Possible secondary apatite fission track age standard from altered volcanic ash beds in the Middle Jurassic Carmel Formation, southwestern Utah

1990

… Society of America …, 2001

Jurassic Temple Cap and Carmel Formations in southwestern Utah record a pulse of active arc-related volcanism between 166 and 171 Ma. A second pulse between 148 and 155 Ma has previously been documented in the Upper Jurassic Morrison Formation. Volcanic and volcaniclastic rocks of these same ages have also been identified closer to or within the arc in California in the Inyo Mountains, the Cowhole Mountains, the Palen Mountains, and the central Mojave Desert. The upper part of the volcaniclastic Mount Wrightson Formation and the strata of Cobre Ridge in southern Arizona are ca. 170 Ma in age and appear to be time correlative with the Middle Jurassic formations in southwestern Utah.

Geological Society of America Bulletin, 1971

A suite of igneous rocks from the Sawatch Range, Colorado, known to have a Rb-Sr whole-rock age of 1650-35 m. y., has been dated by the nuclear fission-track method. Analysis yielded an age of about 50 ± 12 m. y. for some samples of apatite; other apatite samples yielded ages of 183 .1 44 m. y. to 248± 68 m. y. Ages of 45 to 50 m. y. agree closely with the age of Laramide igneous events in the Rocky Mountains and suggest that partial to complete annealing of spontaneous fission tracks in the apatites occurred about 50 m. y. ago. Spontaneous fission-track densities in most other minerals are too low to be useful for dating; zircons are strongly metamict (Church, 1967). For assumed durations of the Laramide annealing event, an extrapolated annealing temperature-time curve for apatite provides minimum temperatures. If, for example, the Laramide event lasted 10 m. y., the apatite samples must have been heated to at least 150°C. Rb-Sr ages of minerals from these rocks were not measurably affected by Laramide events; thus, subject to assumptions regarding the duration of the heating, a minimum temperature for Rb-Sr redistribution may be determined.

Central Utah—Diverse …, 2007

A bentonitic ash bed 8.7 m below the top of the Upper Jurassic Morrison Formation near Hanksville, Utah, has been dated by laser ablation multi-collector inductively coupled plasma mass spectrometry and gives a single-crystal U-Pb zircon age of 149.0 +2.5/-2.2 Ma. This age is statistically the same as the 149.3 ± 0.5 Ma age of an ash bed located 0.5 m below the top of the Brushy Basin Member of the Morrison Formation near Notom, Utah, about 25 km to the west. The Hanksville section has a short section of possible Cedar Mountain Formation (between 0.4 and 7.3 m thick) on top of the Morrison section. Two young contaminant zircons with a peak age of 122.5 Ma in the Brushy Basin ash sample indicate that the Cedar Mountain sediments present here are from the lowermost part of the Cedar Mountain Formation. b b b

Geology of the Intermountain West, v. 7, p. 69–96, 2020

A stratigraphic layer containing rhyolite cobbles and boulders in the Middle Jurassic Carmel Formation of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts is about 8 m.y. older than a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma). The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits then remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silt and mud. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc deposited onto an adjacent highland and were then eroded by streams flowing to the east onto a fluvial plain that is near the sea.

Geology, 1986

Depositional ages of sedimentary rocks can be determined using fission-track single grain ages on zircons from layers of volcanic ash or bentonite, even when the layers have been contaminated by older grains. This is done by compiling an age probability distribution or age spectrum for a sample from individual grain ages. An age spectrum is a simple and unambiguous way of testing for contamination and extracting useful age information. The youngest peak in the age spectrum approximates the time of deposition. In most contaminated samples, 30 or more grains should be counted to produce a reliable spectrum. However, useful, reproducible ages can be obtained by counting less than 10 grains in samples where most of the older, contaminating grains can be removed. The few older grains that remain after removing the obviously abraded ones may then be eliminated by examining the age spectrum. Although ages determined in this way are probably not precise enough for use in defining stratigraphic boundaries, they still provide a means of obtaining an isotopic age in sediments that cannot be dated by other radiometric methods.

Tectonics, 1991

Apatite fission track ages and confined track length distributions from the Old Woman Mountains area, southeastem Califomia, record evidence for periods of rapid cooling during the Late Cretaceous following Mesozoic orogenesis and during Miocene crustal extension. Apatites from the Clipper, Marble, and Turtle Mountains give fission track ages between ~62 and -•69 Ma as a result of cooling of these upper crustal rocks after regional Late Cretaceous plutonism and metamorphism. In the Piute Mountains, apatite fission track ages decrease from >70 Ma with mean track lengths of ~13.5 grn to <20 Ma with mean track lengths of > 14 lam from west to east. Intermediate aged samples from the Piute Mountains have mean track lengths between 11 and 13 lam due to their extended residence in the apatite annealing zone during the early Tertiary. This pattem is the result of 200-300 of westward tilting of the Piute Mountains block as indicated by the dip of Tertiary strata. The apatite data indicate that tilting occurred after ~18 Ma and caused rapid cooling of rocks exposed on the eastem side of the mountain range. Knowing the amount of tilting and the apparent paleotemperatures across the Piute Mountains allows us to estimate that the synextensional geothermal gradient was 500 + 20øC/km. Apatites from the northeastern Old Woman Mountains yield fission track ages of -•68 Ma with mean track lengths of 13.8 grn. These samples record cooling after batholith emplacement at 73 Ma. In the northwestern Old Woman Mountains, apatite fission track ages decrease to <20 Ma and have mean track lengths ~15 gm indicating very rapid cooling. We interpret this age pattern to be the result of minor eastward tilting of the Old Woman Mountains block after 18-19 Ma or flexural unroofing of the northwestern Old Woman Mountains due to movement on the Ship Mountains fault. Extension in the Old Woman Mountains area occurred between 19 and 16 Ma, which is coincident with extension in the Colorado River Extensional

1989

The Publication of this report is made possible with fundingfrom the Mineral Lease Special Projects Program. A primary mission of the UGMS is to provide geologic infonnation of Utah through publications; the fonnal publication series is reserved for material whose senior author is a UGMS staff member. The Miscellaneous Publications series provides an outlet for non-UGMS authors without necessarily going through extensive policy, technica~ and editorial review required by the fonnal series. It also provides a means for UGMS and non-UGMS authors to publish more interpretive work with the knowledge that readers will exercise some degree of caution.

Quaternary Research, 1991

Geophysical Research Letters, 1997

We present first results of 4øAr/39Ar multiple single fusion datings on biotites and feldspars from two volcanic ash beds found on Crete, Gavdos and Koufonisi (Greece). Preferred 4øAr/39Ar ages-calculated against TCR sanidine with an age of 27.92 Ma, intercalibrated to an age of 28.09 + 0.10 Ma (l(J) for FCT-3 biotite and an age of 24.99 + 0.07 Ma for DRA sanidineare slightly but consistently younger than astronomical ages obtained independently for the same ash beds. The best fit to the astronomical ages is obtained when the age of TCR sanidine is increased slightly to 27.98 + 0.19 Ma, the age of FCT-3 biotite to 28.15 _+ 0.19 Ma and the age of DRA sanidine to 25.05 _+ 0.17 Ma. The ages for the standards arrive slightly younger if the 4øAr/39Ar age-of 6.936 _+ 0.006 Ma (1 (5)-for a single pure sanidine separate of the lower ash-dated astronomically at 6.941 Ma-is considered most reliable.