejection of zircons that were either too small or contained cracks/inclusions. The use of back-scattered electron and cathodoluminescence images provided assistance in grain selection and final spot placement (Supplementary Fig. S8).

University of Calgary Centre for Pure and Applied Tectonics and Thermochronology (Tapeats samples). Detrital zircon methods for the new LA-ICPMS analyses of samples of the Tapeats Sandstone (F–H) reported here follow those summarized by an earlier study54. Mineral separation and mounting techniques broadly followed those described above for samples of the Sixtymile Formation. To identify Cambrian grains, detrital zircon was first ablated using a high-throughput methodology. Cambrian grains that were sufficiently large to accommodate a second ablation were then ablated again to obtain higher-precision dates. Screening ablations consisted of a 15 s background measurement, obtained with the laser off, followed by an ablation of the calibration reference material FC155. Thirty unknowns were then ablated for 5 seconds each with no background measurement period. A total of 660 unknowns were measured in each sample and the measurement sessions were calibrated using 25 measurements of the calibration reference material. Eleven or 12 measurements of each of the zircon reference materials 91500, 1242b and Temora256–58 and glass reference material NIST610 were used to validate the method and calibrate the U and Th concentrations in the unknowns. A laser fluence of 2 J cm−2 and 10 Hz repetition rate resulted in a pit ~4.5 µ m deep. A beam diameter of 33 µ m was employed.

Higher-precision dates were obtained using an 18.5 s background and longer 25 s ablation period. Eighteen measurements of zircon reference material 91500 were used to calibrate the session and eight measurements of each of the zircon reference materials Temora2, FC1 and 1242 were used to validate the method. To fit the second ablation on the grains, a smaller beam diameter of 22 µ m was employed. A laser energy of 2 J cm−2 and repetition rate of 7 Hz resulted in a pit ~15.8 µ m deep.

Samples were ablated in a Laurin Technic M-50 dual-volume ablation cell using an ASI Resochron 193 nm laser ablation system. For additional details about the ablation cell characteristics and laser ablation system see a previous study59. Isotopic signal intensities were measured using an Agilent 7700 quadrupole mass spectrometer, which was tuned for optimum sensitivity using NIST610 glass before each measurement session.

Uncertainties were propagated in accordance with best practices reported in an earlier study60 using a custom Excel VBA macro (ARS4.0). Data point uncertainties (Sm) were calculated as the standard error of all indications of the isotopic ratio. Excess variance (ε) values for the 206Pb/238U ratios were calculated using measurements of validation reference material 91500 in the screening rounds and Temora2 for the higher-precision rounds. Excess variance values in the 207Pb/206Pb ratios were determined using a calibration curve between excess variance and 207Pb signal intensity for all the validation reference materials in each session (methodology outlined in 61). Long-term excess variance (ε’) for the isotopic ratios was calculated using the long-term reproducibility of reference material 91500 for the 206Pb/238U ratio and 1242 for the 207Pb/206Pb ratio.

Data presentation and calculation of maximum depositional ages. U–Pb analytical data are presented here in age–distribution diagrams, which account for both the age and analytical uncertainty of each analysis. These plots were generated by assuming normal uncertainty distributions for each grain age, followed by the summing of all normal distributions into composites, which were then normalized to subtend equal areas under the curves. A discordance filter of 10% was applied to grains older than 600 Ma.

Estimation of the maximum depositional age of the rock involved analysis of the youngest grain population via several methods that are compared in Supplementary Table 1. Only analyses with 2σ errors overlapping a 1:1 concordance ratio with respect to their 207Pb/235U versus 206Pb/238U age were used to calculate maximum depositional ages61,62. All of the methods give similar results. For example, for the key sample D (K15-52) of the Sixtymile Formation, the weighted mean analysis of LA-ICPMS data includes all 18 ages between 490 and 550 Ma. The oldest grain (546 Ma) was excluded because it is not part of this cluster. The weighted mean age is 510.5 ± 5.8 Ma (2σ ) with a mean square weighted deviation (MSWD) of 1.4, which indicates that all of the accepted ages belong to a single cluster with no sign of inheritance or Pb-loss. The uncertainty of this age increases to 7.7 Ma (2σ ) with the addition of the four main systematic (external) uncertainty components (the age of the primary standard, scatter of measured standard ages, uncertainty of common Pb composition and decay constant uncertainties). For comparison, we also use the Tuffzirc routine of Isoplot63, which also identifies the 546 Ma grain as an outlier, and the rest of the analyses as a coherent group with age of . − .

+ .511 03 2 6 Ma6 6 Ma.

A maximum likelihood analysis (Unmix routine in Isoplot) yields an age of 513.1 ± 2.9 Ma. Finally, the peak in age probability for all young grains from this sample is 511 Ma. Our preferred maximum depositional age based on the LA-ICPMS data for this sample is based on the weighted mean analysis age of 510.5 ± 7.7 Ma (2σ ). This paper places most significance on the weighted mean age and corresponding analytical and systematic uncertainties for all samples in which the maximum depositional age is derived from LA-ICPMS data. For the youngest grains from the Frenchman Mountains and central Arizona, we assign maximum depositional ages of 504.7 ± 2.1 Ma and 501.4 ± 3.8 Ma, respectively, using the same methods.

For more precise dates, selected zircon crystals were plucked from the same mount and analysed by CA-IDTIMS at Boise State University. The selected zircon crystals were removed from the epoxy mount and annealed at 900 °C for 60 hours. Individual crystal fragments were then chemically abraded in 120 µ l of 29 M HF for 12 hours at 190 °C in 300 µ l Teflon PFA microcapsules. The residual crystals after this partial dissolution were fluxed in 3.5 M HNO3 in an ultrasonic bath and on a warm hotplate for 60 minutes, then rinsed twice in ultrapure H2O before being reloaded into microcapsules and spiked with the mixed U–Pb isotope tracer ET53564. Additional details of chemical separations, mass spectrometry and data analysis are described in the literature65. Errors reported in Supplementary Table S2 are analytical errors whereas, to be consistent with reporting of LA-ICPMS-derived maximum depositional age, TIMS errors reported in the main text and Fig. 1 also include systematic error sources, including tracer composition and decay constant errors.

Compilation of detrital zircon data from the Cambrian System of the southwestern US for Fig. 2 involved a total of more than 10,000 dated zircons, as reported in Supplementary Table S1. Data sources, keyed to numbered localities in Fig. 2 are as follows. Localities (1) and (2) from this paper; (3) Sawatch Formation from ref. 66 (4) Bliss Sandstone of New Mexico from ref. 29, (4) Van Horn Formation of Texas from ref. 22, (5) Coronado and Bolsa sandstones of Arizona and Mexico from ref. 32, (6) Proveedora Quartzite of Mexico from refs.32,53, (7) and 8) from this paper and ref. 35, (9) Wood Canyon Formation of California from refs.32,36,53,67,68, Zabriskie Quartzite from refs.32,36,53,67, (10) Harkless, Deep Spring, Reed, Poleta and Campito formations of California from ref. 69, (11) Prebble and Osgood Mountain formations of western Nevada from refs.32,53,70 and (12) Worm Creek and Nounan formations of Idaho from ref. 71, Gibson Jack, Camelback Mountain and Geertsen Canyon formations of Utah from refs.32,44,53.

Code availability. The code used to generate probability density plots is available at http://www.bgc.org/isoplot_etc/isoplot.html.

Data availability. The authors declare that the data supporting the findings of this study are available within the article and Supplementary Information.

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