MAPPING OF UPPER CRETACEOUS AND PALEOGENE STRATA IN BIG BEND NATIONAL PARK, TEXAS

Geologic maps are presented for parts of the McKinney Spring, Grapevine Hills, Sombrero Peak, The Basin, Tule Mountain, Terlingua, Castolon, San Vicente, and Solis quadrangles in Big Bend National Park, Texas. The maps show primarily those areas underlain by Upper Cretaceous and Paleogene sedimentary rocks. Mapping of these quadrangles reveals significant differences with those areas shown on the existing geologic map of Big Bend National Park produced by Maxwell and others in 1967. A small anticline on the northern border of Tornillo Flat is probably a Laramide-age structure. Several small outliers of (?)Canoe Formation rest unconformably on Aguja Formation on the axis of this fold, indicating that deformation there occurred prior to the middle Eocene. A prominent fault along the southwestern flank of the Canoe Syncline juxtaposes Aguja against Black Peaks Formation. This fault continues across Tornillo Flat to the southeast, and cuts the McKinney Hills laccolith. A prominent fault, not recognized previously, runs the length of Exhibit Ridge, juxtaposing Black Peaks against Hannold Hill formations, and this fault also passes to the southeast into the McKinney Hills laccolith. Exposures of Black Peaks Formation are much more extensive across Tornillo Flat, north of Grapevine Hills, and on Onion Flat than shown previously. Many exposures mapped as Javelina and/or Hannold Hill formations are actually part of the Black Peaks. Extensive areas within the Castolon-Terlingua Abaja fault zone shown as Javelina Fm/Alamo Creek basalt, are recognized here as Chisos Fm/Ash Spring basalt. Areas shown previously as Canoe Fm/Javelina Fm around the nose of "Lone Mountain Anticline" from Nugent Mt to Chilicotal Spring and along the east side of Chilicotal Mountain are recognized here as Black Peaks Fm. The western boundary fault of the Estufa Bolson is exposed east of Chilicotal Spring almost continuously southeastward to "San Vicente Anticline" where Black Peaks, Javelina, Aguja, and Pen formations are progressively juxtaposed against Miocene-Pliocene bolson deposits (previously mapped incorrectly as Aguja Formation).