Laramide evaporite tectonics along the Texas-northern Chihuahua border
John C. Gries
A major structural boundary lies approximately parallel to the present political boundary between northeastern Chihuahua and adjacent parts of Trans-Pecos Texas. It coincides with the northern and eastern margin of the Chihuahua Trough (DeFord, 1964), a narrow northwest-trending negative feature flanked by the Aldama Platform of western Chihuahua and the Diablo Platform of Trans-Pecos Texas. From the Late Jurassic until the Late Cretaceous, from 3600 to as much as 7600 m of sedimentary rock was deposited in the Chihuahua Trough while about 1000 m of sedimentary rock accumulated on the adjacent Diablo Platform (cries and Haenggi, 1971). The accumulation of strata, including a thick basal evaporite sequence, was later complexly folded and faulted during formation of the Chihuahua Tectonic Belt. A zone of recurrent faulting along the western margin of the Diablo Platform marks the eastern limit of the Chihuahua Tectonic Belt. This zone of faulting separates the thick, complexly deformed rocks of the basin from the mainly block-faulted platform rocks to the east and north of the basin.
In relation to present geography, the eastern limit of this deformed belt lies immediately east of the eastern flanks of the various sierras extending from La Mula, Chihuahua to Sierra Blanca, Texas, referred to here as the La Mula-Sierra Blanca Range. The northern limit of the belt lies along a line extending from Sierra Blanca, Texas on the east through Sierra Juarez, Chihuahua on the west. Stresses imposed during the Late Cretaceous-early Tertiary Laramide orogeny deformed the thick Mesozoic sequence and the underlying Paleozoic rocks. Evaporites acted as a decollement zone between Mesozoic and Paleozoic rocks. Major thrust faults, overthrust toward the east, developed along the eastern edge of the evaporite basin. Similar thrust faults and accompanying faults occurred along the northern margin of the basin with additional northward movement on the decollement zone. Complex disharmonic folding caused by evaporite, shale and marl flowage is present in thrust plates at all scales up to fold amplitudes of 3500 m. Diapiric injection occurred along three east-trending tear faults and in at least one place on the toe of the thrust complex. Normal faults in Cretaceous rocks developed over void areas left by evaporite flowage into anticlines and diapirs. During Laramide deformation there was probably reactivation of north- to northwest- trending faults that had affected Paleozoic rocks. Pre-Laramide fold-fault trends may have exerted control over Laramide structural trends, which determined the alignment of the front ranges of the Chihuahua Tectonic Belt.
Diapiric features that developed during Laramide deformation continued to be active after most faulting and folding had taken place. Collapse structures developed over diapirs as evaporites were removed by early Tertiary erosion. At several places, erosion removed nonresistant beds and gravity sliding resulted in flaps and detached flaps in adjacent beds of limestone on high-angle flanks of folds.
Late Eocene-early Oligocene volcanism deposited ignimbrites, lava and volcanically derived sediments over much of the area. Regional uplift and block faulting were superimposed on the Laramide structural features during the late Tertiary. Considerable evidence exists that late Tertiary and Quaternary structural features associated with the Rio Grande rift extend into the Chihuahua Tectonic Belt (cries, 1979).
- Gries, John C., 1980, Laramide evaporite tectonics along the Texas-northern Chihuahua border, in: Trans-Pecos Region, Dickerson, Patricia W.; Hoffer, Jerry M.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 31st Field Conference, pp. 93-100.