UPPER CRETACEOUS AND PALEOGENE STRATA SOUTH OF THE CHISOS MOUNTAINS, BIG BEND NATIONAL PARK, TEXAS

Geologic mapping of the Upper Cretaceous and Paleogene Pen, Aguja, Javelina and Black Peaks formations on the Reed Camp, Mariscal Mountain, Glenn Spring, and Solis quadrangles south of the Chisos Mountains, provides new information on the nature and extent of these strata in Big Bend National Park. Exposures of these strata are intermittent in this broad region drained by ephemeral tributaries of the Rio Grande, and mostly covered by Quaternary alluvial deposits. Exposures of the Aguja Fm are far more limited than previously shown, extending no farther north than the nose of Mariscal Mt anticline or west beyond the western flank of Cow Heaven anticline. The incorrect mapping of Aguja Fm in the headwaters of Juniper Draw may have led to mistaken early reports of sauropod bones in the Aguja Fm, which occur instead in the Javelina Fm there. The lower tongue of the Aguja Fm is exposed west of Mariscal Mt, but is not present farther east. A previously unrecognized fault cuts out part of the Aguja Fm along the western flank of Cow Heaven anticline. Exposures of the Javelina Fm are also much more restricted than shown previously; apart from a small outcrop brought up along the eastern boundary fault of the Delaho Bolson northwest of Reed Camp, the Javelina exposure extends from Jewel's Camp and Woodson's up the western flank of Cow Heaven anticline and here exhibits a different friable yellow sandstone facies than observed on Tornillo Flat. The ridge-forming indurated brown sandstone facies more typical of the Javelina Fm picks up north of Talley Mt and extends along the western slope of Chilicotal Mt. Exposures of Black Peaks Fm are much more extensive than previously recognized, including the area south of Punta de la Sierra, and the valleys of Juniper Draw and Glenn Draw northward to beneath Chilicotal Spring. Sills that cap Chilicotal Mt and Talley Mt were emplaced within the Black Peaks Fm. A distinctive sandstone with abundant petrified logs occurs in this region, and is a key bed for correlation with the type section on Tornillo Flat. Much of the upper part of the Black Peaks Fm is covered. The Chisos Fm rests directly on Black Peaks Fm in this area; no exposures of the Paleogene Hannold Hill or Canoe formations occur.