The original age estimate, based on radiometric agesofintrusions and interbedded flows, ranged from 42 to 51 million years.[6] More recent high-precision U-Pb dating gives an age of 45.0±0.7 Ma for an ash fall tuff in the lower part of the formation in the Robledo Mountains and an age of 39.6±0.5 Ma for an ash fall tuff in the upper part of the formation in the Sierra de las Uvas. The interbedded ash fall tuffs and volcaniclastic beds of the formation are interpreted as volcanic activity after the end of Laramide mountain building but before the opening of the Rio Grande Rift. At this time, the remnants of the Farallon Plate began to sink into the deep mantle and hot mantle rock rose to take their place, triggering the Mid-Tertiary ignimbrite flare-up.[7]
The Palm Park Formation contains hot-water stromatolites. Stable oxygen isotope measurements show that the fine layers (laminae) making up the stromatolites were deposited seasonally, with the sparry (coarsely crystalline) layers deposited in the spring and summer, and the micritic (fine-grained) layers deposited in the fall and winter. This suggests that laminae in more ancient stromatolites are also seasonal in nature.[8]
The formation also contains Eocene fossil vertebratesofChadronian age. These are found in deeply weathered volcaniclastic beds of the middle and upper part of the formation that are interbedded with plant- and gastropod-bearing travertines, and most are also badly weathered. However, they include shell fragments of the tortoise Stylemys, jaw fragments of Hyaenodon horriblis, tooth fragments likely of Hyracodon, and a partial skeleton of a protoceratid.[9]
Barite-galena-manganese deposits are present in the Palm Park Formation in the Rincon basin. These had produced 10,520 tons of barite and 1529 tons of 27%-40% manganese ore by 1998. The formation also contains gypsum and travertine, but not in economic amounts for the 1998 market.[10]
The formation was first named by V.C. Kelley and Caswell Silver in 1952 for exposures in Palm Park, southeast of Caballo. Because no complete section exists, a type locality was designated instead of a type section.[1] Steven M. Cather and his coinvestigators assigned the Palm Park Formation to the Spears Group in 1994.[11]
Chafetz, Henry S.; Utech, Nancy M.; Fitzmaurice, Sean P. (1991). "Differences in the 18O and 13C Signatures of Seasonal Laminae Comprising Travertine Stromatolites". SEPM Journal of Sedimentary Research. 61. doi:10.1306/D426782A-2B26-11D7-8648000102C1865D.
Jahns, Richard H.; Kottlowski, Frank E.; Kuellmer, Frederick J. (1955). "Volcanic rocks of south-central New Mexico". New Mexico Geological Society Field Conference Series. 6. CiteSeerX10.1.1.528.3373.
Kelley, V.C.; Silver, Caswell (1952). "Geology of the Caballo Mountains; with special reference to regional stratigraphy and structure and to mineral resources, including oil and gas". University of New Mexico Publications in Geology. 4.
McMillan, Nancy J. \ (2004). "Magmatic record of Laramide subduction and transition to tertiary extension: Upper Cretaceous through Eocene igneous rocks of New Mexico". In Mack, G.H.; Giles, K.A. (eds.). The geology of New Mexico. A geologic history: New Mexico Geological Society Special Volume 11. pp. 249–270. ISBN9781585460106.
