About This Item
Share This Item
The Anacacho Formation is an Upper Cretaceous carbonate-bank sequence that formed in a narrow zone between the waning Cretaceous seaway and the young Gulf of Mexico. The Anacacho crops out along the Cretaceous Gulf coastal trend from Kinney to Bexar Counties, Texas. The most complete and extensive exposures occur in the Anacacho Mountains in Kinney County and the Whites Mine area of Uvalde County where asphalt is actively mined.
In this study area, the Anacacho Formation was deposited in the northern Rio Grande embayment on the fringe of a structural transition zone between compressional features on the west and extensional faulting on the east. The compressional features resulted from Carboniferous continental collision on the southern margin of North America, and Late Cretaceous subduction on the western margin. These events resulted in deep crustal weaknesses and the penetration of mafic intrusions which produced bathymetric highs where Anacacho organisms began to flourish.
In the Anacacho Mountains of Kinney County, the Anacacho is primarily composed of tan-orange recrystallized, molluscan, sorted biosparite. In addition, there are two interbedded chalky micrites which are distinguished from massive calcrete crusts on the biosparites. At the top of the northeast end of the Anacacho Mountains, some patches of molluscan, bryozoan, biosparrudites occur. In the Whites Mine area of Uvalde County, the biosparite also occurs; however, three mines penetrate an asphaltic, molluscan, unsorted biosparrudite.
The diagenetic sequence of the Anacacho Formation began with early diagenetic events such as micritization, authigenic growth of glauconite in foraminiferal tests, and authigenic growth of framboidal pyrite nodules. Postdepositional diagenesis included the development of microspar, syntaxial overgrowths of spar, recrystallization of allochems, and calcite spar fill. Further burial caused severe compaction, evidenced by fracturing and pressure solution features. The sediments of the Anacacho Formation were then uplifted and subjected to ground-water circulation in the freshwater phreatic zone. This resulted in dissolution of spar and microspar followed by precipitation of "dog tooth" sparry cement. This secondary porosity development primarily occurred in the Whites Mine area biosparru ites; however, paleokarstic development which may have occurred during this stage of diagenesis is found throughout the Anacacho in this area. Following the transgression of Upper Cretaceous and lower Tertiary marine shales, the unconformable surface of the Anacacho was sealed and ultimately trapped migrating hydrocarbons. As the hydrocarbons penetrated secondary freshwater porosity, "dog tooth" spar crystals were dislodged and incorporated into the hydrocarbon matrix by mechanical and/or chemical mechanisms. The hydrocarbons were devolitalized to asphalt following exposure of the Anacacho and, possibly, by thermal activity from local mafic intrusions.
The biosparrudites in the Whites Mine area represent biostromal bank buildups on bathymetric highs that produced abundant skeletal debris. This debris migrated to the southwest, primarily as prograding sand waves, and resulted in deposition of biosparite in the Anacacho Mountains. The skeletal debris built up sufficient bathymetric relief into the photic zone that biostromal growth became active. Therefore, through time, the bank migrated to the southwest over skeletal debris.
End_of_Article - Last_Page 570------------