LANDSLIDES AT CHILICOTAL MOUNTAIN, GLENN SPRING QUADRANGLE, TEXAS, BIG BEND NATIONAL PARK

Recent geologic mapping determined that previously unmapped landslide deposits cover much of the western and southeastern flanks of Chilicotal Mountain, a 9-km-long and 4-km-wide landform having relief as great as 360 m. Landslide detachments occurred at the mountain crest where an alkali syenite sill caps mudstones and lesser sandstones of Tertiary-Cretaceous Black Peaks and Javelina Formations. Stratigraphic relationships between landslide deposits and surficial alluvial deposits indicate that multiple landslide events have occurred since probably middle Pleistocene. Precise landslide ages are unknown. Landslide deposits along the west side of Chilicotal Mountain extend between 0.6 and 1.8 km from the main detachment scarps to the landslide toes. Toes of northwestern and southwestern landslide areas are eroded and inset by young alluvial deposits. A 0.5-km-wide, west-central landslide, displaying distinct pressure ridges, incises relatively older landslide deposits and overlies younger alluvial deposits at the landslide toe. A fan-shaped landslide at the southeast side of Chilicotal Mountain has an aerial extent of about 4 square km. Its main detachment occurred where the east-dipping, 50- to 60-m-thick alkali syenite sill that caps the mountain thins into two smaller 10- to 20-m-thick sills that are interbedded with Black Peaks mudstone and minor sandstone. Landslide transport and spread-out areas are well defined. Numerous arcuate pressure ridges formed within the fan-shaped spread-out area. The landslide toe overlies preserved remnants of old alluvial deposits. This large landslide is cut by a relatively younger, narrower 2- to 3.5-km wide landslide that is about 2 km long. Intermediate and young alluvial deposits have been inset into the landscape adjacent to the southeast Chilicotal landslides. Relatively smaller landslides that have formed at the bases of higher relief ridges also exist along the southern flank of the mountain.