RECYCLING OF DETRITAL ZIRCONS FROM JURASSIC EOLIANITES OF THE COLORADO PLATEAU INTO QUARZOSE CRETACEOUS SANDSTONE OF THE BISBEE BASIN IN

SOUTHEASTERN ARIZONAWilliam R. Dickinson and George E. Gehrels (University of Arizona)

Timothy F. Lawton (New Mexico State University)Sandstones in the Jurassic-Cretaceous Bisbee basin of the AZ–Sonora–NM border region include multiple petrofacies: (a) arkosic from internal basement tiltblocks within the Border rift belt, (b) lithic (volcaniclastic) from the Alisitos arc southwest of the basin, (c) subquartzose from sedimentary cover of the rift shoulder north of the basin, and (d) quartzose inferred to reflect reworking of Jurassic eolian sand from the Mogollon highlands of the rift shoulder along the southern rim of the Colorado Plateau (Dickinson and Lawton, 2001). Confirmation that sand was recycled into the Bisbee basin from plateau eolianites is provided by U-Pb ages of detrital zircons (n=100 with <10% age uncertainty and <20% discordance from LA-ICP-MS analysis using a beam diameter of 35 microns) in a sample (KBCR) of quartzose sandstone (Qm91-F6-Lt3) in the Lower Cretaceous (Albian) Cintura Formation of the Bisbee Group at Rucker Canyon in the Chiricahua Mountains (SE AZ). Analysis of plateau Jurassic eolianite provenance has shown that >295 Ma zircon grains were derived from deflation of floodplains lying north and northeast of the Colorado Plateau near the termini of transcontinental drainages heading in the Appalachian province, and include age populations unrepresented by bedrock in SW Laurentia. Comparative K-S statistics indicate that populations of >295 Ma zircon grains in KBCR (n=84 grains) and 10 plateau eolianites (n=887 grains) are statistically indistinguishable (P-value ~0.8), and further that populations of all >115 Ma grains in KBCR (n=98 grains) and four Middle to Upper Jurassic eolianites (n=365 grains) of the eastern Colorado Plateau north of the Mogollon highlands are also statistically indistinguishable (P-value ~0.9). The zircon grains >115 Ma but <295 Ma were derived ultimately from the Permian-Triassic East Mexico and Mesozoic Cordilleran magmatic arcs south of the Bisbee basin, but were apparently recycled into the basin from eastern plateau Jurassic eolianites exposed to the north. Zircon grains of distant ultimate provenance in Jurassic eolianites were recycled into Upper Jurassic and Cretaceous strata of the Colorado Plateau as well as into the Bisbee basin. Two post-Jurassic grains (111-108 Ma) in KBCR represent more juvenile arc contributions compatible with Albian (112-100 Ma) deposition.

Outcrop photographs (left across road, right close up) of sample KBCR sandstone ledge (steep dip to right or east) in Cintura Formation of Bisbee Group at Rucker

Canyon in the southern Chiricahua Mountains of southeastern Arizona

Cintura Data (Sample KBCR): Concordia Plots, Combined Histogram and Age-Probability Plot, Sandstone Petrofacies

(Red Dot)

Geologic Context of Cintura Sample KBCR (Red Dots) in Border Rift Belt and Bisbee Basin

(after Dickinson and Lawton, 2001 Sed Geol 14:475-504)

Regional Geotectonic Setting of Bisbee Basin in Border Rift Belt

Regional Stratigraphy of Border Rift Belt

Distribution of Bisbee Group in Bisbee Core and Flank Basins

Stratigraphy of Bisbee Basin(top:core basin in AZ-NM; bottom: flank basin in Sonora)

Comparative Detrital Zircon (DZ) Age Populations(see column at right for P values from K-S analyses)

[note close match for all eolianite and McCoy pre-285 Ma grains,and tight match for pre-112 Ma eastern plateau eolianite grains]

Distribution of Bisbee Basin Petrofacies(Dickinson and Lawton, 2001 Sed Geol 14:475-504)

Comparative P-values from K-S (Kolmogorov-Smirnoff) Analysis(for P>0.05, cannot be 95% confident that two grain populations were not

sampled randomly from the same parent population where P=1.0 would imply statistical identity) P=0.756 for >285 Ma grains (n=84) in Cintura KBCR vs >285

Ma grains (n=890) in ten samples of Colorado Plateau Jurassic eolianites (Aztec, Bluff, Entrada, Navajo, Nugget, Page, Wingate) – see probability plot at top left

P=0.898 for >112 Ma grains (n=98) in Cintura KBCR vs >112 Ma grains (n=365) in four samples of Middle to Upper Jurassic eolianites (Entrada and Bluff

Sandstones) of eastern Colorado Plateau – see probability plot and cumulative curve middle left P=0.826 for >285 Ma grains (n=84) in Cintura KBCR vs >285 Ma grains (n=54) in two samples (MC7, MC9) of quartzose basal sandstone member of McCoy Mountains Formation – see probability plot at bottom left

Summary of Cintura Provenance Relations(see subregional map above where asterisk denotes Cintura sample site in

Rucker Canyon)1. Age populations of the dominant >285 Ma detrital zircons (ultimate derivation from eastern and central North America) in the Cintura Formation and in Jurassic eolianites of the Colorado Plateau are compatible with recycling of quartzose sand into the Bisbee basin from eolianites exposed in Late Jurassic and Early Cretaceous time along the Mogollon Highlands rift shoulder of the Bisbee basin.2. Age populations of all >112 Ma detrital zircons in the Cintura Formation of the Bisbee Group and in Middle to Upper Jurassic eolianites of the eastern Colorado Plateau (Entrada Sandstone and Bluff Sandstone) are compatible with recycling of both far-travelled (eastern-central North American) and western North American (arc-derived) grains from eastern plateau eolianites into which far-travelled and arc-derived grains were pre-mixed before initiation of the Bisbee basin.3. Eastern plateau eolianites were exposed along the Mogollon Highlands directly north of the quartzose petrofacies tract of the Bisbee basin, whereas direct delivery of arc-derived grains to the quartzose petrofacies tract from arc terranes lying to the west and southwest was precluded by intervening lithic (volcaniclastic) and arkosic petrofacies tracts within the Bisbee basin.4. The dominant >285 Ma age population of detrital zircons in the quartzose petrofacies of the basal sandstone member of the McCoy Mountains Formation deposited at the distal northwest extremity of the Border rift belt are compatible with recycling of detrital zircons from western plateau eolianites exposed to the north (in parallel with recycling of sand from eastern plateau eolianites into the coeval Bisbee Group farther east).

METHODOLOGYsample collection and preparation1) 20-25 kg of fresh sandstone collected from outcrop as chips <5 cm diameter2) sample crushed and pulverized using laboratory jaw crusher and rolling mill3) disaggregated sample placered using Wilfley table and sieved for <350 µm4) grains of high specific gravity separated with heavy liquid (methylene iodide)5) ferromagnetic-paramagnetic grains rejected using a Frantz magnetic separator6) epoxy mount (1” strip) of >90% zircon fraction sanded down to ~20 µm depthdata collection and reduction1) U-Pb ages from laser-ablation inductively-coupled plasma mass spectrometry2) Excimer laser ablation at wavelength of 193 mm with spot diameter of 35 µm3) every fifth data point Sri Lanka standard zircon of known age for calibration4) data corrected for U/Pb and Pb/Pb fractionation and for common lead 204Pb5) ages calculated from 206Pb/238U (<1 Ga grains) and 206Pb/207Pb (>1Ga grains)6) ages with >20% discordance or >10% uncertainty omitted (100 ages retained)data presentation and analysis1) concordia plot for graphical evaluation of age concordance of individual grains2) histogram of best estimates of individual grain ages falling into 50 Ma age bins3) age-probability plot (age-distribution curve) from probability-density function4) cumulative age-probability curve (from 0% to 100%) for array of grain ages5) P-value from K-S statistics (P>0.05=indistinguishable at 95% confidence level)6) comparisons of Cintura zircon age populations with Colorado Plateau Jurassic eolianites and with lower McCoy Mountains Formation of Mojave Desertkey uranium-lead systematics1) let asterisk (*) denote radiogenic lead and italics denote ratios for common lead2) measured isotopic ratios: 206Pb/238U, 206Pb/207Pb, 206Pb/204Pb3) ratio 206Pb*/238U = 206Pb/238U [1 – (206Pb/204Pb)/(206Pb/204Pb)]4) ratio 207Pb*/235U = (206Pb*/238U) x (207Pb*/206Pb*) x 137.88 (from 238U/235U = 137.88 at current stage of Earth history with 207Pb*/206Pb*derived from 207Pb/206Pb by common lead correction analogous to point #3)5) use 206Pb*/207Pb* (instead of 207Pb*/235U) with 206Pb*/238U to control concordia(can measure Pb isotopes more accurately than 235U of which there is so little)6) analytical age uncertainties tabulated at 1σ (highly variable for different grains)