SEDIMENT DISPERSAL IN TRIASSIC FLUVIAL SYSTEMS OF SOUTHWEST

SEDIMENT DISPERSAL IN TRIASSIC FLUVIAL SYSTEMS OF SOUTHWEST

SEDIMENT DISPERSAL IN TRIASSIC FLUVIAL
SYSTEMS OF SOUTHWEST LAURENTIA:
INSIGHTS FROM U-PB AGES OF DETRITAL
ZIRCON GRAINS
William R. Dickinson, George E. Gehrels, Jennifer
D. Fox, Kelley N. Stair, Carl E. Anderson, Jaika
Ojha, Richard A. Brown, Michael B. Norton
(University of Arizona)
Nancy R. Riggs (Northern Arizona University) and
Thomas M. Lehman (Texas Tech University)
U-Pb ages of detrital zircons (DZ) in 20 samples of Triassic fluvial sandstone (Moenkopi, Chinle,
Dockum) on the Colorado Plateau and NM-TX High Plains provide insight into Triassic sediment
dispersal in southwest Laurentia (~100 individual grain ages per sample from LA-ICP-MS with a
beam diameter of 35 microns; average of 92 reliable grain ages per sample after rejection of
ages with >20% discordance or >10% uncertainty; total of 1846 individual DZ zircon ages). U-Pb
age populations fall into four groups: (1) Lowermost Chinle-Dockum (n=3) from the NE fringe of
exposures (TX Santa Rosa, NM Tecolotito, UT Gartra): 35%-70% Cambrian grains (540-500 Ma;
peaks 525-515 Ma) derived from the Amarillo-Wichita uplift or its Paleozoic cover, but no <300 Ma grains, with Gartra of NE UT evidence for Late Triassic sediment transport across eroded roots of Paleozoic Ancestral Rocky Mountains uplifts. (2) Lower to Middle Triassic Moenkopi (n=2), derived from the SE, and lower Chinle (Carnian) Shinarump (n=3) from paleovalleys trending SSE to NNW (AZ-UT): dominated by Proterozoic grains (30%-60%; 1785-1400 Ma) derived from Yavapai-Mazatzal basement (intruded by anorogenic plutons) and Permian-Triassic grains (20%-40%) derived from both Cordilleran (250-220 Ma) and East Mexico (295-260 Ma) magmatic arcs; similar DZ in upper Chinle (Norian) Sonsela also derived from the south. (3) Lower Chinle (Carnian) Shinarump (n=3) and correlative Santa Rosa (NM; n=1) along the ChinleDockum trunk paleoriver (course ESE to WNW across Colorado Plateau): heterogeneous DZ including Grenville (40%-50%; 1290-1000 Ma) and Neoproterozoic-Paleozoic (15%-20%; 640320 Ma) grains recycled from the Ouachita orogen, with arc-derived grains sparse (<10%); closely matching DZ downstream in a Chinle outlier at Currie NV within the Great Basin. (4) Upper Chinle (Norian) of the Cottonwood paleovalley (n=4), subparallel to the lower Chinle paleoriver but farther northeast: also heterogeneous DZ with even more Neoproterozic-Paleozoic grains (30%-40%; 690-310 Ma) but Grenville (20%-30%) still dominant over Yavapai-Mazatzal (10%-15%); broadly similar DZ in correlative upper Chinle-Dockum upstream on the High Plains (n=3), though with more arc-derived grains (275-210 Ma; 15%-25%). Our study shows the utility of detrital zircon ages for helping to delineate paleodrainage patterns in ancient fluvial systems. Methodology SAMPLE COLLECTION AND PREPARATION -About thirty pounds of clean/fresh rock was collected from outcrop -Sample was crushed and pulverized using a jaw crusher and roller mill -Pulverized sample placed on Wilfley table to separate grains by density and sieved to collect zircons of <350 microns -Grains further separated by density using heavy liquids (Methylene Iodide) -Final grain separation using a Frantz magnetic separator -Sample with >90% purity of zircon grains was mounted in epoxy within 1 diameter rings and
sanded down ~40 microns depth to expose zircon interiors
DATA COLLECTION AND REDUCTION
-U-Pb ages were obtained using a Laser-Ablation Multicollector ICP Mass Spectrometer
-1000 zircon grains (100 per sample) were individually ablated (laser diameter = 35-50
microns) with standard zircon grains (age = 564 Ma) being ablated between every five (5)
unknowns grains
-Data was corrected for U/Pb and Pb/Pb fractionation and for common lead (using measured
204Pb)
-Ages were calculated from the isotope ratios of 206Pb/238U and 206Pb/207Pb
-Data accuracy and precision was evaluated using a Pb/U Concordia diagram. Data with
>20% discordance or >10% uncertainty were removed (see Concordia plot)
-206Pb/238U ages (for <1.0 Ga grains) and 206Pb/207Pb ages (for >1.0 Ga grains) were
plotted on an age probability plot (see age probability plots)

CP 42: Shinarump (lower Chinle) in
Wolverine Canyon of Circle Cliffs UT

Confreres for Collecting
Ronald C. Blake
Spencer G. Lucas
John H. Stewart

Key Conclusions
The Moenkopi deposystem occupied the distal cratonward flank of the Sonoma proforeland
basin, and the Chinle-Dockum deposystem occupied an elongate backarc basin formed by
dynamic subsidence behind the Cordilleran arc in response to subduction of a mantle slab
beneath the retroarc foreland.
On the northeast, basal Chinle-Dockum strata contain abundant Cambrian detrital zircons
derived from granite forming part of the deformed aulacogen floor of the Amarillo-Wichita uplift,
and grains from the same source dominate the Gartra DZ population of the Eagle paleoriver
which flowed across or through eroded Pennsylvanian uplifts of the Ancestral
Rocky Mountains Province in Colorado.
The principal lower Chinle-Dockum paleodrainage (Carnian) was a trunk paleoriver flowing
from the Ouachita foreland of the High Plains through the site of the San Juan basin and across
the Colorado Plateau toward the Chinle outlier at Currie NV, and carried large DZ fractions of
Grenville and Neoproterozoic-Paleozoic grains recycled from the Ouachita orogen.
Multiple southern tributaries entering the trunk paleoriver from the south contributed DZ
populations similar to those in older Moenkopi sands derived in large part from Yavapai-Mazatzal
basement and the Permian-Triassic East Mexico arc of southwest Laurentia lying southeast (uppaleocurrent) from the Colorado Plateau.
The upper Chinle-Dockum Cottonwood paleovalley (Norian) lay geographically between the two
older (Carnian) paleorivers, with its headwater reaches in the Ouachita foreland, and also carried
a high proportion of Neoproterozoic-Paleozoic-Grenville DZ recycled from the Ouachita orogen.

Chronostratigraphy of Colorado
PlateauHigh Plains Triassic
(showing Moenkopi samples)
Church Rock/Rock Point interval
treated here as basal Glen
Canyon Group
Timescale: pre-220 Ma after
Brack et al. (2005); post-220 Ma
after Gradsteinh et al. (2004)

Schematic transects of sampled Upper Triassic ChinleDockum sandstone lithosomes (variably formations or
members of group or formation) on Colorado Plateau (1-4) and High Plains (5-6) [note multiple paleovalley fills]

Triassic outcrop-subcrop on Colorado Plateau and
High Plains

Preserved extent of lower Chinle-Dockum (Carnian)
deposystem on Colorado Plateau and High Plains (crosses
denote sample localities except asterisk for Currie inlier of
Great Basin (NV)

Preserved extent of upper Chinle-Dockum (Norian)
deposystem on Colorado Plateau and High Plains (crosses
denote sample localities and course of Cottonwood
paleovalley shown)

Preserved extent of Lower to Middle Triassic Moenkopi
deposystem of Colorado Plateau and High Plains
(crosses denote sample localities)
Age-distribution curves of
samples with
dominant/prominent peaks for
Cambrian grains derived from the
Amarillo-Wichita uplift (Gartra
also includes subordinate
Proterozoic grains contributed in
transit across basement of the
Ancestral Rocky Mountains
Province in Colorado)

Chinle-Dockum paleorivers (trends subparallel to tectonic axis of dynamic backarc basin)

sandstone
detrital
modes

Age-bin histograms (blue) and age-distribution curves (red) for lower Chinle-Dockum (Carnian)
samples collected along course of trunk paleoriver (note the relative abundance of recycled
Paleozoic-Neoproterozoic-Grenville grains from Ouachita orogen)

Age-bin histograms (blue) and age-distribution curves (red) for upper Chinle-Dockum (Norian)
samples collected along course of Cottonwood paleovalley and its headwaters (note relative
abundance of recycled Paleozoic-Neoproterozoic-Grenville detritus from Ouachita orogen)

CP26: Poleo (upper Chinle) in roadcut
on US84 near Abiquiu Reservoir NM

Research Support
NSF Grant EAR-0341987
Arizona LaserChron Center

Permits for Collecting
Navajo Nation and Hopi Tribe
Grand Staircase-Escalante NM
Anton Chico Land Grant

Composite DZ populations in Moenkopi and southern Chinle
(N=samples; n=grain ages)
Age-bin histograms (blue) and age-distribution curves (red) for Moenkopi and southern Chinle
samples (note abundance of 1400-1900 Ma Yavapai-Mazatzal and anorogenic granite detritus
from south of the Colorado Plateau, as well as spikes of arc-derived grains from the
southeast)

Composite age-distribution curves for Carnian Lower ChinleDockum (trunk paleoriver and terminus) and Norian Upper
Chinle-Dockum (Cottonwood paleovalley and headwaters)
(N = samples; n = grain ages)

CP42: Shinarump (lower Chinle)
quartzose sandstone
Wolverine Canyon of Circle Cliffs UT

CP5: Trujillo (upper Chinle)
sublitharenite, roadcut on NM104 near
Conchas Reservoir NM

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