New Mexico Geological Society Annual Spring Meeting — Abstracts

Between 1988 and 1996, eight altered volcanic ash beds in Upper Cretaceous strata were sampled in the San Juan Basin of New Mexico and Colorado and their ages determined using ⁴⁰Ar/³⁹Ar analyses of sanidine crystals separated from the ash layers. Nearly all the samples were dated using single-crystal methodology. All samples were collected by the author and processed by John Obradovich at the USGS campus at the Denver Federal Center. Samples were irradiated in the USGS reactor at the Federal Center and then taken to the USGS ⁴⁰Ar/³⁹Ar lab in Menlo Park, California for final processing. Sample ages were based on the monitor age of 28.02 Ma for Fish Canyon Tuff and 28.32 Ma for Taylor Creek Rhyolite (these standards yield an average age of 65.51 ± 0.01 for the K-T boundary). The stratigraphically lowest sample was collected from the Huerfanito Bentonite Bed of the Lewis Shale at an outcrop north of Cuba, NM and had an age of 75.76 ± 0.34 Ma; this bed is 380 m below the Cretaceous-Tertiary interface (base of the Ojo Alamo Sandstone) in the southern part of the basin. Six other samples were collected from ash beds in the Fruitland and Kirtland Formations and one sample was collected from another altered ash bed near the top of the Lewis Shale. The highest of these samples from an ash bed about 5 m below the K-T interface (the base of the Ojo Alamo Sandstone) yielded an age of 73.04 ± 0.25 Ma. The six intermediate ages ranged from 75.56 ± 0.41 Ma to 73.37 ± 0.18 Ma.

This data set of eight precise ages for uppermost Cretaceous rock strata in the San Juan Basin clarifies some previous interpretations of these rocks and allows for precise estimations of subsidence rates: 100-125 m/m.y.; rate of sediment accumulation: 140 m/m.y.; and rate of shoreline regression across the basin area for the shoreface Pictured Cliffs Sandstone: 53 km/m.y. Earlier authors had suggested that there was no unconformity at the base of the Ojo Alamo Sandstone at the K-T interface, but these age data prove that there is a hiatus of nearly 8 m.y. at this contact representing 7.54 m.y. of latest Cretaceous time (all of the Maastrichtian and the upper part of the Campanian) plus about 0.3 m.y. of earliest Paleocene time. (Palynologic data had previously indicated the presence of this hiatus.)

These ash-bed ages bracketed the magnetic polarity reversal from C33n to C32r allowing for a precise determination of the age of this reversal of 75.50 ± 0.19 Ma.

Vertebrate fossils identified from uppermost Cretaceous strata are largely endemic to the San Juan Basin, thus their ages have never been accurately known. The data set of eight dated ash beds in this interval now allows for precise age assignments of these fossils and thus now permit comparisons of these species with similar species from the northern part of the Western Interior, the radiometric ages of which have been known for some time.