Recent mapping of the Oligo-Miocene Los Pinos Formation and associated units in the Tusas Mountains, New Mexico
— Scott B. Aby, Kirt Kempter, and Daniel Koning

 The Los Pinos Formation is a volcaniclastic unit derived mostly from intermediate volcanic sources but contain­ing mafic-to-silicic components and some primary volcanic intervals. First described by Atwood and Mather (1932), the Los Pinos Formation is found in the Tusas Mountains of northern New Mexico and the San Juan Mountains of southern Colorado. Butler (1946) first divided the Los Pinos into members, and several other authors have revised his member subdivisions in their mapping. Recent mapping of the Los Pinos on the Las Tablas quadrangle (Aby et al., 2010) has given new insights into the Los Pinos and provides some new context for member-rank subdivisions. Most importantly, a poorly welded tuffaceous unit found near Las Tablas (The Las Tablas tuff) has been recognized as partly(?) a ‘megabreccia’ containing abundant volcanic and sedi­mentary debris and large blocks of older granitic(?), intermediate volcanic, volcaniclastic and clastic rocks. Some of these large blocks have previously been interpreted as distinct units (either individual members of the Los Pinos or other, older formations) and this has lead to a lack of uniformity in previous mapping. Additionally, the Los Pinos has traditionally been defined mostly by its stratigraphic position relative to an ignimbrite (the Treasure Mountain Tuff) that is not present in all areas. We propose that member-rank subdivisions eventually be formally defined by the clast types each member contains so that, even in poorly exposed areas, individual members can be objectively identified. This type of definition will then allow the source areas of the various members to be distinguished and the geologic history of the area to be reconstructed more easily. Additional mapping is needed before a final, formal definition of the Los Pinos Formation is possible.