DETERMINING THE DEPOSITIONAL SETTING OF A LIMESTONE UNIT IN BIG BEND RANCH STATE PARK, TEXAS

A thin (~1 m) but relatively widespread Oligocene – Eocene limestone unit occurs within the Solitario Conglomerate (Henry, 1998) on the Big Bend Ranch State Park, West Texas. Cenozoic limestones are unusual in this region of Trans-Pecos, Texas because the area is dominated by volcanics and clastics, and the limestone is between two volcanic units. The limestone is thin-bedded, light-colored, locally dolomitic, and contains only 10 – 12 % insolubles. Travertine textures are abundant as well as stromatolitic laminations, birdseyes, oncoids, and intraclasts. No bioclastic allochems were observed. SEM and EDS observations of fractured surfaces revealed calcified filaments (1 µm) of cyanobacteria penetrating the matrix.

The absence of any Cenozoic marine units in the region suggests a lacustrine depositional setting for the limestone. The thinness of the unit, the light color (lack of organics) and the lack of variation in the carbonate facies indicates a closed lake. Stromatolites, lime mudstone rip-ups, and oncoids also indicate very shallow conditions. Birdseye structures appear to be of both stromatolite and travertine origin. An ephemeral lake or playa may have been fed by inflow from springs and groundwater seepage which would explain the lack of clastics. The minor amount of clay-sized clastics is likely due to eolian processes. Besides seasonal variations, longer term changes in climate can cause the level of the water table to rise and fall through time causing a lake to form and then revert back to a clastic setting when the water table drops. The lake may have been similar to modern playas in tectonically active areas which contain travertine and grade laterally into alluvial plains or fans.