EARLY CORDILLERAN ARC MAGMATISM RECORDED BY DETRITAL ZIRCON GRAINS IN LOWER TO MIDDLE TRIASSIC STRATA OF SOUTHWESTERN NORTH AMERICA

Abstract A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of this transition and the inception and tectonic development of the Permo-Triassic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America, in successions in the distal back-arc region on the Colorado Plateau, and in the more proximal back-arc region in the rocks of the Buckskin Formation of southeastern California and west-central Arizona (southwestern North America). The Buckskin Formation is correlated to the Lower–Middle Triassic Moenkopi and Upper Triassic Chinle Formations of the Colorado Plateau based on stratigraphic facies and position and new detrital zircon data. Calcareous, fine- to medium-grained and locally gypsiferous quartzites (quartz siltstone) of the lower and quartzite members of the Buckskin Formation were deposited in a marginal-marine environment between ca. 250 and 245 Ma, based on detrital zircon U-Pb data analysis, matching a detrital-zircon maximum depositional age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An unconformity that separates the quartzite and phyllite members is inferred to be the Tr-3 unconformity that is documented across the Colorado Plateau, and marks a transition in depositional environments. Rocks of the phyllite and upper members were deposited in wholly continental depositional environments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz wacke) in the phyllite member indicate deposition in fluvial systems, whereas the fine- to medium-grained beds of quartzite (quartz arenite) in the upper member indicate deposition in fluvial and shallow-lacustrine environments. The lower and phyllite members show very strong age and Th/U overlap with grains derived from Cordilleran arc plutons. A normalized-distribution plot of Triassic ages across southwestern North America shows peak magmatism at ca. 260–250 Ma and 230–210 Ma, with relatively less activity at ca. 240 Ma, when a land bridge between the arc and the continent was established. Ages and facies of the Buckskin Formation provide insight into the tectono-magmatic evolution of early Mesozoic southwestern North America. During deposition of the lower and quartzite members, the Cordilleran arc was offshore and likely dominantly marine. Sedimentation patterns were most strongly influenced by the Sonoma orogeny in northern Nevada and Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism and the “shoaling” of the arc. The phyllite and upper members were deposited in a sedimentary system that was still influenced by a strong contribution of detritus from headwaters far to the southeast, but more locally by a developing arc that had a far stronger effect on sedimentation than the initial phases of magmatism during deposition of the basal members.

Download Full-text

Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America

Geology ◽

10.1130/0091-7613(1995)023<0831:dzrfct>2.3.co;2 ◽

1995 ◽

Vol 23 (9) ◽

pp. 831 ◽

Cited By ~ 140

Author(s):

George E. Gehrels ◽

William R. Dickinson ◽

Gerald M. Ross ◽

John H. Stewart ◽

David G. Howell

Keyword(s):

North America ◽

Detrital Zircon ◽

Western North America

Download Full-text

The birth of a forearc: The basal Great Valley Group, California, USA

Geology ◽

10.1130/g46283.1 ◽

2019 ◽

Vol 47 (8) ◽

pp. 757-761 ◽

Cited By ~ 4

Author(s):

Devon A. Orme ◽

Kathleen D. Surpless

Keyword(s):

North America ◽

Detrital Zircon ◽

Forearc Basin ◽

Early Stages ◽

Structural Style ◽

Basin Development ◽

Great Valley Group ◽

Extensional Model ◽

Great Valley

AbstractThe Great Valley basin of California (USA) is an archetypal forearc basin, yet the timing, structural style, and location of basin development remain controversial. Eighteen of 20 detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples are consistent with central North America sources and do not require a southern provenance. We interpret that diachronous initiation of sedimentation reflects the growth of isolated depocenters, consistent with an extensional model for the early stages of forearc basin development.

Download Full-text

On the presence ofMixosaurus(Ichthyopterygia: Reptilia) in the Middle Triassic of Nevada

Journal of Paleontology ◽

10.1017/s0022336000042396 ◽

1990 ◽

Vol 64 (1) ◽

pp. 161-164 ◽

Cited By ~ 9

Author(s):

P. Martin Sander ◽

Hugo Bucher

Keyword(s):

British Columbia ◽

North America ◽

Middle Triassic ◽

Lower Triassic ◽

High Arctic ◽

Western Tethys ◽

History Of ◽

Northwestern North America

The small ichthyosaurMixosaurusis the most widely distributed ichthyosaur. It is known from Lower Triassic rocks of British Columbia, Canada (Callaway and Brinkman, 1989), and Middle Triassic rocks of northwestern North America (Alaska, British Columbia), China, Timor, the western Tethys (Switzerland, Italy, Turkey), the Germanic Triassic, and the high Arctic (Spitsbergen, Exmouth Island) (Mazin, 1986; Callaway and Brinkman, 1989; Callaway and Massare, 1989). The presence ofMixosaurusin one of the richest ichthyosaur provinces, the Middle Triassic of Nevada (Merriam, 1908), has been difficult to establish. The history of this problem is very colorful and is the topic of this note together with the description of a new specimen from the Nevada Middle Triassic.

Download Full-text