Guacio Member (informally used by M.D. Turner, unpublished) is here named in the Rio Culebrinas Formation, here adopted and redefined. Is the basal of 4 lithic types in the Rio Culebrinas: Guacio Member, tuff breccia, crystal-lithic tuff, and volcanic sandstone and mudstone. Three upper lithic types grade into one another and are interbedded. Guacio consists of thin-bedded siliceous and calcareous mudstone and impure limestone. Conformably overlies Mal Paso Formation. Age is middle Eocene based on Foraminifera in the Rio Culebrinas. Contact between rocks of the Rio Culebrinas and underlying Milagros Formation in the eastern San Sebastian and Bayaney quads is conformable and gradational. The Guacio Member, always present at the base of the Rio Culebrinas to the southwest, is not present where the Rio Culebrinas rests on the Milagros. The Milagros may be in part equivalent to the lower part of the Rio Culebrinas of the southern area. Early middle Eocene Foraminifera collected from the Matilde Formation which underlies the Milagros, support this interpretation and indicate that the Milagros is probably middle Eocene, same as the Rio Culebrinas. An unconformity between Cretaceous and middle Eocene rocks is recognized in west-central PR. The unconformity may also be present in northwestern PR as Paleocene and possibly lower Eocene rocks are missing there.

Source: GNU records (USGS DDS-6; Reston GNULEX).