Stratigraphy and sedimentology of the Upper Cretaceous terrigenous rocks and coal of the Sabinas-Monclova area, northern Mexico

Late Campanian to late Maastrichtian rocks of the Sabinas-Monclova area consist of nine lithostratigraphic units with a composite thickness of 2,000 m. They form a complex of intertonguing deltaic sandstone lithosomes with fine-grained shaly rocks. New stratigraphic and sedimentologic data permit most formations to be redefined, and new stratigraphic units to be introduced, in order to reconcile the stratigraphy of the Rio Grande area to the north with that of the La Popa and Parras basins to the south of the study area.;Variation in sediment influx and basin subsidence produced four major transgressive-regressive episodes. Lithologic, paleontologic, and petrographic data support the correlations of episodes which also seem to be related to tectonic activity in the source area. This source was located in southeastern Arizona and underwent a major lithologic change, reflected by a dramatic increase in volcanic detritus during the late Campanian. Detritus from southeastern Arizona permited a 400-km progradation that passed from the Ojinaga-Big Bend area to the study area and yielded the diachronous Upson- San Miguel- Olmos Formations. They represent the deposits of an initially fluvial-dominated, lobate deltaic system that evolved into a wave-dominated system as southeastward progradation continued. Younger stratigraphic units contain deposits of wave-dominated deltas whose sandstones indicate an increase in metamorphic and especially sedimentary detritus at the expense of volcanics. The morphology, texture and microfossils of chert pebbles indicate that some rivers drained areas of Lower Cretaceous and Paleozoic sedimentary rocks located 300 to 500 km west and northwest of the study area during late Maestrichtian time.;The Olmos Formation is the coal-bearing unit of the study area and contains deposits of upper and lower delta-plain facies and of coastal and lagoon/back-barrier facies. Coal rank ranges from sub-bituminous to low-volatile bituminous (ASTM); according to maceral content most coals are vitric-bituminous.;The term sabinite is proposed to replace the ambiguous term as tonstein. In the study area these clay partings are the result of pedogenetic alteration of an illitic clay parent material interacting with organic colloids. Sabinites are particularly concentrated in interdistributary and flanking areas of major fluvial axes. Their restricted distribution is not compatible with an origin as volcanic ash-fall deposits. (Abstract shortened with permission of author.)...