STRATIGRAPHY OF MIOCENE AND PLIOCENE SEDIMENTARY AND VOLCANIC ROCKS BENEATH THE PAJARITO PLATEAU, WESTERN ESPANOLA BASIN, NEW MEXICO

The Pajarito Plateau is located in the western part of the Española basin, where rocks of the Jemez and Cerros del Rio volcanic fields overlie and interfinger with Neogene basin-fill sedimentary rocks. The plateau formed above the deepest parts of the basin, and geologic relations of Oligocene to Pliocene basin-filling rocks are obscured by thick deposits of Pleistocene Bandelier Tuff and Pliocene Cerros del Rio basalt. The stratigraphy of the upper basin-fill deposits is known from over 60 new groundwater characterization and monitoring wells and surface geologic information. During the early-middle Miocene, >130 m of fine-medium sands and silty sands of the Chama-El Rito Member of the Tesuque Formation were deposited in the western part of the Española basin by south-flowing, low-energy streams. Chama-El Rito deposits are overlain by the Chamita Formation (400-1000(?) m-thick). The Chamita Formation (~6–13 Ma) is primarily comprised of sands and gravels deposited by two merging, south-flowing rivers: the ancestral Rio Chama (Hernandez Member) and ancestral Rio Grande (Vallito Member). Thickness variations of the Chamita Formation were probably controlled by subsidence along the segmented Pajarito fault system. Ages of basalts and phreatomagmatic deposits intercalated within the Chamita Formation cluster between 11.5–13.2 Ma and 8.8–9.3 Ma indicating sedimentation and mafic volcanism were coincident. The upper part of the Chamita Formation interfingers westward with alluvial fans of intermediate to felsic volcanic gravels and sands shed into the western Española basin during the early stages of Jemez volcanism. The upper part of these fan deposits contains abundant crystal-poor rhyolite pumice and lava detritus. Glass compositions and ⁴⁰Ar/³⁹Ar results from the rhyolitic pumice indicate these deposits represent reworked Bearhead Rhyolite tephra (~6.8–7 Ma). During the Pliocene, thick volcanogenic alluvial fans were shed into the western Española basin concurrent with voluminous dacitic volcanism in the eastern part of the Jemez volcanic field. These volcaniclastic deposits interfinger eastward with ancestral Rio Grande deposits of the Totavi Lentil, and together they form the Puye Formation. The Puye Formation interfingers with basaltic to dacitic rocks of the Cerros del Rio volcanic field.