LAKE LEVELS AND DELTAS IN THE MID-CRETACEOUS BORDER RIFT, INDIO MOUNTAINS WEST TEXAS

The Aptian/Albian Yucca Formation of West Texas and Northern Chihuahua was deposited during rifting within the Chihuahua Trough basin of the Mid-Cretaceous Border Rift. In the Indio Mountains of West Texas, these strata are spectacularly exposed. The upper part of the Formation, the Informal Upper Yucca Member is up to 650 m thick and contains interbedded fluvial and lacustrine intervals that define 5 sequences that include higher frequency sequences. The lacustrine strata are similar in appearance, but sedimentologic and stratigraphic analysis reveals differences in depth and chemistry, that allow an understanding of the hydrology of the lake basin. The basal two and uppermost strata expose low-relief lake floors that were deposited in lakes with alkaline waters. The middle two sequences expose deltas that filled portions of a deeper lake. Deltaic strata are exposed in three locations in the upper Yucca in the Indio Mountains. Detailed mapping and high-resolution photographs and 3-D models of the deltaic strata allow a more complete interpretation of these intervals. The lacustrine deltas expose foresets with restored slopes of 30° with paleo water depths of at least 40 m. The foresets are arranged in high-frequency cycles containing 20-m thick foresets that separate lowstand strata deposited on the floor beneath the foresets. Strata alternate between sandstones, deposited at the base of the foresets, and representing fluvial incursions across a dry lake floor. Deltaic progradational intervals 40 m above this level, and mudstones that drape the deltas, representing lake highstands of unknown, but greater depths. Microbial limestones on the top of the deltas indicates abandonment of the deltas as the lakes began to fill above this level. The repeated return to a fixed intermediate lake level suggests that the strata were deposited in a sub-basin of a larger lake system, with a spillover point that allowed repeated filling to the same level.