NEW 1:100,000-SCALE MAPPING OF THE SURFICIAL GEOLOGY IN BIG BEND NATIONAL PARK, TEXAS

Surficial deposits and Quaternary faults in Big Bend National Park previously have not been mapped in detail. The USGS currently is mapping them as part of a collaborative effort with the National Park Service and academic geologists to produce a new 1:100,000-scale geologic map of the park. The goal, in addition to a printed map in popular format, is a detailed GIS database to aid park service management. The existing map of the park (Maxwell et al., 1967, Univ. Texas Pub. 6711; 1:62,500) portrays a two-fold subdivision of surficial deposits. New mapping by the USGS subdivides the deposits into active (Holocene) wash and river deposits (Qaw), young (Holocene) wash and river deposits (Qyw), young alluvial fan deposits (Qya), intermediate (late and middle Pleistocene) wash and river deposits (Qiw), intermediate pediment and alluvial fan deposits (Qia), old (middle Pleistocene) pediment and alluvial fan deposits (Qoa), extremely old (early Pleistocene to Pliocene) alluvial fan and valley-fill deposits (QTa), colluvium (Qc), rock fall deposits (Qrf), landslides (Qls), eolian sand and silt (Qe), and mixed eolian and alluvial deposits (Qea). Units Qyw, Qya, Qiw, and Qia are subdivided where applicable. To expedite mapping of surficial deposits over a large area (3,185 km2), the USGS is simultaneously interpreting and digitizing directly from aerial photography and satellite imagery using computerized photogrammetric instruments. Digitized lines are plotted on a topographic or orthophoto base and taken to the park for checking, thus maximizing efficiency of limited field time. Surface morphology, tone, relative height above stream channels, and map pattern are used to differentiate units. General characteristics of alluvial deposits Qa and Qy include some preservation of bar-and-swale topography and low relative height. Deposits of unit Qi are higher, with planar surfaces that are partly incised and slightly rounded at the edges. Deposits of unit Qoa are higher still, with surfaces that are deeply dissected into broad, rounded ridges. Deposits of unit QTa typically have little or no depositional morphology preserved. Detailed mapping of the surficial deposits should lend valuable insight into the Quaternary geologic history of the park.