New Mexico Geological Society Annual Spring Meeting — Abstracts

In west-central New Mexico and east-central Arizona, the TR-4 unconformity is at the base of the Sonsela Member of the Petrified Forest Formation (Chinle Group). Multiple lines of evidence, including tetrapod biochronology and palynostratigraphy, indicate that the TR-4 unconformity approximates the Carnian-Norian boundary. Correlation of measured stratigraphic sections extending from the Petrified Forest
National Park (Apache County, Arizona) eastward through the Zuni Mountains (McKinley and Cibola Counties, New Mexico) and continuing to the Lucero uplift (Cibola, Valencia, and Socorro Counties, New Mexico) demonstrate that up to 100 m of erosional relief was generated from west to east along this transect.

In eastern Arizona, conglomerates and conglomeratic sandstones of the Sonsela Member are underlain by as much as 90 m of mudstone-dominated flood-plain and paleosol deposits of the Blue Mesa Member of the Petrified Forest Formation. To the east, the strata of the Blue Mesa Member are progressively thinner, averaging approximately 35 m throughout the Zuni Mountains. In the Lucero uplift, Blue Mesa Member strata are entirely absent, and the Sonsela rests disconformably on "red beds" of the older Bluewater Creek Formation or its lateral equivalent, the San Pedro Arroyo Formation. Measured sections and field observation in the Zuni Mountains and Lucero uplift show that the Bluewater Creek Formation does not thicken appreciably at the expense of the Blue Mesa Member, maintaining a nearconstant thickness of approximately 60 m, and at no place do the two units interfinger or otherwise grade into each other. Similarly, in the southernmost portion of the Lucero uplift, San Pedro Arroyo Formation sediments laterally replace Bluewater Creek Formation strata, again with little if any increase in stratigraphic thickness. We interpret these relationships to indicate that the decreasing thickness of the Blue Mesa Member is related to a period of erosion due to a regional fall in base level, with subsequent infilling of the resulting incised topography with coarse clastics of the Sonsela Member.