Santa Fe Group is a term like sparrow or hummingbird. It allows you to describe things fairly accurately when, in fact, you don’t really know enough to be specific.
Ted Galusha and John Blick say that at one time or another it’s been used to describe almost anything along the Rio Grande. They narrow the term to middle Miocene and early Pliocene sediments found between the Sangre de Cristo and Jemez.
They identify two general areas, one they call Chamita, the other Tesuque. Within the second, which is the one found where I live, they identified five major strata: Nambé, Skull Ridge, Pojoaque, Chama-el Rito, and Ojo Caliente sandstone.
The laminated formations you see along 285 when you drive through Arroyo Seco between Santa Fé and Española are from the Skull Ridge and Pojoaque members. According to Daniel Koning, the first was deposited 16.2 to 14.6 million years ago; the second is dated between 14.6 to 11.6 million years ago.
Koning also indicates that rocks came from two sources in the late Miocene. Those towards the north and west are from the Peñasco embayment between the Picuris and Santa Fe Ranges of the Sangre de Cristo, while those to the south and east generally arrived from the Santa Fe Range.
The Los Barrancos fault zone runs to the west of the highway. Sediments to the east, the ones you see, are older than the ones to the west, which are the ones that come close to my house.
The rocks on the west side of the road in the top picture are from the Peñasco embayment with the Skull Ridge member exposed in Arroyo Seco. The rocks on the east side in the second picture have the same provenance, but are older.
The third picture was taken on Pojoaque pueblo land. Koning identifies them as Skull Ridge layers from the Santa Fe range. In the picture below rocks from the Pojoaque member of the Peñasco embayment rise behind the wall of the far arroyo.
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