New Mexico Geological Society Annual Spring Meeting — Abstracts

Approximately 625 m of Cretaceous strata are expose NE of Virden in secs. 1-9, 16-18 and 20-21, T18S, R20W, Hidalgo County. The oldest Cretaceous strata here pertain to the Sarten Formation (= Beartooth Quartzite) and rest nonconformably on coarse microcline granite of Precambrian age. Sarten strata are 18-45 m thick and are dominated by medium gray and reddish brown,coarse-grained, poorly sorted, trough-crossbedded quartzarenite. The sarten is disconformably overlain by 70-96 m of Mancos Formation which. are mostly olive-gray and yellowish brown, calcareous siltstone. About 10 m above the base Mancos are 5.5 m of gray limestone and brown calcarenite we assign to the Bridge Creek Member. These Bridge Creek strata contain abundant Pycnodonte newberryi and fossil-shark teeth and rare inoceramids and ammonites. The Atarque Sandstone overlies the Mancos Formation near Virden; it is 24-39 m of qreenish gray and pale yellowish brown calcarenite and calcareous siltstone dominated by biostromal layers of Crassostrea. We assign the 103 m of yellowish brown and pale orange beds of sandy siltstone and trough-crossbedded quartzarenite above the Atarque Sandstone to the Moreno Hill Formation. Above them are 340 m of strata dominated by beds of gray siltstone and pale olive lithic and subarkosic sandstone. The upper 80 m of these strata include volcanic boulder conglomerates and volcaniclastic sandstones. Approximately 64 m below the top of this interval, a volcaniclastic sandstone in the SE1/4 SW1/4 SE1/4, sec. 8, T18S, R20W produces fossil leaves ("Araucarites, Ficus, Juglans", etc.) previously considered to be of Maastrichtian age. We believe the upper 340 m of the cretaceous section near Virden may be equivalent to the Ringbone Formation and/or Hidalgo Volcanics of the northern Little Hatchet Mountains, 100 km to the southeast. For now, we restrict use of Elston's (NMBMMR Geologic Map 15) name Virden Formation to these 340 m of strata; Elston originally used the name Virden Formation to refer to all the post-Mancos Cretaceous strata exposed NE of Virden. The Virden Formation sensu stricto is disconformably overlain by andesite and latite flows and tuffs of the lower Datil Group (Eocene-Oligocene).