INSIGHTS INTO THE PALEOGEOGRAPHY OF LAURENTIA DERIVED FROM U-PB AGES OF DETRITAL ZIRCONS IN MESOZOIC STRATA OF

THE COLORADO PLATEAUWiliam R. Dickinson and George E. Gehrels (University of Arizona)

Individual U-Pb ages for 5199 detrital zircon (DZ) grains in 59 samples of Mesozoic sandstone from the Colorado Plateau, plus the contiguous High Plains and eastern Great Basin, provide insight into paleogeographic relations across southwest Laurentia. U-Pb ages were determined for ~100 DZ grains per sample by LA-ICP-MS using a beam diameter of 35µ, but ages >20% discordant or with analytical uncertainties >10% were rejected (average of 88 grain ages retained per sample). Middle Triassic fluviodeltaic facies (Moenkopi) and Upper Triassic fluvial facies (Chinle) of the southern plateau contain DZ populations derived dominantly from Proterozoic basement of SW Laurentia and either the Permian-Triassic East Mexico arc or the nascent Cordilleran arc within a provenance lying up-paleocurrent from the Colorado Plateau. Upper Triassic Chinle-Dockum sandstones from SSE-to-WNW paleodrainages crossing the High Plains and Colorado Plateau from headwaters in the Ouachita foreland contain more heterogeneous DZ populations that reflect recycling of detritus from the Ouachita orogen plus contributions of arc-derived DZ. Even more heterogeneous DZ populations in Jurassic eolianites are dominated by Paleozoic, Neoproterozoic, and Grenvillean grains blown southward across the Colorado Plateau from floodplains or deltas of transcontinental paleorivers heading in the Appalachian region, but also contain non-Appalachian DZ grains contributed by tributaries to the paleorivers during transit across the craton. Western plateau eolianites lack arc-derived grains, but eastern plateau eolianites contain admixed DZ derived ultimately from the East Mexico arc. Lower Jurassic fluvial facies displaying SSE-to-NNW paleocurrents contain DZ reflecting either mixture of East Mexico arc detritus with reworked eolian sand (Kayenta) or derivation largely from the Cordilleran arc assemblage (Springdale). Upper Jurassic and Lower Cretaceous fluvial and Upper Cretaceous fluviodeltaic facies derived from the west and southwest contain varied DZ populations that reflect both recycling of eolianite DZ, off the Sevier thrustbelt to the west and the Mogollon highlands to the south, and derivation from Proterozoic basement of the Mogollon highlands, together with variable admixtures of arc-derived DZ of Cordilleran origin.

ACKNOWLEDGMENTSSample Collection and Analysis(students and ex-students in NSF-supported undergraduate Colorado Plateau Seminar in Department of Geosciences at University of Arizona)Joseph R. Amar Linette C. Ancha Carl E. Anderson Erin V. Brenneman Jessica J. Bressmer Richard A. Brown Carla M. Eichler Jennifer D. Fox Erin H. Gleeson Owen V. Hurd Jennifer L. McGraw Michael B. Norton Jaika Ojha Gregory R. Schmidt Kelley N. StairSelected Sample ProvidersGerald Bryant (University of Toronto) – Sample DOLThomas M. Lehman (Texas Tech University) – Sample COONancy Riggs (Northern Arizona University) – Sample SRStratigraphic Context ConfreresRonald C. Blakey (No Ariz Univ) Brian S. Currie (Miami Univ) Peter G. DeCelles (Univ Ariz) Charles M. Gilbert (Univ Okla) Timothy F. Lawton (NM State Univ) Thomas M. Lehman (Texas Tech Univ) Spencer G. Lucas (NMMNHS) Fred Peterson (USGS) John H. Stewart (USGS) Robert J. Stern (Univ Texas Dallas) Christine Turner-Peterson (USGS)Sample Collection PermitsAnton Chico Land Grant Hopi Tribe Grand Staircase-Escalante NM Navajo Nation Pueblo of LagunaResearch Funding SupportNational Science Foundation Grants EAR-0341987 and EAR-0443387

Bedrock Zircon Sources in North America

Special Note: calculation of P-values from Kolmogorov-Smirnoff statistics allows objective comparison of two DZ age populations to supplement visual impressions of age-probability plots and cumulative-age curves – where P>0.05, one cannot be 95% confident that two age populations were not sampled randomly from the same parent

population (P=1.0 for identity)

Crossing North America: Transcontinental Fluvial Transport of Laurentian Sand to Floodplains and Deltas Deflated to Feed Jurassic Ergs

[Colorado Plateau Eolianite DZ Signature and Interpretation)

Cumulative Age Curves of Dominant >285 Ma DZ Grains in Ten Colorado Plateau Jurassic Eolianites (Aztec, Bluff, Entrada, Navajo, Nugget, Page, Wingate)

Net Eolianite DZ Age Distribution (>285 Ma Grains)

Age Distributions of >285 Ma DZ Grains in Western Plateau Eolianites Lacking Arc-Derived Grains and Eastern Plateau Eolianites Containing Arc-Derived Grains

[grains derived from Cordilleran Triassic-Jurassic and/or East Mexico Permian-Triassic magmatic arcs admixed with older grains from eastern and central North America]

Cumulative Age Curves of >285 Ma DZ Grains in Western and Eastern Plateau Eolianites(showing common provenance for pre-arc grains)

Linked Transcontinental Fluvial (to WNW)–Eolian (to SSW) Sand Dispersal System for >285 Ma DZ Grains in Colorado Plateau Jurassic Ergs (Stippled)

Grenville-Aged GrainsAge distributions of Mesoproterozoic grains of Grenvillean age that form prominent age peaks in DZ populations of a majority (~75%) of

the Colorado Plateau–High Plains Mesozoic sandstones are effectively indistinguishable in groups of samples of varying ages (see age-

probability plots above and cumulative age plots below) – the integrated DZ signal of detritus from the Grenville orogen provides no reliable guide to the segment or segments of the orogen from which detritus was ultimately derived (plus or minus varied dispersal paths

and intaregional sediment recycling)

RECYCLING OF EOLIANITE SAND

Reworking of Eolian Sand into Marine Curtis Formation from Underlying Entrada Sandstone(sharp Curtis age spike at 165 Ma reflects minor contributions from backarc eruptive centers of

Great Basin to west – not present in Entrada Sandstone with paleowinds from northeast)

Recycling Lower-Middle Jurassic Eolian Sand into Fluvial Upper Jurassic Morrison Formation(Tidwell-Recapture-Salt Wash-Fiftymile Members)

[sources along Sevier thrust belt and/or Mogollon highlands; P=0.7 from K-S analysis)

Similar Eolianite Recycling into Fluvial Lower Cretaceous Strata(Cedar Mountain Fm off Sevier thrust belt; Burro Canyon Fm off Mogollon highlands)

[see accompanying youngest age poster for arc-derived grains <285 Ma]

Similar DZ Signatures in Redistributed (Lower Jurassic Kayenta) or Recycled (Upper Jurassic-Cretaceous) Eolianite Zircons (Eolianite Standard at Bottom)

[prominent 1600-1800 Ma age peak for fluviodeltaic strata of Sevier foredeep probably reflects additional reworking of zircons from Paleozoic and/or Neoproterozoic strata also

incorporated into Sevier thrust belt]

DZ Age Populations of Paleozoic-Neoproterozoic Grains in Colorado Plateau–High Plains Mesozoic Strata

[full interpretation uncertain but Upper Triassic Chinle-Dockum deposystems with headwaters in Ouachita foreland of Texas display somewhat different age

peaks than Jurassic eolianites and younger recycled strata with DZ derived ultimately from eastern North America]

Mixing Pre-Arc and Arc-Derived Grains[see accompanying youngest age poster for age populations of grains derived from Cordilleran Mesozoic and East Mexico Permian-Triassic arc assemblages]

Redistribution of Eolian Sand from Coeval Glen Canyon Ergs into Fluvial Kayenta Fm(P=0.65 for >285 Ma grains)

[note admixture of 288-231 Ma grains from 284-232 Ma East Mexico arc lying to southeast of Colorado Plateau with >285 Ma grains in Kayenta Formation with WNW paleocurrents]

Age-Distribution Curves (Above) and Cumulative Age Curves (Below)showing similarity of mixed DZ grain populations (pre-arc and arc-

derived) in Middle to Upper Jurassic Entrada-Bluff Sandstones (eastern Colorado Plateau ergs) and Lower Jurassic Kayenta Formation (fluvially redistributed erg sand admixed with minor arc detritus from southeast)

[P=0.3 from K-S Analysis]

Age-Distribution Curves (Above) and Cumulative Age Curves (Below)showing similarity of mixed DZ grain populations (pre-arc and arc-derived) in Middle to Upper Jurassic Entrada-Bluff Sandstones (eastern Colorado Plateau ergs) and Upper

Jurassic Morrison Formation fluvial sandstones containing recycled eolianite sand admixed with minor arc detritus

[P=0.7 from K-S Analysis]

Mogollon and Ouachita Provenances

Age-Distribution Curves of DZ Populations in Jurassic Eolianite Samples and Samples Derived from Mogollon Highlands Provenance South of Colorado Plateau[note subdued Paleozoic-Neoproterozic-Grenville peaks in latter (top) coupled with

enhanced peaks for Yavapai-Mazatzal basement (1725 Ma) and associated anorogenic granite (1450 Ma)]

DZ Populations in Age Groups of Samples with Mogollon Provenance(note consistent Yavapai-Mazatzal and anorogenic granite age peaks)

Principle Paleorivers of Chinle-Dockum Deposystem

Age-Distribution Curves for Chinle-Dockum Samples of Northeastern Paleodrainages[prominent Cambrian (515-525 Ma) age peaks from Amarillo-Wichita granite

provenance; Cambrian zircon grains probably reworked from Permian granite-wash sediments overlying Pennsylvanian Amarillo-Wichita uplift]

Age-Distribution Curves for DZ Populations of Chinle-Dockum Samples from Trends of Major Paleostreams with Headwaters in Ouachita Foreland

(Paleozoic-Neoproterozoic-Grenville grains probably recycled from sedimentary assemblages of Ouachita orogen exposed along Eagle Mills rift shoulder in Texas)

[see accompanying youngest age poster for younger arc-derived grains]