Transcript in Border Rift Belt and Bisbee Basin

RECYCLING OF DETRITAL ZIRCONS FROM JURASSIC EOLIANITES OF THE COLORADO
PLATEAU INTO QUARZOSE CRETACEOUS SANDSTONE OF THE BISBEE BASIN IN
SOUTHEASTERN ARIZONA
William 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-F6Lt3) 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 (111108 Ma) in KBCR represent more juvenile arc contributions compatible with Albian (112-100 Ma)
deposition.
METHODOLOGY
sample collection and preparation
1) 20-25 kg of fresh sandstone collected from outcrop as chips <5 cm diameter
2) sample crushed and pulverized using laboratory jaw crusher and rolling mill
3) disaggregated sample placered using Wilfley table and sieved for <350 µm
4) grains of high specific gravity separated with heavy liquid (methylene iodide)
5) ferromagnetic-paramagnetic grains rejected using a Frantz magnetic separator
6) epoxy mount (1” strip) of >90% zircon fraction sanded down to ~20 µm depth
data collection and reduction
1) U-Pb ages from laser-ablation inductively-coupled plasma mass spectrometry
2) Excimer laser ablation at wavelength of 193 mm with spot diameter of 35 µm
3) every fifth data point Sri Lanka standard zircon of known age for calibration
4) data corrected for U/Pb and Pb/Pb fractionation and for common lead 204Pb
5) 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 analysis
1) concordia plot for graphical evaluation of age concordance of individual grains
2) histogram of best estimates of individual grain ages falling into 50 Ma age bins
3) age-probability plot (age-distribution curve) from probability-density function
4) cumulative age-probability curve (from 0% to 100%) for array of grain ages
5) 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 Desert
key uranium-lead systematics
1) let asterisk (*) denote radiogenic lead and italics denote ratios for common lead
2) measured isotopic ratios: 206Pb/238U, 206Pb/207Pb, 206Pb/204Pb
3) 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)
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)
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)
Regional Geotectonic Setting of Bisbee Basin in Border Rift Belt
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
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)
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).