DEPOSITIONAL CHANGES DUE TO ACTIVE RIFTING IN THE CRETACEOUS (ALBIAN) BLUFF MESA, INDIO MOUNTAINS, TRANS-PECOS, TEXAS

Located roughly 160 miles southeast of El Paso, Texas, the Indio Mountains provide excellent outcrop exposure of Cretaceous sediments deposited in an active rift basin. During the Aptian and Albian (125 to 100 Ma), several kilometers of sediments accumulated, First in an alluvial fan system, then in a coastal, narrow sea setting. The Albian Bluff Mesa Limestone consists of interstratified limestones and sandstones deposited in a shallow marine setting. The Limestone has a conformable, intertonguing contact with the underlying clastic marine strata of the Upper Yucca Formation and is overlain by an unconformity that separates it from the Cox Sandstone. Bluff Mesa offers clues to depositional changes during active rifting. Lateral facies changes within units are relatively abrupt, reflecting the narrow sea in which they were deposited. Bluff Mesa can be divided into four main units. The lower Bluff Mesa is a fossiliferous, cliff-forming limestone, with lenticular oyster-bearing beds and more continuous cross-stratified bioclastic grainstones. Above this unit is a thick interval of sandstone and limestone, dominated by lenticular, cross-stratified sandstones and finally, a wackesone with abundant Orbitolina During the Laramide, the Indio Mountains were subjected to thrust faulting and the deeper, thicker parts of the basin were thrust, to the Northeast, near the basin margin. Because of this, two 7 km long continuous exposures of distinctly different thicknesses reside in close proximity to one another. Each of the formation exposures will be studied in detail both in the field and through aerial photographs. Changes in facies characteristics will help to describe depositional processes and environments during this Mid-Cretaceous failed rift.