New Evidence Confirms the ~250 K.y. Duration for Deposition of the Paleocene Ojo Alamo Sandstone in the Southern San Juan Basin, New Mexico

Abstract

The dinosaur-bearing Ojo Alamo Sandstone (OAS) of the San Juan Basin (SJB), NM and CO, is earliest Paleocene based on palynologic data from multiple localities. An unconformity of 7-8 m.y. separates the Paleocene OAS from underlying Cretaceous strata in the southern SJB – Maastrichtian-age strata are absent there. Previous publications show that OAS dinosaur bones were not reworked but were fossilized in place. The OAS averages 15-30 m thick in the southern SJB and is up to 130 m thick further north in the basin. The first altered volcanic ash bed ever found in the Nacimiento Fm. was discovered in the southeast SJB 64 m above the top of the OAS near Cuba, NM. Sanidine grains from this ash bed had a 40 Ar/ 39 Ar age of 64.60 Ma (adjusted). Based on an estimated rate of deposition for the lower Nacimiento Fm., the age for the top of the underlying OAS was determined to be 65.7 Ma. Subsequently, a second Nacimiento ash bed was found only 10.5 m above the top of the Ojo Alamo with a reported 40 Ar/ 39 Ar sanidine age of 65.49 Ma . This ash bed was found near De-na-zin Arroyo in the southwest part of the SJB. Again, based on an estimated rate of deposition for the lowermost Nacimiento Fm, the top of the underlying OAS was calculated to be 65.7 Ma. In addition, a detrital sanidine age for the top of the OAS in the southern part of Cuba, NM was reported to be 65.67 Ma, in agreement with the ages above. And a very recent paper (2020) discussing the paleomagnetism of the Nacimiento Fm., suggests an age of 65.67 Ma for the base of magnetochron C29n, a few meters above the top of the OAS. These new data also support an age of 65.7 Ma for the top of the OAS in the southern SJB. Previous publications have estimated the base of the OAS to be ~65.95 Ma, thus the duration of OAS deposition in the southern SJB must have been ~250 k.y. This duration of OAS deposition in the southern SJB of about a quarter-million years is thus confirmed by recent data. It must be cautioned that in those parts of the basin to the north, where the OAS is thicker, the time interval for its deposition could have been proportionally greater and have a younger upper boundary. Some workers (mostly vertebrate paleontologists) have suggested that the OAS consists of two members: a lower, dinosaur-bearing member of Cretaceous age separated from the upper part by an imagined unconformity of millions of years. This report shows that the undivided OAS was deposited over about 250 k.y. in the southern SJB. There is no stratigraphic evidence on OAS outcrops around the basin for a significant break in deposition within this formation; indeed, OAS outcrop observations clearly show otherwise. Voluminous palynologic data, published heretofore, unequivocally support the top-to-bottom Paleocene age for the dinosaur-bearing OAS throughout the SJB. None of these data have ever been falsified.