SEDIMENT DISPERSAL IN TRIASSIC FLUVIAL SYSTEMS OF SOUTHWEST LAURENTIA:

1
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)

• 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
  (up-paleocurrent) 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)
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 gt20 discordance or gt10 uncertainty total
of 1846 individual DZ zircon ages). U-Pb age
populations fall into four groups (1) Lowermost
Chinle-Dockum (n3) 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 lt300 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 (n2), derived from the SE, and lower
Chinle (Carnian) Shinarump (n3) 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 (n3)
and correlative Santa Rosa (NM n1) along the
Chinle-Dockum trunk paleoriver (course ESE to WNW
across Colorado Plateau) heterogeneous DZ
including Grenville (40-50 1290-1000 Ma) and
Neoproterozoic-Paleozoic (15-20 640-320 Ma)
grains recycled from the Ouachita orogen, with
arc-derived grains sparse (lt10) closely
matching DZ downstream in a Chinle outlier at
Currie NV within the Great Basin. (4) Upper
Chinle (Norian) of the Cottonwood paleovalley
(n4), 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 (n3),
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.
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)
Chinle-Dockum paleorivers (trends subparallel to
tectonic axis of dynamic backarc basin)
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 lt350 microns -Grains further
separated by density using heavy liquids
(Methylene Iodide) -Final grain separation using
a Frantz magnetic separator -Sample with gt90
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 gt20 discordance or
gt10 uncertainty were removed (see Concordia
plot) -206Pb/238U ages (for lt1.0 Ga grains) and
206Pb/207Pb ages (for gt1.0 Ga grains) were
plotted on an age probability plot (see age
probability plots)
Preserved extent of Lower to Middle Triassic
Moenkopi deposystem of Colorado Plateau and High
Plains (crosses denote sample localities)
sandstone detrital modes
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)
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-Gre
nville detritus from Ouachita orogen)
CP 42 Shinarump (lower Chinle) in Wolverine
Canyon of Circle Cliffs UT
CP26 Poleo (upper Chinle) in roadcut on US84
near Abiquiu Reservoir NM
CP42 Shinarump (lower Chinle) quartzose
sandstone Wolverine Canyon of Circle Cliffs UT
CP5 Trujillo (upper Chinle) sublitharenite,
roadcut on NM104 near Conchas Reservoir NM
Confreres for Collecting Ronald C. Blake Spencer
G. Lucas John H. Stewart
Permits for Collecting Navajo Nation and Hopi
Tribe Grand Staircase-Escalante NM Anton Chico
Land Grant
Research Support NSF Grant EAR-0341987 Arizona
LaserChron Center
Composite DZ populations in Moenkopi and southern
Chinle (Nsamples ngrain ages)
Composite age-distribution curves for Carnian
Lower Chinle-Dockum (trunk paleoriver and
terminus) and Norian Upper Chinle-Dockum
(Cottonwood paleovalley and headwaters) (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)