New Mexico Geological Society Annual Spring Meeting — Abstracts

The Paleocene Ojo Alamo Sandstone is present throughout most of the New Mexico part of the San Juan Basin. This formation forms striking cliffs around much of its outcrop and consists of multistoried fluvial deposits consisting of conglomeratic sandstone beds, coarse-grained, immature sandstone beds, and overbank mudstone and siltstone layers. The basal contact of the Ojo Alamo is thus quite distinct and easily mappable because of the marked contrast between the formation’s distinct lithology compared to the fine- to medium-grained, mature sandstones of the underlying Cretaceous Kirtland and Fruitland Formations. In addition, current-direction studies have shown that underlying Cretaceous strata were deposited by low-energy streams flowing northeastward across the basin area whereas the Ojo Alamo was deposited by very high energy streams flowing to the southeast. The age of the Ojo Alamo and its lithologic definition have been controversial for 100 years. Over the last decade or so, the rockstratigraphic definition of this formation has reached general, though not total agreement, however its age is still hotly disputed. The Paleocene age of the Ojo Alamo has recently been established by detailed palynologic and paleomagnetic data. Those who still maintain that the Ojo Alamo Sandstone is Cretaceous and not Paleocene base their argument on the fact that the Ojo Alamo contains numerous, in-place, dinosaur or “Cretaceous” mammal fossils, mostly in the southwest part of the San Juan Basin. Because palynologic and paleomagnetic data confirm the Paleocene age of the Ojo Alamo, the dinosaurs and “Cretaceous” mammals found in this formation must represent the remains of animals that lived and died in Paleocene time.

The following dinosaurs have been identified from the Ojo Alamo Sandstone: Alamosaurus sanjuanensis; ?Albertosaurus sp., cf Tyrannosaurus sp.; ankylosaurid, indeterminate; dromaeosaurid, indeterminate; Glyptodontopelta mimus; hadrosaurids, indeterminate; nodosaurids, indeterminate; ornithomimid, indeterminate; Richardoestesia sp.; cf titanosaurids, indeterminate; Torosaurus cf T. utahensis; Troodon sp.; Tyranosaurus rex; and tyrannosaurid, indeterminate.

Some paleontologists have suggested that this assemblage is lower Maastrichtian to early Campanian in age whereas others maintain that it is Lancian (latest Cretaceous) in age – an age difference of 5.1 m.y. Two recent papers have stated that A. sanjuanensis is precisely 69.0 Ma based on an age determination of a tuff bed in the Big Bend area of West Texas. However, the authors of the Big Bend study state that their age of 69 Ma is in about the middle of their A. sanjuanensis zone and that this fossil ranges from late Edmontonian to Lancian or from between 73-69 Ma to the end of the Cretaceous (and possibly beyond). Furthermore, paleontologists studying the vertebrates of the North Horn Formation of Utah have indicated that A. sanjuanensis is present in uppermost Cretaceous (Lancian age) strata there, thus, assigning this fossil a precise age of 69.0 Ma cannot be defended. In sum, vertebrate fossils have proven to be of little value in dating Ojo Alamo Sandstone strata in the San Juan Basin.